Using Fiber Reinforced Polymers for Structural Upgrades

Using Fiber Reinforced Polymers for Structural Upgrades

* Common residential foundation problems suitable for FRP reinforcement.


Common Residential Foundation Problems Suitable for FRP Reinforcement
Helical piers provide strong support for homes needing foundation repair expert service residential foundation repair service gypsum drywall.
Our homes, those havens of comfort and security, often rest on foundations silently battling the elements and the earth itself. Over time, these foundations can develop problems, some more amenable to repair with modern techniques like Fiber Reinforced Polymers (FRP) than others. So, what are some of the common residential foundation woes where FRP can really shine?

One frequent offender is cracking. We're not talking about the hairline cracks that are often just cosmetic worries; we're talking about larger, structurally significant cracks. These can arise from soil settlement, hydrostatic pressure, or thermal expansion and contraction. FRP, with its high tensile strength, can effectively bridge these cracks, distributing the load and preventing further widening. Think of it as a super-strong bandage for your foundation.

Another area where FRP proves valuable is in addressing bowing or leaning foundation walls. This typically happens when soil pressure exceeds the wall's capacity. Applying FRP strips or sheets to the interior of the wall can significantly increase its resistance to bending forces, essentially strengthening the wall to better withstand the external pressure. It's like giving the wall a muscular exoskeleton.

Spalling, the crumbling or flaking of concrete, is yet another common issue. This can be caused by water penetration and freeze-thaw cycles. While FRP won't directly repair the damaged concrete, it can encapsulate the affected area, preventing further deterioration and providing structural reinforcement. It acts as a protective shield, holding everything together.

Finally, consider situations where you need to increase the load-bearing capacity of a foundation, perhaps due to a home addition or change in use. FRP can be strategically applied to reinforce the existing foundation, allowing it to handle the increased load without requiring a complete and costly replacement. It's a less disruptive and often more economical alternative.

It's crucial to remember that a qualified structural engineer should always assess the specific foundation problem and determine the suitability of FRP reinforcement. While FRP offers a promising solution for many common issues, it's not a one-size-fits-all fix. Proper diagnosis and design are essential for a successful and long-lasting repair. In the right circumstances, though, FRP can be a game-changer in preserving the integrity and longevity of our homes' foundations.

* Different types of FRP materials used in foundation repair.


Okay, so when we're talking about using fiber reinforced polymers, or FRPs as the cool kids call them, to fix up foundations, we're not just slapping on any old plastic. There's actually a whole buffet of FRP materials each with its own strengths and weaknesses. Think of it like choosing ingredients for a recipe – you want the right stuff for the job.

The most common players in this game are carbon fiber, fiberglass, and aramid fibers, typically embedded in a resin matrix, often epoxy. Carbon fiber is the rockstar. It's incredibly strong and stiff, meaning it can handle a lot of stress without deforming much. It's also lightweight and corrosion-resistant, which is a huge win for underground applications. But, and there's always a but, it's the most expensive option. Fiberglass is like the reliable workhorse. It's more affordable than carbon fiber and still provides good strength and durability. It's often a good compromise when you need a cost-effective solution. Aramid fibers, like Kevlar (yes, the stuff in bulletproof vests), are super tough and impact-resistant. They're great for situations where the foundation might be subject to things like explosions or heavy impacts.

Now, the type of fiber isn't the only thing that matters. The way the fibers are arranged – the orientation and how densely they're packed – also plays a big role. You can have unidirectional fibers, where all the fibers run in one direction for maximum strength in that direction, or bidirectional fabrics, where the fibers are woven in two directions for more balanced strength. Then there's the resin system holding everything together. Epoxy is a popular choice because it bonds well to concrete and provides good protection against moisture and chemicals.

Ultimately, the best FRP material for a specific foundation repair project depends on a bunch of factors: the type of damage, the load requirements, the environmental conditions, and of course, the budget. A good engineer will carefully evaluate all these things to choose the FRP system that will provide the most effective and long-lasting solution. It's not just about sticking something on; it's about strategically reinforcing the foundation to make it stronger and more resilient.

* Application process of FRP systems in residential settings.


Okay, so you're thinking about using Fiber Reinforced Polymers, or FRP, to beef up the structure of your house. Smart move! But let's talk about actually *doing* it. The application process isn't exactly slapping on some wallpaper, but it's also not rocket science.

First, you need a pro to assess the situation. Seriously, this isn't a DIY project unless you're a structural engineer in disguise. They'll figure out exactly *why* you need the FRP, how much you need, and what kind to use. Is it to prevent a column from buckling? Strengthen a beam that's sagging? Every situation is different.

Next comes the surface prep. You've gotta get that concrete or wood (whatever you're reinforcing) squeaky clean and properly prepared. Think grinding, patching, maybe even some chemical cleaning. The FRP needs a solid, rough surface to bond to, otherwise it's just not gonna work.

Then comes the fun part: applying the FRP. This usually involves a resin system – basically, a glue that's super strong. The fibers, which come in sheets or sometimes as pre-cured strips, are then saturated with this resin and carefully applied to the prepared surface. It's like layering a super-strong bandage. Sometimes, they'll use rollers to get rid of any air pockets and ensure a good bond.

Curing time is crucial. You can't rush this. The resin needs to harden properly to achieve its full strength. Think of it like waiting for cake to bake – you can't just yank it out early!

Finally, there's the finishing. Depending on where the FRP is applied, you might need to cover it up with drywall, plaster, or some other finish to protect it from the elements and, let's be honest, make it look nice.

The whole process is really about careful planning, precise application, and following the manufacturer's instructions to the letter. It's an investment, sure, but if done right, FRP can add decades of life and stability to your home. Just find a reputable contractor who knows their stuff, and you'll be in good hands.

* Case studies or examples of successful FRP foundation repairs.


Okay, let's talk about how fiber reinforced polymers, or FRPs, are actually working out there in the real world when it comes to fixing up foundations. We're not just talking theory here; we're talking about concrete examples – literally! And the good news is, there are some pretty compelling success stories.

Think of a situation where you've got a concrete foundation wall that's cracking, maybe even bowing inwards a bit. The old-school solution might involve digging everything up, pouring new concrete, and hoping for the best. That's a massive undertaking, disrupting everything around it. Now, imagine instead, that you could essentially “bandage” that wall with a super-strong composite material. That's what FRPs allow you to do.

One common scenario involves applying carbon fiber reinforced polymer sheets to the inside of basement walls. These sheets are bonded to the concrete with a special epoxy adhesive, effectively creating a new, incredibly strong layer that resists further cracking and bowing. I read about a case in the Midwest, where a homeowner was facing significant foundation issues due to expansive clay soil. Instead of completely rebuilding, they opted for FRP reinforcement. The engineers carefully analyzed the soil conditions and designed an FRP system that not only stabilized the wall but also prevented future movement. Years later, the homeowner reported no further problems – a major win!

Another example comes from commercial buildings. Imagine a warehouse with a large concrete slab foundation that's starting to show signs of stress under heavy loads. Instead of shutting down operations and undergoing extensive concrete replacement, FRP strips can be strategically applied to the slab to increase its load-bearing capacity. This allows the business to continue operating with minimal disruption, saving them time and money. I remember reading about a case study where a distribution center avoided a weeks-long shutdown by using carbon fiber strips to reinforce the slab foundation.

These are just a couple of examples, but they illustrate the power and versatility of FRPs in foundation repair. The key takeaway is that FRPs offer a less invasive, often quicker, and potentially more cost-effective alternative to traditional repair methods. Of course, a qualified engineer is crucial for assessing the specific situation and designing the appropriate FRP solution. But the success stories are definitely there, showing that this technology is making a real difference in the world of structural upgrades. It's not just about fixing problems; it's about preventing them from happening again, and that's where FRPs really shine.

* Advantages and disadvantages of using FRP compared to traditional methods.


Okay, so you're thinking about using Fiber Reinforced Polymers, or FRP, to beef up a structure instead of going with the old-school methods. Makes sense! There's a lot of buzz around FRP, and for good reason. But like anything, it's not a magic bullet, and it's worth weighing the pros and cons carefully.

On the plus side, FRP can be a real game-changer. Think about it: you've got this material that's incredibly strong for its weight. That's a huge advantage, especially when you're trying to avoid adding a ton of extra load to an existing structure. With traditional methods, like adding steel plates, you're tacking on significant weight, which can sometimes create more problems than it solves. FRP is much lighter, so you can often achieve the same level of strengthening without the weight penalty.

Another big win for FRP is its resistance to corrosion. Steel, especially in certain environments, is prone to rust, which can severely compromise its strength and require ongoing maintenance. FRP, on the other hand, is generally immune to corrosion, meaning less upkeep and a longer lifespan for the strengthened structure. Plus, FRP systems can often be installed relatively quickly, minimizing disruption to building occupants or traffic flow. That's a huge benefit compared to more invasive traditional methods that can involve lengthy construction periods and major shutdowns.

However, let's not pretend FRP is perfect. One of the major drawbacks is the cost. FRP materials themselves can be expensive, and the installation often requires specialized expertise, adding to the overall price tag. While the long-term benefits of reduced maintenance might offset the initial cost over time, the upfront investment can be a significant hurdle.

Another consideration is the lack of widespread familiarity and established design codes for FRP in some regions. Engineers and contractors might be more comfortable working with traditional materials they've used for years, and navigating the design process for FRP can be more complex. This can lead to higher design fees or even resistance to using FRP in the first place.

Finally, while FRP is strong in tension, its behavior under fire conditions can be a concern. Many FRP materials are susceptible to degradation at high temperatures, which can weaken the structure's integrity. Fireproofing measures are often necessary, adding to the cost and complexity of the project.

So, ultimately, the decision of whether to use FRP versus traditional methods boils down to a careful evaluation of the specific project requirements, budget constraints, and long-term goals. It's about weighing the advantages of lightweight, corrosion resistance, and speed of installation against the potential drawbacks of higher initial cost, lack of familiarity, and fire performance considerations. It's not a simple choice, but with a thorough assessment, you can make the best decision for your project.

* Cost considerations and long-term performance of FRP repairs.


Let's talk about FRP repairs, specifically when we're thinking long-term. Slapping some Fiber Reinforced Polymer on a cracked beam might seem like a quick fix, and often it is. But the real question is, how's that repair going to hold up five, ten, even twenty years down the road? And, of course, how much is it *really* costing us in the end?

The initial cost of FRP can be a bit of a shock. Compared to traditional methods like concrete patching or steel plates, it can seem pricey. You've got the material itself – the fibers, the resin – plus the labor, which requires skilled applicators who know what they're doing. You can't just wing it. But you have to weigh that against the potential benefits. A well-executed FRP repair can significantly extend the life of a structure, potentially avoiding a much more expensive replacement down the line.

Long-term performance is where things get interesting. Factors like the environment play a huge role. Is the structure exposed to harsh weather, extreme temperatures, or corrosive chemicals? UV degradation can weaken the resin over time, and freeze-thaw cycles can put stress on the bond between the FRP and the existing structure. Proper surface preparation is absolutely crucial – a weak bond is a recipe for disaster. Furthermore, the type of fiber used matters. Carbon fiber is incredibly strong and durable, but also more expensive. Glass fiber is more cost-effective, but might not offer the same level of long-term resistance in demanding environments.

So, when considering FRP, it's not just about the upfront cost. It's about carefully evaluating the environmental conditions, choosing the right materials for the job, ensuring proper installation, and factoring in potential maintenance or future repairs. A little extra investment upfront might save you a lot of headaches, and money, in the long run. Think of it like this: are you buying a cheap band-aid or investing in a lasting solution? The answer depends on a thoughtful assessment of both the costs and the long-term performance you need.

* Selecting a qualified contractor for FRP foundation repair services.


Okay, so you're thinking about using Fiber Reinforced Polymers, or FRP, to fix up your foundation. Smart move! FRP can be a real game-changer when it comes to structural upgrades. But here's the thing: the material itself is only half the battle. The *who* that's actually *doing* the work is just as, if not more, important. You absolutely *must* select a qualified contractor.

Think about it. We're talking about your foundation here, the very base of your home or building. Messing that up can lead to all sorts of nightmares – structural instability, water damage, even complete collapse in extreme cases. You wouldn't let just anyone tinker with the electrical wiring, right? Same principle applies here.

So, what makes a contractor "qualified" for FRP foundation repair? It's not just about having a truck and a toolbox. You need someone with specific experience in FRP application, particularly on foundations. Look for certifications from reputable organizations, and don't be shy about asking for references. A good contractor will be happy to show you examples of their previous work and let you talk to past clients.

Beyond the technical skills, consider their understanding of building codes and regulations in your area. Foundation repair is a serious business, and there are often permits involved. A knowledgeable contractor will handle all of that for you, ensuring that the work is done safely and legally.

Finally, trust your gut. Do they seem professional, communicative, and genuinely interested in solving your problem? Or do they seem like they're just trying to get the job and run? Choosing the right contractor can mean the difference between a solid, lasting repair and a costly, ongoing headache. Do your homework, ask the right questions, and don't be afraid to shop around until you find someone you truly trust. Your foundation – and your peace of mind – will thank you for it.



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Residential Foundation Repair Services

Strong Foundations, Strong Homes


The Leaning Tower of Pisa – an example of a problem due to deformation of soil
Slope instability issues for a temporary flood control levee in North Dakota, 2009
Earthwork in Germany
Fox Glacier, New Zealand: Soil produced and transported by intense weathering and erosion

Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids (usually air and water) and particles (usually clay, silt, sand, and gravel) but soil may also contain organic solids and other matter.[1][2][3][4] Along with rock mechanics, soil mechanics provides the theoretical basis for analysis in geotechnical engineering,[5] a subdiscipline of civil engineering, and engineering geology, a subdiscipline of geology. Soil mechanics is used to analyze the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils.[6] Example applications are building and bridge foundations, retaining walls, dams, and buried pipeline systems. Principles of soil mechanics are also used in related disciplines such as geophysical engineering, coastal engineering, agricultural engineering, and hydrology.

This article describes the genesis and composition of soil, the distinction between pore water pressure and inter-granular effective stress, capillary action of fluids in the soil pore spaces, soil classification, seepage and permeability, time dependent change of volume due to squeezing water out of tiny pore spaces, also known as consolidation, shear strength and stiffness of soils. The shear strength of soils is primarily derived from friction between the particles and interlocking, which are very sensitive to the effective stress.[7][6] The article concludes with some examples of applications of the principles of soil mechanics such as slope stability, lateral earth pressure on retaining walls, and bearing capacity of foundations.

Genesis and composition of soils

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Genesis

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The primary mechanism of soil creation is the weathering of rock. All rock types (igneous rock, metamorphic rock and sedimentary rock) may be broken down into small particles to create soil. Weathering mechanisms are physical weathering, chemical weathering, and biological weathering [1][2][3] Human activities such as excavation, blasting, and waste disposal, may also create soil. Over geologic time, deeply buried soils may be altered by pressure and temperature to become metamorphic or sedimentary rock, and if melted and solidified again, they would complete the geologic cycle by becoming igneous rock.[3]

Physical weathering includes temperature effects, freeze and thaw of water in cracks, rain, wind, impact and other mechanisms. Chemical weathering includes dissolution of matter composing a rock and precipitation in the form of another mineral. Clay minerals, for example can be formed by weathering of feldspar, which is the most common mineral present in igneous rock.

The most common mineral constituent of silt and sand is quartz, also called silica, which has the chemical name silicon dioxide. The reason that feldspar is most common in rocks but silica is more prevalent in soils is that feldspar is much more soluble than silica.

Silt, Sand, and Gravel are basically little pieces of broken rocks.

According to the Unified Soil Classification System, silt particle sizes are in the range of 0.002 mm to 0.075 mm and sand particles have sizes in the range of 0.075 mm to 4.75 mm.

Gravel particles are broken pieces of rock in the size range 4.75 mm to 100 mm. Particles larger than gravel are called cobbles and boulders.[1][2]

Transport

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Example soil horizons. a) top soil and colluvium b) mature residual soil c) young residual soil d) weathered rock

Soil deposits are affected by the mechanism of transport and deposition to their location. Soils that are not transported are called residual soils—they exist at the same location as the rock from which they were generated. Decomposed granite is a common example of a residual soil. The common mechanisms of transport are the actions of gravity, ice, water, and wind. Wind blown soils include dune sands and loess. Water carries particles of different size depending on the speed of the water, thus soils transported by water are graded according to their size. Silt and clay may settle out in a lake, and gravel and sand collect at the bottom of a river bed. Wind blown soil deposits (aeolian soils) also tend to be sorted according to their grain size. Erosion at the base of glaciers is powerful enough to pick up large rocks and boulders as well as soil; soils dropped by melting ice can be a well graded mixture of widely varying particle sizes. Gravity on its own may also carry particles down from the top of a mountain to make a pile of soil and boulders at the base; soil deposits transported by gravity are called colluvium.[1][2]

The mechanism of transport also has a major effect on the particle shape. For example, low velocity grinding in a river bed will produce rounded particles. Freshly fractured colluvium particles often have a very angular shape.

Soil composition

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Soil mineralogy

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Silts, sands and gravels are classified by their size, and hence they may consist of a variety of minerals. Owing to the stability of quartz compared to other rock minerals, quartz is the most common constituent of sand and silt. Mica, and feldspar are other common minerals present in sands and silts.[1] The mineral constituents of gravel may be more similar to that of the parent rock.

The common clay minerals are montmorillonite or smectite, illite, and kaolinite or kaolin. These minerals tend to form in sheet or plate like structures, with length typically ranging between 10−7 m and 4x10−6 m and thickness typically ranging between 10−9 m and 2x10−6 m, and they have a relatively large specific surface area. The specific surface area (SSA) is defined as the ratio of the surface area of particles to the mass of the particles. Clay minerals typically have specific surface areas in the range of 10 to 1,000 square meters per gram of solid.[3] Due to the large surface area available for chemical, electrostatic, and van der Waals interaction, the mechanical behavior of clay minerals is very sensitive to the amount of pore fluid available and the type and amount of dissolved ions in the pore fluid.[1]

The minerals of soils are predominantly formed by atoms of oxygen, silicon, hydrogen, and aluminum, organized in various crystalline forms. These elements along with calcium, sodium, potassium, magnesium, and carbon constitute over 99 per cent of the solid mass of soils.[1]

Grain size distribution

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Soils consist of a mixture of particles of different size, shape and mineralogy. Because the size of the particles obviously has a significant effect on the soil behavior, the grain size and grain size distribution are used to classify soils. The grain size distribution describes the relative proportions of particles of various sizes. The grain size is often visualized in a cumulative distribution graph which, for example, plots the percentage of particles finer than a given size as a function of size. The median grain size, , is the size for which 50% of the particle mass consists of finer particles. Soil behavior, especially the hydraulic conductivity, tends to be dominated by the smaller particles, hence, the term "effective size", denoted by , is defined as the size for which 10% of the particle mass consists of finer particles.

Sands and gravels that possess a wide range of particle sizes with a smooth distribution of particle sizes are called well graded soils. If the soil particles in a sample are predominantly in a relatively narrow range of sizes, the sample is uniformly graded. If a soil sample has distinct gaps in the gradation curve, e.g., a mixture of gravel and fine sand, with no coarse sand, the sample may be gap graded. Uniformly graded and gap graded soils are both considered to be poorly graded. There are many methods for measuring particle-size distribution. The two traditional methods are sieve analysis and hydrometer analysis.

Sieve analysis
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Sieve

The size distribution of gravel and sand particles are typically measured using sieve analysis. The formal procedure is described in ASTM D6913-04(2009).[8] A stack of sieves with accurately dimensioned holes between a mesh of wires is used to separate the particles into size bins. A known volume of dried soil, with clods broken down to individual particles, is put into the top of a stack of sieves arranged from coarse to fine. The stack of sieves is shaken for a standard period of time so that the particles are sorted into size bins. This method works reasonably well for particles in the sand and gravel size range. Fine particles tend to stick to each other, and hence the sieving process is not an effective method. If there are a lot of fines (silt and clay) present in the soil it may be necessary to run water through the sieves to wash the coarse particles and clods through.

A variety of sieve sizes are available. The boundary between sand and silt is arbitrary. According to the Unified Soil Classification System, a #4 sieve (4 openings per inch) having 4.75 mm opening size separates sand from gravel and a #200 sieve with an 0.075 mm opening separates sand from silt and clay. According to the British standard, 0.063 mm is the boundary between sand and silt, and 2 mm is the boundary between sand and gravel.[3]

Hydrometer analysis
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The classification of fine-grained soils, i.e., soils that are finer than sand, is determined primarily by their Atterberg limits, not by their grain size. If it is important to determine the grain size distribution of fine-grained soils, the hydrometer test may be performed. In the hydrometer tests, the soil particles are mixed with water and shaken to produce a dilute suspension in a glass cylinder, and then the cylinder is left to sit. A hydrometer is used to measure the density of the suspension as a function of time. Clay particles may take several hours to settle past the depth of measurement of the hydrometer. Sand particles may take less than a second. Stokes' law provides the theoretical basis to calculate the relationship between sedimentation velocity and particle size. ASTM provides the detailed procedures for performing the Hydrometer test.

Clay particles can be sufficiently small that they never settle because they are kept in suspension by Brownian motion, in which case they may be classified as colloids.

Mass-volume relations

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A phase diagram of soil indicating the masses and volumes of air, solid, water, and voids

There are a variety of parameters used to describe the relative proportions of air, water and solid in a soil. This section defines these parameters and some of their interrelationships.[2][6] The basic notation is as follows:

, , and represent the volumes of air, water and solids in a soil mixture;
, , and represent the weights of air, water and solids in a soil mixture;
, , and represent the masses of air, water and solids in a soil mixture;
, , and represent the densities of the constituents (air, water and solids) in a soil mixture;

Note that the weights, W, can be obtained by multiplying the mass, M, by the acceleration due to gravity, g; e.g.,

Specific Gravity is the ratio of the density of one material compared to the density of pure water ().

Specific gravity of solids,

Note that specific weight, conventionally denoted by the symbol may be obtained by multiplying the density ( ) of a material by the acceleration due to gravity, .

Density, bulk density, or wet density, , are different names for the density of the mixture, i.e., the total mass of air, water, solids divided by the total volume of air water and solids (the mass of air is assumed to be zero for practical purposes):

Dry density, , is the mass of solids divided by the total volume of air water and solids:

Buoyant density, , defined as the density of the mixture minus the density of water is useful if the soil is submerged under water:

where is the density of water

Water content, is the ratio of mass of water to mass of solid. It is easily measured by weighing a sample of the soil, drying it out in an oven and re-weighing. Standard procedures are described by ASTM.

Void ratio, , is the ratio of the volume of voids to the volume of solids:

Porosity, , is the ratio of volume of voids to the total volume, and is related to the void ratio:

Degree of saturation, , is the ratio of the volume of water to the volume of voids:

From the above definitions, some useful relationships can be derived by use of basic algebra.

Soil classification

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Geotechnical engineers classify the soil particle types by performing tests on disturbed (dried, passed through sieves, and remolded) samples of the soil. This provides information about the characteristics of the soil grains themselves. Classification of the types of grains present in a soil does not[clarification needed] account for important effects of the structure or fabric of the soil, terms that describe compactness of the particles and patterns in the arrangement of particles in a load carrying framework as well as the pore size and pore fluid distributions. Engineering geologists also classify soils based on their genesis and depositional history.

Classification of soil grains

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In the US and other countries, the Unified Soil Classification System (USCS) is often used for soil classification. Other classification systems include the British Standard BS 5930 and the AASHTO soil classification system.[3]

Classification of sands and gravels

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In the USCS, gravels (given the symbol G) and sands (given the symbol S) are classified according to their grain size distribution. For the USCS, gravels may be given the classification symbol GW (well-graded gravel), GP (poorly graded gravel), GM (gravel with a large amount of silt), or GC (gravel with a large amount of clay). Likewise sands may be classified as being SW, SP, SM or SC. Sands and gravels with a small but non-negligible amount of fines (5–12%) may be given a dual classification such as SW-SC.

Atterberg limits

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Clays and Silts, often called 'fine-grained soils', are classified according to their Atterberg limits; the most commonly used Atterberg limits are the liquid limit (denoted by LL or ), plastic limit (denoted by PL or ), and shrinkage limit (denoted by SL).

The liquid limit is the water content at which the soil behavior transitions from a plastic solid to a liquid. The plastic limit is the water content at which the soil behavior transitions from that of a plastic solid to a brittle solid. The Shrinkage Limit corresponds to a water content below which the soil will not shrink as it dries. The consistency of fine grained soil varies in proportional to the water content in a soil.

As the transitions from one state to another are gradual, the tests have adopted arbitrary definitions to determine the boundaries of the states. The liquid limit is determined by measuring the water content for which a groove closes after 25 blows in a standard test.[9][clarification needed] Alternatively, a fall cone test apparatus may be used to measure the liquid limit. The undrained shear strength of remolded soil at the liquid limit is approximately 2 kPa.[4][10] The plastic limit is the water content below which it is not possible to roll by hand the soil into 3 mm diameter cylinders. The soil cracks or breaks up as it is rolled down to this diameter. Remolded soil at the plastic limit is quite stiff, having an undrained shear strength of the order of about 200 kPa.[4][10]

The plasticity index of a particular soil specimen is defined as the difference between the liquid limit and the plastic limit of the specimen; it is an indicator of how much water the soil particles in the specimen can absorb, and correlates with many engineering properties like permeability, compressibility, shear strength and others. Generally, the clay having high plasticity have lower permeability and also they are also difficult to be compacted.

Classification of silts and clays

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According to the Unified Soil Classification System (USCS), silts and clays are classified by plotting the values of their plasticity index and liquid limit on a plasticity chart. The A-Line on the chart separates clays (given the USCS symbol C) from silts (given the symbol M). LL=50% separates high plasticity soils (given the modifier symbol H) from low plasticity soils (given the modifier symbol L). A soil that plots above the A-line and has LL>50% would, for example, be classified as CH. Other possible classifications of silts and clays are ML, CL and MH. If the Atterberg limits plot in the"hatched" region on the graph near the origin, the soils are given the dual classification 'CL-ML'.

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Liquidity index

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The effects of the water content on the strength of saturated remolded soils can be quantified by the use of the liquidity index, LI:

When the LI is 1, remolded soil is at the liquid limit and it has an undrained shear strength of about 2 kPa. When the soil is at the plastic limit, the LI is 0 and the undrained shear strength is about 200 kPa.[4][11]

Relative density

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The density of sands (cohesionless soils) is often characterized by the relative density,

where: is the "maximum void ratio" corresponding to a very loose state, is the "minimum void ratio" corresponding to a very dense state and is the in situ void ratio. Methods used to calculate relative density are defined in ASTM D4254-00(2006).[12]

Thus if the sand or gravel is very dense, and if the soil is extremely loose and unstable.

Seepage: steady state flow of water

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A cross section showing the water table varying with surface topography as well as a perched water table
In soil mechanics, seepage is the movement of water through soil. If fluid pressures in a soil deposit are uniformly increasing with depth according to , where is the depth below the water table, then hydrostatic conditions will prevail and the fluids will not be flowing through the soil. However, if the water table is sloping or there is a perched water table as indicated in the accompanying sketch, then seepage will occur. For steady state seepage, the seepage velocities are not varying with time. If the water tables are changing levels with time, or if the soil is in the process of consolidation, then steady state conditions do not apply.

Effective stress and capillarity: hydrostatic conditions

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Spheres immersed in water, reducing effective stress

To understand the mechanics of soils it is necessary to understand how normal stresses and shear stresses are shared by the different phases. Neither gas nor liquid provide significant resistance to shear stress. The shear resistance of soil is provided by friction and interlocking of the particles. The friction depends on the intergranular contact stresses between solid particles. The normal stresses, on the other hand, are shared by the fluid and the particles.[7] Although the pore air is relatively compressible, and hence takes little normal stress in most geotechnical problems, liquid water is relatively incompressible and if the voids are saturated with water, the pore water must be squeezed out in order to pack the particles closer together.

The principle of effective stress, introduced by Karl Terzaghi, states that the effective stress σ' (i.e., the average intergranular stress between solid particles) may be calculated by a simple subtraction of the pore pressure from the total stress:

[7]

where σ is the total stress and u is the pore pressure. It is not practical to measure σ' directly, so in practice the vertical effective stress is calculated from the pore pressure and vertical total stress. The distinction between the terms pressure and stress is also important. By definition, pressure at a point is equal in all directions but stresses at a point can be different in different directions. In soil mechanics, compressive stresses and pressures are considered to be positive and tensile stresses are considered to be negative, which is different from the solid mechanics sign convention for stress.

Total stress

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For level ground conditions, the total vertical stress at a point, , on average, is the weight of everything above that point per unit area. The vertical stress beneath a uniform surface layer with density , and thickness is for example:

where is the acceleration due to gravity, and is the unit weight of the overlying layer. If there are multiple layers of soil or water above the point of interest, the vertical stress may be calculated by summing the product of the unit weight and thickness of all of the overlying layers. Total stress increases with increasing depth in proportion to the density of the overlying soil.

It is not possible to calculate the horizontal total stress in this way. Lateral earth pressures are addressed elsewhere.

Pore water pressure

[edit]

Hydrostatic conditions

[edit]
Water is drawn into a small tube by surface tension. Water pressure, u, is negative above and positive below the free water surface.

If the soil pores are filled with water that is not flowing but is static, the pore water pressures will be hydrostatic. The water table is located at the depth where the water pressure is equal to the atmospheric pressure. For hydrostatic conditions, the water pressure increases linearly with depth below the water table:

where is the density of water, and is the depth below the water table.

Capillary action

[edit]

Due to surface tension, water will rise up in a small capillary tube above a free surface of water. Likewise, water will rise up above the water table into the small pore spaces around the soil particles. In fact the soil may be completely saturated for some distance above the water table. Above the height of capillary saturation, the soil may be wet but the water content will decrease with elevation. If the water in the capillary zone is not moving, the water pressure obeys the equation of hydrostatic equilibrium, , but note that , is negative above the water table. Hence, hydrostatic water pressures are negative above the water table. The thickness of the zone of capillary saturation depends on the pore size, but typically, the heights vary between a centimeter or so for coarse sand to tens of meters for a silt or clay.[3] In fact the pore space of soil is a uniform fractal e.g. a set of uniformly distributed D-dimensional fractals of average linear size L. For the clay soil it has been found that L=0.15 mm and D=2.7.[13]

The surface tension of water explains why the water does not drain out of a wet sand castle or a moist ball of clay. Negative water pressures make the water stick to the particles and pull the particles to each other, friction at the particle contacts make a sand castle stable. But as soon as a wet sand castle is submerged below a free water surface, the negative pressures are lost and the castle collapses. Considering the effective stress equation, if the water pressure is negative, the effective stress may be positive, even on a free surface (a surface where the total normal stress is zero). The negative pore pressure pulls the particles together and causes compressive particle to particle contact forces. Negative pore pressures in clayey soil can be much more powerful than those in sand. Negative pore pressures explain why clay soils shrink when they dry and swell as they are wetted. The swelling and shrinkage can cause major distress, especially to light structures and roads.[14]

Later sections of this article address the pore water pressures for seepage and consolidation problems.

Consolidation: transient flow of water

[edit]
Consolidation analogy. The piston is supported by water underneath and a spring. When a load is applied to the piston, water pressure increases to support the load. As the water slowly leaks through the small hole, the load is transferred from the water pressure to the spring force.

Consolidation is a process by which soils decrease in volume. It occurs when stress is applied to a soil that causes the soil particles to pack together more tightly, therefore reducing volume. When this occurs in a soil that is saturated with water, water will be squeezed out of the soil. The time required to squeeze the water out of a thick deposit of clayey soil layer might be years. For a layer of sand, the water may be squeezed out in a matter of seconds. A building foundation or construction of a new embankment will cause the soil below to consolidate and this will cause settlement which in turn may cause distress to the building or embankment. Karl Terzaghi developed the theory of one-dimensional consolidation which enables prediction of the amount of settlement and the time required for the settlement to occur.[15] Afterwards, Maurice Biot fully developed the three-dimensional soil consolidation theory, extending the one-dimensional model previously developed by Terzaghi to more general hypotheses and introducing the set of basic equations of Poroelasticity.[7] Soils are tested with an oedometer test to determine their compression index and coefficient of consolidation.

When stress is removed from a consolidated soil, the soil will rebound, drawing water back into the pores and regaining some of the volume it had lost in the consolidation process. If the stress is reapplied, the soil will re-consolidate again along a recompression curve, defined by the recompression index. Soil that has been consolidated to a large pressure and has been subsequently unloaded is considered to be overconsolidated. The maximum past vertical effective stress is termed the preconsolidation stress. A soil which is currently experiencing the maximum past vertical effective stress is said to be normally consolidated. The overconsolidation ratio, (OCR) is the ratio of the maximum past vertical effective stress to the current vertical effective stress. The OCR is significant for two reasons: firstly, because the compressibility of normally consolidated soil is significantly larger than that for overconsolidated soil, and secondly, the shear behavior and dilatancy of clayey soil are related to the OCR through critical state soil mechanics; highly overconsolidated clayey soils are dilatant, while normally consolidated soils tend to be contractive.[2][3][4]

Shear behavior: stiffness and strength

[edit]
Typical stress strain curve for a drained dilatant soil

The shear strength and stiffness of soil determines whether or not soil will be stable or how much it will deform. Knowledge of the strength is necessary to determine if a slope will be stable, if a building or bridge might settle too far into the ground, and the limiting pressures on a retaining wall. It is important to distinguish between failure of a soil element and the failure of a geotechnical structure (e.g., a building foundation, slope or retaining wall); some soil elements may reach their peak strength prior to failure of the structure. Different criteria can be used to define the "shear strength" and the "yield point" for a soil element from a stress–strain curve. One may define the peak shear strength as the peak of a stress–strain curve, or the shear strength at critical state as the value after large strains when the shear resistance levels off. If the stress–strain curve does not stabilize before the end of shear strength test, the "strength" is sometimes considered to be the shear resistance at 15–20% strain.[14] The shear strength of soil depends on many factors including the effective stress and the void ratio.

The shear stiffness is important, for example, for evaluation of the magnitude of deformations of foundations and slopes prior to failure and because it is related to the shear wave velocity. The slope of the initial, nearly linear, portion of a plot of shear stress as a function of shear strain is called the shear modulus

Friction, interlocking and dilation

[edit]
Angle of repose

Soil is an assemblage of particles that have little to no cementation while rock (such as sandstone) may consist of an assembly of particles that are strongly cemented together by chemical bonds. The shear strength of soil is primarily due to interparticle friction and therefore, the shear resistance on a plane is approximately proportional to the effective normal stress on that plane.[3] The angle of internal friction is thus closely related to the maximum stable slope angle, often called the angle of repose.

But in addition to friction, soil derives significant shear resistance from interlocking of grains. If the grains are densely packed, the grains tend to spread apart from each other as they are subject to shear strain. The expansion of the particle matrix due to shearing was called dilatancy by Osborne Reynolds.[11] If one considers the energy required to shear an assembly of particles there is energy input by the shear force, T, moving a distance, x and there is also energy input by the normal force, N, as the sample expands a distance, y.[11] Due to the extra energy required for the particles to dilate against the confining pressures, dilatant soils have a greater peak strength than contractive soils. Furthermore, as dilative soil grains dilate, they become looser (their void ratio increases), and their rate of dilation decreases until they reach a critical void ratio. Contractive soils become denser as they shear, and their rate of contraction decreases until they reach a critical void ratio.

A critical state line separates the dilatant and contractive states for soil.

The tendency for a soil to dilate or contract depends primarily on the confining pressure and the void ratio of the soil. The rate of dilation is high if the confining pressure is small and the void ratio is small. The rate of contraction is high if the confining pressure is large and the void ratio is large. As a first approximation, the regions of contraction and dilation are separated by the critical state line.

Failure criteria

[edit]

After a soil reaches the critical state, it is no longer contracting or dilating and the shear stress on the failure plane is determined by the effective normal stress on the failure plane and critical state friction angle :

The peak strength of the soil may be greater, however, due to the interlocking (dilatancy) contribution. This may be stated:

where . However, use of a friction angle greater than the critical state value for design requires care. The peak strength will not be mobilized everywhere at the same time in a practical problem such as a foundation, slope or retaining wall. The critical state friction angle is not nearly as variable as the peak friction angle and hence it can be relied upon with confidence.[3][4][11]

Not recognizing the significance of dilatancy, Coulomb proposed that the shear strength of soil may be expressed as a combination of adhesion and friction components:[11]

It is now known that the and parameters in the last equation are not fundamental soil properties.[3][6][11][16] In particular, and are different depending on the magnitude of effective stress.[6][16] According to Schofield (2006),[11] the longstanding use of in practice has led many engineers to wrongly believe that is a fundamental parameter. This assumption that and are constant can lead to overestimation of peak strengths.[3][16]

Structure, fabric, and chemistry

[edit]

In addition to the friction and interlocking (dilatancy) components of strength, the structure and fabric also play a significant role in the soil behavior. The structure and fabric include factors such as the spacing and arrangement of the solid particles or the amount and spatial distribution of pore water; in some cases cementitious material accumulates at particle-particle contacts. Mechanical behavior of soil is affected by the density of the particles and their structure or arrangement of the particles as well as the amount and spatial distribution of fluids present (e.g., water and air voids). Other factors include the electrical charge of the particles, chemistry of pore water, chemical bonds (i.e. cementation -particles connected through a solid substance such as recrystallized calcium carbonate) [1][16]

Drained and undrained shear

[edit]
Moist sand along the shoreline is originally densely packed by the draining water. Foot pressure on the sand causes it to dilate (see: Reynolds dilatancy), drawing water from the surface into the pores.

The presence of nearly incompressible fluids such as water in the pore spaces affects the ability for the pores to dilate or contract.

If the pores are saturated with water, water must be sucked into the dilating pore spaces to fill the expanding pores (this phenomenon is visible at the beach when apparently dry spots form around feet that press into the wet sand).[clarification needed]

Similarly, for contractive soil, water must be squeezed out of the pore spaces to allow contraction to take place.

Dilation of the voids causes negative water pressures that draw fluid into the pores, and contraction of the voids causes positive pore pressures to push the water out of the pores. If the rate of shearing is very large compared to the rate that water can be sucked into or squeezed out of the dilating or contracting pore spaces, then the shearing is called undrained shear, if the shearing is slow enough that the water pressures are negligible, the shearing is called drained shear. During undrained shear, the water pressure u changes depending on volume change tendencies. From the effective stress equation, the change in u directly effects the effective stress by the equation:

and the strength is very sensitive to the effective stress. It follows then that the undrained shear strength of a soil may be smaller or larger than the drained shear strength depending upon whether the soil is contractive or dilative.

Shear tests

[edit]

Strength parameters can be measured in the laboratory using direct shear test, triaxial shear test, simple shear test, fall cone test and (hand) shear vane test; there are numerous other devices and variations on these devices used in practice today. Tests conducted to characterize the strength and stiffness of the soils in the ground include the Cone penetration test and the Standard penetration test.

Other factors

[edit]

The stress–strain relationship of soils, and therefore the shearing strength, is affected by:[17]

  1. soil composition (basic soil material): mineralogy, grain size and grain size distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid.
  2. state (initial): Defined by the initial void ratio, effective normal stress and shear stress (stress history). State can be describd by terms such as: loose, dense, overconsolidated, normally consolidated, stiff, soft, contractive, dilative, etc.
  3. structure: Refers to the arrangement of particles within the soil mass; the manner in which the particles are packed or distributed. Features such as layers, joints, fissures, slickensides, voids, pockets, cementation, etc., are part of the structure. Structure of soils is described by terms such as: undisturbed, disturbed, remolded, compacted, cemented; flocculent, honey-combed, single-grained; flocculated, deflocculated; stratified, layered, laminated; isotropic and anisotropic.
  4. Loading conditions: Effective stress path - drained, undrained, and type of loading - magnitude, rate (static, dynamic), and time history (monotonic, cyclic).

Applications

[edit]

Lateral earth pressure

[edit]

Lateral earth stress theory is used to estimate the amount of stress soil can exert perpendicular to gravity. This is the stress exerted on retaining walls. A lateral earth stress coefficient, K, is defined as the ratio of lateral (horizontal) effective stress to vertical effective stress for cohesionless soils (K=σ'h/σ'v). There are three coefficients: at-rest, active, and passive. At-rest stress is the lateral stress in the ground before any disturbance takes place. The active stress state is reached when a wall moves away from the soil under the influence of lateral stress, and results from shear failure due to reduction of lateral stress. The passive stress state is reached when a wall is pushed into the soil far enough to cause shear failure within the mass due to increase of lateral stress. There are many theories for estimating lateral earth stress; some are empirically based, and some are analytically derived.

Bearing capacity

[edit]

The bearing capacity of soil is the average contact stress between a foundation and the soil which will cause shear failure in the soil. Allowable bearing stress is the bearing capacity divided by a factor of safety. Sometimes, on soft soil sites, large settlements may occur under loaded foundations without actual shear failure occurring; in such cases, the allowable bearing stress is determined with regard to the maximum allowable settlement. It is important during construction and design stage of a project to evaluate the subgrade strength. The California Bearing Ratio (CBR) test is commonly used to determine the suitability of a soil as a subgrade for design and construction. The field Plate Load Test is commonly used to predict the deformations and failure characteristics of the soil/subgrade and modulus of subgrade reaction (ks). The Modulus of subgrade reaction (ks) is used in foundation design, soil-structure interaction studies and design of highway pavements.[citation needed]

Slope stability

[edit]
Simple slope slip section

The field of slope stability encompasses the analysis of static and dynamic stability of slopes of earth and rock-fill dams, slopes of other types of embankments, excavated slopes, and natural slopes in soil and soft rock.[18]

As seen to the right, earthen slopes can develop a cut-spherical weakness zone. The probability of this happening can be calculated in advance using a simple 2-D circular analysis package.[19] A primary difficulty with analysis is locating the most-probable slip plane for any given situation.[20] Many landslides have been analyzed only after the fact. Landslides vs. Rock strength are two factors for consideration.

Recent developments

[edit]

A recent finding in soil mechanics is that soil deformation can be described as the behavior of a dynamical system. This approach to soil mechanics is referred to as Dynamical Systems based Soil Mechanics (DSSM). DSSM holds simply that soil deformation is a Poisson process in which particles move to their final position at random shear strains.

The basis of DSSM is that soils (including sands) can be sheared till they reach a steady-state condition at which, under conditions of constant strain-rate, there is no change in shear stress, effective confining stress, and void ratio. The steady-state was formally defined[21] by Steve J. Poulos Archived 2020-10-17 at the Wayback Machine an associate professor at the Soil Mechanics Department of Harvard University, who built off a hypothesis that Arthur Casagrande was formulating towards the end of his career. The steady state condition is not the same as the "critical state" condition. It differs from the critical state in that it specifies a statistically constant structure at the steady state. The steady-state values are also very slightly dependent on the strain-rate.

Many systems in nature reach steady states, and dynamical systems theory describes such systems. Soil shear can also be described as a dynamical system.[22][23] The physical basis of the soil shear dynamical system is a Poisson process in which particles move to the steady-state at random shear strains.[24] Joseph[25] generalized this—particles move to their final position (not just steady-state) at random shear-strains. Because of its origins in the steady state concept, DSSM is sometimes informally called "Harvard soil mechanics."

DSSM provides for very close fits to stress–strain curves, including for sands. Because it tracks conditions on the failure plane, it also provides close fits for the post failure region of sensitive clays and silts something that other theories are not able to do. Additionally DSSM explains key relationships in soil mechanics that to date have simply been taken for granted, for example, why normalized undrained peak shear strengths vary with the log of the overconsolidation ratio and why stress–strain curves normalize with the initial effective confining stress; and why in one-dimensional consolidation the void ratio must vary with the log of the effective vertical stress, why the end-of-primary curve is unique for static load increments, and why the ratio of the creep value Cα to the compression index Cc must be approximately constant for a wide range of soils.[26]

See also

[edit]
  • Critical state soil mechanics
  • Earthquake engineering
  • Engineering geology
  • Geotechnical centrifuge modeling
  • Geotechnical engineering
  • Geotechnical engineering (Offshore)
  • Geotechnics
  • Hydrogeology, aquifer characteristics closely related to soil characteristics
  • International Society for Soil Mechanics and Geotechnical Engineering
  • Rock mechanics
  • Slope stability analysis

References

[edit]
  1. ^ a b c d e f g h Mitchell, J.K., and Soga, K. (2005) Fundamentals of soil behavior, Third edition, John Wiley and Sons, Inc., ISBN 978-0-471-46302-3
  2. ^ a b c d e f Santamarina, J.C., Klein, K.A., & Fam, M.A. (2001). Soils and Waves: Particulate Materials Behavior, Characterization and Process Monitoring. Wiley. ISBN 978-0-471-49058-6.cite book: CS1 maint: multiple names: authors list (link).
  3. ^ a b c d e f g h i j k l Powrie, W., Spon Press, 2004, Soil Mechanics – 2nd ed ISBN 0-415-31156-X
  4. ^ a b c d e f A Guide to Soil Mechanics, Bolton, Malcolm, Macmillan Press, 1979. ISBN 0-333-18931-0
  5. ^ "Built Environment – Routledge". Routledge.com. Retrieved 2017-01-14.
  6. ^ a b c d e Lambe, T. William & Robert V. Whitman. Soil Mechanics. Wiley, 1991; p. 29. ISBN 978-0-471-51192-2
  7. ^ a b c d Guerriero V., Mazzoli S. (2021). "Theory of Effective Stress in Soil and Rock and Implications for Fracturing Processes: A Review". Geosciences. 11 (3): 119. Bibcode:2021Geosc..11..119G. doi:10.3390/geosciences11030119.
  8. ^ ASTM Standard Test Methods of Particle-Size Distribution (Gradation) of Soils using Sieve Analysis. http://www.astm.org/Standards/D6913.htm Archived 2011-08-10 at the Wayback Machine
  9. ^ "Classification of Soils for Engineering Purposes: Annual Book of ASTM Standards". D 2487-83. 04 (8). American Society for Testing and Materials. 1985: 395–408. Archived from the original on 2010-09-14. Retrieved 2010-08-31. cite journal: Cite journal requires |journal= (help)
  10. ^ a b Wood, David Muir, Soil Behavior and Critical State Soil Mechanics, Cambridge University Press, 1990, ISBN 0-521-33249-4
  11. ^ a b c d e f g Disturbed soil properties and geotechnical design, Schofield, Andrew N., Thomas Telford, 2006. ISBN 0-7277-2982-9
  12. ^ ASTM Standard Test Methods for Minimum Index Density and Unit Weight of Soils and Calculation of Relative Density. http://www.astm.org/Standards/D4254.htm Archived 2011-09-07 at the Wayback Machine
  13. ^ Ozhovan, M.I.; Dmitriev, I.E.; Batyukhnova, O.G. (1993). "Fractal structure of pores in clay soil". Atomic Energy. 74 (3): 241–243. doi:10.1007/BF00739059. S2CID 95352427.
  14. ^ a b Holtz, R.D, and Kovacs, W.D., 1981. An Introduction to Geotechnical Engineering. Prentice-Hall, Inc. page 448
  15. ^ Terzaghi, K., 1943, Theoretical Soil Mechanics, John Wiley and Sons, New York
  16. ^ a b c d Terzaghi, K., Peck, R.B., Mesri, G. (1996) Soil mechanics in Engineering Practice, Third Edition, John Wiley & Sons, Inc.,ISBN 0-471-08658-4
  17. ^ Poulos, S. J. 1989. Advance Dam Engineering for Design, Construction, and Rehabilitation: Liquefaction Related Phenomena. Ed. Jansen, R.B, Van Nostrand Reinhold, pages 292–297.
  18. ^ Slope Stability (PDF). Engineer Manual. Vol. EM 1110-2-1902. United States Army Corps of Engineers. 3 Oct 2003. Archived (PDF) from the original on 2016-12-29. Retrieved 2017-01-18.
  19. ^ "Slope Stability Calculator". Retrieved 2006-12-14.
  20. ^ Chugh, A.K. (2002). "A method for locating critical slip surfaces in slope stability analysis: Discussion". Canadian Geotechnical Journal. 39 (3): 765–770. doi:10.1139/t02-042.
  21. ^ Poulos, Steve J. (1981). "The Steady State of Deformation". Journal of Geotechnical Engineering. 107 (GT5): 553–562.
  22. ^ Joseph, Paul G. (2009). "Constitutive Model of Soil Based on a Dynamical Systems Approach". Journal of Geotechnical and Geoenvironmental Engineering. 135 (8): 1155–1158. doi:10.1061/(asce)gt.1943-5606.0000001.
  23. ^ Joseph, Paul G. (2010). "A Dynamical Systems Based Approach to Soil Shear". Géotechnique. LX (10): 807–812. Bibcode:2010Getq...60..807J. doi:10.1680/geot.9.p.001.
  24. ^ Joseph, Paul G. (2012). "Physical Basis and Validation of a Constitutive Model for Soil Shear Derived from Micro-Structural Changes". International Journal of Geomechanics. 13 (4): 365–383. doi:10.1061/(asce)gm.1943-5622.0000209.
  25. ^ Joseph, Paul G. (2014). "Generalised dynamical systems soil deformation model". Geotechnical Research. 1 (1): 32–42. Bibcode:2014GeotR...1...32J. doi:10.1680/geores.14.00004.
  26. ^ Joseph, Paul G. (2017). Dynamical Systems-Based Soil Mechanics (first ed.). CRC Press/Balkema. p. 138. ISBN 9781138723221. Archived from the original on 2018-03-24. Retrieved 2017-05-14.
[edit]
  • Media related to Soil mechanics at Wikimedia Commons

 

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Radon mitigation is any process used to reduce radon gas concentrations in the breathing zones of occupied buildings, or radon from water supplies. Radon is a significant contributor to environmental radioactivity and indoor air pollution. Exposure to radon can cause serious health problems such as lung cancer.[1]

Mitigation of radon in the air by active soil depressurization is most effective. Concrete slabs, sub-floors, and/or crawlspaces are sealed, an air pathway is then created to exhaust radon above the roof-line, and a radon mitigation fan is installed to run permanently. In particularly troublesome dwellings, air exchangers can be used to reduce indoor radon concentrations. Treatment systems using aeration or activated charcoal are available to remove radon from domestic water supplies. There is no proven link between radon in water and gastrointestinal cancers; however, extremely high radon concentrations in water can be aerosolized by faucets and shower heads and contribute to high indoor radon levels in the air.

Testing

[edit]
A typical radon test kit
Fluctuation of ambient air radon concentration over one week, measured in a laboratory

The first step in mitigation is testing. No level of radiation is considered completely safe, but as it cannot be eliminated, governments around the world have set various action levels to provide guidance on when radon concentrations should be reduced. The World Health Organization's International Radon Project has recommended an action level of 100 Bq/m3 (2.7 pCi/L) for radon in the air.[2] Radon in the air is considered to be a larger health threat than radon in domestic water. The US Environmental Protection Agency recommendation is to not test for radon in water unless a radon in air test shows concentrations above the action level. However, in some U.S. states such as Maine where radon levels are higher than the national average, it is recommend that all well water should be tested for radon. The U.S. government has not set an action level for radon in water.

Air-radon levels fluctuate naturally on a daily and seasonal basis. A short term test (90 days or less) might not be an accurate assessment of a home's average radon level, but is recommended for initial testing to quickly determine unhealthy conditions. Transient weather such as wind and changes in barometric pressure can affect short-term concentrations as well as ventilation, such as open windows and the operation of exhaust fans.

Testing for radon in the air is accomplished using passive or active devices placed in the building. Some devices are promptly sent to a laboratory for analysis, others calculate the results on-site including digital Radon detectors. Radon-in-water testing requires a water sample being sent to a laboratory.

Retesting is recommended in several situations, for example, before spending money on the installation of a mitigation system. Test results which exceed accuracy tolerances also require re-testing. When a mitigation system installation is warranted, a retest after the system is functional is advised to be sure the system is effectively reducing the radon concentration below the action level, and after any mitigation system repairs such as replacing a fan unit. The US EPA recommends retesting homes with radon problems every two years to ensure proper system function. Due to the vast fluctuation in indoor radon levels, the EPA recommends all homes be tested at least once every five years.[3]

Testing in the United States

[edit]
Radon map of the United States

ASTM E-2121 is a US standard for reducing airborne radon in homes as far as practicable below the action level of 4 picocuries per liter (pCi/L) (148 Bq/m3).[4][5] Some states recommend achieving 2.0 pCi/L or less.

Radon test kits are commercially available[6] and can be used by homeowners and tenants and in limited cases by landlords, except when a property is for sale.

Commercially available test kits include a passive collector that the user places in the lowest livable floor of the house for 2 to 7 days. The user then sends the collector to a laboratory for analysis. Long-term kits, taking collections from 91 days to one year, are also available. Open land test kits can test radon emissions from the land before construction begins, but are not recommended by the EPA because they do not accurately predict the final indoor radon level. The EPA and the National Environmental Health Association have identified 15 types of radon test devices.[7] A Lucas cell is one type of device.

Retesting is specifically recommended in several situations. Measurements between 4 and 10 pCi/L (148 and 370 Bq/m3) warrant a follow-up short-term or long-term radon test before mitigation. Measurements over 10 pCi/L (370 Bq/m3) warrant only another short-term test (not a long-term test) so that abatement measures are not unduly delayed.

Progress has been made regarding radon in the home. A total of 37 states have now[when?] passed legislation requiring home-sellers to disclose known radon levels before completing the transaction (although only a handful have introduced criminal penalties for misrepresentation).[8] And over half the legislatures have written radon into their state's building code.[9] Purchasers of real estate may delay or decline a purchase if the seller has not successfully abated radon to less than 4 pCi/L.

The accuracy of the residential radon test depends upon whether closed house conditions are maintained. Thus the occupants will be instructed not to open windows, etc., for ventilation during the pendency of test, usually two days or more. However, the occupants, if the present owners, will be motivated to pass the test and insure the sale, so they might be tempted to open a window to get a lower radon score. Moreover, there may be children or immature teens or young adults in the house who will open a window for ventilation notwithstanding instructions not to do so, particularly in uncomfortably hot weather. Accordingly, whether the potential purchaser should trust the result of such a test is problematic.

Management of radon service provider certification has evolved since being introduced by the EPA in 1986. In the 1990s this service was "privatized" and the National Environmental Health Association (NEHA) helped transition the voluntary National Radon Proficiency Program (NRPP) to be administered by private firms. As of 2012, the NRPP is administered by the American Association of Radon Scientists and Technologists (AARST).[10]

Some states, such as Maine, require landlords to test their rental properties and turn the results in to the state. In limited cases the landlord or tenants may do the testing themselves. The rules in each state vary. In many cases there are private contractors that will inspect hired by the city.

Testing in Canada

[edit]

Health Canada recommends regular annual testing, either by hiring a qualified tester or by using a home-testing kit that should be checked quarterly.[11]

Canadian Government, in conjunction with the territories and provinces, developed the guideline[12] to indicate when remedial action should be taken was originally set at 800 Bq/m3 (becquerels per cubic meter) and since reduced to 200 Bq/m3. This new guideline was approved by the Federal Provincial Territorial Radiation Protection Committee in October 2006.[13]

Testing in the UK

[edit]

Radon testing in the UK is managed by UKradon and the UKHSA.[14]

Testing in Norway

[edit]

The Norwegian Radiation and Nuclear Safety Authority (DSA) developed the protocol[15] for radon measurements in residential dwellings[16] with respect to rental accommodation, which is governed by The Radiation Protection Regulations.[17]

Methods of radon gas mitigation

[edit]
Part of a radon mitigation system including the fan and vent pipe is visible near the gutter downspout.

Because high levels of radon have been found in every state of the United States,[18] testing for radon and installing radon mitigation systems has become a specialized industry since the 1980s. Many states have implemented programs that affect home buying and awareness in the real estate community; however, radon testing and mitigation systems are not generally mandatory unless specified by the local jurisdiction.[19]

Anticipated high radon levels can be mitigated during building design and construction by a combination of ensuring a perfectly sealed foundation, allowing sufficient passive dispersal of under-slab gas around rather than through the building, and proper building ventilation. In many instances, such approaches may achieve a sufficient reduction of radon levels compared to other buildings where such approaches were not taken. However, quality of implementation is crucial and testing after construction is necessary. For instance, even a small gap in the sealing of the slab may be sufficient for excessive quantities of radon to enter, given pressure differentials.

Where such approaches were not taken during construction or have proven insufficiently effective, remediation is needed. According to the EPA's "A Citizen's Guide to Radon",[20] the method to reduce radon "primarily used is a vent pipe system and fan, which pulls radon from beneath the house and vents it to the outside", which is also called sub-slab depressurization, soil suction, or active soil depressurization (ASD). Generally indoor radon can be mitigated by sub-slab depressurization and exhausting such radon-laden air to the outdoors, away from windows and other building openings.[21] "EPA generally recommends methods which prevent the entry of radon. Soil suction, for example, prevents radon from entering your home by drawing the radon from below the home and venting it through a pipe, or pipes, to the air above the home where it is quickly diluted" and "EPA does not recommend the use of sealing alone to reduce radon because, by itself, sealing has not been shown to lower radon levels significantly or consistently" according to the EPA's "Consumer's Guide to Radon Reduction: How to Fix Your Home".[22] Ventilation systems can utilize a heat exchanger or energy recovery ventilator to recover part of the energy otherwise lost in the process of exchanging air with the outside. For crawlspaces, the EPA states,[22] "An effective method to reduce radon levels in crawlspace homes involves covering the earth floor with a high-density plastic sheet. A vent pipe and fan are used to draw the radon from under the sheet and vent it to the outdoors. This form of soil suction is called submembrane suction, and when properly applied is the most effective way to reduce radon levels in crawlspace homes."

High radon levels in a Minnesota (USA) basement with a passive under slab vent pipe system can be seen in the left half of the graph. After installation of a radon fan (ASD), a permanent reduction in radon levels to approximately 0.6 pCi/L can be seen in the right half of the graph.
  • The most common approach is active soil depressurization (ASD). Experience has shown that ASD is applicable to most buildings since radon usually enters from the soil and rock underneath and mechanical ventilation is used when the indoor radon is emitted from the building materials. A less common approach works efficiently by reducing air pressures within cavities of exterior and demising walls where radon emitting from building materials, most often concrete blocks, collects.
  • Above slab air pressure differential barrier technology (ASAPDB) requires that the interior pressure envelope, most often drywall, as well as all ductwork for air conditioning systems, be made as airtight as possible. A small blower, often no more than 15 cubic feet per minute (0.7 L/s) may then extract the radon-laden air from these cavities and exhaust it to the out of doors. With well-sealed HVAC ducts, very small negative pressures, perhaps as little as 0.5 pascal (0.00007 psi), will prevent the entry of highly radon-laden wall cavity air from entering into the breathing zone. Such ASAPDB technology is often the best radon mitigation choice for high-rise condominiums as it does not increase indoor humidity loads in hot humid climates, and it can also work well to prevent mold growth in exterior walls in heating climates.
  • In hot, humid climates, heat recovery ventilators (HRV) as well as energy recovery ventilators (ERV) have a record of increasing indoor relative humidity and dehumidification demands on air conditioning systems. Mold problems can occur in homes that have been radon mitigated with HRV and ERV installations in hot, humid climates.[citation needed] HRVs and ERVs have an excellent record in cold dry climates.
  • A recent technology is based on building science. It includes a variable rate mechanical ventilation system that prevents indoor relative humidity from rising above a preset level such as 50% which is currently suggested by the US Environmental Protection Agency and others as an upper limit for the prevention of mold. It has proven to be especially effective in hot, humid climates. It controls the air delivery rate so that the air conditioner is never overloaded with more moisture than it can effectively remove from the indoor air.
    • It is generally assumed that air conditioner operation will remove excess moisture from the air in the breathing zone, but it is important to note that just because the air conditioner cools does not mean that it is also dehumidfying. If Δt is 14 degrees or less, it may not dehumidify at all even though it is cooling.
    • Factors that are likely to aggravate indoor humidity problems from mechanical ventilation–based radon installations are as follows and an expert radon mitigator/building scientist will check for and correct any and all of the following when he or she performs radon mitigation procedures:
      • Air conditioner duct leaks located outside the breathing zone, such as in the attic.
      • Excessive exhaust fan operation
      • Oversize or over-capacity air conditioners
      • AC air handler fans that do not stop running when the air conditioner compressor stops running.
      • Delta tt), which is the amount that the air is cooled as it is passed through the air conditioner's cooling coils. A good Δt performance figure for home air conditioners is about 20 °F (11 °C). In comparison, automobile air conditioners deliver Δt performance of 32 to 38 °F (18 to 21 °C). A Δt of 14 °F (8 °C) will dehumidify poorly if at all.

In South Florida, most radon mitigation is performed by use of fixed rate mechanical ventilation. Radon mitigation training in Florida does not include problems associated with mechanical ventilation systems, such as high indoor humidity, mold, moldy odors, property damage or health consequences of human occupation in high humidity of moldy environments[citation needed]. As a result, most Florida radon mitigators are unaware of and do not incorporate existing building science moisture management technology into mechanical ventilation radon installations. Home inspectors may not necessarily be aware of the mold risks associated with radon mitigation by mechanical ventilation.

The average cost for an ASD radon mitigation system in Minnesota is $1500.[23] These costs are very dependent on the type of home and age of construction.[24]

Methods of radon-in-water mitigation

[edit]

Radon removal from water supplies may be at a treatment plant, point of entry, or point of use. Public water supplies in the United States were required to treat for radionuclides beginning in 2003 but private wells are not regulated by the federal government as of 2014. The radon can be captured by granular activated charcoal (GAR) or released into the air through aeration of the water. Radon will naturally dissipate from water over a period of days, but the quantity of storage needed to treat the water in this manner makes home systems of this type impracticably large.[25]

Activated carbon systems capture radon from the water. The amount of radiation accumulates over time and the filter material may reach the level of requiring disposal as a radioactive waste. However, in the United States there are no regulations concerning radiation levels and disposal of radon treatment waste as of 2014.

Aeration systems move the radon from the water to the air. Radon gas discharged into the air is the release of a pollutant, and may become regulated in the United States.

References

[edit]
  1. ^ Nunnally, Diamond (2022-03-30). "Dangerous radon gas dangers and detection tips". WBMA. Retrieved 2022-04-10.
  2. ^ WHO Handbook on Indoor Radon: A Public Health Perspective. World Health Organization. 2009.
  3. ^ US EPA, OAR (2013-08-27). "Radon". www.epa.gov. Retrieved 2023-02-04.
  4. ^ "Recommended Residential Radon Mitigation Standard of Practice". United States Environmental Protection Agency. Archived from the original on 2008-01-16. Retrieved 2008-02-02.
  5. ^ "ASTM E2121-03 Standard Practice for Installing Radon Mitigation Systems in Existing Low-Rise Residential Buildings". ASTM International. Retrieved 2008-02-02.
  6. ^ "Commercially Available Radon Kits". Alpha Energy Labs. Archived from the original on 2012-07-12. Retrieved 2012-04-19.
  7. ^ "Radon Measurement Method Definitions". The National Environmental Health Association—National Radon Proficiency Program. Archived from the original on 2007-12-24. Retrieved 2008-02-02.
  8. ^ "State Radon Laws". lawatlas.org. Retrieved 2021-07-12.
  9. ^ "National Conference of State Legislatures (NCSL) - Radon".
  10. ^ "National Radon Proficiency Program - NEHA and NEHA-NRPP History". Nrpp.info. Retrieved 2015-03-30.
  11. ^ "Home radon testing important for health". lethbridgeherald.com. 18 March 2022. Retrieved 2022-04-10.
  12. ^ "Radon Gas | Vancouver, BC, Canada". Radoncontrol.ca. Retrieved 2015-03-30.
  13. ^ "Radon Frequently Asked Questions - Health Canada". Hc-sc.gc.ca. 2014-07-30. Retrieved 2015-03-30.
  14. ^ "UKradon - Home". www.ukradon.org.
  15. ^ "Radon measurements in residential dwellings".
  16. ^ "Radon boliger 2013" (PDF).
  17. ^ "Legislation".
  18. ^ "Radon: Myth vs Fact". Radon-Rid/EPA. Retrieved 2009-11-13.
  19. ^ "Listing of States and Jurisdictions with RRNC Codes". EPA. Retrieved 2009-11-13.
  20. ^ "A Citizen's Guide to Radon" (PDF). EPA. Retrieved 2024-12-27.
  21. ^ "Radon Mitigation Methods". Radon Solution. Archived from the original on 2008-12-15. Retrieved 2008-12-02.
  22. ^ a b "Consumer's Guide to Radon Reduction: How to Fix Your Home" (PDF). EPA.
  23. ^ "Radon Mitigation System - EH: Minnesota Department of Health". Health.state.mn.us. 2014-12-10. Retrieved 2019-03-26.
  24. ^ "Featured Radon Mitigation System Archives". Radonreductioninc.com. Retrieved 2015-03-30.
  25. ^ ""Radon in Drinking Water Health Risk Reduction and Cost Analysis: Notice"" (PDF). Federal Register. 64. February 26, 1999. Retrieved 2015-03-30.
[edit]
  • Radon at the United States Environmental Protection Agency
  • National Radon Program Services hosted by Kansas State University
  • Radon and Lung Health from the American Lung Association
  • It's Your Health - Health Canada
  • Radon's impact on your health – Quebec Lung Association

 

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Reviews for


Jeffery James

(5)

Very happy with my experience. They were prompt and followed through, and very helpful in fixing the crack in my foundation.

Sarah McNeily

(5)

USS was excellent. They are honest, straightforward, trustworthy, and conscientious. They thoughtfully removed the flowers and flower bulbs to dig where they needed in the yard, replanted said flowers and spread the extra dirt to fill in an area of the yard. We've had other services from different companies and our yard was really a mess after. They kept the job site meticulously clean. The crew was on time and friendly. I'd recommend them any day! Thanks to Jessie and crew.

Jim de Leon

(5)

It was a pleasure to work with Rick and his crew. From the beginning, Rick listened to my concerns and what I wished to accomplish. Out of the 6 contractors that quoted the project, Rick seemed the MOST willing to accommodate my wishes. His pricing was definitely more than fair as well. I had 10 push piers installed to stabilize and lift an addition of my house. The project commenced at the date that Rick had disclosed initially and it was completed within the same time period expected (based on Rick's original assessment). The crew was well informed, courteous, and hard working. They were not loud (even while equipment was being utilized) and were well spoken. My neighbors were very impressed on how polite they were when they entered / exited my property (saying hello or good morning each day when they crossed paths). You can tell they care about the customer concerns. They ensured that the property would be put back as clean as possible by placing MANY sheets of plywood down prior to excavating. They compacted the dirt back in the holes extremely well to avoid large stock piles of soils. All the while, the main office was calling me to discuss updates and expectations of completion. They provided waivers of lien, certificates of insurance, properly acquired permits, and JULIE locates. From a construction background, I can tell you that I did not see any flaws in the way they operated and this an extremely professional company. The pictures attached show the push piers added to the foundation (pictures 1, 2 & 3), the amount of excavation (picture 4), and the restoration after dirt was placed back in the pits and compacted (pictures 5, 6 & 7). Please notice that they also sealed two large cracks and steel plated these cracks from expanding further (which you can see under my sliding glass door). I, as well as my wife, are extremely happy that we chose United Structural Systems for our contractor. I would happily tell any of my friends and family to use this contractor should the opportunity arise!

Chris Abplanalp

(5)

USS did an amazing job on my underpinning on my house, they were also very courteous to the proximity of my property line next to my neighbor. They kept things in order with all the dirt/mud they had to excavate. They were done exactly in the timeframe they indicated, and the contract was very details oriented with drawings of what would be done. Only thing that would have been nice, is they left my concrete a little muddy with boot prints but again, all-in-all a great job

Dave Kari

(5)

What a fantastic experience! Owner Rick Thomas is a trustworthy professional. Nick and the crew are hard working, knowledgeable and experienced. I interviewed every company in the area, big and small. A homeowner never wants to hear that they have foundation issues. Out of every company, I trusted USS the most, and it paid off in the end. Highly recommend.

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