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Expansive Soils

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Marketing disclaimer -  this is neither entertaining nor entertainment - it is a small excerpt from a book I am writing and is meant as advance publicity. 
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Expansive Soils
 
Most soils are formed, at least partially, through physical and chemical decomposition of rock. Physical weathering (erosion, abrasion,  freezing and thawing) break the solid rock into fragments; chemical weathering (oxidation and attack by natural acids) further decomposes the minerals that are not chemically stable.
 
 In engineering work, soils are typically classified by particle size, with the amount of each particle size determined by a sieve analysis.  Particles that are smaller than the No. 200 (75 µm) sieve (silts and clays) are referred to as fines.  (A No. 200 sieve has 200 openings per inch in each direction, for a total of 40,000 openings per square inch.)
 
The No. 200 sieve is the smallest sieve used to separate particle sizes.  Smaller particles must be separated by a hydrometer analysis that determines particle size based on how long it takes the particle to settle.  Particles larger than 2 micrometers (2 µm) are considered to be silt; particles smaller than 2 µm are clays. A given soil will typically have a wide range of particle sizes; a soil is usually considered fine-grained if 50% or more passes the No.  200 sieve.  Fine-grained soils are classified as having either high or low plasticity according to their Atterberg limits (Liquid Limit, Plastic Limit, and Plasticity Index).
 
The soil classification system typically used in the US is based on a classification developed by Casagrande in the 1940s and subsequently published by the U.S. Army Corps of Engineers as The Unified Soil Classification System.   The Corp of Engineers publication included information on appropriate uses of various soils including their value as foundation support when subject to frost action, their value as a base directly under a wearing surface, their potential for compression and expansion, their drainage characteristics, and recommended compaction equipment. ASTM D 2487, Classification of Soils for Engineering Purposes, is a modified version of the Corps of Engineers publication. ASTM D 2487 classifies soils according to ASTM D 422, Method for Particle-Size Analysis of Soils and ASTM D 4318, Test Method for Liquid Limit, Plastic Limit, and Plasticity Index of Soils.
 
Silts and Clays
 
Although silts and clays are jointly referred as "fines",  they have distinctly different behaviors.  Clays are typically cohesive and are often highly plastic, meaning they have putty-like properties over a wide range of water contents.  Silts are not cohesive and are usually only plastic if they have a high organic content. 
 
These differences are due to differences in the particle shapes and mineralogical composition as well as particle size.  Silts are very small particles of disintegrated rock.  Like sands and gravel, they are composed of the same minerals as the original rock and are typically found as bulky grains, meaning  their dimensions (length, width, and height) are approximately the same order of magnitude.  Clays, however, are the result of chemical changes that alter the original rock minerals. The flat, plate-like particles consist of layers of silica (silicon-oxygen molecules) alternating with layers of alumina (aluminum-oxygen molecules). The attraction and bonding between these layers and between the particles and surrounding water molecules is responsible for the cohesion and plasticity that is characteristic of clays.
 
Because clay particles have a very high surface-to-mass ratio, electrical charges on the surface of the particles strongly influence the particle behavior.  Although the particle edges have both positive and negative charges, the faces have a net negative charge.  The faces thus attract the positive end of water molecules (water molecules are polar, meaning they have a positive and negative end.)  The faces also attract positive ions (cations) from dissolved salts in the groundwater; the cations attract the negative ends of additional water molecules.  Water can be absorbed between the layers of the particles as well as on the surface of the particles; as the moisture content of the soil increases, the amount of absorbed water increases, causing the soil to expand.
 
Atterberg Limits
 
The consistency of a clay soil varies according to its water content.  At high water contents, the soil-water mixture posses the properties of a liquid; as the water content decreases, the material exhibits the properties of a plastic; at still lower water contents, the mixture behaves as a semisolid, and finally as a solid.  ASTM D 4318 is used to determine when a soil acts as a solid and when it acts as a liquid.  The water contents at which the material properties change are referred to as Atterberg limits.  Because the liquid and plastic limits of different types of clays are very different, the water contents at these transitions can be used for identification and comparison of different clays.
 
The liquid limit (LL) is the water content at which soil behavior changes from a viscous liquid to a plastic solid.  It is determined by forming a groove in a soil sample, then subjecting the sample dish to the impact of sharp blows.  When the soil is at the liquid limit, the two sections of soil will barely touch each other but will not flow together.
 
The plastic limit (PL) is the water content at which the soil changes from a plastic solid to a semisolid; it is the minimum water content at which soil can be rolled into a 1/8-in. diameter thread without crumbling.  The soil's plasticity index (PI), the difference between its liquid and plastic limit, is the range over which the soil acts as a plastic material.  The plasticity index is a reflection of the clay's ability to absorb water and swell; a high plasticity index is an indicator of an expansive clay. 
 
Expansive Clays
 
It should be emphasized that not all clays are expansive.  The behavior of a particular clay is a function of its mineralogical composition.  A clay's mineralogical composition, in turn, depends on the type of rock it was formed from and the conditions it was formed under.  It also depends on the ground water chemistry, in particular the type of cations in the water. 
 
All clays have the same general structure - layers of silica alternating with layers of alumina.  However, the layers, typically referred to as sheets, can be arranged in different orders and can be either weakly or strongly bonded.  In addition, other elements can occur in place of the silicon and aluminum, or aluminum may occur in place of the silicon.  If the elements have different electrical charges, these substitutions will alter the electrical charge of the particle.  For example, since silicon has a valence of +4 and aluminum has a valence of +3,  substitution of aluminum for silicon increase the negative charge of the particle and thus increases the attraction for cations.
 
The ability of a clay mineral to attract cations is referred to as its cation exchange capacity (CEC); expansive clays tend to have very large cation exchange capacities.   
 
Expansive clays are mostly found in arid and semi-arid regions.  In areas with high rainfall, the active clay minerals typically weather to less-active minerals.  The clay minerals also tend to leach down into the soil strata and thus have less effect on soil movement at the surface.