SOIL CLASSIFICATION METHODS IN FOUNDATION DESIGN BASICS AND TUTORIALS

SOIL CLASSIFICATION METHODS IN FOUNDATION DESIGN BASIC INFORMATION
What Are The Methods Of Classifying Soils In Foundation Design?

It is necessary for the foundation engineer to classify the site soils for use as a foundation for several reasons:

1. To be able to use the database of others in predicting foundation performance.
2. To build one's own local database of successes (or any failures).
3. To maintain a permanent record that can be understood by others should problems later develop and outside parties be required to investigate the original design.
4. To be able to contribute to the general body of knowledge in common terminology via journal papers or conference presentations. After all, if one is to partake in the contributions of others, one should be making contributions to the general knowledge base and not be just a "taker."


The Unified Soil Classification System (USCS) of Table 2-1 is much used in foundation work. A version of this system has been standardized by ASTM as D 2487 (in Volume 04.08: Soil and Rock; Dimension Stone; Geosynthetics). The standardized version is similar to the original USCS as given by Casagrande (1948) but with specified percentages of sand or gravel passing specific sieves being used to give the "visual description" of the soil.

The original Casagrande USCS only classified the soil using the symbols shown in Table 2-1 (GP, GW, SM, SP, CL, CH, etc.), based on the indicated percentages passing the No. 4 and No. 200 sieves and the plasticity data. The author has always suggested a visual description supplement such as the following:

It is evident in this table that terms "trace" and "with" are somewhat subjective. The soil color, such as "blue clay," "gray clay," etc., is particularly useful in soil classification.

In many areas the color—particularly of cohesive soils—is an indication of the presence of the same soil stratum as found elsewhere. For example the "soft blue clay" on the soil profile of Fig. 2-4 for Chicago has about the same properties at any site in the Chicago area.

In foundation work the terms loose, medium, and dense, , and consistency descriptions such as soft, stiff, very stiff, etc., are also commonly used in foundation soil classification. Clearly, all of these descriptive terms are of great use to the local geotechnical engineer but are somewhat subjective.

That is, there could easily be some debate over what is a "medium" versus a "dense" sand, for example. The D 2487 standard removed some of the subjectiveness of the classification and requires the following terminology:

< 15% is sand or gravel use name (organic clay, silt, etc.)
15% < x < 30% is sand or gravel describe as clay or silt with sand, or clay or silt with gravel
> 30% is sand or gravel describe as sandy clay, silty clay, or gravelly clay, gravelly silt

The gravel or sand classification is based on the percentage retained on the No. 4 (gravel) sieve or passing the No. 4 and retained on the No. 200 (sand) sieves. This explanation is only partial, as the new standard is too lengthy to be presented in detail.

Although not stated in D 2487, the standard is devised for using a computer program3 to classify the soil. Further, not all geotechnical engineers directly use the ASTM standard, particularly if their practice has a history of success using the original USC system.

FIVE (5) MAJOR FACTORS THAT AFFECT THE ENGINEERING PROPERTIES OF SOILS

MAJOR FACTORS THAT AFFECT THE ENGINEERING PROPERTIES OF SOILS
What Are The 5 Major Factors That Affect The Engineering Properties of Soils?


Most factors that affect the engineering properties of soils involve geological processes acting over long time periods. Among the most important are the following.

1. Natural Cementation and Aging

All soils undergo a natural cementation at the particle contact points. The process of aging seems to increase the cementing effect by a variable amount. This effect was recognized very early in cohesive soils but is now deemed of considerable importance in cohesionless deposits as well.

The effect of cementation and aging in sand is not nearly so pronounced as for clay but still the effect as a statistical accumulation from a very large number of grain contacts can be of significance for designing a foundation. Care must be taken to ascertain the quantitative effects properly since sample disturbance and the small relative quantity of grains in a laboratory sample versus site amounts may provide difficulties in making a value measurement that is more than just an estimate.

Field observations have well validated the concept of the cementation and aging process. Loess deposits, in particular, illustrate the beneficial effects of the cementation process where vertical banks are readily excavated.

2. Overconsolidation

A soil is said to be normally consolidated (nc) if the current overburden pressure (column of soil overlying the plane of consideration) is the largest to which the mass has ever been subjected. It has been found by experience that prior stresses on a soil element produce an imprint or stress history that is retained by the soil structure until a new stress state exceeds the maximum previous one.

The soil is said to be overconsolidated (or preconsolidated) if the stress history involves a stress state larger than the present overburden pressure.

Overconsolidated cohesive soils have received considerable attention. Only more recently has it been recognized that overconsolidation may be of some importance in cohesionless soils. A part of the problem, of course, is that it is relatively easy to ascertain overconsolidation in cohesive soils but very difficult in cohesionless deposits.

The behavior of overconsolidated soils under new loads is different from that of normally consolidated soils, so it is important— particularly for cohesive soils—to be able to recognize the occurrence.

3. Mode of Deposit Formation

Soil deposits that have been transported, particularly via water, tend to be made up of small grain sizes and initially to be somewhat loose with large void ratios.

They tend to be fairly uniform in composition but may be stratified with alternating very fine material and thin sand seams, the sand being transported and deposited during high-water periods when stream velocity can support larger grain sizes.

These deposits tend to stabilize and may become very compact (dense) over geological periods from subsequent overburden pressure as well as cementing and aging processes.

Soil deposits developed'where the transporting agent is a glacier tend to be more varied in composition. These deposits may contain large sand or clay lenses. It is not unusual for glacial deposits to contain considerable amounts of gravel and even suspended boulders.

Glacial deposits may have specific names as found in geology textbooks such as moraines, eskers, etc.; however, for foundation work our principal interest is in the uniformity and quality of the deposit. Dense, uniform deposits are usually not troublesome. Deposits with an erratic composition may be satisfactory for use, but soil properties may be very difficult to obtain.

Boulders and lenses of widely varying characteristics may cause construction difficulties. The principal consideration for residual soil deposits is the amount of rainfall that has occurred. Large amounts of surface water tend to leach materials from the upper zones to greater depths. A resulting stratum of fine particles at some depth can affect the strength and settlement characteristics of the site.

4. Quality of the Clay

The term clay is commonly used to describe any cohesive soil deposit with sufficient clay minerals present that drying produces shrinkage with the formation of cracks or fissures such that block slippage can occur.

Where drying has produced shrinkage cracks in the deposit we have a fissured clay. This material can be troublesome for field sampling because the material may be very hard, and fissures make sample recovery difficult. In laboratory strength tests the fissures can define failure planes and produce fictitiously low strength predictions (alternatively, testing intact pieces produces too high a prediction) compared to in situ tests where size effects may either bridge or confine the discontinuity.

A great potential for strength reduction exists during construction where opening an excavation reduces the overburden pressure so that expansion takes place along any fissures. Subsequent rainwater or even local humidity can enter the fissure so that interior as well as surface softening occurs.

A clay without fissures is an intact clay and is usually normally consolidated or at least has not been over consolidated from shrinkage stresses. Although these clays may expand from excavation of overburden, the subsequent access to free water is not so potentially disastrous as for fissured clay because the water effect is more nearly confined to the surface.

5. Soil Water

Soil water may be a geological phenomenon; however, it can also be as recent as the latest rainfall or broken water pipe. An increase in water content tends to decrease the shear strength of cohesive soils. An increase in the pore pressure in any soil will reduce the shear strength.

A sufficient increase can reduce the shear strength to zero—for cohesionless soils the end result is a viscous fluid. A saturated sand in a loose state can, from a sudden shock, also become a viscous fluid. This phenomenon is termed liquefaction and is of considerable importance when considering major structures (such as power plants) in earthquake-prone areas.

When soil water just dampens sand, the surface tension produced will allow shallow excavations with vertical sides. If the water evaporates, the sides will collapse; however, construction vibrations can initiate a cave-in prior to complete drying.

The sides of a vertical excavation in a cohesive soil may collapse from a combination of rainfall softening the clay together with excess water entering surface tension cracks to create hydrostatic water pressure. In any case, the shear strength of a cohesive soil can be markedly influenced by water.

Even without laboratory equipment, one has probably seen how cohesive soil strength can range from a fluid to a brick-like material as a mudhole alongside a road fills during a rain and subsequently dries. Ground cracks in the hole bottom after drying are shrinkage (or tension) cracks.