The equation for shearing strength S (lb/ft2 or kPa) of a soil may be taken as follows:

S = c + a tan ф (8.3)

where c = cohesion, lb/ft2 (kPa)

a = confining pressure or normal stress, lb/ft2 (kPa) ф = angle of internal friction of the soil, degrees

The shearing strength of the soil should account for the effect of pore water pressure when present. Equation (8.3) can be modified as:

S = c + (a — u) tan ф (8.4)

where u = pore water pressure, lb/ft2 (kPa)

Soil consolidation is produced by load and is associated with changes in soil mois­ture. It is also a function of time. The time required for drainage to occur, which results from the change in soil moisture, is a function of the permeability of the soil and the distance the water must travel in the material to be released. It is clear that consolidation of coarse-grained materials will occur fairly rapidly. This explains the often used assumption that consolidation of such materials under applied load, for example, the load of a retaining wall, generally occurs during the construction of the wall. Thus, long-term settlement is not normally considered to occur. On the contrary, clays and/or silts are relatively impermeable, so that long-term settlement should be anticipated in the design. The designer must consider various options to accommodate this projected long-term settlement. For example, the designer may (1) require preloading to effect the settlement before the wall is constructed, (2) accelerate the consolidation by drilling for and placing sand drains, and (3) decide to build the structure with pile or caisson support systems that are independent of the consolidation.

Bedrock is divided by geologists into three large groups, namely (1) igneous, (2) meta – morphic, and (3) sedimentary. Igneous rocks are those that have resulted from the cooling and crystallization of molten masses of mineral matter and gases either at or below the earth’s surface. Sedimentary rocks consist of the transported and subse­quently indurated products of weathering of previously existing rock types, while metamorphic rocks are frequently defined as those having characteristic textures and mineral compositions that have resulted from high temperatures and pressures and/or hot mineralizing solutions acting on a parent rock. Figures 8.16, 8.17, and 8.18 indi­cate easily recognizable descriptions for field classification of igneous, metamorphic, and sedimentary rock, respectively.

8.3.2 Soils Laboratory Tests

Grain size, shape, and gradation are generally established by sieve analysis. For the finer clays, a hydrometer analysis is necessary. Figure 8.19 depicts a classification of sediment based on grain size.

Atterberg limit tests are performed on fine-grained soils and represent the amount of water present in the voids. The liquid limit (LL), plastic limit (PL), and plasticity index (PI) constitute the Atterberg limits.

The triaxial shear test is used to find the shear strength of a soil for the determina­tion of pile lengths and of bearing capacity for spread footings or drilled shafts. Triaxial shear test results are also needed to give soil parameters for the design of retaining walls. High-quality, undisturbed samples are required for triaxial shear tests. Poor samples should be discarded rather than tested, as they will give misleading results.

The direct shear test is sometimes performed in lieu of other shear tests, and the use of its results is the same as that noted above for the triaxial shear test. It is impor­tant to remember that direct shear test results are usually less reliable than those obtained from the triaxial shear test, since the failure line in the direct shear test is imposed by the method of testing, whereas the triaxial method allows the sample to fail in its weakest plane. On occasion, it is desirable to shear soil or rock along a par­ticular plane. In these cases, a direct shear test may be used. High-quality, undisturbed samples are needed for this test.

The unconfined compression test of a soil is a uniaxial compression test in which the test specimen is provided with no lateral support while undergoing vertical com­pression. The test measures the unconfined, compressive strength of a cylinder of cohesive or semicohesive soil, which, indirectly, may be indicative of the shearing strength. The test is usually performed on an undisturbed sample of soil at its natural moisture content. It may also be performed on a remolded sample to evaluate the effects of disturbance and remolding upon the shearing strength.

Unconfined compression tests are relatively quick to perform and relatively inex­pensive. When used in conjunction with the triaxial test, the unconfined compres­sion test is of value. Also, it is sometimes used as an index test because it is easy to conduct.

(a)

If gravel is present in appreciable amounts, the term “gravelly” may be added to the class name, vis. “gravelly sand”. The terms “coarse”, “medium”, and “fine”, when used to describe gravel, sand, and silt, refer to standard grade size limits.

(b)

FIGURE 8.19 Classification of soil based on (a) grain size of sediment and (b) standard grain size limits. (From C. H. Harned, Some Practical Aspects of Foundation Studies for Highway Bridges, U. S. Bureau of Public Roads, January 1959)

Retaining wall design engineers not fully trained in soil mechanics need to be acquainted with certain basic principles, in order to understand the data developed by the geotechnical engineer or geologist responsible for the subsurface exploration. Soil is a nonhomogeneous earthen material that varies laterally and vertically in mineral context, grain size, density, grain shape, moisture content, strength, consistency, and compressibility. For the design of retaining walls and other structure-type foundations, the engineering properties of the soil must be evaluated. Such an evaluation will

always require consideration of foundation soil classification, bearing capacity, and compressibility.

Soil Classification. Since the types of soils are so numerous and variable, a classifi­cation system is important. The Unified Soil Classification System (USCS) has been generally accepted by engineers. It is based upon the sizes of the particles, the distrib­ution of the particle sizes, and the properties of the fine-grained portion. Only particle sizes of 3 in (75 mm) or less are included in the USCS. Materials greater in size are generally indicated in the log of borings as cobbles or boulders. Figure 8.14 shows the unified soil classification chart. The basic classifications include coarse-grained and fine-grained soils.

Coarse-Grained Soils. Coarse-grained soils are classified as either gravels or sands, dependent upon the fraction of the material retained on a no. 200 sieve. The classification threshold is 50 percent; i. e., if more than 50 percent of the fraction retained on a no. 200 sieve is retained on a no. 4 sieve, the soil is classified a gravel. If more than 50 percent passes the no. 4 sieve, the soil is classified a sand. There are many groupings of these coarse-grained soils, as indicated in the chart.

Fine-Grained Soils. Fine-grained soils are subdivided by plasticity and compress­ibility rather than by grain size. Fine-grained soils are classified as silt or clay, and as lowly or highly compressible. Criteria for classification are based upon the relation­ship between the liquid limit and the plasticity index. The relationship is given in the form of a plasticity chart shown by the inset in Fig. 8.14 and reproduced in Fig. 8.15. The “A” line on the chart divides clays from silts. Soils whose Atterberg limits plot above the line are clays, designated C; limits that plot below the line are silts, designated M.