Soils Analysis

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.