Civil and Building Engineering.

Among the first problems is to investigate the spring constant of the soil. A soil sample of height H is placed in an apparatus such as the one shown in Figure 3.6. A series of loads are applied, and for each load the settlement and the elapsed time are measured when equilibrium is established. For convenience, S is divided by H to obtain strain and a stress-strain curve is plotted. The curve is likely to have the shape shown in Figure 3.21. The spring representing the model must be nonlinear to fit measured soil properties.

If the stress-strain curve is really representative of the soil in the field, the curve should be useful for analysis. A thin stratum of soil at depth zj in the soil deposit is considered. The stratum has a thickness Hj , and before placement of the embankment, it is in equilibrium under a vertical stress, The . j condition of the soils is indicated by point j in Figure 3.21. If a stress from the embankment is added, the strain results. The settlement of the surface caused by the compression of the jth layer is then
The settlement of the surface caused by compression of the entire subsoil is then
where
Although the foregoing analysis is simple, many necessary quantities remain undefined. For example, how is the added effective stress calculated with depth below a foundation? How is the initial effective stress calculated? How are the proper - curves obtained? These quantities and many others must be determined before any realistic field problems can be analyzed.