Soil Samples and Soil Profiles.

It is a wise precaution to take more soil samples than necessary to determine the ground conditions (and increasing the frequency of samples does not proportionally increase the cost of the soil survey). It is not however necessary to test every single sample. If the surface soil is weak and underlain by good rock or dense gravel there may be little point in testing the weak surface soil if piling down to the good strata is proposed.

Soil profiles (section through boreholes) are extremely helpful in enabling the designer to visualize the ground
conditions. This valuable aid is, in the authors’ opinion, too often given inadequate attention in site investigations.

Many foundation failures can be traced back to faulty  visualization of the ground conditions due to inadequate soil profiles or misinterpretation of them. A typical soil profile is shown in Fig. 3.7.

Most experienced designers would tend to study the soil profile first before reading the site report, studying the test results and checking other data. This makes for efficiency, better assessment of site conditions, improved judgement of data, it warns of problems and can indicate the need for possible further investigation.

Some typical misinterpretations or inadequate data leading to false conclusions and similar errors are shown in Fig. 3.8 (see also Figs 2.28, 2.29 and 2.31).

Fig. 3.7 Soil profile for a typical site.

Fig. 3.8 Misinterpretation of soil profile 

(Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983),

Fig. 2.28 Mistaken bedrock.

Fig. 2.29 Unchecked fault.

Fig. 2.31 Folded strata mistaken for level strata.