Classify the soil stratum between depths of 66 and 80 ft in

Classify the soil stratum between depths of 66 and 80 ft in Figure 4.14. What is the significance of the spike in the plots at a depth of 77 ft? Note: Fig. 4.14 from 2nd Edition, same figure is Fig. 4.22 in 3rd Edition

Solution

Solution:-

Most of the soil between 66 and 80 ft has the following characteristics: q_c = 17 tsf = 17 kg/cm², R_f = 2.6%. Per Figure 4.15, this soil is a clayey silt or silty clay.

According to Figure 4.15, the spikes between 76 to 78 ft indicate a sand seam.

In Detail:-

Figure 4.14, Sample CPT test results. These results were obtained from a piezocone, and thus also included a plot of pore water pressure, u, versus depth. All stresses and pressures are expressed in tons per square foot (tsf). For practical purposes, 1 tsf = 1 kg/cm².

Refer figure 4.14 for sample CPT test results, in the textbook.

Consider the values of cone bearing and friction ratio for the soil stratum between of 66 ft and 85 ft

Cone Bearing, q_c = 20 tsf

= 20 kg/cm²

Friction ratio, R_f = 1.2%

Refer figure 4.15 for Classification of soil based on CPT test result, in the textbook.

Consider the type of sand corresponding to 20 kg/cm³ of q_c and 1.2% for R_f.

The soil stratum is Partially drained - sandy silt to clayey silt.

Therefore, the soil stratum is Partially drained.