Refer to Figure 13.62. Shear strength parameters are needed for the design of a foundation placed at a depth of 2 m in the silty sand layer. Soils collected from this sand were compacted in the direct shear mold (diameter = 63.5 mm) at the same dry unit weight as the field and subjected to four direct shear tests. Results are as follows:

a. Determine the shear strength parameter ϕ for the soil.
b. Determine the shear strength at the bottom of the silty sand layer.

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Test no. 123 st 2 4 Normal force (N) 200 400 800 1600 Shear force at failure (N) 105 205 414 830