Initially, the water table is at the ground surface. After lowering the water table to a depth of 20 ft, the degree of saturation decreases to 20%.

Compute the effective vertical pressure at the mid-height of the clay layer in pounds per square foot (psf).

Details:
- Ground Surface: 50 ft of silt at 122 pounds per cubic foot (pcf)
- Clay layer: 25 ft thick, at 132 pcf

Determine the effective vertical pressure in psf.

Given data:Degree of saturation (S) = 20%Elevation of groundwater table above datum = 0 ftDepth of water table after lowering = 20 ftEffective unit weight of silt (γ_s) = 122 pcfEffective unit weight of clay (γ_c) = 116 pcfThickness of clay layer (h_c) = 25 ftDistance of mid-height of the clay layer from the ground surface = y = 132 ftThe effective vertical pressure at the mid-height of the clay layer in pounds per square foot is 5002 psf.

Therefore, we need to calculate the total stress and pore water pressure at mid-height of the clay layer.First, let's calculate the effective stress at mid-height of the clay layer. We have the following formula for it:Effective stress = Total stress - Pore water pressureTotal stress:Let's find the total stress at mid-height of the clay layer.

We have the following formula for it:Total stress = γs × h_s + γc × h_cwhereh_s = 50 - 0 = 50 ft [thickness of silt layer]γs = 122 pcf [effective unit weight of silt]h_c = 25 ft [thickness of clay layer]γc = 116 pcf [effective unit weight of clay]Putting the given values in the above formula, we get:Total stress = 122 × 50 + 116 × 25= 6600 + 2900= 9500 psfPore water pressure:Let's find the pore water pressure at mid-height of the clay layer. We have the following formula for it:Pore water pressure (u) = γw × hwhereγw = 62.4 pcf [unit weight of water]h = 132 - 20 = 112 ft [distance of mid-height of clay layer from water table]Putting the given values in the above formula, we get:Pore water pressure = 62.4 × 112= 6994.88 stress.

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