Shear strength on horizontal plane, WT 3.5 m

Assumptions: $\gamma_w = 9.81$ kN/m³; the soil above the water table is partly saturated ($S_r = 0.5$); $c=0$ and capillary effects are neglected. Strength $= \sigma'\tan\phi$ on the horizontal plane.

Case 1: Water table at 3.5 m (below the 3 m depth, so there is no pore pressure at the plane)

$$\gamma = \frac{(G_s + S_r e)\gamma_w}{1+e} = \frac{(2.70 + 0.5\times0.50)(9.81)}{1.50} = 19.29\ \text{kN/m}^3$$ $$\sigma' = \sigma = 19.29\times 3 = 57.9\ \text{kPa}$$ $$s = \sigma'\tan\phi = 57.9\tan30^\circ = 33.4\ \text{kPa}$$

Case 2: Water table at the ground surface (fully saturated, $S_r = 1$)

$$\gamma_{sat} = \frac{(G_s + e)\gamma_w}{1+e} = \frac{(2.70+0.50)(9.81)}{1.50} = 20.93\ \text{kN/m}^3$$ $$\sigma' = (20.93 - 9.81)\times 3 = 33.4\ \text{kPa}$$ $$s = 33.4\tan30^\circ = 19.3\ \text{kPa}$$

Answer: shear strength is about 33.4 kPa with the water table at 3.5 m, and falls to about 19.3 kPa (a reduction of about 42%) when the water table rises to the ground surface.