The depth of water in a well is 3 m. Below the bottom of the well lies a layer of sand 5 meters thick overlying a clay deposit. The specific gravity of the solids of sand and clay are respectively 2.64 and 2.70. Their water contents are respectively 25 and 20 percent. Compute the total,

Question

The depth of water in a well is 3 m. Below the bottom of the well lies a layer of sand 5 meters thick overlying a clay deposit. The specific gravity of the solids of sand and clay are respectively 2.64 and 2.70. Their water contents are respectively 25 and 20 percent. Compute the total, intergranular and pore water pressures at points A and B shown in Fig. Ex. 5.1.

Accepted Answer

The formula for the submerged unit weight is

[latex]\gamma_{b}=\frac{\gamma_{w}\left(G_{s}-1 ight)}{1+e}[/latex]

Since the soil is saturated,

[latex]e=w G_{s}, \quad \gamma_{b}=\frac{\gamma_{w}\left(G_{s}-1 ight)}{1+w G}[/latex]

For sand, [latex]\gamma_{b}=\frac{9.81(2.64-1)}{1+0.25 imes 2.64}=9.7 kN / m ^{3}[/latex]

For clay, [latex]\gamma_{b}=\frac{9.81(2.70-1)}{1+0.20 imes 2.70}=10.83 kN / m ^{3}[/latex]

Pressure at point A

(i) Total pressure [latex]=3 imes 9.7( ext { sand })+6 imes 9.81=29.1+58.9=88 kN / m ^{2}[/latex]

(ii) Effective pressure [latex]=3 imes 9.7=29.1 kN / m ^{2}[/latex]

(iii) Pore water pressure [latex]=6 imes 9.81=58.9 kN / m ^{2}[/latex]

Pressure at point B

(i) Total pressure [latex]=5 imes 9.7+2 imes 10.83+10 imes 9.81=168.3 kN / m ^{2}[/latex]

(ii) Intergranular pressure [latex]=5 imes 9.7+2 imes 10.83=70.2 kN / m ^{2}[/latex]

(iii) Pore water pressure [latex]=10 imes 9.81=98.1 kN / m ^{2}[/latex]

Question 5.1: The depth of water in a well is 3 m. Below the bottom of the...

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