DHARMSOIL MOISTURE–PERMEABILITY AND CAPILLARITY 159
Pressure at X = + hcγw = 15 981
100..× = 0.147 kN/m (^2) = 147 N/m 2
Pressure at Y = – hcγw =
−× 1981
100
. = – 0.098 kN/m (^2) = – 98 N/m 2
Pressure at Z = – hcγw =
−× 4981
100
. = – 0.392 kN/m (^2) – 392 N/m 2
Example 5.21. Sketch the variation in total stress, effective stress, and pore water pres-
sure up to a depth of 6 m below ground level, given the following data. The water table is 2 m
below ground level. The dry density of the soil is 17.66 kN/m^3 , water content is 12% ; specific
gravity is 2.65. What would be the change in these stresses, if water-table drops by 1.0 m?
(S.V.U.—B.Tech., (Part-time)—May, 1983)
γd = 17.66 kN/m^3
G = 2.65
w = 12% (assumed that it is at saturation)
γd =
G
e
γw
()1+
1.8 =
265
1
.
()+e ∴ (1 + e) =
265
180
.
.
= 1.472
∴ e = 0.472
γsat =
2 65 0 472
1 472
..
.
- × 9.81 kN/m^3 = 20.81 kN/m^3
Total stress at 2 m below GL = 2 × 17.66 kN/m^2 = 35.32 kN/m^2
Total stress at 6 m below GL = (2 × 17.66 + 4 × 20.81) = 118.56 kN/m^2
Neutral stress at 2 m below GL = zero.
Neutral stress at 6 m below GL = 4 × γw = 4 × 9.81 kN/m^2 = 39.24 kN/m^2
Effective stress at 2 m below GL, σ = σ – u = 35.32 – 0 = 35.32 kN/m^2
Effective stress at 6 m below GL, σ = σ – u = 118.56 – 39.24 = 79.32 kN/m^2
The variation in the total, neutral, and effective stresses with depth is shown in Fig. 5.32.
G-L
GWL
2 2mm
4 4mm
35.32 35.32
39.24
118.56118.56 79.3279.32
All pressures
are in kN/m^2
(a) Initial conditions (b) Total stress (c) Neutral stress (d) Effective stress
Fig. 5.32 Soil profile and pressure diagrams (Example 5.21)
Immediately after the water table is lowered by 1 m, the conditions are shown in Fig. 5.33.