Geotechnical Engineering

DHARM

160 GEOTECHNICAL ENGINEERING

2 2mm

3 3 mm Submerged

Capillary conditions

(assume 100% saturation)

Dry (assumed)

Original WT

1m 35.32

56.13

118.56118.56

29.43 89.13

+ +

56.13

45.13

?9.81

+?

(a) Conditions after lowering the water table by 1 m( (b) Total stress
(c) Neutral stress (d) Effective stress
Fig. 5.33 Conditions and pressure diagrams (Example 5.21)
The top 2 m is assumed to be dry.
The next 1 m is under capillary conditions.
With suspended water is may be assumed to be 100% saturated.
The next 3 m is submerged
Total stress:
σ at 2 m below GL = 2 × 17.66 kN/m^2 = 35.32 kN/m^2
σ at 3 m below GL = (2 × 17.66 + 1 × 20.81) = 56.13 kN/m^2
σ at 6 m below GL = (2 × 17.66 + 4 × 20.81) = 118.56 kN/m^2
Variation is linear.
Neutral stress:
u up to 2 m below GL is uncertain.
u at 2 m below GL is due to capillary meniscus.
It is given by – 1 × 9.81 kN/m^2.
u at 3 m below GL is zero.
u at 6 m below GL = + 3 × 9.81 kN/m^3 = 29.43 kN/m^2
Effective stress :
σ at 2 m below GL = 35.32 – (– 9.81) = 45.13 kN/m^2
σ upto 2 m below GL is uncertain.
σ at 3 m below GL = 56.13 kN/m^2.
σ at 6 m below GL = 118.56 – 29.43 = 89.13 kN/m^2
The variation of total, neutral, and effective stresses is shown in Fig. 5.33.
The variation of the letter two from the surface up to 2 m depth is uncertain because the
capillary conditions in this zone cannot easily be assessed.

SUMMARY OF MAIN POINTS

1. Soil moisture or water in soil occurs in several forms, the free water being the most important.


2. The total stress applied to a saturated soil mass will be shared by the pore water and the solid
   grains ; that which is borne by pore water is called the ‘neutral stress’ (computed as γw. h), and