Geotechnical Engineering

DHARM

CAISSONS AND WELL FOUNDATIONS 783

Sc = HC

e

oc

o

.

()

.log ()

1 10

0

+ 0

σσ+

σ

∆ ...(Eq. 7.14)

where Sc = Consolidation settlement (mm),

Cc = compression index of the soil,

eo = initial void ratio (prior to the application of the stress increment),

σo = effective vertical stress at the centre of the cohesive stratum prior to the applica-

tion of the stress increment (kN/m^2 ),

∆σ = increment in the effective vertical stress due to the application of the loads

(kN/m^2 ),

and Ho = initial thickness of the cohesive layer (mm).

If the well foundation rests on a stratum of rock, the crushing strength of rock can be

obtained by testing rock cores in the laboratory. However, the effect of structural defects like

faults, fissures, joints, and other discontinuities cannot be easily ascertained. Teng (1962)

suggests that the allowable pressure of rock should not exceed that of concrete seal. The allow-

able strength of concrete placed under water is usually taken as 3.5 MN/m^2 , and since a high

factor of safety of 10 is desired for rock, the crushing strength of rock should be at least 35 MN/

m^2. Bowles (1968) recommends the compressive strength values as given in Table 19.4:

Table 19.4 Compressive strength of rocks (Bowles, 1968)

S.No. Rock Type Compressive Strength, MN/m^2

1. Granite 70 – 177


2. Basalt 177 – 283


3. Schist 36 – 106


4. Sandstone 18 – 70


5. Shale 7 – 36


6. Limestone 36 – 106


7. Porous limestone 7 – 36

19.9.4Design Aspects of the Components of a Well Foundation

The overall design aspects of the well foundation have been discussed in the preceding subsec-

tions. Certain design aspects of the individual components of a well foundation will be given in

this subsection.

(1) Cutting Edge

The cutting edge should have a sharp angle for cutting the soil. It should be strong so that it

does not bend even when boulders are encountered. An angle of 30° with the vertical or a slope

of one horizontal to two vertical is generally used. A cutting edge with stub-nose may also be

used if a sharp cutting edge is likely to be damaged (Fig. 19.12). The cutting edge should be

properly anchored to the curb.