Geotechnical Engineering

DHARM

CAISSONS AND WELL FOUNDATIONS 761

19.2.3Allowable Bearing Pressure

Caissons are carried to a hard stratum, such as compact sand, hard clay, gravel, or rock and

never to a soft stratum or weathered rock.

The Net allowable bearing pressure, qna, for a Caisson in cohesionless soil may be ob-

tained from the following equation:

qna = 0.22N^2 BRγ + 0.67(100 + N^2 ) Df · Rq ...(Eq. 19.1)

where B = Smaller dimension of the Caisson, m

Df = Depth of Foundation below scour level, m

N = Standard penetration number (corrected)

and Rγ and Rq = Correction Factors for Water Table (Refer Fig. 14.14)

The factor of safety is 3 and qna will be got in kN/m^2. In the case of pure clays, undis-

turbed samples should be tested to determine the value of cohesion, c.

The ultimate bearing capacity is obtained from

qult = c.Nc ...(Eq. 19.2)

where qult = Ultimate bearing capacity, kN/m^2

c = Unit Cohesion, kN/m^2

and Nc = Bearing capacity factor (Refer Eqs. 16.11 and 16.12)

The allowable bearing pressure of Caissons resting on Rock should not exceed that for

the concrete seal. Since the seal is in water or in adverse working conditions, the allowable

bearing pressure is usually limited to 3,500 kN/m^2.

19.2.4Skin Friction and Sinking Effort

Skin Friction is the shearing resistance between the soil and the exterior surface of the Cais-

son, encountered during the process of sinking. Caissons are usually designed to have suffi-

cient weight in each lift to overcome skin friction to facilitate the sinking process. If the self-

weight is not adequate, additional ballast, known as ‘Sinking Effort’ would become necessary

to sink the Caisson. Occasionally, the use of water jets on the sides tends to reduce the skin

friction. Even the injection of bentonite solution on to the exterior of the well has been found to

reduce skin friction.

Values of the skin friction vary within a wide range for each type of soil. Terzaghi and

Peck (1948) give the following values (Table 19.1):

Table 19.1 Values of skin friction (after Terzaghi and Peck, 1967)

S.No. Type of Soil Skin Friction kN/m^2

1. Silt and soft clay 7.3 to 29.3


2. Very stiff clay 49 to 195


3. Loose Sand 12.2 to 34.2


4. Dense Sand 34.2 to 68.4


5. Dense Gravel 49 to 98