WORLD FOR CIVIL

Shaft Settlement

Empirical correlations or load-deformation compatibility analyses are usually done to estimate shaft settlements . Other methods used to estimate settlement of drilled shafts, singly or in groups, are identical to those used for piles.

These include elastic, semiempirical elastic, and load-transfer solutions for single shafts drilled in cohesive or cohesionless soils.
Resistance to tensile and lateral loads by straight-shaft drilled shafts can be estimated similarly as done in case of pile foundations.

For bearing capactiy :

Q_ul=?(D_b²-D²)/4 -N_cwc_u+W_p

The shear-strength reduction factor w considers disturbance effects and ranges from 1/ 2 (slurry construction) to 3 /4 (dry construction). The c_u represents the undrained shear strength of the soil just above the bell surface, and N_c is a bearing capacity factor.

The failure surface of the friction cylinder model is conservatively assumed to be vertical, starting from the base of the bell. Qut can then be determined for both cohesive and cohesionless soils from:

Q_ul=?f_utL+W_s+W_p

where
f_ut is the average ultimate skin-friction stress in tension developed on the failure plane; that is, f_ut= 0.8c_u for clays or K? _vo tan? for sands. W_s and W_p represent the weight of soil contained within the failure plane and the shaft weight, respectively.

SHAFT RESISTANCE IN
COHESIONLESS SOILS

The shaft resistance stress f_s is a function of the soil-shaft friction angle ? , degree, and an empirical lateral earth- pressure coefficient K:

f_s= K ? ‘_vo tan?<= f_l

At displacement-pile penetrations of 10 to 20 pile diameters (loose to dense sand), the average skin friction reaches a limiting value f_l.
For relatively long piles in sand, K is typically taken in the range of 0.7 to 1.0 and ? is taken to be about ?-5, where ? is the angle of internal friction, degree. For piles less than 50 ft (15.2 m) long, K is more likely to be in the range of 1.0 to 2.0, but can be greater than 3.0 for tapered piles.