Monday, December 3, 2012


For the purpose of estimating the ultimate bearing capacity, the subsoil is divided into layers (Fig. 17.12) based on judgment and experience (O'Neill and Reese, 1999). Each layer is assigned one of four classifications.

1. Cohesive soil [clays and plastic silts with undrained shear strength cu <= 250 kN/m2 (2.5 t/ft^2)] .

2. Granular soil [cohesionless geomaterial, such as sand, gravel or nonplastic silt with uncorrected SPT(N) values of 50 blows per 0.3/m or less].

3. Intermediate geometerial [cohesive geometerial with undrained shear strength cu between 250 and 2500 kN/m^2 (2.5 and 25 tsf), or cohesionless geomaterial with SPT(N) values > 50 blows per 0.3 mj .

4. Rock [highly cemented geomaterial with unconfmed compressive strength greater than 5000 kN/m^2 (50 tsf)].

The unit side resistance fs (=fmax) is computed in each layer through which the drilled shaft passes, and the unit base resistance qb (=qmax) is computed for the layer on or in which the base of the drilled shaft is founded.

The soil along the whole length of the shaft is divided into four layers as shown in Fig. 17.12.

Effective Length for Computing Side Resistance in Cohesive Soil 
O'Neill and Reese (1999) suggest that the following effective length of pier is to be considered for computing side resistance in cohesive soil.

Straight shaft: One diameter from the bottom and 1.5 m (5 feet) from the top are to be excluded
from the embedded length of pile for computing side resistance as shown in Fig. 17.13(a).

Belled shaft: The height of the bell plus the diameter of the shaft from the bottom and 1.5 m (5 ft)
from the top are to be excluded as shown in Fig 17.13(b).

Figure 17.12 Idealized geomaterial layering for computation of compression load
and resistance (O'Neill and Reese, 1999)
Figure 17.13 Exclusion zones for estimating side resistance for drilled shafts in
cohesive soils

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