SETTLEMENT OF DRILLED SHAFTS
Description:
The short-term settlement of drilled shafts, based on fi eld data, is shown in Figure. The load– settlement curves for fi ne-grained soils due to skin friction show plateau values between 0.2% and 0.8% of the pile diameter, but for end bearing, the plateau values are between 2% and 5%. For coarse-grained soils, the load–settlement curves show plateau values between 0.1% and 1% for skin friction, but for end bearing, no plateau value was observed.
Rather, the load–settlement curve shows increasing settlement. These latter results illustrate the diffi culty in predicting the load–settlement response of piles in particularly coarse-grained soils. Consequently, confi dence in the prediction depends on experience with similar soil and construction conditions.
Negative skin friction.
PILES SUBJECTED TO NEGATIVE SKIN FRICTION:
Piles located in setting soil layers (e.g., soft clays or fi lls) are subjected to negative skin friction called downdrag (Figure ). The settlement of the soil layer causes the friction forces to act in the same direction as the loading on the pile. Rather than providing resistance, the negative friction imposes additional loads on the pile.
The net effect is that the pile load capacity is reduced, and pile settlement increases. The allowable load capacity is given, with reference to Figure , as
For a soft, normally consolidated soil, the negative skin friction is usually calculated over one-half its thickness. Negative skin friction should be computed for long-term condition; that is, you should use an ESA.