Example

A 5m thick layer of medium sand overlies a deep deposit of dense gravel. A series of standard penetration tests carried out through the depth of the sand has established that the average blow count, N, is 22. Further tests show that the gravel has a standard penetration value of N = 40 in the region of the interface with the sand. A precast pile of square section 0.25 x 0.25 m2 is to be driven down through the sand and to penetrate sufficiently into the gravel to give good end bearing.

Adopting a safety factor of 3.0 determine the allowable load that the pile will be able to carry.

Solution

Ultimate bearing capacity of the pile = Qu = Qs + Qb

Qb: All end bearing effects will occur in the gravel. Now

D , , qb = 40 N — kN/m or 400 x N kN/m (whichever is the lesser) B

40 x 40

Qs in sand: Qs = fsAs = 22 x 5 x 0.25 x 4 = 110 kN Qs in gravel: Qs = fsAs = 4.0 x 2.5 x 0.25 x 4 = 100 kN

Note It is seen that, as discussed earlier, the end bearing effects are much greater than those due to side friction. It can be argued that, in order to develop side friction fully, a significant downward movement of the pile is required which cannot occur in this example because of the end resistance in the gravel. There have been suggestions that, because of this phenomenon, the factor of safety applied to the skin friction resistance should be different from that applied to the end bearing resistance. If this argument prevailed then Qu= 1000 kN and the allowable load = 333 kN. However, general practice is to use the same value of safety factor for both resistance components and, considering the uncertainties associated with pile design, this seems a reasonable approach.

Negative skin friction

If a soil subsides or consolidates around a group of piles these piles will tend to support the soil and there can be a considerable increase in the load on the piles.

The main causes for this state of affairs are that:

(i) bearing piles have been driven into recently placed fill;

(ii) fill has been placed around the piles after driving.

If negative friction effects are likely to occur then the piles must be designed to carry the additional load. In extreme cases the value of negative skin friction can equal the positive skin friction but, of course, this maximum value cannot act over the entire bedded length of the pile, being virtually zero at the top of the pile and reaching some maximum value at its base.

8.10.10 Action of pile groups

As indicated in the previous paragraph, piles are usually driven in groups (see Fig. 8.23).

In the case of end bearing piles the pressure bulbs of the individual piles will overlap (if spacing < 5d - the usual condition). Provided that the bearing strata are firm throughout the affected depth of this combined bulb then the bearing capacity of the group will be equal to the summation of the individual strengths of the piles. However, if there is a compressible soil layer beneath the firm layer in which the piles are founded, care must be taken to ensure that this weaker layer is not overstressed.

Pile groups in cohesionless soils

Pile driving in sands and gravels compacts the soil between the piles. This compactive effect can make the bearing capacity of the pile group greater than the sum of the individual pile strengths. Spacing of piles is usually from two to three times the diameter, or breadth, of the piles.

Pile groups in cohesive soils

A pile group placed in a cohesive soil has a collective strength which is considerably less than the summation of the individual pile strengths which compose it.

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