Page 60 - Unit2.docx

• pile spacing
• independent calculations, showing bearing capacity of the block and
bearing capacity of individual piles in the group should be made.
• relate the ultimate load capacity of the block to the sum of load capacity of
individual piles in the group ( the ratio of block capacity to the sum of
individual piles capacity) the higher the better.
• In the case of where the pile spacing in one direction is much greater than that in
perpendicular direction, the capacity of the group failing as shown in Figure 6-2 b)
should be assessed.

6.2.2 Pile groups in cohesive soil

For pile groups in cohesive soil, the group bearing capacity as a block may be
calculated by mans of e.q. 4-5 with appropriate N c value.

6.2.2 Pile groups in non-cohesive soil

For pile groups in non-cohesive soil, the group bearing capacity as a block may
be calculated by means of e.q. 4-7

6.2.3 Pile groups in sand

In the case of most pile groups installed in sand, the estimated capacity of the block will
be well in excess of the sum of the individual pile capacities. As a conservative approach
in design, the axial capacity of a pile group in sand is usually taken as the sum of
individual pile capacities calculated using formulae in 4-8.
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Worked Example 6-2

Calculate the bearing capacity and group efficiency of pile foundation installed in uniform
3
2
clay of bulk unit weight, γ of 20kN/m and undrained shear strength of C u of 50kN/m .
The foundation consists of 25 piles each 28m long ,0.4m in diameter and weight 60kN.
The weight of the pile cap is 600kN and founded 2m below the ground level. The
adhesion factor α for the soil/pile interface has a value of 0.8