The resultant of the vertical and horizontal forces has an eccentricity of 150035/30 000
1.75 m at ground level. From equation 6.54, the load on each of the outer four piles due to
wind loading from an east–west direction is given by
Therefore uplift does not occur on the windward side and the maximum pile load is 2 364 kN.
Checking the maximum pile load for wind in a north–south direction:
Therefore, maximum pile load 2 326 kN.
For piles with a shaft diameter of 1 m:
working stress on concretewhich is within safe limits.
Adopting an under-reamed base to a diameter of 1.8 m, from equation 4.4,
ultimate base resistance
For a safety factor of two on the combined base resistance and skin friction, the required
ultimate skin friction (22 364) 4 351 377 kN.
If the required depth of penetration into the glacial clay to mobilize the required ultimate
resistance is Lm, ignoring the small skin friction in the very soft clay and adopting an
adhesion factor of 0.3 (to allow for delays in under-reaming), then from equation 4.7,
Qs 0.3 190 1 Land if Qs 377, L 2.10 m. Thus the allowable pile load for a factor of safety of 3 in base
resistance and unity in skin friction is , which
is insufficient. Taking Las 4.9 m,
Qs 0.3 190 1 4.9 877 kNand the allowable pile load is which is satisfactory.
It is necessary to add two shaft diameters and the depth of the under-ream to arrive at the
total penetration of the piles below ground level. Thus
D 6 m (soft clay)4.92.00.8 13.7 mAn adhesion factor of 0.5 is used for straight-shafted piles in a glacial clay (Figure 4.8).
Therefore the allowable load on a straight-shafted pile drilled to the same depth as the
1
3 4 351^877 2 328 kN,1
3 QbQs^ (1
3 4 351)^377 1 827 kNQb 9 190 14 1.8^2 4 351 kN2 364 1 000
1
4 1 000
2 3 N/mm2V 1364 30 000 1.75 5.20
(8 5.20^2 )(10 2.60^2 )
1364 962 kN1 3641 000 kNV 30 000
22
^30 000 1.75 ^6
(4 62 )(6 4.5^2 )(4 32 )(6 1.5^2 )
Piles to resist uplift and lateral loading 369