What resists the pull-out of an uplift pile?

Skin friction along the shaft plus the pile weight

End bearing at the tip plus skin friction

Passive resistance of the pile cap only

The uplift capacity of a pile group is taken as:

The lesser of the sum of individual piles and the block of soil plus cap weight and shear

The greater of the sum of piles and the soil block value

Always the sum of individual piles

Only the weight of the pile cap

2076 Chaitra · 2 marks

Piles subjected to uplift loads

Short answer

Uplift piles resist pull-out (towers, buoyant basements) using shaft friction plus pile weight, no end bearing. Under-reamed bulbs increase resistance. Group uplift capacity is the lesser of sum of individual capacities or block weight plus perimeter shear; factor of safety 2–3.

Describe the piles subjected to uplift loads.

Uplift (tension) piles resist upward or pull-out forces in a structure, where the load tries to lift the foundation.

Uses: transmission towers and chimneys (wind overturning), basement floors and tanks under the water table (buoyancy), anchoring of docks and offshore structures, and bridge piers resisting overturning.
Resistance: only the skin friction along the shaft acts (there is no end bearing), plus the weight of the pile. It is about 70 to 80% of the friction in compression. Ultimate uplift: $Q_{up} = \alpha c_u\pi DL + W_p$ (clay) or $\sum K\sigma'_v\tan\delta\,\pi D\Delta L + W_p$ (sand).
Under-reamed (belled) piles have a larger resistance from the bulb. The pile is also anchored to the cap by reinforcement for full tension.
Group: the uplift capacity of a group is the lesser of the sum of individual piles, and the weight of the block of soil plus pile cap and the shear around the block.
A factor of safety of 2 to 3 is used.

✓ Answer checked by Claude Sonnet 5.5

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