Pile Group Settlement Under a Storage Tank
A 22 m diameter steel water tank is to be founded on a 3 x 3 group of 600 mm diameter bored piles, spaced at 2.0 m centres in a square grid. The site consists of 4 m of loose fill over 18 m of normally consolidated soft clay (cu = 25 kPa at mid-depth, increasing linearly), underlain by dense sand. The structural engineer has sized the tank for a working load of 28 MN, giving 3.1 MN per pile against a single-pile capacity of 4.5 MN, so the design appears to satisfy a factor of safety of 1.45. The client's process requires a maximum differential settlement of 25 mm across the tank base and total settlement under 100 mm. A peer reviewer has flagged the design as unsafe.
1. Explain why checking single-pile capacity alone is insufficient here, and outline the additional failure and serviceability checks that must be performed.
Read the worked answer
Single-pile capacity checks only the ultimate axial resistance of an isolated pile. In soft clay the governing issues are group effects, not individual capacity.
Block failure: the 3 x 3 group at 2.0 m spacing forms a block roughly 4.6 m square. The block perimeter shear plus base bearing must be checked; in soft clay with cu = 25 kPa the block capacity can be lower than 9 times the single-pile capacity, so the applied 28 MN may exceed it.
Settlement: load is transferred to the clay over the full pile length and the group acts as a deep foundation. Consolidation settlement of the 18 m clay layer under the net group load is likely to be large (hundreds of mm), far exceeding the 100 mm limit. Group settlement must be estimated (equivalent raft method) and compared with both limits.
Differential settlement: the 25 mm criterion controls the tank shell and base plate stresses. A 3 x 3 group under a flexible circular load will settle non-uniformly at the edges; this needs a proper soil-structure interaction check.
Other checks: negative skin friction from the loose fill, pile structural capacity, and downdrag on the clay-sand interface. The 1.45 factor on single-pile capacity gives false assurance.
2. The client proposes reducing pile diameter to 450 mm to cut cost while keeping the same number of piles. Evaluate this proposal.
Read the worked answer
Reducing diameter to 450 mm reduces each pile's shaft area and base area proportionally, cutting single-pile capacity by roughly 25% (capacity scales with perimeter and base area). The applied load is unchanged at 28 MN, so the demand per pile rises relative to capacity and the already marginal 1.45 factor falls to about 1.1 - unacceptable.
More importantly, group behaviour worsens. Smaller piles at the same 2.0 m spacing give a group block of similar plan dimensions, so block capacity is largely unchanged while individual pile contribution drops. Settlement is not reduced: the equivalent raft depth and loaded area are similar, and the soft clay still consolidates under the same net load.
The proposal addresses neither the block failure risk nor the settlement criterion, which are the actual governing issues. A better direction is to increase pile length to reach the dense sand (mobilising end bearing and reducing load shed to the clay), widen spacing, or reduce tank load. Diameter reduction should be rejected.