Module 17 · Lesson 17.1
Group capacity
Efficiency, and the failure mode that ignores the individual piles.
Why this matters
Piles are almost never used singly. Once they are in a group their stress bulbs overlap, and the group develops less capacity than the piles would individually — sometimes much less, and occasionally by a completely different mechanism.
By the end of this lesson you should be able to
- Compute group efficiency from geometry
- Check the block failure mode
- Say which of the two governs and when
Group efficiency η is the group's capacity as a fraction of the sum of the single-pile capacities. The Converse–Labarre expression is a geometric fit: it depends only on the number of rows and columns, the spacing and the diameter, and it falls as the piles get closer or the group gets larger.
For a 3×3 group of 0.6 m piles at three diameters' spacing, η = 0.73. Tighten to two diameters and it falls to 0.61. A 5×5 group at two diameters gives 0.53 — the group is worth barely half its members.
The reason is stress overlap. Each pile sheds load into the soil around it, and where those zones overlap the soil is being asked to carry the same load twice.
Worked example
Two groups, and whether the block governs
Given
- Piles D = 0.6 m, 12 m long, in soft clay with su = 40 kPa, α = 1.0
- Single pile capacity 1007 kN
- Group A: 5×5 at 1.2 m centres — an outer dimension of 5.4 m
- Group B: 3×3 at 1.5 m centres — an outer dimension of 3.6 m
Find
Whether the group fails as individual piles or as one buried block
Check yourself
Why does block failure become more critical as a pile group gets larger, at constant spacing?
Practice
A 3×3 group of 0.6 m piles at 1.8 m centres has an efficiency of 0.73. If a single pile carries 1001 kN, what is the group capacity by the efficiency route, in kN?
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint