Pile group capacity
A group of piles does not carry the sum of its members. The stress bulbs overlap, so each pile is partly working in soil the others have already loaded, and the group can also fail as a block — the whole cluster punching down with the soil between the piles. Whichever is smaller governs.
| Check | Value | Note |
|---|---|---|
| Pile count | 9 | 3 × 3 |
| Diameter | 400 mm | as entered |
| Spacing | 1200 mm | 3.0 diameters |
| Group efficiency | 72.7% | Converse-Labarre |
| Sum of single piles | 6300 kN | before efficiency |
| Group capacity | 4579 kN | sum × efficiency |
| Footprint | 2.80 × 2.80 m | centres plus one diameter |
| Block capacity | 4771 kN | perimeter shaft plus base, over FS |
| Governing | Individual piles | the smaller of the two |
| Allowable | 4579 kN | what the group carries |
Individual pile capacity governs at 4,579 kN. The efficiency is 73%, so the group carries 73% of the sum of its members.
1 − θ[(n−1)m + (m−1)n]/(90mn) · θ = atan(D/s) in degrees.
n × Q_single × Eg · Sum reduced by efficiency.
cu(perimeter × L) + 9cu(B × L) · Undrained.
min(group, block) · The smaller governs.
A clear path from inputs to results.
A group of piles does not carry the sum of its members. The stress bulbs overlap, so each pile is partly working in soil the others have already loaded, and the group can also fail as a block.
How to use
- Enter the group dimensions and the pile geometry.
- Set the spacing, length and soil strength.
- Add the single-pile allowable capacity.
- Read the efficiency, the block capacity and which governs.
The formula
Reference: Converse-Labarre for group efficiency; undrained block failure from the group perimeter and base.
Worked example
A 3 × 3 group of 400 mm piles at 1.2 m spacing
- 3 × 3, 400 mm diameter, 1.2 m spacing, 15 m long.
- cu = 50 kPa, single pile allowable 700 kN.
- Block factor of safety 2.5.
- 1
The spacing is 3 diameters, and θ = atan(1/3) = 18.4°.
- 2
The efficiency is 1 − 18.4 × 12 / 810 = 0.727.
- 3
The group capacity is 9 × 700 × 0.727 = 4,580 kN.
- 4
The block check uses a 2.8 m footprint: 50 × 11.2 × 15 + 9 × 50 × 2.8² = 11,928 kN, or 4,771 kN at a factor of safety of 2.5.
- 5
Individual pile capacity governs, but only just — at 800 mm spacing the block would take over.
73% efficiency · 4,580 kN · individual piles govern.
Interpreting the result
The spacing is the design variable that matters. Below about 2.5 diameters the efficiency falls quickly and block failure usually takes over, which is why the rule of thumb exists rather than being an arbitrary preference.
Assumptions & limitations
- A rectangular group in uniform clay.
- Converse-Labarre for efficiency, which is empirical.
- Undrained block failure with Nc = 9 at the base.
- No settlement calculation.
Frequently asked questions
Why does the group carry less than the sum?
Because the stress from each pile spreads into soil that the neighbouring piles are also loading, so the effective stress increase per pile is smaller and the soil fails sooner.
Does settlement matter more than capacity?
Usually yes. A group at half its ultimate capacity can settle more than the structure allows, and settlement is not part of this calculation.
How does sand differ?
A sand group is usually checked with a different block expression and a settlement calculation, because drained behaviour has no undrained strength to work with.
What is block failure?
The whole cluster punching down as a unit, with the soil between the piles moving with it. It is checked by treating the group perimeter as a single large shaft and the base as a single large footing.
What spacing should I use?
2.5 to 3 diameters for friction piles in clay, and 3 to 4 for end-bearing piles. Closer than 2.5 and the group efficiency and installation interference both become problems.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.