Foundation Engineering / Pile groups, settlement and negative skin friction

Group capacity

Efficiency, and the failure mode that ignores the individual piles.

Lesson review: expert verification remains open

This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.

Corrections checked in this pass

Corrected the pile-plan scale and removed an unsupported total-settlement ratio. The available movement components are labelled separately.

Checks needed for project use

A geotechnical reviewer must confirm drainage and stress basis, bearing-factor formulation, correlations and settlement influence factors. Partial factors and project-specific ground conditions are outside these teaching checks.
Equivalent-raft depth/spread are selected empirical assumptions; embedded layered-ground movement and group load sharing need verification. Single-pile shaft-induced soil displacement is omitted, so no complete total or ratio is available.

These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.

Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.

Read the lesson register and outstanding review notes

Why this matters

Piles in a group interact through the surrounding ground. Depending on installation, spacing and soil conditions, group resistance can be less than or greater than the sum of isolated-pile resistances. Settlement and block mechanisms also need assessment.

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 s_u = 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?