Pile Group Analysis

A pile group is not nine single piles. The cap ties them together so they move as one, and the soil in front of a trailing pile has already been pushed by the pile ahead of it. Both effects change how much load each pile actually carries, and both are what this calculator computes.

What a p-multiplier is

A pile in a closely spaced group attracts less soil resistance than an isolated one. The pile ahead of it has already displaced the same soil, so the trailing pile meets ground that is disturbed and partly unloaded - the shadowing effect. The standard treatment scales the whole p-y curve by a p-multiplier Pm less than one. This tool uses AASHTO LRFD Table 10.7.2.4-1: at three-diameter spacing the leading row keeps 0.80 of its resistance, the second row 0.40 and the third and beyond 0.30. At five diameters those rise to 1.00, 0.85 and 0.70, and beyond five diameters the piles are treated as isolated.

Which row is which depends on the load direction

Rows are counted along the direction the load acts, and row 1 is the row the load pushes toward. That means a group which is symmetric in plan is not symmetric in its multipliers: reverse the load and the leading row becomes the trailing one. The tool derives rows from the plan layout and the load direction rather than asking you to number them, so the numbering cannot drift out of step with the geometry.

Shear carried by each pile

Because the cap is rigid, every pile head moves the same distance - so the piles do not share the lateral load equally, they share it in proportion to how much resistance each one can still mobilise. On the default 3x3 group at 3D spacing under 3,600 kN, the leading row carries 585 kN per pile, the second row 341 kN and the trailing row 274 kN. The leading row takes more than twice what the back row takes, from an identical pile in identical soil.

Axial force and uplift

An overturning moment on the cap is resisted mainly by axial push and pull between the front and back rows, not by bending in the piles - the couple works over the group plan dimension in metres, while a pile bending against its own diameter has a far shorter lever. On the default group the leading row takes 2,114 kN of compression while the trailing row goes into 114 kN of tension. A pile in uplift is a different design case: it needs shaft resistance and a tension connection to the cap, and the tool flags it rather than leaving a minus sign to be noticed.

Cap movement - translation, settlement and rotation

The rigid cap has three degrees of freedom, and the tool reports all three: lateral translation, settlement and rotation. On the default group these come out at 20.6 mm, 5.4 mm and 3.35 milliradians. Translation is usually the number a serviceability check cares about; rotation matters when the group supports something sensitive to tilt, such as a bridge pier or a crane base.

Group efficiency is load-dependent

Group efficiency - the group stiffness divided by the stiffness of the same number of isolated piles - is often quoted as a single figure, but it is not constant. At small load the piles sit in the near-linear part of the p-y curve, initial stiffness dominates and efficiency is close to one. Once the springs mobilise, the multipliers bite. On the default profile efficiency is about 0.95 at 1.5 mm of cap deflection and about 0.33 at working load near 23 mm. An efficiency figure quoted without a load level does not mean much.

What this tool does not check

It distributes load between piles. It does not check axial capacity, settlement, block failure or the strength of the cap itself, and the axial response of each pile is treated as linear. A pile shown carrying 2,114 kN has not been shown to be able to carry it - that is a separate axial capacity check. The axial stiffness is estimated from the section and an ultimate capacity, and real axial stiffness varies widely with installation method and soil type, so a pile load test is the reliable source.

Common questions