P-multipliers and shadowing in lateral pile groups

Lateral group action does not reduce to a single η. Leading row pushes into fresh soil; trailing rows push into soil already disturbed — shadowing. Scale the p-values of each row's p-y curves by a row-specific p-multiplier less than 1. Deflections stay compatible (the cap forces a common head deflection); trailing rows attract less shear. Theory and the published table live on the pile group efficiency hub. Not a PE stamp, not a design example for a real project.

Why lateral groups are not one efficiency factor

Vertical Converse-Labarre is a different check — geometric screening on axial capacity. Clay equivalent-block is a third check. Do not mix them with lateral row interaction. When a group is pushed sideways, the leading row still sees relatively fresh soil, but each trailing row works against soil already displaced by the row ahead. A single axial η cannot capture that directional shadowing.

The standard treatment keeps compatible head deflections under the cap and reduces soil resistance row by row. Trailing piles then carry less head shear at the same deflection.

FHWA GEC 9 (2018) Table 7-1

PileCalc uses the row-specific multipliers of FHWA GEC 9 (2018), Table 7-1, tabulated at 3B and 5B loading-direction spacing, linearly interpolated between them. Copy these hub numbers; do not invent others:

A 3×3 group at s/B = 3 runs its leading row at 0.8 and its trailing rows at 0.4 and 0.3 — trailing rows deliver less than half the leading row's resistance at the same deflection. PileCalc applies the table over its published 3B–5B range and reports pile resistance only, conservatively leaving any cap/soil contribution out.

What this is not

Vertical Converse-Labarre on the hub's 3×3 at s/d = 3 gives η ≈ 0.73 — layout-only axial screening. That is a different check. Clay equivalent-block failure is a third check. See Converse-Labarre equation and block failure in clay. Do not substitute a single axial η for row p-multipliers under lateral load.

Keep reading

• Live calculator — Converse-Labarre and the block comparison in the browser
• Pile group efficiency hub — formulas, η table, block failure, lateral p-multipliers
• Block failure in clay — the check Converse-Labarre misses
• Converse-Labarre equation — worked reduction factors from the hub table
• What is a p-y curve? — single-pile lateral soil springs before group scaling

Source: FHWA GEC 9, Design and Analysis of Laterally Loaded Deep Foundations (Table 7-1 row p-multipliers). Engineering judgment stays with the user.