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The number of rows in the group.
A rectangular grid of rows by columns under one cap; an irregular group needs its own block outline.
The number of piles in each row.
Rows times columns is the number of piles; the block's plan is set by the outermost piles.
The distance between pile centres, the same in both directions.
Three diameters is the conventional minimum. The closer the piles, the more likely the group fails as a block rather than pile by pile.
The pile's diameter, or the width of a square pile.
The block's plan dimensions run out to the outer faces of the edge piles, one diameter beyond the outer centres.
The embedded length the block's sides shear along.
Measured below the cap, through the clay; a soft layer that contributes nothing, or pulls down on the piles, is left out of this length.
The clay's undrained shear strength averaged over the pile length.
From the ground investigation's strength profile. The block shears soil against soil, so the full strength is used here, without the adhesion factor a single pile's shaft takes.
The clay's undrained strength at the level of the pile toes.
Stiffer clay at depth is common, which is why the base value is entered on its own.
One pile's ultimate capacity, shaft plus base, to compare the block against.
From the single-pile calculation. The page reduces the sum of the piles by the Converse-Labarre efficiency and sets it beside the block, because the smaller of the two is the group's capacity.
Block failure capacity (ultimate)
2,510 kips
The reduced sum of the single piles gives the smaller capacity; the block is stronger than the piles acting individually. Both figures are ultimate capacities. The working load follows from the factor of safety or the partial factors the design applies, and settlement of the group often governs before either is reached.
- Block width, out to out
- 9.33 ft
- Block length, out to out
- 9.33 ft
- Bearing capacity factor Nc
- 9
- Shear round the block's sides
- 1,528.26 kips
- End bearing under the block
- 982.45 kips
- Converse-Labarre efficiency
- 0.73
- Sum of the single piles
- 1,820.95 kips
- Sum reduced by the efficiency
- 1,323.63 kips
- The smaller of the two
- 1,323.63 kips
They open the calculator with your figures already in it
Pile Group Block Failure Calculator (Clay): 2,511 kips — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Block failure of a pile group in clay (Terzaghi and Peck; Tomlinson and Woodward, Pile Design and Construction Practice): Q = 2(B + L) × D × c̄u + B × L × cu × Nc, where B and L are the group's plan dimensions out to out, D the pile length, c̄u the average undrained strength along the shaft and cu the strength at the base
- Skempton's bearing capacity factor for a rectangular base in clay: Nc = 5 × (1 + 0.2 D/B) × (1 + 0.2 B/L), with D/B taken no greater than 2.5, so Nc reaches 9 under a deep square block
- The Converse-Labarre group efficiency E = 1 − θ × [(n − 1)m + (m − 1)n] ÷ (90 × m × n), θ = arctan(d/s) in degrees, for m columns and n rows (Bowles, Foundation Analysis and Design)
Inputs used
- Rows of Piles
- 3
- Piles in Each Row
- 3
- Centre-to-Centre Spacing
- 4 ft
- Pile Diameter
- 16 in
- Pile Length in the Clay (D)
- 49 ft
- Average Undrained Strength Along the Shaft (c̄u)
- 835.42 psf
- Undrained Strength at the Base (cu)
- 1253.13 psf
- Ultimate Capacity of One Pile (0 to skip)
- 202.33 kip
Intermediate steps
- Block width, out to out
- 9.33 ft
- Block length, out to out
- 9.33 ft
- Bearing capacity factor Nc
- 9
- Shear round the block's sides
- 1,528.26 kips
- End bearing under the block
- 982.45 kips
- Converse-Labarre efficiency
- 0.73
- Sum of the single piles
- 1,820.95 kips
- Sum reduced by the efficiency
- 1,323.63 kips
- The smaller of the two
- 1,323.63 kips
Confidence note: The reduced sum of the single piles gives the smaller capacity; the block is stronger than the piles acting individually. Both figures are ultimate capacities. The working load follows from the factor of safety or the partial factors the design applies, and settlement of the group often governs before either is reached.
What this calculation does not cover
- Undrained, total-stress capacity of friction piles in clay in the short term. Long-term drained behaviour, piles in sand, and end-bearing piles on rock are different calculations.
- The block's weight and the weight of soil it replaces are taken to cancel, as the method conventionally does. A block founded in soft clay under a heavy cap needs that assumption checked.
- The Converse-Labarre efficiency knows only the pile diameter, the spacing and the grid, nothing about the soil or how the piles were installed. It is a screening figure from the foundation literature, not a design rule.
Add the equipment this sizes
This result is a specification — 2,510 kips — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-22 · v1.0.0
Regulatory standards & verification citations3
- Block failure of a pile group in clay (Terzaghi and Peck; Tomlinson and Woodward, Pile Design and Construction Practice): Q = 2(B + L) × D × c̄u + B × L × cu × Nc, where B and L are the group's plan dimensions out to out, D the pile length, c̄u the average undrained strength along the shaft and cu the strength at the base
- Skempton's bearing capacity factor for a rectangular base in clay: Nc = 5 × (1 + 0.2 D/B) × (1 + 0.2 B/L), with D/B taken no greater than 2.5, so Nc reaches 9 under a deep square block
- The Converse-Labarre group efficiency E = 1 − θ × [(n − 1)m + (m − 1)n] ÷ (90 × m × n), θ = arctan(d/s) in degrees, for m columns and n rows (Bowles, Foundation Analysis and Design)
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