Pile Group Efficiency Calculator

The calculator reports group efficiency and then estimates capacity using the selected method.

• Converse–Labarre efficiency: Eg = 1 − (θ/90) · [ (m−1)/m + (n−1)/n ], where θ = arctan(d/s) in degrees.
• Efficiency-adjusted capacity: Qsum = Eg · N · Q1, with N = m·n.
• Optional clay block check (short-term): Qblock = α·cu·P·Le + Nc·cu·A, where P is group perimeter, A is group plan area, and Le is embedment length.
• Allowable capacity: Qallow = Qgoverning / FS.

1. Choose the efficiency method that matches your design approach.
2. Enter pile layout (rows and columns), diameter, and spacing.
3. Provide the single pile ultimate capacity from your geotechnical design.
4. Set a factor of safety consistent with your project requirements.
5. Enable the block check for cohesive soils when appropriate.
6. Click Calculate to view results above the form.
7. Use CSV or PDF downloads to document your estimate.

1) Why pile group efficiency matters

Group action can reduce the capacity of closely spaced piles because overlapping stress zones mobilize soil resistance together. An efficiency factor Eg helps translate single-pile capacity to a realistic group estimate for preliminary design, especially when pile spacing is below about 3d.

2) Core inputs and typical data ranges

This calculator uses pile layout (m × n), diameter d, spacing s, single pile ultimate capacity Q1 (kN), and a factor of safety. In practice, FS commonly falls between 2.0 and 3.0, depending on investigation quality and design standard.

3) Spacing ratio benchmarks for interaction

The spacing ratio s/d is a quick indicator of pile interference. At s/d ≤ 2, interaction is typically strong, and group efficiency may be noticeably lower. At s/d ≥ 6, interaction often becomes minor and Eg trends toward 1.0 for many cases.

4) Converse–Labarre interpretation

The Converse–Labarre option computes an interaction angle θ = arctan(d/s) and reduces efficiency as group size grows. Because the reduction depends on both m and n, a 4×4 group may show lower efficiency than a 2×8 group at the same spacing. Use it as an informed estimate, not a substitute for full analysis.

5) Empirical spacing model guidance

The empirical model maps s/d to an efficiency reminder: tight spacing gives a lower base value, and wider spacing approaches unity. A small layout penalty is applied for larger groups to reflect practical diminishing returns. Treat this method as a screening tool when you need a fast comparison of layout options.

6) Block failure check in cohesive soils

For short-term clay behavior, the optional block check compares the efficiency-adjusted sum to a block capacity. It uses perimeter adhesion α·cu·P·Le plus base bearing Nc·cu·A. Typical reference values include Nc ≈ 9 for undrained bearing and α in the range 0.5–1.0, depending on pile type and clay sensitivity.

7) What the outputs tell you

The key results include group dimensions (B, L), Eg, ultimate capacities (Eg·N·Q1 and optional Qblock), and allowable capacity Qallow. If Qblock governs, review soil parameters, embedment assumptions, and the chosen failure mechanism.

8) Practical design actions to improve efficiency

Improving efficiency usually means increasing spacing, reducing group size, or switching to a layout with fewer adjacent piles sharing the same influence zone. During constructability review, confirm minimum clear distance, rig access, and tolerance. For final design, supplement efficiency checks with settlement evaluation, load sharing assumptions, and load test evidence where available.

1) What does pile group efficiency represent?

It is the ratio between the estimated group ultimate capacity and the sum of individual pile capacities. Values below 1.0 indicate interaction effects reduce group performance compared with isolated piles.

2) Which method should I choose?

Use Converse–Labarre for classic interaction-based reduction, often applied to friction pile groups. Use the empirical spacing model for quick layout comparisons when you mainly want sensitivity to spacing ratio.

3) Why is spacing so important?

Spacing controls overlap of stress and shear zones in soil. Small s/d increases interaction, lowering efficiency and increasing settlement risk. Larger spacing reduces interaction and usually raises Eg.

4) When should I enable the block check?

Enable it for cohesive soils where a group may act like a single block in undrained conditions. It is a comparison check to see whether block failure could govern over the efficiency-adjusted sum.

5) What single pile capacity should I enter?

Enter a geotechnical ultimate capacity for one pile consistent with your method and ground model. Keep units consistent (kN). If you only have allowable capacity, convert it back using your chosen safety factor.

6) Does this replace settlement analysis?

No. Efficiency addresses capacity interaction, while settlement depends on soil stiffness, layering, drainage conditions, and load distribution. Use the outputs as a preliminary step before running settlement and serviceability checks.

7) Are CSV and PDF results identical?

Yes. Both downloads export the same latest inputs and outputs generated after you click Calculate, so you can archive a consistent record for design notes, reviews, or client reporting.