CALIBRATION OF CONVERSE-LABARRE EQUATION FOR BORED PILE GROUPS IN BANGKOK SUBSOIL USING 3D FEM

Abstract

Conventional pile-group efficiency equations are used widely in foundation design; however, their applicability remains uncertain to bored pile groups in Bangkok soft clay, because simplified formulations do not account explicitly for soil-dependent pile-soil-pile interactions. This study evaluated and calibrated the Converse-Labarre equation for Bangkok subsoil using three-dimensional finite element analyses (3D FEM). Two representative bored pile series were considered: a smaller pile terminating in the first sand layer and a larger pile extending to the second sand layer. Pile-group efficiency was interpreted from FEM-derived load-settlement curves using the Chin method, with the Mazurkiewicz method used as an independent graphical check. The results showed that FEM-derived efficiency was consistently lower than unity, decreasing with an increasing number of piles, and increasing with an increasing spacing ratio. Among the classical capacity-based methods considered, the Converse-Labarre equation better captured the effects of both pile number and spacing ratio, although further correction was needed, especially at close spacing. Therefore, a Bangkok-specific correction factor was developed as a multiplicative adjustment to the original Converse-Labarre efficiency, with regression terms based on ln(n) and 1/ln(s/D). The selected model had R² = 0.587, reduced the mean absolute error from 3.46% to 2.61%, and reduced the maximum absolute error from 19.53% to 12.03%. The proposed correction preserves the simplicity of the Converse-Labarre equation and provides geotechnical engineers with a practical preliminary adjustment for estimating bored pile-group efficiency in Bangkok subsoil.