Axial Capacities of Drilled Displacement Piles Predicted by Spt and CPT Based Methods-Paper

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The study evaluates methods for predicting the axial capacity of drilled displacement piles (DDPs), which improve surrounding soil through lateral displacement and compaction. Researchers compared direct CPT- and SPT-based methods against 33 static load tests from 21 construction sites, covering sand-like, clay-like, and mixed, stratified soils.<br /><br />Before comparing predicted and measured capacities, the study assessed ways to interpret pile failure when tests did not reach plunging behavior. The Butler–Hoy method and Hansen 80% criterion most closely matched observed failure loads; Chin–Kondner and Decourt tended to overestimate failure, while Davisson tended to underestimate it.<br /><br />Among CPT methods, Bustamante and Gianeselli (1993, 1998), developed specifically for displacement piles, gave the closest overall capacity predictions. Most other CPT methods were conservative, generally underpredicting tested capacity. SPT-based methods showed less consistent performance and could overpredict capacity; their applicability is limited because many were developed for other pile types. For sandy soils, Decourt performed best among the SPT methods, while O’Neill and Reese performed best for mixed soils.<br /><br />The gap between predicted and measured capacity was linked to soil improvement from installation. Greater improvement—especially densification of initially loose soils—was associated with larger capacity gains and less reliable predictions based on pre-installation data. Post-installation CPT measurements produced more accurate estimates, highlighting the value of accounting for installation effects. Overall, the findings support careful selection of predictive methods and further development of methods calibrated specifically for DDPs.

Keywords

drilled displacement piles

axial capacity prediction

CPT methods

SPT methods

static load tests

pile failure criteria

soil densification

installation effects

Bustamante and Gianeselli

post-installation CPT