Modeling the Effect of Excess Pore-Water Pressure on the Bearing Capacity of Full Displacement Piles in Group: A Case-Study in Parma (Italy)-Paper

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The study examines how excess pore-water pressure generated during installation of full-displacement piles affects their bearing capacity over time, particularly when many piles are installed close together in saturated clay.<br /><br />The case study involved two raft foundations in Parma, Italy, supported by 756 concrete auger-screw piles, each 32 m long and 0.6 m in diameter. Static load tests showed that several piles tested about one month after installation had substantially lower capacities than expected, with considerable variation across the foundations. A later retest of one pile, six and ten months after installation, showed improved performance. The authors attribute this recovery mainly to dissipation of excess pore pressure and soil consolidation.<br /><br />To interpret the results, the authors developed a simplified model that estimates pore-pressure contributions from nearby piles based on their distance from the tested pile and the time since installation. The contributions are summed, then related linearly to pile capacity. The model predicts that capacity recovers as excess pore pressure dissipates. For some piles, pressure at the shaft initially increased after installation because later-installed neighboring piles added to the local pressure effects.<br /><br />Fitted to the site’s load-test data, the model had an average prediction error of about 5.5%, though individual errors ranged from −14% to 28%. Its assumptions include constant soil permeability and undrained shear strength, radial consolidation around individual piles, and simple addition of neighboring piles’ pressure effects.<br /><br />The authors present the method as a preliminary tool for estimating time-dependent capacity in pile groups, not as a validated general solution. More load tests, instrumentation, and numerical analysis are needed to verify the model and better represent the complex soil response to pile installation.

Keywords

excess pore-water pressure

full-displacement piles

bearing capacity

pile group effects

saturated clay

soil consolidation

static load tests

time-dependent capacity

augur-screw piles

Parma Italy