Effect of Pile-Head Connection Conditions and Groundwater Level on the Seismic Response of Piled-Raft Foundations

The seismic response of piled-raft foundations depends on the pile-head connection and the groundwater level, but their combined effect under different ground motions has received little attention. Three-dimensional finite element analyses (PLAXIS 3D, Hardening Soil model, drained conditions) were performed for a 4 × 4 pile group beneath a raft in medium-dense sand, comparing a raft without piles (RF) with connected (CPRF), semi-connected (SCPRF) and unconnected (UNCPRF) piled rafts under the El Centro (1940) and Erzincan (1992) records, the latter at its original intensity (0.496 g) and scaled to 0.319 g, with the groundwater level at −0.5, −7.5 and −50 m. The study compared the records using a difference ratio (DR). The CPRF gave the lowest absolute displacement under El Centro (15.6% below the RF), but its benefit decreased under the stronger Erzincan record. The UNCPRF gave the lowest displacement relative to the soil base, the lowest settlement, and the smallest pile moments under all records, whereas the CPRF and SCPRF amplified the raft acceleration several times. A shallow water table increased settlement by a factor of 1.6–2.0 and increased the governing pile moment under both records, with the magnitude depending on the record. The conclusions refer to the case studied and to non-liquefaction conditions.