BEHAVIOR OF SETTLEMENT REDUCING PILES UNDER STATIC AND SEISMIC LOADINGS-Paper

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The paper evaluates unconnected piled-raft foundations, in which a compacted sand or gravel cushion separates the raft from the pile heads. Unlike conventional connected piles, these piles reinforce the soil and reduce settlement rather than acting as directly connected structural members. The study uses three-dimensional ABAQUS finite-element analyses to compare connected and unconnected systems under static loading and an earthquake record scaled to 0.2 g. It also assesses an existing 14-story building and estimates construction costs.<br /><br />In the seismic analysis, the cushion redistributed loads more evenly among the piles and reduced pile-head stresses. Compared with connected piles, unconnected piles had about 38% lower static pile-head loads and 43% lower peak vertical loads during earthquake loading. Pile-head bending moments were reduced by approximately 50–100%, while horizontal and shear stresses at the pile head largely diminished. Despite these reductions, settlement under seismic loading was nearly the same for both foundation types, and lateral displacement was not appreciably affected by whether the piles were connected.<br /><br />For the building case study, both piled-raft options substantially reduced settlement compared with a raft alone. The unconnected system reduced pile load sharing and maintained settlements within the cited allowable limit, even in design variations with shorter piles or fewer piles.<br /><br />The cost estimates indicate savings of 57.8% for an unconnected system using the same number and length of piles, rising to 63.7% with shorter piles and 72.5% with fewer piles. Savings arise partly from eliminating pile reinforcement and pile-head demolition, and avoiding complex pile-to-raft connections.<br /><br />The authors conclude that unconnected piled rafts can provide an economical alternative with lower pile stresses and comparable settlement performance. Their findings are based on the modeled soil profiles and case study; project-specific validation is needed before design.

Keywords

unconnected piled-raft foundation

connected piles

finite element analysis

ABAQUS

seismic loading

pile-head stress

settlement reduction

earthquake response

cost savings

building case study