Approximate Assessment of Basement Wall Effects on Foundation Performance-Paper

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The paper examines how basement walls contribute to the stiffness and load resistance of piled-raft foundations. Although walls are commonly designed mainly to retain soil during excavation, they can also support building loads and reduce foundation movements and pile stresses.<br /><br />The authors propose a simplified method that represents basement walls as rows of equivalent piles. These piles are assigned properties chosen to match the walls’ axial and bending stiffness, geotechnical resistance, and structural capacity. The approach is analyzed using GARP for vertical loading and CLAP for lateral and moment loading, then compared with three-dimensional PLAXIS analyses.<br /><br />In the example studied—a 16 m square piled raft in stiff clay—the simplified results generally agreed reasonably with the finite-element results. As wall depth increased, vertical settlement and horizontal displacement decreased, while vertical, horizontal, and rotational stiffness increased. Walls also shielded foundation piles from lateral loading, reducing pile-head bending moments; most of this reduction occurred with relatively shallow walls. Basement walls redistributed axial loads modestly, reducing loads on corner and mid-side piles while slightly increasing loads on central piles.<br /><br />Differences between methods arose partly because the simplified programs use assumptions such as a rigid cap in CLAP, whereas the finite-element model represents the raft and continuous walls more directly. The simplified approach may therefore be useful for preliminary design or checking detailed analyses, but it does not capture every aspect of wall–raft interaction. The authors also caution that wall effects become less significant for larger foundation footprints and that wall stiffening can increase differential settlement near raft edges relative to some simplified predictions.<br /><br />Overall, incorporating basement walls into foundation design can avoid overly conservative estimates of pile forces and moments, while helping identify settlement and rotation effects that might otherwise be overlooked.

Keywords

basement walls

piled-raft foundations

stiffness

load resistance

equivalent piles

settlement

pile stresses

lateral loading

finite-element analysis

foundation design