When Is a Constant Compression Index Sufficient for Pile Downdrag? Joint Drag and Settlement Criteria with Laboratory and Field Checks

We propose a joint response-based procedure to determine when one constant compression coefficient is sufficient for both pile drag increment and head settlement. Ordered compression-strain bounds are propagated through a finite-stiffness pile with fixed monotone shaft and toe laws; acceptable coefficient intervals are then intersected. Of 500 synthetic configurations, 339 pass a physical screen (upper strain at most 5% and soil–pile movement at most 0.3 m); in this subset, 335 admit a constant matching their prescribed nonlinear reference within 10%, whereas only 35 satisfy the conservative response-box test. Nine published compression curves provide approximation and withheld-stress checks. Explicit smooth whole-curve witnesses and curvature-constrained enclosures show which conclusions require additional shape information. A Bangkok stage-transfer reconstruction fails; a settlement-independent force-capacity bound identifies incompatibility of the frozen fitted transfer model with later observations. A worked single-pile example with explicit effective stress shows that groundwater lowering can change the decision and separates approximation accuracy from conservative design demands. The procedure supports conditional simplification decisions while making missing compression or transfer information explicit.