Shallow foundation load testing on natural Florida limestone: single rock layer and rock-over-sand subsurface

This article presents three shallow foundation load tests under various layer conditions (single rock layer and rock-over-sand subsurface), rock strengths, and limestone formations. Rock coring with laboratory triaxial testing, standard penetration tests, seismic shear tests, and measuring while drilling tests were performed to identify the representative mass bulk dry unit weight ( γ dt ) and the spatial variability of γ dt . The geometric mean and median γ dt , together with seismic shear results, were used to characterize rock strength and stiffness. Each footing test was sized at 1.07–1.83 m to fit the 12.20 m load frame (8.01 MN capacity) using laboratory-assessed bilinear strength envelopes of the rock. Predicted bearing capacities with Florida bearing capacity equations provided reasonable estimates for three load tests. Mean and differential settlement, including spatial variability for a single footing on a heterogeneous rock layer, were captured with the Fenton and Griffiths method. For the settlement prediction of rock-over-sand subsurface, a parametric study was conducted for typical footing size, rock thickness, and mass properties with the finite element method (FEM), and a Winkler approach with a stress-dependent weighted harmonic mean modulus ( E eq ). The proposed method achieved good agreement with the FEM results and was validated with load tests.