Static Load Testing with Elastic Cushion Layer: A Case Study in Foshan Highway Project

To address limitations in conventional static load tests for composite foundations, a method utilizing an elastic cushion layer is proposed to accurately simulate subgrade behavior, focusing on a highway project in Guangdong Province. Initial finite element calculations justify the elastic cushion layer introduction. Functional correlations are established between the stab deformation of the piles and the thickness, modulus and the pile-soil stress ratio. The reasonable value of the elastic modulus is taken as 4–10 MPa when the elastic cushion layer thickness is 60 mm. Lab and field tests of cement fly-ash gravel (CFG) pile composite foundations assess settlement variations and pile-soil stress ratios from the proposed improvements. Results reveal the superior performance of the elastic cushion layer over the sand cushion layer, effectively mimicking embankment subgrade layers and reducing rigid constraints on the loading plate. The elastic cushion layer exhibits a significant stress-regulating effect, allowing the transition from the original “100–150 mm sand cushion layer + loading plate” to “less than 100 mm sand cushion layer + 60 mm elastic cushion layer (8 MPa) + loading plate”. This adjustment enhances the practical role of the subgrade in the composite foundation, improving the accuracy and feasibility of static load testing methods.