Impact-induced transient strain in dense sand: fibre optic sensing and numerical modelling

Transient impact loads during construction and operation are critical for evaluating the safety and performance of geotechnical infrastructures, as they act directly on foundation soils. In this study, an integrated experimental–numerical framework was developed and validated to characterise the transient strain response of dense sand under impact using fibre Bragg grating (FBG) sensors and the material point method (MPM). Laboratory free-fall sphere tests were conducted, with steel spheres dropped from various heights onto an instrumented sand foundation. The MPM was employed to simulate the impact and optimise the test design. Results show that the strain–time history exhibits four distinct phases: non-linear ascending, linear increasing, non-linear descending and residual stabilisation. The peak strain consistently occurred at ≈100 ms, indicating a critical inertial timescale. At a given depth, the peak strain increased linearly with the falling height, while it decayed exponentially with depth for the same impact energy. The effective influence depth was 4–5 times the sphere diameter. These findings demonstrate the capability of the FBG-MPM framework to capture high-frequency strain responses reliably. The fundamental soil behaviour under impact determined using the framework provides benchmark data for constitutive model calibration and the design of shallow foundations subjected to impact loads.