Study on Crack Propagation Characteristics of Foam Concrete–Soil Composite with Different Height Ratios under Dynamic and Static Loading Conditions

Foam concrete is often used for the backfill of subgrade in road engineering to decrease differential settlement, thus forming the foam concrete-soil composite that bears the road dynamic and static loads together.Crack propagation of foam concrete-soil with different backfill depths is a critical factor that influences subgrade stability, and a reasonable study on crack propagation is of great importance to road traffic safety.To explore the crack propagation characteristics of foam concrete-soil subgrade with different backfill depths in traffic dynamic and static loading environments, foam concrete-soil samples with varying ratios of height of 3:1, 1:1, and 1:3 were designed to perform compression and vibration tests.Crack propagation processes of foam concrete-soil samples with different height ratios under dynamic and static loading conditions were also investigated.Crack damage modes and crack propagation scales were analyzed by combining the Gaussian mixture model and the acoustic emission b value.Results demonstrate that, the porous structures inside the foam concrete interlock mutually upon compression; thus, the thicker the foam concrete layer is, the greater the bearing capacity of the combination after failure.The bearing capacity after the fracture failure of the composite increases from 0.278 MPa to 2.036 MPa.In the composite, cracks initiate from soil mass under dynamic and static loading conditions and spread upward gradually, breaking through the interface and forming through cracks of the foam concrete-soil composite.With the increase in thickness of the foam concrete layer, the proportion of shear cracks under vibration increases from 19.1% to 75.4%, and the fracture mode changes from the tensile fracture-dominated mode to the shear failure-dominated mode.Compared with uniaxial compression, the acoustic emission b value of the composite increases substantially due to the intense energy storage behavior before the development of macroscopic fracture under vibration loads.The proposed novel method provides remarkable evidence for analyzing the crack propagation characteristics of foam concrete-soil subgrade with different backfill depths and evaluating and enhancing its stability.