Evaluating soil compaction in the field using a solitary strain wave device with a modified contact interface

Non-destructive tests are used to evaluate the mechanical properties of construction materials. Solitary strain wave (SSW) devices estimate Young's modulus of a material by measuring the time of flight (TOF), defined as the time between the incident and reflected wave propagating through a chain of steel spheres. This study evaluates the incorporation of a PLA plate between the last element of the chain and the tested medium (soil). This plate aims to evenly distribute contact forces and reduce plastic deformations at the soil surface. This is addressed by modifying the Hertz contact equation using a finite element approach. Then, the dynamic equilibrium equations of the device were solved using a finite difference method to establish a relationship between TOF and the soil's Young's modulus. This relationship was validated by comparing the modulus of seven control polyurethane foams with measurements obtained from the SSW device. After validation, the device was tested on compacted soil samples using SSW and wave velocity methods. Finally, the device was employed for field compaction assessment. Results show that the modified setup improves the consistency of TOF measurements and opens new possibilities for using TOF as a parameter for compaction control.