Using a Seismic Method to Study the Dynamic Soil Behavior During Repeated Loading With An Agricultural Tire.

Soil compaction caused by agricultural field traffic increases the mechanical resistance for root growth resulting in reduced crop growth, and modifies the soil pore size distribution and connectivity and therefore negatively affects key soil functions and processes such as water and gas transport and storage. Vehicular traffic transmits dynamic stresses to the soil. However, our understanding of dynamic soil deformation processes remains limited. Yet such knowledge is essential for better predictions of effects of soil management practices such as agricultural field traffic on soil functioning. The main objective of this study was to investigate whether seismic measurements could be used to assess the dynamic soil behavior during repeated loading. Moreover, we aimed at linking the velocity of P-waves, V_p, to traditionally measured soil properties associated with soil compaction, namely bulk density and penetrometer resistance. A wheeling experiment was carried out with an agricultural tire (60 kN wheel load) on a gleyic Cambisol. We measured V_p using an acoustic (micro-seismic) device at various depths before, during (i.e. below the tire) and after wheeling. In addition, we measured bulk density and penetrometer resistance before and after wheeling, and simulated the evolution of bulk density due to the wheeling using a soil compaction model. The dynamic soil response during loading-unloading-reloading cycles could be well captured with the seismic method. The measured V_p related to bulk density, and the compaction-induced increase in V_p correlated with the increase in penetrometer resistance. The seismic measurements seem a promising in situ method for gaining new insights into the dynamics of soil behavior. The method may be especially powerful when combined with traditional measurements and with modeling of soil deformation.