Experimental mechanical compaction to measure the influence of pore pressure on compressional wave velocity in unconsolidated sands

This study describes uniaxial mechanical compaction tests performed on two naturally occurring unconsolidated sands with different mineralogy, quartz-rich and quartz-poor, in order to investigate the effect of pore pressure on the P-wave velocity. The compaction experiments simulated burial of sands with increasing vertical stress and pore pressure, and also simulated the effect of overpressure. For both sands the P-wave velocity increased and porosities decreased with increasing pore pressure when the applied vertical stress was increased at the same rate as the pore pressure. These effects may result from grain rearrangement, with decreasing radial effective stress offsetting the contraction of sand grains caused by pore pressure increase. The P-wave effective stress coefficient n_p was computed using a simple procedure. A decrease in n_p of the quartz-poor sand above 10 MPa vertical effective stress was observed. This decrease is attributed to compaction, mainly due to grain crushing. The experimental results show that pore pressure can have significant effects on P-wave velocity and n_p, both directly and indirectly, depending upon the stress loading paths and mineralogy.