Presenter: Melissa Marry (University of New Hampshire)

Description:
Momentary liquefaction, a temporary loss of effective stress between individual sand grains, is caused by the upward vertical pressure gradient of the excess pore-water pressure in the soil during the passage of a wave (Sumer et al., 2002). While extensive work has been done both through model experiments and field measurements to calculate the wave-induced pore-water pressure response in the seabed, there has been few field experiments to definitively observe the occurrence of momentary liquefaction in nature (Foster et al., 2006). Observations of vertical pore water, water column, and surface pressure are obtained in the inner surf of a moderately sloping, macro-tidal New Hampshire Beach, over a range of conditions. The beach has a 2.5 m tidal range with offshore wave conditions over the experiments varying from 0.5 - 2 m high and periods of 5 - 17 seconds. A self-recording pressure array composed of 0.3 mm diaphragm pressure sensors spanning from 0.35 m above the bed to 0.35 m below the bed were sampled at 18 Hz and able to measure pressure variability due to the tides, free surface gravity waves, and turbulence. Our observations show the vertical gradient of pressure overcomes the effective weight of the soil. The vertical gradient of excess pore water pressure becomes negative, implying a decrease in effective stress between sand grains as far as 15 cm into the sediment bed. Combined these observations demonstrate evidence for the occurrence of momentary liquefaction events (Mory et al., 2007). The results suggest that hurricane conditions may increase the frequency of these events.