Uniaxial reconsolidation tests were conducted on samples from four whole-round cores of similar hemipelagic silty mudstones from the Shikoku Basin sequence recovered at DSDP Site 582 and ODP Site 808. Two cores from Site 808 were from below the d?collement, in the prism toe, and two from Site 582 were from the undeformed trench wedge. The objectives of testing included the estimation of maximum past consolidation stress, the in-situ stress, the exploration of relationships between physical properties and mechanical state of the naturally consolidated mudstones, and comparison of the behavior of these natural sediments with similar but experimentally consolidated mudstone. Testing also determined the effect of pressure between laboratory (1 bar) and in-situ conditions on seismic velocity and on porosity.On the basis of their yield stresses (sigma c'), samples from Site 582 were overconsolidated, whereas those from Site 808 were highly underconsolidated. The apparent overconsolidation at Site 582 is attributed to incipient cementation, which contributes strength and stiffness to the sediment. Porosity in these samples was much higher than that in the experimentally consolidated analog at the same sigma v', which probably reflects higher than hydrostatic pore pressure. However, the effect of excess pore pressure on sigma c' is masked by that of cementation.In part, the underconsolidation at Site 808 is thought to reflect inhibited drainage, primarily occurring as these sediments were subducted beneath the tip of the décollement. The extreme apparent underconsolidation of the sample closest to the décollement is attributed to a reduction of sigma c' caused by destruction of cementation, possibly by isotropic stress pulses of high pore-fluid pressure.The enhanced cementation at Site 582 relative to that at Site 808, which behaves similarly to the uncemented, experimentally consolidated mudstone, results in much higher constrained moduli and higher seismic velocities at equivalent porosities. This demonstrates that velocity-porosity relationships are strongly influenced by very subtle differences in sediment properties. Seismic velocities increased between 5% and 10% between laboratory and in-situ pressures. Most of this increase reflects crackclosing at low pressure. Porosity rebound at both sites was from <1%-2%, far less than for the experimentally consolidated sediment or than was predicted from published values for mudstones.

Supplement to: Karig, Daniel E (1993): Reconsolidation tests and sonic velocity measurements of clay-rich sediments from the Nankai Trough. In: Hill, IA; Taira, A; Firth, JV; et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 131, 247-260