Uniaxial reconsolidation-consolidation tests on porous sediments have been run on core samples from many Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) legs but the results on the whole have not been particularly productive. This admittedly very subjective conclusion arises not from a perceived lack of importance of such testing but from the sense that a lack of an adequate understanding of the basis for these tests by the geoscience community has sometimes led to poor-quality results and unwarranted interpretations on one hand, but on the other hand it has also led to a lack of appreciation for the range of geological implications that can be drawn from such tests. To be sure, several papers in ODP volumes have been written by trained geotechnical engineers (e.g., Moran and Christian, 1990; Feeser et al., 1993), but these tend to be couched in terminology that is difficult for most geologists to follow.
Uniaxial consolidation tests involve the variation of effective vertical stress on a sample for which the lateral (horizontal) strain is constrained to be zero. Such tests are presumed to mimic aspects of the deformational history in sedimentary basins, where the lateral strain is assumed to be insignificant. These tests have been assumed to provide a method of estimating the maximum applied in situ effective vertical stress, and, if certain conditions are met, for the estimation of pore fluid pressure (P). More sophisticated tests can include determination of the horizontal effective stress associated with consolidation, which might be a measure of the in situ horizontal stress. Vertical strains associated with consolidation are also valuable in estimating settlement resulting from loading by structures or caused by fluid withdrawal.
Still more consolidation tests, from ODP Site 897, are reported here, but because the general ODP community could benefit from a review of the subject, this paper includes a short section concerning the bases and methodologies of uniaxial consolidation testing. In large part the material presented here is drawn from the extensive and advanced literature on consolidation by the geotechnical engineering community. This literature has been largely ignored by geologists, not merely because it belongs to a different discipline but also because it is couched in a terminology and approach that is difficult for most geologists to follow. One objective of this review is to translate geologically pertinent aspects of this material on consolidation with the hope that it will lead to more sophisticated and more useful consolidation studies by the ODP community. In a sense, this review is an update of that by Bryant et al. (1981), which covered a wider range of mechanical studies on marine sediments.
This paper then reports the results of consolidation tests on a mid-Eocene calcareous mudstone from ODP Hole 897D. Uniaxial strain tests were performed on undisturbed samples as well as on this same lithology after disaggregation. The results are interpreted in terms of constraints on in situ stress and suggest very high pore-fluid pressures near the base of the sediment section. Comparison between test results of the two analogous materials and with those of a silty clay from the Nankai Trough (Karig, 1993; Karig and Morgan, 1994) elucidate problems in extrapolating from laboratory to in situ conditions.
Because acoustic velocities (Vp) were measured during most of these consolidation tests, a short section is included that describes velocity-stress relationships. The difference in Vp between shipboard and in situ stress conditions have important implications for the use of shipboard velocity measurements.