The relationship between permeability and effective stress prior to deformation is shown in Figure F10A. The gradient of each line indicates the level of stress dependence of permeability, and it can be observed that increasing the mean effective stress from 175 kPa to a maximum of 1200 kPa leads to a decrease in permeability of approximately half an order of magnitude in most cases. This permeability reduction corresponds to a porosity loss of only several porosity units (Fig. F10B). Given the limitations of the test equipment (maximum stresses of approximately 1.6 MPa are achievable), all samples tested are overconsolidated with respect to the in situ effective stress acting upon the sediment. Any porosity loss due to loading therefore represents the elastic portion of a typical consolidation curve and may explain the relatively small change in both porosity and permeability with increasing effective stress.
The range of permeabilities measured before, during, and after shear are summarized in Figure F11. Permeability evolution during shear in all cases bar one displays small-scale changes in permeability (less than one order of magnitude). Similarly, in the majority of cases, the permeability for a given effective stress after shear did not differ significantly from the equivalent preshear permeability. In other words, deformation predominantly has little effect on the overall permeability of the material, even when effective stresses approach zero magnitude (in other words at extreme levels of overpressure).
The sample that showed the greatest variation in permeability was a stiff, friable intensely overconsolidated claystone (Sample 170-1040C-13R-5, 88-105 cm). This sample, although extracted from within the prism, contained intact fabrics that typically characterized the upper part of the décollement. Geochemical and physical properties variations (Kimura, Silver, Blum, et al., 1997) confirm that this interval represents a fault zone acting as an efficient conduit for fluid flow. In this instance, axial deformation induced a markedly different fluid pressure response and permeability change than observed in any of the other samples. Whereas compactive shear, typical of the majority of these sediments, induced both increases in fluid pressure and bulk decreases in permeability, shearing of this heavily overconsolidated sediment induced a rapid drop in fluid pressure (Fig. F12A), consistent with mechanical principles of dilation in overconsolidated sediments (Karig, 1990). Although porosity varied by little more than 6% (from 44% at the highest levels of effective stress before shear to 49.4% at low effective stress after shear), the permeability of the sample increased by almost four orders of magnitude. Furthermore, specimen failure was intensely localized and brittle and induced a permeability increase from 3 × 10-18 m2 to 1 × 10-15 m2 (Fig. F12C) directly before and after shear. Increased effective stress after failure induces permeability reduction, which may reflect the closure of newly formed dilatant fractures. The permeability of this sample therefore ranges by nearly four orders of magnitude through the whole stress-strain path, from 6 × 10-19 m2 before shear (900 kPa effective stress) to 2 × 10-15 m2 after shear (50 kPa effective stress).
It was observed in the majority of cases that deformation had an insignificant effect on the permeability of these samples (summarized in Fig. F11). Even though all samples were tested in an overconsolidated state (due to equipment limitations), they gradually consolidated during shear in a diffuse manner. We infer that significant permeability variations do not occur in the absence of any significant microfabrics.
Two samples that deformed in a predominantly brittle manner (Samples 170-1040C-50R-3, 115-130 cm, and 13R-5, 88-105 cm) were notably more indurated, and yet displayed individually vastly different hydrological response during and after shear. The overconsolidated chalk (Sample 170-1040C-50R-3) contained a relatively high initial permeability, which displayed very little permeability reduction before, during, or after shear even though the material failed along single plane with a well-defined peak stress. In other words, brittle failure has little influence on hydrological properties when the initial permeability is high, even at low effective stresses. In contrast, the permeability of the friable claystone increased by nearly four orders of magnitude when both brittle fractures formed and fluid pressures were raised. Postshear permeability was more sensitive to changes in effective stress than porosity variations, which may reflect the opening and closure of fractures in response to fluid pressure fluctuations (and hence effective stress) that allows increased interconnectivity and fluid flow. Fracture closure results in fluid flow that is controlled by matrix properties (e.g., porosity) as opposed to stress. Similar permeability behavior has been observed in situ within the Barbados décollement (Fisher et al., 1996; Screaton et al., 1997) and during laboratory tests (Brown, 1995; Zwart et al., 1997) on various sediments from different margins.