Pore water pressure

About: Pore water pressure is a research topic. Over the lifetime, 11455 publications have been published within this topic receiving 247670 citations. The topic is also known as: pwp.

Slip behavior of simulated gouge‐bearing faults under conditions favoring pressure solution

Abstract: Geophysical observations as well as deformation experiments indicate that under hydrothermal conditions, crustal faults can be significantly weakened with respect to conventional brittle-plastic strength envelopes. Pressure solution has long been proposed as a mechanism leading to fault weakness. However, pressure solution has also been proposed as contributing to interseismic fault healing, and the competition between the weakening and healing effects of pressure solution is unclear. To investigate this issue, we have conducted rotary shear experiments on synthetic faults containing granular halite (NaCI) gouge using NaCI-saturated mixtures of water and methanol as pore fluid. The NaCI-water-methanol system was chosen as a rock analogue because pressure solution is known to be important in this system at ambient conditions. We explored the influence of varying pore fluid composition (hence pressure solution rate), gouge grain size, and wall rock surface roughness, as well as normal stress and sliding velocity on slip behavior. All experiments were done under drained conditions. An acoustic emission detection system allowed detection of brittle events in the gouge. The results show no evidence for steady state pressure solution-controlled fault slip. Frictional, rate-insensitive behavior was observed, whereas the microstructures and compaction behavior clearly demonstrated that pressure solution was active in the gouge. Our data show that fluid-assisted healing effects dominated over weakening, causing fault strength to be controlled mainly by brittle-frictional processes. Existing models describing pressure solution-controlled fault creep may not be applicable to a porous gouge undergoing compaction as well as slip.

A mathematical model for predicting moisture flow in an unsaturated soil under hydraulic and temperature gradients

Abstract: A theoretical model is presented to predict the moisture flow in an unsaturated soil as the result of hydraulic and temperature gradients. A partial differential heat flow equation (for above-freezing conditions) and the two partial differential transient flow equations (one for the water phase and the other for the air phase), are derived in this paper and solved using a finite difference technique. Darcy's law is used to describe the flow in the water phase, while Pick's law is used for the air phase. The constitutive equations proposed by Fredlund and Morgenstern are used to define the volume change of an unsaturated soil. The simultaneous solution of the partial differential equations gives the temperature, the pore water pressure, and the pore air pressure distribution with space and time in an unsaturated soil. The pressure changes can, in turn, be used to compute the quantity of moisture flow.

Character of sediments entering the Costa Rica subduction zone: Implications for partitioning of water along the plate interface

Abstract: Sediments deposited off the Nicoya Peninsula advect large volumes of water as they enter the Costa Rica subduction zone. Seismic reflection data, together with results from Ocean Drilling Program Leg 170, show that hemipelagic mud comprises the upper ∼135 m of the sediment column (ranging from 0 to 210 m). The lower ∼215 m of the sediment column (ranging from 0 to 470 m) is pelagic carbonate ooze. We analyzed samples from 60 shallow (<7 m) cores to characterize the spatial variability of sediment composition on the incoming Cocos Plate. The bulk hemipelagic sediment is 10 wt% opal and 60 wt% smectite on average, with no significant variations along strike; the pelagic chalk contains approximately 2 wt% opal and <1 wt% smectite. Initially, most of the water (96%) in the subducting sediment is stored in pore spaces, but the pore water is expelled during the early stages of subduction by compaction and tectonic consolidation. Approximately 3.6% of the sediment's total water volume enters the subduction zone as interlayer water in smectite; only 0.4% of the water is bound in opal. Once subducting strata reach depths greater than 6 km (more than 30 km inboard of the subduction front), porosity drops to less than 15%, and temperature rises to greater than 60°C. Under those conditions, discrete pulses of opal and smectite dehydration should create local compartments of fluid overpressure, which probably influence fluid flow patterns and reduce effective stress along the plate boundary fault.

Analysis of the landslide triggering mechanism during the storm of 20th–21st November 2000, in Northern Tuscany

Abstract: A severe rainstorm of high intensity occurred on 20th–21st November 2000, in the region of Pistoia, Tuscany, Italy, which triggered, within the entire province, over 50 landslides. These landslides can be broadly defined as complex earth slides—earth flows, originating as rotational slides that develop downslope into a flow. In this paper, two such landslides have been investigated by modelling the process of rainwater infiltration, the variations in both the positive and negative pore water pressures and their effect on slope stability during the storm. For both sites, results from morphometric and geotechnical analyses were used as a direct input to the numerical modelling. A modified Chu, 1978 approach was used to estimate the surface infiltration rate by adapting the original Green and Ampt, 1911 equations for unsteady rainfall intensity in conjunction with the surficial water balance. For transient conditions, a finite element analysis was used to model the fluctuations in pore water pressure during the storm, with the computed surface infiltration rate as the surface boundary condition. This was then followed by the application of the limit equilibrium Morgenstern and Price, 1965 slope-stability method, using the temporal pore water pressure distributions derived from the seepage analysis. From this methodology, a trend for the factor of safety was produced for both landslide sites. These results indicate that the most critical time step for failure was a few hours following the rainfall peak.