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Marine structures placed in the shallower seabed can experience pore water drainages with more complexity than those in onshore environments, particularly in coarse-grained soils where drainage is neither purely “drained” nor “undrained,” but Partially Drained (PD). However, current laboratory approaches for characterizing soil behavior are limited to modeling drainage conditions as fully drained or undrained. This paper presents results from a series of confined monotonic saturated simple shear tests under various drainage conditions on reconstituted medium dense to dense Monterey sand specimens to fill this knowledge gap. Although others have performed limited PD element-level tests under triaxial conditions, no documentation exists for tests using a simple monotonic shear configuration. To achieve PD, a special filter was fabricated and connected between the bottom of the specimen and the backpressure controller. The hydraulic filter comprises a series of needle valves to provide various hydraulic impedances. All simple shear tests in this paper were backpressure-saturated. Two different degrees of PD were considered and compared with fully drained and undrained conditions. Results show that the excess pore water pressure generation and measured volumetric changes in the PD tests are bounded between those measured from fully drained and undrained, proving the PD filter provided the hydraulic resistance to achieve PD condition.
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Homogeneous and Membrane Pore Fluid Diffusion in Spring Block Simulations of Fault Slip With Rate and State Friction
Abstract Spring block, and sometimes continuum, models of the effects of the coupling of fluid flow and frictional slip often employ the membrane approximation. This approximation assumes that the fluid flux between the slipping layer and a remote pore fluid reservoir is proportional to the difference between the value of pore pressure in the reservoir and in the slipping zone. In contrast, Darcy's law states that the fluid flux is proportional to the gradient of the pore pressure. We analyze and compare these two formulations by asymptotic analysis of the fluid flow equations and numerical simulations of a spring–block model using rate and state friction. This analysis shows that membrane diffusion agrees with homogeneous diffusion in the limit of undrained conditions and for nearly drained conditions. For homogeneous diffusion and essentially undrained conditions, both the asymptotic analysis and numerical simulations indicate the formation of a boundary layer near the shear zone where gradients of pore pressure are large. Outside this layer the pore pressure rapidly approaches the drained solution. A linearized stability analysis derives the dependence of the non‐dimensional critical stiffness () on fluid diffusivity, dilatancy factor (), and shear zone thickness. The homogeneous and membrane diffusion models exhibit nearly identical in the limits of drained and undrained conditions, but differ between the two limits. In this intermediate range, numerical simulations show that the two models produce similar slip behavior for but significant differences in slip velocity and recurrence patterns for .
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- Award ID(s):
- 2120374
- PAR ID:
- 10673624
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 130
- Issue:
- 12
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2025JB032174
