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.

Interpretation of observed fluid potential patterns in a deep sedimentary basin under tectonic compression: Hungarian Great Plain, Pannonian Basin

Abstract: The ≈ 40 000 km2 Hungarian Great Plain portion of the Pannonian Basin consists of a basin fill of 100 m to more than 7000 m thick semi- to unconsolidated marine, deltaic, lacustrine and fluviatile clastic sediments of Neogene age, resting on a strongly tectonized Pre-Neogene basement of horst-and-graben topography of a relief in excess of 5000 m. The basement is built of a great variety of brittle rocks, including flysch, carbonates and metamorphics. The relatively continuous Endrőd Aquitard, with a permeability of less than 1 md (10−15 m2) and a depth varying between 500 and 5000 m, divides the basin's rock framework into upper and lower sequences of highly permeable rock units, whose permeabilities range from a few tens to several thousands of millidarcy. Subsurface fluid potential and flow fields were inferred from 16 192 water level and pore pressure measurements using three methods of representation: pressure–elevation profiles; hydraulic head maps; and hydraulic cross-sections. Pressure–elevation profiles were constructed for eight areas. Typically, they start from the surface with a straight-line segment of a hydrostatic gradient (γst = 9.8067 MPa km−1) and extend to depths of 1400–2500 m. At high surface elevations, the gradient is slightly smaller than hydrostatic, while at low elevations it is slightly greater. At greater depths, both the pressures and their vertical gradients are uniformly superhydrostatic. The transition to the overpressured depths may be gradual, with a gradient of γdyn = 10–15 MPa km−1 over a vertical distance of 400–1000 m, or abrupt, with a pressure jump of up to 10 MPa km−1 over less than 100 m and a gradient of γdyn > 20 MPa km−1. According to the hydraulic head maps for 13 100–500 m thick horizontal slices of the rock framework, the fluid potential in the near-surface domains declines with depth beneath positive topographic features, but it increases beneath depressions. The approximate boundary between these hydraulically contrasting regions is the 100 m elevation contour line in the Duna–Tisza interfluve, and the 100–110 m contours in the Nyirseg uplands. Below depths of ≈ 600 m, islets of superhydrostatic heads develop which grow in number, areal extent and height as the depth increases; hydraulic heads may exceed 3000 m locally. A hydraulic head ‘escarpment’ appears gradually in the elevation range of − 1000 to − 2800 m along an arcuate line which tracks a major regional fault zone striking NE–SW: heads drop stepwise by several hundred metres, at places 2000 m, from its north and west sides to the south and east. The escarpment forms a ‘fluid potential bank’ between a ‘fluid potential highland’ (500–2500 m) to the north and west, and a ‘fluid potential basin’ (100–500 m) to the south and east. A ‘potential island’ rises 1000 m high above this basin further south. According to four vertical hydraulic sections, groundwater flow is controlled by the topography in the upper 200–1700 m of the basin; the driving force is orientated downwards beneath the highlands and upwards beneath the lowlands. However, it is directed uniformly upwards at greater depths. The transition between the two regimes may be gradual or abrupt, as indicated by wide or dense spacing of the hydraulic head contours, respectively. Pressure ‘plumes’ or ‘ridges’ may protrude to shallow depths along faults originating in the basement. The basement horsts appear to be overpressured relative to the intervening grabens. The principal thesis of this paper is that the two main driving forces of fluid flow in the basin are gravitation, due to elevation differences of the topographic relief, and tectonic compression. The flow field is unconfined in the gravitational regime, whereas it is confined in the compressional regime. The nature and geometry of the fluid potential field between the two regimes are controlled by the sedimentary and structural features of the rock units in that domain, characterized by highly permeable and localized sedimentary windows, conductive faults and fracture zones. The transition between the two potential fields can be gradual or abrupt in the vertical, and island-like or ridge-like in plan view. The depth of the boundary zone can vary between 400 and 2000 m. Recharge to the gravitational regime is inferred to occur from infiltrating precipitation water, whereas that to the confined regime is from pore volume reduction due to the basement's tectonic compression.

Thermochemical pressurization of faults during coseismic slip

Abstract: [1] During earthquakes, frictional heating on the fault plane induces a temperature rise and thus a pore pressure rise, which is known as thermal pressurization (TP). Coseismic mineral dehydrations may occur because of this temperature increase and are included within the TP framework. Dehydrations are modeled as a source term for pore pressure because of the total volume change and as a sink term for temperature because they are endothermic. The reaction occurs within the slipping zone when a threshold temperature Ts is reached. Dehydration reaction kinetic is modeled using a first-order reaction rate. Using energy and fluid mass conservation, we derive analytically the equations of evolution of pore pressure, temperature, and reaction extent in the undrained, adiabatic case using a constant reaction rate. We investigate the values of the kinetic rate constant required to produce a significant effect, which are much higher than laboratory data reported in the literature on clay, serpentine, and phyllosilicate dehydration. We show, however, that such high values can be reached if the temperature dependency of the rate constant is taken into account. Next, we include fluid and heat transport and use an Arrhenius law to calculate the rate constant as a function of temperature. The subsequent set of differential equations is then solved numerically. The main effect of dehydration reactions is an increase of pore pressure and a stabilization of the temperature during slip. We explore a wide range of parameters in order to determine in which cases dehydration can be considered as a nonnegligible process. For high-permeability rocks (>10−18 m2) and when the amount of water that can be released is of the order of 10%, dehydration is an important mechanism as it delays the onset of melting, which would normally occur even within the TP framework. If the onset temperature is low compared to the initial temperature T0 (Ts − T0 ≲ 150°C), overpressure can occur. If the reactions are highly endothermic and if their kinetic is fast enough, frictional melting would not occur unless the dehydration reactions are completed within the slipping zone.

Observed performance of a deep excavation in clay

Abstract: A test section was established next to a 40-ft-deep, braced excavation made through soft to medium-stiff, saturated clays in Chicago. Surface and subsurface three-dimensional ground movements, pore water pressures, sheet pile deformations and strut loads were measured. Results of these observations were correlated with construction activities at the test section. Larger than expected ground-surface settlements adjacent to the excavation were observed. The large movements occurred as a result of overexcavation during construction. The largest incremental ground movements occurred when the excavation was approximately half completed. Base stability computations indicated that the factor of safety against basal heave at that point in construction was 1.1. Two distinct shear zones developed in the soil mass; their initiation corresponded to the times when the largest incremental movements occurred. Soil displacements were always directed towards the excavation with magnitudes, at later stages of construction, larger than those measured on the sheet pile. Pore water response was markedly influenced by sheet pile installation and strut preloading; as a result, little net change in pore pressures was observed at the end of construction. Magnitudes of measured strut loads were within the levels expected based on standard design procedures.