Showing papers on "Pore water pressure published in 1997"

The physics of debris flows

Abstract: Recent advances in theory and experimen- tation motivate a thorough reassessment of the physics of debris flows. Analyses of flows of dry, granular solids and solid-fluid mixtures provide a foundation for a com- prehensive debris flow theory, and experiments provide data that reveal the strengths and limitations of theoret- ical models. Both debris flow materials and dry granular materials can sustain shear stresses while remaining stat- ic; both can deform in a slow, tranquil mode character- ized by enduring, frictional grain contacts; and both can flow in a more rapid, agitated mode characterized by brief, inelastic grain collisions. In debris flows, however, pore fluid that is highly viscous and nearly incompress- ible, composed of water with suspended silt and clay, can strongly mediate intergranular friction and collisions. Grain friction, grain collisions, and viscous fluid flow may transfer significant momentum simultaneously. Both the vibrational kinetic energy of solid grains (mea- sured by a quantity termed the granular temperature) and the pressure of the intervening pore fluid facilitate motion of grains past one another, thereby enhancing debris flow mobility. Granular temperature arises from conversion of flow translational energy to grain vibra- tional energy, a process that depends on shear rates, grain properties, boundary conditions, and the ambient fluid viscosity and pressure. Pore fluid pressures that exceed static equilibrium pressures result from local or global debris contraction. Like larger, natural debris flows, experimental debris flows of ;10 m 3 of poorly sorted, water-saturated sediment invariably move as an unsteady surge or series of surges. Measurements at the base of experimental flows show that coarse-grained surge fronts have little or no pore fluid pressure. In contrast, finer-grained, thoroughly saturated debris be- hind surge fronts is nearly liquefied by high pore pres- sure, which persists owing to the great compressibility and moderate permeability of the debris. Realistic mod- els of debris flows therefore require equations that sim- ulate inertial motion of surges in which high-resistance fronts dominated by solid forces impede the motion of low-resistance tails more strongly influenced by fluid forces. Furthermore, because debris flows characteristi- cally originate as nearly rigid sediment masses, trans- form at least partly to liquefied flows, and then trans- form again to nearly rigid deposits, acceptable models must simulate an evolution of material behavior without invoking preternatural changes in material properties. A simple model that satisfies most of these criteria uses depth-averaged equations of motion patterned after those of the Savage-Hutter theory for gravity-driven flow of dry granular masses but generalized to include the effects of viscous pore fluid with varying pressure. These equations can describe a spectrum of debris flow behav- iors intermediate between those of wet rock avalanches and sediment-laden water floods. With appropriate pore pressure distributions the equations yield numerical so- lutions that successfully predict unsteady, nonuniform motion of experimental debris flows.

Injection-induced earthquakes and crustal stress at 9 km depth at the KTB deep drilling site, Germany

Abstract: A fluid injection-induced seismicity experiment was conducted in the German Continental Deep Drilling Program (KTB) main borehole at 91 km depth (in situ temperature of 260°C) to extend knowledge about stress magnitudes and brittle faulting to depths and temperatures approaching the brittle-ductile transition Almost 400 microearthquakes were induced at an average depth of 88 km by injection of KBr/KCl brine into a ∼70 m open hole section near the bottom of the borehole Although most focal plane mechanisms were poorly constrained due to the very small size of the induced earthquakes, several different clusters of microearthquakes with distinct mechanisms were defined Most of the microearthquakes for which focal plane mechanisms were determined were strike-slip events with a NNW trending P axis, essentially parallel to the direction of maximum horizontal compression observed in the borehole The largest induced earthquake, M 12, occurred 18 hours after injection was started This event was a strike-slip/reverse faulting event which also had a NNW trending P axis Utilization of a precise relative location technique indicates that many of microearthquakes occurred relatively far (50–100 m) from the well bore Modeling of the pore pressure disturbance caused by injection suggests that many of the earthquakes were induced by extremely small pore pressure perturbations (<1 MPa) less than 1% greater than the ambient, approximately hydrostatic pore pressure at depth Thus it is apparent that there are critically stressed, permeable fault zones in the crust, even at great depth and temperature A frictional analysis of the focal plane mechanisms of the induced microearthquakes indicates that fault slip is consistent with the stress magnitudes and orientations determined in situ at depths to 77 km in the borehole and relatively high coefficients of friction (∼06–07) reported by Brudy et al [this issue] This and the observation that very small pore pressure perturbations were able to trigger seismicity appear to confirm the hypothesis that “Byerlee's law” (ie, that differential stresses in situ are limited by the frictional strength of well-oriented, preexisting faults) is valid to great crustal depth and that the crust is in brittle failure equilibrium at depths and temperatures approaching the brittle-ductile transition, even in this relatively stable intraplate area

Hydrologic response of a steep, unchanneled valley to natural and applied rainfall

Abstract: Observations from natural rain storms and sprinkling experiments at a steep zero-order catchment in the Oregon Coast Range demonstrate the importance offlow through near-surface bedrock on runoff generation and pore pressure development in shallow colluvial soils. Sprinkling experiments, involving irrigation of the entire 860 m 2 catchment at average intensities of 1.5 and 3.0 mm/h, permitted detailed observation of runoff and the development of subsurface saturation under controlled conditions. A weir installed to collectflow through the colluvium at the base of the catchment recovered runoff equal to one third to one half of the precipitation rate during quasi-steady irrigation. Three key observations demonstrate that a significant proportion of storm runoffflows through near-surface bedrock and illustrate the importance of shallow bedrockflow in pore pressure development in the overlying colluvial soil: (1) greater discharge recovery during both the experiments and natural rainfall at a weir installed approximately 15 m downslope of the weir at the base of the catchment, (2) spatially discontinuous patterns of positive pressure head in the colluvium during steady sprinkling, and (3) local development of upward head gradients associated withflow from weathered rock into the overlying colluvium during high-intensity rainfall. Data from natural storms also show that smaller storms produce no significant runoff or piezometric response and point to a critical intensity-duration rainfall to overcome vadose zone storage. Together these observations highlight the role of interaction betweenflow in colluvium and near- surface bedrock in governing patterns of soil saturation, runoff production, and positive pore pressures.

Basal hydraulic system of a West Antarctic ice stream: constraints from borehole observations

Abstract: Pressure and tracer measurements in boreholes drilled to the bottom of Ice Stream B, West Antarctica, are used to obtain information about the basal water conduit system in which high water pressures are developed. These high pressures presumably make possible the rapid movement of the ice stream. Pressure in the system is indicated by the borehole water level once connection to the conduit system is made. On initial connection, here also called "breakthrough" to the basal water system, the water level drops in a few minutes to an initial depth in the range 96- 117 m below the surface. These water levels are near but mostly somewhat deeper than the flotation level of about 100m depth (water level at which basal water pressure and ice overburden pressure are equal), which is calculated from depth-density profiles and is measured in one borehole. The conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed; the thickness of the gap δ has to be about 2 mm to account for the water-level drop on breakthrough, and about 4 mm to fit the results of a salt-tracer experiment indicating downstream transport at a speed of 7.5 mm s^-1. The above gap-conduit model is, however, ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole, and also by the large hole-to-hole variation in measured basal pressure, which if present in a gap-conduit system with δ = 2 or 4 mm would result in unacceptably large local water fluxes. An alternative model that avoids the e objections, called the "gap opening" model, involves opening a gap as injection proceeds: starting with a thin film, the injection of water under pressure lifts the ice mass a round the borehole, creating a gap 3 or 4 mm wide at the ice/bed interface. Evaluated quantitatively, the gap-opening model accounts for the volume of water that the basal watery system accepts on breakthrough, which obviates the gap-conduit model. In order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits, of which the most promising type is the "canal" conduit proposed theoretically by Walder and Fowler (1994): flat, low conduits incised into the till, ~0.1 m deep and perhaps ~1 m wide, with a flat ice roof The basal water-pressure data suggest that the canals are spaced ~50-300 m apart, much closer than R-tunnel would be. The deepest observed water level, 117 m, i the most likely to reflect the actual water pressure in the canals, corresponding to a basal effective pressure of 1.6 bar. In this interpretation, the shallower water levels are affected by loss of hydraulic head in the narrow passageway(s; that connect along the bed from borehole to canal (s). Once a borehole has frozen up and any passageways connecting with canals have become closed, a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till, given enough time for pressure equilibration. This pressure varies considerably with time, over the equivalent water-level range from 100 to 113 m. Basal pressure sensors 500 m apart report uncorrelated variations, whereas sensors in boreholes 25 m apart report mostly (but not entirely) well-correlated variations, of unknown origin. In part of the record, remarkable anticorrelated variations are interspersed with positively correlated one, and there are rare, abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m. A diurnal pressure fluctuation, intermittently observed, may possibly be caused by the ocean tide in the Ross Sea. The lack of any observed variation in ice-stream motion, when large percentagewise variations in basal effective pressure were occurring according to our data, suggests that the observed pressure variations are sufficiently local, and so randomly variable from place to place, that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed.

Water film flow along fracture surfaces of porous rock

Abstract: This study shows that hydraulic properties of individual fracture surfaces can be meaningfully defined and measured, and that water film flow is a mechanism contributing to fast, unsaturated flow in fractures. The hydraulic conductivity of an unconfined block of Bishop Tuff was measured over a range of near-zero matric potentials, where differences between hydraulic conductivities obtained without and with wax sealing of its lateral sides allowed isolation of film flow effects. Tensiometer and flux measurements showed that surface film flow in this system was significant for matric potentials greater (more positive) than about −250 Pa. In this range the average film thickness was shown to be potential dependent and proportional to the observed enhanced hydraulic conductivity. Measured average surface film thicknesses ranged from 2 to 70 μm, with average film velocities in the range of 2 to 40 m d−1 (about 103 times faster than that of the pore water under unit gradient saturated flow). Our experiments demonstrate that hydraulic properties of macroscopic surfaces of porous media are quantifiable, related to surface roughness, and potentially important in the flow of water in vadose environments. This study further shows that contrary to existing conceptual models, unsaturated flow in fractures cannot generally be predicted solely on the basis of aperture distribution information. The high velocities of these surface films suggest that film flow can be an important mechanism contributing to fast flow in unsaturated fractures and macropores, especially in media characterized by low-permeability matrix and along regions of convergent flow in partially saturated fractures.

The permeable crust: Geohydraulic properties down to 9101 m depth

Abstract: Both boreholes of the German Continental Deep Drilling Program encountered a fluid-conducting crust over a 9101 m vertical profile. Low matrix permeability was observed on the laboratory scale (geometric mean value 7×10−20 m2 with something more than one decade standard deviation) and in situ higher values of the order of 5×10−18 to 3×10−16 m2. With respect to the scatter of measured matrix permeability there was no lithology dependence observed, whereas permeability parallel to the foliation (geometric mean value 3×10−19 m2) is significant higher than those perpendicular to the foliation (2×10−20 m2). With increasing depth the in situ permeability varies within a few orders of magnitude, showing, however, no clear depth dependence. The in situ permeability increases over 3 orders of magnitude while the effective pressure decreases by 50 MPa, whereas in contrast, the permeability of laboratory core specimens changes by 1 order of magnitude. The pilot drill hole “Vorboh-rung” and the main drill hole “Hauptbohrung” communicate through a network of conductive fractures at the bottom hole level of the Vorbohrung as well as between both bottom hole sections. The formation pressure increases with a mean gradient of 11.5 MPa km−1 from surface to 103 ± 3 MPa to 9101 m depth. Overhydrostatic increasing pore pressure and so a strong stress reduction can be excluded as a reason for the low permeability decrease observed at the bottom of the hole. Formation pressure remains hydrostatic with respect to probable increasing salinity.

Effects of pore and differential pressure on compressional wave velocity and quality factor in Berea and Michigan sandstones

Abstract: Compressional‐wave velocity (VP) and quality factor (QP) have been measured in Berea and Michigan sandstones as a function of confining pressure (Pc) to 55 MPa and pore pressure (Pp) to 35 MPa. VP values are lower in the poorly cemented, finer grained, and microcracked Berea sandstone. QP values are affected to a lesser extent by the microstructural differences. A directional dependence of QP is observed in both sandstones and can be related to pore alignment with pressure. VP anisotropy is observed only in Berea sandstone. VP and QP increase with both increasing differential pressure (Pd=Pc-Pp) and increasing Pp. The effect of Pp on QP is greater at higher Pd. The results suggest that the effective stress coefficient, a measure of pore space deformation, for both VP and QP is less than 1 and decreases with increasing Pd.

Methane hydrate stability in pore water: A simple theoretical approach for geophysical applications

Abstract: Geophysicists have recently expressed an interest in understanding how pore water composition affects CH4 hydrate stability conditions in the marine environment. It has previously been shown in the chemical engineering literature that CH4 hydrate stability conditions in electrolyte solutions are related to the activity of water (aw). Here we present additional experimental data in support of this relationship and then use the relationship to address issues relevant to geophysicists. Pressure and temperature conditions of CH4 hydrate dissociation were determined for 10 solutions containing variable concentrations of Cl−, SO42− Br−, Na+, K+, Mg2+, NH4+, and Cu2+. The reciprocal temperature offset of CH4 hydrate dissociation between the CH4-pure water system and each of these solutions (and for other electrolyte solutions in literature) is directly related to the logarithm of the activity of water (lnaw). Stability conditions for CH4 hydrate in any pore water system therefore can be predicted simply and accurately by calculating lnaw. The effect of salinity variation and chemical diagenesis on CH4 hydrate stability conditions in the marine environment can be evaluated by determining how these processes affect lnaw of pore water.

X-ray diffraction studies of freezing and melting of water confined in a mesoporous adsorbent (MCM-41)

Abstract: In order to study the freezing/melting behavior of pore water, we performed x-ray diffraction measurements of water confined inside the cylindrical pores of two kinds of siliceous MCM-41 with different pore size and one kind of aluminosilicate MCM-41 as a function of temperature. The results show that its freezing/melting behavior is not affected by the incorporation of Al into the pore wall and the hysteresis effect between freezing and melting is very small or negligible. On cooling the water in the middle of the pores with a pore diameter of 4.2 nm, that is, the free water freezes abruptly around 232 K to give rise to cubic ice while the water confined in the pores with a pore diameter of 2.4 nm freezes very gradually at lower temperatures. The diffraction profile after the freezing of the free water suggests that the interfacial water confined between the surface of the pore wall and the frozen phase of the free water consists of randomly displaced water molecules.

Equilibrium partitioning of heavy metals in dutch field soils. II. Prediction of metal accumulation in earthworms

Abstract: To evaluate the adequacy of the equilibrium partitioning concept in predicting metal bioaccumulation, a soil invertebrate species was exposed in 20 Dutch field soils with moderate metal contamination. Earthworms (Eisenia andrei) were kept in the soils for 3 weeks under laboratory conditions. Bioconcentration factors (BCFs) for six metals (Zn, Cu, Pb, Cd, Cr, Ni) and for As were calculated as the ratio of body- and solid-phase metal concentrations. Multivariate statistical analyses suggested that the BCFs for As, Cd, Cu, and Zn are governed by the same soil characteristics that determine equilibrium partition coefficients between the soil solid phase and the pore water. This suggests that uptake of metals is either direct from the pore water or indirect through an uptake route closely related to pore water. Regression equations were derived for predicting BCF values as a function of easily determinable soil characteristics. By means of internal validation it was shown that the equations obtained can be used for predictive purposes within the range of soil properties encountered in the dataset. Due to a lack of data, external validation was possible only in a qualitative sense.

Generation and maintenance of pore pressure excess in a dehydrating system 2. Theoretical analysis

Abstract: Fluid is released by dehydration reactions during prograde metamorphism. If the dilation of the pore space is insufficient to provide storage for all the released fluid, then pore pressure excess is generated. Whether the excess can be maintained over long duration hinges on the hydraulic transport properties of the rock. Motivated by recent experimental and microstructural observations, we developed a theoretical model which incorporates dehydration and porosity production rates as source terms in the hydraulic diffusion equation. The permeability was assumed to be sensitively dependent on the porosity. The finite difference technique was used to analyze the generation and maintenance of pore pressure excess for several types of boundary conditions of importance in laboratory and crustal scales. Analytic estimates of the pore pressure anomaly were also obtained. The model is in reasonable agreement with experimental observations on dehydration-induced weakening and transient buildup of pore pressure in a nominally drained sample. It provides hydrogeological constraints on the development of pore pressure excess in metamorphic and tectonic settings. The maintenance of a nearly lithostatic pore pressure requires the permeability to be below a critical value which increases with increasing dehydration rate and thickness of the dehydrating layer, and with decreasing porosity production rate. If these constraints are not met, the pore pressure excess can only occur as a transient pulse, the amplitude of which may approach lithostatic for sufficiently large dehydration rate and layer thickness, or sufficiently low permeability.

A coupled heat-moisture transfer theory for deformable unsaturated soil and its algorithmic implementation

Abstract: This paper presents a formulation for coupled heat and moisture transfer in a deformable partially saturated soil. The research is based on a mechanistic phase interaction model coupled to a state surface approach. The method takes into account the coupling effect of temperature gradient and deformations on flows in porous media. Pore water pressure, pore air pressure, temperature and displacement are treated as the primary unknowns. A numerical solution of the formulation is then achieved via the finite element method. An example of the use of the new model is then presented. © 1997 by John Wiley & Sons, Ltd.

Settling, dissolution and burial of biogenic silica in the sediments off Somalia (northwestern Indian Ocean)

Abstract: Particle fluxes of biogenic silica through the water column, silica burial fluxes into the sediments, and the flux of dissolved silica across the sediment-water interface estimated from pore water profiles are used to assess the behaviour of biogenic silica at two stations 80 and 270 km offshore along a transect off the Somali coast in the northwestern Indian Ocean. Particulate biogenic silica fluxes varied from 0.3 mmol m−2 day−1 in the non-upwelling season to 6 mmol m−2 day−1 during upwelling on the Somali slope. Fluxes were lower in the Somali Basin, from 0.2 to 2.3 mmol m−2 day−1. Evaluation of the dissolution curves derived by wet chemical leaching in sediment trap and sediment samples shows that the Km values, the apparent reactivity rates in alkaline medium, are higher for the shallow sediment traps than for deep trap and boxcore sediments. Modelling of pore water profiles shows that in the sediment most dissolution occurs in the top halfcentimetre, and pore water effluxes are in close agreement with those from in situ benthic incubations. Our results show that less than 10% of the biogenic silica arriving on the Somali Margin is buried in the sediments, giving a burial efficiency lower than the approximately 20% reported from the open Arabian Sea.

Generation and maintenance of pore pressure excess in a dehydrating system 1. Experimental and microstructural observations

Abstract: The generation and maintenance of excess pore pressures in dehydrating gypsum aggregates were investigated using experiments and microstructural analyses. A triaxial deformation apparatus, was equipped with a pore fluid system connected directly to the dehydrating sample. This system was operated in constant fluid volume mode to monitor pore pressure increase under undrained conditions, and in constant pore pressure mode to monitor fluid expulsion under drained conditions. X ray diffraction and backscatter scanning electron microscopy were used to characterize the spatial relationship among gypsum, the product phase bassanite, and the pores. In addition, we measured the permeability and pore compressibility of the starting material and explored the influence of effective and pore pressures, temperature, and axial load on fluid expulsion. Three stages of fluid expulsion and microstructural evolution during dehydration of an initially low-porosity, low-permeability gypsum aggregate are defined: (1) Initially, fluid released by the reaction is trapped in isolated or discontinuous pore networks and high pore pressures are possible. (2) An interconnected pore network eventually develops and fluid readily escapes. (3) Fluid expulsion slows down drastically as the reaction nears completion. As a result of coupling between dehydration and porosity production, both the cumulative volume of fluid expelled and the expulsion rate increase with increasing temperature, effective pressure, and axial load and with decreasing pore pressure. Our hydrological and microstructural data, combined with previous mechanical data, provide a better understanding of the relationships among changes in fluid volume, porosity, and pore pressure excess, and the deformation behavior of a dehydrating system where drainage evolves with time.