Showing papers on "Pore water pressure published in 2002"

Pore pressure and poroelasticity effects in Coulomb stress analysis of earthquake interactions

Abstract: [1] Pore pressure changes are rigorously included in Coulomb stress calculations for fault interaction studies. These are considered changes under undrained conditions for analyzing very short term postseismic response. The assumption that pore pressure is proportional to fault-normal stress leads to the widely used concept of an effective friction coefficient. We provide an exact expression for undrained fault zone pore pressure changes to evaluate the validity of that concept. A narrow fault zone is considered whose poroelastic parameters are different from those in the surrounding medium, which is assumed to be elastically isotropic. We use conditions for mechanical equilibrium of stress and geometric compatibility of strain to express the effective normal stress change within the fault as a weighted linear combination of mean stress and fault-normal stress changes in the surroundings. Pore pressure changes are determined by fault-normal stress changes when the shear modulus within the fault zone is significantly smaller than in the surroundings but by mean stress changes when the elastic mismatch is small. We also consider an anisotropic fault zone, introducing a Skempton tensor for pore pressure changes. If the anisotropy is extreme, such that fluid pressurization under constant stress would cause expansion only in the fault-normal direction, then the effective friction coefficient concept applies exactly. We finally consider moderately longer timescales than those for undrained response. A sufficiently permeable fault may come to local pressure equilibrium with its surroundings even while that surrounding region may still be undrained, leading to pore pressure change determined by mean stress changes in those surroundings.

Impact of Soil Type and Compaction Conditions on Soil Water Characteristic

Abstract: Tests were conducted to determine the variation of water content and pore water suction for compacted clayey soils. The soils had varying amounts of clay fraction with plasticities ranging from low to high plasticity. The unsaturated soil behavior was investigated for six conditions, covering a range of compactive efforts and water contents. The experimental data were fit to four commonly used models for the water content-pore water suction relationship. Each model provided a satisfactory fit to the experimental data. However, the individual parameters obtained from the curve fits varied significantly between models. The soil water characteristic curves (SWCCs) were more sensitive to changes in compaction effort than changes in compaction water content. At similar water contents, the pore water suction increased with increasing compaction effort for each compaction condition and soil type. For all compaction conditions, the lowest plasticity soils retained the smallest water content and the highest plasticity soils retained the highest water content at a specified suction. In addition, SWCCs for soils compacted in the laboratory and the field were similar.

Detecting high overpressure

Abstract: Normal pressure is pore fluid pressure that equals the hydrostatic pressure of a column of formation water extending to the surface. Overpressure is pore fluid pressure greater than normal pressure. However, no standard definition exists for what constitutes high overpressure. What can be said is that high overpressure often means trouble. For an explorationist, it could mean blown reservoir seals; for a driller, it could mean excessive time spent fighting formation fluid influxes and/or drilling fluid losses. A practical upper limit for pore pressure is the overburden stress. Pore pressures in this range are on the verge of opening fractures that can vent fluid and bleed off pressure like a pressure relief valve. Therefore, criteria for defining high overpressure are sometimes expressed in terms of a percentage of the overburden stress, say, pore pressure greater than 90% of the overburden stress. In this article, high overpressure will be defined simply as pore pressure that approaches the overburden stress. All but one potential cause of overpressure can produce high pressure. Fortunately, the mechanism that cannot generate high pressure is the most common cause of overpressure. Therefore, detecting high overpressure basically boils down to determining where extraordinary overpressure mechanisms may be encountered. Overpressure detection is based on the premise that pore pressure affects compaction-dependent geophysical properties such as density, resistivity, and sonic velocity. Shales are the preferred lithology for pore pressure interpretation because they are more responsive to overpressure than most rock types. Consequently, overpressure detection centers around shale deformation behavior. For stress ranges of practical interest, shale compaction is controlled by the difference between total applied stress and pore fluid pressure. This difference, termed the effective stress , represents the portion of the total stress carried by the rock grains. Figure 1 illustrates the effective stress concept with laboratory data for …

Porosity loss within the underthrust sediments of the Nankai accretionary complex: Implications for overpressures

Abstract: Subduction complexes provide an opportunity to examine the interactions of deformation and fluid flow in an active setting. Ocean Drilling Program Leg 190 investigated the relationship between deformation, physical properties, and fluid flow in the toe of the Nankai Trough accretionary complex. With three sites (two from Leg 190, one from a previous leg) penetrating the decollement zone at various stages of development along the same transect, it is now possible to examine the change in porosity during rapid loading by trench turbidites and subsequent underthrusting. Results indicate inhibited dewatering and probable overpressure development seaward of the frontal thrust. Comparison of a reference site porosity versus depth curve to data from a site located within the protothrust zone indicates an overpressure ratio, λ * , of ∼0.42, where λ * = [(pore pressure - hydrostatic pressure)/(lithostatic pressure - hydrostatic pressure)]. These overpressures suggest that the hemipelagic sediments have insufficient permeability for fluid escape to keep pace with the rapid loading by turbidite deposition within the trench. At a site 1.75 km farther arcward, an excess pore pressure ratio of λ * = ∼0.47 was estimated, reflecting the additional loading due to recent thickening by the frontal thrust.

Aspects of the behaviour of compacted clayey soils on drying and wetting paths

Abstract: A relatively large number of drying and wetting tests have been performed on clayey soils compacted at the standard or modified Proctor optimum water content and maximum density and compared with tests on normally consolidated or overconsolidated soils. The results show that drying and wetting paths on compacted soils are fairly linear and reversible in the void ratio or water content versus negative pore-water pressure planes. On the wet side of the optimum, the wetting paths are independent of the compaction water content and can be approached by compaction tests with measurement of the negative pore-water pressure. Correlations have been established between the liquid limit of the soils and such properties as the optimum water content and negative pore-water pressure, the maximum dry density, and the swelling or drying index. Although based on a limited number of tests, these correlations provide a fairly good basis to model the drying–wetting paths when all the necessary data are not available.Key wor...

A numerical study of the effect of groundwater movement on long-term tunnel behaviour

Abstract: Soft ground tunnelling beneath the water table induces pore water pressure changes and alters the hydraulic regime in the ground. There are various factors that control the tunnel/soil interaction behaviour during the equilibration period after tunnel construction through to the long-term steady state. Through the presentation of results from coupled non-linear finite element analyses, this paper considers the differences in predicted ground and lining responses in the long term for different post-tunnelling flow conditions. A 4 m diameter circular tunnel constructed 20 m below ground level is adopted for the geometry, with the soil conditions based on those found in London, UK, and Seoul, South Korea. The lining permeability is varied from fully permeable to impermeable, and an approach to modelling finite lining permeability is presented and assessed. For the permeable lining analyses two flow regimes are modelled, one in which the water table is maintained at its original elevation and one in which the...

Piezometric response in shallow bedrock at CB1: Implications for runoff generation and landsliding

Abstract: [1] Experimental observations comparing two steep unchanneled valleys in the Oregon Coast Range, one intensively instrumented (CB1) and the other monitored for runoff but which produced a debris flow (CB2), shed light on the mechanisms of shallow flow in bedrock, its interaction with the vadose zone, and its role in generating landslides. Previous work at CB1 led to the proposal that during storms pulses of rainfall transmit pressure waves through the vadose zone and down to the saturated zone to create rapid pore pressure response and runoff [Torres et al., 1998]. Here, we document the associated rapid pore pressure response in the shallow fractured bedrock that underlies these colluvium-mantled sites and examine its influence on the generation of storm flow, seasonal variations in base flow, and slope stability in the overlying colluvial soil. Our observations document rapid piezometric response in the shallow bedrock and a substantial contribution of shallow fracture flow to both storm flow and seasonal variations in base flow. Saturated hydraulic conductivity in the colluvial soil decreases with depth below the ground surface, but the conductivity of the near-surface bedrock displays no depth dependence and varies over five orders of magnitude. Analysis of runoff intensity and duration in a series of storms that did and did not trigger debris flows in the surrounding area shows that the landslide inducing storms had the greatest intensity over durations similar to those predicted by a simple model of piezometric response. During a monitored storm in February 1992, the channel head at the base of the neighboring CB2 site failed as a debris flow. Automated piezometric measurements document that the CB2 debris flow initiated several hours after peak discharge, coincident with localized development of upward spikes of pressure head from near-surface bedrock into the overlying colluvial soil in CB1. Artesian flow observed exfiltrating from bedrock fractures on the failure surfaces at CB2 further implicates bedrock fracture flow in both runoff generation by subsurface storm flow and suggests a connection to landslide initiation. Our observations show that the timing of shallow landslide initiation can be delayed relative to both peak rainfall and peak runoff and support the argument that the influence of fracture flow on shallow landsliding helps explain the wide variability in the occurrence of slope instability in topographically analogous locations.

Aftershocks and Pore Fluid Diffusion Following the 1992 Landers Earthquake

Abstract: [1] We model the evolution of regional stress following the 1992 Landers earthquake in order to test the importance of pore fluid flow in producing aftershocks. Rising fluid pressure due to pore fluid flow and the resulting Coulomb stress changes were found to be strongly correlated with the time and location of aftershock events. Regional aftershock frequencies computed by integrating pore pressure decay rates also agreed quite well with aftershock data. Calculations show that regions of rising postseismic poroelastic Coulomb stress overlap considerably with regions of positive coseismic Coulomb stress. Thus pore fluid flow, which affects pore pressure within faults and causes regional poroelastic stress evolution following earthquakes, gradually evolves the initial coseismic stress changes. Together these changes provide a reasonable physical mechanism for aftershock triggering which agrees with data for the 1992 Landers earthquake.

Analytical expressions for transient specific yield and shallow water table drainage

Abstract: [1] New closed-form expressions are introduced to capture the dependence of specific yield on time and depth to water table. The expressions allow the user to convert observations of water table fluctuations to volumes of water released from storage in a shallow water table aquifer. Whereas a linear relationship between water table fluctuations and released volumes holds for a deep water table aquifer, this relationship is nonlinear for shallow water table aquifers. The dependence of specific yield on time stems from the slow drainage of soil water from pores above the water table. The new expressions allow determination of transient specific yield and time to drain the soil water profile for a given water table fluctuation. If the time step in a numerical groundwater model is longer than the time for limiting specific yield, then a constant (time independent) specific yield can be justifiably adopted. The new expressions are easy to use and require knowledge of soil hydraulic properties which are readily available from soil water retention data and surveys.

Hydrologic controls on the morphology and mechanics of accretionary wedges

Abstract: At many subduction zones, accretionary complexes form as sediments are offscraped from the subducting plate. Existing mechanical models that treat accretionary complexes as critically tapered wedges of sediment demonstrate that pore pressure controls their taper angle by modifying rock strength. We combine a model of groundwater flow with critical-taper theory to show that permeability and plate-convergence rate are important controls on accretionary wedge geometry through their influence on pore pressure. Low permeability and rapid convergence sustain nearly undrained conditions and shallowly tapered geometry, whereas high permeability and slow convergence result in steep geometry. Our results are generally in good agreement with data from active accretionary complexes, but also illustrate the importance of other factors, such as incoming sediment thickness and stratigraphy. One key implication is that strain rate and hydrologic properties may strongly influence the strength of the crust in a variety of geologic settings.

Soil Deformation and Excess Pore Pressure Field around a Closed-Ended Pile

Abstract: Numerous field and laboratory research projects have examined loading conditions, total soil stresses, and excess pore pressures during and following pile installation. Recently, a large number of seismic retrofitting projects for bridges in California have revealed the need for well-documented field tests evaluating the effect of pile installation on the static and dynamic properties of soft clays. This paper documents the geotechnical field investigation and field monitoring program, which included measurements of excess pore pressure and horizontal deformations at three radial distances from a full-scale close-ended pile driven into a deep deposit of Young Bay Mud. Significant excess pore pressures developed as a result of pile driving and they are a function of distance to the pile wall. Lateral deformations decrease with increasing distance from the pile and can be approximately described by the cylindrical cavity expansion method. iclinometer measurements show lateral deformation towards the pile wall as the excess pore pressure dissipates. These measurements are essential in the calibration and validation of analytical techniques to predict changes in properties of the foundation soil.

Mechanism for Postliquefaction Water Film Generation in Layered Sand

Abstract: Soil investigations at two sites demonstrate that layered structure or stratification in sand deposits is prevalent not only in reclaimed ground but also in natural alluvial ground. One-dimensional liquefaction tests in a lucite tube are then carried out for models of several types of layered sand, indicating that water films will develop under most circumstances beneath or within less-permeable sublayers. A basic mechanism for the water film generation is discussed based on the measurements of soil settlement and excess pore pressure. The development of the water film and the associated soil settlement are numerically simulated by a simple sedimentation analysis and a rational explanation of the test results is found. Thus a significant involvement of water films in liquefied sand deposits and their basic mechanism are clarified. It is highly probable that water films are involved as a part of a sliding surface and play a significant role in a seismically induced flow failure in loose and layered sand deposits during liquefaction.

Temperature Dependence of Water Retention Curves for Wettable and Water-Repellent Soils

Abstract: The capillary pressure (ψ) in unsaturated porous media is known to be a function of temperature (T). Temperature affects the surface tension (α) of the pore water, but possibly also the angle of contact (y). Because information on the temperature dependence of (γ) in porous media is rare, we conducted experiments with three wettable soils and their hydrophobic counterparts. The objectives were (i) to determine the temperature dependence of the water retention curve (WRC) for wettable and water-repellent soils, (ii) to assess temperature effects on the apparent contact angle γ A derived from those WRCs, and (iii) to evaluate two models (Philip-de Vries and Grant-Salehzadeh) that describe temperature effects on (. Columns packed with natural or hydrophobized soil materials were first water saturated, then drained at 5, 20, and 38°C, and rewetted again to saturation. Capillary pressure and water content, 0, at five depths in the columns were measured continuously. The observations were used to determine the change in γ A with T, as well as a parameter β 0 that describes the change in ( with T. It was found that the Philip-de Vries model did not adequately describe the observed relation between ( and T. A mean value for β 0 of -457 K was measured, whereas the Philip-de Vries model predicts a value of -766 K. Our results seem to confirm the Grant-Salezahdeh model that predicts a temperature effect on γ A . For the sand and the silt we studied, we found a decrease in γ A between 1.0 to 8.5°, when the temperature was increased from 5 to 38°C. Both β 0 and γ A were only weak functions of 0. Furthermore, it seemed that for the humic soil under study, surfactants, i.e., the dissolution of soil organic matter, may compound the contact angle effect of the soil solids.

Effects of pipe flow and bedrock groundwater on runoff generation in a steep headwater catchment in Ashiu, central Japan

Abstract: [1] It has been suggested that pipe flow and bedrock groundwater play important roles in storm runoff generation. We examined the effects of pipe flow on storm runoff generation and the roles of bedrock groundwater in rainfall-runoff phenomena in a steep headwater catchment in central Japan. Measurements of pore water pressures, pipe flow, and streamflow showed that when the total rainfall amount was 70 mm, the dominant runoff process shifted to pipe flow. Temperature measurements indicated four key points: (1) During base flow conditions (streamflow 0.2 mm h−1) the source of pipe flow was the same as the streamflow; (3) the transient groundwater at the upper hillslope was commonly dominated by the preevent soil water; and (4) only after the large storms with wet antecedent conditions, was water emerging from the bedrock and mixing with preevent soil water in the transient saturated area at the upper hillslope. Both the hydrometric and temperature measurements indicated that once pipe flow occurs, the contributing area of streamflow extended to upper hillslope, and the transient groundwater at the upper hillslope was delivered to the stream via preferential flow paths, shortcutting the normal mixing process through the soil matrix.

Undrained poroelastic response of sandstones to deviatoric stress change

Abstract: Deformation of porous crustal rock, through diagenesis, tectonic loading or other processes, can change pore volume and affect fluid pressure. The largest stress- induced pore pressure changes occur when fluid is trapped in pores in an 'undrained' condition. We have measured the undrained poroelastic response of two sandstones to changes in mean and deviatoric stress. Pore pressure was found to respond to mean stress σ m in the usual manner: ∆ p = B∆ σ m (B ranging from 0.4 to 0.7), nearly independent of the ambient deviatoric stress state. However, variations in deviatoric stress ( σ d = (σ 1 - σ 3)/2) at constant mean stress were also found to induce a reversible (elastic) pore pressure response to stress levels up to and exceeding 80 percent failure stress ( i.e., ∆ p = η∆ σ d |σ m =const.). The coefficient η became more negative with increasing deviatoric stress level in sandstone and Ottawa sand samples. That is, η represents a dilatant response where increased deviatoric stress causes a decrease in pore pressure. The poroelastic response to deviatoric stress is explained in terms of anisotropic matrix stiffening due to closure of crack-like pore space or flattening of grain contacts at high ambient stress levels and can be important in calculations of earthquake stress transfer.

Laboratory investigation of variable-density flow and solute transport in unsaturated-saturated porous media

Abstract: In many groundwater systems, fluid density and viscosity may vary in space and time as a function of changes in concentration and temperature of the fluid. When dense groundwater plumes interact with less dense ambient groundwater, these density variations can significantly affect flow and transport processes. Under certain conditions, gravitational instabilities in the form of lobe-shaped fingers can occur. This process is significant because it can lead to more rapid and spatially extensive solute transport. This paper presents new experiments carried out in a sand filled glass flow container under both fully saturated and variably-saturated conditions and focuses upon the processes that occur at the capillary fringe and below the water table, as affected by a dense contaminant plumes migration through the unsaturated zone. Source fluids stained with Rhodamine-WT were introduced at the upper boundary of the tank at a range of low and high densities. In addition to the fluid density gradients and porous medium permeability that determine the onset conditions for instabilities in fully saturated experiments, volumetric water content appears critical in the variably-saturated laboratory runs. Plume behaviour at the water table appears dependent upon the density of the fluid that accumulates there. For neutral and low density fluids, plumes accumulate at the water table and then spread laterally above it and the water table forms a barrier to further vertical flow as pore water velocities reduce with increasing water content. For medium and high density fluids, vertical movement continues as instabilities form at the capillary fringe and fingers begin to grow at the water table boundary and move downwards into the saturated zone. In these cases, lateral spreading of the plume is small. Despite these more qualitative observations, the exact nature of the relevant stability criteria for the onset and growth of instabilities in variably-saturated porous media presently remain unclear. All experimental results suggest, however, that the unsaturated zone and position of the water table must be considered in contaminant studies in order to predict the migration pathways, rates and ultimate fate of dense contaminant plumes. It is possible that the results of experiments presented in this paper could form a useful basis for the testing of variable-density (and variably-saturated) groundwater flow and solute transport numerical codes because they offer controlled physical laboratory analogs for comparison. They also provide a strong basis for the development of more rigorous mathematical formulations that are likely to be either developed or tested using numerical flow and solute transport simulators.

Two-Dimensional Small-Scale Variability of Pore Water Phosphate in Freshwater Lakes: Results from a Novel Dialysis Sampler

Abstract: Vertical concentration profiles of soluble reactive phosphorus (SRP) in the upper sedimentary zone of freshwater lakes are an important means for studying internal phosphorus (P) loading and to gain insight into early diagenetic processes. The interpretation of such pore water profiles generally neglects the occurrence of horizontal variability at a specific sampling site. To further examine this variability, we have designed a novel two-dimensional sampler (2D peeper) consisting of 2280 chambers at a spatial resolution of 9 mm providing a sampling area of 43 x 44 cm. This new device was deployed in three eutrophic lakes in north-eastern Germany. The resulting 2D images of the SRP concentrations, diffusive fluxes, and turnover rates revealed systematic vertical and horizontal structures with local niches of increased phosphorus release. Thus, the extrapolation of P flux calculations based on one-dimensional pore water profiles may lead to a considerable error. The observed small-scale horizontal heterogeneity, probably mainly caused by organisms, was larger in the biologically more active Lake Mullrose and Susser See than in the deeper Arendsee where meio- and macrozoobenthos were missing. In all cases, the variability was highest at the sediment-water interface and diminished with sediment depth.

Evaluation of preconsolidation pressure and the overconsolidation ratio from piezocone tests of clay deposits in Quebec

Abstract: The preconsolidation pressure and overconsolidation ratio profiles are the most important factors related to the mechanical behaviour of clay deposits. They are interpreted on the basis of a limite...