Showing papers on "Pore water pressure published in 1994"

Laboratory measurement of compaction-induced permeability change in porous rocks: Implications for the generation and maintenance of pore pressure excess in the crust

Abstract: Permeability exerts significant control over the development of pore pressure excess in the crust, and it is a physical quantity sensitively dependent on the pore structure and stress state. In many applications, the relation between permeability and effective mean stress is assumed to be exponential and that between permeability and porosity is assumed to be a power law, so that the pressure sensitivity of permeability is characterized by the coefficient γ and the porosity sensitivity by the exponent α. In this study, we investigate experimentally the dependence of permeability on pressure and porosity in five sandstones with porosities ranging from 14% to 35% and we review published experimental data on intact rocks, unconsolidated materials and rock fractures. The laboratory data show that the pressure and porosity sensitivities differ significantly for different compaction mechanisms, but for a given compaction mechanism, the data can often be approximated by the empirical relations. The permeabilities of tight rocks and rock joints show relatively high pressure sensitivity and low porosity sensitivity. A wide range of values for α and γ have been observed in relation to the mechanical compaction of porous rocks, sand and fault gouge, whereas the porosity sensitivity for chemical compaction processes is often observed to be given by α≈3. We show that since the ratio γ/α corresponds to the pore compressibility, the different dependences of permeability on porosity and pressure are related to the pore structure and its compressibility. Guided by the laboratory data, we conduct numerical simulations on the development of pore pressure in crustal tectonic settings according to the models ofWalder andNur (1984) andRice (1992). Laboratory data suggest that the pressure sensitivity of fault gouge is relatively low, and to maintain pore pressure at close to the lithostatic value in the Rice model, a relatively high influx of fluid from below the seismogenic layer is necessary. The fluid may be injected as vertically propagating pressure pulses into the seismogenic system, andRice's (1992) critical condition for the existence of solitary wave is shown to be equivalent to α>1, which is satisfied by most geologic materials in the laboratory. Laboratory data suggest that the porosity sensitivity is relatively high when the permeability is reduced by a coupled mechanical and chemical compaction process. This implies that in a crustal layer, pore pressure may be generated more efficiently than cases studied byWalder andNur (1984) who assumed a relatively low porosity sensitivity of α=2.

Soil physical properties related to soil structure

Abstract: The aim of this paper is to clarify the effect of soil aggregation on soil physical and chemical properties of structured soils both on a bulk soil scale, for single aggregates, as well as for homogenized material. Aggregate formation and aggregate strength depend on swelling and shrinkage processes and on biological activity and kinds of organic exudates as well as on the intensity, number and time of swelling and drying events. Such aggregates are, most of all, more dense than the aggregated bulk soil. The intra-aggregate pore distribution consists not only of finer pores but these are also more tortuous. Thus, water fluxes in aggregated soils are mostly multidimensional and the corresponding water fluxes in the intra-aggregate pore system are much smaller. Furthermore, ion transport by mass flow as well as by diffusion are delayed, whereby the length of the flow path in such tortuous finer pores further retards chemical exchange processes. The chemical composition of the percolating soil solution differs even more from that of the corresponding homogenized material the stronger and denser the aggregates are. The rearrangement of particles by aggregate formation also induces an increased apparent thermal diffusivity as compared with the homogenized material. The aggregate formation also affects the aeration and the gaseous composition of the intra-aggregate pore space. Depending on the kind and intensity of aggregation, the intra-aggregate pores can be completely anoxic, while the inter-aggregate pores are already completely aerated. The higher the amount of dissolved organic carbon in the percolating soil solution, the more pronounced is the difference between the gaseous composition in the inter- and in the intra-aggregate pore system. From the mechanical point of view, the strength single aggregates, determined as the angle of internal friction and cohesion, depends on the number of contact points or the forces, which can be transmitted at each single contact point. The more structured soils are, the higher the proportion of the effective stress on the total stress is, but even in single aggregates positive pore water pressure values can be revealed. Dynamic forces e.g. due to wheeling and/or slip processes can affect the pore system as well as the composition of the soil by: (1) a rearrangement of single aggregates in the existing inter-aggregate pore system resulting in an increased bulk density and a less aerated and less rootable soil volume, (2) a complete homogenization, i.e. aggregate deterioration due to shearing. Thus, the smaller texture dependent soil strength coincides with a more intensive soil compaction due to loading. (3) Aggregate deterioration due to shearing results in a complete homogenization, if excess soil water is available owing to kneading as soon as the octahedral shear stresses and the mean normal stresses exceed the stress state defined by the Mohr-Coulomb failure line. Consequently, normal shrinkage processes start again. Thus, the rearrangement of particles and the formation of well defined single aggregates even at the same bulk density of the bulk soil both affect, to a great extent, various ecological parameters. Environmental aspects can also be correlated, or at least explained with the processes in soils, as a major compartment of terrestial ecosystems, if the physical and chemical properties of the structure elements and their composition in the bulk soil are understood.

Wave-induced soil response in an unsaturated anisotropic seabed of finite thickness

Abstract: An analytical solution for the wave-induced soil response is developed for a seabed of finite thickness subject to a three-dimensional (3-D) wave system produced by two intersecting waves of equal properties. These 3-D exact solutions for the pore pressure and effective stresses, proposed for a non-cohesive soil matrix of finite depth in a homogeneously unsaturated and anisotropic condition, are readily reducible to the limiting two-dimensional cases of progressive and standing waves, for which no explicit solutions are available for finite thickness. The effects of soil isotropy, degree of saturation, seabed thickness and grain size on the wave-induced pore pressure are discussed in detail. The explicit solutions presented in this study for the wave-induced pore pressure and effective stresses should benefit the laboratory experiments and field monitoring programs carried out in soil of finite depth.

Analysis of Site Liquefaction Using Earthquake Records

Abstract: The surface and downhole accelerations, and pore-water pressures recorded during the 1987 earthquakes at a site in the Imperial Wildlife Management Area (Imperial County, Calif.) are used to obtain direct estimates of the average seismic shear stress-strain and effective stress-path histories. These histories provide valuable insight into the site seismic behavior during liquefaction and associated loss of soil stiffness. As the pore pressure increases due to seismic excitation, site stiffness is found to gradually decrease. During the high-pore-pressure phase, site behavior is characterized by cycles of large shear strain and very small shear stress. At these large strains, evidence of hardening response, possibly due to a dilative-type soil behavior, is observed. The results of this study demonstrate that acceleration and pore pressure histories recorded by downhole arrays represent a valuable direct source of information on site response during seismic excitation.

Flow and solute transport through the soil matrix and macropores of a hillslope segment

Abstract: Subsurface flow from various portions of a soil profile on a steep, forested hillslope was evaluated by two sets of step-change miscible displacement tests at different application rates and antecedent hydrologic conditions. Solutions of NaCl (1000 mg L−1 Cl−) were applied at steady state rates (equivalent to 20 and 30 mm h−1 of standing water over the entire plot area) using a line irrigation source located 1.5 m upslope (lateral distance) from an excavated soil pit. Subsurface flow and tracer breakthrough from five portions (the organic-rich soil layer including macropores, the mineral soil matrix, and three groups of macropores in the mineral soil layer) of the soil profile were individually measured and analyzed using a convective-dispersive model. Matrix flow dominated discharge from the soil pit during tracer tests (70–93% of total discharge). However, during wet periods with upslope drainage, macropores (including organic-rich soil) contributed proportionally more flow than during periods when upslope drainage was minimal. Outflow from macropores during the test with wet antecedent conditions had lower Cl− concentrations than drainage from the soil matrix, suggesting dilution in macropores from upslope drainage. Effective pore volumes calculated for the flow-averaged breakthrough data from the entire profile were much less (<40%) than the estimates (measured by tensiometers) of total volume of pore water, suggesting that preferential flow significantly contributed to subsurface transport of tracer. The pore volume for the entire profile increased only slightly with increasing application rate; however, the relative proportions of pore volumes calculated for individual portions varied proportionally to antecedent hydrologic conditions. These changes are attributed to the expansion of individual macropores with surrounding soil and the lateral extension of macropore networks during wetter conditions.

Porosity/depth trends in reservoir sandstones; assessing the quantitative effects of varying pore-pressure, temperature history and mineralogy, Norwegian Shelf data

Abstract: Empirical porosity/depth trends from the Norwegian Continental Shelf are discussed in the light of models on compactional processes during burial. Linear best-fit lines are useful as a first approximation, illustrating the general decline in porosity with depth. Porosity increases non-linearly with pore-pressure, and notably higher than normal porosity is found in very high overpressure reservoirs. Hydrostatic pressure sandstones from the Haltenbanken area are on average more porous than equivalents from the northern Viking Graben. This may be related to the burial history, as generally more subsidence occurred in Late Pliocene and Pleistocene in the Haltenbanken basins. The porosity/depth data show considerable scatter, and this is to a large degree related to the initial mineralogical composition. Correlated with mineralogical analyses, the reservoir sandstones show a trend towards lower porosity with increasing clay content at shallow burial, probably due to more extensive mechanical compaction. At depths >3.0–3.5 km, the porosity loss is highest in sandstones with low clay contents. This may be explained by a lower rate of quartz cementation in sandstones with abundant intergranular clays or clay coatings around quartz grains. These data show that pre-burial mineralogy is a very important factor in porosity prediction of sandstones, in many cases more important than the degree of overpressure.

Hydrocarbon‐generation‐induced microcracking of source rocks

Abstract: Laboratory measurements of ultrasonic velocity and anisotropy in kerogen-rich black shales of varying maturity suggest that extensive, bedding-parallel microcracks exist in situ in most mature source rocks undergoing the major stage of hydrocarbon generation and migration. Given the normal faulting regime with the vertical stress being the maximum principal stress typical of most sedimentary basins, this microcrack alignment cannot be accounted for using simplified fracture mechanics concepts. This subhorizontal microcrack alignment is consistent with (1) a model of local principal stress rotation and deviatoric stress reduction within an overpressured formation undergoing hydrocarbon generation, and with (2) a strong mechanical strength anisotropy of kerogen-rich shales caused by bedding-parallel alignment of kerogen microlayers. Microcracks originate within kerogen or at kerogen-illite interfaces when pore pressure exceeds the bedding-normal total stress by only a few MPa due to the extremely low-fracture toughness of organic matter. P-wave and, especially, S-wave anisotropy of the most mature black shales, measured as a function of confining pressure, indicate the effective closure pressure of these microcracks in the range from 10 to 25 MPa. Estimates of pore pressure cycles in the matrix of the active hydrocarbon-generating/expelling part of the source rock formation show that microcracks can be maintained openmore » over the sequence of these cycles and hence be detectable via high-resolution in-situ sonic/seismic studies.« less

Borehole Stability in Shales

Abstract: Downhole mud/shale interaction can only be properly understood if rock mechanical, shale hydration, and fluid transport phenomena are taken into account. This paper presents a review of Koninklijke Shell E P Laboratorium's research on borehole stability in shales. Mechanisms relevant to shale stability, including pore pressure penetration (the gradual increase in pore pressure resulting from high mud weight), capillary threshold pressures, compressive and tensile failure, postfailure stabilization, hydration stress, inhibition, and osmotic phenomena are discussed. The authors attempt to integrate these mechanisms into a comprehensive model for shale behavior.

Liquefaction mechanism for layered soils

Abstract: Results from four centrifuge model tests are presented. Three of the model tests involve layered soil deposits subject to base shaking; one model test involves a uniform soil deposit of sand subject to base shaking. The layered soil models consisted of fine sand overlain by a layer of relatively impermeable silica flour (silt). Pore-water pressures, accelerations, and settlements were measured during all four tests. Results from the model tests involving layered soils suggest that during liquefaction a water interlayer or very loose zone of soil may develop at the sand-silt interface due to the difference in permeabilities. In each layered model test, boils were observed on the surface of the silt layer. These boils were concentrated in the thinnest zones of the overlying silt layer and provided a vent for the excess pore-water pressure generated in the fine sand.

Simulations of the Origin of Fluid Pressure, Fracture Generation, and the Movement of Fluids in the Uinta Basin, Utah

Abstract: The Altamont oil field in the deep Uinta basin is known to have reservoir fluid pressures that approach lithostatic. One explanation for this high pore-fluid pressure is the generation of oil from kerogen in the Green River oil shale at depth. A three-dimensional simulation of flow in the basin was done to test this hypothesis. In the flow simulation, oil generation is included as a fluid source. The kinetics of oil generation from oil shale is a function of temperature. The temperature is controlled by (1) the depth of sediment burial and (2) the geothermal gradient. Using this conceptual model, the pressure buildup results from the trade-off between the rate of oil generation and the flow away from the source volume. The pressure increase depends primarily on (1) the rate of the oil-generation reaction and (2) the permeability of the reservoir rocks. A sensitivity analysis was performed in which both of these parameters were systematically varied. The reservoir permeability must be lower than most of the observed data for the pressure to build up to near lithostatic. The results of the simulations indicated that once oil generation was initiated, the pore pressure built up rapidly to near lithostatic. We simulated hydrofractures in that part of the system in which the pressures approach lithostatic by increasing both the horizontal and the vertical permeability by an order of magnitude. Because the simulated hydrofractures were produced by the high pore pressure, they were restricted to the Altamont field. A new flow system was established in the vicinity of the reservoir; the maximum pore pressure was limited by the least principal stress. Fluids moved vertically up and down and laterally outward away from the source of oil generation. The analysis indicated that, assuming that one is willing to accept the low values of permeability, oil generati n can account for the observed high pressures at Altamont field.

Conductive heat flow and thermally induced fluid flow around a well bore in a poroelastic medium

Abstract: Transient analytical solutions for temperature and pore pressure changes near a circular borehole under instantaneous temperature and fluid pressure changes inside the borehole are presented. The solutions couple conductive heat transfer with Darcy fluid flow, and a borehole under a nonhydrostatic far-field stress state is simulated. The heat conduction equation is decoupled from the coupled system of isothermal governing equations, and the complete solution is obtained by superimposing this decoupled solution on the isothermal one. The solution is therefore applicable to low-permeability media, where heat transfer is dominated by conduction only. Both cold and warm injection processes are studied, and the applications to hydraulic fracture initiation and thermally induced fluid flow are discussed. Taking Westerly granite as an example, it is concluded that the maximum thermally induced pore pressure inside the rock formation can be 30% higher than the isothermal pore pressure, with a borehole temperature and fluid pressure change ratio (ΔT/Δp) = 1°C/MPa. It is emphasized that the thermally induced pore pressure change can be significant inside a low-permeability porous medium, and a coupled solution must be obtained to address the mechanical, hydraulic, and thermal responses appropriately.

A study of the geoelectrical properties of peatlands and their influence on ground‐penetrating radar surveying1

Abstract: Geophysical surveys and chemical analyses on cores were carried out in three Ontario peatlands, from which we have gained a better understanding of the peat properties that control the geophysical responses. The electrical conductivity depends linearly on the concentration of total dissolved solids in the peat pore waters and the pore waters in turn bear the ionic signatures of the underlying mineral sediments. The ionic concentration, and thus the electrical conductivity, increase linearly from the surface to basement. The average bulk electrical conductivity of peatlands at Ellice Marsh, near Stratford, and at Wally Creek Area Forest Drainage Project, near Cochrane, are of the order of 25 mS/m. The Mer Bleue peatland, near Ottawa, has extremely high electrical conductivity, reaching levels of up to 380 mS/m near the base of the peat. The Mer Bleue peatland water has correspondingly high values of total dissolved solids, which originate from the underlying Champlain Sea glaciomarine clays. The dielectric permittivity in peats is largely controlled by the bulk water content. Ground penetrating radar can detect changes in water content greater than 3%, occurring within a depth interval less than 15 cm. The principal peatland interfaces detected are the near-surface aerobic to anaerobic transition and the peat to mineral basement contact. The potential for the successful detection of the basement contact using the radar can be predicted using the radar instrument specifications, estimates of the peatland depth, and either the bulk peat or the peat pore water electrical conductivities. Predicted depths of penetration of up to 10 m for Ellice Marsh and Wally Creek exceed the observed depths of 1 to 2 m. At Mer Bleue, on the other hand, we observe that, as predicted, a 100 MHz signal will penetrate to the base of a 2 m thick peat but a 200 MHz signal will not.

Porosity and permeability evolution during hot isostatic pressing of calcite aggregates

Abstract: Porosity, permeability, and storativity were measured during isostatic hot-pressing of fine-grained calcite aggregates at temperatures of 633 to 833 K, confining pressures of 200 to 300 MPa, and argon pore pressures of 100 to 250 MPa. The progressive changes in total porosity were measured in situ by monitoring the sample length changes. The connected porosity and the permeability and storativity were measured in situ by incrementing and oscillating pore pressure techniques, respectively. In a given test, there was a decrease with time in the rate of reduction of porosity, the rates being higher at higher temperature and effective pressure. The permeability k was nonlinearly related to the total porosity ϕ in the form k ∝ ϕn. The exponent n was approximately equal to 3 and thus consistent with the prediction of the “equivalent channel” model, in the range of porosity from 0.18 down to 0.07. Below 0.07, n became much larger (around 14), an effect that can be attributed to loss of connectivity and which is qualitatively similar to that observed by Bernabe et al. [1982] in post-hot-pressing measurements. However, a cube law continues to apply below 0.07 total porosity if the permeability is related to the connected porosity itself. The storativity is also nonlinearly related to the porosity. Model analyses of the permeability and storativity results indicate both that the pore apertures decrease and that the pore shapes become more equant as the porosity decreases. The marked downturn in the permeability-porosity relationship at total porosities below 0.07 appears from microscopical observation to correspond to a change in pore geometry from largely connected, irregular pores between grains to isolated, tubular pores at junctions of several grains. Application of the “Swiss-cheese” continuum percolation model indicates a percolation threshold of about 0.04 porosity. Microstructural evidence, the apparent activation energy for densification, and the stress dependence of densification rate suggest that porosity reduction has occurred mainly by dislocation creep.

Estimating seismic velocities at ultrasonic frequencies in partially saturated rocks

Abstract: Seismic velocities in rocks at ultrasonic frequencies depend not only on the degree of saturation but also on the distribution of the fluid phase at various scales within the pore space. Two scales of saturation heterogeneity are important: (1) saturation differences between thin compliant pores and larger stiffer pores, and (2) differences between saturated patches and undersaturated patches at a scale much larger than any pore. We propose a formalism for predicting the range of velocities in partially saturated rocks that avoids assuming idealized pore shapes by using measured dry rock velocity versus pressure and dry rock porosity versus pressure. The pressure dependence contains all of the necessary information about the distribution of pore compliances for estimating effects of saturation at the finest scales where small amounts of fluid in the thinnest, most compliant parts of the pore space stiffen the rock in both compression and shear (increasing both P- and S-wave velocities) in approximately the same way that confining pressure stiffens the rock by closing the compliant pores. Large-scale saturation patches tend to increase only the high-frequency bulk modulus by amounts roughly proportional to the saturation. The pore-scale effects will be most important at laboratory and logging frequencies when pore-scale pore pressure gradients are unrelaxed. The patchy-saturation effects can persist even at seismic field frequencies if the patch sizes are sufficiently large and the diffusivities are sufficiently low for the larger-scale pressure gradients to be unrelaxed.

Effects of lithology and depth on the permeability of core samples from the Kola and KTB drill holes

Abstract: Permeability measurements were conducted on intact core samples from the Kola drill hole in Russia and the KTB drill hole in Germany. Samples included granodiorite gneisses, basalts and amphibolites from depths up to 11 km. The tests were intended to determine the pressure sensitivity of permeability and to compare the effects of stress relief and thermal microcracking on the matrix permeability of different rock types and similar samples from different depths. The pore pressure Pp was fixed at the estimated in situ pressure assuming a normal hydrostatic gradient; the confining pressure -Pc was varied to produce effective pressures (-Pe - -Pc- -Pp) of 5 to 300 MPa. The permeability of the basaltic samples was the lowest and most sensitive to pressure, ranging from 10 -2o to 10-23m 2 as effective pressure increased from 5 to only 60 MPa. In contrast, the granodiorite gneiss samples were more permeable and less sensitive to pressure, with permeability values ranging from 10 -l? to 10 -22 rn 2 as effective pressures increased to 300 MPa. Amphibolites displayed intermectiate behavior. There was an abundance of microfractures in the quartz-rich rocks, but a relative paucity of cracks in the mafic rocks, suggesting that the observed differences in permeability are based on rock type and depth, and that stress relief/thermal-cracking damage is correlated with quartz content. By applying the equivalent channel model of Walsh gcl Brace (1984) to the permeability data of the quartz-rich samples, we can estimate the closure pressure of the stress-relief cracks and thereby place bounds on the in situ effective pressure. This method may be useful for drill holes where the fluid pressure is not well constrained, such as at the Kola well. However, the use of crack closure to estimate in situ pressure was not appropriate for the basalt and amphibolite samples, because they are relatively crack-free in situ and remain so even after core retrieval. As a result, their permeability is near or below the measurable lower limit of our apparatus at the estimated in situ pressures of the rocks.

Calibration chamber studies of piezocone test in cohesive soils

Abstract: Results of miniature piezocone penetration tests (PCPT) on cohesive soil specimens in a calibration chamber system are presented. The influence of soil type, stress history, penetration boundary conditions, and filter locations on PCPT data are investigated. An increase in the strength of the soil around the cone penetrometer is observed when penetration is resumed after a dissipation test. The penetration depth required to attain a steady excess pore pressure value at the tip of the cone is influenced by the overconsolidation ratio and the lateral stress coefficient. The empirical values of the cone factor and the pore pressure factor are found to be strongly influenced by the lateral stress and the plasticity index. The tip resistance measured during the miniature quasi‐static friction cone penetration tests (QCPT) are consistently lower than the values obtained from the PCPT. Interpretation of the dissipation results to evaluate the radial coefficient of consolidation should be based on the initial dis...

Resonant fluidization of silty soil by water waves

Abstract: A new resonant stage is identified in the laboratory during the complex process of soil fluidization by water waves. Massive fluidization failure is consistently observed following internal resonance events inside the silt bed. The weakening effect of this resonance is observed to endure long periods of consolidation. A wave reloading, following such a consolidation period, on a preresonated soil usually results in a recurrence of a similarly massive fluidization failure. Analysis of pore pressure records indicates that the resonance events are localized in space and the associated wave modes are short in wavelengths. It is proposed that these resonant events are a consequence of a strong channeling of the seepage flow within the silt bed.

Comparison of techniques for the isolation of sediment pore water for toxicity testing

Abstract: The objective of this study was to define an interstitial (pore) water isolation technique suitable for sediment toxicity testing and toxicity identification evaluation (TIE) research. Pore water was prepared from sediments collected at two fresh-water sites (Saginaw River, Keweenaw Waterway) using four or five different techniques, and the samples were compared with respect to toxicity to Ceriodaphnia dubia and several inorganic chemical parameters. Methods evaluated were: high speed (10,000×g) and low speed (2,500×g) centrifugation, syringe extraction, compression, and dialysis. Both high speed and low speed centrifugation resulted in adequate volumes of pore water for routine testing with relatively minimal effort, while the other three techniques were labor intensive and/or yielded small quantities of pore water. Filtration of samples either during preparation (syringe-extraction, compression) or subsequent to initial isolation (centrifuged samples) resulted in significant losses of toxicity, apparently due to the adsorption of contaminants onto the filter or particles retained by the filter. Low speed centrifugation generally resulted in much higher metal concentrations than the other four techniques. However, compared to samples prepared by high speed centrifugation or dialysis, a relatively high proportion of the metals in pore water isolated by low speed centrifugation were biologically unavailable, as inferred from the results of the toxicity tests. Based on these studies, as well as the work of others, we recommend that sediment pore water for toxicity testing and/or TIE analyses be prepared by centrifugation (preferably at 10,000×g) without subsequent filtration.

Seepage-induced effective stresses and water pressures around pressure tunnels

Abstract: The change in stresses in the rock mass generated by the internal loading of a pressurized conduit, has traditionally been estimated assuming that the rock mass is impermeable and that the internal...

Velocity and inferred porosity model of the Oregon accretionary prism from multichannel seismic reflection data: Implications on sediment dewatering and overpressure

Abstract: A two-dimensional model of seismic velocity derived from multichannel seismic data collected off Oregon in 1989 shows that as sediments are carried from Cascadia Basin into the accretionary prism, there are measurable changes in velocity-depth profiles. In the seaward most area of the basin, where no thrust faults are observed, there is a landward (and downward) increase of velocity in the sedimentary section. We attribute the velocity increase in the basin to a reduction of porosity resulting from consolidation and cementation, accompanied by diffusive flow of pore water driven by lateral tectonic as well as gravitational stress. Near the base of the slope there is an area of incipient thrusting (the protothrust zone) where protothrusts sole out into a protodecollement. Synthetic seismogram modeling of the reverse-polarity reflection from the protodecollement shows a 100-m-thick layer with a slightly lower velocity relative to the sediments above it. Above the protodecollement, velocity continues to increase landward. We suggest that in this area the diffusive flow of pore water out of the sediment is augmented above the protodecollement by fault-focused flow. Below the protodecollement a reversal in velocity may be due to an increase in porosity resulting from overpressuring of pore fluid trapped by reduction of the permeability of the sediment above the protodecollement. Farther landward, where thrusting has formed a fault-bend fold, velocity values are lower in the accreted section of sediments relative to the velocity at a comparable subbottom depth in the protothrust zone. The decrease in velocity is a result of microfracturing of the highly consolidated sediments accompanying uplift and folding and reflects the increasing role of fracturing and faulting in the control of dewatering of the sediments.

Oscillatory bending of a poroelastic beam

Abstract: An analytical solution of the oscillatory axial and bending loading of a poroelastic beam is presented. The pore pressure behavior in the beam is explored as a function of frequency of the applied load, the ratio of the bending to axial applied loading, and the leakage at the top and bottom of the beam. The conditions under which the pore pressure carries its largest fraction of the total applied loading are determined. The solution is illustrated using the values of the material parameters appropriate for living bone, which is a poroelastic medium. At high frequencies, in the free leakage case, our results are consistent with the notion that the percentage of the applied load carried by the pore fluid pressure is equal to the porosity of the medium.

Use of radium isotopes to examine pore-water exchange in an estuary

Abstract: The measured distributions of four isotopes of Ra along the estuary of the Bega River are used to examine sediment-water column exchange. Ra is created in estuarine sediments by the radioactive decay of insoluble Th parents residing close to or on the surfaces of the sediment grains. Ra desorbed into the pore water is continuously lost to the water column due to the cyclical draining and filling of the sediments by the tides. The distribution of Ra in the estuary is governed by its rate of loss from the sediments, its advection along the estuary resulting from river discharge into the estuary’s head, tidal mixing, and radioactive decay. These processes are all described in a model. Matching of model-predicted Ra concentrations with measurements allows an estimate of the effective depth in the sediments to which the pore water is exchanged every tidal cycle. This depth is large (15 cm), but it is shown to be reasonable for the Bega estuary.

Chemistry of Sediment Pore Water in Aquaculture Ponds Built on Clayey Ultisols at Auburn, Alabama

Abstract: The composition of sediment pore water was determined for ponds constructed on clayey Ultisols at Auburn, Alabama. Pore water was anaerobic and contained much higher concentrations of ferrous iron (Fe2+), soluble reactive phosphorus (SRP), total phosphorus (TP), total ammonia-nitrogen (TAN), and sulfide (S2−) than surface or bottom waters. Concentrations of SRP and TP in pore water were higher in ponds with high soil phosphorus concentrations than in a new pond with less soil phosphorus. Increased concentrations of organic matter in soil or larger inputs of feed to ponds favored greater microbial activity in soils and higher concentrations of TAN in pore water. The pH of pore water was 6.5–7.0, and pH was apparently controlled by the equilibrium: Movement of Fe2+, SRP, and S2− from pore water into pond water apparently was prevented by the oxidized layer of soil just below the soil-water interface. Pond managers should concentrate on maintaining this oxidized layer to reduce the tendency for toxic substances to diffuse into the pond water.

Modelling unanticipated pore-water pressures in soft clays

Abstract: Field observations in thin soft clay layers may show pore-water pressures that increase for some time after the loading is applied. Reasons for these observations are not well understood. The paper shows how an elastic viscoplastic constitutive model incorporated into the consolidation equation can predict these pore-water pressure increases in soils that exhibit significant creep behaviour (or secondary compression). The phenomenon has been related to relaxation in regions of the profile from which drainage has not yet begun. Key words : clay, consolidation, creep, secondary compression, viscous, relaxation, pore-water pressure, elastic–plastic.

Excess pore pressure in a poroelastic seabed saturated with a compressible fluid

Abstract: This paper presents an analytical investigation on the excess pore-fluid pressure in a finite seabed layer by taking into account the influence of a compressible pore fluid. The seabed layer is modeled as a poroelastic layer saturated with a compressible pore fluid and resting on a rough, rigid impermeable base. The surface of the poroelastic seabed layer is either completely pervious or completely impervious, and subjected to a normal surface traction induced by offshore structures. The paper presents analytical and numerical results to illustrate the time-dependent behaviour of excess pore pressure in the poroelastic seabed. The results demonstrate that the presence of a compressible pore fluid reduces the generation of excess pore pressure in the poroelastic seabed layer. Key words : excess pore pressure, poroelastic seabed layer, soil consolidation, compressible pore fluid, integral transforms.