Showing papers on "Pore water pressure published in 1979"

Driven piles in clay - the effects of installation and subsequent consolidation

Abstract: This paper describes the results of numerical analysis of the effects of installing a driven pile. The geometry of the problem has been simplified by the assumption of plane strain conditions in addition to axial symmetry. Pile installation has been modelled as the undrained expansion of a cylindrical cavity. The excess pore pressures generated in this process have subsequently been assumed to dissipate by means of outward radial flow of pore water. The consolidation of the soil has been studied using a work-hardening elasto–plastic soil model which has the unique feature of allowing the strength of the soil to change as the water content changes. Thus it is possible to calculate the new intrinsic soil strength at any stage during consolidation. In particular the long-term shaft capacity of a driven pile may be estimated from the final effective stress state and intrinsic strength of the soil adjacent to the pile. A parametric study has been made of the effect of the past consolidation history of the soil...

An analytical solution for the consolidation around a driven pile

Abstract: Field studies have shown that the driving of a displacement pile into cohesive soil generates large excess pore pressures in the vicinity of the pile. These pore pressures are often larger than the effective overburden pressure and facilitate the installation of the pile. The subsequent increase in bearing capacity of the pile is largely controlled by the dissipation of the excess pore pressures and a consequent increase in the effective stresses acting on the pile. The paper presents a closed formanalytical solution for the radial consolidation of the soil around a driven pile, assuming that the soil skeleton deforms elastically. This assumption is examined in the light of the predicted effective stress changes in the soil and is shown to lead to, a realistic model for the decay of pore pressure near the pile with time after driving. Although the solution may be applied to any initial distribution of excess pore pressure, attention is focussed on that due to the expansion of a cylindrical cavity in an ideal elastic, perfectly plastic soil. The resulting logarithmic variation of excess pore pressure with radius is considered to be close to that generated around a pile as a result of driving. In addition to giving estimates of the time needed for a driven pile to achieve its maximum strength, the solution may also be used in the analysis of pressuremeter tests to provide in-situ measurements of the coefficient of consolidation of the soil.

Stress and pore pressure changes in clay during and after the expansion of a cylindrical cavity

Abstract: The disturbance of a clay mass, due to either the installation of a driven pile or the expansion of a pressuremeter membrane, is often modelled as a cylindrical cavity expansion. In addition, it is usual (and convenient) to assume that the expansion occurs under conditions of plane strain. For this problem a method of analysis is presented which considers the soil to be a saturated two-phase material with a pore fluid which flows according to Darcy's Law. Non-linearity in material behaviour is permitted as long as the effective stress–strain law can be written in an incremental or rate form. The use of a consolidation analysis allows the changes in effective stress and pore pressure to be determined at any stage during both the cavity expansion and the subsequent period of reconsolidation. Expansions may occur at any prescribed rate, including the very fast (undrained) and the very slow (fully drained) case. The technique is illustrated by considering the expansion of a cavity in two different types of elastoplastic soil. It is shown how these solutions may be used to model the disturbance of the soil due to pile driving.

Pore Pressure in Heated Concrete Walls: Theoretical Prediction

Abstract: Synopsis Pore-water pressures in concrete can be calculated by a previously developed theory which is based on thermodynamic properties of water and takes into account the huge changes in permeability and sorption isotherm with temperature, as well as the changes of pore space due to temperature and pressure. After reviewing the theory, finite-element solutions are com-pared with weight-loss tests of Chapman and England, and theoretical predictions are made for rapid heating of thick walls, either sealed or unsealed. A two-dimensional axisymmetric finite-element solution is developed to analyse theffect of a hot spot on the wall. The pore-pressure peaks are found to be much higher than for slow heating (25 atm versus 8 atm), and still about 50% higher when the heating is confined to a hot spot. The moisture movement in regions where the pressure gradient is opposite to the temperature gradient is found to be rather irregular and to exhibit oscillations. The theory predicts the phenomenon of ‘moisture clog...

Compressional and shear waves in saturated rock during water-steam transition

Abstract: Compressional and shear wave velocities were measured in water-filled Berea sandstone as a function of pore pressure, with a constant confining pressure of 300 bars. The measurements were made at room temperature, 145°C, and 198°C. At 145°C, compressional velocity increased from vapor-saturated (low pore pressure) to liquid-saturated (high pore pressure) conditions, whereas shear wave velocity decreased. For Compressional waves there was a velocity minimum and increased attenuation near the liquid-vapor transition. The results at 198°C show decreases of both compressional and shear velocities and a small velocity minimum for compressional velocity without marked attenuation. At both temperatures, Vp/Vs and Poisson's ratios increased from steam- to water-saturated rock. The results are compatible with the mechanical effects of mixing steam and water in the pore space near the phase transition and may be applicable to in situ geothermal field evaluation.

Shearing of saturated clays in rock joints at high confining pressures

Abstract: Saturated clays are sheared between rock joints at various pore water pressures and at confining pressures up to 3 kb (300 Mpa). Sliding on these joints is stable. For a given clay, the shear stress required to initiate sliding increases linearly with the effective normal stress across the sliding surface, with a slope of 0.08 +- 0.01 for joints filled with saturated montmorillonite, 0.12 +- 0.01 with saturated chlorite, 0.15 +- 0.01 with saturated kaolinite, and 0.22 +- 0.02 with saturated silty illite. Thus at high confining pressures the shear stress required to initiate sliding on joints filled with saturated clays are very much smaller than that required to initiate sliding on clean rock joints or on joints filled with dry gouge materials. In the crust, saturation of gouge materials along active faults would greatly lower the frictional resistance to faulting and would stabilize fault movement. Different fault behaviors such as stable creep along some faults and intermittent but sudden slip along others may reflect in part different degrees of saturation of fault zones at depth.

Pore water sampling in anoxic carbonate sediments: oxidation artefacts

Abstract: ANY exposure of anoxic estuarine sediments to the atmosphere during sample handling and processing can lead to decreased values of ferrous iron1, reactive phosphate2 and reactive silicate3 in the pore waters of these sediments. Thus, the mishandling of anaerobic sediments and their pore fluids will in turn lower calculated diffusional fluxes of these compounds from the sediments into the overlying water. To date, this oxidation effect has only been documented for clastic sediments rich in iron. We present data here which indicate that a similar phenomenon occurs when anoxic iron-poor, carbonate-rich sediments are exposed to an oxidising milieu.

The mechanics of drumlin formation with particular reference to the change in pore-water content of the till

Abstract: Most drumlins are composed of till that appears similar in all characteristics to non-drumlin till. Evidence, however, in the form of till layer sequences and from palaeomagnetic Koenigsberger ratios indicate that drumlin till may have a slightly higher pore-water content at the time of lodgement than nondrumlin till. In order that deposited till remains at the ice-glacier-bed interface to initiate drumlin formation, it must undergo rapid geotechnical changes whereby its pore-water content is dissipated thereby increasing the shear strength of the till. Two mechanisms of pore-water removal are possible. Firstly, pore water may be removed from localized patches within a mobile layer of till at the ice-glacier-bed interface, thus creating nuclei of higher-strength till around which deforming till may adhere. Secondly, removal of water initially from the thin water film at the base of a glacier may result in increased pressure melting of the ice leading to till melt-out and subsequent loss of pore water from the deposited till. The loss of water at the glacier bed may be either via permeable subglacial deposits or bedrock, or within subglacial depressions.

Calculated Compaction States of Thick Abnormally Pressured Shales

Abstract: Density/depth and pore pressure/depth profiles of thick tectonically undisturbed shales show that, during deposition and during burial by a permeable overburden having hydrostatic pore pressures, compaction is transient, controlled not only by the mechanical process of pore pressure dissipation, but also by such depositional events as the rate of sedimentation, shale thickness, overburden density, and overburden thickness. These calculated compaction profiles define several aspects of compaction history. For example, they show that shale density inversions may form early if the rate of burial by a hydrostatically pressured overburden is sufficiently rapid; density inversions also may disappear with additional burial. The profiles also show that thin zones of high-density shale overlying less compacted shale (sometimes called "seals") are features that can form late in compaction history and, therefore, are not necessarily responsible for abnormal pore pressure. The profiles also show that abnormal pressures may be present either during shale deposition, or may originate during burial beneath an overburden having normal pore pressures. These compaction states have been calculated by integrating empiricism with principles of soil mechanics (consolidation theory). The attempt is to use the depositional history (e.g., sedimentation rate, duration of deposition, and unconformities) and the material properties of the shale (e.g., permeability coefficient) to determine the present compaction state and to unravel the compaction history. Although other techniques utilize the depositional history, this technique differs in including the material properties of shale and not using empirical density-depth relations such as the Athy curve. Others have utilized consolidation theory to model compaction of thick shales. The calculations shown here, however, differ from them primarily by showing the effect of depositing a normally pressured overburden on top of thick shales. Thus, an important result has been to show the effect of using different boundary conditions of pore pressure dissipation.

Pore-pressure dissipation during excavation

Abstract: A finite element analysis procedure is presented that allows analysis of the actual degree of pore-pressure dissipation that occurs during and after excavation. Allowances are made for nonlinear soil behavior, variations in rate of the excavation process, and the presence of imprevious support wall systems. The validity of the program is checked by comparing predicted results with closed-form one-dimensional solutions, and data obtained in partially drained plane strain tests on a clayey soil. The computed response is found to agree exactly with theoretical results and to closely approximate that observed in the soil tests. In order to investigate dissipation of excess pore presssures for excavations, the results of a series of analyses of one-dimensional and two-dimensional problems are presented. It is found that significant pore-pressure dissipation is likely to occur during excavation except where the soil deposit is of low permeability. /ASCE/

A study of the effects of pore-water salinity on some physical properties of sedimentary rocks at permafrost temperatures

Abstract: The effect of pore-water salinity on the elastic wave velocities and electrical resistivities of frozen, saturated specimens of two sandstones and a limestone have been studied in the temperature r...

Stress-deformation and strength characteristics of a compacted shale

Abstract: When shales are encountered in road cuts, economic and environmental considerations usually dictate that they be used in adjoining embankments However, unless special precautions are taken, the stability of a shale embankment can deteriorate with time on account of the nondurable nature of most shales Previous investigators have developed various tests to classify shales with respect to their hardness, durability and degradability This paper presents the results of a laboratory investigation to define the effective stress strength parameters of mechanically hard but nondurable shale pieces of a lower Mississippian age formation (new Providence) from the state of Indiana, USA The initial gradation of crushed shale aggregate, the molding water content, the compaction pressure, and the pre-shear consolidation pressure were adopted as the independent test variables Triaxial specimens formed by kneading compaction were back pressure saturated under a low effective confining pressure, consolidated to the desired isotropic effective stress, and sheared undrained at a constant rate of deformation The compaction characteristics, the volume changes during saturation and consolidation, and the undrained shearing response including pore water pressure changes were studied The effective stress strength parameters evaluated at maximum deviator stress were found to be essentially independent of the initial conditions, except for loose uncompacted aggregate The compaction characteristics, the volume change characteristics, the induced pore water pressure changes, and the consolidated undrained strength were found to be greatly dependent on the initial conditions (TRRL)

Response and Stability of Earth Dams during Strong Earthquakes.

Abstract: : This earth dam study considers the response of an earth dam resting on a layer underlain by a rigid base and subjected to a strong ground motion. The dam and the layer are assumed to be elastic, homogeneous, and of simple geometric form. The properties of the dam and the layer may be different. Several combinations of the properties of the dam and the layer, as well as several ratios of the height of the dam to the depth of the layer, are studied. All of these are subjected to nine strong motion acceleration recrods, and their response is calculated. At the same time, seismic coefficients for various sizes of sliding wedges are computed. The results of individual cases and their envelopes are presented in the spectral form for design purposes. Methods of stability analysis for static and pseudostatic conditions are examined, and two new methods are presented. One of these methods is suitable for analyzing existing slip surfaces, and the other for quick computation of the critical acceleration for a given surface. Since pore water pressure developed in the dam during an earthquake is an important factor, a method of stability analysis is developed that takes the excess dynamic pore water pressure into account. The method is based on limit equilibrium principle. Pore water pressures are introduced in the form of dynamic parameter A sub n. The result is obtained in the form of a critical acceleration required to cause failure as a function of the number of cycles of earthquake load.

Temperature variation of the acoustic properties of laboratory sediments

Abstract: The compressional wave speed and attenuation and the shear wave speed and attenuation of unconsolidated artificial laboratory sediments have been measured as a function of temperature. No overburden or pore pressure in excess of that found at a few centimeters depth was applied. The compressional wave speed of the sediment was found to vary approximately as would water with a slight increase in the slope of the curve of the sediment over that of water. Shear wave speed appears to be independent of temperature within the accuracy of the measurement. Attenuation of both types of acoustic waves also appears to be independent of temperature. [Work supported by the Office of Naval Research.]

Device for determining the pore water pressure in a soil

Abstract: A device for measuring pore water pressure in soils employs a pore pressure sound (2) as a sensing member and a closed measuring system (5, 6) for creating the least possible disturbance in soils having a low degree of permeability, such as clay. The closed liquid-filled measuring system comprises a hypodermic needle type (20) of connection between the pore pressure sound and a measuring device (3), said needle being operative to penetrate a member of resilient material to extend into a liquid-filled chamber in said pore pressure sound and transfer a reading of the pore water pressure to said measuring device.

Use of the osmotic tensiometer to measure negative pore water pressure

Abstract: The osmotic tensiometer was developed to provide a method of direct measurement of tension in the pore water of soils. A study was undertaken at the University of Saskatchewan to investigate the instrument's operational characteristics and its applicability to practical engineering problems. The study covered long-term stability, response time to changes in pore water pressure, and response to changes in ambient temperature. It was found that the internal prestress pressure tended to diminish with time, probably due to leakage of the confined solute through the semipermeable membrane. However, it was possible to individually calibrate each instrument to correct for this effect. Second, the pressure response of the osmotic tensiometers indicated that a transient flow process was occurring; the equilibration time of the device was primarily a function of the configuration and materials of the unit and the compressibility of the solution in the chamber. Third, it was found that changes in ambient temperature considerably affected the internal reference pressures. These results indicate that osmotic tensiometers may be useful to measure negative pore water pressures, providing that the temperature is controlled and the pore water pressure is constant. This may restrict their usefulness to research applications. (Author)

Pore-pressure response beneath a ring foundation on clay

Abstract: A 9 m diameter and 22 m high concrete silo with a ring footing was instrumented and its performance monitored. The instrumentation included 12 pressure cells, 7 piezometers, 9 settlement cells, and...

Soil water hysteresis as a cause of delayed yield from unconfined aquifers

Abstract: Hysteresis in the relation between soil water content and (negative) pressure head of the soil water produces a lag in the release of pore water from a rewetted soil when the pressure heads are lowered (made more negative). For an unconfined aquifer this means that the water table must drop some distance in response to pumping a well before pore water is fully released, if the water table prior to pumping had been rising. Since water tables around active wells usually have been rising before a pumping test is performed (to establish equilibrium), soil water hysteresis may well be a common cause of delayed yield from unconfined aquifers.

Construction pore pressures in two earth dams

Abstract: The shear failures at Chingford and Muirhead Dams in 1937 and 1941 could be attributed to high pore pressures. Rates of construction had increased and there had been insufficient time for the dissipation of construction pore pressures. Measurements of pore pressures were made with standpipes at Knockendon: a dam similar to Muirhead being built at about the same time. During the designs of embankment dams on the rivers Usk and Daer consideration was given to pore pressures and twin tube hydraulic piezometers were installed during construction in 1951-3. The dams had similar sections and were both built from boulder clay, but while negligible pore pressures were measured at Daer, values greater than the overburden pressure were observed in the fill of the usk dam. Tests showed that there were marked differences in the values of coefficient of consolidation and water content of the two fills. The slip surface at Chingford passed through a layer of soft clay in the foundation. A layer of silt was found in the foundation at Usk and to ensure adequate dissipation of pore pressure from it to ensure stability when the dam was built, a simple system of sand drains was used. (TRRL)

Effects of restricted flow at the surface of saturated clay

Abstract: The deflection of, and the pore pressure at, the surface of a saturated clay mass are computed by modelling the mass mathematically as a consolidationg half-space Specifically, attention is focused on the effect of a surface cover which impedes, though does not entirely prevent, the flow of the pore water A number of results of interest are reported, generalizing work reported earlier by McNamee and Gibson8