Development Of Pore Water Pressure Research Articles

A Pore Pressure Model for Cyclic Straining of Clay

The paper focuses on the development and modelling of residual cyclic pore water pressure, u, in clay during undrained cyclic strain-controlled loading. Data obtained from a series of NGI type cyclic simple shear tests were employed. In addition to the amplitude of cyclic shear strain, γ c , the number of cycles, N, and the overconsolidation ratio, OCR, a new component is incorporated in the conventional characterization of u. This new component is the volumetric threshold shear strain, γ tv , of clayey soils, below which for all practical purposes u does not develop. The lack of understanding of complex clay microstructure and the associated interaction between clay particles during cyclic shearing makes the modelling of u in a manner other than curve fitting rather difficult. This is especially true in regard to the development of negative u in overconsolidated clays. A model based on the systematic curve fitting of the pore pressure data expressed in terms of γ c , γ tv , N and OCR is presented. The possibilities of the incorporation of such a model into existing numerical tools for simulation of the seismic and ocean wave loading response of natural soil deposits are briefly discussed as well.

The role of vegetation clearing in the mass failure of hillslopes: Moresby Ranges, Western Australia

The paper attempts to establish the hillslope stability response to vegetation clearing in the Moresby Ranges, Western Australia. Since the turn of the century the area has been extensively cleared of native vegetation. At present, mass failure of hillslopes, through a variety of failure modes, is widespread in the area. Stability calculations show that the “immediate” cause of failure are debris mantle saturation and the development of high positive pore water pressures. But by considering the root strength that the native vegetation imparted to the debris mantle, it is clear that the “ultimate” cause of hillslope failure is the clearing of native vegetation.

Microplane Model for Triaxial Deformation of Saturated Cohesive Soils

A microplane model, in which the constitutive properties are characterized independently on planes of various orientations (microplanes), is presented. It is found that the basic scheme previously developed for concrete is also valid for clays, but with certain modifications. The micro‐macro constraint is kinematic, and the stresses on each microplane are defined as explicit functions of the volumetric and deviatoric normal and shear components of the macroscopic strain tensor on the microplane. To account for undrained behavior, a pore water pressure term that affects only the volumetric equations is introduced. This makes possible the uncoupling of the stress‐strain and the pore water pressure formulations. The model is calibrated and verified by comparisons with numerous data for both drained and undrained tests, and good agreement is attained, including volume changes, pore water pressure evolution, and various stress‐strain diagrams. Although the model involves nine material parameters, four (or five...

A permeameter for unsaturated soil

A permeameter for unsaturated soil was developed by observing the way in which pore water recovers hydrostatic equilibrium. It works like an hour glass that is turned upside-down everytime the state of reference (or hydrostatic equilibrium) is reached. The hydraulic conductivity is deduced from the curves of evolution of pore-water pressure and from the distribution of partial density of water at hydrostatic equilibrium.

Influence of the testing technique on the pore water pressure development in clay under wave loading : De Campos, T M P Proc 5th International Symposium on Offshore Engineering, Rio de Janeiro, 16–20 September 1985V5, P225–237. Publ London: Pentech, 1986

Influence of the testing technique on the pore water pressure development in clay under wave loading : De Campos, T M P Proc 5th International Symposium on Offshore Engineering, Rio de Janeiro, 16–20 September 1985V5, P225–237. Publ London: Pentech, 1986

Influence of the testing technique on the pore water pressure development in clay under wave loading : De Campos, T M P Proc 5th International Symposium on Offshore Engineering, Rio de Janeiro, 16–20 September 1985V5, P225–237. Publ London: Pentech, 1986

Influence of the testing technique on the pore water pressure development in clay under wave loading : De Campos, T M P Proc 5th International Symposium on Offshore Engineering, Rio de Janeiro, 16–20 September 1985V5, P225–237. Publ London: Pentech, 1986

Effect of inertia term on pore-water pressure development in seabed due to waves

Effect of inertia term on pore-water pressure development in seabed due to waves

Prediction of Lateral Displacement of Anchored Bulkheads Induced by Seismic Liquefaction

Numerical analysis was carried out on earthquake-induced displacement of anchored bulk-heads. The permanent displacement of several meters experienced in liquefied areas during the Niigata earthquake was large enough to damage the functions of nearby structures. Since the conventional methods of predicting seismically-induced displacement could not deal with thin sheet-pile walls subject to liquefaction, they were modified and improved. The new method analyzes the limit equilibrium of a backfill soil wedge under the action of pseudostatic seismic force, and calculates the critical acceleration of an earthquake which is required for the sliding mechanism to be activated. By following Newmark’s sliding block theory, the seismic acceleration in excess of the critical value is integrated twice with time so that permanent displacement may be obtained. Effects of pore water pressure development is taken into account by allowing the variation of critical acceleration with time. Finally parametric studies were carried out by using the method derived here, and their results are explained briefly.

A LIQUEFACTION ANALYSIS OF SATURATED SAND GROUND BASED ON SHEAR WORKS

A simple formulation is proposed by using shear works to express the development of pore water pressure under cyclic stresses. The proposed formulation is derived from data of the cyclic triaxial tests with various confining stresses and relative densities on a saturated sand. The pore water pressure model can be easily coupled with a nonlinear model to perform the liquefaction analysis for saturated sand deposits. The analysis, called Ws method, is a simplified effective stress analysis, calculating pore water pressure every half cycle. The method is compared with a nonlinear effective analysis, which is called the equi-Wst-line method and evaluates the pore water pressure by every time step.

Undrained Strength of Sand Undergoing Cyclic Rotation of Principal Stress Axes

This study concerns the effects of continuous rotation of principal stress axes on excess pore water pressure development during cyclic loading. Continuous rotation of principal stress axes has been known to exert some influence on the development of excess pore water pressure and failure of sand during cyclic loading. In an effort to investigate this aspect of the problem, several series of cyclic undrained tests were carried out in a static manner on saturated sampes of sand using a triaxial torsion shear apparatus. In this test apparatus, a hollow cylindrical soil specimen is subjected to a simultaneous application of both triaxial and torsional modes of shear stresses, which brings about the continuous rotation of principal stress axes. The test results indicated that, while the angle of internal friction remains unaffected, the continuous rotation of principal stress axes substantially reduces the resistance of sand to liquefaction by generating a greater amount of excess pore water pressure than in the case without the rotation. Some special tests were also performed to examine the effects of previous liquefaction on the undrained behavior of sand during the subsequent cyclic loading. The results show that the sand having previously experienced large deformations due to liquefaction exhibit a highly anisotropic stress-strain characteristics, whereby reducing the resistance of the sand drastically when sheared in certain directions.

Pore pressure response of earth dams in random seismic environment

The progressive development of pore water pressure due to earthquake loading and associated loss of strength may be a major cause of instability in earth dams consisting of low to medium dense saturated sands and hydraulic fill materials. In this study a procedure is developed for the analysis of pore pressure response of an earth dam subjected to a random seismic environment. The ground motion is modelled as a portion of the stationary random process which may be defined alternatively by (a) Kani—Tajimi spectral density function, or (b) a design response spectrum. The earth dam is modelled as a series of shear slices and is considered to be homogeneous. The nonlinearity in shear modulus and damping are incorporated by using an equivalent linear model for the stress-strain behavior of soils. The dynamic response to vertically propagating shear waves is formulated in the frequency domain providing the transfer functions for various parameters, viz, stress, strain and acceleration, etc. The power spectra of these parameters are then readily obtained. Using a simple linear model for the generation of pore water pressure the expected value of associated damage is obtained. Numerical results of an example analysis and a parametric study are presented.

Sand Liquefaction: Inelastic Effective Stress Model

An effective stress numerical model for soil liquefaction, which provides interaction between shearing deformation and transient pore-water pressure development, is presented. Some attractive features of this one-dimensional model are the preservation of the inelastic soil character during shearing, the coupling between shear wave propagation and pore-water pressure development and seepage, and the association of excess pore-water pressure development with the inelastic volumetric deformation of the solid matrix. A comparison is made with results from another effective stress model and a second comparison is made with experimental results from a large scale shaking table.

Consolidation Behavior of Peats

Abstract Four peat samples, covering a wide range of fiber contents, were subjected to one-dimensional consolidation tests. The development and dissipation of excess pore water pressure, effluent outflow, and permeability were also monitored during some of the consolidation tests. The other test variables included the application of back pressure, specimen thickness, and specimen orientation. Significant compression and effluent outflow take place after the dissipation of measurable excess pore water pressure, while permeability decreases drastically during consolidation. The rate of secondary compression, after a certain time, increases with the logarithm of time before gradually decreasing until it vanishes for very large times, resulting in a new stage termed “tertiary” compression. This behavior, along with the observations of peat microstructure, supports the presence of a two-level structure for peats, possibly consisting of macropores and micropores. The rate of compression of peats depends primarily on void ratio; peat type appears to be of secondary importance. Test results provide certain insights into the nature and the mechanisms of peat consolidation. The consolidation behavior of peats is quite distinct from that of other soils and requires special considerations in laboratory testing procedures and interpretation of results.

Seismic Response and Liquefaction of Sands

An effective stress analysis has been developed for determining the dynamic response of horizontal saturated sand deposits to earthquake motions consisting of vertically propagating shear waves. A hyperbolic stress-strain law is used for sands in shear and during the earthquake motions the modulus and damping properties of the sands are modified continuously for the effects of dynamic shear strains and pore-water pressures. The pore-water pressures are continuously updated using equations which relate pore-water pressures to dynamic shear strain history. Comparisons between data from the effective stress analysis and current total stress methods show that only the effective stress method can predict and reproduce the phenomena that occur in saturated sands during earthquakes. It gives the time to liquefaction, the time history of the development of pore-water pressures and surface accelerations comparable in form to those recorded on saturated sands during earthquakes.

Effect of the amplitude of shear strain on the energy density at the onset of liquefaction

Earthquakes cause liquefaction in sands of low to medium relative density. The hollow cylinder torsional shear device has been used in the laboratory to study this phenomenon; and in particular the effect of strain on the onset of liquefaction. To examine the influence of the various parameters affecting the development of liquefaction a series of undrained torsional shear tests were conducted on long, thin hollow cylinders of sand at several relative densities, confining pressures and sinusoidally applied shear strain amplitudes. An analysis of variance was used to study the interaction among the various parameters influencing liquefaction. The shear strain amplitude was found to have a significant influence on the pore water pressure development; and its relation to the number of cycles at the onset of liquefaction can be described quite well by a power function. It was found that the relation between the dissipated energy density and the developed pore water pressure was not significantly affected by the strain amplitude. This opens the door to the use of the energy method in predicting liquefaction. Such method lends itself ideally to the study of the effects of random excitations generated by earthquakes and the results of some preliminary tests support this hypothesis.

Pore Pressures, Consolidation, and Electrokinetics

The pore-water pressures that develop when an electric field is introduced into a soil mass are shown to depend upon the voltage distribution and the ratio of the electrokinetic and hydraulic permeabilities of the soil. Thus, the magnitude of the pressures at a point depends upon electrode geometry. The time necessary for the development of these pore-water pressures is shown to be predictable by use of the theory of consolidation. The rate of pore-pressure development is dependent upon the hydraulic permeability of the soil and not upon its electrokinetic permeability. Whether soil stabilization occurs as a result of the development of negative porewater pressures or destabilization occurs due to positive pore water pressures, depends upon the boundary conditions at the electrodes. Secondary effects, from the point of view of large-scale engineering works, are associated with alterations in soil pH and conductivity. Laboratory and field test data are presented to substantiate the arguments presented.

Embankment Pore Pressures During Construction

Theoretical methods for predicting pore pressures in earth dams are reviewed, and observed pore pressure data from selected dams constructed by the Corps of Engineers (CE) and other agencies are summarized to indicate the development and magnitudes of construction pore water pressures in earth dams. Construction characteristics and pore pressure data from 10 CE dams, 24 USBR dams, and 9 foreign dams are summarized and compared. The study determined that because of the numerous factors which influence pore pressure buildup, broad conclusions for all earth dams are difficult to make and each dam must be treated individually with respect to predicting construction pore pressures. It is concluded that: (1) Provisions for internal drainage effectively relieve construction pore pressures in earth embankments; (2) pore-pressure ratios in embankment materials increase rapidly as placement-water content increases, especially above optimum water content; and (3) pore pressures increase with increasing dam height, but even low dams (less than 100-ft high) can develop large pore pressures.

A new odontograph

Coseismic landslides are important effects of strong earthquakes and one of the most poorly understood. Such landslides are driven by inertial forces acting on slopes and can vary in size from individual rock blocks of a few m3 in volume to catastrophic rock avalanches of many Mm3, which can claim many thousands of lives. The capability of an earthquake to trigger a landslide is a function of energy arriving at the site, which is itself related to earthquake magnitude and epicentral distance. However, as much as these two parameters can be shown to exert a limit on the ability of the earthquake to cause landslides, it is clear from the spatial distribution of landslides caused by earthquakes that these are not the controlling factors. A detailed consideration of ground accelerations recorded during earthquakes indicates that threshold or yield accelerations are required to initiate movement on slopes. These yield accelerations are strongly dependent on the degree of stability of the slope under ambient stress conditions, development of pore-water pressures during shaking, and degradation of strength and stiffness under cyclic loading. Traditional engineering analyses have tended to focus on horizontal accelerations; however, a growing body of evidence points to the importance of vertical accelerations in seismic slope stability. In addition to the polarity of ground motions, there is an important influence from topographic amplification of shaking. This can give rise to ground motions that are 5–10 times higher than free-field accelerations, meaning that ground shaking at slope crests and ridges can be considerably higher than the vibration at the foot, or behind the crest, of the slope. This variation in ground motions means that shaking recorded on free-field instruments is unlikely to be representative of ground motions driving slope instability. Therefore, several significant unknowns severely limit understandings of this group of dynamic slope processes.