Excess Pore Pressure Generation Research Articles

The role of sedimentation rate and permeability in the slope stability of the formerly glaciated Norwegian continental margin: the Storegga slide model

Despite the gently dipping slopes (ca 1°), large-scale submarine slope failures have occurred on the mid-Norwegian continental margin (Storegga, Sklinnadjupet, Traenadjupet), suggesting the presence of special conditions predisposing to failure in this formerly glaciated margin. With a volume estimated between 2,400 and 3,200 km3 and an affected area of approximately 95,000 km2, the Storegga slide represents one of the largest and best-studied submarine slides of Holocene age known worldwide. Finite element modeling of slope failure indicates that a large (6.5 < Ms < 7.0) seismic triggering mechanism would not be sufficient to cause failure at more than 110 m below the seabed as observed for the slip planes at Storegga (northern sidewall). This implies that other factors (e.g., liquefaction, strain softening, gas charging, rapid burial) are needed to explain the occurrence of the Storegga slide with a deep surface of failure. In this paper, we discuss the importance of the compaction effect of rapidly accumulated sediments in the slide area. During compaction, sediment grains reorganize themselves, thereby, expelling pore water. Consequently, depending on sedimentation rate and permeability, excess pore pressures might result beneath less permeable sediments. Our modeling and cross-checking illustrate how excess pore pressure generation due to high sedimentation rate could explain the development of layers of weakness, and thus, how such a large slide might have been initiated in deep sediments. Using the highest sedimentation rate estimated in the area (36 and 27 m/kyr between 16.2 and 15 kyr BP), 1D modeling shows excess pore pressure values of around 200 kPa at a depth of 100 m below the seafloor 15 kyr BP and 60 kPa at a depth of 100 m at the time of the slide (8 kyr BP). Excess pore pressure apparently drastically reduced the resistance of the sediment (incomplete consolidation). In addition, 2D modeling shows that permeability anisotropies can significantly affect the lateral extent of excess pore pressure dissipation, affecting, that way, normally consolidated sediments far from the excess pore pressure initiation area.

Influence of landslide geometry and kinematic deformation to describe the liquefaction of landslides: Some theoretical considerations

The generation of excess pore pressure and possible liquefaction can be explained by different mechanisms. In literature, less attention is given to the effects of landslide geometry and kinematic deformation on the generation of excess pore pressure and possible liquefaction of sliding blocks. The objective of this paper is to discuss these factors by proposing a simple analytical model of landslide liquefaction using classic soil mechanics theory. The model describes the initial state of failure and solves forces for each slice with the Bishop's limit equilibrium method. Immediately after failure the difference in movement for each slice is calculated assuming a viscous shear band and using the Coulomb-viscous model. The differential movements conduct to differential strains which are transferred to excess pore pressures. In slides characterized by curved slip surfaces, the lower parts showed compaction and increase in pore pressures during displacement, while the upper parts showed dilatation and a decrease in pore pressures. The potential liquefaction is then evaluated for each slice in relation to the displacement. The model is applied to a slump-type failure that occurred on the Super-Sauze mudslide (Southeast France) in compact clay-rich material. It was observed that the slump completely liquefied into a flow. However, the model simulations were not able to completely liquefy all the slices of the slump in case a decrease in pore pressures through dilatation was assumed in the upper part of the slump. The decrease in pore pressures caused a rapid stabilisation of the remaining sliding body. In case a neutral pore pressure was assumed in the upper part with dilatation, movement and deformation of the remaining slices could continue leading to a nearly complete liquefaction of the slump. Model simulations for many geometries of curved slip surfaces reveal that the liquefied volume increases with steeper slopes and more curved slip surfaces. It is discussed also how planar slides, which in theory show small differential movement of the individual slices and hence no large compaction, can completely liquefy through deformation at the toe.

Role of input motion in excess pore pressure generation in dynamic centrifuge modelling

Dynamic centrifuge testing is widely used to model geotechnical earthquake engineering problems. This is especially helpful in generating data to validate numerical models or to understand failure mechanisms in a given boundary value problem. The input motions used in many of these tests to simulate the earthquake loading vary from simple sinusoidal motions at a single frequency to more realistic earthquake motions recorded in previous earthquakes. These realistic earthquakes normally contain different amplitude cycles as well as different frequency components. This paper investigates the effect of choice of input motion on the behaviour of loose, saturated soil layers in terms of excess pore pressure generation and settlement. It presents results from centrifuge tests carried out using specially designed input motions that are comparable, with either uniform or non-uniform amplitude cycles. It will be shown that both these types of motions resulted in very similar peak excess pore pressures. However non-uniform amplitude cycles resulted in large, dilation induced suction spikes that can have an effect on accumulated soil deformation. It is concluded that choice of input motion may depend on the type of problem being investigated and the purpose of centrifuge testing.

Liquefaction Phenomena underneath Marine Gravity Structures Subjected to Wave Loads

The foundation of a vertical breakwater or offshore gravity platform may fail due to wave loading. The question is discussed whether liquefaction phenomena in a sandy subsoil, such as the generation of residual excess pore pressures and cyclic mobility, may significantly contribute to such failure. Theory, descriptions of prototype failure cases, and several scale tests are analyzed. It is concluded that the most spectacular failure type “liquefaction flow failure” is only possible in the case of a subsoil of very loose sand or silt combined with a low drainage potential, e.g., by the presence of a clay layer or large structure dimensions. Three other potential failure types are more likely in other cases: stepwise liquefaction failure, stepwise failure, and wobble failure. Their relevance increases with decreasing relative density and decreasing drainage potential. Brief recommendations for assessment of each failure type are presented.

Ground Motion Intensity Measures for Liquefaction Hazard Evaluation

The requirements of performance-based earthquake engineering place increasing importance on the optimal characterization of earthquake ground motions. With respect to liquefaction hazard evaluation, ground motions have historically been characterized by a combination of peak acceleration and earthquake magnitude, and more recently by Arias intensity. This paper introduces a new ground motion intensity measure, CAV5, and shows that excess pore pressure generation in potentially liquefiable soils is considerably more closely related to CAV5 than to other intensity measures, including peak acceleration and Arias intensity. CAV5 is shown to be an efficient, sufficient, and predictable intensity measure for rock motions used as input to liquefaction hazard evaluations. An attenuation relationship for CAV5 is presented and used in an example that illustrates the benefits of scaling bedrock motions to a particular value of CAV5, rather than to the historical intensity measures, for performance-based evaluation of liquefaction hazards.

Laboratory Evaluation of Pullout Capacity of Reinforced Silty Sands in Drained and Undrained Conditions

Abstract This paper presents results of laboratory pullout tests performed to determine the drained and undrained pullout capacity of reinforced silty sands. The pullout tests were conducted for different soil types: clean sand and silty sand with 5, 10, 15, and 35 % silt, and overburden pressures: 30, 100, and 200 kPa. Pullout experiments were performed on a large pullout box with dimension 1.5 m long, 0.5 m high, and 0.4 m wide, where a single steel inclusion was placed inside a mass of homogeneous soil. Both drained and undrained pullout capacities depended on the silt content and increased with internal friction angle of the soil and with overburden pressure. Undrained conditions could significantly reduce the pullout capacity. The reduction was caused by the generation of excess pore pressures in the soil under rapid loading, which in turn decreased the effective normal stress at the soil-reinforcement interface.

Evidence for excess pore‐water pressure generated in subglacial sediment: Implications for clast ploughing

Pore water pressure in front of an object pulled through subglacial sediment and water pressure at the base of a borehole were simultaneously recorded at Unteraargletscher, Switzerland, to investigate the generation of excess pore pressures downglacier from ploughing clasts. Analysis of the strongly correlated pressure records revealed that the amplitude of the pore‐pressure fluctuations was larger than that of the corresponding borehole pressure fluctuations. Guided by high temporal resolution measurements of glacier surface motion from a different year, this finding suggests that temporal changes in glacier speed lead to changes in the degree of excess pore‐pressure in front of the ploughing object. The generation of excess pore water pressure has the potential to weaken the sediment downglacier from ploughing clasts. Instead of these clasts providing the roughness at the ice–bed interface to resist glacier motion, the ploughing process may in fact help to decouple the ice from the bed reducing basal resistance.

Undrained shear behaviour of sands subjected to large shear displacement and estimation of excess pore-pressure generation from drained ring shear tests

Undrained shear behaviour of fine silica and weathered granitic sand subjected to large shear displacement is examined. Parallel experiments using ring shear and the triaxial compression tests on soil specimens through a wide range of initial void ratios were conducted to investigate undrained shear strength as the key factor in the flow-like motion of landslides. The steady-state undrained shear strengths achieved in ring shear tests were, in general, smaller than those in the triaxial compression tests, probably because of the excess pore-pressure generation by grain crushing within the shear zone that occurred in ring shear. Very low steady-state shear strengths were achieved, however, in triaxial compression tests on the dense silica sand in which well-defined shear surfaces developed in the cylindrical specimens. In these triaxial compression tests, shear deformation must have been concentrated on these surfaces to generate excess pore pressure similar to that found in ring shear tests. An attempt was made to estimate excess pore pressure generated in undrained ring shear tests using the results of drained ring shear tests. The equivalent normal stress calculated as the ratio of volumetric strain in the drained test to the coefficient of volume change was introduced as a parameter for the estimation of excess pore-pressure generation for the large shear displacement that is usually found in landslides. Equivalent normal stress from drained tests was almost the same as the generated excess pore pressure in undrained tests with up to 1 m of shear displacement, at which the steady state was reached.Key words: undrained shear strength, excess pore pressure, equivalent normal stress, ring shear test, triaxial compression test, liquefaction.

Importance of time step discretisation for nonlinear dynamic finite element analysis

The behaviour of saturated soils subjected to earthquake loading has been investigated using the finite element method in the time domain. Such analyses are able to capture the excess pore pressure generation and subsequent nonlinear behaviour of the soil. It is important, however, when carrying out such analyses to use adequately small time steps. It has been recognized by previous researchers that small time steps are needed for the numerical solution of wave propagation problems, whereas in this technical note, the importance of using very small time steps for the analysis of nonlinear liquefying soil is demonstrated. It is shown that satisfactory results can only be obtained from finite element analyses when the time steps used are shorter than one twentieth of the time taken for a shear wave to travel across the smallest element in the finite element mesh.Key words: dynamic finite element analysis, liquefaction, time discretisation.

Explaining the Storegga Slide

The Storegga Slide occurred 8200 years ago and was the last megaslide in this region where similar slides have occurred with intervals of approximately 100 ky since the onset of continental shelf glaciations at 0.5 Ma. A geological model for the Plio-Pleistocene of the area explains the large scale sliding as a response to climatic variability, and the seismic stratigraphy indicates that sliding occurs at the end of a glaciation or soon after the deglaciation. The slides are in general translational with the failure planes related to strain softening behaviour of marine clay layers. The destabilisation prior to the slide is related to rapid loading from glacial deposits with generation of excess pore pressure and reduction of the effective shear strength in the underlying clays. Basin modelling has shown that excess pore pressure generated in the North Sea Fan area is transferred to the Storegga area with reduction of the slope stability in the old escarpments in distal parts of the Storegga Slide. The slide was most likely triggered by a strong earthquake in an area 150 km downslope from the Ormen Lange gas field and developed as a retrogressive slide. The unstable sediments in the area disappeared with the slide 8200 years ago. A new ice age with infilling of glacial sediments on top of marine clays in the slide scar would be needed to create a new unstable situation at Ormen Lange.

Pore pressure response of saturated soils around a penetrating object

It is known that the excess pore pressure response of saturated soils around a penetrating object varies depending on the location of the pore pressure measuring point. In this work, the pore pressure response of soils measured during the penetration test is analyzed by the finite element analysis based on the coupled theory of mixtures. It is also experimentally investigated using the Louisiana State University calibration chamber test. From this study, it is found that the pore pressure distribution along a penetration object is higher at the face and lower at the shaft. This phenomenon is typically believed to be due to two different mechanisms of excess pore pressure generation. The first is the shear induced pore water pressure and the second is the compression induced pore water pressure. It is shown that the interaction and accompanying dissipation of the shear-induced pore pressure and compression-induced pore pressure play a major role for the excess pore pressure response at the shaft of the penetrating object.

Slope stability assessment of the Helland Hansen area offshore the mid-Norwegian margin

Numerous large-scale submarine slides affected the continental slope offshore mid-Norway during the Holocene. One of them, the Storegga slide, is located south of the Vøring Plateau, stretching from the North Sea Fan to the Helland Hansen arch. With a volume of 3200 km 3 and an influenced area of 95,000 km 2, the Storegga slide, which occurred about 8250 years before present (BP), represents one of the largest submarine slides worldwide. The slope stability assessment concentrates on the northern sidewall (Helland Hansen area) where gas hydrates are suspected from the presence of a bottom-simulating reflector (BSR) on reflection seismic data and where few slope stability investigations have been performed compared to the Ormen Lange area. The limit equilibrium and finite element methods (FEM) were used for the evaluation of static and dynamic (seismic) stabilities of the dipping seabed. To account for the uncertainties in the soil parameters, a probabilistic approach was applied by coupling the limit equilibrium model with the first- and second-order reliability methods (FORM and SORM) and the Monte Carlo simulation. The finite element simulation for the seismic loading indicates that a strong earthquake (>6.5 Ms) could be a potential trigger for slope failure but only down to 30 m subsurface. Obviously, one needs a significant pore pressure build-up and loosening of the sediments to explain the slide on such gentle slopes. More critical preconditions to failure in the deeper sediments should exist to explain the thickness of the sliding slab, knowing that the northern sidewall is approximately 150 m high. Cyclic loading due to a series of earthquakes could explain the slide, affecting the shearing resistance in a marine clay unit (weak layer) by excess pore pressure generation (drained conditions). The degradation of undrained shear strength with cyclic loading plays a major role in the instability process.

Excess pore pressure and grain crushing of sands by means of undrained and naturally drained ring-shear tests

An investigation of excess pore-pressure generation of a weathered granitic sand, taken from the source area of a typical landslide caused as a result of liquefaction, and a fine silica sand was conducted, in which grain crushing within the shear zone of ring-shear test specimens was examined as the key phenomenon of rapid long-runout motion of landslides. In order to investigate and explain the low average apparent-friction angle, mobilized in a liquidized landslide, speed-controlled ring-shear tests were conducted under undrained conditions on weathered granitic-sand specimens, formed under a wide range of initial void ratios. It was revealed that very small steady-state shear resistances were obtained irrespective of the initial void ratios, which can explain the low average apparent-friction angle. In addition, two series of ring-shear tests on weathered-granitic and fine-silica sands were conducted under naturally drained conditions by keeping the upper drain valve of the shear box open during the tests. The first series of tests was performed under differing total normal stresses, but at the same shear speed, and the second series was conducted at differing shear speeds, but under the same total normal stress. In order to investigate and analyze excess pore-pressure generation and dissipation within the shear zone that is associated with grain crushing, permeability analyses were conducted by passing water through the sample box of the ring-shear test apparatus before and after shearing. In addition, grain-size distribution analyses of samples taken from the shear zone after shearing were carried out. For the weathered granitic-sand samples, a significant change in bulk permeability and large amount of grain crushing were observed. In these tests on the above soil, a considerable reduction of shear resistance, which increased proportionally to the total normal stress and shear speed, were obtained. It was observed that due to grain crushing, finer grains that lowered the permeability of the soil in the shear zone, were formed. It is likely that the decrease in permeability facilitated the generation of high excess pore pressures by reducing the pore-pressure dissipation rate from the shear zone; thus, flow behaviour was exhibited even under naturally drained conditions.

15 m diameter tunnelling under Netherlands Polders

In order to become part of the Trans-European network of high-speed rail links, Netherlands is working on the transport of the future: a high-speed link (HSL) between Amsterdam and the Belgian border, connecting the Dutch capital with Antwerpen, Brussels and Paris. A solution with a bored tunnel was chosen to minimize the line's impact on the characteristic fenlandzGM4PIHNgpiv landscape of the Groene Hart (the Green Heart of Holland, situated between The Hague, Amsterdam, Utrecht and Rotterdam). Seven kilometers long (8.5 km including approaches), under the Dutch polders, the tunnel will allow the line to follow the fastest route between Amsterdam and Rotterdam while leaving the countryside largely undisturbed. Designed as a single tube, with a bi-directional train circulation, this tunnel, almost 15 m diameter was constructed with the help of Aurora, the specially designed world largest confining tunnel boring machine (TBM). Yet the realization of such an impressive tunnel required an accurate and efficient risk management policy. This papers deals with the treatment of one particular risk, due to the specific hydro geological conditions of the project, that is to say boring under polders. Aurora, as a slurry shield-boring machine, generates excess pore pressure (EPP) in the soil during the boring process. This EPP phenomenon is originated by the dissipation of the confining slurry overpressure in the cutter head chamber amplified by an accumulation process taking place in a captive aquifer as in the Groene Hart tunnel. This phenomenon was early identified as a potential risk for the project, as the generation of excess pore pressure may induce local instability in the impermeable peat layer and thus connections between the brackish captive water table and the free water table. As the first aquifer piezometric level is higher than the ground level, there is a risk of flooding the polder situated above the tunnel. Even if the communication between the two aquifers is not sufficient to get to this point, the inflow of brackish water in the free water table is a risk as the lands above the tunnel are mainly intended for agriculture. Therefore, a first measurement campaign was implemented very early in the tunnel progression and showed that the phenomenon was of such a magnitude that the risk could not be neglected. A second instrumented plot was thus implemented, to be able to improve the knowledge and understanding of the phenomenon, qualitatively as well as quantitatively. The plot instrumentation was formed of 11 settlement points, 19 pressure cells and two extensometers. The measurement period extends from February 4th to March 11th (2003) (i.e., they cover the boring of about 600 m tunnel length). The experimental results associated with 3D transient numerical modeling, by allowing a better understanding of the phenomenon, gave the tool to identify the risk level and determine the required mitigation measures. This analyze of the phenomenon proved to be accurate as Aurora passed the identified risk area without any trouble, and the last measurements in the risk area are concordant with the predicted settlement and pore pressure levels. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.

A numerical investigation into effects of single and multiple frequency earthquake motions

During dynamic centrifuge modelling for earthquakes there is a decision to be made about the type of base input motion that should be imparted to the soil model. Motions can either be a tone burst of single frequency or a multi-frequency event simulating a real earthquake. In this paper a series of numerical analyses is reported which endeavours to capture the effects of loading rate on liquefiable soil. Non-linear dynamic finite element analysis in terms of effective stress was carried out using the FE code SWANDYNE. The numerical model was subjected to four types of input motion. Results are presented in terms of excess pore pressure ratios for different frequencies. It was seen that the interplay between frequency content of the seismic motion and generation of excess pore pressure could change the dynamic response of a system. It was concluded that simplicity of the input motion leads to a soil response that is less difficult to analyse.

Interaction mechanism between deep mixing column and surrounding clay during installation

Laboratory and field investigation has shown that the shear strength of the surrounding clay increased in the vicinity of a deep mixed (DM) column in the short period following installation. Field observations indicated that significant excess pore pressure, which may be higher than the hydraulic fracture pressure, was generated during installation.The behaviour between a DM column interacting with the surrounding clay during installation can be simulated as the shearing-expanding process of a cylindrical cavity. A simple approach is proposed to calculate the excess pore pressure around a DM column during installation. The proposed method of calculation considers the effect of a shearing force caused by rotating blades during mixing. In the proposed approach, the excess pore pressure is expressed in terms of undrained strength of clay, shearing force, injection pressure, and a pore pressure parameter. An approach is developed to analyze clay fracturing using a tensile failure mechanism. Analytical results indicate that the rotation of the mixing blades has a significant effect on the generation of excess pore pressure and clay fracturing in a close region around the column. The proposed approach has been verified against laboratory model tests and in situ DM column installation in clay ground at two construction sites. Clay fracturing around a DM column was observed in the laboratory and is analyzed based on field data. This paper reveals that clay fracturing is the major factor behind the strength increase and property changes of the surrounding clay in a short period.Key words: soft clay, deep mixing column, shearing-expanding process, excess pore pressure, clay fracturing, laboratory test, field tests.

Partially-Drained Cyclic Behavior and its Application to the Settlement of a low Embankment Road on Silty-Clay

This study examines the phenomenon of settlement induced by traffic loading on low embankment roads built on soft cohesive soils deposited in a reclaimed lowland. A series of cyclic triaxial tests were conducted on undisturbed soft Ariake silty-clay under undrained and partially-drained conditions. A model based on the test results is proposed to simulate the partially drained behavior of silty-clay under a variety of lateral stresses. It is shown that the maximum excess pore pressure generation ratios and volumetric changes increase until the cyclic stress ratio reaches a value of 0.73, which is independent of the effects of lateral stresses under the same anisotropic consolidation ratio. Numerical analyses applying the proposed model were performed to compare the effects of soil thickness and cyclic loading time during one day and one cycle on partially drained cyclic behavior. It was found that a large ratio of cyclic to non-cyclic load­ing time during one cycle exerts a smaller influence on volumetric strain than the total time of cyclic loading during one day. Observed settlement results of a low embankment road are analyzed to verify the applicability of the proposed model.

A Displacement Prediction Method for Retaining Walls under Seismic Loading.

This paper describes a new calculation method for seismic displacement of retaining walls. A macroscopic failure surface and a plastic displacement potential in the general load space are considered in the method to evaluate the subgrade reaction force from foundation ground. The method is capable of calculating not only horizontal, vertical or rotational displacement alone, but also their combined effect. The method is validated through comparison with centrifuge test results of a gravity retaining wall with dense backfill sand subjected to strong base shaking. The calculated displacement components, that is vertical, horizontal and rotational displacement at the end of shaking, compared well with those measured. It also appeared that previously proposed conventional displacement calculation methods based on Newmark's sliding block analogy might underestimate the displacement. The assumption of the constant frictional coefficient may be responsible for this. The proposed method is limited to retaining walls resting on soils where dramatic degradation of soil strength due to the generation of excess pore pressure does not occur.

Excess pore pressure generation potential within shear zone by means of ring shear tests

Excess pore pressure generation potential within shear zone by employment of the ring shear tests have been investigated on the saturated samples which have quite different grain crushing susceptibility. In naturally drained tests, the sample the grains of which are crushable showed the great reduction of apparent internal friction during shearing to generate rapid flow phenomena. Based on the results of permeability and grain size analyses, it is estimated that the crushed particles formed the less-permeable shear zone and excess pore pressure generation rate exceeded the dissipation rate to yield flow behaviour, and it occurred heavily in high shear speed or normal stress tests. And to express the susceptibility of the samples against flows, the new parameter “potential for rapid flow phenomena” was proposed as the ratio of internal friction angle (φm) to apparent friction angle (φa).In order to estimate the excess pore pressure in undrained condition from the drained tests, the concept of “equivalent normal stress” was introduced as the ratio of volumetric strain to the compressibility. Equivalent normal stress using compressibility which was obtained by changing normal stress during shearing was almost equal to excess pore pressure at the same shear displacement up to 1 m at which steady state was reached. And Excess pore pressure ratio in undrained tests and the logarithm of equivalent normal stress divided by normal stress with compressibility obtained during consolidation process showed the linear relationship.

Experimental Study of the Compaction of Phyllosilicate-Bearing Sand at Elevated Temperature and with Controlled Pore Water Pressure

Relative to the time-dependent compaction behavior of clean quartz sands, little experimental work has been carried out on phyllosilicate-rich siliciclastic rocks (mudstones). Compaction is expected to be produced by intracrystalline plastic deformation of the phyllosilicate matrix, coupled with pressure solution of embedded quartz grains and with silica reprecipitation in the adjacent pore spaces or loss from the system via an advective flux of pore water. Time dependence of compaction also arises in nature owing to the generation of excess pore pressure, which lowers effective stress and hence compaction rate until such pressure excess becomes dissipated by permeation. A suite of experiments was performed on synthetic mixtures of quartz sand in a matrix of fine grained illite + muscovite, hydrostatically compacted at a range of effective pressures between 10 MPa and 210 MPa, with a constant pore water pressure of 70 MPa and at temperatures ranging between 300 and 450°C. A small adjacent volume of porous quartz sand was provided as a precipitation sink for silica dissolved from the phyllosilicate + quartz mixture. Volume strains were measured by (a) direct measurement of sample length and (b) measurement of amount of pore water expelled at constant pore pressure, and interpreted with the aid of quantitative petrography on compacted samples. Control experiments were performed on (i) dry, clean sand, (ii) dry sand + phyllosilicate, and (iii) wet, clean sand. Dry, clean sand showed almost no compaction. Wet, clean sand showed a degree of compaction similar to dry phyllosilicate + sand, but wet phyllosilicate + sand showed dramatically more compaction. Compaction rate φ for the wet, mixed phyllosilicate/sand samples is described by ![Formula][1] in which σ is effective stress (MPa), φ is porosity (%), R is the gas constant, and T is temperature (K). Petrographic study showed interpenetration of grain contacts, formation of quartz overgrowths, a decrease in the amount of quartz embedded in the phyllosilicate matrix, and an increase in mean grain size of quartz with progressive compaction. These effects are attributed to solution and redeposition of quartz. The empirical compaction law was integrated over a natural burial history. The porosity evolution is extremely sensitive to the evolution of pore fluid pressure and relatively less to temperature gradient and burial rate. There is reasonable qualitative agreement when compared with porosity/depth relations reported in the literature. [1]: /embed/graphic-1.gif