Showing papers on "Pore water pressure published in 1973"

Porosity dependence and mechanism of brittle fracture in sandstones

Abstract: Brittle fracture tests of 105 fine-grained quartz arenites were conducted at 25°C, 1.0-kb confining pressure, a constant strain rate of 6.5 × 10−5/sec, and pore pressure ranging from 0 to 750 bars. Orientation of planar anisotropy (bedding or cross-bedding) with respect to principal stresses has little influence on the fracture strength. The Donath orientation effect depends on rock type. Strong dependence of fracture strength on porosity is of the form y = axb (where y equals stress difference at failure, x equals porosity, and a > 0 > b; in our samples, values for a ranged between 16 and 25 kb, and b between −0.8 and −1.0). Through-going shear fractures result from coalescence of grain boundary cracks, extension fractures within grains, and void space. Rocks with low porosity develop through-going shears only after many grains are extension fractured. The functional relationship between porosity and fracture strength derives from the lower energy required for propagating cracks to use void space rather than forming extension fractures.

Premonitory Changes in Seismic Velocities and Prediction of Earthquakes

Abstract: Large premonitory changes in ratio of seismic P and S velocities correlate with time of occurrence and size of earthquakes at Blue Mountain Lake, New York. These changes may be related to the opening of cracks or to changes in pore pressure.

pH BUFFERING OF PORE WATER OF RECENT ANOXIC MARINE SEDIMENTS

Abstract: A model is proposed to explain the relative pH stability in pore water of recent anoxic marine sediments. Oceanic data are not inconsistent iwth a model which assumes that the pH of the pore waters is controlled by the byproducts of organic decomposition, sulfate reduction, and precipitation of sulfide and carbonate. The model predicts that the pH of pore waters should remain in the range 6.9 to 8.3, which is in agreement with measured values.

Note on effective pressure

Abstract: When a pore pressure Pp and a confining pressure Pc are applied to a porous solid, various physical properties are affected. When dealing with these properties, the concept of effective pressure is often used. On the basis of the examples of pore volume and of bulk volumetric strain for several elastic solids, we emphasize that (1) there is no unique law of effective pressure, (2) effective pressure may not be a very useful concept, unless it has the simple form Pe = Pc − Pp, and (3) when a property does not vary linearly with pore or confining pressure, effective pressure does not in general have any simple analytic expression, unless again it reduces to Pe = Pc − Pp.

Interstitial Water Composition and Geochemistry of Deep Gulf Coast Shales and Sandstones

Abstract: Interstitial water from shales and sandstones shows a contrast in concentration and composition. Sidewall cores of shales were taken every 500 ft between 3,000 and 14,000 ft in a well in Calcasieu Parish, Louisiana, which encountered abnormally high fluid pressures just below 10,000 ft. Significant differences between the total dissolved solids concentrations in waters from normally pressured sandstones (600-180,000 mg/l) and highly pressured sandstone (16,000-26,000 mg/l) were noted. Shale pore water has a lower salinity than the water in the adjacent normally pressured sandstones, but the concentrations are more similar in the high pressure zone. Shale water generally has a concentration order of SO4 = > HCO3- > Cl-, whe eas water in normally pressured sandstone has a reversed concentration order. Conversion from predominantly expandable to non-expandable clays accelerates near the top of the high pressure zone, which appears correlative with a major temperature gradient change, an increase in shale porosity (decrease in shale density), a lithology change to a massive shale, an increase in shale conductivity, an increase in fluid pressure, and a decrease in the salinity of the interstitial waters. The data presented suggest that the clays subjected to diagenetic change release two layers of deionized water and that this released water may be responsible for the lower salinity of the water found in the high pressure section.

The influence of an undrained change in stress on the pore pressure in porous media of low compressibility

Abstract: The author describes some earlier work directed at predicting and measuring the pore pressures that are set up by changes in the state of stress of soils, under undrained conditions. He shows how he has derived from first principles an expression of pore porosity ratio in terms of porosity, compressibility of the pore water, compressibility of the soil grains and compressibility of the soil skeleton. It is indicated how the same expression can be derived from Geertsma's equation relating the change in volume of pore space to the changes in total stress and pore pressure. It is stated that the methods of analysis used by Gassman led to conclusions that were consistent with the author's and Geertsma's findings. This is followed by a discussion of the principle of effective stress and, in particular, the importance of using the excess of total stress over pore pressure in the calculations rather than the effective stress relating to volume change. The author then discusses the predicted range of values of pore pressure ratio and concludes with a derivation of the pore pressure parameter as defined by Skempton.

Pore pressure changes and the delayed failure of cutting slopes in overconsolidated clay

Abstract: Failure of slopes cut in overconsolidated clay may be delayed by the rate of equilibration of pore pressure after excavation. Other mechanisms, such as decrease in drained strength with time, have been suggested as reasons for delayed failure. Previous studies of cutting slopes in the London Clay have suggested that equilibration of pore pressure is relatively rapid and that other mechanisms control eventual failure. Recent observations of pore pressure in a cut slope nine years after excavation suggest that equilibration of pore pressure may be the principal mechanism controlling delayed failure in the London Clay. The studies made of failures in London Clay slopes have played a significant part in the general study of long-term failure of cut slopes in clay, so this conclusion has some general implication. La rupture des talus decoupes dans de l'argile surconsolidee peut etre retarde par la vitesse d'equilibrage de la pression interstitielle apres excavation. D'autres mecanismes tels que la reduction de...

Effects of air pressure on water flow in an unsaturated stratified vertical column of sand

Abstract: By the use of a vertical stratified column of unsaturated soil the effects of air pressure on constant flux infiltration and gravity drainage were studied experimentally. Water content was measured by γ ray attenuation; water pressures were obtained simultaneously at 10 different levels in the column by using tensiometers connected to pressure transducers. In addition, the differential ports of some transducers were connected by a ‘scanivalve’ either to the external atmosphere or to the soil air phase by using hypodermic needles inserted through the column wall. The soil column consisted of three layers: fine sand over coarse sand over fine sand. The wall of the column was also provided with a series of adjustable air ports to allow experiments either with or without lateral air movement. It was shown that (1) the local soil air pressure can differ significantly from the external atmospheric pressure, e.g., +50 mb for rain with an intensity of 3 cm/hr or −15 mb for gravity drainage, and (2) the water flow depends not only on the boundary conditions in terms of water content and/or water pressure but also on the air pressure conditions. We conclude that the air pressure must be taken into account in determining the soil water suction, and we suggest that the flow equations must be written in terms of two-phase immiscible fluid flow.

Lüders' Bands in Experimentally Deformed Sandstone and Limestone

Abstract: Planar deformation features inclined along planes of high shear stress and along which cataclasis is concentrated are here called “Luders9 bands.” In Coconino Sandstone (deformed dry and with pore pressure at effective confining pressures to 2.4 kb, room temperature, and at a strain rate of 10 −4 per sec) the bands begin to develop in the transitional regime and are the major deformation feature in the macroscopically ductile regime. At axial shortenings of 5 percent and more they pervade the specimen and become closer spaced and thicker with increasing strain. The bands are formed by two or more layers of moderately to highly fractured quartz grains. They are markedly different from shear fractures (or faults) of similar size that typically contain quartz gouge. The average angle between conjugate sets of bands bisected by the greatest principal compressive stress σ 1 increases with effective confining pressure from 75° to 109°. The corresponding angles between conjugate macroscopic shear fractures average 60°. The angle between Luders9 bands is essentially independent of strain at fixed effective confining pressure. In Solenhofen Limestone (deformed dry at confining pressures to 3.0 kb, 24°C, and strain rates from 10/sec to 10 −4 /sec) the Luders9 bands are developed only in the outer shell of the solid cylinders; however, in coarser grained limestones, the bands are pervasive as in the sandstone. In cylinders of Solenhofen, the bands are best developed in the transitional regime, as noted previously by Heard (1960). They do not form in the ductile regime. The average angle between conjugate sets is independent of strain and strain rate; but as for the sandstone, it increases with confining pressure from 75° to 103°, and it is at least 20 degrees larger than the corresponding angle between conjugate shear fractures which form after the Luders9 bands. Optical and SEM studies indicate that the features in both rocks are zones of intergranular and intragranular cataclasis, along which shear displacements are negligible. Because the angle between conjugate Luders9 bands is a function of the effective pressure, the bands might be used to derive depth of burial at time of deformation, provided that (1) the pore fluid pressure is assumed to be hydrostatic, and (2) the orientation of CTI, which can be the acute or obtuse bisector, is independently known. If (1) is unwarranted, then the angle between the bands could be used to infer the pore fluid pressure, provided the depth of burial is known. This approach is illustrated with reference to an occurrence of Luders9 bands in naturally deformed Entrada Sandstone, Trachyte Mesa, Utah.

Chemico-Osmotic Effects in Fine-Grained Soils

Abstract: A theory to describe the simultaneous coupled diffusional flow of salt and water in soils is summarized. Examination of the coupling effects indicated by the theory and a computer analysis of consolidation caused by chemico-osmotic coupling shows that the effects of coupling should increase as soil void ratio decreases and soil compressibility and salt concentration differences increase. Test results have shown that chemico-osmotic effects do not exist, and the rates of solution flow are in reasonable accord with theoretical predictions. Chemico-osmotic consolidation is likely to be small for most soils, except for very fine-grained active clays like bentonite. However, these and other studies have shown that chemico-osmotic coupling is capable of moving soil pore water and dissolved salts and that chemico-osmotic soil stabilization might be feasible in certain cases.

Undrained shear testing of jointed rock

Abstract: Undrained Shear Testing of Jointed Rock Water pressures must change inside joints undergoing shear without perfect drainage. This paper describes a new direct shear machine in which jacketed samples with oriented joints can be tested under consolidated undrained conditions with pore pressure measurement. Triaxial compression techniques for such tests are also described and typical results with intact and jointed sandstone samples are compared with results from the direct shear tests. Whereas intact specimens displayed increasing pore pressure followed by dilatancy and pore pressure decline before peak loading, the pore pressure in jointed specimens continued increasing right up to the peak load.

Pore pressure changes during creep events on the San Andreas Fault

Abstract: A 152-meter-deep well was drilled into the San Andreas fault zone near Hollister, California, to examine the possibility of any variations in fluid pore pressure during fault creep movement. Anomalous water level changes were recorded that coincide, within hours, with the only creep episodes recorded at a nearby site. The ratio of maximum water level change to total creep offset observed is 14 mm/mm. The calculated change in pore pressure for creep events is about 20 mb. Assuming 1.2 cm of total creep displacement per year along this section of the San Andreas fault, we obtain a total stress drop of 60 mb/yr associated with creep at shallow depths. The results are most encouraging, indicating the possibility that deeper wells could be used to monitor stress changes along active fault zones.

Excess pore pressures during undrained clay creep

Abstract: The objective of this presentation is to examine experimentally how the excess pore-water pressure is related to the mechanism for undrained creep of San Francisco Bay mud. The results are discusse...

Hydrofracture mechanisms in rock during pressure grouting

Abstract: Hydrofracture Mechanisms in Rock During Pressure Grouting. The paper examines the basic meachnisms controlling the initiation of fractures in rocks and layered soils during pressure grouting, and their subsequent propagation into the ground mass. Previous analyses of fracture initiation have tended to concentrate on simplified models in which the ground is treated as an impervious elastic or Mohr-Coulomb continuum. The present method allows for the porous or fissured nature of the ground by considering the effect of seepage forces induced by the pore pressure gradient. The effect is quantified by use of a parameterN such that the ratio of fluid force used in expanding the injection hole, to that used in forcing fluid through void spaces, isN to (1—N).

Elasticity of water‐saturated rocks as a function of temperature and pressure

Abstract: Compressional and shear wave velocities of water-saturated rocks were measured as a function of both pressure and temperature near the melting point of ice to confining pressure of 2 kb. The pore pressure was kept at about 1 bar before the water froze. The presence of a liquid phase (rather than ice) in microcracks of about 0.3% porosity affected the compressional wave velocity by about 5% and the shear wave velocity by about 10%. The calculated effective bulk modulus of the rocks changes rapidly over a narrow range of temperature near the melting point of ice, but the effective shear modulus changes gradually over a wider range of temperature. This phenomenon, termed elastic anomaly, is attributed to the existence of liquid on the boundary between rock and ice due to local stresses and anomalous melting of ice under pressure.

An experimental study of vertical infiltration into a structurally unstable swelling soil, with particular reference to the infiltration throttle

Abstract: The infiltration phenomena associated with a structurally unstable swelling soil are compared with those of a two-layer stable system of a fine sand layer over coarse sand, the fine sand simulating a slaked layer at the soil surface. Water content and bulk density are measured using dual source gamma ray attenuation, pore water pressures by means of individual tensiometer-transducer systems, and soil temperatures by means of individual thermistor-bridge systems. Analysis of the sand column using well-established principles shows that after the wetting front has passed the texture boundary, infiltration is controlled by Kmax of the fine sand layer and the negative moisture potential in the coarse sand at the texture boundary. After the wetting front penetrates the column, the moisture potential at the texture boundary becomes steady and is unaffected by the development of a capillary fringe and outflow at the base of the column. The negative moisture potentials at the texture boundary give rise to potential gradients up to 6.0 in the simulated slaked layer, and an infiltration rate several times that of Kmax. The low flow rates caused by the fine sand layer give rise to an unstable wetting front in the coarse sand and severe 'fingering' occurs. In the swelling soil column, with aggregates of the same size as the coarse sand, the infiltration throttle occurs immediately below the visibly slaked layer and not at the ground surface. Potential gradients through the throttle reach a maximum of 5.9 similar to that in the layered sand column, but the infiltration behaviour of swelling soil differs from the latter in other respects. Infiltration into the former does not occur under isothermal conditions, a 'hot front' 3°C above ambient occurring 2-3 mm ahead of the wetting front, and infiltration does not reach a constant rate because of changes in the hydraulic properties of the throttle with time. Moisture profiles in the swelling soil column during infiltration show the various zones described by Bodman and Colman (1944) for non-swelling soils. An enlarged apparent transition zone extend to 12 cm below the soil surface. Other properties such as density, moisture content, and total potential suggest that much of this apparent transition zone is really part of a transmission zone made up of layers of soil which have different properties because of swelling.

Role of Pore Fluids in Faulting

Abstract: We consider three in situ processes which involve fluid flow in the crust: fault creep, aftershocks and dilatancy. Measurements of water level in wells suggest that creep events on the San Andreas fault are coupled with pore pressure changes. Readjustment of transient pore pressure, induced by large shallow earthquakes, possess the correct time constants and magnitudes to explain the occurrence of aftershocks. And finally, temporal changes of travel times in the Gram district (U.S.S.R.) imply that dilatancy may occur in situ.

Foundation failure of new liskeard embankment

Abstract: A CASE HISTORY OF AN EMBANKMENT FOUNDATION FAILURE IN VARVED CLAY IS PRESENTED. STABILITY ANALYSES BY TOTAL STRESS, PARTIAL TOTAL STRESS, EFFECTIVE STRESS, AND FINITE ELEMENTS HAVE BEEN OBTAINED AND ARE DISCUSSED. ATTENTION IS FOCUSED ON THE PERFORMANCE OF THE STIFF CRUST, THE SELECTION OF THE SOIL PROPERTIES, AND THE PLACEMENT OF THE INSTRUMENTATION. THE SITE GEOLOGY AND SOIL PROPERTIES OF SAMPLES TAKEN WITH A COMMERCIAL SAMPLE TUBE ARE DISCUSSED. THE INSTRUMENTATION TO MEASURE PORE RESPONSE (ILLUSTRATED) WAS PLACED ON THE EMBANKMENT SITE A YEAR PRIOR TO CONSTRUCTION. FIGURES ILLUSTRATE THE RESPONSE MEASURED IN THE PIEZOMETERS. THE METHOD USED IN THE TOTAL STRESS AND PARTIAL TOTAL STRESS ANALYSIS IS DESCRIBED. DETAILS ARE GIVEN OF EFFECTIVE STRESS ANALYSIS AND FINITE ELEMENT TOTAL STRESS ANALYSIS. CONCLUSIONS DRAWN FROM THE EXPERIENCE ARE PRESENTED. THE NEAR FAILURE EXCESS PORE PRESSURES IN THE FOUNDATION MATERIAL MAY BE CALCULATED WITH SUFFICIENT ACCURACY FOR PRACTICAL PURPOSES FROM A BOUSSINESQ STRESS DISTRIBUTION INCREASE AND SKEMPTON'S PORE PRESSURE COEFFICIENTS. THE ASSIGNMENT OF SOIL STRENGTHS TO THE CRUST MATERIAL IS SEEN AS THE MAJOR PROBLEM IN USING UNDRAINED TEST RESULTS IN A TOTAL STRESS ANALYSIS. BOTH ACTUAL FAILURE AND THE ESTIMATED FAILURE CIRCLES WERE FOUND TO BE VERY SHALLOW, EXTENDING GENERALLY TO A DEPTH LESS THAN 19.7 FEET (6 M). THE USE OF THE MOST REALISTIC EFFICTIVE STRESS ANALYSIS PARAMETERS (I.E. WITH UNDRAINED EXCESS PORE PRESSURES THROUGHOUT THE MASS) RESULTED IN A VERY SHALLOW, UNREALISTIC SLIP CIRCLE ALTHOUGH THE FACTOR OF SAFETY WAS ONLY SLIGHTLY TOO LARGE. THE FINITE ELEMENT ANALYSIS WAS FOUND TO BE USEFUL IN PREDICTING THE FAILED AREA BY USING A PSEUDO TOTAL STRESS ANALYSIS, PROVIDED MULTILINEAR STRESS- STRAIN PROPERTIES WERE USED.

The effect of height of sample and confinement on the moisture characteristic of an aggregated swelling clay soil

Abstract: The pore pressure/moisture content relationship of 0.5-1.0 mm aggregates of Narrabri soil, initially 1.66 cm high, was examined at 1/3 cm height intervals by means of dual source gamma ray scanning, for confined and unconfined conditions. In the unconfined sample, absorption and desorption of water to or from the interaggregate pore space occurred at less negative pore water pressures than in the confined sample. In the latter, the swelling aggregates deformed into the inter-aggregate pore space during absorption and reduced the size of the pores. In the unconfined sample, absorption into the inter-aggregate pore space occurred at less negative water pressures with increasing height in the sample. During absorption, bulk density decreased with height in the sample, and bulk density at all points within the sample decreased until pore water pressures were between -20 to - 10 cmH2O. At pore water pressures closer to zero, bulk density increased as the sample compacted under its own weight. During desorption, bulk density increased at all heights as the pore water pressures decreased from zero to - 8 cmH2O, because of isotropic stress imparted by the hanging water column. During this phase normal shrinkage occurred, therefore overburden potentials, and hence moisture potentials, could be calculated. Between - 10 and - 50 cmH2O, the inter-aggregate pore space drained rapidly, and no further increase in bulk density was detectable and hence moisture potential equates to manometric pressure. It was found that the derived moisture characteristics (O versus e) are different for different heights in the aggregate mass. This result is of consequence to the theory of hydrostatics in swelling media. In the unconfined sample, no differences in the pore water pressure/moisture content relationships occurred with height during absorption or desorption, and bulk density remained constant throughout. The application of these results to the behaviour of swelling soils in the field is briefly discussed. The use of dual source gamma ray scanning seems an essential tool for further investigations.

Frost susceptibility of massachusetts soils-evaluation of raid frost susceptibility tests

Abstract: Two promising approaches to the development of a rapid laboratory procedure for evaluation of frost susceptibility of soils are investigated in detail. Measurement of pore water pressure change during freezing of soil in a closed system under constant temperature gradients is shown to be extremely sensitive to experimental technique so that its usefulness as a rapid routine test is limited. Heave pressure measurement during the freezing of soil in an open system, at essentially constant volume under constant temperature gradients, is shown to be a simpler, faster and less sensitive approach to experimental technique than the pore water pressure approach. The heave pressure method is therefore recommended. Evaluation of frost susceptibility is based upon the characteristics shape of the heave pressure of pore pressure versus log time curves. Once a constant temperature gradient has been established, the characteristic curve shows a straight line segment. The slope and the duration of the straight line segment is used to rate the continued relative frost susceptibility of soils. To effectively apply this laboratory procedure, data must be collected relating results from this test with detailed field frost susceptibility data. /FHWA/

Main requirements imposed on the quality of skeletal clay cores of rock-fill dams

Abstract: 1. Skeletal clay soil should be placed in the cores of high dams, with a water content which provides solid consistency of its fine portion (fractions with a diameter less than 2–5 mm). 2. Since the range of variation of the plastic limit of the fine portion can be close to the plasticity index, special attention must be given to elimination of the possibility of the fine portion, or some part of it, changing to a fluid-plastic state during construction or operation, which promotes undercompaction of the ocil, the occurrence of pore water pressure, delay of consolidation of the core, and occurrence of long-time deformations. 3. A comparison of the calculated values of the density and water content of the fine portion of skeletal clay soil of the core of the Nureksk dam with the results of laboratory investigations and an analysis of foreign experience in compaction showed that the main requirements imposed on the density-water content of soil are met in the case of its impact consolidation in the laboratory by work of 28,000 g-cm/cm3 according to the modernized AASHO method. 4. The parameters of density and water content of skeletal clay soils established by calculation and laboratory experiments should serve as the basis when selecting the optimal technological process of compaction, type of roller, and vehicles for delivering the soil to the dam core.

Tensiometer Use in Shallow Ground-Water Studies

Abstract: Tensiometers installed with a ground surface or other reference can be used to determine ground-water equipotentials and the position of the water table in the field. Tensiometer determined water tables were roughly comparable to those measured with perforated wells in a sprinkler irrigated Coachella fine sandy soil. Water differences depended on the flow conditions and proximity of the tensiometers to a drain. Shallow ground-water equipotentials can be determined above and below the water table with a tensiometer system.