摘要:

A model to predict the hydraulic conductivity of consolidated clay, simulating clay liners compacted wet of optimum, is presented. The concept that clays exist as clusters and the electrical double layer theory are used to predict the hydraulic conductivity of clays for permeants of known composition. The model relates the physical properties of clays, such as its surface area, with the overburden pressure and the concentration of ions in the permeant. The model can be used to predict the hydraulic conductivity of bentonitic clays with monovalent as well as divalent exchangeable cations. The model is valid within the range of applicability of the Gouy-Chapman electric double layer theory. The variation in the number of clay particles per cluster with the consolidation pressure and concentration of ions in the permeant has been discussed. The model has been calibrated and verified using published experimental data. However, the model in its present form is valid only for homoionic clays and permeants with the same valency. With an increase in concentration of ions in the permeant, the precision of the model has been found to decrease.Key words: clay, clusters, hydraulic conductivity, double layer, model, permeant, concentration.

ISSN:0008-3674    

DOI:10.1139/t99-052

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Laboratory model tests have been carried out using a rigid strip footing supported on dense sand overlying soft clay with and without a layer of geotextile reinforcement at the interface. The study aimed at determining the effect of geotextile reinforcement and the thickness of a sand layer on the ultimate bearing capacity and settlement characteristics of the footing resting on a granular fill - soft soil system. It was found that the bearing capacity increases with an increase in the ratio of sand thickness to footing width until it reaches a critical value, which can be considered as the optimum limit of improvement of the bearing capacity of the layered soil. The installation of a geotextile reinforcement at the interface resulted in an appreciable increase in bearing capacity and decrease in settlement of the footing. The optimum thickness of the sand layer for a geotextile-reinforced foundation was found to be 0.8 times the width of the footing, which was significantly lower than that of an unreinforced foundation. The results of the laboratory model tests were validated by a comparison with the results of a finite element analysis. The results obtained using the finite element model compared well with data obtained from the laboratory tests. Additional parametric study was carried out by the finite element model to supplement the results of the laboratory model tests. Design recommendations are given based on the results of the finite element model and laboratory model studies for a rigid footing supported on a reinforced granular fill - soft soil system.Key words: model tests, footing, bearing capacity, granular fill, clays, finite elements, geotextiles.

ISSN:0008-3674    

DOI:10.1139/t99-053

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

J.B. Burland proposed that the void index aids in assessing the compressibility of various sedimentary clays. The extensive data on sensitive Ariake clays are used in this study to investigate the compression behavior using the void index. The analysis results indicate that the compression curves of remolded Ariake clays can be normalized by the void index proposed by Burland. It is found, however, that most natural Ariake clays lie above the sedimentation compression line. Salt removal and cementation are considered to be the most probable causes for such a phenomenon for Ariake clays. Furthermore, a relatively good relationship is found between the in situ void ratio of Ariake clays and the effective overburden pressure. The relationship can be expressed by a straight line in a bilogarithmic plot, herein designated field state line (FSL) for Ariake clays.Key words: Ariake clays, compression curve, intrinsic compression line, field state line, oedometer test, sedimentation compression line, void index.

ISSN:0008-3674    

DOI:10.1139/t99-058

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Clay barriers normally function at hydraulic gradients much lower than are commonly used in laboratory tests for hydraulic conductivity. This paper describes low-gradient tests on compacted illite, Na-bentonite, and sand-bentonite specimens at a range of dry densities. The tests examined the effects of deionized or saline pore fluid, the initial degree of saturation at the time of compaction, and back-pressuring to achieve saturation during permeation. No "critical" or "threshold" gradients were observed. In low-density materials, "transitional" gradients defined two separate regions of Darcian flow. Low flow rates and low hydraulic conductivities were associated with hydraulic gradients below the transitional gradient. Higher conductivities were associated with gradients above the transitional gradient. Hydraulic conductivities decreased with increasing initial density. They appeared independent of initial degree of saturation or back-pressuring. Tests on Na-rich bentonite showed that deionized and saline permeants produced similar hydraulic conductivities. Measured hydraulic conductivities were in reasonable agreement with values computed using the Poiseuille and Kozeny-Carman equations.Key words: hydraulic conductivity, gradient, Darcian, illite, bentonite.

ISSN:0008-3674    

DOI:10.1139/t99-057

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Laboratory experiments were performed to assess the contribution of resuspension to the oxidation of pyrrhotite mine tailings flooded with a shallow water cover (up to 80 cm). Resuspension was produced by a paddle rotated at prescribed speeds in the water cover. The quality of the water cover was monitored over a 126 day period and the results were compared to the case where the tailings were kept still (no resuspension). The results showed that resuspended tailings oxidized considerably more than tailings at rest. The pH and dissolved oxygen concentration of the stirred water covers were 2.6 and 14 times lower, respectively, than those of the static water covers where there was no resuspension. Sulphate production increased by about 26-64 times, while the release of metals such as zinc, copper, and nickel was 1020, 318, and 138 times greater, respectively. The suspended tailings were finer and had lower pyrrhotite content at the end of the experiments than the tailings at rest and the original tailings. Tailings oxidation and metal release increased with increasing stirrer speed and decreasing depth of water cover. These findings pertain to laboratory conditions and the water-quality data cannot necessarily be extrapolated to the field, except in a general sense.Key words: acid generation, oxygen flux, mass transfer, precipitation, ripples, hydraulic sorting.

ISSN:0008-3674    

DOI:10.1139/t99-044

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

An experimental soil cover constructed near London, Ontario, 23.2 m × 15.2 m in plan area, has been monitored for 2 years for percolation and water-content data. The cover was a multilayer system consisting of a compacted till barrier soil placed between evaporation and drainage barriers of sandy gravel. Half of the cover was capped with coarse stone to prevent erosion and the other half was covered with topsoil to facilitate revegetation. High percolation rates and substantial desiccation of the barrier soil were reported under the topsoil protection layer. Unsaturated-saturated liquid and vapour flow modelling shows that the topsoil may act as a capillary barrier to infiltration, thereby promoting desiccation of the underlying compacted till. Percolation through the cover was measured using lysimeters filled with 5-16 cm (2-6 in.) diameter stone placed directly underneath the compacted till. Though the field lysimeters report water and laboratory tests show no evidence of flow partitioning between coarse stone and gravel, two-dimensional unsaturated-saturated liquid flow modelling of the lysimeter-cover interaction predicts that the lysimeters should not report significant water. Phenomena not simulated by the modelling, which include macrostructure flow in the barrier soil, trickle flow in the coarse stone, and vapour diffusion, are investigated and discussed. Consideration of vapour flow yields results that are inconsistent with the predictions of the liquid flow modelling.Key words: capillary barrier, lysimeters, soil covers, unsaturated flow, vapour flow

ISSN:0008-3674    

DOI:10.1139/t99-046

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Hydraulic fracturing can occur in the clay core of an earth and rock-fill dam if the vertical effective stress in the core is reduced to levels that are small enough to allow a tensile fracture to occur. This situation may arise if the total stress in the core is reduced by the "arching effect" where the core settles relative to the rock-fill shoulders of the dam. Water pressure increases in the core which occur on first impounding of water will reduce effective stresses further, and if they reach low enough values, a fracture will occur. The design of earth dams to resist hydraulic fracture is therefore of great importance (especially those dams with thin vertical or near-vertical central cores), as there have been several dam failures in the past that have been attributed to hydraulic fracture. This paper presents a method of predicting hydraulic fracture in the core of earth and rock-fill dams by using a numerical procedure based on the finite element method. The finite element procedure makes use of special joint elements that allow fluid flow and fracture to be modeled and is an advance over previous methods in that it allows the complete history of pore-pressure development in the core of a dam to be simulated. A study of the behaviour of the Hyttejuvet Dam, which was thought to have failed due to hydraulic fracturing, is also carried out, and the results of the analysis suggest that the failure of the dam was probably due to hydraulic fracturing that occurred during first filling of the reservoir. The fractures predicted occur at about the location that the actual fracture was thought to have been located.Key words: hydraulic fracture, earth and rock-fill dams, finite element method.

ISSN:0008-3674    

DOI:10.1139/t99-049

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

The evaluation of critical state strength parameters is important, especially with the introduction of limit state design. The modified Cam clay (MCC) model is often used, but it is suitable mainly for evaluating the critical state strength parameters from triaxial compression tests on isotropically consolidated soils. The initial stress condition of a natural soil is usually anisotropic, and the stress paths imposed by external loading could deviate from that of a simple triaxial compression. The use of MCC in practice deserves careful consideration. This paper describes a proposed extension of the MCC model for the evaluation of critical state strength parameters from undrained triaxial and plane strain tests on anisotropically consolidated clays. Using Lade's failure criterion and the plastic potential of MCC, a generalized yield surface is determined and the relationships between the critical state internal friction angles from various triaxial and plane strain tests are obtained. By relating the isotropic overconsolidation ratio to the conventional overconsolidation ratio (OCR), a procedure is suggested for the prediction of critical state undrained shear strength (su) for clays. The undrained strength normalized by the preconsolidation pressure is not a constant but a function of the angle of internal friction and the OCR. For overconsolidated clay, the predicted undrained strength ratio (su/&sgr;VO) for the triaxial compression condition is larger than that for the plane strain compression condition. Comparisons of predicted results with published data indicate that the procedure is applicable to various compression tests on normally and lightly to moderately overconsolidated clays.Key words: stress anisotropy, clay, constitutive model, critical state, laboratory test, shear strength parameters.

ISSN:0008-3674    

DOI:10.1139/t99-050

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

This paper attempts to evaluate the undrained shear strength of sand during flow failures, based on both laboratory testing and field observations. In the laboratory, the minimum shear resistance during monotonic loading was taken as the undrained strength, based on the criterion of stability. Triaxial compression, triaxial extension, and simple shear test data on clean sand were examined and it was revealed that the undrained shear strength ratio could be related to the relative density of the material provided that the initial stress,piprime, was less than 500 kPa. Three previous flow failures involving sand layers with relatively low fines contents and reliable cone penetration test (CPT) data were studied. Using existing calibration chamber test results, the Toyoura sand specimen densities in the laboratory tests were converted to equivalent values of CPT penetration resistance. The undrained shear strengths measured in the laboratory for Toyoura sand were compared with those from the case studies. It was found that the behaviour of sand in simple shear in the laboratory was consistent with the field performance observations. Triaxial compression tests overestimated the undrained strengths, and triaxial extension tests underestimated the undrained strengths. From both the simple shear test result and the CPT field data, the threshold value of clean sand equivalent cone resistance for flow failure was detected. Based on these observations, a CPT-based guideline for evaluating the potential for flow failure of a clean sand deposit is proposed.Key words: liquefaction, flow, laboratory testing, in situ test, case histories.

ISSN:0008-3674    

DOI:10.1139/t99-047

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

When sandy soils respond in a strain-softening manner to undrained loading, an estimation of the resulting undrained shear strength (Su) is required to determine the potential for flow liquefaction at a given site. One of the most commonly used methods for estimating the undrained strength of liquefied sand is an empirical standard penetration test (SPT) based chart (originally proposed by H.B. Seed), which was developed using a number of case histories. The original interpretations of these case histories are viewed by many workers and regulatory agencies as the most authoritative measure of the liquefied strength of sand. Consequently, in comparison, other less conservative methods are generally held in an unfavourable light. This paper reexamines the original database of case histories in view of some more recent concepts regarding soil liquefaction. The objectives of this paper are to explore and reassess the issues involved in the original assessment and to offer alternative views of the case records. The conclusions presented here indicate that alternative explanations of the liquefied strength of sand are not inconsistent with the original case histories.Key words: sandy soils, soil liquefaction, undrained strength, standard penetration test (SPT).

ISSN:0008-3674    

DOI:10.1139/t99-048

出版商:NRC Research Press    

年代:1999

数据来源: NRC