The finite element method has been used to analyze two geogrid reinforced embankments on Muar clay deposit in Malaysia. One embankment is with a berm and the other is without a berm. In finite element modelling, two different soil/reinforcement interaction modes, i.e. direct shear and pullout have been considered. The large deformation phenomenon has been also approximately simulated. Comparing the finite element results with the field data in terms of excess pore pressures, settlements, and lateral displacements, the performance of the stage constructed reinforced embankment can be systematically simulated using the finite element method. The reinforcement tension force and soil/reinforcement shear stress mobilization process aud distribution pattern, the soil/reinforcement interaction mode (direct shear or pullout), and the effect of reinforcement on embankment stability are also analyzed.

View full abstract

The paper deals with the prediction of lower bound bearing capacity of smooth (surface) and rough (embedded) strip footings based on finite elements and nonlinear programming. For embedded footings the influence of mesh pattern on the solution has been highlighted. The analysis is conducted for different values of the dimensionless parameter G(γB/2C) and φ values. The results so obtained are compared with the reported limit analysis, limit equilibrium and finite element solutions. For φ=0 case the predictions using the present analysis are in very close agreement with the available solutions, but as φ increases the difference from the known solutions also increases.

View full abstract

Long-term loading tests were performed on newly developed short friction piles during the period of four months. Eight test piles of 4.5 m long and 250-430 mm in diameter were installed by grouting mortar under the pressure of 0.7-1.0 MPa in bags woven by a synthetic fiber that had previously been inserted in drilled holes in a soft clayey soil. The long-term vertical loads were applied on the pile heads ranging from 21% to 44% of the pull-out capacity of the piles. Drained soil modulus and creep coefficient that describe the long-term settlement behavior of piles were derived from the observed pile head settlement vs. log time relationship. Both parameters are influenced by the ratio of the long-term applied load to the pull-out capacity, P/P_u. Drained soil modulus reduces with an increase of P/P_u, demonstrating the soil non-linear characteristic, while creep coefficient remains in a fairly constant value. The ratio of the creep coefficient to the soil flexibility (inverse of soil modulus) lies in the range of 0.1-1.5 under P/P_u less than 0.4.

View full abstract

A fully coupled numerical method for the analysis of excavation, which has previously been developed by the authors, is used for the back-analysis of a large-scale excavation in silty clays. The drawdown of the water table is considered in the analysis and a method which can correctly calculate the excavation forces due to overburden removal is used. The excavation was intensively instrumented and the measurements were well documented. Modified Cam Clay theory is used in the analysis to simulate the non-linear behaviour of the soil during excavation. Comparisons between the numerical results and the measured data are given in this study.

View full abstract

In this paper, several theoretical aspects for preparation of constitutive equations of unsaturated soils are discussed. First, possible pore water states are described. Three states, insular air, fuzzy and pendular saturation are taken into account to examine the mechanical behavior of unsaturated soils. Suction effects are clarified for each state and these are used to interpret the overall mechanical behavior of unsaturated soils. The suction effects can be classified into two categories. One is that an increase in suction induces an increase in effective stresses and the other is that an increase in suction induces an increase in both yield stress and the stiffness of the soil skeleton against plastic deformations. The first suction effect can be estimated by formulating the relationship between suction and shear strength at the wet side of critical state . The second suction effect can be estimated from the state surface concept. The modified Cam clay model with these suction effects is proposed. The performance of this model will be examined in the paper (Kohgo et al., 1993).

View full abstract

We have already examined about the theoretical aspects of constitutive models for unsaturated soils. From the results, it was recognized that the consideration of two suction effects was necessary to explain the mechanical behavior of unsaturated soils. Here a more general version elastoplastic model with these two suction effects is proposed. The schematic estimations of volume change behavior of unsaturated soils were performed using this model. The verification of the model was also examined simulating typical element test series. The model could well express them.

View full abstract

Through chemical process weathering, unwelded tuff pyroclastic flow deposits from Aso volcano have been altered to allophane and halloysite in geological time. As a result, natural slopes in areas abundant in Weathered soils of unwelded tuff are prone to catastrophic failure in periods of heavy rainfall. Mineralogical, chemical and electron microscopy study of soil samples from unfailed slope and actual case of failed slopes have shown that the main clay mineral of these soils is halloysite. The results from strength tests have proved that the complex interaction between weathering and leaching processes transforms the soil grain structure into a collapsible structure considerably weak in a saturated state. The collapsible structure is identifyable from the low density in undisturbed state and hence, the low density criterion may be used as an indicator on the loss of strength that may result from field saturation during heavy rainfall. Besides, leaching accelerates internal erosion in the natural slopes, and hence risk of failure by the seepage forces.

View full abstract

To use the distinct element method to perform stress analysis in three dimensions (3D), the domain considered has to be discretized into distinct polyhedral blocks by the joints present in rock masses. Unfortunately, most of the joint networks present in rock masses do not produce distinct polyhedral blocks. Therefore, to use the distinct element method to perform stress analysis in actual jointed rock masses, it is necessary to find a procedure to discretize the rock mass into polyhedral blocks starting from a 3D joint network which does not discretize a rock mass into polyhedral blocks. Also, this procedure should not alter the mass deformation and strength properties of the jointed rock. This paper provides needed procedures to link 3D stochastic or deterministic joint geometry modeling schemes to a 3D distinct element method to perform stress analysis in jointed rock masses which contain finite size joints. The procedure is illustrated through an example.

View full abstract

Berms are often used to improve the stability of embankments. The analysis for rotational stability is usually carried out by using limit equilibrium approach assuming a circular slip surface. Since the embankments on soft soils are not very high and are constructed quickly, a total stress analysis is carried out to evaluate the factor of safety at the end of construction. Most of the available methods require a trial and error procedure to determine the minimum factor of safety. Solutions are presented in the paper in the form of simple equations using which the location of the critical slip circle aud the minimum factor of safety can be calculated directly for a given limiting tangent. In the first part of the paper, the slip circle has been assumed to pass through the foundation soil and the embankment. Due to the rigidity of the embankment, tension cracks may develop in the embankment. The second part of the paper considers the presence of full height dry tension crack in the embankment. The solutions make possible a quantitative evaluation of the effect of berms and of tension crack on the stability of embankments. The optimum dimensions of the berms also can be chosen.

View full abstract

Equipment for the crosshole permeability test was developed, and the large-scale permeability test and the sinusoidal pressure test performed in low permeability deep bedrock and other area. The results were summarized as follows : 1) Large-scale permeability tests are useful testing methods to evaluate the average permeability of a relatively large rock mass. 2) The results of sinusoidal pressure tests in low permeability deep bedrock show that the bedrock storage properties are strongly influenced by the joint system, as demonstrated by the fact that the direction of the response of sinusoidal pressure tests coincides with the dominant direction of joints. The sinusoidal pressure tests are also useful to calculate the hydraulic diffusivity and specific storage coefficient and to investigate the hydraulic characteristics related to the joint system in bedrock. 3) A method for calculating the coefficient of permeability using the sinusoidal pressure test was proposed. The in-situ application achieved favorable results showing the effectiveness of this method.

View full abstract

This paper presents a series of tests performed on uncemented and cemented sands along conventional and unconventional stress paths using a true (or cubical) triaxial testing device. The tests were planned such that the influence of cementation on constitutive behavior of sands could be comprehended. The true triaxial device used for this study was a flexible boundary type that could allow testing of a 4 in. (10.2 cm) cubical soil specimen under any stress path in the first octant of stress space. The stress paths considered in this investigation included hydrostatic compression, conventional triaxial compression, and tests along different directions on different octahedral planes. All the tests were stress controlled, and performed under drained condition. The specimens were prepared by the method of undercompaction using Monterey No.0 sand and portland cement type I. The test results show that the effect of cementation on failure, shear strength, stress-strain, and volume change behavior of sands is significant and depends strongly on the stress path.

View full abstract

Plastic drains generally band or strip shaped, are often installed in square or triangular arrays. The horizontal coefficient of permeability measured in situ from around a plastic board drain indicated three distinct zones, viz., an inner smear zone consisting of highly remolded soil, an outer transition zone, and soil unaffected by the drain installation. Thus, the plastic drain treated soil is modelled as a two dimensional consolidation problem consisting of these three distinct zones. The resulting equations are solved numerically to obtain the degree of consolidation versus time factor curves. The extent and the value of the coefficient of permeability of the inner smear zone are shown to be the primary factors controlling the consolidation behavior of the soil. Based on the results from this study, it is recommended that the response of plastic drain treated soil can be improved by reducing the mandrel size and/or modifying the shape of the drain to a circle or an ellipse, rather than by having only a closer spacing of the drain.

View full abstract

Laboratory tests were conducted on Toyoura sand and the sand mixed with two kinds of fines to investigate the disturbance caused by a freeze and thaw cycle. Specimens whose liquefaction resistance had been increased by pre-straining under drained conditions were frozen unidirectionally. The liquefaction resistances of the specimens after thawing were compared with the resistance before freezing. It was found that : (1) The clean sand sample was not disturbed by a freeze and thaw cycle under drained conditions, if the confining stress during freezing and thawing was higher than 20 kPa; (2) the liquefaction resistances of the samples with fines were affected by the freeze and thaw cycle under drained conditions; and (3) "sample quality index" that indicates the degree of disturbance due to a freeze and thaw cycle was nearly uniquely related with volumetric expansion during freezing.

View full abstract

The purpose of this study was to determine the pipeline lateral soil restraint due to relative movement between the pipeline and the dry sand. A large scale drag device with dimensions of 6'×6'×4' (1.8m × 1.8m × 1.2m) had been fabricated to study the primary variables such as sand density, pipe diameter, pipe burial depth and relative velocity on the pipe lateral soil restraint. All 120 test results indicated that the dimensionless maximum soil restraints and the corresponding displacements exhibited the power law relationship with the pipe velocity, V/D. In dense sand, the pipe velocity exponents, n, were found constant and be irrespective of pipe burial depth, however, values of n increased with burial depth in loose sand and emerged with the dense sand constant exponent at the burial depth of 10.5 times of pipe diameter. Also, the generalized maximum soil restraints increased with depth of burial, until the burial depths with H/D of 10.5 in loose sand and of 12.5 in dense sand were reached, at which points, the soil restraints became constant. Force-displacement relationship of pipe-soil interaction could be represented by a two-constant hyperbolic equation. These two constant values of a and b were found to have the power law relationship with the pipe velocity, V/D.

View full abstract

Laboratory tests were conducted to investigate the volume change of clays induced by elevated temperature. A coventional consolidometer equipped with a heater and a thermostat was employed for a study on the interaction between clay particles and pore water. Consolidation tests on normally consolidated specimens revealed volume contraction upon heating and, when stress application continued further, the samples behaved like overconsolidated clays. Volume contraction occurred similarly, when a sample in the course of secondary compression was heated. Although reloading overconsolidated samples showed similar volume change as well, samples during stress reduction swelled in contrast. Finally, an attempt was made to propose a hypothesis that could explain the mechanism of thermal volume change of clays as observed in the present study.

View full abstract

This is a report of a preliminary study which was conducted prior to consolidation tests on the same kinds of clay undergoing elevated temperatures. It was considered imperative to examine at the beginning of study whether or not the planned heating affects the fundamental nature of clays. Therefore, the tested clay powders were heated in an oven under at the highest 200°C, followed by cooling and running those conventional tests on Atterberg limits, free swelling, and unconfined compression. Those tests revealed that pre-heated powders behaved in the same manner as unheated powders. Moreover, microscopic investigation did not detect a perceptible change in the appearance of clay after heating. Consequently, it was concluded that the employed range of heating does not influence the soil-mechanic nature of clay particles, and that the thermal effects should occur only through the interaction between clay grains and pore water.

View full abstract