At present, finite-element methods are frequently used in the analysis of geotechnical engineering problems. The selection of an appropriate constitutive law primarily involves balancing simplicity with accuracy. Given that many practitioners still use the hyperbolic model, the adequacy of a modified version of this simplified stress-strain relation-ship to predict the nonlinear behavior of granular soils, is herein examined. Drained triaxial tests were performed on specimens composed of Maipo River sand at different relative densities. These specimens were subjected to different stress-paths in order to an extensive comparison of the measured strains and the predicted values. In addition, laboratory plate-load tests were conducted on rough, circular and strip, surface footings, which were 100-mm in diameter and 60-mm wide, respectively. The testing box, with dimensions 1.05m × 1.05m × 0.65m, was filled with air-dried sand by using a raining apparatus. A field plate-load test performed on dense sandy gravel was also analyzed. From the good agreement achieved between the empirical observations and both the calculated strains and load-settlement relationships, it is concluded that the proposed hyperbolic model predicts with sufficient accuracy the nonlinear response of granular soils in most practical cases, as long as the soil mass is not close to failure.

View full abstract

In order to establish practical design procedures to evaluate seismic stability of different types of retaining walls against high seismic loads, a series of shaking table tests were conducted with irregular excitation on retaining wall models consisting of six different types. Reinforced-soil retaining wall models with a rigid full-height facing exhibited a ductile behavior, when compared with conventional type retaining wall models such as gravity, leaning and cantilever-types. When the conventional type wall started to tilt, the subsoil reaction force at the toe of wall suddenly decreased due to the loss of bearing capacity. On the other hand, under similar conditions, the tensile force in the reinforcement of reinforced-soil walls could be mobilized effectively to resist overturning.

View full abstract

The 1999 Kocaeli earthquake of August 17, 1999 hit the western part of Turkey causing significant damage to buildings and sivil engineering structures. This paper reports geotechnical aspects of damage mainly along the south coast of the lzmit Bay. Ground subsidence and loss were observed in the widespread area in the waterfront areas in the south of the Izmit Bay. It is supposed to have ben caused by slope instability of the seabed and a fault dislocation. Significant ground subsidence was also observed along the southern coast of the Sapanca Lake. Site investigations of these areas were carried out by the reconnaissance teams from the Japanese Geotechnical Society. Ground survey and seabed depth measurement were carried out in order to clarify the cause of these damages. Three different factors were identified as possible causes for the damage. A fault which appeared on the ground surface caused direct damage to structures by the relative displacement between the two sides. A pull-apart dislocation by main and branch faults caused ground subsidence in a wide area. A submarine landslide caused land flows in a fill and an alluvial fan deposit flowed out. Soil liquefactions, though observed from place to place, did not seem to have caused significant geotechnical damage.

View full abstract

This paper presents the analytical solutions for estimating the displacements and stresses in a cross-anisotropic half-space due to various loading types of an axially loaded pile. The loading types include an embedded point load for an end-bearing pile, uniform skin friction, linear variation of skin friction, and non-linear variation of skin friction for a friction pile, respectively. The planes of cross-anisotropy are assumed to be parallel to its horizontal surface. The derived solutions have not yet appeared in the literature and can be obtained by integrating the point load solutions in a Cartesian co-ordinate system for the cross-anisotropic half-space. A part of the solutions are identical with the Mindlin's and Geddes's solutions when the medium is isotropic. The presentation of these proposed solutions is concise and systematized ; also, they indicate that the displacements and stresses in a cross-anisotropic half-space are affected by the type and degree of material anisotropy, and the loading types. Furthermore, two illustrative examples are given to investigate the effect of the type and degree of soil anisotropy, and four different loading types on the vertical surface displacement and vertical normal stress, respectively. The results show that the displacement and stress accounted for soil anisotropy are quite different from those estimated by isotropic solutions.

View full abstract

A new field test method was proposed for the purpose of measuring average stress-strain relationships of rock masses. The test is conducted on a hollow cylindrical specimen prepared at the bottom of a drill-hole. Average axial as well as lateral strains can be measured in a center hole and an outer slit by a noel technique of instrumentation for cavity deformation. A set of test equipment for this test method was developed at Central Research Institute of Electric Power Industry, CRIEPI, in Japan for prototype tests. Specimens of 400 mm in outer diameter and 1050 mm in height can be sheared under confining pressures as high as 5.0 MPa. Accuracy of strain measurements was designed to be of the order of 10^-4. Trial series of tests were carried out at the site of rhyolitic tuffacious rock formation. The results, similar to conventional laboratory triaxial tests, proved that the proposed test method was successful to measure average stress-strain relationships of large rock specimens. Considering the significant size of the specimens, the volume of rock involved in the tests is of the same order or greater than those in conventional field rock tests, such as plate load tests and rock shear tests. As a consequence, the test results are considered as representative in evaluation of strength and deformation characteristics of rock masses but not rock cores.

View full abstract

The stabilisation of the Tower of Pisa has been a very difficult challenge for geotechnical engineering. The Tower is founded on weak, highly compressible soils and its inclination has been increasing inexorably over the years to the point at which it was about to reach leaning instability. Any disturbance to the ground beneath the south side of the foundation is very dangerous ; therefore the use of conventional geotechnical processes at the south side, such as underpinning, grouting etd., involved unacceptable risk. The internationally accepted conventions for the conservation and preservation of valuable historic buildings, of which the Pisa Tower is one of the best known and most treasured, require that their essential character should be preserved, with their history, craftsmanship and enigmas. Thus any intrusive intervention on the Tower had to be kept to an absolute minimum and permanent stabilisation schemes involving propping or visible support were unacceptable and in any case could have triggered the collapse of the fragile masonry. In 1990 the Italian Government appointed an International Committee for the safeguard and stabilisation of the Tower. It was conceived as a multidisciplinary body, whose members were experts in arts, restoration and materials, structural engineers and geotechnical engineers. After a careful consideration of a number of possible approaches, the Committee adopted a controlled removal of small volumes of soil from beneath the north side of the Tower foundation (underexcavation). This technique provided an ultra soft method of increasing the stability of the Tower, which is completely consistent with the requirement of architectural conservation. The paper reports the analyses and experimental investigations carried out to explore the applicability of the procedure for the stabilisation of the Leaning Tower of Pisa. All the results having been satisfactory, the actual underexcavation of the monument was carried out in the years 1999-2001 ; the results obtained are presented and discussed.

View full abstract

A highway embankment built in Akita prefecture using ground stabilization methods was constructed on very soft peat and clay deposits on a sloping base. This paper presents a case study of a soil/water coupled elasto-viscoplastic finite element simulation of an actual embankment construction, that is, the deformation analysis of a soft clayey foundation deposited on an inclined base with embankment loading. For execution of the numerical simulation, determining the input parameters needed in the analysis and evaluating the shape of sloping bedrock under the ground were problems. The former is discussed for a practical procedure of determining input parameters using unconfined compression strengths and the latter is accomplished by drawing a contour map of the top face of the bedrock under the ground, with reference to the geographical features of the area around the site and to a topographic map. The performance of the numerical simulation is discussed in comparison with in-situ monitored records. The effectiveness of ground stabilization methods employed at the site is also a topic of discussion.

View full abstract

The paper presents a study of the static liquefaction of granular soils, particularly the existence and uniqueness of the so-called steady-state and the conditions leading to the steady-state. The study is carried out using a simple constitutive model for granular soils established using a few well-known relationships based on results of drained triaxial tests on sands and silts. The paper clarifies the necessary theoretical and experimental conditions in determining whether a soil has achieved the steady-state. The results of the analytical study strongly emphasize the importance of dilatancy and deformability on the steady-state response of granular soils. It is shown that, in the absence of instability, liquefaction should be treated simply as another aspect of the stress-strain response of granular soils, and liquefaction should be treated not just as a strength problem but also as a deformation problem.

View full abstract

In this paper, by introducing a tij concept and Matsuoka-Nakai failure criteria into the model proposed by Oka and Adachi, a modified strain-softening model is developed to consider the influence of the intermediate principal stress that may greatly affect the strength and stress-dilatancy relation of geologic materials under different loading paths. The original characteristics of the model, such as the ability to describe not only the strain softening behavior of soft rock but also the stress-dilatancy relation, the ability to obtain a unique solution for initial value and boundary value problems in a finite element analysis, remain valid in the modified model. Its validity is verified by compression tests on artificial soft rock under plane-strain condition.

View full abstract

In the present paper, based onan elastoplastic model with strain hardening and strain softening, a soil-water coupled finite element analysis is conducted to investigate the progressive failure of a cut slope in a model ground. In order to verify the validity of the analyses related to the strain-softening behavior, numerical analyses are firstly conducted for plane-strain compression in different meshes and loading steps under complete drained condition. It is confirmed by the analyses that the analysis conducted in this paper has a small dependency on the mesh size. Then, the mechanical behaviours of a cut slope, such as the change of excessive pore-water pressure, the redistribution of stress in ground due to strain softening, the propagation of shear band and the progressive failure are discussed in detail by the soil-water coupled finite element analysis. It is found that a soil-water coupled analysis based on an elastoplastic model can describe the time dependent behavior of soft rock in boundary-value problems. It is also found that a soil-water coupled analysis based on a strain-softening model can simulate the progressive failure of a cut slope.

View full abstract

This paper aims to reveal the size effects of earth pressure cells on measurements in sand and gravels and propose a guideline for the selection of an appropriate size. In the case of gravel containing large particles, point loadings at several contact points in the cell make it difficult to measure the earth pressure with sufficient accuracy and reliability. Four types of earth pressure cells were used in this study with different sizes of sensitive panels and principles of measurements. A series of loading tests were conducted with sand and four types of gravels. The reliability of the test results was first examined ; the influence of parameters such as the stiffness of the earth pressure cell, and the sidewall friction of the test container were investigated. From a comparative study of the measured data, the appropriate size of the earth pressure cell was determined as a function of the grain size of the gravel and accuracy needed in the measurements.

View full abstract

This research establishes quantitative relationships between soil's anisotropy and liquefaction resistance for granular materials. Uniform medium density (D_r=50%) sand specimens were prepared using three different sample preparation techniques (air pluviated (AP), moist tamped (MT), and moist vibrated (MV) to create different initial soil fabrics. Undrained cyclic triaxial tests were then performed to determine the liquefaction resistance of each soil specimen. On the same specimens in the triaxial cell, vertical and horizontal compression wave velocities and vertical shear wave velocity (V_s) were measured using piezoelectric bender elements. Anisotropic (transversely isotropic) elastic constants of the soil specimens were determined from the elastic wave measurements and additional consolidation test data. With the aid of additional data from earlier discrete element model (DEM) simulations, anisotropic parameters, which influence the liquefaction resistance, were examine. It was found that when liquefaction resistance is divided by G₁₂/(E₁/E₂)³, by G_average/(E₁/E₂)³, or by V^5.0_s, liquefaction resistance curves converge to a unique curve regardless of the sample preparation techniques. Liquefaction Stress Ratio Reduction Factor (LSRRF) was introduced to estimate the reduction of liquefaction cyclic stress ratio of an anisotropic specimen from the isotropic specimen as simple functions of (E₁/E₂)^-5.0 or (V_s/V_s(iso.))^5.0.

View full abstract

Sixteen different soils were tested in an NGI type simple shear device to examine the effect of strain rate, γ^^·, on cyclic secant shear modulus, G_s, at very small to small cyclic shear strain amplitudes, γ_c ≈ 0.0003-0.01%. The effect of γ^^·on G_s was characterized by strain-rate shear modulus parameter, α_G, and strain-rate shear modulus factor, N_γ-G. Parameter α_G describes the slope of G_s-log γ^^· data for a given γ_c, while N_γ-G is α_G normalized by reference G_s. For the clayey soils and their conditions tested α_G ≈ 2.6-6.0% were obtained. For the sandy soils α_G ≈ 0.3-1.2 MPa and N_γ-G ≈ 0.5-3.0% were obtained. Data revealed that α_G decreases with increasing γ_c, while N_γ-G either slightly decreases with increasing γ_c or stays about constant. The trends of N_γ-G with plasticity indes, I_p, liquid limit, w_L, clay content, and activity were examined. It was found that at a given γ_c, N_γ-G generally increases with increasing all of these properties and indices. Most consistent and strongest correlations were obtained between N_γ-G and I_p and N_γ-G and w_L. These two correlations can be useful for the soil dynamics engineering practice and are presented in the paper

View full abstract

This paper reports results of in-situ tests and undrained cyclic shear tests on high-quality undisturbed samples obtained by the in-situ freezing method at three sites where foundation soils had been improved with sand compaction piles. Triaxial specimens obtained from the frozen samples were fully saturated in the triaxial cell and then subjected to cyclic loadings. In the current design practice of ground improvement with the sand compaction piles, the SPT N-value at a mid point of a rectangular area surrounded by four adjacent sand piles, at which the N-value, and thus the liquefaction resistance is considered to be the smallest, is set as a target for the degree of compaction. It was found, however, that distributions of N-values and N_d-values in a horizontal plane at a certain depth appeared to be rather random. This suggests that the V-Value at the center of a rectangular area does not always provide a conservative evaluation. The relationship between liquefaction resistances and the N-value, which was developed based mainly on field evidences of earthquake-induced liquefaction of natural soil deposits and reclaimed lands, compared quite well with that obtained from tests on fully saturated high quality specimens and mean values of N_d at several locations in the improved sands. Degree of saturation of the frozen sample was revealed to be considerably low, in the range between about 70 and 90%. This fact indicates that the liquefaction resistances of improved sands are significantly higher than those obtained from the relationship which is available only for fully saturated soils.

View full abstract

Constitutive equations have to be formulated in an indifferent form independent of the frame (i. e. coordinate systems) by which they are described or so as to be independent of the superposition of rigid body rotation. This fact is required by the principle of material-frame indifference (Oldroyd, 1950) and is attained conveniently by describing rate variables in terms of rate tensors with objectivity in constitutive equations in rate form. A plastic strain rate is derived by substituting the plastic flow rulr into the consistency condition given as the material-time derivative of yield condition. In this note the mathematical process demonstrates the fact that rate variables involved in the material-time derivative of yield function can be directly replaced with their objective rate tensors. Here, the yield function involves arbitrary tensors, whilst the special case that the yield function involves only a single tensor or second-order tensors has been discussed in the past.

View full abstract