During deep mixing (DM) column installation, excess pore pressure arises in the surrounding clay due to the injection of chemical admixtures and the shearing action caused by blade rotation. The excess pore pressure is greater than the in-situ effective stress, resulting in fractures in the surrounding clay. Pore water infiltrates into fractures, and the consolidation of clay is accelerated. The chemical admixture penetrates into the fractures and thereby exchangeable cations diffuse into the surrounding clay effectively. All of these effects accelerate the strength increase of the surrounding clay after the installation of DM columns. The main objective of this study is to confirm the aforementioned phenomena. These phenomena are verified through (1) a laboratory vane shear test, (2) a laboratory test on a model column made in a model clay ground, and (3) full-scale field tests on DM column installation.

抄録全体を表示

This paper describes a method for the analysis of bending interaction between a thin plate and unilateral elastic foundation with particular emphasis on the non-contact phenomenon between the element and the medium. The boundary element-linear complementary equation method (BE-LCEM) is presented to solve the underlying problem. Following numerical discretization using the boundary integral equation method for this contact problem, an effective linear complementary equation is then established with two complementary variables for each contact node. Complementary variables are taken as the normal contact force and deflection of the plate. The resulting linear complementary equations are solved by using mathematical programming. Details of the transformation are described. A number of examples are presented to demonstrate the effectiveness of the features implemented. The method is shown to provide good agreement with analytical solutions obtained by other investigators.

抄録全体を表示

Stress-strain behavior of sand in plane strain compression, extension and cyclic loading was studied. To attain plane strain extension stress states in a conventional plane strain compression apparatus, a pair of intermediate confining platens were added between the specimen and the specimen and the conventional confining platens. They together were enveloped within a specimen membrane so that the intermediate principal stress can be larger than the vertical stress under plane strain extension stress conditions with the lateral confining pressure larger than the vertical stress. To simulate active and passive earth pressure conditions and those under horizontal cyclic loading conditions with the constant vertical stress in the field, specimens were rotated by 90 degrees before each test so that the horizontal stress when the specimen was prepared became the vertical stress in the test. Typical stress-strain relationships are presented from tests performed on saturated specimens of air-pluviated Toyoura sand measuring strains locally. It is shown that although the relationships between the vertical stress and the vertical strain are considerably non-symmetric about the neutral stress condition under the plane strain compression and extension stress conditions, those between sin φ_mob and the shear strain are essentially symmetric, and therefore, relevant for modeling. Stress-dilatancy relationships are also presented.

抄録全体を表示

The bearing capacity of plane strain model footings on sand is numerically investigated with a finite element method and a polar hypoplastic constitutive law. The constitutive law is obtained through an extension of a non-polar hypoplastic constitutive law by polar quantities : rotations, curvatures, couple stresses and a characteristic length. It can reproduce the salient features of granular bodies while taking into account shear localisation. The hypoplastic constants are calibrated with element test calculations on the basis of plane strain and triaxial compression tests. The FE-analyses are performed for both different mean grain diameters and initial densities of dry medium sand, and footing widths. The effect of material constants on the load-displacement curve is also investigated. The FE-results are compared with model tests. Advantages and limitations of a polar hypoplastic approach are outlined.

抄録全体を表示

It has been known that the penetration resistance depends on the grain size of soils, and that fines-containing sands have smaller SPT N-values than clean sands. Previous investigations on the link between the penetration resistance and relative density have indicated that the ratio between the normalized N-value and the square of the relative density, N₁/D²_r, is dependent on the grain size of sands. While this dependence has not been properly quantified, it has been customary to utilize the well-known expression of Meyerhof with N₁/D²_r being fixed at 41. In this paper, an attempt was made to correlate the penetration resistance and relative density by accounting for the grain size properties of soils. The void ratio range (e_max-e_min) was used as a measure indicative of the grain size and grain size composition. It was found that N₁/D²_r is highly dependent on the value of (e_max-e_min) and that this ratio gradually decreases with increasing void ratio range from a value of about 100 for gravels to a value of about 10 for silty sands. This dependence is mathematically formulated and used to establish an empirical correlation between the SPT N-value and D_r that is applicable to various kinds of soils ranging from silty sands to gravels. The correlation is developed by using data of high-quality undisturbed samples and results of SPT measurements on natural deposits of sandy soils and gravels.

抄録全体を表示

The advantages and disadvantages of using a back analysis of slope failures to evaluate soil shear strength are discussed. A methodology is presented herein that allows the implied level of reliability associated with soil shear strength parameters back calculated from slope failures to be estimated. A reliability approach is also used to estimate the probability of failure for a given limit equilibrium slope stability method, design factor of safety, and combination of back calculated Mohr-Coulomb shear strength parameters, c' and φ'. The methodology is illustrated using 39 landslides in the Orinda Formation in the San Francisco Bay area. The impact of additional case histories in the same geologic setting, i.e., a larger data set, on the required design factor of safety for a given probability of failure is also investigated.

抄録全体を表示

The surface and downhole accelerations records of "Central de Abasto Oficinas (CAO)" array at Mexico City, have been analyzed to determine the soil stiffness and damping ratio as a function of shear strain amplitude. The 3/31/93, 24/10/93, 23/05/94, 10/12/94 and 09/10/95 seismic events have been used for this purpose. The shear stress-strain histories have been evaluated directly from the field downhole acceleration records, employing a technique of system identification, and have been used to obtain the variation of shear modulus and damping ratio with shear strain amplitude. A shear-beam model, calibrated by the identified properties, is found to represent the site dynamic response characteristics. The results have been compared with values obtained by previous authors using field and laboratory tests.

抄録全体を表示

Hatanaka and Uchida (1996) developed a simple method (named G₀-equal method) for estimating K₀-value of cohesionless soils by equalizing the initial shear modulus calculated from the shear wave velocity measured in the field (G_OF) and that measured in laboratory (G_OL) for high-quality undisturbed samples. It is clear that to directly equalize the shear wave velocity obtained both in the laboratory and the field is more convenient. As a result, the G₀-equal method was modified as a V_S-equal method in the present study. In the V_S-equal method, the K₀-value can be described in Eq. (1). K₀={(3/σ'_v)·(V_SF/a')^1/n'-1}/2 (1) V_SL=a'(σ'_m)^n' (2) where, K₀ is the coefficient of earth pressure at rest, σ'_v is the effective vertical stress at depth for measuring the shear wave velocity, V_SF is the shear wave velocity measured in the field, σ'_m is the effective mean principal stress, and α' and n' are the soil constants in Eq. (2). In order to examine the validity of the V_S-equal method, the effects of the principal stress ratio and the stress history on the V_SL-σ'_m correlation were verified by performing a series of laboratory tests on undisturbed and reconstituted sand and gravel samples. The K₀-value of in-situ gravelly soils was measured by using the V_S-equal method. High-quality undisturbed gravelly samples for the determination of the K₀-value were recovered at six sites. Based on the test results and discussion, the following were concluded. 1. A simple testing method was successfully created for reliably measuring the shear wave velocity of gravel samples in the laboratory. This method has the following advantages ; (1) without bedding error, (2) little personal error, (3) easy to use and (4) adjustable to specimen height. 2. The effect of the principal stress ratio (R=σ'₃/σ'₁, σ'₃ : radial stress, σ'₁ : axial stress) on the V_SL-σ'_m relation was found to be negligibly small in the range of R tested (R=0.5∼1.5). The effect of the stress history on the V_SL-σ'_m relation was also found to be very small. These results indicate that the V_S-equal method modified from the G₀-equal method is a useful tool to determine the K₀-value for sandy and gravelly soils. 3. The K₀-values measured by the V_S-equal method were 0.19 to 0.40 for untreated gravelly fill, 0.55 to 1.0 for compacted gravelly fill, and 0.83 to 1.14 for Holocene gravelly soils. Test results indicate that the K₀-value of compacted gravelly soils almost correspond to the range of the K₀-value usually adopted in practical use (K₀=0.5 to 1.0). However, the K₀-values for gravelly fill are much lower than that for common use. Holocene gravel has relatively high K₀-values. 4. Based on the test results, a simple equation (Eq. (3)) is newly proposed for the estimation of the K₀-values for insitu gravelly soils by only using the shear wave velocity measured in the field (V_SF). K₀=0.0058V_SF-0.53 (150≤V_SF≤350 (m/s)) (3)

抄録全体を表示

An FEM simulation of plane strain compression tests of dense Toyoura sand reinforced with reinforcement having a wide range of stiffness is described. Strain localisation is taken into account by modelling a shear band having a specific thickness and specific strain-softening properties determined based on experimental results. Global and local behaviour of the unreinforced and reinforced sand observed in plane strain compression tests are properly simulated.

抄録全体を表示

In this study, an extension of the variational model proposed by Vallabhan and Mustafa (1996) for the settlement analysis of an axially loaded pile embedded in a multi-layered soil profile is presented. In this model, the soil profile and the embedded pile are divided into a number of sub-layers according to the actual number of soil layers observed in the field. The displacement shape function of each soil layer is given as the product of an exponential equation along the pile depth and Bessel's solution in the radial direction. The displacement relationship in each layer can be derived through transformation matrices. One of the major features of this method is that the total number of pile elements is the same as the total number of soil sub-layers. All the field components, such as displacements, stresses, and strains in the soil, can be calculated by closed-form solutions except the only unknown variable, β, which is a variable as expressed by the modified Bessel's functions of the second kind, and which can be determined by iteration techniques. Comparisons were made with the results of finite element analysis and field pile-loaded tests. Reasonable results were obtained for all cases.

抄録全体を表示

A permeameter for soil-cement mixtures was developed and the equation for estimating the coefficient of permeability was given. With this permeameter, the coefficient of permeability can be tested accurately, fast and conveniently. The coefficient of permeability of soil-cement mixture is generally in the order of 10^-8 cm/s and the confining pressure and seepage pressure has little effect on it in the specified range. The effect of shear stress on the permeability was studied with the hollow cylinder sample carrying confining pressure, seepage pressure and vertical load. The results show that, with the increase of the vertical load, the coefficient of permeability increases markedly if the confining pressure is low, and it only changes slightly or even decreases if the confining pressure is high.

抄録全体を表示