Settlement Behavior Evaluation of Soft-Clay Layer Reinforced with Sand Piles

In this study, the performances of the sand piles in Istanbul's Bağcılar and Zeytinburnu districts has been analyzed using Finite Element Method (FEM). Single and group (triple) piles with various length/diameter ratios (L/D) were placed in the water-saturated soft clay soil. Sand piles were modeled in various L/D ratios (10, 5.71, and 8.57). The distance between the piles was chosen as 2 meters and the group effect was also investigated. A uniformly distributed load of 162 kN/m2 is placed on the ground. In addition, the soil was modeled with the Soft-Soil soil model, the hardening soil model for the infill part, and the sand piles with the Mohr-Coulomb soil model. According to the results , the settlement of the soil decreases by 52.8% for a single pile with an L/D ratio of 8.57. However, the best L/D ratio for triple piles was found to be 5.71. In this case, the settlement decreases by 52.8% compared to the pileless situation. Finally it was concluded that the model with the L/D ratio of 8.57 reduced settlement in the best and the most efficient way.

Upper Bound Analysis of the Stability of 3D Slopes in the Saturated Soft Clay Subjected to Seismic Effect

In order to study the three-dimensional stability problem of the saturated soft clay slope under earthquake loads, based on the three-dimensional rotation failure model, the seismic force was introduced into the calculation by the quasi-static method. The work rate of external loads and the internal energy dissipation rate of the saturated soft clay slope were calculated using the upper bound method of limit analysis, and the analytical solution of stability coefficient of saturated soft clay slopes was derived based on the fictitious power principle. By virtue of the exhaust algorithm, the optimal solution of stability coefficient of saturated soft clay slopes was obtained. The influence of the slope angle and the horizontal and vertical seismic forces on the stability coefficient of saturated soft clay slope was analyzed. The results show that the slope angle has a great influence on the stability coefficient, and the relative difference is up to 35.7%. Therefore, the stability coefficient of saturated soft clay slopes can be effectively increased by a proper slope setting. The horizontal and vertical seismic forces also have a significant influence on the stability of saturated soft clay slopes. The relative differences of the stability coefficient under horizontal and vertical seismic forces are as high as 41 and 14.7%, respectively. If they are ignored, the stability coefficient of saturated soft clay slopes will be seriously overestimated. It is suggested that the effects of horizontal and vertical seismic forces must be considered simultaneously in the seismic design of saturated soft clay slopes.

Cyclic Loading Test for the Small-Strain Shear Modulus of Saturated Soft Clay and Its Failure Mechanism

Small-strain shear modulus, G max , is a key evaluation index to study the dynamic characteristics of soil in geotechnical engineering. It is widely adopted to evaluate the stiffness of soft soil in soil dynamic engineering. In this paper, the cyclic triaxial tests and resonance column tests were carried out to explore the variation of G max of soft clay with respect to various confining stresses, cyclic shear stress ratios, pore pressures, and effective stress paths. Test results indicated that the effective stress decreased gradually with the increase of the cycle shear stress ratio. The failure points were mainly concentrated in a rectangular area, defined by the normalized effective stress from 0.56 to 0.64 and the normalized shear modulus from 0.72 to 0.78. Additionally, a short pause caused a small increase of 1-2% in G max as well as pore pressure. This study demonstrates that G max can be effectively used to characterize the failure of saturated soft clay in a more intuitive and convenient way, compared to the commonly used strain failure standards.

Relationship between monotonic and cyclic behavior of saturated soft clay in undrained triaxial compression tests

Cyclic loading-induced deformation of soil is a common problem in the engineering practice. In the current practice, however, monotonic triaxial tests are more commonly used in the practice, due to the availability of apparatus and ease of operation. Thus, it will be very useful and practical if the monotonic triaxial tests can be used to evaluate the behavior of soil under cyclic loading. This study aims to find an explicit relationship between monotonic and cyclic behavior of saturated soft clay. Six monotonic and nine cyclic triaxial compression tests were conducted on undisturbed saturated soft clay under an undrained condition. The test results showed that the monotonic and cyclic tests shared the same stress-strain surface in a three-dimensional space p^'-q-ε_a. It is also found possible to evaluate the effective stress states of cyclic tests at two specific numbers of cycles, using corresponding monotonic tests. Based on these two findings, a simple procedure was then proposed to predict the peak axial strain for the saturated soft clay under different cyclic loadings based on the monotonic tests and only one cyclic test, which was further verified against more test data from the previous literature.

The influence of soft clay saturation characteristics on unconfined compressive strength in Guangdong-Hong Kong-Macao Greater Bay Area

In order to study the influence of saturated characteristics of soft clay on unconfined compressive strength, the soft clay of Guangdong-Hong Kong-Macao Bay Area is taken as the research object, comparing and contrasting on unconfined compression test of saturated clay and unsaturated clay in laboratory, studying the variation law of unconfined compressive strength and sensitivity of unsaturated and saturated soft clay. The test results show that: 1. During the failure of unsaturated soft clay samples, oblique fractures appear, showing brittle shear failure, while the saturated clay samples appear constant bulging in the middle, and finally a “cross” is broken in the central bulging part, showing plastic shear failure.2. The unconfined compressive strength of unsaturated clay is about 10kPa higher than that of saturated soft clay, and its corresponding sensitivity is also about 0.4 higher. It can be shown that the soil saturation has a certain influence on the soil strength. The difference between saturated soil and unsaturated soil is the existence of gas phase. To be more precise, the existence of gas phase in unsaturated soil, i.e. the existence of suction, makes the soil stronger and presents the brittle shear failure form, while the saturated soil basically presents the plastic shear failure form.

Settlement simulation of soft clay in the subway under dynamic load based on Midas GTS NX

The subway has become the main way for people to travel nowadays. The saturated soft clay area has a large population and subway construction is the most extensively distributed. The saturated soft clay foundation will settle under the dynamic load of the subway train, which will affect the service life and structural safety. To study the settlement characteristics of soft clay under the dynamic load of the subway, a three-dimensional dynamic finite element model was established based on the finite element software Midas GTS NX, and the soft clay under the normal design speed (80 km/h) and the high-speed (120 km/h) were compared and analyzed. The research results show that the higher the train speed, the larger the lateral influence range of the surface settlement trough, but the settlement decreases with the increase of the subway running speed. The settlement of the subway line after one year of operation is about 45mm, and the settlement after 20 years is about 58mm. The growth rate of the settlement prediction curve decreases gradually, and the settlement increment is very small for a long time in the later period of operation.