The strain of soils measured by the conventional triaxial apparatus can at best reach the value as small as 0.1～0.05%, beyond which is out of the functional limitation in the triaxial apparatus. The fact also limits the development of soil models and the selection of soil parameters. More and more field observation data, however, showed that soil strains induced by construction were mostly less than 0.05% except those within the stress concentration zone near the construction site. Accordingly, over the last twenty years, international geotechnical engineering circles have been devoted to developing small strain tests, including the improvement of the triaxial apparatus and the development of small strain instruments. Researchers thus gradually recognize the importance of the small strain behavior of soils, which deeply influences the development of later advanced soil models. This paper introduces the development of small strain triaxial tests in detail and its application to developing the soil models and the analysis of engineering behavior.
Soil modulus (shear or Young’s modulus) and damping ratio are important parameters in geotechnical dynamic analyses. Test results are typically presented as relationships between secant modulus or damping ratio and strain, these relationships are referred to as the non-linear dynamic soil properties. In order to provide the necessary parameters for analyses, the non-linear dynamic soil properties must include the soil behavior under a range of strain from 10-4% to 1.0%. Because of the limitation in the experimental capabilities, when the strain was less than 10-2%, the dynamic soil parameters were usually determined by resonant column tests. When the strain exceeded 10-2%, a separate specimen tested under a torsional, simple shear or triaxial shearing mode would be required to establish the dynamic properties. Due to discrepancies in stress path and/or those between soil specimens, this procedure requires the expenses to set up a multiple-unit experimental system and a discontinuity often occurs in the modulus or damping ratio – strain curves where the test method changes. Thus, using a single specimen to perform the dynamic testing under the required strain strange can avoid some of these experimental problems and improve the quality of test data. Using a high torque stepper motor as the driving system, coupled with a high resolution non-contact proximiters and bender element shear wave velocity measurement devices, the authors developed a triaxial test apparatus capable of performing strain controlled cyclic triaxial tests under small to medium strain range. This paper describes the development of this apparatus and presents a series of results to demonstrate its capabilities.
The matric suction that exists in the unsaturated soil slopes will significantly affect the stability of soil slopes. How to measure matric suction accurately and reliably is still being under extensive research worldwide. This paper introduces some major techniques for measuring matric suction in the field and illustrates the influence of matric suction to the stability of soil slopes. Firstly, this paper describes the principle, measuring method and major equipment such as tensiometers and thermal conductivity sensors. Subsequently, case histories of measuring matric suction in the field are described and the effects of matric suction on the behavior of unsaturated soil slopes are reported and discussed. In addition, a series of finite element transient seepage analyses have been conducted to investigate the climate and vegetation conditions of slope surface on slopes. The results of the transient seepage analysis are input to slope stability analyses to study the influence of rainfall to slope stability of unsaturated soil slope.
For the study of the soil behavior under earthquake shaking, a physical model test using a large-scale shear box on the shaking table at the National Center for Research on Earthquake Engineering (NCREE) is developed. A laminar shear box movable in 2 axes, with a specimen size of 1.88m×1.88m in plan and 1.52m in height, is designed and manufactured in cooperation with Mechanical Industry Research Laboratories at Industrial Technology Research Institute. The shear box is composed of 15 layers of aluminum alloy frames. Each layer consists of an inner frame and an outer frame, which are allowed to move freely without torsion on the horizontal plane subjecting to the two-dimensional shear wave action induced by the shaking table. The sand specimen inside the shear box is prepared by the wet sedimentation method from a specially made pluviation device. The uniformity, density and saturation of the sand were checked by mini-cone penetration tests and P-wave velocity measurements. Pore pressures within the soil, displacements, accelerations and velocities of various depths of the frames were measured during tests under both one- and two-dimensional shakings. The preliminary test results show that the performance of the biaxial shear box is satisfactory. This shear box on shaking table will be used to study the problems of soil liquefaction, ground motions, and soil-structure-interaction during earthquakes.
It is a thought that gravelly ground is so hard that commonly seen sounding methods, such as SPT and CPT, are not applicable. As a consequence, evaluation of such ground condition is very rare in real practices. In Taiwan, gravelly ground covers most central area, and liquefaction occurred in the Ji-Ji earthquake reveals that gravelly ground is liquefiable once subjected to strong seismic motion. A suitable sounding method for the relevant evaluation of liquefaction potential is thus desired. This paper describes a Large Penetration Test (LPT), which was proposed and adopted in an in situ test project performed at a site by Darli river side near Futen bridge. Becker Penetration Test (BPT) is described herein as a comparison for the development of devices and the associated technical details. Testing results are shown and discussions with recommendations are stated for further development and improvement of LPT in local practices.
Flexural toughness is one of the pivotal mechanical characteristics of Steel Fiber Reinforced Shotcrete (SFRS) when it serves as part of tunnel support system. It enables effective and timely support for tunnels under excavation, being capable of tolerating relatively large deformation and in-situ stress, and thus is applicable to soft or squeezing rock conditions in Taiwan. Nowadays, a variety of concepts and technologies toward the toughness indices of SFRS has been developed. Serving as checkpoints for quality control of on-site operation, a precise toughness index is essentially important to justify the actual performance of SFRS. This article presents the toughness behavior of SFRS through a series of systematically designed experiments conducted on-site in a railway tunnel project. It aims at identifying the controlling factors and associated influence to the performance of SFRS, which is frequently measured from third-point tests. Accordingly, evaluation is implemented to highlight which indices are actually indicative of the toughness of SFRS made from various premixed formula. The results demonstrate that indices involving greater deflection, instead of accumulated absorbed energy, tend to have better elucidation of the actual trend. To contribute to national standard establishment in the near future, suitable indices and associated rough classifications are recommended based on aforementioned experimental results.
A large-scale dip-slope landslide, the Chiufenershan landslide, was triggered by the 921 Chi-Chi earthquake on Sept. 21, 1999. On the landslide, over thirty million cubic meters volume of rock masses slid down, covering a area about 200 hectares, blocking streams to form two dammed lakes, and causing people deaths in excess of 20. Based on geological investigations and various field landslide features collected , the classification and factors causing landslide were elucidated.