3
Stress-strain behavior of soils at small strain has become one of the most important geotechnical engineer topics for the last two decades. According to the data measured in-situ, many researchers (Jardine et al. 1986; Burland 1989; Mair 1993) pointed out that the strain caused by construction was always less than 0.05% in the regions far from construction areas, which belongs to the range of small strains. Generally, the strain measurements used in conventional triaxial tests are measured externally, which may result in overestimating the axial strain.
In recent years, the study of soil stress-strain behavior at small strain has been improved by the development of high-resolution local measurements, which allow strain measurements on specimens to be resolved to the order of 10-5 and bring out realistic soil behavior.
As highlighted above, strains concerned in geotechnical problems at small strain are mostly less than 0.05%. Thus, the way of measuring axial strains in triaxial tests becomes significantly important. Considerable research has proved that the high stiffness of soils at small strain can only be observed through measuring the strain on the sample locally (Jardine et al. 1984; Goto et al 1991;
Lo Presti et al. 1995). The recommended method to fix the local sensor on a soil specimen is either sticking the sensor on the membrane for stiff clays, which may result in a relative displacement between the sample and the local sensor, or piercing a pin into the specimen through the membrane for soft clays. The use of pierced-pin placement may reduce the relative displacement between the
specimen and the local sensor, but the pin piercing the membrane will increase the risk of leakage within tests, even if silicone is used to seal the puncture. In
addition, the installation of local sensors on a specimen may cause undesirable disturbances to the tested soil sample. The degree of this disturbance greatly depends on the technique of setup of local sensors, i.e. it varies from researchers to researchers.
In order to eliminate possible errors of measurement, an alternative strain
4
measurement, making use of a high-precision servo motor, namely the
Direct-Drive motor (D.D. motor), in the axial loading system is presented in this study, which not only provides the needed axial displacement but also can be used to measure the axial strain. A modified triaxial testing system with the mentioned D.D. motor and local strain measurements is thus developed to perform the test.
Then, the stress-strain behaviors of natural Taipei silty clay obtained by two different strain measurements were compared to investigate the feasibility of measuring the strain by means of the D.D. motor.
In addition to the strain measurement used in triaxial tests, testing procedures for sampled soil have been recognized as important factors in determining soil properties. As a result, work on developing and comparing techniques for reproducing the in-situ state of soil have been undertaken (e.g., Bjerrum 1973;
Ladd and Foott 1974; Ladd and De Groot 2003; Santagata and Germaine 2005).
Most studies have focused on improving the reconsolidation stage. However, as observed by Cho et al. (2007), the effect of swelling during saturation of clay specimens causes changes in soil structure and affectes stress-strain responses at a strain of less than 0.01%. Saturation is thus considered as important as the
reconsolidation stage in a triaxial test.
To maintain the good quality of the soil sample during the saturation stage, an apparatus capable of controlling the suction in soil specimens and connecting to the triaxial testing system should be developed. Studies related to setting up a suction control system have been done by many researchers (e.g., Cunningham et al. 2003, Jotisankasa et al. 2007). However, these suction control systems focused on studies related to unsaturated soil. The application of existing suction control systems to saturated soil tests has been limited because suction force directly applied to saturated soil will result in the decrease of degree of saturation, which is undesirable in testing on a saturated soil sample.
Thus, this study also describes the improvement of saturation in triaxial tests
5
considering soil suction due to sampling. An apparatus capable of suction control in triaxial tests has been developed to improve the saturation condition. Triaxial tests with and without suction control were conducted on reconstituted samples of Taipei silty clay. The effects of saturation with suction control are verified in terms of (1) the void ratio change (e e0 ) after recompression (2) the shear modulus obtained from bender element tests during K0 consolidation, and (3) the
stress-strain characteristics at small strains during undrained shearing.
After the improvement of testing apparatus and procedure, triaxial tests conducted on Taipei silty clay were carried out. The stress-strain behavior observed in test results were used in modifying the soil constitutuve model. The accuracy of ground movement predictions during underground construction is improved by using the soil models which can simulate the soil behaviors at small strain. However, another attempt has been made to achieve the improvement of accuracy by considering the anisotropic properties of soils in numerical
simulations (Ng et al. 2003; Nishimura et al. 2005). The work mentioned above carried out that the prediction of deformation in construction is greatly improved with consideration of the anisotropic behavior for clayey soils. Therefore, the anisotropic properties of soils should be investigated for the purpose of getting better prediction and analytic results for geotechnical problems, especially for deep excavations with structures nearby.
The existence of inherent anisotropic stiffness for soft clays under K0
consolidation state can be proven and investigated by performing triaxial tests with local measurements and multi-orientation bender elements. The bender element, embedded in triaxial apparatus and proposed by Dyvik and Madshus (1985), has become one of the major laboratory tests in the study of the soil stiffness. Recent advances in laboratory techniques enable measurements of the horizontal propagation of shear wave velocity through the soil specimen in triaxial tests (Pennington et al. 1997). The shear waves obtained by bender element tests
6
can be used in calculating the shear moduli, GBE, by applying the following equation:
2
( ) ( )
BE ij s ij
G V (1)
where i is the direction of wave propagation and j is the direction of wave
polarization. is the bulk density and Vs is the measured shear wave velocity.
Finally, this study focused on the development of anisotropic shear modulus during K0 consolidation and undrained shearing. A series of CK0UAC triaxial tests were conducted on reconstituted Taipei silty clay. The variation of anisotropy of shear moduli under different consolidation stress, void ratio, and shear strain was investigated by performing bender element tests. The empirical equations for estimating the shear moduli and anisotropic ratio for shear moduli in the function of stress state, void ratio, and OCR were established.
7