Fu-Chen Teng

Measurements and Numerical Simulations of Inherent Stiffness Anisotropy in Soft Taipei Clay

AbstractSoils are generally known not to be isotropic materials. The inherent anisotropy of clays results from the deposition process, which tends to induce a horizontal bedding plane in the soil layer. In this paper, the anisotropy of clays was studied to obtain more accurate analytical results for geotechnical problems, especially for deep excavations in soft clay with nearby structures. A series of K0-consolidated undrained triaxial compression (CK0UC) tests thus was conducted on tube samples of natural Taipei silty clay with multidirectional bender elements. A newly designed triaxial testing system equipped with a high-precision servo motor and local strain sensors was developed. A new soil suction-control system was also developed to perform soil saturation for triaxial tests. The suction-control system was used to reduce the change in the void ratio during saturation in the triaxial tests. Soil samples were retrieved from a site near a well-documented excavation case in Taipei. The anisotropy ratios...

Electro-Osmotic Chemical Treatment of Clay on Shear Modulus

Electro-osmosis chemical treatment (ECT) is one of the soil improvement methods used to improve the engineering properties of soft soils. The injected chemical solutions are usually a calcium chloride solution in combination with sodium silicate solution. Previous laboratory studies have shown that the injection of a calcium chloride solution followed by the injection of a sodium silicate solution during electro-osmosis is effective in strengthening soft silty clay. Very stiff cemented soil near the anode is formed after treatment due to the cementation of two chemical solutions and soil particles under applied electric field. However, most works focused on the improvement of the strength of the soil with treatment. The variation of the shear modulus on soils after electro-osmosis chemical treatment is rarely discussed. Thus, a field test of electro-osmosis chemical treatment on Taipei silty clay was carried out in this study. Six electrodes were installed with electrode spacing of 1.5 and 2.0 m. The shear wave velocity of the treated clay was measured by multi-orientation bender elements. The bender element test results show that the maximum improvement ratio of the shear modulus is about 200 % at positions near the anode and the cathode. The pH value and concentration of Ca2+ was determined by the inductively coupled plasma atomic emission spectrometer (ICP-AES) test at all positions that shear moduli was measured. Strong cementation which enhanced the shear modulus of the soft clay were also observed at the anode and cathode. Results presented in this study demonstrate that the electro-osmosis chemical treatment is effective in improving soil strength and soil stiffness.

Application of a Suction Control System in the Method of Specimen Saturation in Triaxial Tests

An effective stress should be maintained in soil specimens during the saturation stage of a triaxial test in order to minimize changes in the void ratio during recompression. In this paper, a new soil suction control system was developed to saturate soils in triaxial tests. This system was designed to reduce the void ratio change during the saturation stage and to preserve small-strain behavior. A series of triaxial tests were conducted on silty clay, saturated with and without suction control. The stress-strain behavior during consolidation and undrained shearing is described. Bender element tests were also performed during the consolidation stage. The new saturation method with suction control resulted in a better quality soil specimen than conventional saturation. The measured shear modulus and the Young’s modulus of the specimens saturated conventionally were underestimated by 20% and 14% compared with those saturated with suction control for the reconstituted soil. This difference would be much larger for soils sampled at greater depths. Since the in situ shear moduli measured in the laboratory were still lower than those in the field even with suction control, two empirical equations are proposed to estimate the shear modulus of in-situ soils. These equations can be used to compare the in situ sear modulus with the measured shear modulus of sampled soils. Several limitations with the proposed approach are discussed. The proposed approach would only be applicable for high air-entry value soils so that the suction does not cause desaturation in the soil specimen. The proposed suction control system also is not applicable to soil specimens with suctions greater than 100 kPa.

A study on the modification of electroosmotic consolidation theory using electric potential distributions

Abstract Electroosmotic chemical treatment is used for ground improvement. By applying electroosmotic characteristics, water discharge through soil pores is induced; concurrently, electricity is used to facilitate the uptake of a chemical solution by the soil, thereby achieving ground improvement. This approach involves complex electrochemical mechanisms, particularly near the anode and cathode; thus, various electrochemical reactions, including electrode corrosion, electrolysis, hydrolysis, temperature increases, deposition, ion exchange, and cementation, will occur, producing losses in the applied electric potential. Previous research has demonstrated that these problematic losses in potential during electroosmosis are particularly large near the anode and cathode. However, traditional electroosmotic consolidation theory is derived from the assumption that no loss in applied potential will occur, and therefore, this theory does not address the observed issues. In this study, the results of a year of laboratory tests are presented and are placed in the context of the relevant literature; these results indicate that there are many differences between actual potential distributions and the assumptions of traditional electroosmotic consolidation theory due to the existence of losses in applied potential. These results from data compilation and experimental tests are utilized to establish a potential distribution correction model curve of electroosmotic consolidation.

Numerical simulation of triaxial stress probes and recent stress-history effects of compressible Chicago glacial clays

AbstractNumerical simulations and calibration of hypoplasticity constitutive parameters for Chicago clays are presented based on laboratory tests conducted on high-quality block samples and field t...