Experimental study on the interface behavior between unsaturated completely decomposed granite soil and cement grout

Abstract

Soil-structure interaction is an important phenomenon encountered in various geotechnical engineering projects like soil nails, retaining walls, shallow foundations, and pile foundations. One of the most important parameters for the design and safety assessment of these structures is the ultimate interface shear strength at the interface between the structural surface and the surrounding soil surface. The prime focus of this research is to investigate the elementary interface behavior between compacted completely decomposed granite (CDG) soil and cement grout under different matric suctions, net stresses and grouting pressures. Firstly, a series of single-staged consolidated drained direct shear tests are conducted on compacted CDG soil under different matric suctions and net normal stresses. A soil-water retention curve (SWRC) is obtained from the equilibrium water content corresponding to each applied matric suction at zero net stress. The experimental results show that the influence of suction and net stress on shear behavior of soil is significant. Shear strength of soil increases with matric suction and net stress. The suction envelope is observed as nonlinear. Greater dilation angle is found at higher suction with lower net normal stress, while lower or zero dilation angles are observed under higher net normal stress with lower suction, also at saturated condition. A modified model is proposed for predicting the unsaturated shear strength of soils considering the influence of soil-dilation. The experimental shear strength data are found little bit higher than the analytical results at higher suction range under higher net stresses. Secondly, to investigate the elementary behavior of gravity grouted (0 kPa grouting pressure) interface, and compare with the behavior of CDG soil, a number of interface direct shear tests are performed between compacted soil and cement grout under the same matric suctions and net normal stresses. The behavior of stress-displacement curves of soil-cement grout interface tests is similar to those of soil tests. Matric suction and net normal stress have significant influence on the hardening-softening and contractive-dilative behavior of soil-cement interface. The failure envelopes for different matric suctions are observed as linear. The apparent interface friction angle and adhesion intercept increase with matric suction. The apparent interface friction angles for different suctions are equal to the apparent friction angles of soil under the same suctions. However, the apparent adhesion values are higher than the apparent cohesion values of soil in lower suction range, but lower in higher suction range. The suction envelopes for different net normal stresses are nonlinear. The interface shear strength is greater than the strength of soil within the lower suction range for different net stresses. However, the interface shear strength is lower than the strength of soil at higher suction range. A modified model is proposed to consider the influence of dilation on apparent interface friction angle. The experimental shear strength data agrees well with the analytical results for different net normal stresses and matric suctions. Finally, to examine the influence of grouting pressure on elementary interface behavior, a series of interface direct shear tests are performed under the same net stresses at both saturated and unsaturated conditions. At saturated condition, grouting pressure and net stress have significant influence on the behavior of interface. The behavior of stress-displacement curves for pressure grouted interface is similar to those of soil and gravity grouted interface. The failure envelopes for different grouting pressures are linear, the apparent effective interface friction angles are constant, and the apparent effective adhesion intercept increases with grouting pressure. The grouting pressure envelope is approximately linear and declivities are constant for different net stresses. A model is proposed for interface shear strength at saturated condition considering grouting pressure as an independent variable. The predicted interface shear strength of the proposed model agrees fairly well with the experimental data. At unsaturated condition, the apparent interface friction angle increases with matric suction for individual grouting pressure, but decreases with grouting pressure for particular matric suction. The apparent adhesion intercept increases with matric suction. and grouting pressure. The interface strength increases with matric suction at lower suction range, but decreases or remains nearly constant at higher suction range. Similar to CDG soil, the suction envelopes for different grouting pressures are nonlinear. The interface shear strength increases with grouting pressure at lower suctions for particular net stress. On the contrary, a downward trend is obvious for the interface strength under higher suctions for different grouting pressures and net stresses. The interface dilatancy (negative) decreases with grouting pressure. The average interface dilation angles for different grouting pressures are lower compared to those of soil under the same suctions and net stresses. A general model is proposed to predict the shear strength soil-cement interface incorporating the influence of dilation, matric suction, net stress and grouting pressure. The interface shear strength predicted from the proposed model agrees well with the experimental shear strength data. The shear strength of pressure grouted interface is greater than that of soil within the lower suction range for different net stresses. However, the interface strength is lower than the strength of soil at higher suction range. The strength of higher grouting pressure interface is greater than the strength of CDG soil as well as gravity grouted interface under different net stresses and at saturated condition. This indicates that the stability of slopes can be boosted up at saturated condition by the inclusion of pressure grouted soil nails into the slopes instead of gravity grouted soil nails. At saturated condition and lower suctions, the failure of slope may be happened in the soil as the strength of soil is lower than the strength of interface. On the other hand, at higher suctions, failure of slope may be happened in the interface zone rather than in the soil as the strength of soil is greater than the interface. At saturated condition, the interface behaves as a rough interface for different grouting pressures. However, at unsaturated condition, the behavior of soil-cement interface changes from rough interface towards the smooth interface as the grouting pressure is increased.