Prof. D. N. Singh

LALIT BORANA

Ph. D.
The Hong Kong
Polytechnic University,
Hung Hom, Kowloon, Hong
Kong, China.
Indian Institute of Technology Bombay, Powai, India lalitborana@gmail.com

STUDY ON THE INTERFACE BEHAVIOUR BETWEEN UNSATURATED SOIL AND STEEL SURFACE

An interface formed between a structural material and an unsaturated soil is common in various civil engineering projects. The ultimate shear strength at the interface is an important parameter for the design and safety assessment of the structures in the soils and is also a key factor in the design and analysis of the structural interfacial interactions with soil. Matric suction has a predominant effect on the shear strength and volume change behaviour of the soil and soil-steel interfaces, thereby proper characterization of interface behaviour is important for its accurate performance predictions. The experimental study of the interface behaviour plays an important role in advancing understanding of the complex behaviour of soil-steel interfaces. It is believed that the critical interface plane which possesses the minimum shear strength exists on the counterface surface and no definite criterion for selection of interface layer thickness for different soil-structure interfaces is readily available. The main focus of this study is to investigate the behaviour of the interface between compacted completely decomposed granite (CDG) soil and steel counterfaces at (a) different shearing planes and (b) different counterface roughness under the influence of similar matric suctions, net stresses. Considering this main focus, an effort has been made to examine the variation of shear strength for pure soil and soil-steel interface under the controlled conditions by employing a modified suction controlled direct shear testing device. Firstly, to investigate the elementary behaviour of the soil-steel interface (rough) at different shearing planes and compare it with the behaviour of the soil, a number of single-staged consolidated drained direct shear tests were carried out on interface and pure CDG soil specimens, under different matric suctions and net normal stresses. The results confirm that the suction and net normal stress significantly influence the shear behaviour of pure soil and interfaces sheared at different planes. The behaviour of stress-displacement curves of soil-steel interface tests is similar to those of soil tests. The failure envelopes related to matric suction were noted to be nonlinear. The degree of dilatancy contributes to the gain in shear strength and is dependent both on matric suction and net normal stress. It is noted that the matric suction plays an important role in evaluating the critical shear strength, as it directly affects the volume change behaviour. The critical failure plane is dependent on the matric suction and an increase in suction gradually shifts critical shear plane from the soil towards the counterface. The experimental results compares well with shear strength model proposed by Hossain and Yin (2010), which considers the influence of dilation. Secondly, to examine the elementary interface behaviour with different counterface roughness and compare it with the behaviour of pure CDG soil, a series of interface direct shear tests were performed between CDG soil and steel plate with different counterface roughness ( Rn= 0.025 , Rn= 5 and Rn= 10) under the same matric suctions and net normal stresses. The test results show that the counterface roughness, matric suction and net normal stress have significant influence on the hardening-softening and contractive-dilative interface behaviour. The presence of higher net normal stress results in a partial stick slip behaviour during the shearing of soil-steel interfaces. The suction envelopes for different counterface roughness were observed to be non-linear and the apparent interface friction angle increases with matric suction. The experimental shear strength data is compared with an analytical model, which considers the influence of dilation on the apparent interface friction angle. This comparison corroborates well for all the applied stress-state variables. Finally, a series of model pile pull out tests were performed under different unsaturated conditions to evaluate the influence of surface roughness and water content on the elementary interface behaviour. To measure the axial strain along the model pile during pull out, the model piles were installed with fibre Gragg grating (FBG) sensors. The pullout test results are in good agreement with the observations from direct shear tests. The pullout forces are noted to be directly proportional to the surface roughness and indirectly proportional the water content of the soil. The axial strain values along the model pile obtained from FBG sensors reveal that the strain at the top of model pile is notably greater than that at the base.