Study on the interface shear strength of soil nailing in completely decomposed granite (CDG) soil

Abstract

Soil nailing is an in situ soil reinforcing technique used to stabilize existing slopes and excavations. This technique has been widely used during the past two decades due to its technical and economical advantages over conventional stabilization methods. One of the most important parameters governing the design and safety assessment of soil nailing is the ultimate shear strength at the interface between cement grout and the surrounding soil. However, this parameter is estimated from an assumed skin friction on the interface between the soil and soil nail cement grout and is then verified by pull-out tests during construction. The objective of this study is to investigate the interface shear strength behaviour between the soil and soil nail cement grout by pull-out tests and direct shear box interface shear tests. A series of tests are carried out to better understand the interface shear strength behaviour of soil nailing where soil moisture contents, cement surface roughness and stress conditions is varied. The soils used in the study are a Completely Decomposed Granite (CDG) taken from a soil-nail stabilized slope site in Hong Kong. The CDG soil is a common type of slope soils in Hong Kong. A pull-out box apparatus is designed by the author in order to perform laboratory pull-out tests, allowing a sealed condition for the pull-out test of a grouted nail on a water submerged soil with a vertical stress up to 300kPa imposed on the testing soil. The soil nail tested is a 32mm diameter ribbed steel bar surrounded by gravity cement grout in a 100mm diameter drilled hole. A pull-out test is conducted on the grouted nail that is cured inside the soil for 28 days. The influence of soil degrees of saturation on the pull-out shear strength is investigated. A numerical analysis is also carried out to assess the stress conditions within the sample in a pull-out box test. A series of interface shear tests has been performed using a large-size direct shear box machine. Soil-cement grout models are made to simulate the soil and cement grout interface condition of a soil nail. This test series particularly emphasizes the effects of cement surface roughness on the interface shear strength of the soil and cement grout. It is found from the investigation that the interface shear stress-displacement behaviour is dependent on the vertical stress, the degree of saturation of the soil, and the cement grout surface roughness.