Laboratory pull-out testing study on soil nails in compacted completely decomposed granite fill

Abstract

Soil nailing is a technique for stabilizing soil slopes and excavations by installing a large number of closely spaced passive inclusions into the in-situ soil mass. The soil nailing technique has been increasingly used worldwide since its origination in the early 1970's because of its technical and economical advantages. In Hong Kong, soil nailing has been commonly used to stabilize new cut and sub-standard existing slopes since the late 1980's. The interface shear strength between a soil nail and the surrounding soil is a key parameter for design and stability assessment of the soil nailing system. However, in current practice in Hong Kong, this parameter is generally assumed to be the same as the shear strength of the soil and verified by field pull-out tests in the construction stage. Field verification tests are normally subjected to variations of the site conditions and the results are therefore scattered. Laboratory pull-out tests have been carried out to help overcome these problems and precisely investigate the factors influencing the nail-soil interface shear strength. However, there were still some deficiencies in these tests and can be improved. A laboratory study of the pull-out shear resistance of cement grouted soil nails was therefore conducted in compacted completely decomposed granite (CDG) fill. A pull-out box with the internal dimensions of 1.0m in length, 0.6m in width and 0.83m in height was designed and constructed to carry out the pull-out tests. An extension cylindrical chamber was provided to house an extension part of the nail and ensure that a constant 1.0m length of the test soil nail was maintained within the test box during pull-out and no cavity would be left behind the end of the test nail. A waterproof front cap was used to cover the soil nail head and prevent water leakage which made it possible to apply back pressure to saturate the testing soil in submerged tests. Comprehensive instrumentation was used and the earth pressure, suction, and pore water pressure in the soil, the deformation of the testing soil, and the pull-out force and displacement were measured. During the pull-out tests, the overburden pressure was applied before drilling to simulate the actual construction procedure of the soil nailing system. A series of pull-out tests have been conducted using two copies of the above introduced pull-out box. The test results showed that soil stresses around the hole were largely released after drilling and recovery of the stresses due to grouting of the soil nail was minimal. The development of pull-out shear resistance was mainly derived from the constrained dilatancy of the soil. Tests in soil at different degrees of saturation showed that the peak pull-out shear resistance varies with different degrees of saturation of the soil, with higher resistances at the degrees of saturation of 50% and 75%. Pressure grouting tests were carried out and showed that the average peak pull-out shear resistance of the soil nail increased almost linearly with the increase in grouting pressure. Numerical modeling was performed and agreements between the measured and simulated results were good.