Soil nailing has been widely used for improving slope stability over the last three decades around the world and is currently the predominant method used for upgrading the stability of existing soil cut slopes in Hong Kong. Although interaction between soil and nails has been investigated widely, only few studies were conducted on the role of nail heads. Therefore, the objectives of this research are to further improve the fundamental understanding of the behaviour of nailed slopes and to investigate the role of nail heads in improving slope stability and reducing ground deformation....[ Read more ]

Soil nailing has been widely used for improving slope stability over the last three decades around the world and is currently the predominant method used for upgrading the stability of existing soil cut slopes in Hong Kong. Although interaction between soil and nails has been investigated widely, only few studies were conducted on the role of nail heads. Therefore, the objectives of this research are to further improve the fundamental understanding of the behaviour of nailed slopes and to investigate the role of nail heads in improving slope stability and reducing ground deformation.

In this research, a series of CDG dense fill model slopes reinforced with different sizes of nail heads including grillage were destabilized by controlling long-term transient seepage in centrifuge. The prototype slope angle and height were 65⁰ and 15m, respectively. Pore water pressures, slope deformation, axial nail forces and contact pressures between soil and nail heads were measured. Pore water pressure transducers were modified into tensiometers to measure high suction in centrifuge. A four-stage procedure was developed to saturate the tensiometers. Furthermore, an experimental framework was estabilished to identify whether a tensiometer had been desaturated or not. Three-dimensional coupled numerical simulations using ABAQUS were conducted to back-analyse the centrifuge test results. The three-dimensional numerical simulations considered the coupled effects of soil deformation and pore water pressure changes.

It is found that, a global failure was evident in the unreinforced slope after being subjected to long-term transient seepage. Installation of soil nails prevented the formation of the global failure but not a tension crack at the crest. The presence of nail heads prevented the global failure and the formation of the crack after even over two years of transient seepage. With the greater sizes of nail heads, (a) crest settlements decreased, (b) the mobilised axial force at the nail head on each nail and a total of each mobilised maximum axial nail force (T_max) increased, (c) the location of the T_max shifted towards the slope surface, and (d) the normalized contact pressures increased at the quasi-steady state. The global failure which occurred in the unreinforced slope in centrifuge was reasonably consistent with numerical simulations. General trends in crest settlements and axial forces at the quasi-steady state were also captured in the numerical simulations.