||The application of Rubbersoil-Primer to fill slope protection against rainfall infiltration was first initiated in 2004. The engineering properties of Rubbersoil-SB, Rubbersoil-LGM and Rubbersoil-Primer were studied by the former researcher and the capability of Rubbersoils in protecting loose fill slopes were justified by the rainfall model test and a full scale field test in Sau Kei Wan. This thesis serves as a continuation to investigate the function of Rubbersoil-Primer as an interface material for controlling the hydraulic conditions of loose fill slopes and the engineering properties of a new Rubbersoil ------ Rubbersoil-Impermeable Primer were investigated in this thesis. The mechanical and hydraulic properties of Rubbersoil-Primer and Rubbersoil-Impermeable Primer ranging from unconfined compressive strength, saturated and unsaturated permeability, soil water characteristics to confined and unconfined shrinkage were studied by a series of laboratory tests. The performances of Rubbersoil fill slope under hypothetical and real rainfall condition were examined by the rainfall model test, numerical simulation and a full scale field test in Tuen Mun. In light of the fact that the soil-water characteristic (SWCC) measures the water storage capacity of the soil under a given soil suction and it is an essential hydraulic parameter for computing transient seepage in unsaturated soils, an indirect and a direct test making use of MIP (mercury intrusion porosiometry) and modified pressure plate were conducted to determine the SWCC of Rubbersoil-Primer of various densities. The measured SWCC together with the generated permeability functions were used as an input parameters for simulating the transient seepage analysis. Parametric study on the influences of the density and the thickness of Rubbersoil-Primer, the angle of slope inclination and the rainfall pattern on the pore water pressure distribution were performed. The numerical simulation results show that the matric suction of the loose fill soil can be maintained with the use of Rubbersoil-Primer over the density range from 1300 kg/m3 to 1700 kg/m3 and the pore water pressure distribution is found to be insensitive to the thickness of Rubbersoil-Primer. The numerical modeling results suggest that there is a room for reduction to the original design density and thickness of Rubbersoil-Primer. .It is also found that there is no significant change in the pore water pressure distribution with respect to the change in the fill slope inclination and the matric suction of the fill soil is maintained under a 10 hour black rain event as well as a real rainfall event which triggered the 1976 Sau Mau Ping landslide. The capability of Rubbersoil-Primer in protecting fill slope under rainfall infiltration was also assured by the physical rainfall model test and the full scale field test. The matric suction of the loose fill soil was maintained over a 2 hour intense rainfall in the rainfall model test. The lateral and vertical settlements of the fill slope in Tuen Mun Ching Lung Nursery over 1 year monitoring period were also found to be far below the alert level.