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Most research conducted on slope failures can broadly be attributed to the convergence of three factors, i.e. rainfall, steepness of slope, and soil geological profile. The mechanism of the failures is mainly due to the loss of matric suction of soils by rainwater. When rainwater infiltrates into the slopes, it will start to saturate the soil, i.e., reduce the matric suction. A good understanding of landslide mechanisms and the characteristics of unsaturated soil and rock in tropical areas is crucial in landslide hazard formulation. Most of the slope failures in unsaturated tropical residual soil in Malaysia are mainly due to infiltration, especially during intense and prolonged rainfall, which reduces the soil matric suction and hence decreases the stability of the slope. Therefore, the aim of this research is to determine the matric suction for barren soil and to model an unsaturated slope with natural rainfall to evaluate the effects of matric suction on rainfall intensity. A field test was carried out using the Watermark Soil Moisture Sensor to determine the matric suction. The sensor was connected to a program called SpecWare 9 Basic which also used Data Logging Rain gauge Watermark 1120 to measure the intensity and duration of rainfall. This study was conducted at the Research Centre for Soft Soil which is a new Research and Development (R & D) initiative by Universiti Tun Hussein Onn Malaysia, Parit Raja. Field observation showed that the highest daily suction was recorded during noon while the lowest suction was obtained at night and early morning. The highest matric suction for loose condition was 31.0 kPa while the highest matric suction for compacted condition was 32.4 kPa. The results implied that the field suction variation was not only governed by the rainfall, but also the cyclic evaporation process. The findings clearly indicated that the changes in soil suction distribution patterns occurred due to different weather conditions.

The most fundamental of unsaturated soil is matric suction parameter. Matric suction in unsaturated soils plays an important role in controlling physical and mechanical properties. This parameter can influence some other important parameters of soil, such as effective stress, shear strength, swelling pressure, etc. Investigation of matric suction was performed on Ayer Hitam soil, at Batu Pahat District, Johor, Malaysia. Field monitoring works were conducted with and without dolomite treatment at the slopes of Ayer Hitam acidic soil. Field testing used some devices to find out the behavior of matric suction during the rainy session. The result shows that the soil matric suction with dolomite treatment is larger than without treatment and matric suction has a significant difference around 20 – 50 kPa. Enhancement of acidity from the dolomite treatment was not only decreasing pores size caused by the alternating of the microstructure of soil but also leading to increasing the matric suction. The treatment of dolomite material is able to control matric suction and increase the stability of slope acidic Ayer Hitam soil.