Control of Groundwater by Trenching Method for Slope Stability of Granite Soil Area

The objective of this study is to find the influence of lowering groundwater level on
slope stability. The project is examines soil moisture as a factor that influence the slope
failures, soil characteristics, and the design of the control method for residual soil from
granite. The slope stability issues concerning some factors that influenced the slope
failures are investigated and presented. It focuses on granite soil because most landslides
in Malaysia occur in granite soil. Residual soil in granitic area consists of minerals from
the weathering products of feldspar, quartz and mica minerals. Soil are taken from Bukit
Kledang and brought to Geotechnical Laboratory for tests. From the laboratory session,
the soil is a fine soil containing 17.75% of moisture content during normal condition,
and has acidic pH value of 4.71. The specific gravity of granite soil is 2.655. The liquid
and plastic limit tests indicate that the plasticity index of the soil is 7.46%. The content
meaning the soil is silt with low plasticity (ML) according to the plasticity chart. The
soil also has a hydraulic conductivity, k from 0.001 to 0.01 cm/sec which still under the
standard of fine sand. In addition, from the shear box test, the shear strength of the soil
from the study area is 82.907-141.029 kN/m2. The modification of the soil; by adding
5% of Calcium Hydroxide (CaOH) is not really affected the effectiveness of the water
flow. Even though the modified soil has higher k value, immediate test shows that it has
low shear strength. Trenching is an innovative technique to construct a protective trench
at the slope area by using gravel and lined with geotextile. The design of trenching also
consists of perforated pipes and drain to flow the groundwater out. Trenching is a good
option to intercept groundwater which is perched above a relatively impermeable soil.
Laboratory testing was conducted based on a trenching model for granite soil based on
the design. From the tests conducted to the trenching model, it shows that the moisture
content of the soil (15.96-21.22%) is less than the liquid limit (39.50%). It means the
trenching lowers the moisture level. The water that flow out from the perforated pipe is
also clear and without sediment. From the Slope/W analysis, the factor of safety
determined is 1.454 in order to get the stable factor of safety as applied by Malaysia
standard. This control method is considered to be cost effective because the materials to
be used are confined to the trench only