Introduction
Residual soils are found in many parts of the world and are extensively used in the construction of
foundations and as a construction material. In tropical areas, residual soil layers are often extensive and
may measure a few hundred meters until unweathered rock is reached.
The development of residual soils depends on the interaction of three natural features, that is, chemical
composition of the rock, environmental conditions and time, that give rise to state factors responsible
for the formation of soils.
Unsaturated soils are those where the pores are filled with both water and gas (usually air),
unlike saturated soils where the voids are filled entirely with water. Many tropical soils exist in
an unsaturated state due to soil water deficits induced by the tropical climate. While many
tropical regions can have high rainfall, this can be offset by even greater evaporation and
transpiration which removes water from the soil. For this scenario, the ground water table can be
at significant depths (perhaps greater than 10 m) which means that the zone of soil involved in
engineering and construction operations will be above the water table and potentially
unsaturated. This unsaturated zone above the water table is known as the Vadose zone. In
unsaturated conditions, the water phase is held in the soil by a negative pressure (or suction). The
effect of suction is very important in understanding how the soil will behave in an engineering
context. Suction affects the shear behavior and also controls volume changes in response to
wetting and drying. The fact that a soil is unsaturated also has a significant effect on the water
permeability (hydraulic conductivity).
there are two major differences between the water flow in saturated and unsaturated soils.
 Firstly, the ability of the unsaturated soils to retain water varies with soil has to be known.
 Secondly, the coefficient of water permeability is not a constant in unsaturated soils but it
is a function of soil suction.
Therefore, it is essential to determine
a) The so-called soil-water characteristics, which can be described as a measure of the water
storage capacity of the soil for given soil suction, and
b). the water permeability function that varies with soil suction for simulating transient seepage
in unsaturated soil slopes.
The soil water characteristics define the relationship between the soil (matric) suction and either
the gravimetric or volumetric water content or degree of saturation. The soil-water characteristics
are commonly determined in the laboratory using pressure plate apparatus, while the water
permeability function can be estimated from the measured saturated water permeability and the
soil-water characteristics (Brooks and Corey ,1964; Van Genuchten,1980; Mualem,1986;
Fredlund et al…1994)
A theoretical basis for unsaturated soils has been established over the past four decades. The
fundamental accepted principle in this theory is that the unsaturated soil behavior could not be
described just by making use of one stress state variable; in other words, both the net normal stress,
(σ−ua), where σ is the total stress and ua is the pore-air pressure, and the matric suction, (ua − uw),
where uw is the pore-water pressure, are generally required for the constitutive models (Clifton, et al.,
1999). As a result, evaluation of suction is most essential to assess the unsaturated soil behavior. The
direct laboratory testing of suction is time-consuming and costly laboratory tests, therefore estimation
of suction with indirect methods based on other parameters such as water content would be
considered. Consequently, the soil-water characteristic curve (SWCC) that defines the degree of
saturation corresponding to particular suction in the soil is widely used to estimate unsaturated soil
properties. Through the practical applications of the soil-water characteristic 2 curve, the prediction of
shear strength, water storage, and permeability coefficient may be pointed out (Murray, et al., 2000).
SWCC is not only able to describe the relationship between the strength of unsaturated soil and water
content, but also the distribution of water within the body of unsaturated soil. There are different
methods to evaluate the soil-water characteristic curve. However, these methods are divided into two
general branches: laboratory and estimating methods. The researchers, with taking account of the
general form of SWCC, have suggested some equations to approximate these curves.
The unsaturated shear strength of soil is essential for many geotechnical issues that related to
embankment, excavation, slope stability, and bearing capacity. Various equations were introduced
to determine the unsaturated shear strength of soil. One of the approaches used to develop these
equations was utilizing SWCC features together with saturated shear strength parameters. These
equations were successfully predicted shear strength at drying conditions. However, wetting shear
strength was not addressed by any of these published shear strength equations.
In order to predict the permeability function of an unsaturated soil accurately and hence facilities
slope stability assessment. It is essential to have a better understanding of the soil-water
characteristics of unsaturated soils. For transient flows and slope stability problems, osmotic
suction is normally not very important and therefore ignored. Matric suction is generally referred
to soil or total suction (i.e., matric suction plus osmotic suction)(Reference b643178)