Abstract:
Theories and formulations developed for the behaviour of unsaturated soils incorporate
the behaviour of saturated soils as a special case, leading to a generalization of the theory
on mechanics for soils. The net normal stress (a-ua ) and the matric suction (ua-uw ) are the
two stress state variables that are required to describe the state of an unsaturated soil. The
soil-water characteristic curve has been shown to be a key soil property function which
can be used to approximately simulate the behaviour of unsaturated soils. As the soil
moves from a saturated state to a dry state, the distribution of soil, water and air phases
change, giving rise to changes in the stress state. The influence of these phases on the
soil-water characteristic curve and hence on the engineering behaviour of unsaturated
soils, is of importance in the prediction of unsaturated soil properties.
Unsaturated shear strength functions and soil-water characteristic curves of soils obtained
from Pussellawa and Kahagalla landslide sites, Sri Lanka, are developed over a matric
suction value range of 0 - 800 kPa, based on the results obtained from a series of
laboratory tri-axial tests on saturated and unsaturated samples. During tri-axial tests on
unsaturated samples, pore-air pressure and pore-water pressure are controlled to maintain
suction at a constant value in the soil specimen throughout any stage of the test. Saturated
high air entry disks are installed at the bottom of the soil specimen to prevent the entry of
pressurized air into the water supply system maintained at a lower pressure. The soilwater
characteristic curves developed are compared with such typical curves reported in
the literature.
The shear strength function of the soil at Pussellawa landslide site for the matric suction
range of 50 kPa - 575 kPa can be expressed as:
t = 20 + ( s n - Ua) tan 33+ ° ( u a - u w ) tan 11.77°
The shear strength function of the soil at Kahagalla landslide site for the matric suction
range of 100 kPa - 500 kPa can be expressed as:
t = 22.3 + ( s n - u ^ tan 25.2+ ° ( u a - u w ) tan 18.99°
In a parallel study, a 5 bar pressure plate apparatus is used to develop soil-water
characteristic curves of Pussellawa and Kahagalla landslide sites for a limited range of
matric suction values. In the laboratory, a matric suction is applied to the soil specimen
by maintaining a zero excess pore-water pressure (i.e., atmospheric pressure) and
applying a positive pore-air pressure. In this case also, a saturated ceramic plate is used to
prevent the entry of high pressure air into the compartment below the disk.
The soil-water characteristic curves of soil at Pussellawa and Kahagalla landslide sites
were developed by using Modified tri-axial tests and Pressure plate tests.
Slope stability of Pussellawa and Kahagalla landslides are investigated to demonstrate the
effect of partial saturation on the factor of safety against failure. Stability of each slope is
analysed by developing a computer spread-sheet programme to consider saturated and
unsaturated cases. The Modified Janbu's simplified method is used to analyze two slopes
to obtain factor of safety values. For the analysis, location of the water table is changed in
such a way that full saturation and partial saturation of slopes are achieved.
The slope stability analysis of Pussellawa and Kahagalla landslides show that slopes are
currently stable with the parameters obtained from laboratory tests. The slope stability
analysis shows that lowering the water table increases the factor of safety against failure
of a soil slope, due to the influence of matric suction (partial saturation).