Study of the soil-interaction behaviour with underground structures under unsaturated conditions
Abstract
Soil-structure interface behaviour is an interesting topic due to the complexity of the
interface mechanics. The interfaces between cohesionless soils and solid structure
elements are encountered in different geotechnical engineering projects. The shear
strength and stiffness characteristics, the thickness of the interfacial layer, the bonding
and slipping properties are playing significant roles in an understanding the mechanical
behaviour of such interfaces. One of the key parameters for the design and safety
assessment of the engineering structures (e.g. retaining walls, deep and shallow
foundations, tunnels and earth reinforcement) is the shear strength at the interface.
Suction is an important stress-state variable of unsaturated soils. The magnitude of
matric suction affects the shear strength and the volume change of soil and soil-concrete
interfaces, thus the adequate characterization of interface behaviour is significant for its
precise performance predictions.
The prime aim behind this study is to investigate the behaviour of interface between
compacted silty sand soil and concrete counterfaces at (a) different initial void ratios,
(b) different surface roughness (smooth and rough) under the influence of different
levels of applied vertical stress and test conditions (saturated and constant water
content). The second main objective of this study is to investigate the effect of void
ratio and the effect of the applied vertical stress level on the variation of matric suction
during direct shear tests (matric suction stabilisation, consolidation and shearing stages).
To do so, a new loading steel cap of large-scale direct shear apparatus for testing soilsoil and soil-concrete specimens has been manufactured.
Firstly, a series of large-scale (300 mm x 300 mm) direct shear tests were carried out on
compacted soil samples under different levels of applied vertical stress, void ratios and
test conditions (saturated and constant water content). The experimental results confirm
the dependency of shear strength, volumetric behaviour and measured matric suction on
the vertical stress and initial void ratio. Secondly, to investigate the interface behaviour
with different surface roughness (smooth and rough) and compare it with the behaviour
of soil-soil samples, a number of interface direct shear tests were conducted between
silty sand and a concrete pad under the same levels of vertical stress, void ratios and test
conditions. The trend of behaviour of the shear strength versus horizontal displacement
curves of soil-concrete interface tests is similar to those of soil tests. The laboratory
tests results show that the surface roughness, vertical stress, void ratio and test conditions have significant influence on the shearing characteristics of the interface
samples. The study noted that the strain softening behaviour of the tested material is
noticeably influenced by the initial void ratio of specimens and surface roughness for
both test conditions. From the results, it was observed that the initial matric suction has
a clear dependency on the void ratio of the specimens. The shearing behaviour of the
soil samples was higher than the rough and smooth interfaces for both studied void
ratios and test conditions, whereas, the smooth interface showed lower values of shear
strength of all the tested samples. It was noted that there is a remarkable decrease in
matric suction during shearing stage with the level of applied vertical stress and the
most important matric suction evolution was occurred before the horizontal
displacement corresponding to the peak/maximum shear strength achieved.