Hydro-Mechanical Properties of an Unsaturated Frictional Material

CHAPTER 10. BEARING CAPACITY OF A STRIP FOOTING ON UNSATURATED
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HOSTUN SAND
accuracy of the applied load is 0.05%. A maximum load of 40 kN can be applied when using
the loading system. When tensiometer sensor measurements were carried out during testing
procedure adequate data logging system was used to compute pore-water pressure.
10.3 Experimental Program
Bearing capacity tests include the loading of a strip footing model, that is placed on the surface
of the saturated sand specimen (i.e. either fluid or air saturated) as well as the unsaturated
sand specimen (ψ = 2, 3 and 4 kPa). The footing was loaded at a constant rate of 0.002 mm/s.
Also during loading procedure the footing is not fixed to vertical deformations (i.e. horizontal
deformations occur). In a depth of 50 mm the pore-water pressure (i.e. matric suction)
was measured using tensiometer sensor when bearing capacity of unsaturated specimen was
investigated.
The dry specimen with a height of about 36 cm was prepared by uniformly pluviating oven
dry sand with a funnel (500 ml capacity) into the bearing capacity box in several layers. After
each layer the specimen was compacted using a 2 kg hand compactor. The water saturated
and unsaturated specimen with a height of about 36 cm were prepared by uniformly pluviating
oven dry sand with a funnel (500 ml capacity) into the bearing capacity box filled with deaired
water. The box was filled with water successively using a water tank, that was connected at
one side to the water supply during specimen preparation. The deaired water was stepwise
injected into the box from the tank. During the specimen preparation the water level was
always kept above the sand specimen to avoid occlusion of air. Similar to the preparation of
the dense specimen in the sand column device I the falling height of the sand specimen was
approximately 30 cm. To reach unsaturated condition, a matric suction was then induced to
the initially fluid saturated specimen by using hanging water column technique. Therefore the
connection to the water tank was replaced by a connection to a burette. For preparation of
an unsaturated specimen the height of water in both burettes (one at the right hand and one
on the left hand side) was set 200, 300 and 400 mm below the surface of the sand specimen.
The bearing capacity tests were performed on specimen with a void ration e = 0.70. Denser
specimen was not achieved during specimen preparation. When the bearing capacity was
tested on the unsaturated specimen pore-water pressure was measured in the vicinity of the
expected stress bulb near the model footing using tensiometer.
10.4 Experimental Results
Fig. 10.6 shows the failure pattern observed for the present study undertaken with a smooth
footing. The failure pattern is consistent with Gussmann’s study showing a smooth surface
footing. Some horizontal deformation and sliding occurs (also because the footing is not
vertically fixed during loading) along with vertical deformations when a footing has a smooth