Hydro-Mechanical Properties of an Unsaturated Frictional Material

CHAPTER 10. BEARING CAPACITY OF A STRIP FOOTING ON UNSATURATED
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HOSTUN SAND
settlements. With increasing settlement the load and also the stress are linearly increasing.
With larger settlements this behavior becomes non-linear until reaching maximum value (i.e.
ultimate bearing capacity), where the soil fails. After the failure the settlements are further
increasing but the load and the stress are decreasing. From the results clearly can be seen,
that the unsaturated specimens have a higher bearing capacity than the saturated specimens.
While the maximum load reached for the unsaturated specimen is Fmax = 19 kN, that refers
to a stress of σmax = qu = 517 kN/m 2 , the maximum load of the air saturated (S=0) specimen
is Fmax = 4.9 kN, that corresponds to a stress of σmax = qu = 132 kN/m 2 . The maximum
value of the applied load and the stress is reached for the unsaturated specimen for larger
settlement value. The results show that the suction and thus the water content in a sand
is strongly influencing the behavior of the bearing capacity of strip footings. The bearing
capacity of the sand tested under unsaturated condition was found to be approximately 2.5
to 4 times higher then for the saturated specimen. The bearing capacity is increasing with
increase in suction and then after reaching an maximum value decreasing.
10.5 Prediction of Bearing Capacity
Following the proposal given by Vanapalli & Mohamed (2007) the bearing capacity is es-
timated using Eq. 10.3 and compared to the experimental data. Therefore the drainage
soil-water characteristic curve derived from the modified pressure plate apparatus for dense
specimen, the bearing capacity factors derived by Terzaghi and the shape factors derived from
Eqs. 10.5 and 10.6, a friction angle of φ = 46.9 ◦ as well as an air-entry value of ψaev = 1.9 kPa
were used, the average suction is equal to the suction observed by the tensiometer. The pa-
rameter κ is set to 1. This equation takes the same form as Terzaghi’s equation shown in
Eq. 10.1 without the surcharge contribution if (ua − uw) value is set to zero. The contri-
bution of surcharge is not necessary in the present study as the model footing was loaded
placing them directly on the sand surface. Important parameters used for the prediction are
also given in Tab. 10.1. In Fig. 10.8 the experimental results in comparison to the predicted
results are shown. The prediction shows an increase of the bearing capacity with increas-
ing suction, that is consistent with the experimental results. Overall observation is that the
predicted values follow the trend of the experimental results. Whereas after reaching the air-
entry value (transition zone) the experimental results of bearing capacity are still increasing
the predicted values are already decreasing. The experimental results are underestimated by
the predicted results. In the residual zone a decrease of bearing capacity was observed by
Table 10.1: Parameters used for prediction of bearing capacity
Parameter ψaev φ Nc Nγ ξc ξγ
1.9 kPa 46.9 ◦ 223 407 1 0.93