Hydro-Mechanical Properties of an Unsaturated Frictional Material

Chapter 10
Bearing Capacity of a Strip Footing
on Unsaturated Hostun Sand
10.1 General
Most of the shallow foundations are typically located above the groundwater table that re-
quires an understanding of bearing capacity and settlement behavior of soils under unsatu-
rated conditions. The influence of soil suction is however not addressed in conventional design
of foundations and a framework for interpreting these results is not available. The founda-
tions are conventionally designed assuming saturated conditions for the soil. To investigate
the influence of suction on the bearing capacity, a series of model footing tests was carried
out to determine the ultimate bearing capacity of the Hostun sand respectively under both
unsaturated (ψ = 2, 3 and 4 kPa) and saturated (S = 0 as well as S = 1) conditions. A
box was specially designed to load model continuous (i.e. strip) footings under plane strain
condition. The experimental results show that ultimate bearing capacity increases up to a
certain suction value and then decreases with an increase in the suction value.
Further the soil-water characteristic curve as well as saturated shear strength parameter
are used to predict unsaturated bearing capacity. Therefore the model proposed by Vanapalli
& Mohamed (2007) is used.
The bearing capacity is the key parameter required when designing foundations. The
bearing capacity theory on saturated soil has been investigated in the last century by many
researchers. Prandtl (1921) was one of the first author, who studied the behavior of a strip
footing. The author loaded the strip footing until it penetrated into the soil. Prandtl (1921)
introduced the ultimate bearing capacity qu of a soil, that is the stress applied for reaching
failure in the tested soil. Bearing capacity analysis was developed by Terzaghi (1943), Meyer-
hof (1951), Vesic (1973), Bolton (1986). Based on the assumption, that a soil is either in fluid
saturated condition or dry condition several researchers as for instance Prandtl (1921), Terza-
ghi (1943), Meyerhof (1951) proposed approaches for the estimation of the bearing capacity.
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