Hydro-Mechanical Properties of an Unsaturated Frictional Material

CHAPTER 10. BEARING CAPACITY OF A STRIP FOOTING ON UNSATURATED
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HOSTUN SAND
The solution given by Terzaghi (1943) is accepted since decades:
qu = Ncc ′ + Nqq + 0.5BNγγ (10.1)
where: qu is the ultimate bearing capacity, Nc, Nq as well as Nγ are bearing capacity factors
providing the contribution of cohesion in the soil, the surcharge and unit weight, c ′ is the
cohesion, q is the overburden pressure, γ is the weight of the soil and B is the width of the
footing. Bearing capacity factors based on different assumptions were proposed by several
researchers (Terzaghi 1943, Meyerhof 1951, Gussmann 1986, Kumbhojkar 1993, Zhu et al.
2001).
However, foundation designs are complex and due to presence of suction in the soil (i.e.
most foundations are located above the groundwater table) an understanding of the soil
behavior that comprises the role of suction is required. Only view researchers performed ex-
perimental investigations on the influence of suction on the bearing capacity. Broms (1963)
found an influence of suction on the bearing capacity of flexible pavements, Oloo et al. (1997)
carried out the influence of suction of unpaved roads and presented a procedure for the de-
termination of the bearing capacity in pavement systems. Plate load tests were conducted
by Steensen-Bach et al. (1987), Costa et al. (2003), Rojas et al. (2007) on unsaturated sand,
lateritic soils and lean clay. Their field tests showed increasing bearing capacity with increase
in suction. Bearing capacity tests of a square model surface footing were carried out by Mo-
hamed & Vanapalli (2006), Vanapalli & Mohamed (2007) on an unsaturated coarse grained
soil. The authors found the bearing capacity derived from unsaturated specimen 5 to 7 times
higher than the bearing capacity from saturated specimen.
Since several experimental studies show non-linear behavior of unsaturated shear
strength (Gan et al. 1988, Escario & Juca 1989, Vanapalli & Mohamed 2007), Vanapalli
et al. (1996) extended the linear unsaturated shear strength theory proposed by Fredlund
et al. (1978). The authors proposed an non-linear equation for prediction of unsaturated
shear strength using the soil-water characteristic curve in combination with the saturated
shear strength:
τ = [c ′ + (σn − ua) tan φ] + (ua − uw)(S κ tan φ) (10.2)
where: c and φ are the cohesion and friction angle of saturated soil for a particular net stress
σn, (ua − uw) is the difference between pore-air pressure and pore-water pressure, that is the
matric suction, S is the saturation and κ is a fitting parameter. The matric suction and the
saturation are derived from the soil-water characteristic curve. The first part includes the
parameters of saturated shear strength and the second part includes shear strength due to
matric suction. Using the term, that describes the non-linear behavior of the unsaturated
shear strength ((ua − uw)S κ tan φ) following equation was proposed to predict the bearing
capacity of unsaturated soil (Vanapalli & Mohamed 2007):
qu = [c ′ + (ua − uw)S α tan φ] · Ncξc + 0.5BNγγξγ
(10.3)