  We investigate shear strength properties of wet granular materials in the
pendular state (i.e. the state where the liquid phase is discontinuous) as a
function of water content. Sand and glass beads were wetted and tested in a
direct shear cell and under various confining pressures. In parallel, we
carried out three-dimensional molecular dynamics simulations by using an
explicit equation expressing capillary force as a function of interparticle
distance, water bridge volume and surface tension. We show that, due to the
peculiar features of capillary interactions, the major influence of water
content over the shear strength stems from the distribution of liquid bonds.
This property results in shear strength saturation as a function of water
content. We arrive at the same conclusion by a microscopic analysis of the
shear strength. We propose a model that accounts for the capillary force, the
granular texture and particle size polydispersity. We find fairly good
agreement of the theoretical estimate of the shear strength with both
experimental data and simulations. From numerical data, we analyze the
connectivity and anisotropy of different classes of liquid bonds according to
the sign and level of the normal force as well as the bond direction. We find
that weak compressive bonds are almost isotropically distributed whereas strong
compressive and tensile bonds have a pronounced anisotropy. The probability
distribution function of normal forces is exponentially decreasing for strong
compressive bonds, a decreasing power-law function over nearly one decade for
weak compressive bonds and an increasing linear function in the range of
tensile bonds. These features suggest that different bond classes do not play
the same role with respect to the shear strength.
