Scale and water effects on the friction angles of two granular soils with different roughness

An integrated experimental study is presented which aims at relating the frictional properties at the particle scale to the bulk mechanical behavior for two different types of sands. We performed direct shear tests and inter-particle tests on lunar regolith simulant DNA-1A and Ottawa sand (benchmark material) under both dry and wet conditions. We found higher macroscopic friction angles for the lunar simulant in both dry and wet conditions, a smaller strength decay for Ottawa sand during reversal direct shear tests and similar strength envelopes of both materials under wet and dry conditions. Particle-scale tests evidenced higher inter-particle friction for DNA-1A in wet conditions with respect to the dry case for normal force lower than 2–3 N. For the lunar simulant, the differences between bulk and inter-particle friction appeared to be emphasized in dry condition and an evident effect of water on the friction coefficient was found only at the micro-scale.