In simple terms what the difference between drained and undrained condition ?
When soils shear, there is a potential change in the void ratio. If the void ratio increases, water must flow into the sample and if the void ratio decreases, water must flow out of the sample. For the case of sands (i.e., soils with high permeability) this is almost a moot point (unless there’s an earthquake). For the case of clays, this can be a big deal. If the shearing takes place faster than the soil can drain, the buildup of water pressure in the pores of the soil can have a GREAT influence on the soil strength.