A series of the results of triaxial tests on a quartz-rich sand under high pressures is presented, and the effect of pore water on the compressibility and the shear characteristics of the sand is investigated mainly from a mechano-chemical viewpoint. It is shown that pore water increases the compressibility and decreases the shear strength of the sand under high stresses, that is, the water sensitivity of the sand has a close relationship with the surface energy change of cracks which are produced in each particle under high stresses.