The paper presents the results of an experimental work where we analyse the behaviour of an unsaturated quartz sand in a wide range of degree of saturation (from saturated to dry state). The possibility of anticipating the hydro-mechanical behaviour of the soils when they approach the dry state is fundamental in many areas. An extensive experimental program, including controlled-suction and constant water content oedometric tests, was carried out to deeply analyse the water retention behaviour and the relationship between the yield stress and suction (Loading-Collapse curve). All elasto-plastic models provide a monotonically increase of the yield stress with suction. This assumption implies that the yield stress in the dry state is larger than the one relative to the saturated state, in contrast with the classical geotechnical points of view, which suggest that the yield stress of dry granular material must be approximately the same as that of the saturated one. The obtained results show that the yield stress of the sand does not increase monotonically with the suction, as predicted by commons models. In fact, the Loading - Collapse curve showed in this work presents a maximum point, and the yield stress for saturated condition is almost the same of the dried one.