Unsaturated granite residual soil is mainly ferruginous cement

its strength decreases sharply in water

structural significant

showing brittle-elastoplastic damage characteristics

whose soil particle joint function is the core problem of structure and brittle- elastoplastic damage. The unsaturated soil particle joint function is discussed; it is divided into contact and non-contact joints

and these two kinds of joints are expressed as suction effects in soil particle intergranular interactions

that is the suction between grains composed of variable structure suction and wet suction. For unsaturated granite residual soil

particle joint function should be paid more attention. Unsaturated granite residual soil’s contact joint is dominated by ferruginous cementation and all levels of granularity composition mixtures as sand

clay and so on; they cause the suction between grains changing greatly in water or disturbance

resulting in the properties of the brittleness in the mechanics of unsaturated granite residual soil. Then through theoretical derivation

by referencing coordination number

the Smith formula

and the Fisher formula in mineralogy

crystal powder mechanics and engineering

quantitative calculation formula of variable structure suction and the soil accumulation mode

dry density

void ratio

water / saturation are given; and the rationality of the calculation formula is verified by tests. Then we research and realize that free iron oxide cement is the fundamental point of granite residual soil’s property of brittle-elastoplasticity

so a masonry model is used; and then void ratio and structure parameter are applied to construct the brittle damage process function; a theory model which can reflect the damage of unsaturated granite residual soil elastoplasticity is established. Through the comparison of calculation and experiment results. The results show that the model established above can well reflect the unique stress-strain relationship of unsaturated granite residual soil. It can explain well the mechanism of unsaturated granite residual soil which is easily destroyed in water or disturbance according to theory.