Abstract: One special hydrogeologic phenomenon was found when building well drilling for Xiaojihan mine located in Yulin city, north of Shaanxi province. The coal seam is a main aquifer to the geologic structure of the nearby area. Different tests including rock mineral composition test for floor rock samples, rock expansion test and the rock strength test under water pressure have been applied to find out the mechanical properties of the floor rock stratum under this special hydrogeologic phenomenon. The following conclusions have been drawn: 1) The coal seam of Xiaojihan mine is found out to be a fracture aquifer as well as a main aquifer. The floor rock has been exposed to water erosion for a long time, which makes the rock body easier to be deformed or softened. This will cause a fracture expansion of the floor strata crack and damage the stability of the floor strata. 2) Some dilating mineral such as montmorillonite and illite are determined from the floor rock sample. The dilatability of these minerals can be the micro-influencing factor for the expansion deformation of the sample. 3) Three stages were observed in the rock expansion test of floor rock samples: rapid expansion, slow deformation and the end of deformation. Within 2 hours after the test started, a deformation expansion occupied 73.3% of total expansion was observed. From the third to the 12th hour after the test started, the deformation expansion of this stage occupied 24.7% of total expansion. After 12 hours, no deformation was observed. 4) The compressive strength, elastic modulus and shear strength decreased as the moisture content increased. The relationship was calculated to be exponentially declining. 5) After 12 hours’ under-water test, the compressive strength, elastic modulus and shear strength decreased by 71.6%, 72.3% and 65.4% respectively. It is obvious that coal seam fissure water has significant effect on the erosion and softens of the floor rock.

Key words: coal under aquifer, deformation and failure character, floor water inrush, rock strength weakening