The Study Of The Properties And Classifications Of Rock Masses In Enshi District, Hubei Province, China

This work describes the results of rock properties and classification studies that carried out in Enshi District, which situated in southwestern Hubei Province. It is bounded by latitudes 29°50' and 30°39' N, and longitudes 109°04' and 109°58' E, including Enshi city, Jianshi county, Hefeng county, Xianfeng county, Laifeng county, and Xuan'en county with total area of 6500 km². The area is located in the central mountainous area of western Hubei, Yunnan, Guizhou plateau is the eastward extension of landslide disaster prone areas. Landslide disasters have occurred frequently, which lead to the loss of properties and some times people lives. The elevations in the area have three zones: higher in northeast and southwest (1550m); lower in northwest and southeast (800-1300m); and the central part is the river basin (500m). The latter zone is known as landslide area as the local people claimed.It is accessible through a highway, which passes through the mountainous area. Also it can be reached by plane, about 45 minutes from Wuhan city. There is a railway under construction, which in future will link the area with the surrounding places.The studies include both field and laboratory works. The fieldwork includes: geological investigation; rock type identification; rock masses discontinuities; joint surveying; Schmidt hammer; orientation of the sliding masses; in situ point load test; and sampling from more than 24 locations. The laboratory tests were carried under natural and saturated condition including: thin section for mineral identification; specific gravity; water absorption ratio; point load tests; direct shear test; uniaxial compressive strength; triaxial test; and wave velocity.The reconnaissance survey of the study area did reveal the dominant rock types is sedimentary rock consisting of sandstone, limestone, siltstone, dolostone, mudstone, shale, and silicified sandstone. The metamorphic rocks depicted as a minor type including marble and quartzite. Tectonically, the area is folded, faulted, jointed, fractured and bedded strata. These mechanisms resulting in different setting and bedding inclination of the strata. The joint conditions, texture, degree of weathering, the colour of rock formations are determined in the fieldwork. The fractures of the rocks masses are mainly tight, open, filled with clay and/or rock fragments, and unfilled cavities. The description ranges from extremely close spacing (< 0.02 m) to extremely wide spacing (> 6 m)Most joints persistence in the study area is ranging between very low to medium and few locations showed high persistence. The lower and medium persistence were due to the sliding movement. Orientation of a joint plane is represented graphically using hemispherical projection method. Structurally, the area can be grouped into two categories from the stereonet projection. First, two sets group which depicted by a great number of the formation and second, two sets plus random and three sets. In excavation works, the former group gives equi-dimensional blocks and the latter yield different block types. From Rock Quality Designation (RQD), the values ranges are 77.79 - 98.41 % demonstrating good to excellent classes.The ratio of RQD values and Joint set number (RQD/J_n) were put into categories with rating values and given descriptive terms for the block size quality (BSQ), depicting most of the blocks are fair to good block size. From the calculation of the volumetric joint count, and the suggested block size designations, most block sizes (16 samples) are medium size and the rest are small blocks. After the computation of the block volumes from volumetric joint count (J_v) and from the joint set spacing, it concluded that the results more or less are same.The petrographic analysis of the thin sections shows that most of the examined samples in the study area are dominantly sedimentary rock, limestone, dolomitic limestone, mixture of siltstone, shaly sandstone, and quartz sandstone.From the point load test result, the formations can be grouped as following: five Formation including Badong Formation as the poor class; thin limestone layer and Jialing Formation as fair class; the limestone of the Ordovician as good class; finally, The Jurassic limestone as very good class. Actually, most of rock masses are highly degraded by the heavy rainfall and the pre-existing weakness through which the water passes in rocks.The density results show more than 80% of the examined rock formations depicting values above 2.5 g/cm³ that are usable in accordance to the projects specifications under consideration. Since there are roads constructions running in the area, these materials could be used as artificial aggregates. The water absorption test of the intact rocks demonstrated low ratio (0.001～0.025 %), reflecting that only rock surface could be affected by the rainfall.In the present study, the uniaxial compressive strength result was assessed by using two methods: Strength classification of intact rock based on UCS (σ_ci) and engineering classification of intact rock based on modulus ratio E_t/σ_ci. Applying the intact rock strength classification based on UCS test results under natural and saturated condition, the differences are obvious; the class degraded to the lower one and in other formations lower than the next class. Thus, during rainfall the stability factor becoming less and probability of rock mass movement will increase as actually occurred in the study area. The engineering classification of intact rock based on modulus ratio E_t/σ_ci, more or less showed similar result. Similarly, the Chinese Standard for UCS classification for saturated rock shows that most of the rocks types are very hard, 8 formations; fairly hard and soft have similar number of formations, 3; and only KE is very soft (sandstone). The triaxial test result shows major difference between the natural and saturated specimens in maximum principal stress, tensile stress, and shear strength (C andΦ) values. The intact limestone possesses high strength value indicating its usability as cornerstone and hard coarse aggregates in area where no natural gravels.The dynamic method for determining the young modulus, record high values in comparison with the static method values, but it is useful for correlation with the static method.The whole shear test results from the different Formations showing that the values of C scored by the examined formations under natural from higher to lower as follows: Long maxi, Luoreping, Shamao, Daye, Jialing and Badong Formations; in saturated state, the C value from higher to lower are: Shamao, Longmaxi, Luoreping and Badong; the values ofφthrough out the tested natural planar formations depicting similar results with exception of Jialing Formation, which is higher than others. Jialing Formation in natural and Badong Formation in saturated condition scored the high normal stiffness value; the normal stiffness has a significant effect on the peak shear stress with higher normal stiffness generating higher shear strengths; and the more stable formations are Shamao sandstone and Badong Formation (limestone).The correlation between the dry and saturated limestone density scored high coefficient, 0.9703, which reflect the less quantity of water that limestone can absorb. In addition, it means the limestone under consideration could only be weathered by rainfall within long time span. The relationship between the dry and saturated UCS test of limestone yields high R² value indicating the accuracy of the relationship, and implying that the intact limestone properties slightly affected by water during limited time. From the results of P- wave and UCS tests on a large number of sandstone and limestone samples in the study area, under natural and saturated conditions, a good correlation between them was established.The rock masses in the area is classified based on rock quality designation (RQD), rock mass rating (RMR), geomechanics classification system, and geologic strength index (GSI). The RMR classes in the area are showing two classes, good and fair rock quality but the majority is good rock whereas the Q-system results depicting four rock quality masses: very good, good, which is dominant, fair, and poor class. The rock masses qualities from GSI are ranging from very good to fair- poor quality. Comparing the three classifications results, Q-system, RMR, and GSI, more or less the same results were obtained and the correlation between RMR and Q-system to some extend can be used with further studies. From this study, it could be concluded that GSI is useful in preliminary evaluation and in Hoek and Brown criteria.RockLab software program for determining rock mass strength parameters, based on the generalized Hoek-Brown failure criterion was used and yielding good results. From the results of the 17 rock formations samples from RockLab, include Hoek-Brown Criterion (m_b, s, a), Mohr-Coulomb (cohesion and friction angle), and rock mass properties (tensile stress, UCS, and deformation modulus), some correlations were established between the parameters.The general conclusion of the study are, most of the rock masses are highly jointed, folded, and fractured; the heavy rainfall in the area is playing major role in rock mass movement; the intact limestone can be used as crushed aggregates and building stone. The correlation coefficients between dry and saturated rock properties and between uniaxial compressive strength and P-wave are encouraging to be used in all projects that can be established later in the area.The recommendation in further study is petrography and chemical analysis study for fixing types of rock masses that extremely affected by rainfall. This result will put the suggested solution in the right way in accordance to the rock formation and other geotechnical properties. For building material, since there are no natural coarse aggregates in the area, some laboratory tests should be carried out to determine the usability and workability of the intact rock.