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I • .oa I ae+. BRLdP00-65-_ __ _ ~L~-0066 r''~rcBOB _... ._ __ ~._. ___ _ mm - - - - ,. _~ - __ -_ - ~ .. ... _ _.. _.-. A. _ ... _ xaa - __ - ~_ UnMferentizled:._ _.._ ,- _. r .. _ _ _..... _ _ xoa ~-~tawer Q3eam~ - ~- __ ~S G15 cna ` I-egeaartl abne C Seam ~ `AIuNUm sc® ~~ UPParB Seam ~-. coe- _ _ --LOVerB-Seam~~~~.- _ _ ne®,~_ _ _______ _,C $n~fcre _ _ _ _ __ _ WIM 93nmOne____ 560 _. _ __ _ _ _ _ _ __. _. DqO ,rOO IOgO eq0 IDgO 200 WOO ]SqO aq0 Uq0 SO BO SSgO ®~9 4q0 AqO b0O W-E Hubbard Crcek Cross Section Figure 2 presents an east-west geologic cross section passing through longwall study area. The major units present are representative of the Cretaceous age Lower Mesa Verde Formation in this area and include the Rollins Sandstone, the C Sandstone (locally referred to as Bowie Sandstone or Upper Marine Sandstone), and coal seams A through D- Upper. Horizontally bedded mudstone and siltstone sequences, together with the cross-bedded-to- laminarbedded fluvial sandstone channels, are also present, but are shown as undifferentiated on the cross section. The alluvial valley fill is believed to be Quaternary in age and is composed of Tertiary age igneous boulders, cobbles, and gravel. Fig. 2. East-west geologic cross section (after [b]). • Relative motion is 10° to 15° from the horizontal with 1.5 to 3 m (5 to 10 ft) of total throw down to the south. North of the strike-slip fault, the D6-D9 panels are intersected by several N60°E normal faults with limited lateral extent and displacements appearing as tensile gashes. It appears that these faults formed as a result of differential movement along the strike-slip fault. This assertion is based on observations showing that throw decreases with increasing distance from the fault to the north, but no such features are found to the south. Where present, kinematic indicators suggest dip-slip movement and little to no shear [3]. The coal seams and the Rollins and C sandstones are the only truly tabular sedimentary bodies in the section. The two sandstone bodies are even more laterally continuous than the thinner coal seams. Because of strength and lateral persistency, these two marine sandstone bodies have by far the best characteristics for bearing and distributing loads. The C Sandstone is approximately thick 27 m (90 ft) thick in two parts with a 3- to 4.5-m- (10- to 15-ft) thick, mudstone-rich zone near the center ofthe unit. Figure 3 presents histogram frequency diagrams for uniaxial compressive strength and Young's modulus. Data statistics are also provided for the entire database, including laboratory test results from three holes. Using an engineering classification of intact rock [4: table 2], overburden core is classified as having medium strength and, on the average, a medium modulus ratio. (The modulus ratio is obtained by dividing Young's modulus by uniaxial compressive strength.}. In addition, point load tests complemented the uniaxial compressive strength results by providing additional data in a cost-effective manner. Point load strengths were determined both horizontally and vertically to address anisotropic strength properties of overburden rocks. Figure 4 presents calculated RMR [5] values for a continuous core drilled near the longwall block in a drainage area. The core was logged to evaluate the lithology and structure. The boreholes were then used for groundwater monitoring in the intervals above, within, and below the B Seam in the Rollins Sandstone. As illustrated in figure 4, most overburden rocks are classified as fair with an RMR average rating of 55. The rating for the Upper Marine Sandstone and a few other massive units in the overburden exceeds 60, falling into the "good" rock category. A -3