ISSN : 2277-1328 (Online) - ISRM

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20 <strong>ISRM</strong> (India) Journal conducted in spillway drill holes, at increasing and decreasing cycle of pressure, to determine rock mass permeability values. Core samples from the boreholes were tested for physico-mechanical properties of rocks in the laboratory. Table 2 : Prominent joint sets developed in gneisses at dam spillway site Type of Strike or Spacing Persistence Roughness Aperture Infilling Ground Remarks Joint/ Joint Dip (cm) (m) (mm) Water Set Direction/ Dip Foliation J1 N200 o /25 o 12-60 >20 m Smooth Tight to 10 Sheared Dry Shear zones are undulating gaugy some times formed material along this joint, foliation dipping toward downstream side (22°- 30°). Inclined J2 N055 o /55 o 40-60 1 – 5 Rough 12 None Dry Joint dipping undulating toward valley, often controls the valley slope. Subvertical N095 o /80 o Random >20 Smooth planar 4 None Dry Valley dipping joint, J3 perpendicular to dam axis, often control the valley slope. Vertical J4 N055 o -070 o /V >18 >20 Smooth planar 3-5 None Dry Vertical Joint, oblique to dam axis Table 3 : Description of drilling data from spillway alignment area Drill Location Total Angle with Reduced Overburden Weathering Drill Core RQD Brief Hole Depth Horizontal Level of (m) Grade Water Recovery (%) Description No (m) Ground Loss (%) of Rock (m) Types DH-03 71.47m d/s 75.0 90 1849.29 13.50 W-I - W-III Partial – 0-80 0-35 Banded gneiss of dam axis Collar Complete with schistose II-C, centre Elevation parting of river DH-06 162.50m d/s 50.30 90 1849.21 5.0 W-I - W-III Partial 0-100 0-58 Banded gneiss from dam axis Collar with schistose (II-C), river Elevation intercalations bed DH-13 47.00m d/s 150 90 1930.79 0.5 W-I - W-III Partial - 3.3-100 Nil-78.66 Foliated gneiss, perpendicular Complete banded gneiss, to dam axis calc silicate gneiss, amphibolite gneiss DH-14 15.50m d/s 126.0 59 1850.75 1.5 W-I - W-IV Partial - 8-94.7 Nil-74 Foliated gneiss, from dam Complete banded gneiss, axis (II-C), amphibolite right bank gneiss, calc silicate gneiss and schistose quartzite The cores recovered from these bore holes have been examined and logged geologically. Overburden in the river bed varies from 5.0 m to 13.50 m and bedrock occurs at level varying from RL 1832 m to RL 1844 m. Erratic weathering is indicated and level of moderately fresh rock (W-II) varies from RL 1827 m to 1836 m as reported from the drill hole DH-14. Excepting in the limited section of DH-14 drill core recovery is poor and retrieved core are fragmentary such that average RQD values are extremely poor (
Engineering Geological & Geotechnical Evaluation of Dam Spillway (II-C) of Bunakha Hydroelectric Project, Bhutan 21 of schistose interlayers and foliation parallel shear zones (37 Lu. This has been ascribed to presence of close set of schistose interlayers and foliation parallel minor shear zones (
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ISSN : 2277-1328 (Online) - ISRM

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