ISSN : 2277-1328 (Online) - ISRM

20 ISRM (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 (