A GEOLOGICAL NOTE FOR SLOPE STABILIZATION ON LEFT SIDE SLOPE OF SPECIAL BRIDGE ACROSS RIVER CHENAB ON THE KATRA – LAOLE SECTION (USBRL PROJECTS) - JAMMU AND KASHMIR.
The sectional area of the bridge site is given in blow figure:
strata is folded (warping). All the three joint sets are associated with minor and major folding. There are few shear seams recorded at a few places parallel to the bedding joint planes.
Chenab Bridge is the world highest rail bridge (359 m) in the world which is under construction near Village Kauri in district Udhampur, Jammu & Kashmir. The study area (Left abutment) is situated at Katra end, bank of river Chenab. The area is shown in the photographs.
This note includes the overview about geological mapping on 1: 1000 scale of the area and geological 3-log of pits at various locations which have been excavated to know the rock conditions at different depths at katra end of bridge location. Geological features from pits and available surface exposures, were collected
systematically and plotted on map and data projected on Stereonet, which shows structural mean orientation of these features.
GEOLOGY of left abutment slope towards Katra end hill slope is covered with debris/soil, thickness of which varies 1 to 3
meters but at some places rock exposures are also encountered. Geological mapping of the left abutment figure 2, has revealed that right from S 40 location to S10 location, the slope is made up of highly jointed to blocky dolomitic limestone brecciated dolomite/brecciated quartzite. The dolomite is fresh, hard and competent in nature.
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| Joint orientation at Pit Location |
The rock units underwent tectonic movement and local folding/warping has been noticed at drift on S-40 location. The rock units generally strike N10° to 40°W – S10° to 40°E with a dip amount of 10° to 40° towards NE. In the study area, besides fracturing and joints at random following three sets of joints are identified and given in tabel 01:
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| DISCONTINUITIES CHARACTERISTICS |
Most of these joints are open at the surface. Openings of these joints are recorded up to 10 centimeters at some locations. Generally joint"s openings are filled with surface clay and joint planes are stained. Calcareous/siliceous coating is also observed along many joint planes.
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| A outcrop of Dolomitic Limestone with Joints |
All the three joint sets are associated with thin shear zones. But at the surface, these shear zones are covered with grass/debris and visibility is low. Exact thickness and orientation of these shear zones shall be known only during excavation of the slope/pier foundations. Both surface and sub-surface(drift) geological studies reveals that dolomitic limestone is thinly bedded, closely jointed / fractured, folded (warping) and sheared. Except bedding joint set which dip at 10° to 35° towards NE i.e.
upstream side (into the hill) all other joints dip at steep angle (65°to 85°) away from the valley. To avoid any loose fall or rockfall after the excavation of various pits, it is recommended that proper slope cuts with rock bolts and shotcrete should be designed.
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| Geological Map of Left abutment |
EXPLORATORY DRIFT
A drift is excavated at along the axis of the bridge at proposed left abutment location.The dimensions of the drift are 20m x 2m x 2m. The main purpose of drift excavation at this location is to study subsurface geological features likely to beencountered at foundation grade of left abutment and nature of joints which will control the hill slope. Geological data obtained from the drift has been described below.
3D geological log prepared from the drift is enclosed herewith as given in log sheet. From the drift it is observed that well bedded, highly jointed grayish dolomitic limestone has been encountered in the drift. Dolomitic limestone is hard, fresh and competent to take load of the proposed structure.
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| 3-D Log at Drift |
There are mainly three joint sets recorded within the drift. The data of a few random joints have also been recorded. The dip/dip direction of rock strata generally varies 20°/N030° to 50°/N060°. Bedding joint set is most prominent joint set with a spacing of 1 to 6cm. Closely spaced bedding joints are planar and wavy in nature, generally tight and often filled with siliceous and calcareous material. J2 and J3 are open at places and generally filled with clayey gauges and calcareous/ siliceous materials .The joint surface of J3 is found stained. Between the lengths of 12 to 18m the rock
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| Joint Set J1 and J3 at Drift Face |
strata is folded (warping). All the three joint sets are associated with minor and major folding. There are few shear seams recorded at a few places parallel to the bedding joint planes.
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| Warping/Folding at Drift wall |
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