Geological Note

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.

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.

The sectional area of the bridge site is given in blow figure:

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.

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:

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.

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.

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. 

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

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.

Warping/Folding at Drift wall

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Geological  Hammer

Geological hammer is used for splitting of rocks in the field and to check the strength of the rock sample. A Geologist is always try to take a fresh rock sample and determine its mineral composition, Grain size, cleavage etc. in the field. 

Banded Stone Masonry Wall in Hilly Terrain as a slope protection measures, In  Himalaya, India 

Cut slope protection (slope stability)  is a most important part of a newly constructed or widening of a existing highway in hilly area. But the designer or executors is not much concentrate on this topic due to direct high execution cost, even they are not follow the basic rules during cut slope excavation and they are executed the work as a Lehman in most of causes. In hilly area if you are not paying for protection measures then you must be ready to pay double in cut slope maintenance work  in future. it is advised to execution contractor to follow basic rule of cut slope excavation as top to bottom method of excavation with proper safety.

Excavation of hill slope for widening of Existing Highway in Indian Himalaya

Here I am taking about only the Banded dry stone masonry wall which is very helpful for protection of a excavated slope in hilly terrain due to easy availability of major raw material as a stone. It is most cheaper than other works. The water is easily peculate between the stone so no water pressure developed behind the wall. It is cost effective as well as performed better in earthquake condition. It is repaired very quickly as compared to reinforced wall and other option.

Sketch for bonded stone masonry wall as a slope protection for 2-lane highway

we can use a Cement Banded dry stone masonry wall to protect hill side slope, median side as well as valley side slope due to easily availability of raw materials.

Design criteria of banded stone masonry wall:

  There are mainly four types of  load and pressure should be considered to built a

   banded stone masonry wall which are as follow-

• Self weight of banded stone masonry wall (SBC)
• Earth pressure acting on wall (Cut Slope soil/rock pressure)
• Water pressure (water pressure if any)
• Seismic load (seismic zone of the area where  wall is to be built)

After assessment of the above given point the design should be as follow based on  codal provision-

•    Width of wall foundation should be  B= 0.5*H + 0.3 (where H=Height of wall) to get

     desired SBC, top width should be 0.6 m up to 4 m height and 0.5 m more than 4 m

     height of wall.

•  Foundation should be inclined inward 1:3 to 1:6  to safe against overturning

•  keep the front better 1:2 to 1:6, keep back front straight up to 4 m, back front

   reduced more than 4 m to 8 m height.

• Coping placed 0.50 to 0.75 mm. sloping towards valley side.

• There is no need to provide horizontal PVC pipe for drainage as there is sufficient space between dry stone to flow the water.

Construction Methodology  for site:

•  First we should ensure the desired excavation of cut slope is satisfactory or not

   than SBC must be ensure before laying PCC by Geo technical expert.

• Check the Line and level as per drawing for PCC. PCC done 100mm to 300mm thick

   as per   strata.

• Wall foundation must be slopping inward 1:3 to 1:6 to escape overturning.

• Construction materials as Cement mortar should be used 1:4 and size of stone should not be 20*15*10 cm approximately and stone should be clean and clay free.

• Depth of foundation must be 1000 mm when rest on soil and 500 mm if rest on rock strata, depth generally should be 0.1*H + 0.3 .

• The wall should be constructed with 8 meter construction joint up to the coping as  per the given figure.

•  Compacted back fill should be done simultaneously as the height of wall increase, back fill materials should be granular or stone chips (well drained). The back fill layer should be place in such a manner that the back fill load on wall should be Zero. Top surface of back fill to be sealed by impervious 300 mm layer to prevent ingress of rain water.

• Drainage PVC pipe within the wall is not required as space between dry stone is sufficient to release the water pressure behind the wall.

• Coping should be done with 1:4 mortar 50-75 mm sloping towards valley side.

•  Road side drainage should be provided as per discharge quantity of water during monsoon.

• Cement sand mortar ratio should be 1:4

The following points should be remember:

• The foundation area should be compacted properly before laying the PCC.

• If water surcharge is more than proper weep holes should be provided to release  ground water.

• Back filling should be placed properly.

• Mortar quality should be checked at regular interval and curing must be done as per standard.

• Man power should be well experienced in stone masonry works.

1.IRC:SP: 48,1998

2.IRC:40-2002

3. IRC:78-2014

4.IS:14458, Part3