METHODOLOGY FOR SLOPE PROTECTION WORKS
A. METHODOLGY FOR SHOTCRETING:
1. A better base or foundation is necessary for conventional and successful application of shotcrete.
Where the shotcrete is to be placed against earth surfaces as linings, such surfaces shall first be thoroughly compacted and trimmed to line and grade. Shotcrete shall not be placed on any surface which is frozen, spongy, or where there is frequent water.
The surface shall be kept damp for several hours before applying shotcrete.
2. Adequate and safe scaffolding shall be provided so that the operator can hold the nozzle at the optimum angle and distance from the surface for all parts of the work.
The scaffolding shall be also given easy approach to the shotcrete surface for to assist in flattening and finishing, if such is specified. Scaffolding shall be constructed to permit undisturbed applications of the shotcrete wherever possible.
3. Sufficient clearance shall be provided around the wire mesh to permit complete encasement with sound concrete.
The clearance needed depends on the maximum size of aggregate in the mix and the size of wire mesh.
The minimum clearance between the wire mesh and the form or other backup material may vary between 12 mm to 20mm,
4. Each layer of shotcrete is built up by making several passes or loops of the nozzle over the working area.
This may be done by moving the nozzle easily in a series of loops from side to side and uneven.
The shotcrete shall emerge from the nozzle in a steady, undisturbed flow. If the flow becomes irregular due to any cause, the operator shall direct it away from the work until it again becomes constant.
The distance of the nozzle from work (usually between 0.5 and 1.5 m) shall be such as to give the best results for the working conditions.
The nozzle shall be held perpendicular to the surface of application.
5. The mix shall be little wetter than normal, but not so wet as to cause sloughing behind the wire mesh.
This procedure forces the plastic shotcrete behind the mesh while preventing build-up on the front face of the bar.
for wet spraying is mixed, the slump should be measured, and the slump should be 8 to 12 cm.
6. Rebound is the process in which aggregate and cement paste which more times off the surface during the application of shotcrete due to collision with the hard surface, reinforcement, or with the aggregate particles oursleves .
The amount of rebound shall not be more than 20% and rebound material shall not reused.
7. Where a layer of shotcrete is to be covered by a succeeding layer, it shall first be agreed to take its initial set. Then all laitance, loose material and recover, shall be removed by brooming. Any laitance which has achieve final set shall be withdraw by sand-blasting and the surface cleaned with an air water jet. Moreover, the surface shall be thoroughly sounded with a hammer for drummy areas resulting from recover surface or lack of bond.
Dummy areas, sags, or other defects shall be carefully cut out and replaced with the succeeding layer. Surfaces to be shot shall be damp.
8. At the last of every day work, or on ending work for any other reason, the shotcrete shall be do nothing off to a thin edge and then the work shall be restart on next day after cleaning the surface. .
9. The permissible tolerance on the thickness of the work executed by shotcrete shall be 8 mm.
10. For lengths of hose up to 30 m, air pressure at the gun shall be 0.3 N/mm2 or more. Where the length exceeds 30 m, the pressure shall be enlarged by 0.035 N/mms for each moreover 15 m of hose required, and by 0.035 N/mm* for each 7.5 m that the nozzle is raised above the gun.
11. Batching by mass is- to be preferred and is strongly recommended. Aggregates may be batched by volume if periodic checks, are made to ensure that the masses are maintained within a required tolerance. Water may be batched either by mass or by volume. 12. The wet mix of pneumatic feed type of carriage accessories is competent of applying high quality, low-slump mortar or concrete with the constancy needed for general construction and repairing work.
From a pressurized vessel in the equipment, the premixed materials and compressed air are discharged into the delivery hose.
The material and air pass through the hose to the art of shooting gun nozzle which is fitted with an air ring for injecting supplementary compressed air.
13. The bulk density of wet sprayed shotcrete can be 2200-2300 kg/m 3.
The weight ratio of cement to sandstone is ideally 1. 0: 3. 5 to 1. 0: 4. 0, the water-cement ratio is ideally 0.42 to 0.50, and the sand ratio is ideally 50% to 60%.
After the mixture.
14. The coarse aggregate should be made of hard and durable gravel or pebbles.
The particle size should not be more than 10mm, the aggregate grade should be continuous grading, the fine aggregate should be hard and durable medium sand or coarse sand, the fineness modulus should be greater than 2. 5, the water content should be controlled at 5% - 7 %.
15.The suitable accelerator should be selected according to the types of cement , w/c ratio, etc., and the initial setting time should not exceed 5 minutes, and the final setting time should not exceed 10 minutes.
B. Methodology for Drilling and Grouting: Following Methodology shall be adopted for drilling and grouting Flattening/Removal of overburden slope, Cutting of unstable rock blocks and preparation of smooth surface. Scaling of loose materials/blocks, providing drain pipes and drain holes Use of Self Drilling Anchors of 32mm Dia of Strength 200 KN and it should be confirmed by carrying out pull out test on 1% or 10 Nos of bolts (whichever is lesser).
Grouting with cement mortar 1:2,1:3 and 1:4 in sequences till complete refusal. In cement grout Cebex 100 admixture of 250 g can be used for every bag of cement.
Fine aggregate complying with the requirements of grading zone-1 of table 4 of IS 383:1970.
For grouting operation compressor shall be used and pressure shall be monitored, at start pressure shall be not less 0.25 MPa and it shall be increased up to 2.5 MPa in sequential manner.
Pressure chart, duration of grouting and intake of grout shall be maintained. Constructing shotcrete walls with Drainage Holes Constructing rock trap ditches at the toe of the slopes Providing rock catch fences/walls along the slope to make the surrounding locations safe for public usage. Providing hanging chains or webs to slow down toppling of block.
Allowing free-hanging mesh net to direct loose rock pieces to drop down only near the slope toe.
Constructing berms/benches as a rockfall collector.
Providing mesh secured by bolts and grouted to protect friable formation
