5. Selection of Backfill Material for Appropriate Compaction of Backfill over Metro Tunnel Alignment between Barakhamba Road-Connaught Place-Dwaraka Section

December 2005
(i) Central Road Research Institute, New Delhi (R)
(ii) Senbo Engineering Limited, New Delhi (S, I).


Present Status and Progress
Findings/ Conclusions

Senbo Engineering Limited is involved in the construction of tunnel structure for Delhi Metro between chainage (-) 0.800 km to Ch (-) 1.52 km on extension of Barakhamba Road- Connaught Place- Dwarka section using cut and cover method. The construction of tunnel is nearly complete and the entire remaining area left after the construction of tunnel has to be backfilled suitably, so that the proposed road structure, which is to be constructed over the backfill, does not settle. During the construction of the tunnel wooden leggings, sheet piles and steel girders etc have supported the vertical cut slopes. In the upper portion over the completed tunnel, long struts of steel are supporting the vertical cuts. The utility pipes in the upper region are also being supported throughout their length with the help of continuous girders. Such support systems are causing problems and are inhibiting movement in the transportation of appropriate compaction equipment required to backfill and compact the entire backfill in such a manner that no post construction settlement takes place. The time required to backfill the entire area, which amounts to several thousand cubic meter of soil was very limited. Keeping in view the above constraints, it was requested to provide a solution to back fill the area with the following scope and objectives.

1. To analyse method of backfill vis-à-vis time run

2. To analyse backfill material to reduce time overrun

3. To recommend the type and method of backfill suitable for reducing time overrun

4. To have conclusion as justifications of adopting suitable methods and type of backfill considering environment aspects, utility hindrances, compaction factors and space constraints and also few safety observations considering future restoration of roads for traffic.

In this connection the three possible backfill materials, which are locally available in abundance were considered.

a) Local excavated material exhumed during the cutting operation of the ground for the construction of tunnel.
b) Locally available Yamuna sand, and
c) Fly ash available at Indraprastha Thermal Power Plant.

All the above materials are technically suitable for use in backfilling deep excavations but the possibility and time required for achieving the specified degree of compaction varies due to method of placement and compaction deployed due to properties of the individual material. The problem associated with backfill will occur in confined/narrow zones, where only small compaction equipment producing a low compaction effort can be used or where because of confined nature of the backfill zone and other hindrances, even small compaction equipments cannot be operated effectively. Sand has been recommended as the right choice as the backfill material.

In conclusion fine sand preferred over local soil and fly ash for backfill in order to optimize time run considering environmental aspects, utility hindrances, compaction factors and space constraints etc.

1. Soil needs to be placed in layers (20 cm thick) at optimum content before rolling with vibratory rollers, which is difficult considering the space availability at site due to struts and utility network crossing the trench from one side to other. Therefore it is recommended only for top 1.0 m fill at the top before 0.5 m thick sub grade.

2. Fly ash is required to be transported from the ash ponds of near by thermal power plant in slurry form in dumpers, which results in spillage of fly ash on road. This spilled fly ash needs to be cleaned daily which is cumbersome process involving resources and time. Fly ash if gets dry during dumping and handling at site will result in air pollution. More over the permeability of fly ash is in the range of 10-5 cm/sec, so this will take more time after each lift is placed by flooding before it attains stable state of compactness. The fly ash particles are lighter in weight therefore water densification of fly ash will not be as much as can be achieved for fine sand. Therefore fly ash is not recommended in this case.

3. Sand is preferred over soil and fly ash due to its high permeability (10-3 cm/sec), this property will help in downward drainage, which is required to maintain seepage forces in downward direction when the sand is saturated to aid in compaction. Laboratory work has indicated that for achieving 95 percent degree of compaction relative density should not be less than 60 percent, but this relative density is dependent on the ymax of the sand to be incorporated in the work. Generally cohesion less materials having relative density in the range of 70 to 85 percent are termed as in dense state of compaction. In the present case each layer of the fill shall not exceed 1.0 m in thickness after compaction in the fill areas on the sides of the tunnel up to the top of the tunnel roof, at levels above roof top, the fill shall not exceed 0.35 m in thickness after compaction. In the event that application of water alone fails to produce the required relative compaction throughout the entire thickness of the lift, the flooding or jetting shall be supplemented by the use of vibratory compaction equipment. The fine sand to be used shall not have more than 10 percent fraction passing 150 micron sieve.

4. The water used for flooding each layer will seep through the entire thickness of sand and will get accumulated in the space between two consecutive fill portions. This can be reused for flooding of the next layer of sand, this will optimize the water requirement for flooding.

5. Due care needs to be taken during backfilling around the utility pipes, place the material (sand) simultaneously on both the sides of the pipe, keeping the level of backfill the same on each side. Material shall be carefully placed around the pipe so that the pipe barrel is completely supported and that no voids or un-compacted areas are left beneath the pipe. Particular care shall be exercised in placing the material on the underside of the pipe to prevent lateral movement during subsequent backfilling. Compaction of material placed with in 60 cm of the outer surface of the pipe shall be done light tamping to avoid any damage.