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Box Pushing Operation
The thrust beds of required size and designed thickness were cast with M25 concrete. Pin pockets of 0.5m x 0.5m x 0.6m size are provided at suitable spacing for jacking arrangements. The sides of pin pockets are provided with 6mm thick M.S. Plate to take the thrust. The pin pockets are then filled with sand and small layer of screed concrete is placed on them to get smooth surface for casting or Box elements.
The thrust beds of the required size and designed thickness were cast with M25 concrete. Pin pockets of 0.5m x 0.5m x 0.6m size are provided at suitable spacing for jacking arrangements. The sides of pin pockets are provided with a 6mm thick M.S. Plate to take the thrust. The pin pockets are then filled with sand and a small layer of screed concrete is placed on them to get a smooth surface for casting or Box elements.Reference line pillars and TBMs are fixed on the approach to control alignment and level while pushing.
One layer of plastic sheet is spread on the thrust bed where the box is cast. A layer of grease is applied to it and again one mere layer of plastic sheet is kept. Box elements are cast on this, so that friction is reduced while pushing.Entire box concreting done with RMC during nights only since the site is in the heart of the city. The top of the box was painted with epoxy paint after casting to reduce friction.
Within the box, excavation was done with an excavator. The excavated earth was lifted by winches and stacked during the daytime. The same was transported out during nights. Hard granite rock was met with at the bottom layer and also some big boulders at several locations. The same was removed using pavement breakers.
Initially, two elements were cast on both thrust beds. After pushing on the element, the element from the other bed was side slewed to maintain continuity of pushing.Pushing was done with the help of hydraulic jacks. All the S & T cables are protected before pushing.
Sufficient ballast is unloaded with the help of division before taking up the pushing.
Additional safety measures
taken at this site :
Second, cutting edge: As the size of the box is large, based on past experience a second cutting edge has been provided at 2m below the top cutting edge, and both are interconnected with stiffener plates. This helped in supporting the formation/slope in case of soft soils and emergencies.
Additional constraints
at this site:
The approach roads were not ready. There is a difference of 5 m from the existing road level to the box bottom. So the excavated earth has to be lifted to Road level as trucks cannot go since there is no space even for the ramp of Fathenagar side. This is a heavily built-up city area. All transportation has to be done during nights only. A very limited
General safety precautions
taken at site:
Round-the-clock watchman posted to ensure the safety of trains. One P. way gang with experienced supervisor kept on track to attend any track disturbance during pushing.9000 Nos. of filled ballast, sand, and moorum bags kept at the site. Ensuring cutting edge always buried in the earth. Alignment and level of box checked daily. Any correction required was immediately carried out
Problems encountered
while pushing:
Shearing of drag sheets:
This may be due to restricted movement of drag sheets which are pressed between cutting edge and soil of the formation, pressing against boulders present in the formation, not able to withstand the forces developed during pushing, etc.
Conclusion:
Various unexpected situations are likely to occur during the box pushing operations. Since the safety of running trains is directly affected, proper planning and implementation are essential for the smooth completion of work. Advance analysis of site, likely problem that may arise and planning to tackle the same will help the executive for the speedy safe completion of the work.
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