Top Down Method of Construction

TOP DOWN METHOD OF CONSTRUCTIONTOP DOWN METHOD OF CONSTRUCTION  Top Down Method of excavation adopted for the project as deep excavation was to be carried out very near to existing buildings. The top-down construction method enables work to be undertaken above ground while excavating underground. Not only does this method shorten the construction schedule but it also minimise’s associated construction impacts on the community. Service Corridor, being made in Middle Circle of Connaught Place is a tunnel of box section of size (6.3 x 7 mtrs) which is being built by the help of this method.   Introduction Diaphragm Wall is generally reinforced concrete wall constructed in the ground using Underslurry Technique which was developed in Europe.  The technique involves excavating a narrow trench that is kept full of an engineered fluid of slurry.  Walls of thickness between 300 to 1200mm can be formed up to a depth of 45 metres.  STEPS OF TOP DOWN METHOD         Identification of Existing Utilities Shifting of Utilities & Making of Guide Wall Boring for Diaphragm Wall up to the required depth Lowering of R/F cage in the Bored Area Concreting of the Diaphragm Wall Panel Excavation & Chipping of Concrete up to the cut-off level RCC Top Slab for the Service Tunnel Excavation up to the Bottom Slab Identification of Existing Utilities  Making of Pre-trench across the road at regular intervals to identify the utilities running along the Proposed Service Tunnel  After the utilities were identified, methodology for shifting of these utilities were finalized Pre – Trench showing the existing utilities Shifting of Utilities & Making of Guide Wall  The identified utilities were diverted temporarily so as to keep the utilities running & also to make a guide wall for the boring  A Guide Wall was constructed for a depth of 1.6 m along the alignment of Service Corridor – to assist in boring for Diaphragm Wall & to shift any unforeseen utilities which are layed across the road & have not been identified in the pre-trench Boring for Diaphragm Wall  Boring done up to a depth of 14 m  Boring done with the help of bentonite slurry to help stabilize the walls of the boring till concreting is done The grab is tied to the body of the crane as a safety aspect to prevent it from hitting any existing building  Lowering of R/F Cage • R/F Cage for Diaphragm Wall Panels of Size (5.0 M) made ready before boring is commenced • Each R/F cage weighs approximately 10.5 MT Concreting for Diaphragm Wall  Tremie Concreting done for Diaphragm Wall  Each Diaphragm Wall Panel is 56 cum Excavation up to Bottom of Top Slab  After the Diaphragm Wall is casted, Excavation for bottom of top slab is taken up, so that the inconvenience to the public is minimum. Top Slab  A top slab of 600 mm is casted, leaving openings at regular intervals, so that excavation inside the tunnel can be carried out Excavation for Base Slab  After the top slab is casted, excavation inside the tunnel is started for base slab works Utilities Encountered and Shifting • The identified utilities were diverted temporarily so as to keep the utilities running & also to make a guide wall for the boring ,these included electrical cables, communication cables, drainage sewerage etc. FACILITIES INSIDE SERVICE TUNNEL  HT cables  LT cables  Telecommunication cables  Water supply pipe  Fire water pipes -one for hydrant network system and another for water sprinkler systems; tapping to be taken from the latter for providing water spray protection inside the trench.  Irrigation pipeline Gas pipeline.  Air conditioning pipes (chilled water) –one for supply & one for return to the entire Connaught Place areas including P Block, Regal Block & Scindia House Block. Secondary pumping system (one working & one standby) and two heat exchangers required for individual blocks are to be provided in the tunnel