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TOP

DOWN TECHNIQUE

IN CONSTUCTION

Parag P.kesalkar

M0910C04

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INTRODUCTION

n In this method, the underground retaining walls are firstinstalled. In most cases, these retaining walls are concretediaphragm walls. This is followed by excavation to just belowthe roof slab level of the underground structure, with the

retaining walls and struts supporting the soil at the sides. Theroof slab is then constructed, providing a massive supportacross the excavation. Access openings on the roof slab areprovided so that works thereafter could proceed downwards tothe base slab level of the underground structure. Upon

completion of the base slab, the side walls are constructed andthe intermediate struts are progressively removed. The accessopenings on the roof slab are then sealed and the ground issubsequently backfilled and reinstated.

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INSTALLATION OF RETAINING

WALLn The underground retaining wall, which is usually a concrete

diaphragm wall, is installed before excavation commences.

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INSTALLATION OF RETAINING

WALL- DIAPHRAGM WALLn The following pictorial guide illustrates the construction

sequence of a typical diaphragm wall. This type of retaining

wall was used for the construction of underground station in

delhi.

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01 .Construction of Guide Wall

n A guide wall is constructed to set out the position of the

diaphragm wall.

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02 .Excavation of Panel

n The grab/trench cutter cuts and removes the soil to form the

panel. The excavation is stabilized by filling it with bentonite

slurry to support the wall of the excavation.

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03 .Installation of Rebar Cage

n The crane lifts up the reinforcement-bar cage and places it

within the panel.

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04 .Concreting of Panel & wall.

n Concrete is poured into the panel to form the panel wall.n Process 2 - 4 repeats for the remaining soil in between

the panels till the entire length of the diaphragm wall

construction is completed.

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Excavation & Installation of Steel Strut

n The soil is excavated to just below the roof slab level ofthe underground structure. Struts are installed to supportthe retaining walls, which in turn support the soil at the

sides.

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Construction of Underground Structure

n The roof slab is constructed, with access openings provided onthe slab for works to proceed downwards. The roof slab not onlyprovides a massive support across the excavation, it also acts asa noise barrier.

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Construction of Underground Structure

n The next level of slab is constructed, and this process

progresses downwards till the base slab is completed .

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Construction of Underground Structure

n The side walls are constructed upwards, followed byremoval of the intermediate struts. The access openingson the roof slab are then sealed.

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Backfilling & Reinstatement

n After the underground structure is completed, the soil isbackfilled to the top strut level before the strut is removed. This isfollowed by completely backfilling the top of the undergroundstructure and finally reinstating the surface areas.

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CONSTRUCTION OF ROOF SLAB

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UNDERGROUND EXCAVATION

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n Thank u