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Top-down construction – Why it is the new normal
Prof. John Endicott
AECOM Fellow
H.K.U. and H.K.U.S.T.
4th October 2018
Deep excavations normally require temporary walls with supports.
Steel sheet piling as deep as 15m Deeper concrete diapraghm walls
Pile walls
Cutter for diaphragm walls
Excavation for deep basements
Work commences from ground level by installing reinforced concrete walls in slurry filled trenches below ground
Bottom-up sequence for constructing deep basements Stages 1 and 2
Install walls from ground level Excavate and install first strut
Bottom-up sequence for constructing deep basements Stages 3 and 3
Excavate deeper and install second strut Excavate to bottom and cast base
Bottom-up sequence for constructing deep basements Stages 5 and 6
Build walls up to second strut Remove strut cast slab on formwork
Bottom-up sequence for constructing deep basements Stages 7 and 8
Cast roof on formwork Remove strut and backfill
Top-down sequence for constructing deep basements Stages 1 and 2
Install walls from ground level Excavate and install first slab
Sometimes the top slab is at ground surface, sometimes it is blow ground level.
Once it is built it provides a strong level works area
Top-down sequence for constructing deep basements Stages 3 and 4
Excavate below top slab cast second slab Excavate to base and cast base slab
and backfill
Beneath completed slabs the works area is protected.Diaphragm walls can be permanent walls.Slabs are cast on plywood sheets or blinding on the ground.No formwork is required.
Diaphragm walls are sometimes hidden by skin walls to provide a vapour barrier to accommodate slight seepage
Benefits of Top-down sequence
• Early completion and access at ground level• Possible Top-down/Bottom up for tower blocks• No temporary decking for small sites• Little or no temporary strutting good for oddly
shaped sites• No formwork• Much of the work is protected from rain• Openings can be provided for construction access
across the site
Examples
Permanent structure used for site roads on railway station site giving multiple access for hoisting muck at modest radius and though -site haul road and vehicle queuing areas.
22 Presentation to EDC on 26 January 2012
Concept of reference Design
Perimeter diaphragm walls
Excavate to 27m deep (B4)
Temporary Side slopes, no struts
-5.100mPD (B2)
-25.00mPD
G.L. +5.50mPD
-6.35mPD
-21.10mPD (B4)
-6.00mPD
DIAPHRAGM WALL
SHEET PILE WALL
SHEET PILE WALLS
G.L. +5.50mPD
23 Presentation to EDC on 26 January 2012
Bottom-up construction in the middle, B4, B3, B2
-5.100mPD (B2)
-25.00mPD
G.L. +5.50mPD
-6.35mPD
-21.10mPD (B4)
-6.00mPD
DIAPHRAGM WALL
SHEET PILE WALL
SHEET PILE WALLS
G.L. +5.50mPD
24 Presentation to EDC on 26 January 2012
Construct B3, B2 around the edge
-5.100mPD (B2)
-25.00mPD
G.L. +5.50mPD
-6.35mPD
-21.10mPD (B4)
-6.00mPD
DIAPHRAGM WALL
SHEET PILE WALL
SHEET PILE WALLS
G.L. +5.50mPD
25 Presentation to EDC on 26 January 2012
Partial Top-down on eastern and western sides
Continue Bottom-up B1 and G/F on middle part
-5.100mPD (B2)
-25.00mPD
G.L. +5.50mPD
-6.35mPD
-21.10mPD (B4)
-6.00mPD
DIAPHRAGM WALL
SHEET PILE WALL
SHEET PILE WALLS
G.L. +5.50mPD
26 Presentation to EDC on 26 January 2012
Option adopted by the northern half
Completer ground level works and roof structures
-5.100mPD (B2)
-25.00mPD
G.L. +5.50mPD
-6.35mPD
-21.10mPD (B4)
-6.00mPD
DIAPHRAGM WALL
SHEET PILE WALL
SHEET PILE WALLS
G.L. +5.50mPD
Austin
Station
Kowloon
Station
Top-down sequence in soils for first four levels,
then,
Excavation in rock for five levels of Bottom-up sequence
Some thing to take away
Temporary sea walls in a yacht basin.
Then filled with sand.
Temporary island.
Temporary Island, temporary walls, temporary strutting bottom up construction for six lanes of vehicular
tunnels