Wednesday, October 27, 2010




What is top down construction?
1. A method of basement construction whereby the permanent slabs, in the form of perimeter ring, act to resist the lateral forces by soil pressure to retaining wall (d / wall) during excavation of basement.
2. The sequence of slab construction is from the highest basement slab to the lowest basement slab.

Reasons for Opting to Top Down Method
1. Deep basement -not practical to do open cut excavation as site is surrounded by existing utilities/ structures including Common Utility Tunnel ( CUT) on the east & underground LRT tunnel / station on the west
2. Not practical to use ground anchors as tie back to diaphragm wall due to the above

Component of Works for Top Down Construction
1. Earthworks
2. Retaining Structure – Diaphragm Wall
3. Bore Piling + Plunge in Column
4. Deep Excavation
5. R.C Works- Columns, Beams , Slabs

Component of Works for Bottom Up Construction
1. Earthworks
2. Retaining Structure – Diaphragm Wall
3. Piling/ Foundation System- Bored Piling
4. Deep Excavation
5. Temporary Strutting – Steel Beam / Ground Anchor
6. R.C Works- Columns, Beams , Slabs

Table of Comparison

Bottom-Up Sequence

Top-Down Sequence


Diaphragm Wall

Diaphragm Wall

Bore Piling

Bore Piling + PIC

Deep Excavation

Deep Excavation

Temporary Strutting

Temporary Strutting

RC Works – Slab &Beam

1. Earthworks are engineering
works created through the moving of
massive quantities of soil
2. Scope – excavation,cutting & filling,
leveling & platforming
3. Earth shall be excavated to required level
4. Machineries used – excavator, bulldozer,
backhoe, dump truck

Permanent retaining wall structure in substructure construction

Underground structural elements commonly used for retention system & permanent wall

The construction of diaphragm wall is commenced with a 20m wide working platform

Construction Method & Sequence

Construction of Guide Wall
Guide Wall 300 mm x 1.0 m is a temporary reinforced concrete wall constructed along the diaphragm wall alignment. The purpose of guide wall is to support the upper soil and guide the drilling equipment at the right location

Completed guide wall

Stability Fluid
Bentonite Slurry
Bentonite Slurry is used as a stability fluid during excavation. It is prepared by mixing bentonite powder with water in bentonite plant installed on site. Bentonite slurry is frequently recycled during usage and the properties of the slurry is checked prior to concreting.

Mixing & Storage of Bentonite (Silo System)

Bentonite Storage (Pool System)

Bentonite Supply Pipeline Networks

Diaphragm Wall Excavation
  • The excavation is carried out using a suspended mechanical line grab mounted on heavy crawler crane, to the full required depth.
  • The level of bentonite is kept as high as possible between the guide wall
  • Using the grab, excavated materials are loaded on trucks and removed out from site.
  • When excavation is completed, the bentonite slurry shall be replaced or recycled prior to placing reinforcement cage and concreting.

Grab machine

Excavation Sequences

Installation of rubber water stop

End panel & water stop
End panel in-place in the trench

Steel Reinforcement Cage
  • Steel Reinforcement (Rebar) Cage is fabricated on site and delivered by crane into the panel and lowered down into the trench.
  • Steel cage is placed in segments in the trench and positioned on the top of the guide wall at the required location within the excavated panel length.
Fabrication of Rebar Cages
Rebar Cage with Slab Starter Bar
Rebar cage
Hoisting rebar cage
Lowering down rebar cage intro the trench
C.U.T Pipe Penetration Recess Inside Diaphragm Wall Steel Cage

  • Special concrete mix design using 20% PFA (Pulverized Fuel Ash) + 80% OPC (Ordinary Portland Cement for Low Permeability & Low Absorption.
  • Concreting is placed by the tremie method.
  • Concrete is poured into the hopper and is raised from the bottom of the trench, displacing the bentonite slurry.
  • The tremie pipe is kept immersed in the concrete at the bottom of panel to ensure fresh concrete does not mix with bentonite slurry.
Concreting of diaphragm wall panel

Cast in situ pile formed by excavating a shaft of specified size 1,500mmDia & 1,050mmDia to required depth & casting the excavated shaft with concrete of specified strength after reinforcement is lowered down.
Rotary drilling machines with telescopic Kelly bars and augers

Type of Auger

method & construction sequence
  • Kelly bars is mounted with auger
  • Verticality of Kelly bar is checked
  • Auger is lowered & boring commence
  • Temporary casing is used to prevent collapse (top soil)
  • Bentonite fluid is used to prevent collapse
  • If hard materials encountered- other drilling tools used – chiseling
  • After reaching required depth – cleaning bucket is used to clean bottom of b/hole
Installation of temporary casing
Boring work in progress
Drilling Using Cleaning Bucket
Fabrication of steel reinforcement
Steel Reinforcement Installation
Concrete is discharged into the bore holes using tremie method
Concreting Until Bentonite Overflow

4. Plunge-In Column
  • Plunge-In Column (PIC), designed exclusively to support the basement slab during excavation, are installed concurrently with the piling work at the boring platform level
  • PIC are I beams designed to cater slab weight with max live load of 2.5 kPa in a grid of 8.5m x 9 m
Bored Piling work will commence from 1.5m below Ground Floor Level to the require bored pile depth.

Plunge In Columns (350 x 350 Universal Beam) will be installed and embedded 2m into the bored pile immediately upon the concreting of Bored Piles.
Plunge-in Column ( 350 x 350 Universal Beam)
Installation of Guide Frame

Installation of PIC

Connection Plate to Plunge-in Columns

Completed Plunge-in Column inside Permanent Column (Composite)

4. Deep Excavation at Lower Basement

Structural elements:

  • Column – a vertical structural element that transmit through compression the weight of the structure above to other element below
  • Beam- a structural element placed horizontally to carry vertical loads.The loads carries by a beam are transferred to columns, wall or girder, which in turn transfer the force to adjacent structural members
  • Slab – structural member that spanning between beams, walls, girders or column
- slab is used for floors


1.Site is trimmed to a general platform of RL +30.50 to facilitate the commencement of bored pile & diaphragm wall
2.Guide wall for diaphragm wall to constructed along the perimeter of wall with top of guide wall is approx. 300 mm below ground
3. A perimeter working platform of approx. 20m wide perpendicular from d/wall alignment is to be allowed for d/wall activities where center portion to be cut down to RL +27.50 with 1:1 slope to facilitate bored piling works

Temporary Openings
Four (4) Temporary Openings will be provided at the Lift Core Wall area for the purpose of:
1. Ventilation
2. Access for construction materials
& machineries
3. Access for removal of excavated soil
4. Construction of lift pit & core wall
by conventional method (bottom up)

Ground Floor slab will be cast once the Plunge In Columns were completely Installed at the particular Zone

Excavation to 1.5m below the Basement 1 slab will continue after 16 days casting of the Ground Floor Slab

Construction of Basement 1 slabs will then proceed

Preparation of Scaffolding, Formworks & Rebar for Lower Basement Slab

Installation of Chemical Anchors for Slab Starter Bars

Sequence of Works for Lower Basement Slab

Ground Monitoring Instrumentation
The objectives :-
To monitor the effects of Deep Excavation works to Common Utility Tunnel (CUT) & Diaphragm Wall.
The Methods :-
Settlement Marker (inside CUT, Bldg & Adjacent Pavement)
Strain Gauges
Standpipe Piezometer

Settlement Points for Common Utility Tunnel (CUT)

Monitoring Device : Inclinometer

Subsequent excavation below the Basement 3 slab level will then proceed after 16 days casting of the Basement 1 slabs.
Construction of Basement 3 slabs will continue.
Excavation to pilecap level will be carry out after construction of the basement 3 slab.
The Basement 2 slab will construct simultaneously while excavation to the lowest Basement is in progress.
Construction of the Basement 4 slabs will then continue

Completed Basement Slabs

(conventional method)
The construction work for lift core wall & lift pit (previously temporary openings) will be done using conventional method Due to lift pit level is lower than lowest basement slab, Contiguous Bored Piles (CBP) will be installed as retaining wall to facilitate the construction of lift pits

Sequence Of CBP Works

  • Construct CBP concurrently with the foundation bored piles from existing ground level
  • Cast CBP until RL 12.10 which is approximately 900mm from B4 level
  • Upon excavation until B4 level, construct 900mm thick capping beam to tie all CBP prior to the excavation for lift pit
  • Excavate to RL 9.00 for the installation of strutting and water at RL 10.00
  • Upon strutting, excavate further to final excavation at RL 3.00 for the casting of raft slab and build up shear wall
  • Remove strut before casting the remaining of B4 slab
Construction of Lowest Basement Pilecap at B4

Installation of Pressure Relieve Valves

  • Excavation of Lift Pits to the face of the contiguous bored piles will be carry out concurrently with the removal of excavated earth by hoisting out using cranes and buckets.
  • Construction of Lift wall pile caps will then continue.
  • Construction of Basement 4 columns will be carry out upon the completion of Basement 4 slabs.

Construction of Lift Core Pile Cap

Construction of Shear Wall at B3-B4

  • Construction of Lift Pit walls will proceed to the Basement 4 level.
  • Construction of the columns from B3 to B2 will then continue.

  • Construction of Lift Wall and the columns from B4 to B3 will proceed and the temporary opening will be sealed off
  • Construction of columns from B2 to B1 will then continue.

  • Similarly, the construction of Lift Wall and the columns from B3 to B2 will proceed and the temporary opening will be seal off.
  • Construction of columns from B1 to Ground Floor will continue.

Construction of Columns (Tower)

Construction of Columns (Podium)

Construction of Lift wall and the columns from B2 to B1 will commence and the temporary opening will be sealed off.
Construction of Lift Wall and the Columns from B1 to Ground Floor will proceed and finally temporary opening will be sealed off
Finally the construction of the Basement Car park is then complete


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