The document discusses top-down construction, which is required when: 1) The distance between existing buildings and the new building line is small, requiring deep excavation and risking soil collapse. 2) It allows for more building and basement area, suitable for two or more basements. 3) The first basement slab acts as a strut, preventing soil collapse behind retaining walls. The methodology involves: 1) Casting piles with dowels for retaining walls and slabs. 2) Excavating in stages and casting retaining wall panels with dowels between piles. 3) Pouring the first basement slab and then continuing excavation and construction from the bottom up

1. TOP DOWN
CONSTRUCTION
By- Gagan Goswami

2. Why Top Down Construction
Required
► Distance between Boundary wall of Adjacent Existing
Building and Building Line for New Building is too close and
the depth of excavation is more than 6 meter and
possibility of collapse the soil is higher during excavation.
► Using Top Down Construction more Building and Basement
Area can use. Suitable for two or more basement.
► The first base slab level slab will act in such area as a strut
member and the Retaining wall panels will not allow
collapse of soil.
► Water table is too high and difficult to cast Retaining wall,
Diaphragm wall or Continuous Pile can use as a Retaining
wall.

3. METHODOLOGY OF TOP DOWN
CONSTRUCTION
1. Setting out for alignment of bore cast in situ of
piles .
2. Bore Cast in Situ Piles to be cast and dowels bar
to be kept for Retaining wall panels. The center
of piles and Retaining wall panels center line to
be kept in same line. The Diameter of Piles
preferred 600 mm, distance between two piles
center to center preferred 3.33 meters (Suit for
C/C distance of Columns), Depth of Piles to be
confirmed by Structural Consultants.
3. Touching Piles can use as a Retaining wall with
supporting arrangement during Excavation.
4. If area of Basement is more and Span is less
Diaphragm wall can be use as a Retaining wall.

5. 3. Instead of Column C1 to Column C8,Bore
cast in Situ piles to be cast with dowel
bars for different level slab and
connecting Beams, The Diameter of Piles
(suit as per Columns size),
Reinforcement details (with dowels for
Retaining wall and different level slabs
and Beam), Depth of Piles to be
confirmed by Structural Consultants.

6. 4. Excavation to be done around 2 meter
depth and retaining wall panels to be
cast in stages with dowels bars with
adjacent piles and dowel bars for next
Retaining wall panel to be kept .if soil
between piles collapsed soil nails with
wire mesh to be kept on earth side to
retain the same. Retaining wall panels to
be cast with dowels up to the first base
slab bottom.

8. 5. Dressing and compaction of soil to be
done for PCC bed for casting of first base
slab. 50mm PCC (M15 Grade) to be
done, the top levels of PCC to be keep
the base slab level bottom. For beams
excavation to be done in same alignment
and dimensions as required and PCC or
brick work to be done as per beam
dimensions in Vertical face.

10. 6. After PCC 3mm ply to be laid on PCC
Surface and PVC sheet to be nailed with
PCC surface so surface of PCC and
Surface of Slab Concrete will separate.
7. Reinforcement to be completed as per
drawings, for beams dowel bar to be
used from Cast in Situ Piles. Dowel bars
to be keep for adjacent slabs for Bottom
Up Construction.

12. 8. Concreting of Slab and Beam to be
completed.
9. After 14 days of Concreting, PCC below
slab to be break/removed excavation to
be continued and retaining wall panels to
be cast. Till final level of excavation.
10 Waterproofing to be done and base slab
to be cast with dowels from Cast in Situ
Piles.
11.Jacketing of Columns to be done if
required.

13. ►THANKS
►Ifany query please mail -
gagangoswami@yahoo.co.in