When the excavation its come from major cities, particularly in densely develop urban areas. In most common construction methods, buildings with underground parts are built by the bottom-up method where sub-structure and super-structure floors are constructed sequentially from the lowest level of basement to the top of the building. Though this conventional method, also called a bottom-up method, is simple in both design and construction, it is not feasible for the projects with limited construction time or with site constraints (size of location, legal issues, etc). The Top-down construction method is the solution to these problems.
Why Top Down Construction Require ?
- Distance between Boundary wall of Adjacent Existing building and Building line for New building is too close. The depth of excavation is more than 6 meters. The 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. The retaining wall panels will not allow collapse of soil.
- Water table is too high. Difficult to cast Retaining wall, Diaphragm wall or Continuous Pile can use as a Retaining wall.
- Installing of Diaphragm Walls.
- Installations of Barrette Piles.
- Excavation of soil just below the roof slab.
- Provide structural strut to support to provide Diaphragm wall.
- Expose coupler Bars/Dowel bars from Diaphragm.
- R.C.C. slab to be casting with suitable size opening for further construction.
- After casting slab, excavation to continue through opening.
- Excavation to be continue till base slab level.
Procedure of Top Down Construction
- Setting out for alignment of bore cast in situ of pile.
- Bore cast of in situ piles to be casting and dowels bar to be keep for retaining wall panels. The centre of piles and retaining wall panels centre line to be keep in same line. The Diameter of piles prefer 600 mm, distance between two piles centre to centre prefer 3.33 meters ( Suit for c/c distance of columns ). Depth of piles to be confirm by structure consultants.
- Touching piles can use as a Retaining wall with supporting arrangement during excavation.
- If area of basement is more and Span is less. Then, Diaphragm wall can be use as a Retaining wall.
- Excavation to be do around 2 meter depth and Retaining wall panel to be casting in stages with dowels bars with adjacent piles and dowel bars for next retaining wall panel to be keep. If soil between piles collapse soil nails with wire mesh to be keep on earth side to retain the same. Retaining wall panels to be casting with dowels up to the first base slab bottom.
- Dressing and compaction of soil to be do for PCC bed for casting of first base slab. 50 mm PCC (M15 Grade) to be do, the top levels of PCC to be keep the base slab level bottom. For beams excavation to be do in same alignment. Dimensions as require and PCC or brickwork to be do as per beam dimensions in vertical face.
- After PCC 3 mm ply to be lay on PCC surface. PVC sheet to be nail with PCC surface so surface of PCC and Surface of slab concrete will separate.
- Reinforcement to be complete as per drawings, for beams dowel bar to be use from Cast in Situ Piles. Dowel bars to be keep for adjacent slabs for Bottom Up Construction.
- Completing of slab and beam to be complete.
- After 14 days of concreting, PCC below slab to be break/remove excavation to be continue and retaining wall panels to be cat. Till final level of excavation.
- Waterproofing to be do and base slab to be casting with dowels from Cast in Situ Piles.
- Jacketing of columns to be do if require.
- The shorten construction period due to simultaneous construction.
- More operational space gain from advance construction of floor slabs.
- It is highly suitable for construction for tall buildings with deep basements to be construct in Urban areas.
- These can be install in close proximity to existing structure with minimal loss of support to existing foundations.
- Higher cost.
- The lateral displacement of retaining wall or ground settlement may possible.
- The construction quality may influence.
- It requires higher skill supervision and labour force.