Top-Down Basement Construction

Stages of Top-Down ConstructionEssentially involves casting the ground floor slab as early as possible so the superstructure above can be constructed whilst the basement is excavated below. Install perimeter walls (e.g. diaphragm wall, contiguous pile or secant wall) Install bearing piles with columns embedded into low cut-off level concrete. Excavate & cast B2 slab. Extend columns & cast second floor slabs. Cast ground floor slab integral with embedded columns Excavate and cast B3 slab. Extend columns & cast upper floor slabs. Excavate and cast B1 slab Extend columns and cast first floor slab. Top-Down Construction Using CEMLOC CEMLOC system was developed by Skanska. It is a unique system that is an adaption of the traditional approach which dramatically aids top-down construction. The system accurately places & holds plunged columns. CEMLOC is lowered into the casing and aligned with locating dowels. 4 rams at top & bottom of CEMLOC lock it to the casing.  Column is then plunged into the concrete whilst being held accurately & precisely by the CEMLOC. CEMLOC is then removed after the concrete has set. Restraint and column projection is removed once the column has set. Working platform is reinstated.  Grout and backfill is removed later in the process as the basements are constructed.  Case Studies of Top-Down Construction Queensberry House, Mayfair, London - 24m deep, 13 storey basement car park. One Hyde Park, Knightsbridge - housing scheme for the wealthy, four storey basement, saved 6 months on the programme.  The Shard, London - had a top-down core. Top-Down Construction - London Shard Raft foundation was constructed over a 36-hour pour of 700 truckloads and 5,500m3 of concrete. The Shard had already climbed to 21 storeys by the time the foundation was poured.  Stages of Construction The secant pile wall was installed around the perimeter along with the plunge piles and columns. The ground floor slab of the building was cast and excavation began down to level two of the basement. The floor slab at basement level two was cast and the slipform for the construction was erected to 'jump-start' the core. As the core goes up, excavation below basement level 2 continued. As the core construction continued, the raft foundation was cast at basement level three before the concrete walls between the base of the core and the raft were installed. Challenges of the Project Site must operate around the thousands of commuters using London Bridge station. Bus station on the doorstep of the site had to remain running. Hospital located across from the site. Specialised concrete mixture toflow into the densely packed reinforcement and to prevent shrinkage and cracking due to temperature differences.  Narrow roads and large amounts of pedestrians made moving materials to/from the site a major challenge. Must have minimal impact on surroundings. Large Victorian water mains and tunnels of the Jubilee line run beneath the site. Logistical challenges of a constrained site. Benefits of Top-Down Construction Reduced programme Improved health & safety by segregating the excavationa and concreting. 250,000 man hours with no accidents. Saves time by enabling the simultaneous construction of the building's superstructure and substructure. Risks of Deep Basement ConstructionCollapse Case Study - 37m deep diaphragm walls in Cologne collapsed due to the failure of the ground anchors, killing 2 people. Problems with groundwater levels blamed for collapse. Risk Management for Deep Basement Construction Plan tolerances (vertical & horizontal) Understand ground conditions Piling platform stability Watertightness Consider adjacent buildings Female pile concrete mix. Healthy Innovations - Stages of Pile Top Break Down Auger fitted with appropriate head Auger drilled into ground to required depth Concrete poured down hollow core of auger whilst auger is removed Steel reinforcement cage pushed into wet concrete. Wet concrete overspill at ground level removed. Ground level reduced & top section of pile 'broken down' to desired level. Pile cap or capping beam constructed.  Main Piling Health Risks Manual handling aspect of changing auger heads. Contaminated land hazards Dermatitis & other cement-related hazards Manual handling & injury risks in placing rebar cage along with cement-related hazards Cement related & manual handling hazards Major hand-arm or whole-body vibration hazards (e.g. HAVS) Noise and dust hazards (e.g. silicosis) Insitu concrete hazards Method for Addressing Risk of Removing Tops of Insitu Bored PilesNote - There are a number of methods for addressing this risk.Method 1Method used by Elliot reduces the risk of HAVS and other associated health risks by exploiting the physics of crack propagation. Isolating sleeves are fixed to the steel reinforcement bars above the final cut-off level to prevent them bonding with the concrete. A 51mm diameter is drilled horizontally into the concrete cut-off level to just beyond the centre of the pile. A standard hydraulic splitter is inserted & activated & after 30 seconds the concrete cracks across the desired level. A crane or excavator is then used to lift the surplus concrete in a single piece. Elliot's suggest i reduces HAVS risks by more than 90% and takes roughly 10 minutes, bringing productivity & cost benefits.Method 2 - Chemical Pile Break Method (Recepieux)As with Elliot's, this method exploits the principles of crack propagation. Foam sleeves are fixed over the reinforcement over the length to be removed. A series of PVC tubes & cones are assembled. The tube & cone assembly is pushed into the wet concrete. The tube assembly is checked for level & funnels fixed to the top of the tubes. The temperature of the pile concrete is measured. The expanding grout is mixed and batched into containers. The grout is poured into the tubes via funnels. The grout expands in the cones set at cut-off level, propagating a horizontal crack through the pile. The top unwanted section is removed by a crane.

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