Construction of a waterproof reinforced concrete double storey basement extension to an existing Grade II listed terraced town house in central London, including underpinning and the lowering of the existing lower ground floor and vaults to the house.
Due to the location of the site, all excavators and heavy excavation plant was lifted into the rear garden by obtaining a full road closure, back propping the existing vaults and using a 100t crane.
The conveyors (21m in length overall) were setup through the house and supported by the gantry scaffold at the front of the site.
Tension piles were installed from existing rear garden level
1st stage Underpinning to 6m depth were first undertaken, a special hydraulic grab bucket was used to excavate the deep underpins
We were engaged by ME Construction as a domestic Sub-Contractor on an amended form of sub-contact, JCT Design & Build 2011, with design responsibility for temporary works, permanent mini-bore piling and without any other permanent works design responsibility.
The scope of works for the RC double storey basement extension, constructed in the rear garden of an existing Grade II listed terraced townhouse, comprised lifting in all plant to the rear garden of the house due to restricted access, reduced level dig, installation of piling mat and high level mini-bore piles, setting up conveyors through the house and a front gantry for spoil removal, installation of 1st stage reinforced concrete underpinning to a depth of 6m, reduce level dig and installation of the lower ground floor waterproof RC slab cast on existing ground in a top-down method of construction, upper basement excavation and spoil removal by conveyor through the lower ground floor slab apertures, installation of 2nd stage reinforced concrete underpinning to formation level, casting waterproof RC upper basement slab liner walls, sub-basement excavation and spoil removal by conveyor through the upper basement and lower ground floor apertures, lowering a mini piling rig into the sub-basement and installation of mini bore tension piles at sub-basement level, casting waterproof RC sub-basement slab and casting waterproof RC liner walls to the sub-basement.
The existing Grade II terraced townhouse is located in Chelsea, London which is surrounded by congested access roads. All excavators, piling rig and excavation plant were lifted into the rear garden of the house by a 100 tonne capacity mobile crane. We obtained full road closures for the installation and removal. We installed back propping to the existing vaults to facilitate plant traversing over the vaults.
We set up a combination of conveyors (totalling 21m overall) and support scaffolding through the house for the spoil removal from the basement works together with a gantry scaffold and loading platform at the front of the house.
We adopted a top-down method of construction for the double storey basement extension.
Waterproofing to the basement was achieved by using a specified Xypex waterproof concrete additive together with its proprietary hydrophilic waterstops.
The high level mini-bore piles and the sub-basement mini-bore friction piles, all 300mm diameter, were designed and installed by Modebest using our in-house restricted access mini piling rig. The high level piles were installed from a piling mat at the existing rear garden level and cut down at each stage of the basement excavation to their final cut-off levels. We lowered our mini piling rig, using block and tackle, through a stairwell into the excavated basement in order to install the sub-basement mini-bore friction piles.
The reinforced concrete underpinning was deep at over 8m. We constructed the underpinning in two stages. The first stage was 6m deep for each pin which we cast in one vertical depth from the existing ground level. The second stage was cast from the upper basement excavated area. We utilised a special hydraulic grab bucket attachment to the excavator for the deep underpin excavation.
The scope of works entailed:
The lower ground reinforced concrete slab was cast at reduced ground level on a prepared concrete blinding area in a top-down construction method. The upper basement excavation and spoil removal by conveyors was carried out through apertures in the lower ground floor slab. The upper basement reinforced concrete slab was cast in a similar way to the lower ground floor slab and then the excavation and spoil removal to the sub-basement was also carried out in a similar way. The upper basement reinforced concrete liner walls were cast. Following the installation of the sub-basement mini-bore friction piles the sub-basement reinforced concrete slab was cast. The sub-basement reinforced concrete basement liner walls were then cast.
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