City Basements Article Published in Ground Engineering

City Basements' Morph Kassir has article featuring the specialist temporary works used at our contract in Chelsea. The article, as featured below, was published in the October 2008 issue of Ground Engineering.

Chelsea address; a semi-detached Grade II listed villa, built as a private four storey house in London. The £2.2M contract, won by City Basements, involves the creation of a luxury, 540m2 basement – 7m to 8.5m deep. It will house, among other things, a swimming pool and gymnasium.

Excavation went below the foundations of the existing house, along the boundary with a school to the north, the boundary with no 17 The Boltons to the south and a mews house to the west. This meant that the design and installation of the temporary works had to ensure neighbouring properties did not suffer any distress. The contract encompasses all aspects of forming the basement, including piling, underpinning, excavation, building the reinforced concrete structure and waterproofing. Design and installation of all temporary works and some of the permanent works was also carried out in-house by the firm. .

The basement was formed within the River Terrace Gravels formation consisting of well graded medium dense gravels overlying impermeable London Clay. The deeper part of the basement will house the swimming pool and undercroft, both formed below a water table occurring at 7m below ground level. An interesting challenge was the design and installation of the temporary works to support the soil around and under a tree, which was protected under a preservation order. To maximise available space in the basement, while protecting the roots of the tree, the client’s design team needed the ground to be removed from beneath the tree over a limited portion of the basement, below a 1.8m depth of soil which was to be left undisturbed. The external wall of the basement had to be profiled in the area under the tree to provide a 2m radius protected zone. The ground between this and the upper 4.2m radius tree protection zone had to be removed using a mining operation and supported in the permanent case by the basement roof slab .

Although the techniques deployed by City Basements to overcome this challenge are well-established, using them together in this scale of project had not been attempted before by the firm. .

Two types of temporary retaining structures were chosen. First was 127, 111mm diameter Ischebeck Titan micropiles were installed on three sides at a 2.0m radius around the tree. The piles were threaded in between tree roots, which were exposed by hand at ground level. Site workers drove the piles at 500mm centres to 9.5m depth with grout injected through the micropile to create a 300mm diameter pile in the medium dense gravels. Water flushed the top 2.m of each pile to remove the grout and the Ischebeck Titan drilling rod uncoupled and removed. Micropiling was chosen over more conventional minipiling as the technique is less intrusive and quicker. .

The second type of retaining wall used, at a radius of 4.2m from the tree, was a king post type construction. Here, City Basements used the permanent works bearing piles in the temporary condition, to support the soil above, using steel columns plunged into the piles. The soil between the king posts, which were installed at 2m to 3m centers, was held back down to a depth of 2m using trench sheets and a waling beam. .

To create the 2m wide overhang and form the basement under and around the tree, specialist mining techniques were used where trench sheets were pushed horizontally into the ground, reacting off a system of beams welded to the king posts. Once the trench sheets reached the micropile wall where they were placed to rest onto brackets welded to the Ischebeck Titan piles. Installing the micropiling around the tree started at the end of April. Excavation around the tree, including mining beneath it, was carried out concurrently with the bulk excavation for the basement, which completed at end of August. The permanent reinforced concrete structure is currently being formed on a site where no trees were harmed in the making of this feature.

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