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Narrow margins: piling in a highly confined space

An unusual basement and a townhouse are providing a challenge.

Construction of a triple depth basement and two-storey townhouse on an extremely tight land-locked site in the private residential enclave of Rutland Gardens in Knightsbridge was always going to be a tricky task.
The original proposed method of construction for the basement was a combined underpinning and segmental CFA auger mini pile followed by a “top down” sequence.

Contractor Reddington Group was asked to provide an innovative method that would offer reduced construction times and certainty of cost.

Miller Piling and its consultants were involved from the outset as partner of the design development team. Tight settlement constraints imposed by a party wall agreement were a key focus of the project and determined the selection of pile type in the prevailing soil conditions.

A detailed settlement analysis was carried out to determine the effect of excavating a 9m deep hole immediately adjacent to the property on three of the site boundaries.

Rotary bored piles were selected with a 6m long temporary casing to support the loose water bearing sands and gravel overlying the London Clay. The use of temporary casings also gave good control of the position and verticality of the wall over the top section, and mitigated the risk of piles kicking into the basement. Once sealed into the stable underlying strata, piles were open bored through the London Clay to the required design depth.

The physical constraints of the site dictated the logistics of piling, spoil removal and pile arising off site. Six piles were consistently constructed per day with little disruption to the residents of the gated community within Rutland Gardens.

The excavation of the basement was monitored throughout and has been completed within the extremely tight 3mm vertical and 2mm horizontal movement constraints of the adjacent properties.

The construction of the structure is currently progressing well and is on programme, thanks to use of an innovative overhead travelling crane arrangement has assisted the construction from the lowest basement upwards.

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