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Mass market

Spotlight

Underpin & Makegood has been working since March on a contract to stabilise a house in north west London by mass concrete underpinning.

Subsidence to the left hand side of a two-storey traditional masonry extension had been caused by movement of desiccated London clays underlying the foundations.

This had been exacerbated by the felling of a large plane tree nearby.

Access to the site in Hamilton Terrace was difficult: The work had to be carried out below basement level, requiring spoil to be removed and concrete imported via a scaffold tower and hoist.

The traditional 'hit and miss' underpinning method used involves excavating isolated pier bases under the existing foundations - in this instance to a depth of 3m - before mass filling with concrete to within 75mm of the soffit of the existing foundation. After curing, the void is rammed with a semi-dry, cementitious mortar to mitigate any further settlement caused by longer term shrinkage occurring as the concrete cures.

After one underpinning 'base' is completed, the next can start.

To prevent the work causing heave to the foundations, the underpinning sections are lined with an anti-heave material before concrete is placed.

Underpinning depths are evaluated using site investigation data, taking into account the plasticity of the clay, the depth of root activity and an assessment of soil desiccation.

At this site, Atterburg limit test results indicated desiccation to a depth of 2.5m and live tree roots were discovered at 2.4m, hence the design depth of 3m.

The underpinning is due to be completed later this month.

www. underpin. com

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