The ground never really stops moving, which means at some point buildings suffer cracks, subsidence or heave. It’s a common problem for insurance brands and underpinning has been the default solution for decades. But this feature looks at a new system of injecting cementitious grout. It’s a brand new tech for us at Insurance Edge, but if you’re in the insurance sector and want to know more, then here are some extracts from a case study in London.
Two residential buildings in Barnet, London, suffered from subsidence and heave due to the expansion of unsettled London clay soil. Building A was stabilised using the traditional underpinning method. Building B suffered randomised distortion with a lift of 140mm on one side, which presented a challenge for conventional methods and potentially the use of hydraulic jack. The globally successful JOG technology by Mainmark was used to re-level and stabilise the building with minimal damage to the existing structure.
The two identical looking two-storey residential apartments were built in the mid-1990s, by the same developer and to the exact specification. However, the subsidence issues the properties suffered from were quite different.
Why was cementitious grouting selected to remedy subsidence and heave damage?
In 2011, building A showed cracks as an indicator of ground heave damage. The cause of the damage was identified as tree roots and the expansion of unsettled soil. PAH Building and Construction Ltd were appointed to analyse the extent of the damage and find an appropriate solution.
Traditional underpinning with a piled raft successfully stabilised building A with its active movement so that repair works could occur. During the nine-month process to rectify the randomly distributed distortion of building A, it was realised that building B was also impacted by subsidence. Following the monitoring phase, in 2013, surveys identified severe distortion in building B, presented as a 140mm lift to one side.
Multiple stakeholder interests, cost implications for insurers for temporary housing and the cost of remedial building work using traditional piling methods meant that demolition and rebuilding were considered the most effective solution.
Tom Griffiths at PAH Building and Construction Ltd, qualified as both a chartered engineer and a chartered loss adjuster, proposed Mainmark’s multi-point cementitious grouting process, JOG Computer Controlled Grouting, for ground stabilisation and level correction on building B in 2018. Although new to the UK at the time, the technology was and still is widely used worldwide and proven to successfully remedy subsidence to residential and commercial buildings and infrastructure, including extensive damage caused by earthquakes.
The initial survey involved 240 survey points across different sections of building A and showed that 84 points, around 35%, were outside the tolerance levels and NHBC (National House Building Council) standards. This informed the minimum and maximum lift specification, relevant gradient and tolerances.
The JOG Computer Controlled Grouting uses an advanced grout monitoring system with progress reports, allowing for fine-tuning. This helps achieve a high degree of accuracy and minimises stress on separate sections of the building during the process. The sections that were worked upon were supported and precisely lifted using small, sequenced injections of high-mobility cementitious grout that is very fluid but fast curing.
In total, 70 cementitious grout injections were used beneath the block and beam foundations with a split of 40/30 between external and internal placement. The injection depth was 1.1m and 2.1m, respectively, with a distance of 1.5m between injections.
All six of the primary level control stations on the building were monitored, and upon completion, only 9 of the 240 survey points were outside the NHBS standard tolerance level. A lift of 139mm was achieved at the maximum point.
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