{"id":453149,"date":"2022-03-03T06:50:15","date_gmt":"2022-03-03T06:50:15","guid":{"rendered":"https:\/\/www.constructionnews.co.uk\/?p=453149"},"modified":"2022-03-01T17:42:06","modified_gmt":"2022-03-01T17:42:06","slug":"how-underground-pockets-could-hollow-out-your-profits","status":"publish","type":"post","link":"https:\/\/www.constructionnews.co.uk\/consultants\/how-underground-pockets-could-hollow-out-your-profits-03-03-2022\/","title":{"rendered":"How underground pockets could hollow out your profits"},"content":{"rendered":"

Dr Amy Flynn is a design engineer at consultants Robert Bird Group<\/em><\/p>\n

Ground conditions are of vital importance for major construction projects. If assumptions made about the ground underfoot prove to be wrong, it can mean massive additional costs and risks for the contractor or developer.<\/p>\n

One particular ground condition can be a real challenge in London and has affected many high-profile projects, including Thames Tideway, Crossrail, the Northern Line extension and Battersea Power Station. All of these projects encountered sub-surface features called drift-filled hollows and were sometimes forced to divert around these troublesome pockets of \u201canomalous geology\u201d.<\/p>\n

Difficult to detect<\/h3>\n

Drift-filled hollows are scattered right across the London Basin. They range from just 5 metres to 500 metres in width, and up to 75 metres deep. We cannot detect them from ground level, which creates problems for engineers, because we don\u2019t know whether any are there unless we put a borehole down in the right location.<\/p>\n

Given that we need to know what we are building on top of, or what a tunnel is going to meet on its route, the discovery of anomalous geology can have a big impact on cost and risk.<\/p>\n

\u201cEven when a borehole discovers such a hollow it is impossible to know the full extent of the problem. They are highly variable internally\u201d<\/p><\/blockquote>\n

It would certainly help to know where these features are and their extent. For my university PhD research I studied where some of these hollows are in London to try to give better clues about where we might face these underlying ground conditions.<\/p>\n

Drift-filled hollows are anomalous geology features found within the local bedrock, most commonly in London Clay, but they also crop up in the Lambeth Group, Thanet Formation and the Chalk Group.<\/p>\n

If a test borehole finds gravel or sand at a depth usually sufficient to reach the London Clay or local bedrock, it has probably hit a drift-filled hollow.<\/p>\n

Even when a borehole discovers such a hollow it is impossible to know the full extent of the problem. They are highly variable internally, in both horizontal and vertical scale, and in terms of their features. No two seem to be the same.<\/p>\n

My research of 78-85 locations within the M25 shows variations in width, depth and infill material. Sometimes there is also an upwelling of material from below, such as chalk protruding into the hollow.<\/p>\n

As geotechnical engineers, we run tests on the material that comes out of the ground to provide material properties for structural design. But, if the sampled properties are wrong or not representative of the whole feature, how do we characterise or put values on the risks?<\/p>\n

Holes in research<\/h3>\n

My research has registered the identified features in London, but there is a lot more research that could be undertaken. If a developer buys a site and doesn\u2019t know if a hollow is there, or its extent, it could cost a great deal more than expected in ground investigations for the project and even put the viability of a development in question. Having access to research on the nature of the problem may help to minimise these risks and costs.<\/p>\n

The problem for contractors is not only the uncertainty. For example, when piling for foundations, a contractor may have enough concrete, casing or drilling equipment to reach a depth of 10m, where they expect to find bedrock. But if they instead find a hollow that is 75 metres deep, they are unlikely to have enough equipment and materials to reach bedrock.<\/p>\n

Similarly, how can a pile be designed if the pile contractor doesn\u2019t know the properties of the material? A pile securing into bedrock at a depth of 10 metres will have a different capacity to one that runs 75 metres deep.<\/p>\n

A further risk for the contractor is different rates of settlement. On London Clay, where there is good, competent ground, a new building will settle in approximately 15 years, but if half of the building sits on a drift-filled hollow of unconsolidated gravel and sand there will be structural implications due to differential settlement.<\/p>\n

The lesson to be learned is that old adage, buyer beware. Contractors and developers should ensure that the necessary steps have been taken to identify anomalous geology. Expert testing and modelling could save a great deal of money in the long run.<\/p>\n","protected":false},"excerpt":{"rendered":"

Dr Amy Flynn is a design engineer at consultants Robert Bird Group Ground conditions are of vital importance for major construction projects. If assumptions made about the ground underfoot prove to be wrong, it can mean massive additional costs and risks for the contractor or developer. One particular ground condition can be a real challenge …<\/p>\n","protected":false},"author":1,"featured_media":453150,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"ep_exclude_from_search":false},"categories":[57172,567],"tags":[],"yoast_head":"\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\t\n\t\n\t\n