DEWATERING STRATEGIES FOR LONDON | BTS MEETING
QUESTIONS AND ANSWERS
Q: Kate Cooksey: What would you deem as your most complex challenge today among the projects you have described? Toby Roberts (TR): Actually it’s the work we have been doing in Toronto. The soil in Toronto is very similar to the Thanet Sand but with no handy Chalk layer to underdrain it. It is also shallower, so they are building stations using SCL techniques in Toronto ‘Thanet Sand’ with a requirement for significant drawdown to a sand/clay interface. Q: Kate Cooksey: What would be your number one challenge here in the UK? TR: I think the work that we recently carried out at Blackfriars on Tideway was one of the most challenging current projects in the UK. Q: Kate Cooksey: You mentioned that over the past 30 years there has been quite a bit of difference in the technology and equipment that can go underground. What else would you say has been the biggest change in dewatering over this period? TR: The most significant change is the evolving environmental legislation and associated controls and constraints on groundwater pumping and discharge. When I first started my career, we installed dewatering systems and looked around for the nearest manhole or stream to discharge. Things have changed a lot and, quite rightly, environmental requirements are a major driver to what we do now, so it is vital to understand and work with them. Q: Kate Cooksey: What do you predict for the next 30 years in dewatering? TR: The move into a sort of carbon emissions-controlled world is happening now, and because dewatering does not use a lot of steel or concrete it has a relatively low carbon footprint compared to other methods of controlling groundwater. Q: Kate Cooksey: What do you wish construction projects would learn from you about dewatering? TR: The biggest current issue is environmental permissions. If you are going to be doing any serious dewatering you need to be thinking about it around a year in advance and if you are not thinking about it a year in advance it might drive your programme. Q: Mehdi Hosseini : What are the benefits and disadvantages of recharges? TR: You do recharge usually because you have to do it as a means of discharge or settlement mitigation or environmental mitigation. So, to some extent you have to make it work with the constraints that you have. What is interesting about recharge in England is that there is an exemption: if the abstracted water is recharged back to the same aquifer – providing there are no other environmental concerns – it does not need licencing. That is quite a big plus. Q: Cláudio Cabral Dias : What options would you consider when dealing with contaminated ground/ dewatering and in what order of preference?
TR: Contamination covers a lot of different things. We first tend to think about hydrocarbons because they are likely to be a problem wherever you put the water, and we start by looking at activated carbon treatment. If you are pumping 10 litres per second that is quite a small treatment plant but if you are pumping 100 litres per second that requires a big treatment plant and is a big cost. So it all comes down to cost. What are the flows? What are the treatment costs? Is it cheaper to use a physical cut-off or to treat? Also, if you are pumping, are you spreading the contamination? There are real issues around permissions and consents. Q: TunnelTalk : What is your opinion of the fear of surface settlement restricting or disallowing dewatering? TR: Settlement from a dewatering system is usually proportional to the drawdown. The distance of influence for a significant dewatering scheme may be 100m or even kilometres in chalk. This is very different from the settlement from tunnelling or a cavern excavation where the settlement is very local with risk of significant differential settlements. London has been under-drained by pumping from the chalk for 150 years. As a result, London is settling at a few millimetres a year but we all seem to survive. For deep dewatering in the intermediate or lower aquifer in the London Basin, you have to assess it and decide whether you can live with it, and if you can’t, then you cannot dewater and must do something else. The worst settlement problems are caused by soft sensitive soils; for example, when dewatering in the Terrace Gravel there is a risk of under-draining soft alluvium above. Q: Steve Parker: You talked about acid to increase yield. How far does this travel? Is it recovered or lost? TR: We have looked at this quite closely and the answer is not far because the quantities are small. The acid also reacts with chalk, so it is neutralised fairly quickly and, following acidisation, we do clearance pumping and pump it out. Chalk water supply wells have been developed by acidisation for decades. Q: Trevor Carter: How do you make a decision on balancing the settlement/subsidence issues versus dewatering-induced settlement? TR: Once you get below the top of the Terrace Gravels, you are dealing with soils that are very stiff and dense for the most part in the London Basin. The settlement of low permeability clay soils in the time frame of a construction project should be minimal. So, you are really talking about consolidation of the sand layers over a potentially wide area with low differential movement which will occur virtually instantaneously with drawdown, so it happens once. The challenge when estimating settlement is determining the low strain stiffness, and the best way is from case histories. Then it is really a question of looking at structures and finding out what the concern might be. It is worth bearing in mind that most of London has seen this very substantial drawdown which went down and has now gone back up again. It needs to be looked at and considered but it is not immediately obvious why it is a major concern.
November 2021
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