Tony K Posted October 19, 2020 Share Posted October 19, 2020 Hi I have started my SB by reducing the level of the site, digging out some footings and installing a standard soakaway. Recent heavy rain has proved that the soakaway doesn't work as the clay is pretty much impermeable at that depth, which was always a possibility. I have had a drainage engineer produce an alternative design which basically consists of using the soakaway crates as an attenuation tank, connecting a large pump and directing all surface water into the nearby Thames Water sewer network, subject to their permission of course. The Thames Water sewer manhole cover is right outside my site, and the invert level is such that I could connect my downpipes directly into the network and let gravity do the rest. The drainage engineer tells me that Thames Water will not likely accept this approach, as the flow of water will be uncontrolled, whereas with the attenuation tank and pump set up, control can be exercised. I understand that, but its expensive and complicated. Has anyone ever tried to get a mains sewer connection without controlling water flow? Thanks Link to comment Share on other sites More sharing options...
scottishjohn Posted October 19, 2020 Share Posted October 19, 2020 is it not just a case of your soakaway tanks are not big enough -- maybe make them into rainwater harvesting and use it for house toilets+washing machine + garden it would also cut down on your water usage bill? Link to comment Share on other sites More sharing options...
Tony K Posted October 19, 2020 Author Share Posted October 19, 2020 4 hours ago, scottishjohn said: is it not just a case of your soakaway tanks are not big enough -- maybe make them into rainwater harvesting and use it for house toilets+washing machine + garden it would also cut down on your water usage bill? I don't think it matters really how big the soakaway is if the ground is impermeable. I will look into harvesting but I would still have to pump excess into the network even if I have a harvesting set up. Link to comment Share on other sites More sharing options...
Big Jimbo Posted October 19, 2020 Share Posted October 19, 2020 Just a thought, I think you might also have to have a silt catcher before you can discharge rainwater into the sewage system Link to comment Share on other sites More sharing options...
scottishjohn Posted October 19, 2020 Share Posted October 19, 2020 55 minutes ago, Tony K said: I don't think it matters really how big the soakaway is if the ground is impermeable. I will look into harvesting but I would still have to pump excess into the network even if I have a harvesting set up. depending on tank size and usage you might not have any excess --most rainwater systems have a top ballcock from the mains for when you run out of rainwater Link to comment Share on other sites More sharing options...
Tony K Posted October 20, 2020 Author Share Posted October 20, 2020 10 hours ago, scottishjohn said: depending on tank size and usage you might not have any excess --most rainwater systems have a top ballcock from the mains for when you run out of rainwater Thanks, I'll look into it. If I've got to pay for a pump then I might as well pay for a harvesting kit instead if it does the same thing. Link to comment Share on other sites More sharing options...
scottishjohn Posted October 20, 2020 Share Posted October 20, 2020 11 hours ago, Big Jimbo said: Just a thought, I think you might also have to have a silt catcher before you can discharge rainwater into the sewage system there will be one in rainwater harvesting system Link to comment Share on other sites More sharing options...
Oxbow16 Posted October 20, 2020 Share Posted October 20, 2020 Hi. I don't know much about such things so please add the required salt to my comments Two things I have learnt whilst investigating my land for a suitable drainage field (albeit for a septic tank) are: - The land was very different in different areas. My first hole never drained which led me to think it was clay and impermeable. Turns out though that it is likely the land in that area has had human intervention of some sort, as when - by chance - I did a second hole in a different and more out of the way spot, it percolated very well. Obviously this might be irrelevant if your plot is on the smaller side. - When I was reading up on the subject I came across several people talking of a "clay cap". And that if there is one, getting passed that may mean reaching more permeable ground conditions. I didn't pursue this myself because the other hole worked so well so there was no need. And I was digging by hand and so couldn't go too deep. Perhaps if you've had a drainage engineer involved this has already been thought of and/or deemed irrelevant. But it was a couple of things I came across whilst being faced with poorly draining soil and so thought there's no harm in sharing. Good luck! Link to comment Share on other sites More sharing options...
Mr Punter Posted October 20, 2020 Share Posted October 20, 2020 Could you have a fairly flat attenuation tank at high level with a small discharge at the bottom to the sewer and a large sized discharge at the top, so the tank will have to be full before you discharge at full rate and the tank will empty over time? Link to comment Share on other sites More sharing options...
Tony K Posted October 20, 2020 Author Share Posted October 20, 2020 2 hours ago, Mr Punter said: Could you have a fairly flat attenuation tank at high level with a small discharge at the bottom to the sewer and a large sized discharge at the top, so the tank will have to be full before you discharge at full rate and the tank will empty over time? I have considered various set ups like that, but they all fail due to the whereabouts and design of my slab, services, a void for heave, and the sewer connection. Thanks @Oxbow16 but my land has never been touched by human hand, and the clay is consistent all around. My original hope was that someone would say that surface water flowing directly into the main network is demonstrably fine, and that they had experienced as much. Doesn't look like anyone has had that experience though! The next best outcome for me was that someone might come up with a new idea, and though I had discounted harvesting at an earlier stage of my build, that was before all this happened so I will certainly revisit it now, if only to compare the cost of it with the attenuation tank and pump arrangement. Link to comment Share on other sites More sharing options...
scottishjohn Posted October 20, 2020 Share Posted October 20, 2020 do you know how thick the clay layer is? maybe you can go through it into a gravel layer? Link to comment Share on other sites More sharing options...
Mr Punter Posted October 20, 2020 Share Posted October 20, 2020 You are supposed to consider these options in order for surface water: On site via soakaway / permeable surfaces To a watercourse A surface water sewer A combined sewer In exceptional circumstances, a foul sewer Link to comment Share on other sites More sharing options...
Tony K Posted October 20, 2020 Author Share Posted October 20, 2020 1 hour ago, scottishjohn said: do you know how thick the clay layer is? maybe you can go through it into a gravel layer? I don't know for sure, but I can only get a mini-digger on my site and it was pretty close to the limit of how deep it could dig when we put the soakaway in, so going far deeper probably isn't an option. 1 hour ago, Mr Punter said: You are supposed to consider these options in order for surface water: On site via soakaway / permeable surfaces To a watercourse A surface water sewer A combined sewer In exceptional circumstances, a foul sewer That was my understanding, and I can demonstrate that the first three options aren't feasible. I just wondered why direct connection to the sewer is such a no-no? Link to comment Share on other sites More sharing options...
Mr Punter Posted October 20, 2020 Share Posted October 20, 2020 Because if everyone did it the sewer would be overwhelmed in a storm. Also the mix of product at a treatment works becomes very variable. Link to comment Share on other sites More sharing options...
Tony K Posted October 20, 2020 Author Share Posted October 20, 2020 3 minutes ago, Mr Punter said: Because if everyone did it the sewer would be overwhelmed in a storm. Also the mix of product at a treatment works becomes very variable. I can see the general issue of course, but there must be some degree of local variation? If my local network is, for instance, under capacity then I wonder if there is merit in me seeking permission for a direct connection? The line I would connect to is a combined sewer, so I'm not sure I would be affecting the mix of product. I might even be helping to flush things through a little! Link to comment Share on other sites More sharing options...
scottishjohn Posted October 20, 2020 Share Posted October 20, 2020 I know you don,t want to hear this but this is why you should have done percolation tests when making your plans , before you built the house and found a solution then for the planning application and the results should be part of the detail that was submitted to planning and BC not having done this --or maybe your architect fabricated a test to get it passed i don,t see them being too happy with you now It was a thing I did when looking at a site --before i even bought it -- did it in november and the results were not good --so i walked away seeing cows in a field next too it up to the knees in mud made me do this -- and the test confirmed I would have problems , but it something your supposed to do on all sites to prove it can work Link to comment Share on other sites More sharing options...
Mr Punter Posted October 20, 2020 Share Posted October 20, 2020 You may need to do something as a compromise as at the moment you are proposing piping all the surface water direct to the sewer with no attenuation. So a holding tank, a bit of ground infiltration, a few permeable surfaces, directing to a drainage ditch, will all help. Link to comment Share on other sites More sharing options...
ToughButterCup Posted October 20, 2020 Share Posted October 20, 2020 (edited) @Tony K, notwithstanding the above response ( @scottishjohn), now's the time for a bit of 'wider' thinking. If my few years' reading on the subject is to be believed, your problem is common. Look at your topography. Take a deep core maybe , after researching your local topography? Shingle? Sandstone? Mudstone? Flat? Completely flat? If it is not, can you consider making a pond? If so, duct the rainwater into it. Overflow to mains perhaps? How far away is the nearest stream? Discharge to that? Edited October 20, 2020 by ToughButterCup Link to comment Share on other sites More sharing options...
Temp Posted October 20, 2020 Share Posted October 20, 2020 (edited) We have clay soil and soakaways don't work. In such cases you are at the mercy of the water co if there are no other solutions. You can ask if they will allow you to connect to the sewer but many won't allow that at all. You stand much more chance if you are seen to be making an attempt to mitigate the storm surge by using an attenuation tank (with pump if necessary). In our case there was a piped ditch along the road edge. I asked the EA of we could connect into that. They didn't say no but they said "they wouldn't want us to make local flooding worse". So to head off a problem with the planners I proposed an underground rainwater storage tank with overflow to the ditch. This was allowed even though it doesn't provide surge attenuation when full. It's proved very useful in dry summers for watering lawns etc. We have put a pump in it to feed a tap and even run 200m meters of hose to fill up tanks on local allotment. Edited October 20, 2020 by Temp Link to comment Share on other sites More sharing options...
Tony K Posted October 20, 2020 Author Share Posted October 20, 2020 Thanks all. The planning dept didn't ask about drainage, and the building control dept signed off the use of a soakaway, though I was aware that it might fail given the clay. For that reason I've done the ground lowering and installed the SA now, well ahead of any actual building work for the house, just to test it. The drainage engineer feels confident that by proposing the attenuation tank, pump etc, Thames Water will have to say yes, what with the house already having approval and there being almost no other feasible options. I will find out in a couple of weeks whether he is right. If Thames Water say no (and even if they say yes) I will explore the use of harvesting systems whether as a preferable alternative even if I can connect to the sewer, or as a back up alternative if I can't. Link to comment Share on other sites More sharing options...
scottishjohn Posted October 20, 2020 Share Posted October 20, 2020 13 minutes ago, Tony K said: Thanks all. The planning dept didn't ask about drainage, and the building control dept signed off the use of a soakaway, though I was aware that it might fail given the clay. For that reason I've done the ground lowering and installed the SA now, well ahead of any actual building work for the house, just to test it. The drainage engineer feels confident that by proposing the attenuation tank, pump etc, Thames Water will have to say yes, what with the house already having approval and there being almost no other feasible options. I will find out in a couple of weeks whether he is right. If Thames Water say no (and even if they say yes) I will explore the use of harvesting systems whether as a preferable alternative even if I can connect to the sewer, or as a back up alternative if I can't. your lucky down there -- here the BC man would want to come and watch it being done sometimes before he signed off on the drains Link to comment Share on other sites More sharing options...
Mr Punter Posted October 20, 2020 Share Posted October 20, 2020 34 minutes ago, Temp said: We have clay soil and soakaways don't work. In such cases you are at the mercy of the water co if there are no other solutions. You can ask if they will allow you to connect to the sewer but many won't allow that at all. You stand much more chance if you are seen to be making an attempt to mitigate the storm surge by using an attenuation tank (with pump if necessary). In our case there was a piped ditch along the road edge. I asked the EA of we could connect into that. They didn't say no but they said "they wouldn't want us to make local flooding worse". So to head off a problem with the planners I proposed an underground rainwater storage tank with overflow to the ditch. This was allowed even though it doesn't provide surge attenuation when full. It's proved very useful in dry summers for watering lawns etc. We have put a pump in it to feed a tap and even run 200m meters of hose to fill up tanks on local allotment. I have had similar in the past. We drained into an old well and used a pump with a float switch to pump up to a ditch. A few houses along the road had covered over the ditch for their driveway crossovers but I think they have cleared them out since. Link to comment Share on other sites More sharing options...
Temp Posted October 20, 2020 Share Posted October 20, 2020 Its a bit naughty but... Is your foul drain pipe going to be in a trench surrounded by a nice lot of pea gravel? How about the main sewer in the road, is that also surrounded by pea gravel. A little bird told me that in the past they had laid a perforated pipe in along side the foul connection to turn the whole lot into one massive soakaway. Link to comment Share on other sites More sharing options...
Tony K Posted October 21, 2020 Author Share Posted October 21, 2020 21 hours ago, Temp said: Its a bit naughty but... Is your foul drain pipe going to be in a trench surrounded by a nice lot of pea gravel? How about the main sewer in the road, is that also surrounded by pea gravel. A little bird told me that in the past they had laid a perforated pipe in along side the foul connection to turn the whole lot into one massive soakaway. A different little bird whispered some similar things in my ear! Problem is that the local network is prone to blockages (baby wipes seem to be the villain of the piece, even though nobody locally has a baby), and Thames Water are forever around here lifting up manholes and poking about. They would spot any phantom connection pretty quickly. My research into rainwater harvesting is returning some interesting early results. I would need a tank that could hold nearly twice as much rainwater as I would ever likely use - a 7500 lt tank to be precise, measuring some 1.4m deep, 3.3m long and 2.3m wide. That needs a big hole dug for it, though not that much bigger than the one I have already dug for my soakaway, which it would replace. The giant tank is needed because it has to sit half empty all the time, to ensure enough extra capacity to accommodate any 1 in 100 year downpours. I would also still need to connect to the main sewer, because the tank would need to pump water away if it got more than half full. My plans for a green roof would have to be abandoned if I got a harvesting system - you can't harvest from a green roof apparently, or from hard surfaces, which means my patio and artificial turf area would need to be drained separately. The cost for the kit seems to vary from about £3100 to £3700 depending on whether it contains a pump. That actually doesn't compare too badly to the costs of buying the large pump my drainage engineer has specified for the current proposed drainage system. All things considered it is certainly worth investigating further. Link to comment Share on other sites More sharing options...
scottishjohn Posted October 21, 2020 Share Posted October 21, 2020 Just now, Tony K said: (baby wipes seem to be the villain of the piece, even though nobody locally has a baby), thats cos a lot of people use them when they have had a crap bet most are not used for babies Link to comment Share on other sites More sharing options...
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