Browse Forums General Discussion 1 Jul 25, 2017 1:13 am I think I have a problem with groundwater drainage. My house is a single level pole home built on a slope in 2008. I have had it less than a year. The rear edge of the house is near the ground (0.5m above ground level). The front is high above the ground. The rear of the house sits on a small retaining wall. The narrow strip of land just behind this wall is very muddy and wet even though it has not rained here for ages. It seems to be permanently muddy and wet. It seems that the retaining wall is retaining a lot of water in the soil. The soil under the house in front of the retaining wall is dry and rock hard. There is a piece of 10cm underground drainage hose sticking out of the earth just to the side of the retaing wall as if it is supposed to be draining the area to the rear of the house. I think there is some kind of French drain installed under the earth just to the rear of the retaining wall but it does not seem to be working. The hose interior is always bone dry so the hose drainage holes seem to be clogged. The earth down the side of the house is also boggy and damp where the water seems to drain slowly through the earth round the corner at the end of the retaining wall and then down the slope near the pole footings at one side. I am worried that the retaining wall and/or pole foundations could shift if the earth is allowed to remain boggy and wet. Any advice welcome. Re: Ground Water Drainage 3Jul 26, 2017 12:13 am OK thanks. I will maybe get onto the council for the plans. I don't think the drainage arrangements would be too complicated to figure out by just uncovering the dirt from the existing drainage hose (which seems to be blocked) I have put some pictures with more detailed description in this gallery. http://imgur.com/gallery/sCfJ4 Please offer any comments or ask for more info if you think you can advise. Thanks Re: Ground Water Drainage 4Jul 26, 2017 8:12 am Othello Thanks for the photos, did you get a structural report before you purchased the house? There appears to be signs of considerable past soil erosion from under the house make sure you get the contour survey Does your block slope from two sides? Firstly you will need to determine why, then the effect lastly the fix/repair... don't delay Designer,Engineer (Civil,Const & Envir),Builder,Concrete & Masonry Contract.Struct Repairs Re: Ground Water Drainage 5Nov 26, 2017 2:51 am ^I bought the house at the end of 4 months of dry weather. I think the evidence of the problem had been disguised and has now reappeared, so it did not show up in the report. The problem is with the French drain. The folk who laid the original french drain did not lay it very evenly and the flexible drainage pipe has a hump in the middle section. The length of draining pipe upstream of the hump is permanently full of water even though it still works. i.e. The pipe is clear and not blocked and water will drain from the entire length of the pipe but due to the hump one section is full of water all the time. The earth is thick clay and water impermeable. The retaining wall is the foundation for the back of a house built on a slope. The house is 8 years old and the hump has always been there and there has been no subsidence. There is a strip of grass 2m wide to the rear of the retaining wall. The earth on the downslope side of the retaining wall is only 6 inches below the earth to the rear. The retaining wall is set 1m deep into the ground and mostly buried. The drain is 2m back from the retaining wall so as to stop the 2m strip of grass behind the wall from getting boggy. The french drain pipe is only 15cm below ground. The water the drain collects runs off a 7m slope to the rear of the 2m flat section behind the retaining wall. I have discovered all this information by digging the top layer of earth off the french drain. It was clay earth and completely impermeable so the french drain was not doing anything. Water was running over the clay cover and then along the back of the retaining wall and draining round the side. Now I have removed the clay topsoil and uncovered the gravel the drain is working again. It works for its full length as even the part of the pipe that is under water still acts as a conduit from water pressure. I was going to replace the clay top soil with gravel so the french drain channel is gravel all the way to the surface. Is it OK to have gravel all the way to the surface in a French drain instead of the usual top layer of top soil? The soil here is clay and no good as a french drain cover as it is not permeable at all. Rather than dig up the submerged length of pipe that is too low (upstream of the hump) I was thinking of 2 solutions 1. just fill with gravel - the submerged section of pipe still works as a water conduit even though it never empties. although flow is slower from this section 2. add a parallel second section of draining pipe just above the submerged section. should speed up drainage from that section even though there will still be a 2nd section of submerged pipe also acting a conduit (with slower flow) beneath the new section/ Would there be any problems with there being a submerged section in the french drain which never empties but always has a hidden subterranean pool of water in it? I am planning to connect the french drain outlet to an inlet on a new storm water drain box and then connect some non-perforated flexi hose to an outlet to carry the collected water down the slope past the footings to solve the problem of boggy ground around the footings downhill from the retaining wall. . Re: Ground Water Drainage 6Nov 26, 2017 8:16 am It seems to me that trying to fix drainage that does not work is a losing proposition and instead it would be quicker and more effective, reliable and relatively inexpensive to put in a new agi drain in a trench 600mm deep about a meter away from the house.(parallel to the house) Bring crushed rock to the top and make sure that ground either side of the trench slopes toward the trench. That way you will not only get rid of sub soil water but also surface water from heavy rain. Ensure you drainage (new and old outlet is connected to stormwater drain (you don't want drain water discharging under a house) Foremost Building Expert in Australia,assisting with building problems/disputes, building stage inspections,pre-contract review advice for peace of mind 200 blogs http://www.buildingexpert.net.au/blog Re: Ground Water Drainage 7Nov 26, 2017 3:27 pm Agree with B.E. but make sure that the water discharges to a silt pit before it is diverted to stormwater. What is at the top of the slope? Is there room to plant some Callistemons or other water loving shrubs? 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Ground Water Drainage 8Nov 26, 2017 6:56 pm Thanks for the advice fellas. The existing agi drainage pipe is actually clear. It was just half submerged due to the hump halfway along. So it will stilI be serviceable once I have it working. I think it has never really worked since it was installed since they topped it with 5 inches of impermeable clay soil which carried all the water over into a small trench at the back of the retaining wall. I have pulled up 2/3 of the drain pipe from the gravel filled trench. Lifted the drainage matting in the middle and dug out the hump of mud. Put the drainage matting back down. Laid the pipe back down with gravel surround and with a small slope from one end to the other. I tested it with buckets of water and it drains quickly from one end to the other now. The original pipe was not connected to any kind of outlet so even if it had worked it would just discharge to the side of the retaining wall and the run-off would be running down the slope round the house footings. Before I fixed the drain water was discharging at the same spot but it was being channelled by the retaining wall instead of by the french drain. I have rigged up a temporary pit which catches most of the water and then channels it away through flexible hose. I will now do a proper drainage pit with a silt pit at the bottom. Pipe is 90mm, pit is 30cm deep so enough height to offset the inlet and outlet by one diameter plus 10cm for silt. I will be relying on the impermeability of the soil in which the drainage pit will be embedded to make one side of the pit wet (inlet from the french drain) and one side dry (outlet to the runoff channel flexible pipe). I will stop the gravel surrounding the drainage pipe 12 inches before it connects to the pit and use the impermeable clay soil as drainage channel fill instead, to try and get the best seal between wet and dry side of the pit. Does that sound reasonable as an arrangement? Re: Ground Water Drainage 9Nov 26, 2017 9:56 pm I'm impressed! 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Ground Water Drainage 10Nov 27, 2017 12:05 am Thanks.H2O In answer to your question about vegetation. The OP has the link to the original problem when I discovered it with pics of the landscape garden that drains down a slope above the house towards a flat strip of land behind the retaining wall and house. http://imgur.com/gallery/sCfJ4 Here are pics showing the french drain revealed by me digging it up. You can see how the hump half way along was causing the drain to be submerged in water but I have dug the hump out from under the channel base matting and it drains well now. The removal of the impermeable top soil was the main issue though. https://imgur.com/gallery/LyeZl Re: Ground Water Drainage 11Nov 27, 2017 5:28 am Hi Othello. I saw the photos in the opening post. I was wondering about the top of the slope that I couldn't see because planting some suitable shrubs/small trees would supplement the drain if the water is flowing down as a subsurface water course. I has a similar problem a few houses ago except that the water was always flowing onto the footpath at the front but only in a small area. It annoyed me and so I planted an Acacia Floribunda probably 5 metres from the front and it fixed the problem. Am I right in assuming that the clay is only at the top near the rear of the house because the top soil has been removed but this doesn't apply further up the slope? 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Need more photos from around the house including your gutters. 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