Eugene, an avid self-taught DIYer and engineer, has acquired almost 40 years of experience with power/hand tools, plumbing, and woodwork.
This is a drywell I built over the summer to collect rain water from the gully of my sitting room. When the room was given a new pitched roof several decades ago, the builders never reinstalled new gullies at each end of the room to take water from the two downpipes. Over the years rising damp had been an issue, so considering that water draining onto the surrounding ground could be keeping the foundation wet, I decided to route it some distance from the building so that it would soak away safely.
What's a Drywell?
A drywell, also known as a soakaway or soak pit in the UK or soakwell in Australia, is an underground drainage structure, traditionally filled with gravel, larger stones or broken rubble, for collecting rain water from roofs, paved or concreted areas. The drywell acts as a buffer or reservoir, collecting and holding water so that it has time to percolate down through the ground structure. Modern drywells are purpose made, prefabricated, hollow plastic or concrete chambers, with holes in the sides allowing water to drain out to the surrounding soil.
An Overview of My Soakaway
This isn't necessarily constructed to code and shouldn't be built in a location where it will be driven over at any stage in the future because of the nature of the fill. Instead of using solid rubble as an infill which was traditionally employed for these structures and is structurally stable, I tried an experimental method of using cinder blocks (also known as cavity blocks). These have about 40% to 50% air space by volume, meaning that more water can be stored in the hole for a given drywell size. I recycled other material to hand including scrap metal bars, cladding, rubble and carpet.
How Far Should a Drywell Be Located From a Building?
In the US, code says that a drywell should be located no closer than 10' from a building and 5' from the boundary line. This is so that water that collects in the drywell doesn't soak the foundations of your home or seep into a basement.
In the UK, building regulations specify that a soakaway should be at least 5m away from a building or road and 2.5 m from a boundary.
In general a drywell shouldn't be in a location where:
- the water table reaches within 1.2m of the bottom of the soakaway
- near a drainage field or other drywell
- any contamination in runoff draining into the drywell could pollute groundwater
What's a Water Table?
It's the upper surface of a zone of saturation. It can be thought of as the "top" surface of saturated subsurface materials (clay/sand/stone).
Is a Drywell Suitable for Your Garden?
It depends. If you have a naturally high water table, the ground may always be saturated a few feet down and water isn't going to go anywhere. Also if the ground structure is composed of densely packed clay or solid rock, you'll also have a problem.
Establishing Drainage Performance by Doing a Percolation Test
This involves digging a test hole and pouring water into it to investigate the rate of drainage. Percolation test methods differ from country to country. In the UK, this is the procedure:
- At least two test holes should be dug to establish the location with the best drainage.
- Make the hole 300 mm (1 foot ) square and 300 mm deep, measured from the estimated bottom of the incoming drainpipe (the invert level of the pipe). You can establish where the bottom of the pipe will be from the gradient. Dig the hole so that the sides are vertical.
- If you reach the water table and water starts to soak into the hole, a drywell isn't a solution on your property.
- Fill the hole to the top with water and leave overnight. This pre-soaks the ground in case it's overly dry.
- Fill the hole the next day with water to a depth of 300 mm. Measure the time in seconds it takes for water to drop in level from 75 % full (225 mm), to 25 % full (75 mm). This is a drop of 150 mm.
- Divide the time in seconds by 150 to get the average fall rate Vp in seconds per mm.
- The test should be carried out at least three times in total with each of the holes. Take the average value for each hole.
- Tests shouldn't be carried out under abnormal weather conditions such as drought, heavy rain or severe frost.
- If the percolation rate is greater than 100 seconds per mm, the ground is unsuitable for a soakaway.
Sizing a Drywell and Determining Whether It's Suitable for Your Climate
A British source suggests these calculations for the volume required for a soakaway. Note that this is the unfilled volume. Once the drywell is filled, the volume of all the voids that can hold water may be reduced to 30% or less of the volume of the hole, depending on the composition of the fill. I used waste cinder (cavity) blocks as my fill, giving me somewhat more capacity in the voids. I estimated it to be about 40% from measurements of the dimensions of concrete used in blocks. Lets call Vw the volume required to hold the water, then:
Vw = A x (rainfall rate / 3000)
Rainfall rate is in mm/hr and volume calculated is in cubic metres. A is the area that collects water. In the UK rainfall rate for a day or in a shower is taken to be 50 mm.
My gully had to take rainwater from half of the double pitched roof of my sitting room.
The roof area was 11' x 10.5' = 115.5 square feet or 10.7 square metres
Rainfall rate = 50 mm
So space required in drywell to hold this water Vw = 10.7 x (0.05 / 3000) = 0.18 cubic metres
Let's say 40% of the total volume of the drywell is made up of voids which can hold water and VT is the total capacity of the hole. Then the total capacity VT can be worked out from:
VT = Vw / 40% = Vw / 0.4 = Vw x 2.5
In my case, this gave a figure for total volume of 0.18 x 2.5 = 0.45 cubic metres.
I sized my drywell to be about 2' 4" (71 cm) square and 4 feet (1.22 m) deep in total. I adjusted the depth later so that when the drain pipe was laid, there was 3 feet (96 cm) depth below the incoming pipe. So the total usable volume was 28" x 28" x 3' = 12 cubic feet or 0.46 cubic metres.
Note that there may be minimum dimensions for drywells depending on country. For instance this guide for the City of Portland, OR specifies that "Pits for drywells must be at least four feet in diameter and five feet deep. Minimum drywell diameter is 28 inches"
Gradient of Drain Pipe
Typically this should be about 1 in 40 to 1 in 110. 1 in 40 is equivalent to 1/4" per foot or 100 mm in 4 m.
|Units (metric)||Units (Imperial)|
3.281 feet or 3' 3.370"
1 cubic metre
35.315 cubic feet
0.220 UK gallon or 0.264 US gallon
|Units (Imperial)||Units (metric)|
1 cubic foot
0.028 cubic metres
1 US gallon
1 UK gallon
What Materials Are Used to Fill a Drywell?
The traditional method was to dig a hole and fill with rubble(broken masonry, bricks, crockery etc) or gravel. Newer alternatives include soakaway crates (that look like beer crates) or plastic or concrete holding tanks with perforated holes in the sides and bottom for drainage. Ring drywells can also be built from cinder blocks. My drywell consists of multiple courses of blocks, stacked on top of each other.
Preventing Siltup Using a Geotextile Wrap
Geotextile is used to wrap the rubble fill or containers within a drywell. Both the vertical sides and underside should be wrapped. This prevents soils and sand from the surrounding ground structure from getting into the rubble fill, crates or tank system. I didn't use any for my soakwell so possibly I will have to re-excavate in time. However the cinder blocks should keep silt from entering the central area.
My Made Up Gully
You can use either a gully hopper combined with a p-trap as used on a toilet, or a bottle gully. Both systems collect silt and moss from rainwater which can then be cleaned out of the trap before it eventually clogs up the drywell (which could take decades). A trap stops smell from coming up from sewers, however it's probably easier to feed a flexible drain cleaning hose out the side of a bottle gully. My local store didn't have a proper bottle gully, so I made my own from a gully hopper top, a tee and a short piece of 4" (100 mm) soil pipe with a cap on the end.
Installing the Gully
Unfortunately the foundation of my sitting room was close to the surface, so I had to locate the gully out in the middle of the pavement. Getting it flush with the concrete was essential so no one would catch their foot in the grating when installed.
Safety Precautions When Digging a Drywell
Ground was soft clay and sand, easy to dig with spade and shovel. To prevent the ground caving in, I dug the sides as vertical as possible. Remember, in the UK, health & safety rules say that holes deeper than 1.2 m ( 4 feet) should be protected with suitable trench boxes or other shoring so they don't cave in. The equivalent measurement specified in US OSHA rules is 5 feet or 1.5 m. Even when a hole is shallower than this, it can still be a dangerous hazard and a collapse while you're in it can trap your legs so you can't get out. If you crouch down, it's even more hazardous. So ideally don't enter a hole and only excavate material from above.
- Keep excavated soil and heavy equipment away from hole edges
- Know where underground utilities are located.
- A ladder should be placed close to the edge of a hole so that it can be used to climb out.
Digging the Trench for the Drain Pipe
A trench was dug at a suitable gradient for the drain pipe. Once the trench had a reasonly even bottom, a 4 inch (100 mm) soil pipe was laid and sand packed around the underside to support it evenly. I didn't have sufficient waste pipe, so I recycled some stainless steel flue instead to reach the drywell.
Filling the Drywell With Cinder Blocks
Each course of the fill consisted of three cinder blocks, spaced apart by lumps of rock so that pressure from the surrounding ground wouldn't force the blocks together. I also filled around the blocks with rubble as I added each course. Cinder blocks aren't specifically designed to take side pressure, so that's why a drywell with a non-solid fill should not be located where any vehicles are going to be driving on the ground above or collapse of the structure could cause problems. From experience, I know my soil consists of heavy clay and doesn't slump readily, so it's not likely to produce excessive side pressure in the short term.
Evaluating the Performance of the Drywell
Just in case the drywell didn't have enough capacity, I left it open for a couple of months to see how it performed. I could see through the cavities all the way down to the bottom course of blocks and somewhat "disappointingly", water never even collected in the hole. When digging I reached the sand/gravel layer, so I knew it would perform well. The town where I live is built on ground which is a glacial outwash plain, with layers of sand and gravel deposits and excellent drainage.
- Excavations And The 1.2m Rule (UK)
- Occupational Safety and Health Administration (OSHA) - Trenching and Excavation Safety (USA)
- PavingExpert.com - Drainage - Soakaways
- Eden District Council UK - Rainwater Soakaway Design Guidance
- Dig Safe®
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2020 Eugene Brennan