SITE PREPARATION
Going underground
Selecting and installing underground rainwater harvesting tanks can be a complicated process for self-builders, here David Stagg of Graf UK provides advice
P
eople’s increasing interest in rainwater harvesting, which was particularly noticeable during Covid lockdowns, means there is now a perplexing plethora of storage tank options on the UK market. Many are manufactured in the UK, while some are imported from Germany and other European countries. They come in many different shapes and sizes, with some suitable for over-ground storage and some for the more discreet underground. Perhaps the most important consideration when choosing which underground one is right for you, is the groundwater level on the site you are going to be installing the system on. If the water table is high, there are low-
profile shallow tanks which can be completely submerged in groundwater and still be guaranteed to stay in the ground, even though the pressure of the groundwater surrounding them could cause them to pop up out of the ground. Tanks installed in groundwater should have vertical hole features so the groundwater can surround them and pass up through them without building up pressure from underneath and around the sides, which pushes against the tank and causes it to pop upwards.
It is nevertheless important to design the drainage around the tank to try and remove the groundwater from immediately around it. A perforated pipe that circles the tank can collect groundwater and divert it to the nearest drainage point, or ideally somewhere several metres away.
Another important consideration is whether the rainwater harvesting tank you choose requires a concrete or a granular gravel backfill. Sometimes this depends on the type of tank, or the depth in which the tank is being installed; it can also depend on the loading which is going to be imposed on top of the tank. Tanks that require a concrete surround typically take longer to install and so the labour costs can be more expensive. Concrete is also more expensive than gravel. Tanks that require a gravel surround can generally be installed in as
nov/dec 2021
little as half a day. But because the gravel can simply be backfilled and compacted in layers, you don’t have to wait for it to set like concrete so it’s a much quicker process.
Whichever installation method is used,
it’s vital the manufacturer’s instructions are followed so their warranties will stand. Many tanks are provided with warranties of 10+ years but builders and contractors should follow the instructions to the letter to ensure responsibility lies with the manufacturers even once the tank is in the ground.
Another important consideration is whether the tank filter is an internal, integral part of the structure or it’s an external pre-tank filter which requires two access covers and two manhole lids. If the filter is inside the tank, one lock on the top gives easy access to the filter. It helps if the filter package is manufactured by the same company that manufactured the tank.
When it comes to backfilling the tank with gravel or concrete, it’s important to
fill them with water beforehand. Most manufacturers suggest this is done in stages, such as filling the tank with 300 mm of water and surrounding it with 300 mm of aggregate. And repeat until the tank is completely full of water and
Whichever installation method is used, it’s vital the manufacturer’s instructions are followed so their warranties will stand
www.sbhonline.co.uk 27
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84
