6

In the garden I have an access shaft for underground rainwater tank. The shaft itself is a concrete pipe outer diameter 72cm and internal diameter 56cm. The wall thickness varies between 6 and 10 cm (the hole is not centered).

What is the best way to shorten it so it end below the ground level? I want to install proper manhole cover, and I do not like how it is sticking out so much.

Option 1 would be with big grinder w/ concrete blade, but the biggest I could rent will cut only 6cm. Also I would have to dig a wide trench, to get access to the below grade.

Option 2 would be to drill series of hole around the perimeter of the pipe, and then use chisel and wedges to break the top ring off.

I am more comfortable with #2, but I am not sure if there is a risk of breaking the whole pipe. From what I was able to see there is no steel reinforcement in the concrete.

concrete pipe in the middle of the lawn

8
  • 20
    Should check with local regulations if it can be that low. There might be a reason it is that high.
    – crip659
    Oct 30, 2023 at 11:45
  • 12
    What is the rainwater used for? Where is it collected from? There may be good reason to keep the access above the level of the ground around it, if making it low would lead to contamination relative to the usual collection area, or bypass filtration done between the usual collection and the storage tank. Raising the ground level (cut turf, add dirt, replace turf) is the easiest and probably best solution. If you don't want to see it, plant a flowerbed around it.
    – Ecnerwal
    Oct 30, 2023 at 12:56
  • 1
    @crip659 This is private property, no regulations are there about height. The installers just put the concrete saw on the ground when cutting the original height down and in has turned out to be this high.
    – jnovacho
    Oct 30, 2023 at 13:59
  • 24
    Private property doesn't imply no regulations.
    – Reid
    Oct 30, 2023 at 23:28
  • 4
    "no visible reinforcing" <-- this tank might not be legal at all. How big is the tank?
    – Criggie
    Oct 30, 2023 at 23:53

3 Answers 3

20

There exist dedicated concrete cutters. For example the Stihl TS 420 has about 12cm cutting depth. I don't know if those saws allow horizontal cutting.

A preferred solution would be to simply raise the soil if the grading allows it

4
  • Quite possibly the concrete is that high due to contractors using what you suggest. I guess it would be impossible to get horizontal at ground level, the reason it's been left as it is. Might need to dig up soil around it to make clearance for the Stihl, which not many folk would possess, although it could be hired. Still, OP liked the answer.
    – Tim
    Nov 1, 2023 at 15:08
  • Flush cut concrete saws definitely exist, and are often used for cutting curbs.
    – Olivier
    Nov 1, 2023 at 16:11
  • 4
    I cut a lot of concrete with a TS-420. I wouldn't call myself an expert, but I do have some experience. Cutting this sump is going to be noisy and messy. Getting flush will require digging up the area around the sump. You will blast dust all over the yard unless you're doing wet cutting (in which case you'll be blasting wet concrete dust) And you will find it extremely hard to make a high quality top edge for the lid to rest on, that replIcates the current cast concrete edge. In short, I think raising the surrounding earth is the tidier, quieter, easier and cheaper solution. Nov 1, 2023 at 16:28
  • One other suggestion: you could put bushy plants around the sump to help hide it. Nov 1, 2023 at 16:31
7

My answer assumes your local ordinance permits you to achieve your goal.

  1. Mark the inside perimeter with a sharpie or paint line for guidance
  2. Use a masonry bit to drill a few holes next to each other along the paint line
  3. Insert a concrete reciprocating saw (Sawzall) blade into the holes
  4. Make your cut all the way around
    • Reciprocating saw required
    • Don't worry about the dirt, that's only an issue for regular wood blades
    • Make use of small wedges so that the concrete doesn't eventually bind your blade
  5. Lift off the excess concrete shaft
  6. Use a diamond grinding cup to smooth out imperfections
    • Angle grinder required
  7. Chisel and grind out additional concrete to fit a manhole cover

Concrete reciprocating saw blade:

enter image description here

Diamond grinding cup:

enter image description here

5

Obviously, with a cutting wheel, you would cut around the outside, as deep as possible. 6cm, you say. That will cut through at least some of the concrete wall. Don't forget to stuff a cushion or something inside, to catch any debris!

Then, attack what's left of the wall from inside, assuming the tool is short enough to get into the aperature. If not, two stages are called for.

  1. Cut at an angle to reach the right level on the outside.

  2. use the angle grinder/cutter to level off the rim from the outside. Level with the original, hopefully horizontal, cut.

If not, a 'last ditch!' effort of bringing the ground level up would work, over several yards.

2
  • 2
    It almost looks in the picture that is a low spot in the yard.
    – crip659
    Oct 30, 2023 at 12:30
  • 2
    @crip659 - ground probably dropped due to the tank underneath.
    – Tim
    Oct 30, 2023 at 12:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.