Earlier this week we were disconnected from our septic tank and connected to a recently constructed public local sewer system. Everything seemed OK (well, reasonable!) until I looked in the final manhole of our new system. From the outlet of that manhole there is a customary 110 mm diameter pipe which is connected to the "lateral" pipe which is spurred off the sewer. (We are responsible for the system up to where it connects to the lateral pipe. The lateral pipe is 150 mm or more diameter and goes down at 45° to the sewer which is about 2.1 metres below the surface. Even though they're under our back garden, the lateral pipe and the sewer are owned by, and are the responsibility of, the local water authority.)
The thing which concerns me is that there is a residual 9 mm depth of water at the bottom of the final manhole and the start of the final section of drainpipe. Could this residual water be considered acceptable? My own feeling at the moment is that it could indicate that the gradient of the pipe is insufficient and that it could result in solid matter settling out in the pipe and causing a blockage.
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We have about 3" of water constantly in the drain pipe 4ft under the manhole at the back of our house, been here 10 years and its never caused a problem.
I borrowed my mates drain rods and poked along but nothing shifted, i think the pipe was not laid precisely when the house was built.
When the rain butts are full i sluice a few 2 gallon buckets of water down the drain just to make sure that no sediment builds up at the lowest point, nipping down to the next manhole sees only water passing.
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>> ......... nipping down to the next manhole sees only water passing.
>>
Unfortunately, with it being the final manhole and the final bit of pipe before it connects with the sewer I can't see what happens downstream of the manhole. Not only is there the residual water but also the water flows out of the final manhole much slower than it does out of the preceding (upstream) manhole. I rodded it out with a 4" diameter rubber disc at the start of the rods but it didn't make any difference. I'm going to ask the contractor to dig a hole down to the pipe just before it connects with the "lateral" so as to expose the pipe to enable the gradient of the pipe to be checked, with the threat that they won't get paid until I have proof that the gradient is satisfactory.
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Surely you will need to expose the whole pipe to check the gradient? Any hump (or obstruction) in the in the middle of the run will result in a head building up in your manhole.
IIRC the pipes should be only laid at a specific fall (1in 40?) or vertically, steeper is not necessarily better. Not intuitive, but wait for a builder/surveyor comment.
Last edited by: pmh on Sun 22 Jan 12 at 09:23
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>> Surely you will need to expose the whole pipe to check the gradient? Any hump
>> ....... in the in the middle of the run will result in a head
>> building up in your manhole.
I'm sure you're right. After the pipe was laid in the trench the soil above it was compacted using some sort of mechanical rammer and this may have resulted in the pipe getting bent.
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>> IIRC the pipes should be only laid at a specific fall (1in 40?) or vertically,
>> steeper is not necessarily better.
Not an expert but I believe too steep a gradient allows the water to drain before any solids are carried away.
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Too steep is wrong, as too shallow is also wrong.
The rule of thumb used to be:-
4" diameter pipe = 1 in 40 fall
6"...................... = 1 in 60 fall
9" ..................... = 1 in 90 fall
and so on.
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The critical thing with all drainage is a linear fall. Not one which the Contractor/subby/dikhead has laid to the 'Turn of the bubble' on an unchecked spirit level. I see/get it all the time. There is so much to getting it right that it would take an age to write and I for one am heading to the Land of nod soon. The OP however is right to be concerned.
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Thanks Old Navy, Duncan and Martin.
I've spent this morning drawing a plan of the completed system, and calculating the gradients of the pipes. Unfortunately I can't calculate the gradient of the final (dubious) piece which connects to the sewer lateral because there is no manhole at or near the outlet end to enable me to measure the depth. In hindsight I should have asked for one to be put in ~ it would have only cost a relatively small extra amount in relation to the total cost ~ but I left it to the contractor to plan the system. The head honcho of the contractor is coming this afternoon to do an inspection. Here's what I found about gradients of drains. tinyurl.com/6pbv3zm
Last edited by: L'escargot on Mon 23 Jan 12 at 13:31
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Domestic drains question - -
