drainage

[nick1967]

Hi

I hope this is the right section for asking a drainage question

Recently i have had a new building put up, it is a car workshop, i would like a second opinion before i raise my doubts with the contractors, i dont like to complain but if it is wrong i will raise the points with them.

Our waste from the toilets is getting stuck in the first section of drainage
drain A to drain B.

My step son who does building work has been out and measured the depths with a dumpy level at all the drains.

From the measurements we took it would seem there is not enough run on the first section dain A to drain B, and more than enough of a run, from drain B to drain C AND C to D.

I have attached a drawing it is not to scale but the measurements and drops from drain to drain are correct.

I suppose i want to know is could more of run been achieved drain A to B if the run was less between point B to D as the run's dont seem consistent, and the run drain A to B if it were minuim 1 in 10 drop 58mm would only span a distance of 5.8 meters and the distance is 7.9, am i right as i really dont know that much about this kind of thing

.

 

[freddiemercurystwin]

The invert at B needs to be lowered by at least 40mm, the invert at C needs to be lowered by 10mm to achive falls of 1:80.

 

[nick1967]

The invert at B needs to be lowered by at least 40mm, the invert at C needs to be lowered by 10mm to achive falls of 1:80.

Okay thanks for that and by the sizes i have given you could this have been achieved without problem

 

[freddiemercurystwin]

The overall fall is enough to achieve the required falls. Ideally the run would have been consistent for the whole run and would have been 1:68 from A-D.

They have put B in too high for some reason! Realistically lowering invert C for the sake of 10mm is likely to be impracticable.

Are you sure there is nothing in the pipework holding up the turds? Like a blob of mortar or similar.

 

[nick1967]

The overall fall is enough to achieve the required falls. Ideally the run would have been consistent for the whole run and would have been 1:68 from A-D.

They have put B in too high for some reason! Realistically lowering invert C for the sake of 10mm is likely to be impracticable.

Are you sure there is nothing in the pipework holding up the turds? Like a blob of mortar or similar.

No unfortunately there is nothing in the pipe the turds are just sitting right at the end of section A just before drain B

What is the run between A and B as it is at the moment, i dont know how to work it out

In fact if you have the time can you tell me drop of each run

A to B
B to C
C to D

 

[freddiemercurystwin]

this evening

 

[nick1967]

this evening

Thanks it is appreciated.

Any car bodywork questions dont hesitate to ask lol

Maybe i should also mention not sure how important it is, the depths i measured on the drawings are at invert level base of the pipe at each drain/manhole point

 

[freddiemercurystwin]

OK in an ideal world with an overall drop of 520mm over a distance of 35500 (7900 + 9600 + 18000) the fall would be (35500 ÷ 520) 1:68.

Therefore

A to B 116mm (7900 ÷ 68 )
B to C 141mm (9600 ÷ 68 )
C to D 264mm (18000 ÷ 68 )

 

[nick1967]

OK in an ideal world with an overall drop of 520mm over a distance of 35500 (7900 + 9600 + 18000) the fall would be (35500 ÷ 520) 1:68.

Therefore

A to B 116mm (7900 ÷ 68 )
B to C 141mm (9600 ÷ 68 )
C to D 264mm (18000 ÷ 68 )

Thanks i really appreciate the info and taking the time to calculate it for me.
so you will have to excuse me as i ask another couple of layman type questions.

The dimensions you mention above are depths that could be achieved over the drop between drain A and drain D ??

The measurements you mention between b to c and c to d, dont seem a million miles from what is there.

Where did it go wrong at A to B as that point seems to be a mile away.

One more thing i am probably missing the obvious here as i can be a bit thick when it comes to maths lol.

What are my drops as they stand at the moment, particularly intrested in drain A to drain B what is the run on that drain at the moment with 58mm of drop over 7.9 meters.

Again thanks for your help

 

[freddiemercurystwin]

The drops quoted in my last post are what could have been achieved.

Yes you builders have put the invert at B in too high.

At the moment

A to B is a fall of 1:136 (7900 ÷ 58 )
B to C 1:73 (9600 ÷ 151 )
C to D 1:60 (18000 ÷ 298 )

 

[nick1967]

A to B is a fall of 1:136 (7900 ÷ 58 )
B to C 1:73 (9600 ÷ 151 )
C to D 1:60 (18000 ÷ 298 )

so does that mean

A to B drop's 136mm every meter

B to C drop's 73mm every meter

C to D drop's 60mm every meter

And one more layman type question i promise i notice on your calculations you divide length of the run 35.5 meters by the depth 520 mm.

Somebody else told me i could work it out by dividing depth by length witch is the opposite to these calculations, i am getting confused now lol.

I am not doubting your calculations just wondered why i have been told i can divide it the other way round.

This is the last question i promise dont want to waste anybody's time

Thanks for all your help

 

[RedHerring2]

Somebody else told me i could work it out by dividing depth by length witch is the opposite to these calculations, i am getting confused now lol.

That would give you the inverse of the figure that you require, (i.e. 1/x)
(Inverse is nothing to do with invert in this context)

So:
58 ÷ 7900 = 0.007341772 (to 9 decimal places)

But as this is the inverse figure, you'll need to divide that figure into 1
So:
1/0.00734 = 136 approx.

Don't worry if you don't understand it, just accept that fmt is correct.

 

[freddiemercurystwin]

so does that mean

A to B drop's 136mm every meter

B to C drop's 73mm every meter

C to D drop's 60mm every meter

No,

A to B is falling 7.3mm per metre
B to C is falling of 15.7mm per metre
C to D is falling 16.5mm per metre