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Posted: Tue Nov 22, 2011 7:54 am
by nick@dunsdale
Hi just found this forum this morning, i posted this question on another forum last week, just in case anybody from there is here 
I did get a reply from one guy, but was hoping for more than one opinion, please have a look and see what board members think.
Just want to have plenty of info when i take it up with the contractors.
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
Posted: Tue Nov 22, 2011 8:31 am
by local patios and driveway
Well at 1:40 fall which is what it should be, over 36m the total drop should be approx 900mm. I belive yours is 534mm almost half what its supposed to be. The first run should drop 200mm. The second run should be 240mm. And the third at 450mm now its fair to say you wouldnt expect these to be spot on but close.
Im not sure what my piers think of this, ive simply done the maths, more opinions will be listed after 5 ish and some of the guys who sign on at lunch and break time will be on later too.
Posted: Tue Nov 22, 2011 8:48 am
by nick@dunsdale
local patios and driveways wrote:Well at 1:40 fall which is what it should be, over 36m the total drop should be approx 900mm. I belive yours is 534mm almost half what its supposed to be. The first run should drop 200mm. The second run should be 240mm. And the third at 450mm now its fair to say you wouldnt expect these to be spot on but close.
Im not sure what my piers think of this, ive simply done the maths, more opinions will be listed after 5 ish and some of the guys who sign on at lunch and break time will be on later too.
As i know little about this kind of thing, how do you work that sum out
1:40 over 36 meters = 900 drop ??
I was under the impression i could have a run dropping 15mm per meter
as 15mm x 35.5 = 532 (the drop approx) does that not mean i can drop 15 mm per meter
please tell me where i am going wrong
as simple as you can please
Posted: Tue Nov 22, 2011 9:21 am
by local patios and driveway
Over 40 inches the pipe should fall an inch. Or converted you could also say that over 40mm it should drop 1mm. Thats a fall of 1:40
So the maths...
The total length of the run of pipe is 36m. To work out the fall, 36000mm divided by 40. Will tell you how many mm the drop should be.
Or 36m divded by 40 equals 0.9m
I hope this helps
Posted: Tue Nov 22, 2011 9:30 am
by local patios and driveway
Maths isnt my strongest point but with a 15mm drop per meter i think thats a drop of 1:66
Working to that ratio over 36m divided by 66 is 0.54m. Or 540mm
So your run averages 1:66
a to b should be 119mm not 64mm. As it is The first run is 1:123
My brain hurts.....
Posted: Tue Nov 22, 2011 10:35 am
by nick@dunsdale
local patios and driveways wrote:Maths isnt my strongest point but with a 15mm drop per meter i think thats a drop of 1:66
Working to that ratio over 36m divided by 66 is 0.54m. Or 540mm
So your run averages 1:66
a to b should be 119mm not 64mm. As it is The first run is 1:123
My brain hurts.....
That sounds about right to me, hopefully some other posters will chime in to agree this calculation is correct.
The whole thing is upset by drain B as far i can tell this is to high.
The waste is stopping dead as it reaches drain B
The main point i want to put across to the installers of this drain work is what could have been achieved compared to what is there
Posted: Tue Nov 22, 2011 12:51 pm
by Carberry
nick@dunsdale wrote:local patios and driveways wrote:Maths isnt my strongest point but with a 15mm drop per meter i think thats a drop of 1:66
Working to that ratio over 36m divided by 66 is 0.54m. Or 540mm
So your run averages 1:66
a to b should be 119mm not 64mm. As it is The first run is 1:123
My brain hurts.....
That sounds about right to me, hopefully some other posters will chime in to agree this calculation is correct.
The whole thing is upset by drain B as far i can tell this is to high.
The waste is stopping dead as it reaches drain B
The main point i want to put across to the installers of this drain work is what could have been achieved compared to what is there
The calculations are correct. Gold star for Lpad 
Read here for minimum depths, gradients and how to properly bed the pipework.
http://www.pavingexpert.com/drain02.htm
Posted: Tue Nov 22, 2011 5:04 pm
by nick@dunsdale
a to b should be 119mm not 64mm. As it is The first run is 1:123
Just out of interest what are the other runs B to C and C to D as they are at the moment.
And do we think the main problem is that drain B just has not been fitted low enough ??.
And it could have been almost twice as deep ?
.
Posted: Tue Nov 22, 2011 5:10 pm
by local patios and driveway
At 1:66 (which isnt enough of a fall anyway) b to c should be 145mm
C to d should be 272mm
Basically in my opinion there isnt enough fall on any of the pipe.
But at 1:66 you need to raise A by 60mm this will get close to 1:66
I got a gold star yay!
Posted: Tue Nov 22, 2011 5:19 pm
by lutonlagerlout
we have had a job for the local authority recently where we had to lay the foul drains to 1:80
I didnt want to do it but AFAIK they are working
if there are problems the chances are there is a section of pipe or an IC that has a backfall, either that or a blockage
1:40 for foul and 1:100 for rainwater is the ideal gradient
cheers LLL
Posted: Tue Nov 22, 2011 6:24 pm
by seanandruby
has the run had a cctv camera up to see if the pipework is ok and not broke, or sank. Is it plastic drainage, long lengths.
Posted: Tue Nov 22, 2011 6:48 pm
by flowjoe
Sean's bang on the money as usual, you can put plastic pipe work in damn near level and it will push through with the weight of the water, never ideal but fall isn't always there to be won.
I would have a detailed camera survey done, you often find that a loose sealing ring in a joint, or a square junction used instead of an oblique to be the cause of regular blockages.
Posted: Tue Nov 22, 2011 7:15 pm
by nick@dunsdale
seanandruby wrote:has the run had a cctv camera up to see if the pipework is ok and not broke, or sank. Is it plastic drainage, long lengths.
yes it is plastic drainage, i have lifted the cover on drain B and the waste is just sitting there.
When the toilet is flushed the water is just sort of trickling round the side of it, leaving it stranded.
The thing is i want to gather as much info as possible cause i know they are going to try and wriggle out of this.
I have spoke to one guy who simply thinks that if drain B was lower as mentioned above, this would have helped greatly. If it was almost twice a deep, as has been mentioned that surely that would help greatly with the general run.
If i cant come up with a decent argument point of view, i know they will turn round and tell me i am going to need a pumping station or something.
Posted: Tue Nov 22, 2011 9:18 pm
by seanandruby
your invert levels at each drain mean nothing if the pipework isn't laid correctly. If they are laid on uneven bedding you could get dips or high points. Also as flo' suggested seal come loose and snagging the waste. you should get a scan of the drainage to see if there are any snags.
Posted: Tue Nov 22, 2011 9:26 pm
by dignfence
Hi Guys,
My first post on here,been meaning to get involved for ages!!
just to say that a contractor I used to work with always laid sewerage pipe(plastic soil) at 60:1 for years with out a problem but always ensured it was laid accurately and both pressure tested and if possible,used before it was back filled.
I am working on a scheme where I have to lay plastic at 1:80 at the moment,this has been passed by the relavent authorities.(no pun intended).
I was wondering,is the IC at B been knocked back on back filling and ended up running up hill,hence the solids staying there. just athought? ???