2 different engineer opinions

day dreamer

Apologies if this is in wrong area

I am attempting to get planning permission for a site. I engaged an architect and engineer. The land is unsuitable for a septic tank but it is near enought to connect to Town sewage. As the land is low lying the engineer felt that gravity sewer would not work. We sent in the planning permission for a rising main and pump. The Council had np probs with the house but wont allow the rising main and have asked for further info regarding a gravity sewer.

The council have suggested an alternative engineer. The 2nd engineer took levels and says the gravity sewer is not ideal but will work and the council have approved this verbally but we will have to re-submit to get the planning.

Our original engineer was excellent, very helpful and I have no reason to doubt his opinion. He is adamant that the gravity sewer wont work. The 2nd engineer is satisfied that it will work.

I am not looking for engineering advice and dont expect anyone to give an opinion without seeing plans and levels etc. But are such differences of opinion common? We have invested a lot of time and effort and the house is really our dream. I am reluctant to walk away but dont want to buy a site that is unsuitable and have a house that has a sewer backed up either.

thanks in advance

sydthebeat

did the council suggest you engage a different enginner, because they were of the opinion that a gravity fed sewer would work.... or did they specifically name an engineer and say 'go to this guy' ?

this should be a clear cut case. Levels, pipe gradients etc are all definitive figures, so some one has to be wrong.

why is the first engineer adamant that it wouldn't work? what gradient of pipe, if any, does he suggest is available?

what gradient does the second enginner suggest?
does the second engineer suggest lifting the house levels up?
are both working to the same invert level in the existing manhole?

a larger pipe can be laid at a lower gradient.. what pipe sizes are both suggesting?

day dreamer

Thanks syd

I am not an engineer but will try to reply as best i can.

the council did not recommend the 2nd engineer explicitly. Both have established firms locally. It is hard to get to meet any of the council engineers and get a direct answer but they said a gravity sewer should work.

The 1st engineer said originally the pipe would come out of the road so could not work. This was based on raising the site 1.5m, a 225mm PVC pipe @1:225

The 2nd engineer recommended raising the site 2M, 225mm PVC pipe @ 1:250. He says the calculations work and is willing to supervise the construction once the application is through.

We did discuss this with the 1st engineer, he calculated the flow rate and said it did not meet requirements with 1 house discharging and would not be "self cleaning". He would not take on the build based on a gravity sewer. If we build it the Council will take it in charge so others will be using it in time.

They are going to the same manhole but I dont know if it is the same invert level.

thanks again

sydthebeat

i see now.

i think it safe to assume theyre both using the same levels.

225 pipe can be laid to 1:250 gradient.... actually can be allowed down to 1:265 for a flow rate of 0.8 m / s.

i would come at this another way. id accept the second engineers calculation, but i would do what i can to ensure the first engineers fears do not come to pass.
I would do this by minimising the material that usually causes clogging up in foul sewers. In the main this material is the detergent run off from washing machines, dishwasters etc. i would design my foul system so this 'grey' water is allowed to enter a holding tank (say 5000 litres) with a grease trap. manually clean this as required (youd be very surprised how quickly it will fill).
an extra precaution would also be to include rodding eyes at say 30 m intervals along the pipe. personally i wouldnt be overly concerned about a 225 pipe backing up if you took the above precautions.

day dreamer

Cheers syd

It is good to get another view and some helpful tips. I will mention the precautions to the engineer when we meet again

One last thing. The sewer has to run along a road to join the existing council sewer. I am told it will only have 300mm coverage but will be cased in concrete. I know it should have something like 900mm, will this be a problem if it is put in the centre of a narrow road?

thanks again

DonegalMick

1:250 is only 24mm in a 6m length of pipe which may be acceptable on paper but i would advise from experience that a lot of care will be required in laying the pipes. Any small dip will cause a flat spot, even a track machine passing over the trench could cause problems. Raising the house by 2m means that the services will be placed in fill ground, with all the best intentions settlement will occur, and when settlement occurs the services will move. Lots of rodding eyes will be required as advised. I think engineer 1 was being more prudent and allowing some flexibility in the design. In the long term the gravity sewer WILL give problems.

harry21

day dreamer wrote: »

Thanks syd

I am not an engineer but will try to reply as best i can.

The 1st engineer said originally the pipe would come out of the road so could not work. This was based on raising the site 1.5m, a 225mm PVC pipe @1:225

I think you did a good job in replying. The only comment that I would add to what has been said is this. Engineer 1 said it will come out of the road..... won't this happen (if he is right) regardless of wether you raise the site or not?

Have you seen longitudinal sections of the proposed sewer? The suggestion that what is designed may not be exactly what will be constructed, especially where you are using the minimum gradient, is very valid.

The cover seems minimal, and relatisically, RC conrete pipes would be better, but these requires steeper gradients. Also, if there are any other services in the road which need to be crossed it could scupper the whole plan.

A proper survey needs to be caried out and all invert levels determined. I would ask the engineer for a longitudinal section and then maybe you could get another engineer to comment on it?

DomhnallOg

I echo previous posts: 1 in 250 is very flat. A bit of settlement in the pipe or grease build up will cause problems particularly if you're the only one discharging to it.
If you're prepared to put up with the risk in order to get the PP you've no choice.
It will work but you may have intermittent problems. T

Poulgorm

The first engineer was right. I assume that only foul sewage will be permitted in the sewer - storm water will go the sumps.

The flow from a single house will be minuscule in a 225mm diameter pipe - it will not be self cleansing at a gradient of 1:250

day dreamer

thanks for your advice, it seems as if the consensus is that the sewer could cause problems down the line.

Its frustrating as we really want this house to be built but don't want problems down the line either. I was assured by the 2nd engineer that while it was not ideal, it would work and did meet the regulations. The council have also said they will take it in hand when finished so other houses will use it too.

sydthebeat

instead of a rising main in the public road... will the council accept a gravity fed sewer in the public road, but allow you to install a holding tank and pump on your land to pump to this new gravity sewer?

i assume that this configuration would allow for deeper inverts along the public road, thus relieving the depth issue, while also allowing you to install a 100 dia pipe at steeper gradient. This would be a kin to a standard treatment system that pumps to a percolation area.

Frank Black

OP, when you say that land is unsuitable for a septic tank, I presume that it's because your percolation rate is either too low or too high, or is it an issue due to a high water table?
Have you investiaged whether any of the proprietary sewage treatment systems would work on the site and be acceptable to the Council - these systems which treat the waste more than a traditional septic tank can be combined with raised percolation areas.
I know some councils can be wary of these as they fear that they won't be properly maintained but from reading the thread, it would seem to me that you may have problems with achieving a self -cleaning velocity on the foul sewer - as mentioned by a previous poster, a design may look ok on paper, but actually constructing to the falls can give rise to low spots where blockages can occur.

day dreamer

Sorry for the late response, was away last week. Thanks again for the input

Syd, the council refused our initial application involving the tank and pump. This was submitted by the 1st engineer for the reasons outlined already. I was led to beleive that the council would not let us put in a rising main and pump as nobody else would be able to use it and the council did not want the road ripped up to service just 1 house.

BlackFrancis, the water table and soil wont allow a raised bed septic tank so it isnt really an option, even something like a Biocycle.

It seems as if Boards opinion is as divided as the engineers we have spoken with, there probably is no ideal solution. We will have to just go with what we feel we are happy with

thanks again

sydthebeat

day dreamer wrote: »

. I was led to beleive that the council would not let us put in a rising main and pump as nobody else would be able to use it and the council did not want the road ripped up to service just 1 house.

that doesnt make any sense to me...

i dont know the specifics, but surely the situation can be designed so that your house flows by gravity to a holding tank and pumps by rising main to a manhole. Any future house between this manhole and your holding tank can also flow by gravity back to your holding tank before als being pumped to the public sewer system.... thats how rising mains are usually organised anyway in my experience.

admittedly it means more money on your part as youd have to install the holding tank, pump and rising main. you may be able to barter some of the cost against development charges. you would also have to grant the council leeway over your land to gain access to the system should it be taken in charge.

another option is, seeing as your been advised to rise site levels by 1.5 - 2 meters, to rise the site anyway and carry out percolation tests on the new ground. ensure good quality soil to pass EPA tests and requirements. this may be costly, but may actually be less than the cost of the above pumped option.

day dreamer

It doesnt make sense to me either, in fact the whole planning proess is a bit baffling. They say building the house is very stressful but planning is one of the grey arts.

Our initial planning permission was refused for the rising main and pump. The house would be the only one using the rising main as it is close to the gravity sewer.

The other houses use septic tanks except for 2. The tanks seems to give a lot of bother in the area spekaing to the locals. I have a feeling the council would like a new sewer but would like yours truly to pay for it (150m or so) so they can take it in hand and then the other houses with septic tank problems can use it too.