Storm Drain Buildover

[simonniven]

Hi all,

We have planning permission for a single storey extension to our detached property. Building work was set to commence this week, however between Christmas and New Year we were alerted to a Thames Water 825mm Storm Drain running beneath our planned construction site.

I have spoken with Thames Water and despite the size, they will approve a build-over agreement providing that suggestions are suitable.

Whilst the location of the pipe is not 100% accurate, it looks to be running parallel to and 4500mm away from the existing house at a depth of between 2860mm and 3500mm. My current plans are to extend 5063mm away from the house. The pipe can not bear any of the load of the new extension and subsequently, foundations have to be as deep or deeper than the invert level of the pipe (bottom of the pipe).

My question is, how would you good people suggest bridging the pipe whilst trying to maintain the current size of the extension. I appreciate that I can reduce its size to avoid the pipe completely, but I would much rather look at maintaining the 5063mm width or potentially a larger construction to accommodate a bridge. The options I guess are to dig a 3m deep traditional foundation trench before pouring concrete, or to investigate some form of piling (it must be continuous flight/augered piling to comply with Thames Water guidelines).

Is there any reason that piles could not be positioned either side of the pipe, with beams across the pipe to support a concrete covered steel lintel on which the brickwork can be built in the original location?

I would appreciate any input from people that have encountered similar issues or from piling contractors that can elaborate on the likely costs and benefits of either option or alternatives.

I have enclosed the Thames Water Guidelines to a buildover, a site plan or our project and the sewer map.

Thanks very much in advance,

Simon

 

[Ian H]

I'm not sure about building over it but I would get it CCTV'd before and after just incase there are any existing faults on that section.

 

[simonniven]

Thanks for your reply. As part of the Thames Water Buildover Agreement, I have to have a CCTV survey before and after along with a lateral site survey at a cost of c£1,300.

 

[Ian H]

What's the lateral site survey?

 

[noseall]

Hi all,

a Thames Water 825mm Storm Drain running beneath our planned construction site.

it looks to be running parallel to and 4500mm away from the existing house at a depth of between 2860mm and 3500mm.

Sh*ttinhell!!

I'd move mate - immediately. Your foundations (if permissible or even practical) are going to cost thousands.

 

[endecotp]

First work out exactly where it is relative to your plans. Are there manholes at the two corners shown on the plan?

 

[chappers]

Sh*ttinhell!!

I'd move mate - immediately. Your foundations (if permissible or even practical) are going to cost thousands.

agree you're in seriously engineered designed founds territory. Reduce the size of your extension. I wouldn't even want to hazard a guess as to how much all this will add to the overall cost.

 

[Nige F]

I'd move mate - immediately. Your foundations (if permissible or even practical) are going to cost thousands.

Is the quoted depth of the drain to the top( outside seen as you dig down ) or the invert ( inside where the water runs)?

 

[Ian H]

Probably the invert estimated from the nearest manhole.

The CCTV crawler unit will be able to trace and probe the exact line and depth of the drain.

 

[simonniven]

Hi,

Thanks again for replies, the depth quoted is the invert level of the pipe - at one man hole it is 3250mm and at the other it is 2850mm. The corners with the circles on the drain map are the manholes - it is quite difficult to plot the exact location using standard mapping and even Thames Water themselves are unable to confirm the location until the survey, although from speaking with the drain contractor, they don't seem to think that they will be able to confirm the exact location of the pipe, is this correct?

I can potentially reduce the size of the build which clearly would still give us a nice sized extension, but at the same time bigger is better and if the size reduces to 3m as a result of the drain, I'd be tempted to proceed with a build over. Does anyone else have experience with alternative construction methods, as it seems it is an unfortunately uncommon problem?

Thanks,

Simon

 

[Ian H]

With my camera I can exactly locate the camera head see its depth.

Trouble with mine is that you wouldn't be able to push it far up an 825mm pipe and if you could the light wouldn't be powerful enough so show anything.

Crawler units look like remote control cars so they get driven up the pipe and they have much more powerful lights. I'd be supprised if they didn't have the sonde (locator bit) built in.

 

[simonniven]

Thanks very much for your help. It does seem strange if it did not precisely determine the location, otherwise how would they get on with much more intricate building projects such as Crossrail when they need almost millimetre precision.

 

[John D v2.0]

Cross rail has a gigantic budget. Anything is possible given enough money, but for domestic it's going to be prohibitive a long time before for cross rail.

 

[simonniven]

Crawler units look like remote control cars so they get driven up the pipe and they have much more powerful lights. I'd be supprised if they didn't have the sonde (locator bit) built in.

Just spoke to the drain company. They have a crawler unit that will be used but they wish to charge me an extra £220 to have a sonde survey conducted at the same time! This sounds ludicrously expensive?!

 

[r896neo]

Haha, wait til you get quoted for the reinforced foundations.