Drainage to sewer mains - How to determine water table height from

[watt10]

Hello,

I encountered a groundwater-sewer main problem. The groundwater is infiltrating to the sewer main. We want to seal the damage parts of the main, where groundwater is entering. However, we fear that sealing the sewer main will cause to rise the groundwater level, thus would cause problems on the foundations of the residential areas. For that reason, in order to find optimum height where we can seal the sewer main, I would like to make a simulation with simple analytical equation. I have the following information:

- groundwater depth (water table) is located at about three meter below the ground surface
- the water surface in the sewer main is at about 5 meters below the ground surface


Now, if we seal the sewer main by some height, that we do not know for the time being, we have the fear that the water table may rise to two meters below the ground surface.

At various height of sealed surface of the sewer main, acting like an impermeable barrier, we would like to know how the water table height rises.

So, can anyone tell me a simple way of doing it ? A kind of analytical equation would be appreciated. Otherwise, a sort of rule of thumb

Thanking you in advance,
new member !

[flowjoe]

You need some kind of modelling software but i can not see how you can calculate an unknown volume of ground water over an unknown area.

How do you proposed to seal the sewer

[seanandruby]

As the height of the water table changes with the seasons it would be almost impossible to work it out. have you thought about a control system where you put in filter drainage.?

[watt10]

We have some idea on the estimated volume of water infiltrating into the sewer main, i.e. thought out the length of the sewer main.

I though of using leakage equation but getting the leakage factor doesn't seem easy. Because, groundwater is infiltrating through damaged parts of the sewer but we do not know how many damaged parts exist.

"have you thought about a control system where you put in filter drainage.?" Do you mean inside the sewer pipe ?

[flowjoe]

It would be an easy enough job to isolate a section of a sewer from external use and using a flow meter calculate how much water is entering the system via defects.

This however will have little bearing on the volume of ground water at any one time, the area of sub-soil being drained could be a mile square, or it could be ground water tracking through the original cut for the sewer that is entering at the point of least resistance.

I would imagine bore/test holes would be required over a large area and space of time to calculate something like this.

There are several lining and joint sealing methods on the market, but the external pressure of ground water will have to be calculated for most of them, or ground dewatering/freezing may be required during the repair process.

.....and never under estimate local knowledge. look for an old guy walking a dog, they usually know a thing or two ???