micro tunneling/dewatering

[14243]

Hallo Gentlemen:
We have a water pipe line project which requires micro tunneling at a depth of about 20 meter. We plan to push the pipe for a length of about 60 meter. The subsurface soil consists of dense sand/ silty sand. The ground water ia at about 2m below the ground level.
My concerens are as follows:
1. Is it possible to push the pipe for 60 meter and more length?
2. what kind of dewatering system will be approprite?
3. What kind of safety precautions should we take particularly working at 20 meter deep below the ground water level?
4. Any other valuable suggestions.

your comments are greatly appreciated.

Thanks

 

[georam]

I am not sure when you mean "push". Are you thinking about pipe jacking, which is usually done in soft clay soil ?

For dense sand, it will not likely to work. Is the dense soil contains any gravel, cobbles and possible boulders ?

I can realize the problem with high water table, and location at 20m depth. Unless you are doing directional drilling, you will likely create an entrance and exit pits. This need to be dewatered.

Safety for 20m below ground, and in a pit, cohesionless soil, and under water table are a big concerns. Check for boiling at the bottom of the pit. Also the pit requires a sound design, check by a geotech. engineer. Lateral loads is huge here.

I am not sure what type of a person you are. If you are the reading technical paper type, I suggest there are a few papers on micro tunneling . A list is given at

http://www.microtunneling.com/topics/bibliography.htm.

Anither good paper can be viewed in the internet at

http://www.microtunneling.com/education/Papers/ndsu9502.htm

You may also get a linkage to tunneling site from my website at

http://www.pangea.ca/~halim/misc.html

Select Tunneling (last one).

Hope that this helps

 

[Ginger]

As I undertsand your question, you intend to microtunnel at 20m depth. Provided you are using jacking pipes and you have a good bentonite system which can reliably operate in excess of 2 bar pressure, then pushing the pipes 60m should not be a problem. If the groundwater is flowing you run the risk of losing your bentonite curtain. This will drastically increase the jacking loads. You have not stated the pipe diameter which will also have a bearing on this matter.

You should perform a risk assessment to determine whether this is likely. If it is, then consider using an inter-jack station at 30m along the drive.

As Georam states above, your drive and reception pits will be heavily strutted due to the large hydrostatic groundwater pressures involved. My recommendation would be to sink a shaft (possibly by caisson sinking methods, cast a base and ensure that the shaft doesn't float using a combination of weight and skin friction) then drive out of the shaft into a pre-constructed reception shaft.

Regards Andy Machon