Bore and Jack of water line in sand with cobbles 1

[muuddfun]

Does anyone have experience with bore and jack of pipes (24 inch water line)in conditions with cobles and occasional boulders? I reviewed most of the posts on the board on this subject but they were a little different than the sittuation I have.

I have a project with a 24 inch wateline that needs to cross under 48 inch RCP storm drains for about 100 feet. The civil does not want to cut the storm drains and do an open cut, he believes it would still be cheaper to do some pipe jacking. Storm drains are dry 99 percent of the year so diversion of storm water would not be of much concern.

The bottom of the storm drains are about 10 feet below the roadbed. The soils are sand with gravel, cobbles and the occasional boulder. Cobles are about 5 to 15 percent by volume, and the occasional boulder was encountered in my borings. I drilled 3 borings near the location and one terminated at 12 feet on a boulder, and the other 2 were able to get to 30 feet depth with some difficulty. The borings caved to about 7 feet after pulling the augers. There is not groundwater. Most of the borings I drilled for this project terminated on a large cobble or boulder at some point before the planned depth.

I am concerned about groundloss around the bore and jack operation, and how to get past any cobbles or boulders without to much expense, any guidance or experiences anyone could share would be appreciated. I am thinking about grouting first, but if it could be done without?? Any ideas.
Thanks

 

[Rjeffery]

On a project involving putting a 36" casing under active railroad rail bed where the "poor soil conditions" were stabilized by the railroad with ballast (3 to 8 inches in size)to a depth of 8-9 feet, the contractor eventually did a low strength grout injection and successfully pushed the casing 250 feet.

The best way to test something is to squeeze it, slowly, until it breaks!

 

[muuddfun]

What was the strenght of the grout? How large of a grout bulb did they create to be sucessful with the 36 in casing? How much area around the evenutal casing is needed for support?

 

[cvg]

chemical grouting or pressure balanced microtunneling can take care of the unstable sand - but the cobbles and boulders may be just as big of a problem. Especially if they are too large to fit through the casing, you will have a big challenge on your hands to break up the boulder and remove it. One option is to install a larger casing, presumed large enough to fit the largest boulder. But this will increase the cost. Removing the boulders may also create significant voids which will require additional grouting. Additional time and cost as well as the increased risk may make the jacking option less attractive than open cut methods.

 

[motorbiketocrank]

"The civil....believes it would still be cheaper to do some pipe jacking" - does he/she have any practical experience in geotech engineering and various trenchless techniques to have any real idea of what is the best and most economical approach? I'm guessing no because otherwise why would they need your services?

The conditions you discuss with collapsing holes and refusal on boulders pretty much say to me that you shouldn't even think about jack and bore.

Does the waterline definitely have to go under the storm drains with no potential to route it over those lines (I realize frost concerns might prevent that)? And even if it does cross under, why would you have to cut the storm drains for open trench.....couldn't they just be temporarily supported? I'd agree that if trenchless was required you're probably talking signficantly oversized casing with human entry to chip out boulders and some sort of face stabilization. Wouldn't even want to think of the costs related to that type of work.

 

[muuddfun]

Thanks
I have tried to talk the civil out of it but have not been suscesful so far. I will still recommend open cut as my first recommendation, but will need to put in as a secondary option parameters for the bore and jack. The civil does have some experience but I am not sure how much, or maybe he has to much.

There is not enough room over the storm pipes to get the water line and stay below the pavement with any cover. I had not thought about temporary support, that might work.
What is the minimum size cassing you would use if you are going to send a man into it?

 

[motorbiketocrank]

Some of these might help:

http://www.nastt.org/pdf/PowerPoint/JackingBoring.pdf

http://www.asce.org/bookstore/book.cfm?book=4561

http://www.nastt.org/

 

[muuddfun]

Thanks, those are helpful.

 

[Rjeffery]

muuddfun
What was the strenght of the grout? less than 500 PSI

How large of a grout bulb did they create to be sucessful with the 36 in casing? The caseings top was 48 inches or more below the top most elevation of the ballast bed. we did observe some of the grout coming through the ballast. (ballast is very porous after all.)

How much area around the evenutal casing is needed for support? That was not something that I was asked to determine. (my function was to take elevations of the tracks to ensure there were no elevation changes.)

The best way to test something is to squeeze it, slowly, until it breaks!

 

[rchanke]

A typical permeation grouting program (ground improvement) for something like this would involve either microfine or sodium silicate grouting from springline up about 1-dia and about 1.5-dia wide. Sodium silicates in sand/gravel usually give about 60 to 100-psi UCS while microfines will yield greater strengths, typically less than 500-psi.

 

[muuddfun]

How deep below the storm drains should I require the casing to be, if it is grouted I would feel comfortable with it being closer? I have seen in other reports that about 2 cassing diameters is needed to prevent settlements from affecting other utilities when the ground conditions were a lot better than what I have for this site, does anyone have experience with that?