Pipe Jacking Intermediate Jacking Stations

[NAV5]

I am working on a pipe jacking project that has jacking forces of over 750 ton with less than 3' of cover over pipe. Conventional backwall systems are not enough, so we are looking to reduce jacking forces on the reaction wall by use of intermediate jacking stations.

How do you calculate the reduction in jacking forces in the pit, by use of a IJS?

 

[LSPSCAT]

Less than 3 feet of cover? Please list additional parameters for the project to make sure it makes sense. (Diameter, length, materials...etc...)

 

[NAV5]

72" Pipe with less than 3' of cover, length of jack 385'. This is the design from the project engineer, I am working for the contractor making submittals on the jacking and receiving pits.

 

[LSPSCAT]

From my experience this does not seem to make sense. I open to learning new methods if anyone can shed any light on this?

 

[rowingengineer]

I assume that this is for installing a concrete sewer or stormwater line under ground without excavating a trench. I would say this question is really for the civil forum.

How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban

 

[cvg]

IJS consists of a sliding steel can, with same diameter as the pipes and fitted with a set of hydraulic rams. the rams use the pipes behind the IJS for reaction. Once you reach the end you need to excavate, remove the IJS and then use the mainjack to close the gap in the pipeline. reduction in jacking pit wall reaction force is due to the friction force on the pipeline behind the IJS. Your geotech could help with that calculation

 

[LSPSCAT]

My point is with that diameter pipe, length, and shallow overburden you will not be able to develop the confinement to effectively jack the pipe. Typically rule of thumb is a minium of 1.5 diameters for cover.

 

[racookpe1978]

I haven't run pipe this way, so excuse my question.

If I read the process correctly, he's going to have a buried pipe, pushed from one end that's exposed in a pit or evacuated hole, to ram it sideways a short distance until the buried pipe butts up against an existing pipe, already exposed in a different hole. Right?

I'm assuming the problem will be "restraining" the pipe from bending "up" through that little bit of cover through compressive forces (like an unrestrained column would collapse from end pressure). If the pipe were shorter, fatter, stiffer walled, more deeply covered, or needing less force to be rammed sideways (lubricated somehow, or being rammed through less resistant soil), then the threat of sudden collapse would be reduced, right?

 

[LSPSCAT]

Although you can effectively tunnel and excavate as you jack the pipe or structure to minimize the disturbance and decrease jacking forces, the limited depth of cover will not provide the necessary reaction to keep your pipe on line and grade.

In addition you will disturb that shallow depth of three feet of overburden. Pavement will be disturbed, utilities...etc. This installation seems ideal for cut and cover, open trench. Obviously we do not know the full story as I do not understand why this installation would be considered for a pipe jacking.

 

[cvg]

I think the primary problem is with excessive ground loss and settlement of the surface. The effect of ground loss is inversely proportional to the depth of cover over the pipe. Another rule of thumb is minimum 2D cover over the pipe. With only .5D, you are very shall and I would expect you are jacking under utilities or a road. If so, then granular pipe bedding materials may run into your tunnel causing excessive settlement and damage to the existing utilities or to the road above. I would expect that to make this work would require extensive grouting to stabilize the ground and monitoring.

 

[cvg]

http://www.fhwa.dot.gov/bridge/tunnel/pubs/nhi09010/12.cfm

 

[csd72]

cvg covered the main points that I was going to say.

Although I dont think your main issue will be settlement but heave, as you thrust the pipe it is pushing soil inall directions at the front including upward.

The thrust blocks rely on passive resistance to work and I cannot see you getting those sorts of loads at such a shallow depth.

I also fail to see how this could be cheaper than open cut at such a shallow depth.

 

[cvg]

heave - settlement, you can't get one without the other