Jacking Pit Design 2

[cchamberlain]

I am a structural engineer with knowledge of geotechnical engineering. I would not consider myself an expert, but have designed many retaining wall systems for both permanent and temporary applications. I was recently asked by a Contractor to design a jacking/receiving pit for a culvert boring project. I was just wondering if anyone has any experience with this and what is generally required for checks. On the surface it looks like a normally cofferdam design, but I am concerned about unequal forces due to the jacking force. The contractor said the load would be applied to the wall of the pit. Does anyone know how you might check a global stability of the pit. Also the area where the pipe will be will not be available for passive resistance. Probably not a big deal for small pipes, but the pipe I am dealing with is 108" which is somewhat significant. Any suggestions, again for the unequal forces. Thanks

 

[cchamberlain]

I assume you are talking about soldier pile with lagging. If so I would think you could use the full height of the soldier pile and lagging system to provide passive resistance. I used the full height of my sheets even though the concrete block was not full height. Do you have poor soils. My soils were all cohesive, but I did a quick check with phi=27 and c=0 to also see if a long term granular state could control. This condition controlled but even dividing the passive pressure by FS=3 I still developed enough for the 1600 kip. In the newer pilebuck manual there is something in there about the failure surface for deadmen which extends in a spherical shape outward from the deadman. I think that the thrust block of jacking pit would be the same, that there is some portion extended beyond the pit width that contributes to the resistance (of course it is conservative not use this). Also I tied my trust block to a concrete slab in the bottom pit and put #6 bar extended from the pit to the slab and checked the shear friction between that connection and said the slab was an additional load path. I think I assumed 25% was transferred thru the slab thru friction and interlock with the teeth of the sheets (you may not have this but you get the idea. All load path.

 

[rcooley]

The contractor is using timber lagging with a steel waler and strut system for the pit. The concrete thrust block is actually located behind the jacking pit. He wants to construct the jacking pit, dig behind the pit and use the lagging to form the front face of the thrust block. I made the concrete block full hieght and I still can not develop enough passive pressure. For soils I have phi=30 amd c=0 and I assumed the distrubition to be triangular, I did check it as a trapezoidal distribution it helped but not enough. I even took into account the sliding frictionof the block but only get a FS=1. Where you using sheet piling driven to a depth or just sheeting with wales and struts? I wish we had pilebuck, I have been begging for that program for months now.

 

[PEinc]

rcooley,

I don't like the contractor's idea of building the concrete block behind and outside of the sheeted pit. If he needs to sheet the jacking pit, then he needs to install more sheeting just to build the concrete block. That would make no sense. Lagging will not be a good way to resist the high jacking loads, unless the lagging is very thick. Also, what hgolds up the front wall when he digs behind the back wall? I used sheet piling behind my concrete block. I checked sheet stresses above and below the concrete block. I don't think you will have a triangular pressure distributiopn against the jacking wall while jacking. I would assume more of a trapezoidal shape.

cchamberlain,

I would not have used a safety factor of 3 for my passive resistance. Maybe 1.5 to 2, but not 3.

 

[cvg]

don't let your FS get too low. I just heard of an incident recently where jacking contractor had to slow down production because of slow delivery on pipe. jacking force then got so high that he eventually heaved the nearby 4 lane arterial roadway and this required an emergency detour and traffic rerouting to get the job done. Luckily this contigency was in the bid, but it was touch and go for a while to get the pipe moving again.

 

[PEinc]

Using an extra high safety factor is not an excuse for poor engineering.

 

[cvg]

I think you are missing the point. jacking pit design is not the place to skimp as skimping on the quality of the pit design can lead to a very costly problem during the jacking. Heaved roads and siezed pipes are to be avoided at all costs. I think your contractor will agree with me here.

 

[PEinc]

I don't think I am missing any point. I believe that you should do a good, safe, economical design with a proper safety factor. Then, it's up to the contractor and inspector to make sure it is built correctly and safely. Some engineers use extra high safety factors to cover their uncertainties, their inexperience, and/or the contractor's potential poor workmanship.

In jacking pit design, if you have the proper soils information and then design it properly with a reasonable safety factor, you should be OK. If there is a problem, it is probably caused by unexpected or unknown conditions or poor contractor workmanship. These factors are not always overcome by picking an extra high safety factor.

 

[cvg]

FS=1 is not acceptable in my opinion

 

[PEinc]

I didn't say to use FS = 1. I would use from 1.5 to 2 depending on the soils. I would not use 3.

 

[cvg]

PEinc - my comment on factors of safety was not directed to you in the first place it was to rcooley who was getting fs=1.

 

[PEinc]

Sorry. Thank you.

 

[rcooley]

That was why I posted in the fist place, I felt my FS was too low. I was shooting for a min FS=1.5 for a safe design.

I am setting up a meeting with the contractor to look into different pit configurations, I agree with you PEinc, I do not think that the contractor's method is the best solution. Hopefully they will go with a more conventional design.

 

[cchamberlain]

I used a FS of 3 on the passive resistance as a check. I used a FS 1.5 for the cofferdam portion (inward loads)which controlled the length of the sheets. I then checked with FS of 3 and it worked. If it wouldn't have worked I would have reduced the FS for the Jacking loads and not tried to increase the lenght of the sheets. I did this because I can somewhat control the design of the cofferdam, but I what the jacking contractor does is less controllable and I didn't get a warm fuzzy with the info he was providing. Another reason is I expected hard driving and even though the Contractor said they could obtain the depths they couldn't (at least with the hammer they brought), so there is really less embedment than was designed, but since the FS was 3 it should be OK when they jack. I really don't feel I have to defend my design as it is a solid design, without being overly conservative.