Jacking Pit Design
Jacking Pit Design
(OP)
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





RE: Jacking Pit Design
RE: Jacking Pit Design
you will need to estimate the force required to jack the pipe which includes the weight of the pipe and friction during jacking. This force must be transferred into reinforced concrete or steel backstop at the back wall of the jacking pit and then transferred into the soil without exceeding the bearing capacity or allowable amount of displacement. Rankine - Bell equation can be used for passive pressure, but requires rotation / movement of the wall which may be quite large. Note that in addition to the backstop / thrust wall you may also need sheet piling or other shoring designed to retain the earth. So your analysis must consider the pressure from the backstop bearing on the sheet piling and any movement or deformation caused by the high pressures. Recommend you find reference material to guide in this complicated design. I have a reference "Pipejacking and Microtunneling" by James Thomson which gives some basic information - but not particularly detailed. Maybe others have something better.
RE: Jacking Pit Design
RE: Jacking Pit Design
The PZ27 sheet piling isn't strong enough to distribute the 2500 kip force to the ground behind. I told them to double up the sheet piling (a double layer sheet pile wall). Also, the concrete block is going to be 4 feet thick by 10 feet high by 21 feet wide (pit width) with a double mat of closely spaced #10 bars. The design of the concrete block highly depends on the size of the steel frame that distributes the jacking force to the concrete. Get this information from the jacking contractor. For my job, the contractor's steel jacking frame wasn't big enough. So, I also had to add some large steel street plates as bearing plates between the steel frame and the front face of the concrete block.
I checked horizontal bearing pressure on the back of the sheeting assuming a trapezoidal distribution. I checked the total passive resistance of the 21 feet high sheet pile wall. I used a safety factor of 1.5 for the passive resistance. I checked the cantilevered moment in the sheet piling above the concrete block.
I also had to design the sheeting wall to support the active earth pressure and any surcharge pressures on the sheet pile walls before the concrete block is built and the jacking force is applied.
Your 108 inch pipe shouldn't affect the design of the reaction wall because you can't jack a pipe through the reaction wall. The pipe will affect the design of the pit bracing and the sheet pile head wall which is a cofferdam wall without any applied jacking forces. My reaction wall needed to be braced initially for the active and surcharge pressures. However, I did not weld the bracing wales to the sheets because the sheets should push backward during the jacking operation. I didn't want to have the sheeting pulling the wales backward.
You also need to make sure that the jacking equipment and the pipe sections can fit between the wales and braces and then into the bottom of the jacking pit.
My design may sound like overkill but consider the magnitude of a 2500 kip jacking force.
At this time, my reaction wall is ready for tunnel jacking. I'll let you know if it works! Wish me luck.
RE: Jacking Pit Design
Our projects sound similar ecept you loads are about 3x bigger. The max jacking force for my project is 800k snd my retained height is about 20'. How many levels of bracing did you use? How are you keeping them in place if you are not welding them to the sheets? Or did you mean just the wales on the reaction wall. When I discussed the pipe in my first post I was concerned about unequal pressure distribution on the soil before the jacking begins. In a normal cofferdam situation you don't have a large opening, so do you just add some steel members around the pipe opening. So the reciving end would just be a cofferdam with a opening, correct? Assuming they don't miss the hole!! Thanks.
RE: Jacking Pit Design
RE: Jacking Pit Design
I am coming up with 30" thrust block with #6 at 4" on the face near the sheeting. Two-way shear controlled, if it is even possible as a failure mode. I checked bending about the reaction plate which is only 26" by 80" but my thrust block is 19.75' (full width) by 8' tall which is below the lowest wale. I am tying my thrust block to the bottom slab to distribute some load to the soil and to the sheets (in plane. I am just getting ready to check the bending of the sheets and the passive pressures. Not sure if I should use undrained condition with phi 27 or so for the passive check since I am mainly in cohesive material. Did you check shear on the sheets or just bending to come up with the double sheets? My sheets are PZC18 Grade 50.
RE: Jacking Pit Design
RE: Jacking Pit Design
I checked the bending and shear above and below the thrust block for the passive soil pressure diagram. This is the max pressure that can devlop even though the actual pressure will be different and may never reach close to that. I checked both the drained and cohesive states, but the availabe passive pressure in the short term was almost 2x the amount applied by the jack so I divided it by two when checking the sheeting. SO when are the going to start jacking on your pit?
RE: Jacking Pit Design
RE: Jacking Pit Design
Contractor did a few small practice pushes so far. Pushed the 11' x 11' x 100'L concrete box tunnel abot 6" in 2" increments. Reportedly, the reaction wall pushed back about 2". The jacking force supposedly was 90 kips. I was told that the tunnel is sitting on a greased work slab to make the jacking easier.
Tunnel weight is about 2 x (11' + 10') x 1't x 100'L x 150 pcf = 630 kips . Jacking force supposedly was 90 kips (I have not confirmed this). Friction factor for concrete to concrete with grease between = 90/630 = 0.14. Does this sound reasonable?
RE: Jacking Pit Design
RE: Jacking Pit Design
Reportedly, 90 kips, not 90 tons. I need to get better info. Concerning deflection, remember that I have two sheet pile walls in front of each other with a little dirt sandwiched between. I would not be surprised if the two walls squashed toward each other.
RE: Jacking Pit Design
RE: Jacking Pit Design
RE: Jacking Pit Design
Tunnel has now been jacked 55' out of 90'.
No jacking today.
Expect to get 20' more tomorrow.
Latest jacking force is now up to 800 kips.
RE: Jacking Pit Design
RE: Jacking Pit Design
Contractor also said that after jacking 55' of 90', the tunnel is 1/4" higher than plan elevation. They also are pumping some type of "slippery mud" under the concrete box tunnel to reduce friction.
RE: Jacking Pit Design
RE: Jacking Pit Design
RE: Jacking Pit Design
RE: Jacking Pit Design
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.
RE: Jacking Pit Design
RE: Jacking Pit Design
RE: Jacking Pit Design
RE: Jacking Pit Design
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.
RE: Jacking Pit Design
RE: Jacking Pit Design
RE: Jacking Pit Design
RE: Jacking Pit Design
RE: Jacking Pit 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.
RE: Jacking Pit Design