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

 

[civilperson]

Install a precast concrete Type III barrier across the back wall of the pit and backfill the space between the barrier and the pit wall with 100% compaction granular material. The area of the barrier, 3.5' x 12', divided by the horizontal loads will give the horizontal soil pressures. Check to see if this pressure is less than Ko or even Kp for the average depth of the barrier. Use a WF shape to spread the load across the concrete barrier, (push against the steel with the hydralic ram).

 

[cvg]

I'm a civil with some experience in jacking and boring...

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.

 

[cchamberlain]

The pit will be somewhat like a cofferdam. Sheet pile on 4 sides because the flow line will be down approximately 20'. So I will have about 20' of retained height, so I was really looking for some insight with how to balance the apparent earth pressures with the jacking force (which I am told by the jacking Contractor could be up to 400 tons), and the proper design loads for the wales and thrust blocks. If the jack loads are only applied to one face of the cofferdam it seems global stability could be an issue. Using the allowable bearing pressure seems unconservative to me since the existing ground could truncate what the typical failure plane would be for say a footing. Even if this is correct how do you develop the design forces for the wales. If the wales are located above the concrete wall (reaction plate or thrust block)wouldn't the loads be higher than just the apparent earth pressure loads?

 

[PEinc]

I just designed a reaction wall for jacking a rectangular concrete pedestrian tunnel 100 feet long. I was asked to design the reaction wall for a jacking force of 2500 kips (= maximum jack capacity). The contractor wanted to use, as a reaction wall, PZ27 sheet piling with a cast-in-place, reinforced concrete pad in front of the sheeting. The pit will be 21 feet deep. Someone may calculate and tell you the expected jacking force but this is just an educated guess. The only accurately known force would be the maximum capacity of the jacks which the contractor may have to apply to jack the pipe.

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.

 

[cchamberlain]

Thanks for the info. What was the trapezodal distribution based on? Was it the force applied to the reaction wall? If so what limits did you use to transition? End of concrete? Did you check the cantilever setion based on the trapezodal pressure or triangular (passive) pressure? I assume you just checked bending and shear for the concrete about whatever frame configuration the jacking contractor supplies. For your case did you assume 2-way bending or did you just check a cantilever about the additional bearing plates.

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.

 

[cvg]

typically there is a floor slab poured and rails installed upon which the pipe and jacking assembly rests. This helps to insure correct alignment so that they don't miss the hole.

 

[cchamberlain]

PEinc,

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.

 

[PEinc]

I needed a double thick wall for bending, not for shear. The contractor had PZ27 Gr50 sheets and wanted to use them instead of getting much heavier sheets. I did not consider any cohesion. I assumed the soils were drained.

 

[cchamberlain]

PEinc,

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?

 

[PEinc]

Not sure. I'll check with the contractor, my client.

 

[PEinc]

Just received a report from the field about the tunnel jacking for which I designed the reaction wall.

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?

 

[cchamberlain]

Intuitavely the friction factor seems high, but not unreasonable. I did not try to verify it with any references. I am curious how they were able to put the 100' of pipe in. Is there a long open distance to get this much pipe in or am I misunderstanding something. My contractor can only get about 12' of pipe in the pit before he starts jacking then once he starts he will be pushing thru the soil with its friction factor against the concrete. 2" seems high for the deflection of your thrust wall, especially with only 90 tons.

 

[PEinc]

It is not a pipe. It is a reinforced concrete pedestrian tunnel. The contractor is digging slightly in front of the rectangular tunnel, from inside, as the tunnel is being jacked. Dig a little, jack a little, repeat. The contractor expects it to take 1 week to cross the road.

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.

 

[cchamberlain]

Sorry, I was using pipe inadvertently. I know what you meant though. 90 kips is very, very low compared to your design loads.

 

[PEinc]

I agree but the jacking contractor wanted to design the reaction for the jack capacity, just in case.

 

[PEinc]

Update on my tunnel jacking project:

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.

 

[cchamberlain]

800 kips. That's getting up there. Contractor is just building my pit. Not sure when jacking is going to start. Have you heard of anymore deflection or any signs of distress? The jacking contractor that I am working with just gave me the size of the reaction plates he will be using, but I did not design them. Hope they are stiff enough to transfer the load like I assumed.

 

[PEinc]

Contractor did not mention anything about deflection of the reaction wall andsheet piling. No news is good news!

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.

 

[rcooley]

I have a similar project, 17' deep trench with (2) 300ton jacks (= 1,200 kip jacking force). The contractor wants to use lagging and not drive sheeting, so basically all I have is the concrete block to resist the force of the jacks. I checked the block by having a driving force of the jacks and a resisting force of the passive pressure of the soil. When I look at it this way, there is no way the soil can resist the force of the jacks, even if I make the block a full 17' tall by 12' wide(trench dims). I guess the first question I have is with your designs the concrete block is pushing against your sheeting and the sheeting is engaging more soil to help resist your jacking forces, correct. I guess I just want to make sure I am looking at this correctly since this is the first jacking pit I have designed and if I should recommend and look into using sheeting instead of lagging which is not helping resist my jacking forces. Any help would be appreciated, thank you