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.
