Tunneling Engineering 101
By Nick Britten | November 29, 2022
At DHC, we try our best to be the construction engineers that you can come to for any design on a construction site. So, when a contractor comes to us to design a jack and bore shoring system, we can help you make sure your casing and carrier pipe are installed safely. From start to finish, we’ve got you covered. Tunneling engineering normally starts with 2 excavation shoring designs. These 2 pits are typically called the jacking pit and the receiving pit but can go by a lot of names (launch pit, exit pit, jack and bore pit, etc.). We’ve basically done it all when it comes to excavation shoring designs for these pits, whether it’s a simple trench shield pit or slide rail pit or a complicated braced soldier pile or braced secant pile shoring pit.
The fun of tunneling engineering doesn’t really start until you have the shoring fully installed and you’re at the base of the excavation. Once you’re at the base, you’ll need to install a thrust block on the side opposite where the pipe is going to go. This block will be pushed against with a jacking machine to create leverage to push the pipe through the soil. We’ve designed thrust blocks for pushing forces ranging from 50 tons to 1200 tons. They are typically constructed of steel plates, steel beams, and/or concrete. The most important thing is that they need to be big and strong enough to distribute the pushing load across enough soil so that it won’t heave.
Now that the shoring and the thrust block are installed, the next big question is whether you have groundwater or not. If so, you’ll need entry and exit seals to ensure water isn’t flooding in around you’re casing as you begin to penetrate the soil or as you exit the soil. These are typically pre-manufactured assemblies that have rubber gaskets tight to the casing. That’s where we come in: you’ll need them anchored to the shoring system or something similar, to ensure the water pressure doesn’t dislodge the seal.
Now it’s time to push the casing pipe into place, assuming you have checked that the casing pipe is strong enough to withstand the installation forces. If not, don’t worry. That’s why you have your construction engineers. The calculations typically include determining the estimated jacking forces, determining the external pressures from soil, water, surcharge, and then checking the casing for the combined loading. There are many other loading conditions that can be checked, and many entities have started to require contractors to have engineered submittals checking them. This includes buoyancy of the casing, settlement, deformation, grouting the annular space between casing and carrier pipe, and checking face pressures at the front of your cutterhead to ensure you’re not heaving or settling the soil.
Once the casing is installed, you then install the carrier pipe that goes inside the casing. Typically, these are on pre-manufactured rollers that make pushing the carrier pipe into place very easy but for bigger diameter carrier pipes and/or longer carrier pipes the forces can add up. This means you’ll need specialty designs to have them installed. These outside-the-box designs take many shapes and forms because the constraints of every job are a little different, but they are our specialty.
DHC has years of experience to bring to the table. We can handle your tunneling engineering needs from the start of a project to its completion, providing a broad scope of engineering services along the way.