Caldecott 4th Bore
The proposed 4th Bore features a two-lane configuration with a 10 foot wide shoulder on the north side and a 2 foot shoulder on the south side, seven cross passages that connect to the 3rd Bore for emergency egress, and an updated tunnel control center. The new tunnel will have a horseshoe shape measuring approximately 3,399 feet long, 50 feet wide, and 32 feet high (excavated dimensions).
Project challenges include designing a safe and efficient way to mine through weak sedimentary rock and unstable fault zones. The 4th Bore will encounter steeply dipping blocky to crushed sedimentary formations consisting of mudstone, shale, sandstone and chert, which have undergone extensive folding and faulting. The unconfined compressive strength of the rocks range from 100 to 17,000 psi.
The design team based their approach on the Sequential Excavation Method, also known as the New Austrian Tunneling Method. This method provides the required flexibility to construct a wide span tunnel in weak and variable ground conditions. Using detailed numerical analyses to predict ground behavior in response to tunnel excavation, the design team developed efficient support systems tailored for specific reaches along the alignment.
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