Geotechnical investigation and analysis of soft layers within a hard rock formation

摘要

Soft layers within much harder rock formations can pose unusual challenges to tunnel design and construction. Identification, analysis and delineation of these features are crucial to maximizing design response to these and related issues. Minimizing unexpected negative impacts due to the presence of soft layers is not unlike the more common case of hard layers within soft formations. The Austin Chalk Formation in Texas, USA, is an example of where soft layers within this relatively hard limestone formation impact the design and construction of tunnels. Proposed are two mined auto tunnels. Current plans indicate the mined tunnels will each be about 3.2 km in length and be on the order of 15 m wide by 9 m in height. According to the Federal Highway Administration, these will be the longest-widest-mined auto tunnels in the United States.