Seeking to add 50 years to the life of its Federal Quarry, Lehigh Hanson began constructing a 920-ft long underground decline tunnel near its center to access deeper Ordovician Age dolomitic limestones. After the original contractor was dismissed, the construction materials supplier hired Deere & Ault (D&A), a Schnabel engineering company, to evaluate the site, design support, and provide services during excavation.

The quarry produces limestone aggregates for construction materials in the Chicago area. Lehigh Hanson constructed the 26-ft wide by 23-ft high tunnel from 2016 to 2018 to access deeper limestones that would be mined using an underground room and pillar operation. The company initially brought us in to evaluate shotcrete and ground conditions in the decline tunnel, which was only partially completed.

After performing this work, we designed new support consisting of shotcrete, rock bolts, and steel sets backfilled with self-consolidating concrete. We then provided full-time construction engineering services during the decline excavation. Particular challenges included backfilling and then mining through the collapsed section of the initial drive, timely ground support in the Scales Shale, and high water inflows near the portal and at the ventilation shaft. During construction, we used a series of multipoint borehole extensometers (MPBXs) to monitor site conditions.