In February 2019, Drilling Service Company completed a drop shaft at the Linden Site for the MSD Deer Creek Sanitary Tunnel (DCST) project. The Linden Site is located near the intersection of I-170 and I-64 in Brentwood, Missouri. The drop shaft is 12 feet in diameter by 127 feet deep. The purpose of the drop shaft is to serve as an access point for an adit connecting nearby sewer lines to the new Deer Creek Sanitary Tunnel.
The geology at the Linden drop shaft location predominantly consists of approximately 20 feet of sandy lean clay overburden overlying 10 feet of shale followed by Limestone bedrock of the St. Genevieve and St. Louis formations. Groundwater was encountered at 3 feet below the surrounding grade.
Drilling Service Company and SAK elected to use conventional drilled shaft excavation methods for the drop shaft in lieu of drilling and blasting in order to minimize disturbance of a nearby water main and surrounding residents and businesses. Drilling Service Company utilized a Watson 4400 drilling rig with 138,000 ft-lbs. of torque for the entire drilling operation. A 13-foot-diameter by 30-foot-long section of corrugated metal pipe (CMP) was installed and grouted in place to restrain the overburden soils and minimize groundwater seepage prior to drilling the bedrock. Drilling Service Company utilized a suite of specialized rock drilling and coring tools during bedrock drilling. A series of pilot holes were advanced through the bedrock to perforate the cross-section of the shaft. Following pilot hole drilling, a 12-foot-diameter core barrel was used to advance the drop shaft to a terminal depth of 127 feet below the surrounding grade.
A total of approximately 411 cubic yards of limestone was excavated during drilling while maintaining shaft verticality within 1%. The drop shaft was completed ahead of schedule and without any safety incidents or equipment breakdowns. The client and owner were very pleased with the quality of the excavated shaft along with the professionalism and experience exhibited by the Drilling Service Company field personnel throughout the duration of the project.