MD 355 CROSSOVER TUNNEL
Montgomery County, MD
The Robert B. Balter Company provided geotechnical services to evaluate the subsurface rock conditions within a new connector tunnel that will connect Walter Reed Medical Center to the nearby Metro Center. The purpose of our study was to collect rock samples from the shaft and the tunnel in an effort to compare them to the previous data collected from the site during design. Contractor efforts had been delayed due to a reported changed rock condition and this evaluation was used to confirm if a change in the rock had occurred or not.
Balter’s work consisted of collecting multiple rock core samples from the accessible sidewalls of the 140-foot deep shaft and 70-foot long tunnel. Much of the exposed rock was covered with reinforcing shotcrete during the study. Cores were collected from both sides of the tunnel and at various points within the shaft. During the coring operation, an experienced geotechnical engineer was onsite to make observations of the exposed rock conditions including fracture and joint patterns, water seepage locations, and general condition of the rock. The collected rock samples were returned to Balter’s AASHTO-accredited laboratory for further documentation and laboratory testing which included Compressive Strength, Brazilian Tensile Strength, and unit weight testing. The stress and strain were measured on a few of the rock core samples in order to estimate Young’s Modulus and Poisson’s Ratio.
The field and laboratory testing identified rock that was similar in nature to the historical data (ie, geologic maps), as well as the historical data collected from previous projects on and near the subject site.