KS Associates Inc., a civil engineering and surveying firm located in Elyria, Ohio, has completed an important surveying and 3D laser scanning assignment to help ensure that two 24-ft tunnels connect more than 200 ft below ground. The two sewer segments – the Euclid Creek Tunnel (ECT) and the Dugway Storage Tunnel (DST) – are part the Northeast Ohio Regional Sewer District (NEORSD)’s Project Clean Lake, a $3 billion program designed to drastically reduce the amount of combined sewage entering local waterways during heavy rains. When constructed, the ECT/DST tunnel system will have the capacity to capture approximately 117 million gallons of stormwater and raw sewage during rain events and transport it to a treatment plant before discharging into Lake Erie and its tributaries
The MWH/Hatch Mott MacDonald joint venture (MWH/HMM) is leading the design of the DST and contracted with KS Associates to perform an as-constructed survey of the tail tunnel portion of the ECT, which was designed by a separate consultant. The ECT’s tail tunnel is the critical connection point of the two tunnel systems. Because the survey controls were completed separately for the two tunnels, the team wanted to be sure that the alignment of the DST would match the ECT.
One of the surveying challenges was having limited access to a project site 200 ft below ground. The only entryway providing access to the tunnels is a 50-ft-wide shaft, which was being utilized by ECT construction crews. To overcome this challenge, KS deployed its Leica ScanStation C10 3D laser scanner. This technology allows for the rapid acquisition of millions of scan points, providing an enormous amount of survey data in a relatively short timeframe.
The ECT construction surveyor had established survey control points on the surface and in the tunnel to control the ECT mining operation. The construction surveyor used a surface-to-tunnel survey transfer system, a common practice in the surveying industry that involves securing heavy plumb bobs from wires around the perimeter of the shaft to the shaft’s ground floor, then surveying the top and bottom of the wires. A considerable amount of time must pass for the plumb bobs to stop moving. Once the plumb bobs are at rest, the control measurements can be recorded and used as the basis for obtaining subsequent survey data.
Surveying the relationship between two tunnel survey controls at ground surface would have been a simple surveying task; however, it was necessary to confirm the datum underground to truly verify the surveys. Because the previous survey transfer system had been removed to make way for ECT construction crews, KS would have had to establish a new survey transfer system and repeat the measurements. This process would have taken several weeks to complete and would have disrupted construction activities at the shaft site. Upon review of the site conditions, the KS survey team developed a viable option to complete the assignment in a fraction of the time, reducing costs and minimizing disruption to the ECT contractor.
The laser scanner was used to scan survey control point targets at the top of the shaft, and from the bottom of the shaft down the tunnel to the ECT’s terminus. Not only did this save time, but the scanner can operate in complete darkness and obtain all necessary survey data of the 24-ft diameter tunnel to the end of the excavation.
According to Mark A. Yeager, P.S., KS Associates’ Director of Surveying Services, “The conventional weight and wire method is a standard and acceptable practice for obtaining the required degree of accuracy; however, providing a separate survey using a different data collection method elevated the reliability of the data for the DST design team and ECT construction contractor. In addition, the entire scanning operation took less than five hours.
“This is the first project for which KS Associates used the scanner to transfer survey control more than 200 ft below ground. The scanner provided a reliable solution and we hope to apply lessons learned on future underground surveying assignments.”