Geotechnical Instrumentation News

Geotechnical applications of time do- main reflectometry (TDR) are continu- ing to evolve and usage is increasing, particularly for monitoring deformation over large lateral distances when a pri- ori knowledge of movement locations is limited. Applications include monitor- ing subsidence along major roadways over active and abandoned mines and monitoring movement along the toe of dams. It has evolved into real time mon- itoring with alarms and a variety of noti- fication schemes. Fully automated sys- tems have been installed which detect when measured deformation on the or- der of millimeters has exceeded speci- fied magnitudes and/or rates, and initi- ate phone calls to responsible parties. The diversity of project-specific details (e.g., cables installed in trenches, in horizontal directionally drilled holes, in angled holes, notification via telephone or radio, etc.) is a reflection of the range of site conditions and owner requirements. This article is based on the paper, “Geotechnical Alarm Systems Based on TDR Technology,” which was pre- sented at FMGM 2007 and is published in GIN with permission from ASCE. Representative Projects The following projects illustrate alarm systems utilizing TDR technology. Common features of the hardware and software include: • 22 mm diameter solid aluminum co- axial cable • robust datalogger and TDR unit • cable lengths up to 610 m • maximized number of interrogation points per cable • baseline reading, or specified refer- ence values, stored on datalogger • difference between baseline reading, or specified reference value, and cur- rent reading computed by datalogger • datalogger initiates alarm sequence when difference exceeds specified threshold value(s), and • external data storage module While there are common features, project-specific installation details, data requirements, and interrogation details reflect the flexibility of TDR technology in these applications. There are also project-specific ratio- nale and objectives that reflect the flexi- bility of TDR technology. I-77 Summit County, Ohio This project was motivated by previous experience of the Ohio Department of Transportation during stabilization of a section of interstate highway impacted by abandoned mine subsidence in Guernsey County. At that location, the highway was closed as the mine was backfilled with grout. As grout was in- jected, water within the mine was dis- placed and subsidence sinkholes devel- oped under the roadway. For the project in Summit County, it was specified that the highway remain open for traffic during grout backfill in- jection and it was necessary to provide an early warning system to detect if sub- sidence was occurring beneath the high- way as water was displaced. Holes were horizontal directionally drilled (HDD) beneath the centerline of each lane of the highway. Coaxial cables were pulled back through the holes and then connected to a remote data acquisition system. Cables were also installed in a trench along the road. When the system detected an alarm condition along any cable, it communicated via phone modem with on-duty GeoTDR personnel. State Route 91 Plasterco, Virginia When the United States Gypsum Com- pany was decommissioning its facility in Plasterco, it was known that subsi- dence would occur when pumps were turned off and water levels rose within the mine. One component of the decommis- sioning plan was construction of a new alignment for SR91 outside the pro- jected influence of subsidence. Con- struction of the new alignment involved blasting for rock excavation and com- pany personnel were concerned that blast-induced vibrations would acceler- ate subsidence of the soft overburden beneath the existing highway. Coaxial cables were installed in angled holes drilled under the road and also in a trench along the road. In addition, mine personnel were concerned that subsidence would occur in the former plant area where excaGeotechnical News, June 2008 41 GEOTECHNICAL INSTRUMENTATION NEWS Table 1. Installation Details Site Orientation Cables I-77 Ohio HDD holes and trench 8 cables 317 to 374 m (1040 to 1227 ft) SR 91 Virginia Angled holes and trench 18 cables 36 to 447 m (118 to 1466 ft) Tuttle Creek Dam Kansas Trench 4 cables 610 m (2000 ft) McMicken Dam Arizona Trench 6 cables 416 to 497 m (1366 to 1630 ft) Table 2. Interrogation Details Site Data Points Interval I-77 Ohio 1 point/ m 3 hrs SR 91 Virginia 3 points/m 3 hrs Tuttle Creek Dam Kansas 2 points/m 10 minutes McMicken Dam Arizona 20 points/m 24 hour Figure 1. TDR data acquisition system. Four coaxial cables are connected to the multiplexer inside the smaller cabinet. The datalogger, TDR unit, external storage module, phone modem, and auto-dialer are also in the smaller cabinet. Figure 2. Holes being drilled for grout injection into abandoned mine along I-77 in Summit County, Ohio.