Kevin M. O'Connor

Comparison of TDR and Inclinometers for Slope Monitoring

As TDR technology grows in acceptance, its use stimulates further innovative applications and comparison with slope inclinometer measurements. This paper presents cases in which the opportunity arose to compare these two technologies to detect and measure subsurface deformation in slopes. Among the main points addressed are (1) the comparison of TDR reflection magnitude and inclinometer incremental displacement to help quantify deformation with TDR technology, and (2) the comparison of the accuracy of the two technologies in detecting and measuring shear deformation in localized versus general shear. Case histories are presented that involve monitoring movement in soil and rock slopes and embankments as well as retrofitting deformed inclinometer casing with coaxial cables. This paper describes installation details. When monitoring to detect narrow shear zones in soils, it is best to use small ratios of hole-to-cable diameter, and prudent use requires that larger diameter, solid, metallic coaxial cables be installed in separate holes. Grout strength should be (1) low enough to fail before bearing capacity of the surrounding soil is reached, and (2) high enough to deform the cable it encapsulates. It is recommended that other users publish cases in which theses two technologies are compared in order to expedite continued assessment. Coaxial Cable Geometry used for TDR Monitoring TDR is analogous to radar in a coaxial cable. Consequently, it is possible to display all reflections along a cable and identify the type and location of cable Figure 1.-Schematic of cable installation and monitoring. deformities producing these reflections. As shown in Figure 1, a metallic coaxial cable can be placed in a dr ill hole and anchored to the walls by tremie pl acement of an expansive cement grout. When localized shear movements in rock or soil are sufficient to fracture the grout, cable deformation occurs and can be detected using a TDR cable tester which launches a voltage pulse along the cable. At each location where deformation has occurred, a portion of the voltage is reflected back to the TDR unit which displays the reflections. Travel time of each reflection distinguishes the locations where cable deformation is occurring, and differences in the reflected signal magnitudes can be employed to quantify the magnitude of cable deformation (O’Connor and Dowding, 1999). When a cable is crimped prior to placement in the hole as shown in Figure 1, a reflection from each crimp serves as a distance reference marker in the TDR record. Strip Mine Highwall Slope (Case 1) The example shown in Figure 2 involved installation of coaxial cable in the highwall slope of an oil sands mine. Details of the installation are compared with the other cases in Tables 1 and 2. The bituminous sands contain numerous thin consolidated clay layers that cause highwall slope instability. Consequently, slope movement is an operational problem and many kilometers of inclinometer casing have

EXPLICIT MODELING OF DILATION, ASPERITY DEGRADATION AND CYCLIC SEATING OF ROCK JOINTS

Abstract This paper presents another development in the discrete element simulation of joint behavior, namely, an appropriate formulation of dilation, asperity degradation, and seating during cyclic motion associated with earthquake and explosive excitation. Nonassociated plasticity is assumed during sliding along local asperity surfaces, however, this causes dilation to occur for the overall joint. The most significant advance is the addition of a continuity condition, the β factor, for normal displacements during joint sliding over local roughness (or asperities) to control seating or bulking during cyclic shear when asperities are degraded or damaged. Three numerical examples are presented to illustrate model calibration and implications for rock mass behavior. The first example involves simulation of a laboratory cyclic direct shear test performed on an artificially-produced tension joint in sandstone. The second example demonstrates use of the seating/bulking parameter, β, to explicitly control continuity of normal displacements for the overall joint. Parameter studies indicate that the value of β should be close to zero and can be uniquely identified from laboratory shear test results. The third example simulates response of a cavern within a moderately jointed rock mass, with and without joint roughness, when subjected to cyclic excitation.

Real-Time Monitoring of Subsidence Along I-70 in Washington, Pennsylvania

Two longwall coalmine panels were mined at a depth of approximately 156 m (510 ft) beneath I-70 east of Washington, Pennsylvania, such that the highway crossed the width of one panel at two locations. The Pennsylvania Department of Transportation (DOT) assumed responsibility for real-time monitoring of both ground deformation and changes in highway conditions. Innovative monitoring of ground deformation was accomplished with time domain reflectometry to interrogate coaxial cables installed in seven deep holes and an array of 32 tiltmeters along the highway shoulder. Surface monitoring was conducted with Global Positioning System measurements at more than 100 locations. Tiltmeters were connected to a central remote data acquisition system that automatically recorded and stored measurements. When specified tilt values were detected, the system initiated a phone call to key Pennsylvania DOT personnel, who then monitored tiltmeter measurements in real time via a phone-line connection. On the basis of this information, they could alert other agencies, if necessary, and intensify visual reconnaissance to determine if lane closures were necessary.

Using Time-Domain Reflectometry for Real-Time Monitoring of Subsidence over an Inactive Mine in Virginia

Closure activities at the United States Gypsum Company facility in Plasterco, Virginia, include realignment of an existing highway outside the predicted limits of long-term subsidence. Concerns about the possibility of subsidence along the existing highway being induced by construction activities, or in the former plant area where excavated rock was being placed, motivated the installation of a real-time monitoring system based on time-domain reflectometry (TDR). This technology involves interrogating solid metallic coaxial cables to determine locations and magnitudes of cable deformation. When the cables are grouted into drill holes and trenches, TDR can be used to determine the location and rate of precursor subsurface movement that is causing cable deformation. Manual monitoring of three cables began in June 2001. Then, during 2002, system capabilities were expanded to automated real-time monitoring of 21 cables with lengths that varied from 10 m (30 ft) to 270 m (886 ft) for a total greater than 2,500 m (greater than 8,200 ft) of cable. The cables were installed in angled holes beneath the existing highway, in trenches along the highway, and in trenches over the former plant area where rock excavated for the new alignment was being placed. The automated monitoring system was controlled by a programmable data logger and incorporated a callback capability. Whenever the difference between the baseline measurement and the current measurement at any location along a cable exceeded a preset alarm threshold, the data logger would initiate a call to responsible personnel.