Charles H. Dowding

EXPERIMENTAL ANALYSIS OF CRACK PROPAGATION IN GRANITE

An experimental approach is used to demonstrate the concept of an effective crack length with double-edge-notched specimens of charcoal and rockville granite. Crack propagation in rock is characterized by microcracking around the crack tip and interlocking along a portion of the crack; this region is called the fracture process zone, and together with the traction free length defines the effective crack length. Twenty closed-loop, strain-controlled fracture tests were conducted on charcoal and rockville granite. Crack growth was monitored with a travelling optical microscope (100 x magnification). By comparing the post-peak behaviour of both granites, it is suggested that the process zone is larger for the larger grain-sized rock (rockville). Conventional linear elastic fracture mechanics techniques are used to calculate the apparent fracture toughness of charcoal granite at various crack lengths. In addition, a j-integral expression is derived for the double-edged-notched geometry in terms of the area under the load-displacement record. The shortcomings of both analyses are discussed. An attempt is made to explain the inadequacies by including the process zone in the calculations of the fracture toughness. (Author/TRRL)

JOINT ASPERITY DEGRADATION DURING CYCLIC SHEAR

Abstract Some 30 real rock granite and limestone joints, sawn with a new numerical control technique, were cyclically sheared to measure asperity degradation under normal stresses and displacements consistent with earthquake loading of underground caverns at depths of 50–300 m. Monotonic shearing resistance and normal dilation of the sawn joints was similar to that for large or field scale joints. Asperity degradation under these conditions was found to be a function of work, joint roughness, normal stress and unconfined compressive strength of the joint wall.

The fracture process zone in granite: evidence and effect

Abstract Evidence of the fracture process zone and its effect on fracture toughness were examined for Charcoal and Rockville granite, average grain sizes of 1 and 10mm. Fracture tests were conducted on wedge-loaded, double cantilever beam specimens, 500 mm long × 200 mm wide × 40 mm thick . The seismic techniques of ultrasonic probing and acoustic emission were utilized to measure the inelastic region. The beginning of the process zone was interpreted by a three-fold increase in transmission of ultrasound from an open crack to a partially closed crack; the end of the damage zone was found by comparing surface wave amplitudes before and after fracture. Following these procedures, the inelastic region in Charcoal granite was about 40 mm; whereas in Rockville granite, a region over 90 mm long was estimated. The major acoustic activity was located within the macrocrack, being released from unbroken or interlocked crystals. The experimental evidence indicated an effective crack to be composed of a traction free length and a ligament process zone, which was observed to form a single, multiconnected region within the macrocrack. The effective crack was used for an R-curve calculation, so that the energy consumed in the process zone was included in an approximate manner by elongating the crack length. A model of crack propagation in rock, consistent with measurements of the inelastic region, describes the effect of a process zone on fracture toughness testing.

Suggested method for blast vibration monitoring

This guideline is separated into three main sections. The first section, Character of Blast Excitation, defines the terminology necessary to describe blasting vibrations and the associated air over pressure. Importance of dominant frequencies of excitation and structural response is introduced here and is emphasized throughout the document. The second section, Measurement Techniques and Instrumentation, describes generic attributes of instruments necessary to measure time histories of the blast- induced disturbances. Special emphasis is placed on computerized systems. Guidance is given for the choice and deployment of instruments, both at the beginning and continuation of a project. The third section, Evaluation of Measurements, presents definitions of structural response. Explanation is given for the need of studies with immediate pre- and post-blast inspection to separate weather- and blast-induced response. Guidance is also given for monitoring response of rock masses and buried structures. (A)

Measurement of rock mass deformation with grouted coaxial antenna cables

SummaryTechniques presented herein show how reflected voltage pulses from coaxial antenna cable grouted in rock masses can be employed to quantify the type and magnitude of rock mass deformation. This measurement is similar to that obtained from a combined full profile extensometer (to measure local extension) and inclinometer (to measure local shearing). Rock mass movements deform the grouted cable, which locally changes cable capacitance and thereby the reflected wave form of the voltage pulse. Thus, by monitoring changes in these reflection signatures, it is possible to monitor rock mass deformation.This paper presents laboratory measurements necessary to quantitatively interpret the reflected voltage signatures. Cables were sheared and extended to correlate measured cable deformation with reflected voltage signals. Laboratory testing included development of grout mixtures with optimum properties for field installation and performance of a TDR (Time Domain Reflectometry) monitoring system. Finally, the interpretive techniques developed through laboratory measurements were applied to previously collected field data to extract hitherto unrealized information.

Principles of time domain reflectometometry applied to measurement of rock mass deformation

Abstract Time Domain Reflectometry (TDR) is an electrical pulse testing technique originally developed to locate faults in coaxial power transmission cables. Recently, this technique has been adapted for monitoring deformation of cables grouted into rock masses. Rock mass movements deform the grouted cable, which locally changes cable capacitance and thereby the reflected wave form of the voltage pulse. By monitoring changes in these reflection signatures, it is possible to monitor both local extension and local shearing. This paper concentrates on the electromagentic wave theory necessary to quantitatively relate changes in cable geometry to changes in reflected voltage signatures. A finite element model is employed to numerically simulate capacitance changes for deformed geometries produced in the laboratory, and the effect of signal attentuation and resolution of two deformities is assessed on the basis of laboratory test results. Finally, these models are employed to extract heretofore unrealized information from previously from previously collected field data.

Lucid dreaming: reliable analog event detection for energy-constrained applications

Existing sensor network architectures are based on the assumption that data will be polled. Therefore, they are not adequate for long-term battery-powered use in applications that must sense or react to events that occur at unpredictable times. In response, and motivated by a structural autonomous crack monitoring (ACM) application from civil engineering that requires bursts of high resolution sampling in response to aperiodic vibrations in buildings and bridges, we have designed, implemented, and evaluated lucid dreaming, a hardware-software technique to dramatically decrease sensor node power consumption in this and other event- driven sensing applications. This work makes the following main contributions: (1) we have identified the key mismatches between existing, polling-based, sensor network architectures and event-driven applications; (2) we have proposed a hardware-software technique to permit the power-efficient use of sensor networks in event-driven applications; (3) we have analytically characterized the situations in which the proposed technique is appropriate; and (4) we have designed, implemented, and tested a hardware-software solution for standard Crossbow motes that embodies the proposed technique. In the building and bridge structural integrity monitoring application, the proposed technique achieves 1/245 the power consumption of existing sensor network architectures, thereby dramatically increasing battery lifespan or permitting operation based on energy scavenging. We believe that the proposed technique will yield similar benefits in a wide range of applications. Printed circuit board specification files permitting reproduction of the current implementation are available for free use in research and education.

Water pressure measurement with time domain reflectometry cables

Attributes of the remote measurement of piezometric water pressure and water level with time domain reflectometry (TDR) techniques are investigated. The evaluation includes a two-wire system for unusually small riser tubes as well as parallel wire and air-filled or hollow coaxial cable for typical installations. Comparison of TDR cable and commercial pressure transducer technology shows coaxial cable TDR systems with distance crimps capable of equaling and exceeding the resolution of pressure transducers. Crimps provide permanent distance markers that allow the TDR system to be self-calibrating.

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

Parallel processing for a discrete element program

Abstract Reconfiguration of a discrete element code for parallel operation provided the opportunity to compare processing speeds on various hardware platforms. The program employed in this comparison, NURBM3DP, is a three dimensional, distinct element code employed to calculate dynamic response of a cavern in a jointed rock mass. On a 16 processor IBM SP2, it is capable of calculating dynamic response with 1000s of explicit time steps of jointed rock masses with up to 2,000,000 blocks. Comparison of single instruction multiple data stream (SIMD) and multiple-instruction multiple-data stream (MIMD) operation showed MIMD processing to provide the best overall parallelization. The full report of the comparisons of operation on different hardware with different data streaming configurations can be found at the research section of the Northwestern University Computational Mechanics site: http://www.tam.nwu.edu/compmech.html. In addition, a color movie of dynamic response of a million block model of a cavern responding to dynamic excitation can be seen at: http://geotech.civen.okstate.edu/ejge/ppr9801/index.htm.