YieldPoint’s d-Rebar v2 measures how the strain and load develops along rigid bolts, such as resin rebar, and hence to assess the Factor of Safety against steel rupture. The technology is applicable to rebar elements, threaded bolts, and a variety of other steel elements used in mining, civil and construction projects. It has the potential to predict the risk of catastrophic structural failure: in fact the d-Rebar data can enhance many aspects of engineering design.
The physical instrumentation of the d-Rebar is recessed into a 3.2mm wide slot which is milled into the steel bolt. Typically the instrumentation consists of up to six (6) wires run as 2 pairs (side A and side B) in diametrically opposed grooves. The translation of the six wires is measured by 6 displacement sensors at the head of the device. As the cable stretches each displacement sensor measures the relative displacement between the head and the nodal point (u1, u2, ….u6). The wires run in tubes which are encapsulated in polyurethane and are designed to withstand the rigors of bolt installation using standard procedure.
The d-Rebar v2 is based on 2 x diametrically opposed (Side A and Side B) miniature borehole extensometers recessed into 3.2mm grooves milled into the bolt. the total of six wires are fixed to the bolt at nodal locations x1,x2……x6. Nodes 1 is always closest to the head. Node 1, 2 and 3 compreise Side A,and 4,5 and 6 Side B. The translation of the six wires is measured by 6 displacement sensors at the head of the device. As the cable stretches each displacement sensor measures the relative displacement between the head and the nodal point (u1, u2 ….u6). The difference between adjacent nodal points can be used to calculate the strain from which the corresponding load can be calculated.
Stacked nodal configuration
Staggered nodal configuration
For applications which require enhanced axial load resolution the strain gauge array should be staggered (lower fig). If bending moments need to be more accurately resolved then a stacked configuration may be preferable ( upper fig).
The specification of the node locations should be based on an understanding of the load distribution along a fully grouted rock bolt. At the collar of the borehole the load depends on the stiffness (i.e. bearing conditions) of the faceplate. Along the length of the bolt the load increases in the bolt along the “pick-up” length and reaches a peak at the neutral point. The load decreases along the remainder of the bolt length, the “anchor length”, and is zero at the free end. In the case of a stiff bearing surface for the plate and a rapid decrease in the displacement magnitude around the excavation then the neutral point may actually occur at the faceplate.
Every instrument is subjected to a multipoint calibration. The calibration coefficients are written into microcontroller memory, and a piecewise linear function is used to enhance linearity the output signal and applied strain.
The d-Rebar v2 has been specifically designed to be spun in epoxy resin using a rock drill. It may be necessary to modify an installation dolly to accept the head of the instrument.
Installing instrumentation with a rock drill can be challenging even for the toughest product. Care must be taken to ensure that the drill is well aligned with the borehole.
- Instrumented rebar that can be spun in resin
Available with 6 nodal points: 3 in two diametrically opposed slotted channels.
- Readout head encapsulated in stainless steel tube for protection.
- Microcontroller provides output directly in mm of stretch
- Each bolt is individually calibrated.
- Immunity to hostile environment
- High survivability to shock and vibration
- Easy to install and maintain
- Low cost peripheral devices for data-logging (BluLink or BluLogger) and data upload using LTE-M. WiFi, or Ethernet (BluGateway)
- Wireless mesh using 900MHz 1for1 radio.
Data download using YieldPoint’s low cost d-Reader readout unit or any Android device using the BluPoint product-line. Both provide the temperature and displacement data directly in °C and mm.
Data from the d-Rebar v2 can be collected using YieldPoint’s BluLink (single instrument) or BluLogger dataloggers. The BluLink and BluLogger and BluGateway also provide datalogging, and BT5 download.
Automated data backhaul:
Clusters of instruments (4 per Logger) that are monitored using YieldPoint’s BluGateway (BT5 beacons only) or d4Blugateway (RS485 or BT5 beacons) can be wireless networked using:
- d-Mesh 1for1 radios: a 900MHz wireless mesh solution,
- BluGateway WiFi if either a WiFi, or Ethernet network is available,
- cellular ( BluGateway LTE) if a cellular network is available
Case Study 1: Monitoring Rock Bolt Loads at an Intersection
As part of the SIUC project outlined above a number of intersections were also monitored. In one particular intersection the bolt were loaded minimally in the 3 months following excavation. However on July 10 th a dramatic increase in microstrain was detected for most bolts ia the intersection. The occurence of the unexpected movement was recorded by a d4logger. Some bolts loaed up most close to the head, others in the middle of the bolt and some towards the toe. Only when the distribution of strain and displacement were contoured across the array could the existence of an inclined structure be inferred. The displacement plot suggests that an interaction of this incled structure with a “cutter” observed in the haunch defined a wedge than needed to be supported by the bolt array.
Temporal plot of bolt and spatial plot for bolt 100555074
Strain in microstrain (Top) and Disp in microns (Bottom) measured by d-Rebar
Case Study 2: Monitoring rock bolt loading at mid-pillar
YieldPoint worked with reasearchers at Southern Illinois University at Carbondale (Prof. Sam Spearing) and NIOSH ( a branch of the CDC (US Center for Disease Control) to develop an innovative instrumented rebar. This technology based on “long baselength” strain gauges was used to monitor bolt loads at several US coal mines operated by Peabody Energy. The minining involved “the room and pillar” mining method. Full results of the study were published in a paper in the IJRMMS. Data were continuously monitored using the d4Logger.
The mining sequence.
Displacement measured by d-Rebar in microns