YieldPoint’s unique 1 to 6-Point Multiple Rod Extensometer (d-EXTO) combines a traditional Multi-rod design and digital signal processing to result in dramatically improved accuracy and reliability compared to similar priced existing technology. This high precision digital instrument comprises up to six (6) Linear Variable Induction Transducers(LVIT) and an on-board digital temperature sensor.

Design features of the d-EXTO include a tough fiber glass tube, stronger and durable than polyethylene cover; An internal rubber spacer ensures that central core which will not be damaged by shear movements of up to 10mm; hermetically sealed electronics that operate indefinitely when submerged in water and robust torsionally straightened stainless steel rods.

The inherently digital nature of the signals eliminates the necessity for expensive analog-to-digital conversion and results in low cost readout unit that reads data directly in real world units (mm and oC). The sensor output is an ASCII (9600,8,N,1) digital signal which can be read by a low cost readout unit (d-READER), dataloggers (d-LOGGER), and wirelessly networked (dMESH and ViewPoint via BluLink) for transmission to cloud based servers (DataPoint and VantagePoint. The signals themselves are robust and can be transmitted over 1000ft of lead-wire. If broken the lead-wire can be twisted and taped together.

  • `Super-Tough` design with high strength filament wound fiber glass tube
  • Multiple Stainless Steel rods hexagonally packed and maintained straight in fiber glass tube for maximum accuracy.
  • 1-6 anchor points
  • 150mm (6 inch) stroke length
  • High individual sensor accuracy (0.3%FS) and resolution (0.01mm FS)
  • Output in real world units
  • Unique instrument ID
  • Calibration Coeffs. in Flash memory
  • Smallest electronics head (25mm diameter 150mm long)
  • On-board digital temperature sensor for accurate compensation
  • High survivability following blasts and vibration even if toe is damaged
  • Easy to install and maintain—Arrives on site fully assembled
  • Key Feature: Fully integrated and preassembled borehole extensometer
  • Dimensions: 25mm diameter can be placed in 50mm hole.
  • Dimensions: Up to 6 Anchor points over lengths up to 30m
  • Disp. Range: 125, 150 (standard), 200 and 250mm.
  • Disp. Accuracy: typically 0.5% F.S(<10m) and 1% linearity (>10m). Depends on distance between anchor point and head.
  • Temp. range: -40 to 125 oC
  • Output Signal: RS485 with transmission up to 500m over 2 x tp.
  • Output Signal: Asynchronous serial signal - 9600,8,N,1. ASCII encoded. Values and units transmitted.
Datalogging and Telemetry:

Data from the d-EXTO can be collected using YieldPoint’s d-LOGGER dataloggers (d1LOGGER and d4LOGGER). The data-loggers require no configuration and are fully interchangeable with any other type of YieldPoint instrument (d-CABLE borehole extensometers, d-GMM, d-TILT, d-PLUCKER etc). Therefore arrays of instruments to monitor cable load and ground movement can easily be combined.

Clusters of instruments (4 per Logger) that are monitored using YieldPoint’s d4LOGGER can be wirelessly networked using dMESH – a 900MHz low-cost Zigbee wireless mesh solution. Each low-cost wireless mesh (up to 20 modems) can be interfaced to a TCP/IP or WiFi network. dEXTO can be configured with BluLink and read using ViewPoint Android App. Wireless datalogging using BluLogger and BluLog app.

Getting started with Borehole Extensometers

This presentation outlines the essential information required to start using borehole extensometers. Firstly, the basic principles of displacement and strain measurement are introduced. Then different designs of borehole extensometer are described. The displacement sensing technology is briefly discussed. Then a section is presented on the installation best practices, including a discussion of grouts. The toe grouting and breather tube method are discussed.

Case study: Pillar monitoring at 8000ft

As part of CEMI’s UDMN project YieldPoint has been working with Vale Canada’s to monitor a pillar in a hard-rock nickel mine located 8000ft underground in Sudbury, Ontario. A dense deployment of instruments was installed, including Borehole Pressure Cells and optical extensometers in addition to d-EXTO’s. By installing a dense deployment of borehole extensometers the deformation pattern in the pillar could be determined in considerable detail. By interpolating and contouring between instruments, Brad Forbes was able to provide the mine with a frame-by-frame playback of progressive deformation as the mining sequence evolved. A shear zone transecting the instrument array had a dramatic effect of the spatial distribution of deformation. The project team has recently submitted an abstract to the upcoming First International Conference on Underground Mining Technology to be held in Sudbury (Punkkinen et al, 2017). One conclusion is:

"Contouring between adjacent multi-point borehole extensometer measurements has provided an excellent depiction of the pillar behavior as development, using the current face destress procedure, approached a known shear zone."

Paper presented at International course on geotechnical and Structural Monitoring.

T. Weinmann and R. Drefus Geocomp Corp

In 2014 YieldPoint Inc (ON, CA). and Geocomp Corp of Acton (MA, USA) embarked on a project to evaluate the potential for using YieldPoint Borehole extensometers for settlement monitoring above metro tunnels in cities. The process involved intensive laboratory testing of the extensometer to confirm specifications (product Evaluation). This was followed by drilling holes at Geocomp (Prototype evaluation). Instruments were then used for a tunneling project in Los Angeles California (Project Implementation) and as part of this a comparison with Geokon Borehole Extensometers was performed (verification Comparison).

The key conclusions were:

  • Met accuracy requirements
  • Passed laboratory evaluation
  • Anchor design for soft grout performed well
  • Passed Prototype performance testing
  • Project Implementation (as expected)
    • Easy Installation
    • Saved approximately 40% in costs
  • Passed Prototype performance testing
  • Project Implementation (as expected)
  • Passed comparison testing