monitoring axial load on rock bolts and roof bolts in … to u-cell.pdf · 2018. 4. 9. · 32...

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Monitoring Axial Load on Rock Bolts and Roof Bolts in Underground Mines

Hani Mitri

McGill UniversityMontreal, Canada

[email protected]

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Outline

Introduction Instrumented Rock Bolt (SAFEBOLT) Instrumented Coupler Load Cell (U-CELL) Data Acquisition Field Installation Programs Conclusions

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INTRODUCTION

The need to measure rockbolt load with instrumentation methods has been recognized by many researchers.

Other measurement techniques were successfully developed and became commercially available, e.g. flat jacks for pressure changes, extensometers for deformations, etc.

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Mining and civil engineering projects Underground openings Slopes and embankments Foundations

INTRODUCTION – MONITORING APPLICATIONS

Long-term monitoring Short-term monitoring Occasional monitoring (e.g., after a seismic event)

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Chamber

Shaft

Drift

INTRODUCTION – MONITORING APPLICATIONSIN MINING

Stope

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INTRODUCTION – CONVENTIONAL LOAD CELL

One of such products is the vibrating wire hollowload cell technology.

Rockbolt

Instrumentation Cable

Hollow Load cellFace plate

Reaction plate

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The vibrating wire strain gauge often breaks prematurelyas it reaches its limit of 2500 to 3000 μɛ, which is oftennot enough to ensure the measurement of the bolt yieldload.

The face and reaction plates must be placedperpendicular to the bolt to capture the correct force,which is not always possible in mining applications.

Must be parallel


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The hollow load cell reduces the headroom of the gateroad by 15 to 20 cm.

Rock surface preparation is required to make sure thatthe plates are parallel.


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INSTRUMENTED ROCKBOLT

Stress And Force Evaluator in Bolts (SAFEBOLT)

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INSTRUMENTED ROCKBOLT - SAFEBOLT

◦ Design concept: 1998◦ First prototype: 1999◦ Underground test: 2000

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Sensor is located past thethreaded portion of the bolt head.


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Mechanical Rebar


Forged head dowel

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Protective cap

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Grouted rebar

b)

Partially grouted rebar

d)

Mechanical rockbolt

a)

Axial load distribution in rock supports


Thus, the maximum axial load occurs at the head of the rockbolt, where the sensor is located.

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Will install on any industry standard rockbolt or rebar

No rock surface preparation is required

Not sensitive to bolt orientation with respect to the rock surface

Measures largest axial load along the bolt in most applications

INSTRUMENTED ROCKBOLT – SAFEBOLT Features

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D

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

P/

P (%

)

0

3

6

9

12

15

18

21

24

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D

4% for 1/8” hole in 5/8” bolt

% reduction of bolt capacity due to borehole


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1. Borehole reduces the strength of the bolt

2. Does not measure the bolt ultimate strength

3. The bolt must be shipped back to the mine after

being instrumented, resulting in added cost.

INSTRUMENTED ROCKBOLT – SAFEBOLT Limitations

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INSTRUMENTED COUPLER LOAD CELL (U-CELL TECHNOLOGY)

Universal Load Cell (U-Cell) – 2005

Universal load cell or U-cell is a load-cell concept to monitor theaxial load on the bolt to its ultimate strength.

Because of its versatile design, U-cell is custom made to fit any sizeof rock anchor (mechanical or rebar).

Acting like a coupler, installation is straightforward. Simply screwthe threaded rock bolt head into the U-cell then install.

A shear-pin nut or a jam nut, or a cap nut allow for bolt spinning forresin applications.

US Provisional Patent – January 10, 2011 US Patent Application - US 13/347,567 filed January 10, 2012 Canadian Patent Application - CA 2763910 filed January 10, 2012

INSTRUMENTED COUPLER LOAD CELL U-CELL TECHNOLOGY

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U-CELL TECHNOLOGY

Hex cap to spin the bolt

Hex nut to tighten the bolt

Embedded strain gauge

Threaded hole to fit the bolt

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U-CELL TECHNOLOGY

Fitting ¾ inch rebar

U78‐Cell for 7/8 inch Mansour resin grouted rockbolt

Hex cap has two functions: 1) to spin the rockbolt to mix the resin in the drill hole, and 2) to protect the U‐cell connector.

1 7/8 inch Hex nut is used to tighten the bolt at the end of the installation.

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U-Cell Model DesignsModel Bolt diameter Bolt type Thread

U58 5/8 inch Mechanical (M)orResin (R)

Left Hand (LH) or Right Hand (RH)

U34 ¾ inch

U78 7/8 inch

Example: U34R-RH

Model Bolt diameter Bolt type Thread

U18 18 mm Mechanical (M)orResin (R)

Left Hand (LH) or Right Hand (RH)

U20 20 mm

U22 22 mm

Example: U20R-RH

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Based on the Wheatstone Bridge circuit

1. Portable strain gauge reader

2. Data logger and internet-connected data logger

3. Wireless transmission of data (In-Progress)

DATA ACQUISITION

In all of the above methods, an instrumentation cable is used to connect the U-cell coupler to either the portable readout or the data logger

DATA ACQUISITION - Portable

• Manual readings• Good for heavy traffic zones• Reader is kept from damage

• Can monitor up to four U-Cells• Monitoring frequency is determined by the user• Battery is good for at least one month

DATA ACQUISITION – Data Logger

Typical calibration test results in the lab

Load Cell Calibration

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Case StudyBirchtree Mine, Thompson, MB

1080 FaultMined & Backfilled

Stopes

609 Fault

Crusher Room4100 Level

Issue: Signs of deterioration in the crusher room were observed during the development of 4000L haulage drift.

4000 Level

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C

U34‐Cell for ¾ inch resin grouted rockbolt

1 7/8 inch hex nut is used to tighten the bolt at the end of the installation. Nut size can be changed to suit available dolley at the mine site

1 1/8 inch square cap has two functions: 1) to spin the rockbolt to mix the resin in the drill hole, and 2) to protect the U‐cell connector. The cap shape and size can be changed to suit available dolley at the mine site.

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Dolly for large hex nut

Demonstration of a U-Cell attached to a rebar

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U-Cell ID Bolt Type Location Axial Load (tons) Temp

110525022 8ft rebar 4000L_904 right wall 3.35 19o

110525023 8ft rebar 4000L_904 back 4.65 21o

110525024 8ft rebar 4000L_904 left wall 3.12 20o

110525021 8ft rebar 4000L_948 right wall 3.57 21o

110525025 8ft rebar 4000L_948 back 3.23 21o

110525015 8ft rebar 4000L_948 left wall 3.42 20o

110525012 8ft rebar 4050 Crusher Ramp right wall 2.17 20o

110525013 16ft ext bolt 4050 Crusher Ramp back 1.01 19o

110525014 8ft rebar 4050 Crusher Ramp left wall 1.53 20o

CASE STUDY

Note: Initial pretension loads vary from 1 to 4.65 tons.

Simple to install and easy to use. Light weight – easy to ship and handle. Designed to fit any rock support. Reliable for long term applications. Stronger than rock bolt itself – will not yield. Will monitor the bolt load until failure. Unlike Safebolt, there is no need to ship the bolt back and forth. Helps decide the timing and/or need for secondary support.

INSTRUMENTED COUPLER LOAD CELL –U-Cell Features

monitoring axial load on rock bolts and roof bolts in … to u-cell.pdf · 2018. 4. 9. · 32...

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