New Rock Fracture Dynamics LabAims to Revolutionize Researchin Engineering Geoscience

The Lassonde Institute at the University of Toronto is now home to the new Rock Fracture Dynamics Laboratory (RFDL). Over 5 years since pen was put to grant application paper, the first experiment in the RFDL has been per-formed, and models are being run on the com-puter cluster. The RFDL is an integrated facility for performing analyzing and visualizing rock fracture experiments, producing high resolution experimental models on the computer cluster, and validating models through numerical and measured result comparison.

The project is headed by Professor Paul Young of the Department of Civil Engineering. The RFDL will boost research partnerships with local Departments at the University including Geology and Physics, and will also be a strong catalyst for collaboration with the top interna-tional researchers in the fields of rock mechanics and geophysics. As well as fundamental research,

the facility will help produce highly trained and qualified people that will be of interest to com-panies in the Canadian mining industry, petro-leum engineering field, and the geophysics and geotechnical sectors. This all fits into the goals of the University and Lassonde Institute which are to foster interdisciplinary research, and increase the potential for innovation and competitiveness in geoscience research. The RFDL was made possible through funding from the Canadian Foundation for Innovation (CFI), Ontario In-novation Trust, and Keck Foundation, as well as industry contributions including MTS Systems Corporation and Dell Canada Inc.

Lassonde InstituteInnovation in Engineering Geoscience

University of Toronto

The RFDL is an integrated rock testing facility with geophysical monitoring, real time results visualization, and numerical modelling capabilities. The figure above shows the overall components and concepts of the facility. The equipment resources and capabilities can be summarized as:

A polyaxial servo-controlled rock deformation system allowing 6800kN axial and 3400kN lateral loading

Polyaxial (true triaxial) and triaxial geophysical imaging cells

Temperature control up to 200ºC; stress and strain recording

Full waveform continuous Acoustic Emission (AE) monitoring

3D velocity measurement system

Pore pressure control and permeability measurement system

Resistivity measurement system

A 256 processor (64 node) parallel computer cluster for modelling and real-time data analysis

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Why Perform Laboratory andModelling Experiments?

There are only a handful of true triaxial rock testing facilities in the world, and the RFDL is the only one for performing true triaxial experi-ments at high load, with the large range of inte-grated sensors and measurement systems men-tioned above. The combined measurements are essential for a complete understanding of what is happening to the sample, for accurate model validation, and for helping the user upscale and predict larger field scale problems.

The combination of controlled experiments and numerical simulation will allow the RFDL to contribute to the following areas of innovative scientific and important industry research.

Investigation of brittle failureunder compression

Understand how different rock types fail under different three-dimensional compressive stress regimes. Go beyond the previous limitations of two independent stresses or low stress level laboratory testing. Use AE measurements and numerical modelling to spatially and temporally visualize the failure process and understand the source mechanics. Prove or disprove some of the existing hypotheses relating to the failure process in the pre- and post-peak regime.

Coupled thermal, hydraulic andmechanical (T-H-M) processes

Improve the understanding of the complex interaction between thermal, hydraulic and 3D mechanical stresses and their effect on rock damage. Tackle problems related to the design of nuclear waste disposal facilities, secondary recovery of hydrocarbons, hot dry rock energy extraction and basic geological processes such as igneous intrusion.

Earthquake mechanics and triggering

Examine fracture formation and propagation under well controlled conditions with adequate sensor coverage, and the ability to directly exam-ine the damaged samples after failure. Investi-gate how temperature and pore pressure condi-tions affect fracture stability, as well as how static stress changes from complex fault networks can ‘trigger’ instabilities. As well as earthquake research, this field can contribute towards rock-burst research and mine safety.

Investigation of long-term strengthof brittle rock

Investigate the fundamentals of time dependent rock behaviour, creep and stress corrosion under different thermal, mechanical, hydraulic and chemical conditions. Improve numerical models such that long term predictions can be made for critical structures and in areas such as nuclear waste disposal.

Development of new techniques formodelling and monitoring rock fracture

Use the continual improvements in computing and electronics, to push forward the capabili-ties and resolution of numerical modelling and experimental monitoring.

The experimental setup includes a custom polyaxial servo-controlled rock deformation system with integral permeability measurement and geophysical imaging capability. For the first time, it is possible to carry out thermome-chanical, geophysical, and hydrological mea-surements, essentially simultaneously, on rock specimens ranging from granite to mudstones, undergoing strains well into the post-failure regime of brittle rocks. The range of the polyax-ial loading machine and geophysical imaging cell covers realistic 3D engineering stresses and

problems to depths of over 4 km at temperatures up to 200C. The use of 8 cm cubical samples loaded between computer controlled servo-hydraulic rams, provides the high degree of op-erational flexibility that is needed for the range of instrumentation, including the monitoring of permeability, seismic velocity, resistivity and AE in 3D. The experimental and measurement capability can be summarized as:

Up to 1063MPa vertical stress, 532MPa lateral stresses

18 AE sensors (dual mode receiver and pulser)

Continuous AE waveform recording and velocity surveys using AE sensors

Dedicated P, S1, S2 velocity measurements in three axes

Resistivity measurements in three axes

Permeability measurements in three axes

Pore pressure control

200ºC temperature control

Stress strain measurement

As well as polyaxial experiments on cubic rock specimens, the facility can perform triaxial experiments (confining pressure up to 104MPa) on cylindrical (5cm diameter, 12.5 cm long) rock specimens including all of the measure-ments indicated above, apart from resistivity. A photo and schematic are shown of the geophysi-cal imaging cell for polyaxial experiments. The photo on the first page is the polyaxial loading machine and acquisition equipment.

Unique ExperimentalSetup

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Photo and schematic on this page: Copyright Ergotech Ltd.

Parallel Computer Cluster

A High-Performance Computing Cluster (HPCC) has been installed consisting of 64 quad-core 64-bit processors and 4-8GB RAM per processor. With a total of 256 processing cores, the cluster has a theoretical peak through-put of a massive 2.7 Teraflops (2.7 trillion float-ing point operations per second). The 64 server nodes and 18.9 terabytes of disk storage (using high-speed 15,000rpm SAS disks) are con-nected by dual gigabit networking allowing for lightning fast data transfer and communication between nodes. Two independent master nodes allow flexibility for the HPCC to be booted completely in the Linux or Windows Cluster O/S, or a combination of both by splitting the nodes into two clusters.

The HPCC will allow much higher resolution discrete element models to be produced than has been possible to date. Additionally the HPCC is being used for near real-time analysis of continu-ous multichannel AE experimental data.

This means it has had to meet the specification of being able to transfer and write 360MB/second of data coming from the streaming ac-quisition computers, and at the same time pass this data to the processing nodes for time and frequency analysis. Data and processed results are displayed on six 46 inch LCD visualization screens, for immediate analysis and discussion. The photos are of the HPCC and visualization room.

Continuous AE Monitoring of Rock Fracture

One limitation of AE monitoring of laboratory rock tests in the past, has been the fact that trig-gered full waveform systems have a certain satu-ration point (generally around 10-20 AE events/second). During critical failure stages, 100’s of AE events/second can occur, and this means that triggered monitoring systems will miss data.

Professor Paul Young’s research group along with collaboration from custom hardware and software companies, has been pioneering con-tinuous AE monitoring and analysis techniques. The RFDL takes advantage of the latest advances in electronics and data storage, and boasts a sys-tem that can monitor AE sensors continuously at 10 million samples per second for periods of hours. This will allow researchers to examine all of the AE information emitted, and draw firm conclusions about the nature of crack coales-cence leading up to failure and into the post peak domain. Continuous AE waveform record-

ing also means that data can be analyzed in the frequency domain. The study of spectral ampli-tude and phase changes as precursors to failure, is an area of future research.

The figure above presents results from a triaxial test on Westerly granite (Young and Thomp-son, 2007). The AE events are harvested from continuously monitored AE sensors, and the po-sitions of AE show the response of the naturally fractured rock to further confinement and axial stress. The AE presented are those occurring in a narrow band relative to the X-ray computer tomography (CT) image shown. The granite sample following the experiment was imaged at the High Resolution CT Facility at the Univer-sity of Texas at Austin. The dense clusters of AE, interpreted to be due to failure of asperities on the fault, occur in structurally complex regions including primary and secondary fault intersec-tion.

A laboratory experiment of stick slip behaviour on a naturally faulted rock sample, that provides an analog to a large scale fault zone. Each red dot is an acoustic emission (AE) represent-ing microcracking in the rock sample. The 3 dense regions of AE Activity are interpreted as due to asperities or locked regions on the fault. The x-ray computed tomography (CT) images show that the asperities are related to areas of structural complexity such as primary and secondary fault intersec-tion (Young and Thompson, 2007).

Examples of Modelling Rock Behaviour

More and more modeling codes are being converted to parallel versions to make use of the power of computing clusters. An example is Itasca’s Adaptive Continuum/ DisContinuum (AC/DC) Code, based on PFC3D and FLAC. The original version of AC/DC was developed as part of the SAFETI project (funded by the EU Euratom grant from 2001-2004 with Profes-sor Paul Young PI) and used on the University of Liverpool UK computer cluster. Discrete element, particle type models such as PFC3D have significant advantages over other model-ing methods. Positions of microcracks do not have to be assumed and added into the model

before hand, but instead are interpreted to occur at any of the particle bonds where tensile or shear strengths are exceeded. In general, by adding more particles to a model, the more realistic the model will be in representing the true mineral structure and micro-behaviour of brittle materials. AC/DC models running on

the new HPCC at the University of Toronto are predicted to reach 40 Million particles, which is a huge increase over the models from the SAFETI project which replicated rock with up to 2.2 Million particles.

The figure above is an example of a 48000 par-ticle PFC3D model of a granitic cube undergo-ing thermal treatment. The red discs mark the position of microcracking and are found to occur mainly between mineral clusters as seen in the 2D slice through the model. This trend, re-lated to differential thermal expansion, leads to a significant loss of sample stiffness and strength.

A model of granitic rock using PFC3D, where three mineral types are represented by clusters of spherical particles. The example shows the location and orientation of microcracking (red discs) that has occurred during a simulated heating test.

The AC/DC concept involves taking these types of cubic ‘Models of Rock’ (aka particle pbricks), and replicating them, allowing large models to be built up (Young et al 2005). As discussed, microcracking can be interpreted in the particle pbricks. Two more computationally efficient types of pbricks (matrix and degenerate ma-trix) are used to build up the outer portion of a model, to ensure the correct boundary and stress conditions. Adaptive logic allows matrix pbricks to convert to particle pbricks based on the sur-rounding stress levels. The figure to the right provides an example of how a large scale model can be built up using AC/DC.

Source mechanisms and magnitudes can be in-terpreted in PFC3D using the change in contact force that occurs when particle bonds break. These model results can be directly compared to moment tensors and magnitudes from real experiments. An important model analysis step is to consider bond breakages that occur closely in space and time to be part of the same seis-mic event. In this way larger magnitude events are interpreted, resulting in a realistic range in magnitudes (and source radii etc), as is seen in actual laboratory rock experiments and at larger scales in nature.

The figure to the left below presents an example of a model of granite with a central thermal source, and the positions of 1081 thermally induced bond breakages in red and blue. Figure to the right is the same model but showing the source mechanisms and magnitudes of the in-terpreted 119 AE seismic events. The combined information of the damage location, magnitude, and orientation of the forces, as well as the time sequence, can be used to successfully interpret the spatial and temporal micromechanics lead-ing up to failure in the modeled sample.

An example of an AC/DC model, made up of the three types of pbricks (Young et al 2005). Following the model setup, an excavation can be made and 3D stresses applied.

A 45000 particle PFC model of a thermal experiment. In (a) the bond breakages are marked in red and blue as a result of the central thermal source. In (b) the acoustic emissions (AE) are interpreted using the bond breakages and clustering criteria. Each AE is marked by a sphere scaled to magnitude and by arrows showing the direction and magnitudes of the forces acting at the source.

Photo of the main experimental equipment, as well as people currently involved in the testing and use of the facility (from left to right: Phil Benson, Winnie Ying, Paul Young, Dave Collins, Farzine Nasseri, Ben Thompson, and Tatyana Katsaga. Missing Toivo Wanne and Xueping Zhao).

SummaryThe new Rock Fracture Dynamics Laboratory at the University of Toronto is now operational. This is the only facility in the world for testing rock samples under true triaxial stresses, with control of pore pressure, and temperature, and the ability to make integrated measurements in-cluding continuous acoustic emission recording, acoustic velocity and permeability. The facility will allow constitutive relationships for rock to be developed under a range of simple to more complicated thermal-hydrological-mechanical

conditions. One aim of the 256 processor computer cluster is to produce exact models of experiments being performed. The stage of model parameter revision and model validation is often inadequately performed due to a lack of real measured data. The RFDL is helping to fill this gap, providing greater confidence in the use of models for the prediction of short and long term rock mass stability.

Further information can be found by contacting Prof. Paul Young or Dr. David Collins directly, or by going to www.lassondeinstitute.utoronto.ca/young.

ReferencesYoung RP and Thompson BD, 2007. Imaging Dynamic Rock Fracture with Acoustic Emission and X-ray Tomography, In Proceedings of 11th Congress of the International Society for Rock Mechanics, Lisbon, July 9-13.

Young RP, Collins DS, Hazzard J, Heath A, Pettitt WS, Baker C, Billaux D, Cundall P, Potyondy D, Dedecker F, Svemar C, and Lebon P, 2005. Seismic Validation of 3-D Thermo-Mechanical Models for the Prediction of the Rock Damage around Radioactive Waste Packages in Geological Repositories - SAFETI,http://cordis.europa.eu/fp5-euratom/src/lib_finalreports.htm#fuel, pp46.

Article prepared for RocNews - Fall 2007