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SAFETI Project

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This study relates to results from work performed within the European Commission (EC) Euratom research project SAFETI.  Within this project, a new innovative 3-D modelling code has been produced to run on a parallel supercomputer, along with software for the 3-D visualization and comparison of model and observational (ultrasonic) data from fracture processes in rock.  The technology was primarily developed for evaluating the concept of deep geological storage of radioactive waste, although has applications in a number of other fields of mining, petroleum, and civil engineering. The modeling code is called AC/DC (Adaptive Continuum/ DisContinuum).

The main basis behind AC/DC is the formation of periodic cells (pbricks) which are a densely packed assembly of PFC particles.  The figure below displays an example of a pbrick representing an isotropic rock (a), as well as a pbrick representing a mudstone with an inclined bedding (b).  A procedure has been developed for producing pbricks that best match the macroproperties of the rock of interest.

pbricks

Each pbrick is formed in such a way that it can be replicated allowing it to fit together perfectly in all three principal directions.  Thus the pbrick forms a building block to produce larger models.

The pbrick described above is the particle pbrick.  Matrix pbricks and degenerate matrix pbricks can also be created which are more computationally efficient than particle pbricks.  The matrix pbrick is essentially a network of masses and springs, which do not allow rotation or loss of bonds.  The degenerate matrix pbrick is a 3-D continuum FLAC zone.

Adaptive logic in AC/DC converts matrix pbricks to particles pbricks as required. Figure (a) below shows a relatively simple example of an AC/DC model with 32 matrix pbricks and 4 particle pbricks, with a tunnel created through the center.  A horizontal stress is applied to the model, which causes three bonds to be broken that are interpreted as microcracks (b).  At the time that the bonds break, the adaptive logic incorporated into the code converts 11 of the matrix pbricks into particle pbricks, since they are now near to regions of microcracking.

pbricks


Stress corrosion logic has been incorporated into AC/DC, to simulate the effect of rock strength degradation due to moisture, stress and time. Also, a method for propagating P- and S-waves through AC/DC (and PFC) models has been successfully determined. The method can be repeated many times during a model run, and directionality of the particle source can be varied to create both P- and S-waves.

The main field scale study to be modeled by AC/DC has been chosen as the Prototype Repository Test (PRT) at SKB’s Hard Rock Laboratory (HRL) in Sweden.  The PRT has been designed to simulate a disposal tunnel in a deep repository for storage of high-level radioactive waste. 
prt
The PRT consists of a 90m long, 5m diameter sub-horizontal tunnel excavated in a dioritic granite using a Tunnel Boring Machine (TBM).  Additional deposition boreholes were excavated into the floor using a TBM converted to vertical boring.  An ultrasonic array of 24 ultrasonic sensors was installed during the excavation of two of the boreholes.  Sixteen of the sensors were used to passively record AE in quiet periods following each excavation increment.  Additionally active velocity surveys were performed after each increment using 8 of the sensors as pulsers and the remaining 16 as receivers.

The figure on the right presents the locations of the AE induced during the excavation of one of the boreholes.  The side view shows some distinct clustering to be occurring along the length of the borehole, suggesting some of the AE to be occurring on pre-existing fractures in the rockmass.  Significant time dependant AE activity is also noticed, with activity in some regions occurring for a number of days after the excavation. A total of 88% of the events locate within 30cm of the borehole perimeter, indicating the main extent of the excavation damage zone.



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