Pillar spalling

THERMO-MECHANICAL SIMULATIONS OF PILLAR SPALLING IN SKB APSE TEST BY FRACOD... 

PLATE 8 Corrosion is a chemical process whose results are easy to see in the world around us. In this picture, corrosion of reinforcing steel has caused the conerete pillars to spall, weakening the bridge and forcing the installation of wooden joists to temporarily support the bridge deck structure. The results of corrosion impose significant economic costs on society—in 1982, these eosts were estimated at about 120 billion. Courtesy, Robert Baboian, Texas Instruments, Inc. 

The Pillar experiment is specifically designed to monitor the brittle failure spalling process. This is a particularly difficult processes to simulate in numerical models as it requires the complete simulation of a rock mass transforming from a continuum to a discontinuum. The scoping... 

For this experiment a tunnel will be excavated at the 450 m depth. Two large holes with a diameter of 1.8 m and a depth of 6 m will be drilled in the tunnel floor to form a pillar with the width of 1 m. The loading configuration is designed to induce stresses in the pillar to a level close to spalling. 

Electric heaters will be used to induce additional thermal stresses that aim to force the rock in the pillar walls to spall. To simulate the effect of confinement from backfill, a water pressure of 1 MPa will be applied in one of the holes. All experimental stages will be monitored by an Acoustic Emission (AE) and micro-seismic system. Convergence and strain measurements will be made to monitor the deformation. 

Major objectives of the Aspo Pillar Stability Experiment are to demonstrate the capability to predict spalling in a fractured rock mass and to investigate the effect of the backfill (confining pressure) on the rock mass response. The FRACOD modelling aims to predict properly the fracturing process and the development of AE events during the planned sequences of excavation-confinement-heating. 

As the stress increase, major fractures develop at the left side (unconfined), but the mid region of the pillar remains stable. The pattern of possible spalling from successive loading becomes more apparent though the pillar remains stable. In spite of the extensive fracturing at left side of the pillar, overall stability seems to be maintained after 120 days of heating. 

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