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Spalling initiation experimentson large hard rock cores
RISE - Research Institutes of Sweden, Safety and Transport, Safety.ORCID iD: 0000-0003-4526-4061
RISE - Research Institutes of Sweden, Built Environment, CBI Swedish Cement and Concrete Research Institute.
RISE - Research Institutes of Sweden, Built Environment, CBI Swedish Cement and Concrete Research Institute.
Swedish Transport Administration, Sweden.
2018 (English)Report (Refereed)
Abstract [en]

A new type of laboratory test method to determine spalling resistance in a situation as in large boreholes in hard rock has been demonstrated. Uniaxial compression tests on large cores with notches were conducted on Äspö diorite from Äspö HRL. Spalling was localized to the notches where the local stress was highest. The crack initiation, crack coalescence and crack damage stresses representing various fracture stages were identified by using acoustic emission monitoring with source localization. The actual stress levels were obtained from the axial forces at which the various fracture stages were identified via FE-calculations representing the actual specimen geometry and loading. The results were compared with uniaxial compression tests carried out on cores with standard size on the same rock type also from Äspö HRL.

The results showed that spalling chips were formed similar to those found in field which indicates that the test is representing a realistic behaviour. However, the results show that the various fracture stress levels found in the tests are higher than the spalling strength found in the field and in the test of small cores. The higher stress levels could be caused by a number of reasons described in the report.

A post characterization of the fractures was carried out on slabs that were cut out from the specimens containing the notch areas. The occurrence of microcracks and how they have propagated through the different minerals and the location in relation to the notches, minerals and grain boundaries were investigated. The patterns of the major fractures and the secondary fractures were analysed. The majority seem to be extension fractures, but shearing could also be verified. The results from the microscopy analysis provide invaluable information of the spalling process at all stages.

Abstract [sv]

En ny typ av laboratorieprovmetod för att bestämma spjälkningsmotstånd i en situation som i stora borrhål i hårt berg har demonstrerats. Enaxiella kompressionsförsök på stora kärnor med notchar (urtag) genomfördes på Äspödiorit från Äspö HRL. Spjälkning lokaliserades till notcharna där den lokala spänningen var högst. Sprickinitierings-, spricksammanslagnings-, och sprickskadespänningen som representerar olika sprickstadier identifierades genom att använda akustisk emmissionsmätningar med lokalisering av källan. De aktuella spänningsnivåerna erhölls från den axiella kraften som de olika sprick stadierna identifierades via FE-beräkningar som representerar det aktuella provets geometri och belastning. Resultaten jämfördes med enaxiella kompressionsförsök som genomförts på borrkärnor med standardstorlek och bergart också från Äspö HRL.

Resultaten visade att spjälkningsbitar bildades liknande sådana som funnits i fält vilket indikerar att provet representerar ett realistiskt beteende. Emellertid visar resultaten att spänningarna vid de olika sprickstadierna i proven är högre än spjälkningshållfastheten som man har funnit i fält och i proven av de små kärnorna. De högre spänningsvärdena kan bero på ett antal orsaker som beskrivs i rapporten.

En post-karaktärisering av sprickorna genomfördes på skivor som sågades ut från proverna innehållande notch-områdena. Förekomsten av mikrosprickor och hur de har växt till genom de olika mineralerna och deras läge i förhållande till notcharna, mineralerna och korngränserna undersöktes. Mönstren av de huvudsakliga sprickorna och de sekundära sprickorna analyserades. Merparten verkar vara dragsprickor men skjuvning kunde också påvisas. Resultaten från mikroskopiundersökningarna ger ovärderlig information om spjälkningsprocessen vid alla stadier.

Place, publisher, year, edition, pages
Solna: Svensk Kärnbränslehantering AB , 2018. , p. 68
Series
SKB, ISSN 1402-3091 ; R-14-12
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:ri:diva-33854OAI: oai:DiVA.org:ri-33854DiVA, id: diva2:1206409
Funder
Swedish Nuclear Fuel and Waste Management Company, SKBAvailable from: 2018-05-16 Created: 2018-05-16 Last updated: 2018-08-22Bibliographically approved

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Jacobsson, Lars

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