Influence of heat on the physical and mechanical properties of selected rock types

Impelled by the increase in the number of tunnel fires in the last decade alone, widespread attention has been drawn towards tunnel fire safety studies. Many of these firesoccurred in road and railway tunnels involving vehicles and trains. These fire incidentshave claimed lives, caused structural damages to the tunnel infrastructure and eveneconomic losses to the government, businesses and communities concerned.When there is a fire in a tunnel, the temperature inside the tunnel increases rapidly tomagnitudes as high as 1500°C. At such high temperatures costly damages to the tunnelstructure is inevitable. Having an understanding of the detrimental effects of such hightemperatures is essential and valuable when carrying out preliminary assessment of thetype and extent of damage in the tunnel. This would in turn provide useful informationin determining the appropriate remedial measures required to make the tunnel safe andusable again in the aftermath of a tunnel fire.In most tunnel fire safety studies, the focus has been on the behaviour of concrete, sinceof course concrete is one of the major support elements in tunnels. However, in severalcases, such as in Scandinavia for example, where the rock mass is competent enough tosupport itself often only a thin layer of shotcrete is used. In such cases the rock will befully exposed to heat a short time after a a fire is fully developed. In this case, whether itis prevention or maintenance of the tunnel, it would require knowledge on the effect ofelevated heat on the rock mass. Hence, it is line with this thinking that a study wasinitiated by the Swedish Transport Administration, (Trafikverket), SvenskKärnbränslehantering AB, SKB, Vattenfall AB, BeFo and Formas to study the effect ofheat on the physical and mechanical properties of some common rock types, and hencethe focus of this study.This study presents the results of a series of laboratory studies which was carried out toinvestigate the effect of heat on the physical and mechanical properties of selected rocktypes, namely; diabase, granite and schist. Samples from these rock types were heattreated at temperature levels of 400°C, 750°C and 1100°C, before investigating theirmechanical and physical properties through mechanical testing and microscopicinvestigations of thin sections. Because the effect of heat on rock can be affected by theheating rate and exposure time, the tests were conducted under controlled conditions in order to avoid significant variation in the results. The results clearly show that the rocktypes behave differently at different temperature levels, which tend to depend on themineral composition and micro crack distribution. As the temperature increases the rockforming minerals undergo changes in their chemical structure thus causing them to alterfrom the original phase they had existed in. With these phase changes different reactionstook place such as re-crystallization, the loss of crystal bound water, thermal expansionand micro cracking of mineral grains as well as the development of voids. Thesemicroscopic changes were manifested in the macro-scale by the variations observed inthe behaviour of strength and stiffness of the samples in the mechanical tests.Ökningen av antalet tunnelbränder under det senaste 10 åren har medfört ökat intresse för studier av säkerheten vid tunnelbränder. Många av bränderna inträffade i väg- och järnvägstunnlar och krävde människoliv. De flesta orsakade dessutom strukturella skador på tunnlarna och infrastrukturen och även ekonomiska förluster. När en brand uppstår i en tunnel, ökar temperaturen i tunneln snabbt och kan nå temperaturer upp mot 1500°C. Vid sådana höga temperaturer är kostsamma skador på tunneln oundviklig. En förståelse för de skadliga effekterna av sådana höga temperaturer är viktig och värdefull när man utför preliminär bedömning av typ och omfattning av skadorna i tunneln. Detta ger användbar information för att bestämma lämpliga åtgärder för att göra tunneln säker och användbar igen efter en tunnelbrand.I de flesta studier av tunnelsäkerhet vid tunnelbränder har fokus legat på betongens beteende, eftersom betong är en av de viktigaste bergförstärkningselementet i tunnlar. Men i flera fall, till exempel i Skandinavien, där bergmassan är kompetent, används ofta endast ett tunt lager av sprutbetong i kombination men bergbultar. I sådana fall, kommer berget att vara helt exponerat för värme en kort tid efter det att branden är fullt utvecklad. Det är därför nödvändigt att öka förståelsen för hur bergmassan reagerar på uppvärmning till höga temperaturnivåer. En studie av bergmekaniska konsekvenser av tunnelbränder initierades därför med stöd av Trafikverket (Banverket), SKB, Vattenfall AB, BeFo och Formas.Denna studie presenterar resultaten av en serie laboratorieförsök som genomfördes för att undersöka effekten av värme på de fysikaliska och mekaniska egenskaperna hos utvalda bergarter, nämligen, diabas, granit och kvartsitisk skiffer. Prover från dessa bergarter värmebehandlades vid temperaturnivåerna 400° , 750°C och 1100°C. Därefter fock proverna svalna. Deras mekaniska och fysikaliska egenskaper bestämdes med hjälp av bergmekaniska tester och mikroskopiundersökningar av tunnslipprover. Eftersom effekten av värme på bergarter kan påverkas av uppvärmningshastigheten och exponeringstiden, utfördes testerna under kontrollerade förhållanden, d v s samma uppvärmninshastighet och avsvalningsmetod samt samma exponeringstid på varje nivå. Resultaten visar tydligt att de studerade bergarterna beter sig olika vid olika temperaturnivåer. Detta beror på olikheter i mineralsammansättning samt mikrouppsprickning. När temperaturen ökar, genomgår vissa mineraler förändringar i sin kemiska struktur, vilket får dem att omvandlas från sin ursprungliga fas. Vid dessa fasförändringar sker olika reaktioner som rekristallisation, förlust av kristallbundet vatten, värmeexpansion och mikrosprickbildning av mineralkorn samt utveckling av hålrum. Dessa mikroskopiska förändringar visar sig i makroskalan genom de variationer som iakttas i hållfasthetens och elasticitetsmodulens beteendet.Godkänd; 2012; 20120328 (ysko

Defining the yield envelope of claystone from 1.1 km depth in Ocean Drilling Program Site 1151, Japan Trench

In this thesis the yield envelope of a claystone from Ocean Drilling Program (ODP) Site 1151 is explored from a range of laboratory tests on undisturbed core samples. The claystone samples tested were obtained from a depth of about 1.1 km below the seafloor, landward of the Japan Trench. During this Ocean Drilling Program (ODP), Leg 186 in the Japan Trench a geophysical observatory consisting of two seismometers, one tiltmeter and one strainmeter was permanently installed at a depth of 1084 to 1095 meters below seafloor (mbsf) near the base of Hole 1151B to investigate the occurrence of silent earthquakes at this portion of the subduction zone. This study provides information of the effective yield stress and mechanical behaviour of sediments near the depth of geophysical observatory. The analysis of the mechanical properties of these cores will provide insights on the deformation processes that occur in this subduction zone particularly the forearc of the Japan Trench. Ask and Kopf (2004) conducted an earlier study in which the physical and mechanical properties of sediments from Sites 1150 and 1151 were analysed and compared. Besides the yield envelope other mechanical properties of the claystone such as the Young’s modulus, the bulk modulus and the effective yield stress were also investigated and obtained. The knowledge of the yield envelope and the mechanical properties form an important aspect to understanding how the sediments may behave during natural consolidation and deformation in this type of geological environments.Validerat; 20101217 (root

Effects of heat on the mechanical properties of selected rock types : A laboratory study

A laboratory study was conducted to study the effect of heat on the mechanical properties of diabase, granite and quartzitic schist at temperatures of 400◦C, 750◦C and 1100◦C. Unheated samples were also studied. The reasoning behind this study was to understand the effect of elevated temperatures on the rock mass, such as in the event of a fire in a rock tunnel. Samples from the aforementioned rock types were heat treated at temperatures shown above, cooled slowly to room temperature and then subjected to uniaxial compression and Brazilian tests. Thin sections were extracted from the heat treated samples for microscopic analyses, which assisted in explaining the reasons for the mechanical behaviour observed from the mechanical test results. The uniaxial compression test showed that the strength of the rock specimens increased by 6% for granite to 29% for diabase at 400◦C when compared to the UCS values of the unheated specimens. From 750◦C to 1100◦C the decay in the strength was very rapid. From the microscopic analyses it was concluded that the increase in the strength of the rock specimens at 400◦C is attributed to the initial reaction of the rock forming minerals, hence the rock specimens were less brittle but more plastic. The rapid drop in the strength from 750◦C to 1100◦C is attributed to the mineralogical changes, micro-cracking and dehydration due to the loss of crystal bound water. At 1100◦C the rocks were highly friable and crumbled very easily when tested mechanically. The effect of mineralogical changes was obvious in diabase where the physical appearance of the samples mimicked that of natural iron, which is believed to be due to the alteration of pyroxene. The result was an increase in strength by 29% at 400◦C compared to the unheated specimens. Even at 750◦C the strength was slightly higher than unheated specimens. In summary; the mechanical behaviour of the rock specimens depended on the temperature level and the mineralogical and physical changes that occur at that temperature.Godkänd; 2011; 20111220 (ysko

Experimental study on thermal spalling of rock blocks exposed to fire

Godkänd; 2010; 20100827 (pinzha