Tensile Strength of Geological Discontinuities Including Incipient Bedding, Rock Joints and Mineral Veins

Geological discontinuities have a controlling influence for many rock-engineering projects in terms of strength, deformability and permeability, but their characterisation is often very difficult. Whilst discontinuities are often modelled as lacking any strength, in many rock masses visible rock discontinuities are only incipient and have tensile strength that may approach and can even exceed that of the parent rock. This fact is of high importance for realistic rock mass characterisation but is generally ignored. It is argued that current ISRM and other standards for rock mass characterisation, as well as rock mass classification schemes such as RMR and Q, do not allow adequately for the incipient nature of many rock fractures or their geological variability and need to be revised, at least conceptually. This paper addresses the issue of the tensile strength of incipient discontinuities in rock and presents results from a laboratory test programme to quantify this parameter. Rock samples containing visible, natural incipient discontinuities including joints, bedding, and mineral veins have been tested in direct tension. It has been confirmed that such discontinuities can have high tensile strength, approaching that of the parent rock. Others are, of course, far weaker. The tested geological discontinuities all exhibited brittle failure at axial strain less than 0.5 % under direct tension conditions. Three factors contributing to the tensile strength of incipient rock discontinuities have been investigated and characterised. A distinction is made between sections of discontinuity that are only partially developed, sections of discontinuity that have been locally weathered leaving localised residual rock bridges and sections that have been ‘healed’ through secondary cementation. Tests on bedding surfaces within sandstone showed that tensile strength of adjacent incipient bedding can vary considerably. In this particular series of tests, values of tensile strength for bedding planes ranged from 32 to 88 % of the parent rock strength (intact without visible discontinuities), and this variability could be attributed to geological factors. Tests on incipient mineral veins also showed considerable scatter, the strength depending upon the geological nature of vein development as well as the presence of rock bridges. As might be anticipated, tensile strength of incipient rock joints decreases with degree of weathering as expressed in colour changes adjacent to rock bridges. Tensile strengths of rock bridges (lacking marked discolouration) were found to be similar to that of the parent rock. It is concluded that the degree of incipiency of rock discontinuities needs to be differentiated in the process of rock mass classification and engineering design and that this can best be done with reference to the tensile strength relative to that of the parent rock. It is argued that the science of rock mass characterisation may be advanced through better appreciation of geological history at a site thereby improving the process of prediction and extrapolating properties

The ESSnuSB design study: overview and future prospects

ESSnuSB is a design study for an experiment to measure the CP violation in the leptonic sector at the second neutrino oscillation maximum using a neutrino beam driven by the uniquely powerful ESS linear accelerator. The reduced impact of systematic errors on sensitivity at the second maximum allows for a very precise measurement of the CP violating parameter. This review describes the fundamental advantages of measurement at the 2nd maximum, the necessary upgrades to the ESS linac in order to produce a neutrino beam, the near and far detector complexes, the expected physics reach of the proposed ESSnuSB experiment, concluding with the near future developments aimed at the project realization.Comment: 19 pages, 11 figures; Corrected minor error in alphabetical ordering of the authors: the author list is now fully alphabetical w.r.t. author surnames as was intended. Corrected an incorrect affiliation for two authors per their reques

Behaviour of blast-induced damaged zone around underground excavations in hard rock mass

The presence of blast-induced damaged zone around an excavation boundary has been an important concern in rock construction. It is generally believed that the presence of this zone can pose problems related to stability and flow and consequently impair the performance and functionality of the excavation. In fact, the immediate consequences of this zone are usually conceived in terms of safety and cost. Hence, some organizations have put in place guidelines for controlling the amount of damage induced by blasting. Since shotcrete or sprayed concrete is a widely used surface rock support, its performance depends primarily on the competence of the damaged zone. However, in some instances the use of shotcrete may unnecessary, but the lack of knowledge of the competency of the damage zone means, it is better not to take chances. It is therefore necessary to increase the knowledge and the understanding of the competency and behaviour of the damaged zone in order to predict the performance and functionality of a rock tunnel. To gain an understanding of the damaged or disturbed zone in general, significant efforts have been made over the last few decades in a broader area; the excavation disturbed zone. These efforts mainly focused on the characterization and classification of the damaged zone. Quantification of this zone has also been done in terms of mechanical, hydraulic and physical parameters, particularly to delineate the extent of the damaged zone. The characterization, classification and quantification of damaged zone were purpose specific and therefore, the definitions for damage zone are different and varying. In this thesis the damaged zone is defined as the zone where the rock has been significantly damaged such that the mechanical properties have been affected and that these changes are measurable by any state of the art measurement techniques. This definition also applies to the blast-induced damage zone. To be able to assess the significance of the blast-induced damage zone and its influence on the performance of an excavation, the mechanical behaviour of this zone must be understood. This thesis is therefore aimed in that direction and thus the objective. Several issues were investigated including; effects and consequences of blast-induced damage zone, most likely failure mechanisms, mechanical parameter sensitivity and their impact on the behaviour of the damaged zone, numerical modelling approach for damaged zone and indicators for failure from a continuum model, etc. A literature review and industrial questionnaire gave the direction for the investigations. Field and numerical methods were employed in the investigations. The results of these investigations are published in a series of papers that make up the thesis. In brief, the main results and conclusions can be summarized as follows: The blast-induced damage zone has been largely defined in terms of its extent and lacks a definition based on its inherent competency parameters, which are directly related to the stability of an excavation. The blast-induced damage zone thickness varies in most practical cases between 0.1 and 1.0 m, with an average ranging from 0.3 m to 0.5 m depending on whether perimeter blasting techniques are used or not. The reduction in the Young's modulus varies anywhere between 10 to 90 % of the undamaged rock value. In a field investigation reported in this thesis the Young's modulus of the damaged rock was found to vary between 50 and 90 % of the value for he undamaged rock mass. The thickness of the damaged zone was between 0.5 and 1.0 m. From the numerical study, the presence of the blast-induced damaged zone did affect the behaviour of the stability quantities, namely; deformation and induced boundary stresses. However, it cannot be concluded if the effects are significant enough to cause problems around an underground excavation in the hard rock mass type studied. The inherent properties of the damaged zone that affected its behaviour, identified in order of their significance are; the deformation modulus, followed by tensile strength and compressive strength. External factors such as the state of the in-situ stresses are seen to significantly influence the behaviour of the damaged zone. With the rock mass type and the in-situ stress regimes used in this thesis, the main failure mechanism within the damaged zone at shallow excavations is tension, while in deep excavations it is compression, as evaluated from the numerical analyses. The influence of the blast-induced damaged zone on failure is evident in shallow excavations but not in deep excavations. This was observed from the coupled continuum-discontinuum models. The study on the shotcrete-rock interface showed that the bond strength of the interface is important for the shear strength. Average bond strength of 0. 5 MPa was determined for interfaces with surfaces roughness with JRC values of 1-3, and 1.4 MPa for those with JRC values of 9-13. These values were determined for low normal load conditions ( MPa) which is often the case when shotcrete is used with rock bolt for rock support. The average adhesion or tensile strength of the interface was determined to be 0.56 MPa.Godkänd; 2008; 20081111 (evan)</p

Back Analysis of Narrow Vein Open Stope Stability and Verification Using Kinematic and Empirical Methods

High walls of open stopes in underground stoping mines can be considered to behave in a similar manner to open pit slopes if stability is largely controlled by geological structures. With this assumption the kinematic method of analyses can be used to assess the stability of the footwall, hangingwall, the roof and floor of an open stope. This paper demonstrates the application of kinematic analyses tools such as Rocscience’s DIPS® and UNWEDGE® to assess the stability of underground stopes in a narrow vein mine. A back analysis was conducted, using both kinematic methods and the empirical stability graph method, after field investigation of stope failures and review of stope closure reports. The stability graph method showed that the designed stopes were stable with support. However, majority of the stopes audited have apparently failed or were in state of failure, i.e., unstable. Kinematic analyses showed that these stopes were certainly at risk of failure which confirmed the observations. The stability chart used by the mine was eventually adjusted based on the kinematic analyses and observations made, resulting in the stability graph having only three regions: stable, unstable and fail.License full text: CC BY-NC 4.0</p

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

Blast-induced damage : a summary of SveBeFo investigations

This report presents a summary of the blast damage investigation carried out by SveBeFo (Swedish Rock Engineering Research) during the period 1991 to 2003 at various hard rock sites in Sweden. The objective of this report is to present a synopsis of the important factors that influence the development and extent of the blast-induced damage zone, nature and characteristics of blast-induced fractures, and a summary of the blast-damage thickness with reference to the existing perimeter blasting damage guidelines for tunnelling and drifting practices in Sweden. Ultimately this report will assist the author in developing numerical models to study the effects of blast-induced damaged zone on stability parameters. It has to be stated clearly that this report is intended as summary and not a reproduction of the works of SveBeFo. It may not also represent the actual conclusions or views of SveBeFo in areas where this author attempted to interpret and conclude from some of the data. Interested readers are strongly urged to consult the SveBeFo publications referenced in this report.The SveBeFo blast damage investigations during the period 1991 to 2003 were carried out at three principle hard rock sites: (i) Vånga granite quarry, (ii) LKAB's Malmberget and Kiruna mines and (iii) SKB's TASQ tunnel. The primary goal of these studies was to improve perimeter blasting guidelines for tunnelling and drifting in hard rock masses in Sweden. Within this scope were several objectives such as: (i) to develop guidelines for blast-induced damage control for drifting and tunnelling in hard rock masses, (ii) devise method for blast damage assessment, (iii) verification of the devised methods in tunnelling and drifting sites. Generally used perimeter blasting techniques, particularly smooth blasting, were employed in the tests. Multiple holes of different, but commonly used, diameters for perimeter holes were used. The explosives used were those commonly used for tunnelling and drifting. After the blasts saw cuts were extracted from the remaining rock, cut into manageable sizes, sprayed with penetrants (to make the blast-induced cracks traceable or visible) and crack parameters (length, quantity and pattern) investigated. The size and pattern of blast-induced damage were observed to depend on various parameters, namely; explosive parameters (explosive type, charge length, initiation method and coupling), blast hole pattern (burden, spacing and hole diameter), in-situ rock mass parameters (geology, in-situ stress, and rock strength and stiffness) and water in the holes. The thickness of the observed damage ranged between 0.1 and 1.2 m. An average damage thickness of 0.5 m was observed in the Kiruna and Malmberget mine drifts and 0.3 m in SKB's ÄSPE/TASQ tunnel and Vånga granite quarry.Godkänd; 2008; 20081114 (davsai

Review of Engineering Geology and Rock Engineering aspects of the operation and closure of a KBS-3 repository at the Forsmark site – Initial Review Phase

The Swedish Radiation Safety Authority (SSM) reviews the Swedish Nuclear Fuel Company’s (SKB) applications under the Act on Nuclear Activities (SFS 1984:3) for the construction and operation of a repository for spent nuclear fuel and for an encapsulation facility. As part of the review, SSM commissions consultants to carry out work in order to obtain information on specic issues. The results from the consultants’ tasks are reported in SSM’s Technical Note series

Blast-induced damaged zone studies : Final Report to Trafikverket

This report is a summary and conclusions from the study on Blast-Induced Damage around underground excavations in hard rock, which was carried out as a PhD research project by the author. The project was funded by Vägverket and the research was conducted at the Division of Mining and Geotechnical Engineering at Luleå University of Technology. The work resulted in several publications; a PhD thesis, journal articles, conference articles and technical reports. The publications can be downloaded from: http://www.ltu.se/staff/d/davsai?l=en. This final report is compilation of the important elements from these various publications.Because the Blast-Induced Damaged Zone (BIDZ) is generally perceived to have a negative impact on the economics and performance of a tunnel many studies have been conducted to delineate the extent of the BIDZ. The primary goal has been to device ways to control or minimise the extent of this zone. As a result guidelines have been developed by concerned authorities, such as for example AnläggningAMA-98 (1999), to control the extent of BIDZ during tunnel construction. These guidelines however do not give recommendations about the strength and stiffness properties of the BIDZ, which are the most important parameters that control its behaviour and ultimately influence the stability and performance of tunnel.The extent of the BIDZ in majority of the practical cases varied between 0.1 and 1.0 m, with an average ranging from 0.3 m to 0.5 m. The stiffness magnitudes of this zone were as low as 10% to as high as 90% of that of the undamaged rock mass. Field investigation at Kiirunavaara underground mine (Malmgren et al, 2007) showed the stiffness of the BIDZ to vary between 50 and 90% of that of the undamaged rock mass, while the thickness ranged from 0.5 to 1.0 m. Although it was possible to deduce the modulus of the BIDZ the strength however, is the most difficult to measure or estimate. The most generic way to estimate the strength of the BIDZ is the use of a rock mass classification system in conjunction with a strength criteria, which was the approach used in the research by this author.Numerical studies of the BIDZ by this author showed that, the presence of this zone does affect the overall response of the near-field rock mass. This therefore implies the need for consideration of this zone during the tunnel design stage. The strength and stiffness of this zone were also found to affect the type of failure around the tunnel and its extent. However, it not is possible to state by what degree the BIDZ will affect the performance of a tunnel as it is relative. It is therefore wise to use guidelines to minimise damage even though it may be costly and unnecessary. In any case it would be beneficial to consider the sensitivity of an excavation, both subjectively and objectively, before emphasising the seriousness to stick to the blast damage guidelines.Godkänd; 2011; 20110214 (davsai

Damaged rock zone study : a progress report

The subject of the damaged rock zone around an underground excavation is very complex. Much of these complexities relate to the physics of the rock properties and the highly anisotropic conditions of the EDZ. In rock physics, for example, the sough after rock properties such as Young's modulus (E) and the strength parameters (óc, c, ö, etc) are tensor quantities - that is they have spatial characteristics (3-dimensional). So the question usually is: Is it sufficient to relate these parameters (which are tensors) to vector quantities such as velocity (which is one- dimensional)? Obviously, there is no mathematical relationship for this. The anisotropy within the EDZ also complicates any process that attempts to estimate the magnitude of these properties. A drill and blast excavation will result in complex fracture patterns (irregularly shaped, cracks ranging from micro to macro sizes, rock bridges, etc). All these complexities will affect the kinematics of rock deformation and strength characteristics. A large volume of publications is available on investigations into the excavation disturbed zone. However, much of these investigations have focussed on identifying the factors affecting the development and extent of this zone, with the primary goal of minimising or if possible to completely eliminate this zone, which though is not possible. A questionnaire on EDZ was send to various organisations and individuals. The responses revealed various views on the EDZ. The mining industry for example, sees the EDZ from two sides of the coin: (i) it can act as protective blanket (destressed zone) by pushing high stresses further into the rock and in doing so protect the excavation, (ii) it can also jeopardise the safety of personnel and equipment as well as resulting in increased support costs. For nuclear waste isolation group the EDZ will provide a flow path for nuclear radiation to reach the atmosphere. Further still, for the civil engineering, the problem is largely related to stability, and maintenance and operational costs. Each of these cases is unique since the requirements are different. Which parameters are important and which factors need attention will depend on the nature and purpose of the excavation.Godkänd; 2008; 20080131 (ysko)</p