assessment of the potential for rock spalling in the ...spalling is, expressly, an event that can...
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P O S I V A O Y
FI -27160 OLKILUOTO, F INLAND
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Top ias S i ren
Dan ie l e Mar t i ne l l i
Lau r i Uot i nen
June 2011
Work ing Repor t 2011 -35
Assessment of the Potential forRock Spalling in the Technical Rooms
of the ONKALO
June 2011
Working Reports contain information on work in progress
or pending completion.
The conclusions and viewpoints presented in the report
are those of author(s) and do not necessarily
coincide with those of Posiva.
Top ias S i ren
Dan ie le Mart ine l l i
Laur i Uot inen
Ka l l i osuunn i t t e lu Oy Rockp lan L td .
Work ing Report 2011 -35
Assessment of the Potential forRock Spalling in the Technical Rooms
of the ONKALO
Base maps: ©National Land Survey, permission 41/MML/11
ABSTRACT It is important to be able to predict the rock spalling in the ONKALO while the excava-tion advances deeper. When stresses at the excavation boundary reach the rock mass spalling strength, a brittle failure occurs that is often called “spalling”. The spalling phenomenon occurs as a strong compressive stress induces crack growth behind the excavated surface. Spalling is, expressly, an event that can create problems in the ON-KALO, not so much for the overall stability of all of the excavations, but rather in par-ticular areas that can cause unnecessary and unintended over-excavations and hazards. For rock engineering and layout design purposes, the knowledge of the predicted spalling in the excavation surface is crucial. Optimization of the design is mainly done by directing the tunnels parallel to the major principal stress direction. However, due to the complex forms and crossing tunnels, especially at the shaft access drift area, sophis-ticated methods are required in order to minimize spalling and to support the unavoid-able spalling that occurs. The complex tunnels require three-dimensional analysis. The software used for the main calculation has been MIDAS/GTS, a geotechnical 3-D FEM that is able to calculate complex geometries rather easily. Most of the models have also been verified with Roc-science Examine3D, which returns the results with a high precision at boundary. The area to model is large, and due to the computational limits, it is divided into six blocks. This analysis, carried out step by step for each block, permitted to draw a map of the spalling depth prevision in the whole tunnel contract 5 (TU5) area. The dominating rock types in the area are migmatitic gneiss and pegmatitic granite. The strength of these rocks has been broadly tested with point load and uniaxial compressive strength tests. The test results show a deviation of the UCS as well as other parameters. Due to this large deviation, a Monte Carlo has been used as an auxiliary analysis method. The results of the Monte Carlo analysis indicate that in the ONKALO 23 percent of the local rock strength and in situ stress combination cases coupled together result in spalling with a mean depth of 0.28 metres. In the three-dimensional models with con-servative parameters, there is spalling in the crown of the tunnels in an unfavourable direction, but moreover, in the shaft access drifts, the tunnel crown is highly stressed with large spalling. At the worst occasions the spalling depth is over one metre. By us-ing average parameters, spalling is predicted only near the shafts. The prediction should be taken into account when designing the rock support for the zones with spalling. Especially in the shaft access drifts, reinforcements that are able to withstand the rock weight of the predicted spalling area, are recommended. Such sup-port measures could be, for example, pre-tensioned anchors together with steel wire mesh and shotcrete. This work underlined how important it is to evaluate the stability of an excavation, even if carried out in very hard rock with low fracture intensity. Keywords: Rock spalling, rock stress, rock strength, rock support, 3-D FEM, 3-D BEM, ONKALO, TU5, depth -430 m.
Selvitys kallion hilseilyn todennäköisyydestä ONKALOn teknisten tilojen alueella TIIVISTELMÄ Louhinnan edetessä yhä syvemmälle ONKALOssa on tärkeää pystyä ennustamaan kal-lion hilseilyä. Tunnelin pinnalla vallitsevien jännitysten ylittäessä kalliomassan hilseily-lujuuden tapahtuu hauras murtuma, jota kutsutaan hilseilyksi. Hilseilyilmiö tapahtuu suuren puristusjännityksen aiheuttaessa raon kasvua louhitun pinnan takana. Hilseily voi aiheuttaa ongelmia ONKALOssa, mutta ei niinkään tunnelien kokonaisstabiliteetille vaan pikemmin paikallisesti tahattomina ryöstöinä ja vaaratilanteina. Kalliorakenne- ja layout-suunnittelua varten hilseilyennuste on välttämätön. Ennuste mahdollistaa tilojen asemoinnin edullisempaan suuntaan, jossa hilseilyä ei juuri esiinny. Pääasiassa tämä tehdään suuntaamalla tilat samansuuntaiseksi suurimman pääjännityk-sen kanssa. Monimutkaiset geometriat ja risteävät tunnelit, etenkin kuiluperien alueella, vaativat kehittyneitä menetelmiä hilseilyn minimoimiseksi ja väistämättömän hilseilyn tukemiseksi. Monimutkaiset tunnelijärjestelmät vaativat kolmiulotteista analysointia. Tärkein käytet-ty ohjelmisto on 3-D FEM MIDAS/GTS, jolla monimutkaisten geometrioiden laskenta onnistuu kohtuullisen vaivattomasti. Suurin osa malleista on myös varmennettu laske-malla ne Rocscience Examine3D:llä, joka mahdollistaa erittäin suuren tulostarkkuuden. Mallinnettu alue on laaja, ja tietoteknisten rajoitteiden takia se on jaettu kuuteen loh-koon. Tämä analyysi, toteutettuna jokaiselle lohkolle erikseen, on mahdollistanut hilsei-lykartan koko tunneliurakka 5 (TU5) -alueelle. Alueen pääkivilajit ovat migmatiittinen gneissi ja pegmatiittinen graniitti. Vallitsevien kivilajien lujuusominaisuuksia on tutkittu laajasti pistekuormitus ja yksiaksiaalisilla kokeilla. Lujuusominaisuuksien hajonta on erityisen laaja. Laajan hajonnan vuoksi Monte Carlo -menetelmää on käytetty kiven lujuuden ja in situ-jännityksen jakaumilla. Monte Carlo -analyysin tulokset osoittavat, että 23 prosenttia lujuuden ja paikallisten in situ -jännitysten jakaumien yhdistelmistä johtaa hilseilyyn ja keskimäärin 0,28 metrin hilseilysyvyyteen. Hilseilyä on myös havaittavissa kolmiulotteisten analyysien tuloksis-sa tunnelien holveissa epäedulliseen suuntaan ja laajasti etenkin kuiluperissä, joissa hol-viin kohdistuu suuri tangentiaalinen jännitys. Pahimmillaan hilseilysyvyys on yli met-rin. Keskimääräisillä parametreilla hilseilyä on ennustettu olevan vain kuilujen ympäris-tössä. Hilseilyennuste tulee ottaa huomioon suunniteltaessa lujituksia mahdollisesti hilseilevil-le alueille. Etenkin kuiluperien alueella suositellaan lujitusrakenteita, jotka pystyvät kantamaan ennustetun hilseilevän kiven painon. Tällaisia lujitusrakenteita voisivat olla esimerkiksi esijännitetyt pultit yhdessä teräsverkolla tuetun ruiskubetonin kanssa. Tämä työ korostaa kalliotilojen vakauden arvioinnin tärkeyttä erittäin kovassa ja lähes rakoi-lemattomassa kalliossa. Avainsanat: Kallion hilseily, jännitys, kiven lujuus, lujitus, 3-D FEM, 3-D BEM, ONKALO, TU5, syvyys -430 m.
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TABLE OF CONTENTS ABSTRACT TIIVISTELMÄ PREFACE ....................................................................................................................... 2 1 INTRODUCTION ......................................................................................................... 3
1.1 Background ........................................................................................................... 3 1.2 Aim of this work .................................................................................................... 3 1.3 The rock spalling phenomenon ............................................................................. 4 1.4 Methods used ....................................................................................................... 5 1.5 Previous work in predicting rock spalling .............................................................. 6
2 INITIAL DATA AND THEORY ..................................................................................... 7
2.1 Geology of the TU5 area ...................................................................................... 7 2.2 Rock stress ........................................................................................................... 8 2.3 Intact rock strength ............................................................................................... 9 2.4 Rock mass strength ............................................................................................ 10 2.5 Constitutive models ............................................................................................ 11 2.6 Rock burst potential ............................................................................................ 12
3 DESCRIPTION OF THE MODELS ............................................................................ 14
3.1 Analysis tools ...................................................................................................... 14 3.2 Divisioning of the models .................................................................................... 14 3.3 Elements and the mesh ...................................................................................... 15 3.4 Monte Carlo simulation ....................................................................................... 16
4 RESULTS OF THE ANALYSIS ................................................................................. 17
4.1 General of the results ......................................................................................... 17 4.2 Brittle spalling simulation .................................................................................... 18
4.2.1 Demonstration tunnels ................................................................................. 18 4.2.2 Crossing area .............................................................................................. 21 4.2.3 Access tunnel – part 1 ................................................................................. 22 4.2.4 Access tunnel – part 2 ................................................................................. 23 4.2.5 Shaft access drift area ................................................................................. 24 4.2.6 Technical rooms .......................................................................................... 27
4.3 Simulation of the tangential stresses at excavation boundary ............................ 28 4.3.1 Demonstration tunnels ................................................................................. 28 4.3.2 Crossing area .............................................................................................. 28 4.3.3 Access tunnel – part 1 ................................................................................. 28 4.3.4 Access tunnel – part 2 ................................................................................. 29 4.3.5 Shaft access drift area ................................................................................. 29 4.3.6 Technical rooms .......................................................................................... 30
4.4 Monte Carlo simulation ....................................................................................... 30 4.5 Comparison of the results of the 3-D FEM and Monte Carlo .............................. 32
5 CONCLUSION ........................................................................................................... 34
5.1 Predicted spalling for TU5 .................................................................................. 34 5.2 Rock reinforcement ............................................................................................. 36 5.3 Monte Carlo -simulation ...................................................................................... 38 5.4 Discussion .......................................................................................................... 38
REFERENCES ............................................................................................................. 39
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PREFACE This working report has been written by Topias Siren and Daniele Martinelli. Also Lauri Uotinen has greatly contributed to the chapters with Monte Carlo simulation. The general content and the models have been done for rock engineering purposes by the authors and by the following people:
- Supervising and commenting the work report: Guido Nuijten (Rockplan) - Modelling and rock mechanics expertise: Jesse Ström (Rockplan) - Rock mechanics expert: Matti Hakala (KMS Hakala Oy) - Reviewing the work report:
- Prof. John Hudson (Rock Engineering Consultants) - Erik Johansson (Saanio & Riekkola Oy) - Kimmo Kemppainen (Posiva Oy)
- Proofreading: Paula Saarelainen.
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1 INTRODUCTION In this chapter, the motivation for the spalling prediction work and the spalling phe-nomenon are explained.
1.1 Background
Currently in Olkiluoto, Posiva is starting the excavation of tunnel contract 5 (TU5) of ONKALO, which is an underground rock characterisation facility for the final disposal of spent nuclear fuel (Figure 1-1). In total, ONKALO consists of one access tunnel, three shafts, and technical rooms.
Figure 1-1. The layout of the final disposal facility (Posiva 2010).
1.2 Aim of this work
The aim of this work has been to verify the stability of ONKALO and to provide initial data for the rock reinforcement design of ONKALO TU5, using mainly 3-D numerical methods and also a Monte Carlo simulation. A potential problem for the tunnel con-struction is excavation-induced spalling, which is expected at this depth (around -430 m from the surface). Thermally-induced spalling has not been evaluated in this work. This work is organised by first giving the initial data used for the different models with the theoretical features necessary for the work, then describing the methods used in the models, such us rock mechanical criteria and statistical and numerical methods, with the overall description of the software used in the calculation. After this, all the results are presented, giving the values both numerically and graphically in order to show the be-haviour of the rock mass during the excavation. The last part is dedicated to conclusions and considerations deriving from the calculation that can be important for other works that are linked to this one.
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1.4 Methods used
Three-dimensional numerical methods were chosen as the primary analysis method be-cause of the complexity of the geometry of the tunnels and niches. In the TU5 area, there are many connecting tunnels and three shaft access drifts with a vertical shaft with a 3.5–4.5-metre diameter. Especially the link between the tunnels and the shafts is the object of this analysis, because of the complex geometry where the spalling phenome-non can possibly reach considerable depths. The layout of ONKALO is presented in Figure 1-4, where the TU4 and TU5 areas are marked.
Figure 1-4. The underground research facility ONKALO. This report describes the spalling prediction for the TU5 area. During the compiling of this report, the TU4 area was excavated. The excavation and the studies of the unmarked areas are described in Posiva 2009 pp. 207–212.
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The accuracy of the values obtained from numerical modelling is important both for the safety during the excavation and for the final support design. For this reason, the results of 3-D finite element method are verified constantly with 3-D boundary element method during the modelling process, to notice possible errors related to main modelling soft-ware. However, there are still uncertainties related to in situ stress parameters (Posiva 2009 pp. 213–214), for example the direction of the major principal stress has some spatial variability. There are also uncertainties related to the spatial variability of strength val-ues. Another general uncertainty is that for the most parts of the TU5 area, the closest core drillings are far away because of the need to avoid any unnecessary hydraulic con-nections. These uncertainties are studied with Monte Carlo simulation. The Monte Carlo simula-tion is a process where random sampling is used to combine independent variables. Of-ten Monte Carlo simulation is used in complex calculations which are hard to solve or there is no exact result.
1.5 Previous work in predicting rock spalling
The latest description of the spalling in ONKALO is Site Description 2008, where the stability of the second and third loop (TU2 & TU3 in Figure 1-4) of ONKALO is de-scribed (Posiva 2009 pp. 207–212). During the compiling of this report, the TU4 area, marked in dark green in Figure 1-4, was excavated. The last spalling assessment by Hakala et al. (2008) reported the potential for rock spalling at the Olkiluoto site. Major work done to study spalling in hard rock has been undertaken especially in URL in Canada, for example by Martin & Chandler (1994), later by Hajiabdolmajid et al. (2002), and lately in Äspö Pillar stability Experiment, for example by Andersson (2007). For the Monte Carlo simulation of spalling in hard rock, the work by Martin & Christiansson (2008) is significant. More about previous work predicting and under-standing spalling is mentioned in Hakala et al. (2008) on pages 5–6.
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2 INITIAL DATA AND THEORY
In this chapter the theory and the data used are presented. The layout of the spring 2010 and the rock stress and strength parameters listed in Site Description 2008 (Posiva 2009 Tables 5-2 and 5-3) were used.
2.1 Geology of the TU5 area
The rock mass present in this area is mainly gneissic with two main different types: migmatitic gneiss and pegmatitic granite. The veined gneiss (VGN), diatexitic gneiss (DGN), mica gneiss (MGN), and tonalitic granodiroritic gneiss (TGG) are grouped as migmatitic gneiss, because their feature deviations are similar. The geology of the area is presented in Figure 2-1 with TU5 layout. The lithology of the section is based to the Geological Model of the Olkiluoto Site (Mattila et al. 2008) and to later minor adjust-ments to be published in the next update of Geological Model.
Figure 2-1. The predicted geology of TU5 and technical rooms with migmatitic gneissic rocks marked in blue and pegmatitic granite marked in red.
Pegmatitic granite
Migmatitic gneiss
8
Most of the layout is dominated by migmatitic gneiss with only minor pegmatitic areas. In Figure 2-1, presented in brown, are the paths of the drilling cores used for geological characterisation, and in red lines, the expected brittle failure zones. In the layout design of the technical rooms, the expected brittle failure zones have been avoided, therefore none are expected to intersect the area.
2.2 Rock stress
The high in situ stress might create problems in the excavations. Using the stress meas-urements carried out during the last years, it has been possible to determine different stress domains to describe the stress distribution at different depths. In this work, carried out to verify the spalling depth, two sets of stress values are used. The mean trend values are used to describe a situation with low spalling. The conserva-tive 90 % fractile values of the stress components have been omitted for the analysis of the excavations in the case of very high stress. These two sets should consider the both ends of the scale in which spalling is likely to occur. The functions used are the follow-ing (Table 2-1): Table 2-1. Principal stress components at the Olkiluoto site (Posiva 2009 Table 5-2).
Range Stress component (MPa) Orientation re-spect to North
Upper limit trend zH ��� 03.06.19� 90° (conservative zh ��� 015.045.13� 0° 90% fractile) zv �� 0292.0� - Mean trend zH ��� 03.06.13� 90° zh ��� 015.045.9� 0° zv �� 0265.0� - Lower limit trend zH ��� 03.06.7� 90° (10% fractile) zh ��� 015.045.5� 0° zv �� 0239.0� -
These values are valid only for the domain of depth z included between 300 and 900 metres. The maximum value of �1 in the upper limit conditions at the TU5 level is around 33 MPa and with the mean stress conditions 27 MPa. These two values are dif-ferent from each other, and so it is easy to expect a different behaviour. The stress in the excavation surface can be expressed, for example, with tangential stress. According to Hoek & Brown (1980), the tangential stress in a round profile can be calculated using the equation ��� � �� � �� � �� 1 where A is a shape parameter and k is the ratio of the horizontal and vertical stress.
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2.3 Intact rock strength
The cores drilled from the area are very good, with only few joints and faults. There-fore, in some parts the RQD of the rock mass is close to 100, a value that presents intact rock. The initial rock mechanics parameters for the intact rock are listed in Table 2-2. Table 2-2. The values of the intact rock mechanics parameters used for the different rock types in TU5 (Posiva 2009 Table 5-3).
Parameter Rock type Mean value Standard deviation
Number of samples
Young’s modulus, E (GPa) MIGM.GN 63* 12 63
PGR 65 9 4
Poisson’s ratio, � (mm/mm) MIGM.GN 0.25 0.05 63
PGR 0.29 0.05 4
Peak strength, �P (MPa) MIGM.GN 115 23 77
PGR 108 12 4
Crack damage stress, �CD (MPa) MIGM.GN 99 26 82
PGR 108 8 4
Crack initiation stress, �CI (MPa) MIGM.GN 52 12 83
PGR 55 12 4
Indirect tensile strength, �T,I (MPa) MIGM.GN 12.0 3.3 53
PGR 5.4 0.9 6
Direct tensile strength, �T,D (MPa) MIGM.GN 7.9 2.2 19
PGR - - 0
* mean anisotropy factor of 1.4 has been reported in Hakala et al. (2005). The most commonly used parameter to describe the strength of the rock is the uniaxial compressive strength (UCS). In the Olkiluoto Migmatitic gneiss the mean value of the UCS is 115 MPa, but it must be noted that the deviation of the strength of the rocks is large (Posiva 2009 Table 5-3). Although it must be noted, as well, that the number of point samples is also numerous, which gives reliability to the results. For the five most common rocks in the Olkiluoto site the deviation of the strength is presented in Figure 2-2.
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2.4 Rock mass strength
Rock mass quality can be expressed for example with Q’-value and GSI-value. For the TU5 area the mean value is calculated as weighted mean from drill cores close in the area using the core logging provided by Posiva. The used drill cores are OL-KR7 (be-tween depths 406…496 m), OL-KR38 (440…450 m), OL-KR24 (432…457 m) and OL-KR48 (414…481 m). The Q’-value is converted to a GSI-value with the equation 2 (Hoek 1995 et al., page 105). GSI = 9 ln Q’ + 44 2 The corresponding GSI value for the TU5 area is 84. This value is used to convert the intact rock parameters with the Rocscience programme RocLab (version 1.031) in Ta-ble 2-2 to rock mass parameters in Table 2-3. The method used by RocLab is described in Hoek & Diederichs 2006. Table 2-3. The rock mass parameters used for the different rock types in TU5.
Parameter Migmatitic gneiss Pegmatitic granite Rock mass Young’s modulus Erm 57,900 MPa 59,700 MPa Poisson’s ratio � 0.25 0.29 Hoek–Brown parameter mb 15.8 15.8 Hoek–Brown parameter s 0.2 0.2
According to Posiva 2009, the used spalling strength is equal to 57 % of the UCS. The used percentile is similar to the values reported in many international studies on the sub-ject in similar rocks. It should be noted that with these considerations spalling occurs before the yielding point of the rock mass. Using the peak strengths listed in Table 2-2 the spalling strength values of the two rock types are calculated using 57 % of peak strength the results are shown in Table 2-4.
0 10 20 30 40 50 60 70 80 90
<50 60 70 80 90 100 110 120 130 140 150 160 170 180 >180
Compressive strength calculated from point load index, (MPa)
VGN
DGNMGN
TGGPGR
Count
VGN Veined gneiss DGN Diatexitic gneiss MGN Mica gneiss TGG Tonalitic granodiroritic gneiss
PGR Pegmatitic granite
Figure 2-2. The distributions of the compressive strength values for the five main rocktypes present at the Olkiluoto site as estimated from point load tests on samples fromdrillholes OL-KR1…KR38 (Hakala et al. 2008).
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Table 2-4. Spalling strength for the different rock types in TU5.
Rock type Rock mass spalling strength �sm
Pegmatitic granite 61.6 MPa Migmatitic gneiss 65.6 MPa
Spalling occurs when the tangential stress expressed with principal stresses reaches the rock mass spalling strength listed in Table 2-4. The tangential stress, and therefore spalling, depends greatly on the direction between the tunnel and the major principal stress direction. If the tunnel is parallel to the major principal stress direction, it is much more unlikely to have spalling compared to a tunnel perpendicular to the major princi-pal stress direction.
2.5 Constitutive models
The media used is isotropic, since the rock mass is considered not have any remarkable faults or slip surfaces. The foliation of the rock is not taken into account in the simula-tions. The 3-D FEM and 3-D BEM analysis are carried out using elastic conditions to calculate the stress state after the excavation. Following the consideration given by Mar-tin & Christiansson (2008), the results are then compared to the known spalling failure limits with the constitutive equation of the differential stress �1 – �3 > �sm 3 where �1 is the major principal stress concentrated around the excavation, �3 the minor principal stress concentrated around the excavation, and �sm the rock mass spalling strength. The method used is very simple, because it permits to use only linear elastic models without having a large amount of data deriving from plastic and non-linear models, but it gives, nevertheless, quite usable results. The spalling depth can be approximated empirically with equation 4 by Martin & Chris-tiansson 2008. For the Monte Carlo simulation, the constitutive spalling depth equation for a round profile can be written as
� � � ���� ������
����� for ��� > �sm 4 where Sd is the spalling depth, a is the radius of the round profile, and ��� is tangential stress. The equation 4 is only valid when tangential stress reaches rock mass spalling strength, which in this case is 57 percent of the UCS. According to Martin & Christiansson (2008), the equation 4 can be used with a Monte Carlo simulation to predict spalling probability—with the probability of tangential stress reaching spalling strength—and spalling depth. Difference from the method de-scribed by Martin & Christiansson is that the tangential stress in this report is calculated by using the equation 1, with actual shape of the excavation profile, instead of assuming a round profile. This ensures the use of more accurate tangential stress values; however the analysis is highly localized. However it should be noted that the spalling depth is still calculated using empirical equation 2 determined for round profile. The relationship between the spalling potential and spalling depth is illustrated in Figure 2-3.
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Figure 2-3. The spalling failure criterion developed for the brittle failure prediction using numerical continuum software (Martin & Christiansson 2008, Figure 5). The factor of safety against spalling can be calculated by using the following equation: �� � ���
��� 5
2.6 Rock burst potential
A rock burst is a violent and really dangerous form of spalling, usually occurring in deep mines where stresses are higher than in the ONKALO. The rock burst potential can be presented with the relationship of UCS and the ratio of the UCS and the tensile strength as presented in Figure 2-4. Using the mean values, the spalling potential of the Olkiluoto rocks can be characterised as low. The deviation of the values is, however, large, which can lead to a situation where some parts are more vulnerable for bursting (spalling).
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Figure 2-4. The spalling potential of ONKALO rocks (modified after Diedrichs 2007 page 1085).
PGR
MIGM.GN
UCS/T
UCS [MPa]
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3 DESCRIPTION OF THE MODELS
To study the problem of spalling in the TU5 area, a numerical three-dimensional finite element method (3-D FEM) and a three-dimensional boundary element method (3-D BEM) are used to simulate the effect and influence of the sequential excavation, tunnel geometry, and of the connecting tunnels (for example between a tunnel or a niche and a shaft).
3.1 Analysis tools
The software used for the main calculation has been MIDAS/GTS (version 3.0.0 R4), a geotechnical 3-D FEM that is able to calculate complex geometries rather easily. With this software, it is possible to carry out almost every type of geotechnical calculation, both for excavation stability and for seepage. The analysis can be conducted in static or dynamic conditions, a good feature when we consider any problem deriving from vibra-tions, such as earthquakes or blasting. The geotechnical model can also be chosen be-tween linear elastic, nonlinear or elastoplastic. In the analysis described in this report elastic conditions have been used. Most of the models have also been verified with Rocscience Examine3D, geotechnical 3-D BEM software, which returns the results with a high analytical precision. Verifying the depth of the spalling with a 3-D BEM model gives high analytical confidence to the current parameters.
3.2 Divisioning of the models
Seeing that the area of interest is large and the requested level of detail is high, the area has been divided into six smaller sub-models. Every part has been developed using the highest density of discretization possible that is able to return the most accurate result using a reasonable quantity of element to avoid overload problems. Every model has been built as a separate block. The distance from the border of the excavation to the border of the block has been cho-sen by following the general rule by the software provider to be 5–6 times of the diame-ter of the excavation. This, however, is quite conservative. Nevertheless, seeing that the geometry is quite complex, a distance of almost 8 times (up to 50–60 metres), depend-ing on the areas, has been used.
The six models used for the TU5 spalling calculation are (shown in Figure 3-1):
1. Demonstration tunnels, two small dimension tunnels located north from the ac-cess tunnel
2. Crossing area, located in the cross area between the demonstration tunnels and the access tunnel to the main level of TU5
3. Access tunnel – part 1, a part of the access tunnel 4. Access tunnel – part 2, a part of the access tunnel with three connecting tunnels 5. Shaft access drift area, including three shafts 6. Technical rooms, an east–west oriented hall with several access tunnels to the
TU6 part.
15
Figure 3-1. Location of the numerical models.
3.3 Elements and the mesh
The elements used in all the models, as presented in Figure 3-2, are tetrahedron solids. The order of these elements is quadratic (second order), using midside nodes that are nodes added at mid-span of edge for the matrix resolution.
Figure 3-2. Example of 3-D meshing in the model 5.
16
In general, midside nodes should be used when results can vary significantly over a sin-gle element. In spalling analysis, the tangential stress is at its maximum at the excava-tion surface and decreases quickly with distance. Using midside nodes allows for get-ting accurate results around the excavation surface with fewer elements, as the element size at excavation boundary can be coarser. The disadvantage with the midside nodes is that they increase the degrees of freedom (DOF) of each element. This increases the complexity of the stiffness matrix and the governing equations, increasing the calcula-tion time. In case of big models with irregular and complex geometries, the number of nodes and elements increases during the discretization in the link areas, and it is necessary to de-crease the density of the mesh (determined by the distance of the nodes) in the less im-portant areas (far from the excavations and close to the model boundaries) and to in-crease the density of the mesh in the important parts. The density of the mesh varies within one model: at the model boundary, the distance between the discretization points is up to 20 metres, whereas in the most critical areas (mainly in the tunnel and shaft links), the discretization is 0.5 metres with midside nodes. The discretization underlines again the motivation to use the quadratic elements: in some areas, for example close to the bottom of the shaft, the stress varies considera-bly in a range of a few centimetres, and discretization of 0.5 metres without midside nodes turns out too sparse in order to obtain a realistic behaviour of the stress distribu-tion. The maximum number of elements used for every model has been limited to around 250,000 to reduce the time of calculation and to make the post-processing of the results easy.
3.4 Monte Carlo simulation
In addition to three-dimensional methods, a Monte Carlo method is used to study the probability of spalling and also to acquire some statistical knowledge on the subject. A Monte Carlo simulation takes pseudo-random samples of the rock stress distribution and the rock strength distribution. The random samples from the separate distributions are combined, and spalling probability and depth in single cases is studied multiple times in order to achieve statistical significance.
17
4 RESULTS OF THE ANALYSIS
In this part the results of the numerical modelling and the Monte Carlo simulation are presented. For a better understanding of the results, also the problem for each model will be discussed in corresponding paragraphs.
4.1 General of the results
The calculations are purely elastic since it is considered to be conservative for the spal-ling prediction. This is because the stresses around the excavations are highest in elastic conditions. If the yielding of the rock would be taken into account, stresses would be relieved due to the rock failure. The models don’t take into account progressive failure during excavation. The calculation has been carried out using both the conservative (90 % fractile) and the average stress conditions (see Table 2-1). Every model is presented with the geometry of the tunnel in dark yellow and the spalling by using an isosurface with a stress limit equal to the spalling stress (as shown in the table 2-3, 65.6 MPa for the migmatitic gneisses and 61.6 MPa for the pegmatitic granite); in green for the conservative models and in red for the average models. It should be also noted that the spalling doesn’t nec-essarily occur as predicted, due to the heterogeneity of the rock stress and the rock strength. In the models, it is sometimes possible to notice spalling in the edges of the tunnels: these stress concentrations are mainly due to the particular geometry of the excavations with sharp angles equal to or less than 90°. These peaks are shown also in the verifica-tion with 2-D BEM software (Figure 4-1). The calculation of the spalling depth does not consider these points, because the rock is broken during the blasting process especially from sharp corners at the bottom.
Figure 4-1. Stress peak in the bottom angle.
18
4.2 Brittle spalling simulation
The results of the 3-D FEM analysis for the whole area are presented in the following sections.
4.2.1 Demonstration tunnels
The demonstration tunnels are built in the disposal depth, and the rock characterisation and classification procedure will be tested there. The demonstrations done there will be a part of Posiva’s final disposal facilities construction licence application that will be submitted in 2012. According to Anttila et al. 2009 the demonstration tunnels will also possibly form a part of the first panel of deposition tunnels. (Anttila et al. 2009 p. 47) The rock in the demonstration tunnels must be as unaffected by the excavation as possi-ble, taking into account also the need to minimize spalling. Because of the uncertainties in the stress direction, a special attention has been paid to orient the demonstration tun-nels parallel to the maximum principal stress. The area has been modelled not only with the expected stress orientation and magnitudes as expressed in Table 2-1, but also ac-cording to the latest unofficial stress results. The new data has been obtained from in-vestigations carried out during the excavation of TU4. During the writing of this report, the demonstration tunnel layout and direction was changed according to these latest stress results. The first case in Figure 4-2 (a) presents an expected, rather high level of spalling, in part due to the presence of the worst type of rock mass (pegmatitic granite) in a part of the curve. The measured maximum depth is equal to 160 millimetres. As shown in the picture, the two tunnels of the in situ tests are not influenced by the spalling phenomenon, mainly because the scenario used for this model has been with the main horizontal stress �1 parallel to the longitudinal axis of the tunnel. Thus in this particular layout, the crowns of the tunnels are not stressed enough to have spalling ar-eas. This scenario is, nevertheless, the worst case possible, seeing that it is calculated considering the highest stress conditions found during the investigations. The average calculation underlines that with the lowest stress field (around 20 % less), there is no spalling, if the sharp corners of the shapes are ignored, as is clearly shown in Figure 4-2 (b). It is possible to notice that the points with spalling are situated exactly around the edges exposed to the flux of the maximum stress, as for example the edges of a house under the action of wind.
19
(a) (b) Figure 4-2. Predicted spalling (green in left and red in right) in the demonstration tun-nel (model 1 in Figure 3-1) in conservative stress conditions (a) in left and average stress conditions (b) in right. No spalling is predicted in average stress conditions. In the calculation with the varied stress directions, a disposal canister hole was added to examine the spalling of the hole. Also a niche was removed from the model to simplify the geometry. Figure 4-3 (a) (conservative stress conditions) and Figure 4-4 (a) (average stress conditions) were calculated with the same stress direction as the previous model (Figure 4-2) to verify the new simplified conditions. As shown in Figure 4-3 (a) and Figure 4-4 (a), spalling occurs exactly in the same place with the same value of depth. However, less spalling occurs in the sharp corners due to the simplifications. The other three models ((b), (c), and (d) in Figure 4-3) are completely different from Figure 4-3 (a), especially because the main stress directions are quite perpendicular. For this reason, all the tunnels that were in the first case perpendicular to the maximum stress, and thus more stressed, are in the other three models parallel to the stress, and thus the crown is not particularly compressed by the ��. The model in Figure 4-3 (b) refers to the stress field measured in TU4 at the level of -345 metres, which presents a maximum stress quite perpendicular to the demonstration tunnels, in which the maximum spalling depth measured has been 550 millimetres. The model in Figure 4-3 (c) is characterised by a stress field rotated by -30°, and so the an-gle between the longitudinal axis of the demonstration tunnel and the maximum stress direction is higher: for this reason the spalling depth in this case is lower than before, with a value equal to 250 millimetres. Lastly, the model shown in Figure 4-3 (d) pre-sents a stress field rotated by +30° to the measured direction from the TU4 level -345 m. In this case the angle is similar to the previous model, although in the opposite direc-tion. The spalling depth in Figure 4-3 (d) is 480 millimetres at the maximum. In Figure 4-3 it is also possible to verify spalling around the deposition holes in the four different cases. It is easy to notice that the behaviour is approximately the same with the spalling areas just rotated according to the direction of the maximum stress. This phe-nomenon can be related to the fact that the holes are linked to the tunnel at the bottom level, where the boundary is slightly stressed. Furthermore, of course, the shape of the hole is perfectly symmetrical, seeing that it is a circle, and thus, in absence of any other external influences, its behaviour is the same when the stress field is rotated.
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21
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23
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18
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25
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den in s. The hat the ilt us-an be
26
reduced by changing the geometry to favourable, in this case elliptical. The ellipse’s major and minor axis ratio is determined as follows with equation 6: A / B = �1 / �2 6 The ellipse axis ratio is 6.0 m / 4.5 m in the personnel shaft. In this case, spalling occurs only in the central part of the shaft, because the link is located in the bottom of the niche where the beginning tangential stress is very low, and so we have a peak of stress just in the middle, as is clearly shown in Figure 4-9.
Figure 4-9. Predicted spalling (in green) in the shaft access drift area in conservative stress conditions (downward shaft). In the calculation carried out with the average stress conditions, the model still pre-sented some spalling areas, so it is easy to expect this phenomenon in this area in all stress conditions. The position of spalling areas, as shown in Figure 4-10, is always in the most critical areas noticed in the conservative model, that is shafts a and b, where the links are particularly delicate for the positions in which they are. By using the same considerations as with the conservative model, the rock around the shafts in this case has the same behaviour: at the bottom, the depth of spalling is high and it rapidly de-creases going upwards, but in this case, the spalling disappears in the middle of the shaft shown in Figure 4-10, because it is not dependent on the fact that the height of the shaft is limited. For this reason, in fact, there is no spalling in shaft c: thus spalling in these stress condi-tions rises only with the compression of the arch of the crown. The area predicted to have spalling is extended over an area of five metres up the shaft, and the maximum spalling depth measured at the bottom is 750 mm for shaft a and 600 mm for shaft b; regarding the spalling along the shafts, the maximum depth is 120 mm for shaft a and 100 mm for shaft b.
20 m 0 m
27
Figure 4-10. Predicted spalling (in red) in the crossing area (model 5 in Figure 3-1) in average stress conditions.
4.2.6 Technical rooms
The last model performed for this work has been done considering the series of tunnels linked to the TU6 part, shown in Figure 4-11 (a). It is composed by one hall driven in the direction of the maximum main stress �1 and it does not present, as in the previous models, any spalling areas. In addition to this, the model comprises of two other tunnels that are perpendicular to the hall and one that is inclined at 60° with respect to the hall. In Figure 4-11 (a) is also shown the spalling distribution in conservative stress condi-tions: the maximum depth measured in the perpendicular upper tunnel is 120 mm and in the tilted tunnel 80 mm. On the contrary, there are no spalling areas in the central tun-nel, probably because the shape is lower (only 4.4 m high) and because the tunnel is located between two other tunnels that reduce the stress flow. Instead, the model in average stress conditions does not present any spalling areas, as already noticed in the majority of the previous models analysed. Even in this case this scenario can be considered to be quite realistic compared to the conservative, because the spalling depth is low also with the high stress conditions.
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redicted spa): conservat
on of the t
hing to do wf the numerfrom bad prods or usins been verifiof the resulte spalling dodels and th
ications havpossible. Taccuracy in
ulation makeore it is not oundary or
he models rding. In the n
tration tunn
was carried
g area
was carried
unnel – pa
nd with the Fit is possiblss of 65.6 Mbetween 30spalling canhe spalling d
m
120 mm
alling (greetive (a) and
tangential
when we prrical code anrocesses or
ng software ied by usingts obtained.
depth has behe shaft are
ve been donThe main adn which thees it imposspossible to inside prede
eturned resunext paragra
nels
d out on this
d out on this
rt 1
FEM modele to read thMPa, reachi0 and 500 m
n be considdepth and fo
m
80 mm
�1
�2
28
(b)
n in left andaverage (b)
l stresses
roceed to annd the verifmissing dausing diffe
g 3-D BEM This verifieen found qa.
ne using thedvantage ine 3-D data csible to incluverify everyefined calcu
ults very simaphs are sho
s area.
s area.
el, this part phe value of ing high valmm, as alreadered valid, or the ���.
�1
N �2
d red in rig) stress.
at excava
nalyse casesfication of thata. This prerent proced
M software inication has bquite notabl
conservativn using thiscan be manude rock heywhere the ulation poin
milar to eacown the res
presents qutangential slues close toady found ialso becau
ght) in the T
ation boun
s with a numhe model, torocedure cadures. In thn order to cbeen done ile for the w
ve data in os software inaged and ceterogeneity
level of strnts.
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uite a notablstress ��� tho 90 MPa. Tn the mode
use the valu
TU6 links (m
ndary
merical moo avoid any
an be done his case, theheck the anin the three
work: both o
order to estais the speedcalculated. Hy or to use pess in point
d we can ash model.
le spalling dhat is higherThe extent ol with 3-D F
ues are quit
model
del is y mis-using e 3-D nalyti-
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ssume
depth. r than of the FEM. te the
(
Figutunne
4.3.4
The FEMthe lextenstres
(
Figutunne
4.3.5
Evenmeth
a)
ure 4-12. Tael – part 1 (
4 Access t
3-D BEM iM in Figure 4limit of 65.nt of the sps.
a)
ure 4-13. Tael - part 2 (
5 Shaft acc
n with the chods returne
angential str(model 3 in
unnel – pa
in Figure 4-4-7 (a), with6 MPa. Thpalling area
angential str(model 4 in F
cess drift a
complex geoed the same
ress (a) andFigure 3-1)
rt 2
-13 shows eh the indica
he predicteda is quite b
ress (a) andFigure 3-1)
area
ometry in the scenario w
�1
�2
29
(b)
d predicted s).
exactly the ation of the d maximumbig in the t
d predicted s).
he shaft accwith many s
�
N �2
�1
�2
spalling (b)
same behavmaximum
m stress staytunnels per
(b)
spalling (b)
cess drift arspalling area
�1
�1
N �2
) in the mod
viour as alrtangential s
ys moderaterpendicular
) in the mod
rea, the diffas in the tu
1
del of the ac
ready seen istress foundely high, buto the prin
del of the ac
ferent calculunnel crown
ccess
in the d over ut the ncipal
ccess
lation ns and
30
at the links between tunnels and shafts. Also the measured spalling depth is comparable to the results found before. In Figure 4-14, the attention is focused on the most critical area of the whole TU5, the two shaft access drifts where the main access tunnel is linked to the ventilation shafts. The main thing visible in this figure is the level of tangential stress around the excava-tion that is higher than the spalling strength: the maximum value at the excavation sur-face is close to 130 MPa. This is higher than the uniaxial compressive strength of the intact rock, so failure is probable. The area in these conditions is subject to plastic fail-ures with possible crack propagations around the shafts.
Figure 4-14. Tangential stress in the shaft access drift area (model 5 in Figure 3-1).
4.3.6 Technical rooms
No BEM study was carried out on this area.
4.4 Monte Carlo simulation
The Monte Carlo simulation can take into account the large deviation of the uniaxial compressive strength and the uncertainties in the in situ stress. The sufficiency of the random samples (n=10000) was confirmed by repeating the process and by examining the deviation of the results. The used method is described more detailed in section 2 and in references by Martin 2005 and Martin & Christiansson 2008. The VGN and DGN samples (a total of 772 samples) were chosen for this study because of their representativeness and low strength. Five percent of the highest and lowest frac-tiles were removed from the sampling to reduce disturbance (n=691 samples). For each profile a shape factor is calculated using a 2-D BEM method. The shape factors are listed in Table 4-1.
�2
N �2 �1
�1
31
Table 4-1. Shape factors (A and B) for the excavation profiles.
Profile 1 A = 2,91 Profile 1A A = 2,99 Profile 2 A = 2,60 Profile 3 A = 2,28 Profile 3A A = 2,34 Profile 3B A = 2,64 Profile 4 A = 2,20 Profile 5 A = 2,48 Profile 7 A = 3,21 Profile 13B A = 2,65 Profile 13C A = 2,32 Profile 13E A = 2,33 Bnom = 11,0 m Profile 14 A = 2,91 Bnom = 11,0 m After Uotinen et al. (2009) for the stress a triangular distribution was used with the 10 %, 50 %, and 90 % fractiles (see Table 2-1) as the tips of the triangle. The triangular distribution was corrected with the shape factors and the direction of the stress to calcu-late the tangential stress distribution for the area with equation 1. The distribution of the strength is scaled with factor 0.57, which presents the rock mass spalling strength. The statistical spalling probability of a random case is presented in Figure 4-15.
Figure 4-15. The results of the statistical approach to spalling probability is shown in figure (in green the deviation of strength parameters and in red the corrected tangential stress distribution). The factor of safety is calculated from the mean values.
0 %2 %4 %6 %8 %
10 %12 %14 %16 %18 %20 %22 %24 %26 %28 %30 %32 %34 %
2,5
12,5
22,5
32,5
42,5
52,5
62,5
72,5
82,5
92,5
102,
5
112,
5
122,
5
132,
5
Prop
ortio
n of
the
sam
ples
Stress [MPa]
Spalling P = 23 % No spalling P = 77 %
FOSmean = 1,26
Tangential stress
Strength
32
According to the Monte Carlo simulation, 23 percent of the local rock strength and in situ stress combination cases coupled together will have spalling. This implies that for a 100 metre long tunnel, there is 23 metres of tunnel in which some spalling will occur. The mean factor of safety (FOS) is 1.26. The mean spalling depth for the cases with spalling can be calculated using equation 4. The calculated depths of spalling are pre-sented in Figure 4-16. The mean depth of spalling is 0.28 metres with a range from 0 to over 1 metre.
Figure 4-16. The distribution of predicted spalling depth and the mean depth of spalling are illustrated in the figure.
4.5 Comparison of the results of the 3-D FEM and Monte Carlo
The results of the 3-D FEM and Monte Carlo can be compared in order to investigate the reliability of the results respective to each other. In the demonstration tunnel with the conservative stress conditions (Figure 4-3 (a)), according to the 3-D FEM model, the spalling depth is 160 mm. Respectively calculated with equation 4, as described in the previous chapter, the mean spalling depth is 120 mm, the mean factor of safety calcu-lated with equation 5 is 1.13, and spalling probability is 34.7 %. In part 1 of the access tunnel with the conservative stress conditions (Figure 4-6 (a)), according to the 3-D FEM model, the spalling depth is between 200 mm and 300 mm. Respectively, the mean spalling depth is 130 mm and the mean factor of safety is 1.12 with a spalling probability of 36.5 %. Taking into account the differences in the used methods the results are quite close to each other.
0,00 %
0,50 %
1,00 %
1,50 %
2,00 %
2,50 %
3,00 %
3,50 %
0,02
50,
075
0,12
50,
175
0,22
50,
275
0,32
50,
375
0,42
50,
475
0,52
50,
575
0,62
50,
675
0,72
50,
775
0,82
50,
875
0,92
50,
975
>1,0
00
Prop
ortio
n of
the
sam
ples
(n=1
0000
)
The depth of the spalling, [m]
ds,mean (0,28 m)
33
Previously Hakala et al. (2008, p. 25) compared the spalling prediction with Monte Carlo and fuzzy number approaches and concluded that there is also acceptable agree-ment on the spalling percentage between the two methods. The spalling depth resulted in similar shape of the distribution tail however the mean depths differenced. It was concluded that the empirical relation for spalling depth needs to be calibrated to the Olkiluoto circumstances.
34
5 CONCLUSION
The aim of this work has been to predict areas that will have spalling in the TU5 part of ONKALO and to provide initial data for rock reinforcement design of the area. The methods used were a Monte Carlo simulation and 3-D FEM, and 3-D BEM for verifica-tion. The parameters used were mostly acquired from Olkiluoto Site Description 2008 (Posiva 2009) and the layout of the spring 2010. The thermal stresses were not taken into account in this analysis. There are still uncertainties related to in situ rock stress, and the parameters used for the TU5 models are not acquired directly on the site. The major principal stress direction has a lot of variation, and the deviation of the rock strength is large. Although the pa-rameters used are the best available knowledge of the area, this analysis is valid only for the current parameters used and must be redone if parameters change significantly. Due to the uncertainty of the major principal stress direction and because of the recent meas-urements differing from the assumed direction, a sensitivity study on a demonstration tunnel was included in this work. Spalling is, expressly, an event that can create problems, not so much for the overall stability of the underground facilities, but rather in particular areas, where spalling can cause unnecessary over-excavations and hazards when not supported properly fast enough. When this phenomenon is expected during the excavation, because the in situ stress is high enough to create tangential stress that can damage the shapes, it is impor-tant to verify the position of spalling areas. With complex shapes as in this case, the only way is to use numerical methods that can easily establish the behaviour of the boundaries, because with any other method it is quite impossible to consider the influ-ences between the excavations. Thus this analysis underlined strongly the necessity to create this kind of models. In fact, it is the only effective method to have a distinct idea of the behaviour of the excavation from the stress’s point of view. The models showed the necessity to use three-dimensional methods, since the behaviour of the excavations is notably influenced by the proximity of any other tunnels or cav-erns. For sure, in a large area like the one considered in this work, it is impossible to create only a single model for everything, because to obtain a sufficient level of detail it might be necessary to use millions of elements, a number that is quite impossible to manage using current modelling software and computer hardware in market.
5.1 Predicted spalling for TU5
The results of the calculations show spalling areas that are quite scattered to be ex-pressed unambiguously. One solution found for this problem is the creation of a spalling map that collects all of the information about spalling in a graphic way. This particular map of TU5 is shown in Figure 5-1.
Figu Manynels, strestial srock The mdriftsdoubured const
ure 5-1. Pre
y of the spa have the los. In these a
stresses, butpieces in a
models unds. In those abled the val
spalling detruction cha
dicted spall
alling areas ongitudinal areas the spat the area iswide area.
derlined thatareas the levlue obtainedepth is highallenge.
1250 mm
ling (green)
presented iaxis perpenalling depth quite exten
t another imvel of spallind in the samher than one
m
500 mm
�1
�2
35
) for TU5.
in Figure 5-ndicular to th is not extrnsive, which
mportant areng is very h
me areas wie metre, wh
m
N �2
-1, especialthe directionremely highh can create
ea where sphigh; in somithout the s
hich creates
�1
2
lly in the crn of the ma, and neithee a possible
alling occurme cases theshafts. More a difficult
0m
rowns of theaximum priner are the tane fall of dam
rs is shaft a tangential eover, the mengineering
5
e tun-ncipal ngen-
maged
access stress meas-g and
50m
36
A probabilistic assessment of spalling can be created by using calculated results of the tangential stress with equation 5 regionally. This way the probability of spalling can be presented as a map. While drawing the map some generalisation was done as can be noted in Figure 5-2.
Figure 5-2. Risk of spalling in the TU5 area. The sensitivity analysis of the demonstration tunnels with conservative stress parame-ters clearly shows that the tunnels must be oriented parallel to the major principal stress direction. A mistake in direction of 45 degrees shows minor spalling at the roof of the demonstration tunnel. This problem must be tried to be solved mainly by turning the layout to advantageous direction, but secondly with instant rock support measures.
5.2 Rock reinforcement
To meet the engineering challenge the rock support has to be able to withstand the rock weight of the predicted spalling area (Martin 2005, p. 64). Secondarily, a rock support can provide minor support stress and in some minor cases even prevent the spalling from happening. Structures that can withstand movements without breaking are the best solutions as support, for example pre-tensioned anchors together with steel wire mesh
37
and shotcrete. The volume of spalling can be measured or estimated from the models as presented in Figure 5-3. Also the rock bolt lengths can be checked to stretch beyond the spalling.
Figure 5-3. The calculation of spalling volume. In the shafts the area predicted to have spalling will have to be supported thoroughly, for example as presented in Figure 5-4 by Siren et al. (2009, Figure 10), where the rock bolts are installed to intermediate stress direction.
Figure 5-4. Supports are installed to intermediate stress direction in order to control the spalling rock.
38
5.3 Monte Carlo -simulation
The deviation of the rock strength samples leads to a situation where a tunnel can have local spalling although the main parts of the excavations are stable. Although the con-servative option affects only stress magnitude, the models with conservative parameters can actually predict spalling locally in the weakest band of rock, possibly between rock contacts. To study this possibility a Monte Carlo simulation was done. The results of the Monte Carlo simulation show that with the realistic distributions of in situ stress and rock strength, spalling will occur in 23 percent of the rock strength and in situ stress combination cases coupled together with a mean spalling depth of 0.28 me-tres. These results can be more realistic because the actual conditions are most probably within the Monte Carlo simulation range. The results of the Monte Carlo simulation were compared to the 3-D FEM results, and the results were observed to be quite close to each other.
5.4 Discussion
This work underlined how important it is to evaluate the stability of an excavation, even if carried out in hard rock with low fracture intensity. Although the rock quality is very good, the deviation of the rock strength parameters is large. More research is needed on how to take the large deviation into account in rock mechanics modelling and especially in spalling prediction. Also one of the most critical parameters to verify during the construction of TU5 is the level and direction of in situ stress in order to check the possible failure of the rock mass and to possibly change the layout. More research is needed on this subject and as this report is being published and the access tunnel is advancing deeper, Posiva has per-formed more LVDT-cell measurements, which indicate major changes in the in situ stress direction at the TU5 level. The spalling predictions clearly need to be revised later on according to the latest test results.
39
REFERENCES
Andersson, C. J. 2007. Rock mass response to coupled mechanical thermal loading. Äspö Pillar stability Experiment. Doctoral Thesis, KTH, Sweden. Anttila, P., Arenius, M., Haapala, K., Hansen, J., Hellä, P., Jalonen, T., Lahdenperä, J., Lyytinen, T., Mellanen, S., Vuorio, P., & Äikäs, T. 2009, Testing and Demonstrations in ONKALO - Aims and Needs. Working Report Posiva 2009-24. Diederichs, M. 2007. The 2003 Canadian Geotechnical Colloquium: Mechanistic inter-pretation and practical application of damage and spalling prediction criteria for deep tunnelling. Canadian Geotechnical Journal 44(9): (2007) pp. 1082–1116. Hajiabdolmajid, V., Kaiser, P.K., Martin, C.D. 2002. Modelling brittle rock failure. In-ternational Journal Rock Mechanics and Mining Science, 39(6): pp. 731–742. Hakala, M., Hudson, J.A., Harrison, J.P. & Johansson, E. 2008. Assessment of the Po-tential for Rock Spalling at the Olkiluoto Site. Working Report Posiva 2008-83. Hakala, M., Kuula, H. & Hudson, J. A. 2005. Strength and strain anisotropy of Olkilu-oto mica gneiss. Working Report Posiva 2005-61. Hoek, E., Brown E. 1980. Underground excavations in rock. Institute of Mining and Metallurgy, London, p. 527. Hoek, E., Diedrichs, M. S., 2006. Empirical estimation of rock mass modulus. Interna-tional Journal of Rock Mechanics & Mining Sciences 43 (2006) pp. 203–215. Hoek, E., Kaiser, P. K. & Bawden, W. F. 1995. Support of Underground Excavations in Hard Rock. Rotterdam: A.A.Balkema. Martin, C.D., 2005. Preliminary assessment of potential underground stability (wedge and spalling) at Forsmark, Simpevarp and Laxemar sites. SKB Rapport R-05-71. p 74. Martin, C.D. & Chandler N. A. 1994. The progressive fracture of Lac du Bonn Granite. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., pp. 31, 6, 643–659. Martin, C.D. & Christiansson, R., 2008. Estimating the potential for spalling around a deep nuclear waste repository in crystalline rock. International Journal of Rock Mechan-ics & Mining Sciences 46 (2009) pp. 219–228. Mattila, J., Aaltonen, I., Kemppainen, K., Wikström, L., Paananen, M., Paulamäki, S., Front, K., Gehör, S., Kärki, A. & Ahokas, T. 2008. Geological Model of the Olkiluoto Site, Version 1.0. Working Report Posiva 2007-92. Posiva 2009. Olkiluoto Site Description 2008. Report Posiva 2009-01. Posiva 2010. Posiva Oy –Internet page. Tietopankki, Kuvapankki. http://www.posiva.fi/files/1183/100325_loppusijoitustilat.JPG, 22.9.2010.
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Siren, T.,Uotinen, L.,Ström, J.,Lehmusjärvi, R., Rinne, M., 2009. Spalling prediction methods in high stress conditions. Rock Engineering Seminar 4.-5.11.2009, Finnish Tunneling Association FTA and The Finnish National Group of ISRM. pp. 207-216. Uotinen, L., Siren, T., Lehmusjärvi, R. 2009. Stochastically determined safety of under-ground structures according to Eurocode. Rock Engineering Seminar 4.-5.11.2009, Fin-nish Tunneling Association FTA and The Finnish National Group of ISRM. pp.149-158.