In situ strength and failure mechanisms of migmatitic gneiss and pegmatitic granite at the nuclear waste disposal site in Olkiluoto, Western Finland

International Journal of Rock Mechanics & Mining Sciences 79 (2015) 135–148

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In situ strength and failure mechanisms of migmatitic gneiss and pegmatitic granite at the nuclear waste disposal site in Olkiluoto, Western Finland T. Siren a,n, M. Hakala b, J. Valli c, P. Kantia d, J.A. Hudson e, E. Johansson f a

Aalto University, Espoo, Finland KMS Hakala Oy, Nokia, Finland c Pöyry Finland Oy, Vantaa, Finland d Geofcon, Rovaniemi, Finland e Rock Engineering Consultants and Imperial College London, UK f Saanio & Riekkola Oy, Helsinki, Finland b

art ic l e i nf o

a b s t r a c t

Article history: Received 24 March 2015 Received in revised form 20 July 2015 Accepted 11 August 2015

The primary goals of the experimental work described in this paper were to establish the in situ spalling/ damage strength of the intact rock, to establish the state of the in situ stress, and for the work to act as a Prediction–Outcome exercise within the context of confirmatory underground site investigations being conducted at the nuclear waste disposal site at Olkiluoto, in Western Finland. To establish the in situ mechanical properties, Posiva (the Finnish implementer) formulated the in situ Posiva Olkiluoto Spalling Experiment (POSE) in 2009. The outcome of this experiment was that rock failure mainly occurred due to structurally controlled factors, rather than being dictated solely by the expected location of the maximum stress. The POSE experiment also showed that the onset of fracture initiation in the Olkiluoto rock occurs at 40 MPa, and the rock mass strength is ca. 90 MPa-compared to the mean laboratory value of 104 MPa. In view of these observations made at the projected disposal depth, the vertical disposal concept, KSB-3V, will involve initiation of new fractures, although the horizontal disposal concept would be less affected by such fracture initiation. However, neither of the disposal concepts is expected to suffer any major rock mass failure, and the vertical disposal holes are not particularly sensitive to fracture initiation if the disposal tunnels are oriented within 30° trend of the major principal stress direction. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Rock mass strength In situ experiment Structurally controlled failure Spalling potential Nuclear waste disposal Olkiluoto ONKALO

1. Introduction Currently in Finland and Sweden, the nuclear waste disposal projects are heading towards the implementation phase in the next ten years or so. At Olkiluoto Island, located in Western Finland, the Finnish nuclear waste management company, Posiva Oy, has created the ONKALO rock characterization facility (Fig. 1) in the future nuclear waste disposal site, for confirming site studies. The Olkiluoto site has been subject to thorough research for many years and, at the end of year 2012, Posiva Oy submitted an application for the construction licence for the future nuclear waste repository. There are still confirmatory studies being carried out, related, e.g., to the in situ stress and the rock strength, which are the most important rock mechanics parameters determining the n Correspondence to: Aalto University, Rock Engineering, P.O. Box 12100, 00076 Aalto, Finland. E-mail address: topias.siren@aalto.fi (T. Siren).

http://dx.doi.org/10.1016/j.ijrmms.2015.08.012 1365-1609/& 2015 Elsevier Ltd. All rights reserved.

potential for rock failure and the extent of failure in the vertical KBS-3V and horizontal KBS-3H (Fig. 2) nuclear waste disposal concepts.1 In these two concepts, the nuclear waste is packed in a copper canister having an iron insert and surrounded with compacted bentonite as a deposition hole and tunnel filling.2 The particular interest lies in understanding the potential for continuous rock damage (i.e. spalling) around the circular disposal holes and the disposal tunnels in the Olkiluoto rock mass. The long timespan of the project (ca. 100 years) and the long term safety aspects (of the order of 100,000 years), necessitate a more comprehensive understanding of the rock mass properties and emphasize the need for in situ experiments. Continuous rock damage/ spalling events can potentially create radionuclide transport routes in situations where the nuclear waste canisters are damaged. The KBS-3V concept uses ca. 1.8 m-diameter boreholes drilled from the floor of a disposal tunnel to a depth of 8 m. The Atomic Energy Canada Limited (AECL) and Swedish Nuclear Fuel and Waste Management Company (SKB) carried out extensive in situ experiments to establish the rock mass strength in isotropic

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Fig. 1. Experimental area in the ONKALO underground facility with OL-BFZ020a, OL-BFZ020b and OL-BFZ099 brittle failure zones for the work described in this paper.

Fig. 2. Vertical KBS-3V and horizontal KBS-3H disposal concepts for nuclear waste disposal.

granitic crystalline rocks for nuclear waste disposal. AECL conducted several in situ experiments at their Underground Research Laboratory (URL).3–8 The in situ rock mass strength was studied in the Mine-By Experiment (MBE) and subsequent Heated Failure Tests (HTF) at -420 m depth in highly stressed Lac du Bonnet granite.3 The MBE test tunnel was orientated perpendicular to the maximum principal stress (s1 ¼607 3 MPa 145°/11°) while the other principal stress components were s2 ¼45 MPa (54°/08°) and s3 ¼ 11 MPa (290°/77°).4 Notch formation was initiated when the tangential stress reached 120 MPa which is ca. 56% of the mean peak laboratory uniaxial compressive strength (UCSm).9 The HTF tests were executed in the MBE test tunnel floor in four phases.6 In the tests, 0.6 m diameter holes, and in one stage a pillar between holes, were heated up to 85 °C to simulate the thermal boundary conditions during disposal. Breakouts observed in the experiments exhibited a radial extent of 70 mm.7 The experiments were monitored using acoustic emission (AE) equipment, which indicated that with additional confinement (100 kPa), the AE activity decreased.7 SKB conducted the in situ Äspö Pillar Spalling Experiment (APSE) in SKB’s Äspö Hard Rock Laboratory (HRL), in Sweden, at the  450 m level.10 The in situ spalling strength (also in situ rock mass strength) of Äspö diorite was found to be 122 MPa, which is ca. 5874% of the UCSm (211 MPa).10 Andersson back-calculated the maximum principal in situ stress at the experimental site and concluded that it is 30 MPa.10 Based on the experiments, both

from Canada and Sweden, the methodology for estimating the spalling potential in crystalline rock was further developed by Martin and Christiansson.9 However, the rock mass at the Olkiluoto facility consists of migmatitic gneiss with pegmatitic granite inclusions. This Olkiluoto rock behaves in a brittle manner in the laboratory11; however, it is unlikely that the isotropic granitic rock mass response encountered during the AECL and SKB experiments will be fully representative of the anisotropic and heterogeneous Olkiluoto migmatitic gneiss/pegmatite rock mass. Consequently Posiva, using the experience from AECL’s HTF and SKB’s APSE experiments, designed the in situ Posiva’s Olkiluoto Spalling Experiment (POSE) in 2009. In this paper, we present the results of this POSE in situ experiment conducted between 2011 and 2013 and study of the the relation of the observed damage with the determined critical rock stress in the migmatitic gneiss. The back-calculation campaign of the in situ experiment and detailed descriptions of the failure mechanisms are currently being conducted and are thus not reported in this paper.

2. POSE experiment The location for conducting the spalling experiment, and later other rock mechanics in situ experiments, was the rock mechanics investigation niche in the ONKALO facility at a depth of  345 m,

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Fig. 3. Schematic representation of the experimental area in the ONKALO underground facility with the most important monitoring holes indicated.

located mainly in migmatitic gneiss with minor pegmatitic granitic inclusions. The niche is close to the anticipated repository depth of about  430 m. Previously, the niche was used for excavation damage zone (EDZ) investigations and was originally excavated using state-of-the-art blasting techniques to minimize the blast damage. Before the experiment, the niche was expanded from a width of 5.0 m to 7.5 m and from a height of 4.5 m to 9.2 m, the new geometry providing a greater stress concentration and a more uniform stress state below the tunnel. The three primary goals of the POSE experiment were (a) to establish the in situ spalling/damage strength of Olkiluoto migmatitic gneiss, (b) to establish the state of in situ stress at the  345 m depth level, and (c) to act as a Prediction–Outcome (P–O) exercise.

boreholes having temperature sensors and four boreholes with a 24 channel AE system. A three months heating period was chosen, based on the prediction calculations, with the total heating power of 6400 W to be ramped up from the initial 3200 W for the first three weeks. Based on previous knowledge, when only access to the deep investigation boreholes and laboratory testing data were available to estimate the rock mass strength and the damage mechanism, the experiment was designed with the laboratory testing data indicating brittle behaviour, and the spalling was assumed to occur at around 115 MPa (0.56UCSm).16 2.2. Evaluation of the failure

2.1. Experiment phases The experiment consisted of three phases (Fig. 3), each of which was executed separately. Thermomechanical 3D predictions were developed for all of the experimental phases12 and fracture mechanics predictions for Phases 1 and 3.13–15 The phases were as follows. 1. Pillar damage test, in which the emerging pillar between two experimental holes (∅1.524 m) was monitored during the boring of the second experimental hole ONK-EH2. Before executing the test, the first experimental hole, ONK-EH1, was bored, mapped and instrumented with a camera system, strain gauges and temperature metres. Also, a microseismic system was installed in four boreholes around the pillar. 2. Pillar heating damage test, in which the pillar was heated with four heaters installed in boreholes at two sides of the pillar and the damage in the pillar was monitored. The stress was amplified by using heaters to heat up the rock mass and to induce thermally increased stresses. The monitoring system used in the first phase was reused and the experimental hole ONK-EH2 was instrumented with a pressure metre and filled with sand to create a minor support pressure. A two-month heating period was chosen based on the prediction calculations, with the total heating power of 8000 W, ramped up in 2000 W per day steps in the beginning. 3. Single hole heating damage test, which was executed by heating a separate experimental hole ONK-EH3 (∅1.524 m) from the inside. The hole was instrumented with strain gauges and temperature metres, and filled with tabular alumina chips, and eight heaters were inside the experiment hole. The rock mass around the experimental hole was instrumented with five

When establishing rock mass strength within the context of long-term safety, a conventional design criterion may be unsuitable; thus the failure mechanism and criticality of the failure must be considered in detail. For a jointed rock mass and anisotropic rock the rock mass, strength criteria was formulated already in 1980 by Hoek and Brown.17 Later on, Brown et al. 198318 provided a comprehensive list of references for solutions in a uniform initial stress field for damage around circular openings. The nonuniform stress field was addressed and popularized by Detourney and St. John19 with a first design chart which introduced a relation between failure modes and stress state. It introduced different damage mechanisms for rocks with different strength characteristics and stress conditions. Since then, research on rock failure mechanisms related especially to brittle failure and spalling have advanced with the latest major contribution being by Diederichs.20 In conditions where the stress state reaches the rock mass strength, rock failure occurs. The failure is highly dependant on the differential stresses (s1/s3) around the tunnel periphery with the tunnel geometry changing the in situ stress field. For a circular tunnel, Detourney and St. John19 identified four failure modes with different behaviour, which Martin et al.21 modified a decade later. To evaluate the failure, the European Standards for structural design are considered as a reference. In those standards, the ultimate limit state is defined as the condition where the safety of people and/or a structure is at stake, and they include limit states that concern the protection of the contents.22 When the limit state is exceeded, the structure no longer fulfils the design criteria.22 The serviceability limit state is relevant when safety is not at stake but rather the functioning of the structure or structural members under normal use.22 This can be, for example, caused by extensive cracking (where aesthetics are not concerned).22 A progressive

Table 1 Distribution of rock types at ONKALO from surface to relevant disposal depth.26 Rock type

0…  457 m depth

Veined gneiss (VGN) Diatexitic gneiss (DGN) Mica gneiss (MGN) Tonalitic–granodiorittic–granitic gneiss (TGG) Pegmatitic granite (PGR) Other (SGN, QGN, MFGN, DB) All (PGR, VGN, DGN, MGN, TGG, other)

43% 21% 7% 8% 20% 1% 100%

70 19 15 12 24 116 145 10.8 11.8 18.3 11.0 12.8 10.8 11.3 32.5 34.0 33.4 27.4 32.2 32.4 32.4 50.1 50.0 47.1 46.8 53.2 49.2 50.1 70 19 14 12 25 115 145 26.1 24.7 38.9 19.7 15.6 25.1 23.7 51.6 50.4 41.9 76.1 61.7 50.8 50.9 91.7 81.2 90.8 98.1 86.2 88.7 88.4 83 19 16 16 28 134 167 28.0 27.0 44.1 15.7 25.2 27.3 26.9 66.4 55.4 42.8 85.7 71.2 55.7 56.5

CIsd(MPa) CIk(MPa) CIm(MPa) nCD (dimensionless) CDsd(MPa) CDk(MPa) CDm(MPa) nUCS (dimensionless) UCSsd(MPa)

108.2 93.5 101.7 106.9 108.2 103.0 104.3

A thrust faulting stress regime is present at Olkiluoto.36 In the

Veined gneiss (VGN) Diatexitic gneiss (DGN) Mica gneiss (MGN) TGG gneiss (TGG) Pegmatitic granite (PGR) Gneissic rocks (weighted) All (weighted)

2.4. Rock stress at the Olkiluoto site

UCSk(MPa)

At Olkiluoto, around 80% of the total rock volume is migmatitic gneiss, and the remainder is pegmatitic granite. The migmatitic gneiss, the over-arching term used to simplify terminology, can be classified into veined gneiss, diatexitic gneiss, mica gneiss, tonalitic-granodioritic-granitic gneiss, quartz gneiss, mafic gneiss and stromatic gneiss. Also diabase dykes locally exist at the site. The gneisses at Olkiluoto site are anisotropic and have their lowest strengths in the laboratory for medium angles of anisotropy (see11 for details). The anisotropy has a demonstrable effect on the rock mass response and induces anisotropy in the UCS and tensile (T) strengths.25 However, the anisotropy is not taken in account in this analysis because there is a large number of data without clear description of the anisotropy. Anisotropy has nevertheless been taken into account in fracture mechanics prediction calculations and will also be accounted for in the back-calculation campaign. The distribution of different rock types in the central part of the Olkiluoto area is presented in Table 1 using tunnel mapping data.26 The distribution of the rock uniaxial compressive, crack damage and crack initiation strengths with mean, 5% fractile and standard deviation values are presented in Table 2. The strengths are calculated using the existing uniaxial testing11,27–32 data from the ONKALO and the recent test results33,34 from samples acquired from the rock mechanics investigation niche. The full strength distributions for uniaxial compressive, crack damage and crack initiation strengths of the Olkiluoto rocks are illustrated in Fig. 4. However, there have been observations that the excavation stability might not be directly correlated with lithology25; the assessment of the strength parameters of a rock mass with a complex lithology requires synthesis of the rock components and their geometry. The representative weighted mean uniaxial compressive strength of the ONKALO rock (104.3 MPa; Fig. 5) is calculated using the rock type distribution data (Table 1). The mean uniaxial compressive strength of the gneissic rocks (GN), which are 79% of the volume, is 103.6 MPa and for pegmatitic granite (PGR), which is 20% of the volume, is 108.2 MPa. The indirect and direct tensile strengths are listed in Table 3. The characteristic rock strength values (the 5% strength fractions) are calculated using a simple Monte Carlo simulation.35

UCSm(MPa)

2.3. Geology and intact rock strength at the Olkiluoto site

Rock type

brittle failure, as in AECL’s URL,23 is clearly an ultimate limit state failure; however, discontinuous single crack growth with no significant consequences can be considered as a serviceability limit state failure. According to the European Standard on Geotechnical design24: If statistical methods are used, the characteristic value should be derived such that the calculated probability of a worse value governing the occurrence of the limit state under consideration is not greater than 5%. This approach is applied in this paper.

nCI (dimensionless)

T. Siren et al. / International Journal of Rock Mechanics & Mining Sciences 79 (2015) 135–148 Table 2 The distribution of rock uniaxial compressive strength (UCS), crack damage strength (CD), crack initiation strength (CI) and number of samples for each (n) with their mean values (m), 5% fractiles (k) and standard deviations (sd).

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139

Fig. 4. Uniaxial compressive (left), crack damage (centre) and crack initiation (right) strength distributions of Olkiluoto rocks. Table 4 Interpreted stress field at Olkiluoto based on in situ stress data (99% confidence interval).1 Range

rH

rh

rv

Mean (MPa) Min (MPa) Max (MPa) Orient. (° from N)

13.0 þ 0.031z 2.0 þ0.030z 24.0 þ0.033z 87 (8–165)

8.5 þ 0.024z 1.0 þ0.020z 16.0 þ 0.028z 177 (98–255)

0.0265z 0.0240z 0 to OL-BFZ020 0.0290z –

Mean (MPa) Min (MPa)

10.9 þ 0.033z 0.1þ 0.032z

5.3 þ 0.027z  1.7 þ 0.027z

10.8 þ 0.033z 3.0 þ0.030z 18.6 þ 0.036z 84 (53–115)

2.1 þ0.026z  3.4 þ 0.026z 7.6 þ0.026z 174 (143–205)

Max (MPa) Orient. (o from N)

Mean (MPa) Min (MPa) Max (MPa) Orient. (oN)

Vertical depth range

0.0265z 0.0240z OL-BFZ020 to OLBFZ099 21.7 þ 0.033z 12.3 þ 0.027z 0.0290z 112 (53–171) 202 (143–261) –

0.0265z 0.0240z OL-BFZ099–900 m 0.0290z –

Fig. 5. Weighted strength distribution of Olkiluoto rocks at the disposal depth.

proposed in situ stress model 1 based on hydraulic and semi-integration results,1 the stress state is divided into three domains (see Table 4), determined by the locations of Brittle Deformation Zones (BDZ) OL-BFZ020 and OL-BFZ099 (see Fig. 1 for locations). The gently dipping (ca. 20° towards the SE) OL-BFZ020 consist of splays of two sub-parallel fault zones, OL-BFZ020a and OLBFZ020b, located a maximum of tens of metres from each other, consisting of various clay-filled and densely fractured sections.26 The moderately dipping (40° towards the SE) OL-BFZ099 is a more distinct zone with abundant fracturing, clay-filled fractures and slickensides.26 In the rock above the major fracture zone OLBFZ020 (  156 m to  265 m), the stress component magnitudes are close to each other and the stress field is not clear. Below OLBFZ020 ( 345 m to  408 m), the major in situ stress is roughly in line with the regional NW–SE orientation of the major principal stress, as typically found in Scandinavia.37 The intermediate stress component is also horizontal and the minor is almost vertical. In the ONKALO area, the disposal depth is between the OL-BFZ020

and the OL-BFZ099 zones, and that stress domain is mostly considered in this article—unless otherwise specified. In the rock mechanics investigation niche, two local stress interpretations exist (see12). During disposal, the spent nuclear fuel containers induce a thermal rock stress increase above the in situ rock stress (Fig. 6). The expansion of the rock increases the horizontal stresses significantly, while the vertical stress increases more modestly. For the Olkiluoto repository, Hakala et al.38 calculated the thermal and stress increase during the operational life time of the facility at the  420 m depth level. During that time, the buffer and backfill are not expected to swell and produce backfill pressure due to low inflow to the facility (in total ca. 33 l min  11). In the model, the rock cover had a magnitude of 422 m, while the horizontal dimensions of the heated rock volume are 2.5 km and 1 km.38 The highest temperature increase was about 28 °C in the back-filled disposal tunnels38 and the maximum stress increase from 25 MPa to 42 MPa.

Table 3 The indirect (Ti) and direct tensile (Td) strengths with standard deviations (Ti;sd and Td;sd) of Olkiluoto rocks and number of samples (y).1 Rock type

Ti(MPa)

Ti;sd(MPa)

n (dimensionless)

Td(MPa)

Td;sd(MPa)

n (dimensionless)

Pegmatitic granite (PGR) Gneissic rocks (GN)

12.1 8.9

2.9 2.1

98 51

7.6 –

1.5 –

18 0

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Fig. 6. Contours presenting the maximum compressive stress (s1), (A) after 60 years from the start of deposition,38 (B) after 120 years from the start of deposition,38 and (C) the thermally induced evolution of the in situ stress field in disposal hole at the location of the temperature maximum.

3. The results of the POSE experiment The experimental data from the POSE experiment Phases 1 and 2 have been published in Ref.39 and the outcome of the third phase in detail in Ref.40 The back-calculations for the POSE experiment are to be executed from 2015 onwards. Below is brief summary of the main findings from each Phase. 3.1. Outcome of the first POSE phase In the first phase, only local minor damage was noticed during the boring of the two holes. The damage consisted of three new fractures that initiated; however, there was no spalling or surface type damage. The first two sub-vertical fractures formed in the wall of hole ONK-EH1 two and four weeks after the boring of the hole. The second hole was not bored at the time the two first fractures were observed. The fractures were localized in mica rich layers and rock type contacts which were known to be relatively weak. The third, sub-vertical fracture was observed in the wall of ONK-EH2 after boring had been completed. The maximum tangential stress at the damage locations is estimated to be between

40 MPa and 45 MPa. In the pegmatitic granite, a region which has significantly displaced can be observed (Fig. 7) thus indicating partial stress relaxation due to the shearing of the second fracture. In the region, the whitish lines in Fig. 7 are located parallel to the shear plane. 3.2. Outcome of the second POSE phase In the second phase, the heating lasted two months with a power drop in the last three weeks due to the failure of one heater unit. To retain symmetry, the second heater was turned off and the individual heating power in the two remaining heaters was increased. The observed damage was well localized around the holes and controlled by the foliation, mica rich layers and rock type contacts (Fig. 8). Surface type failures were not observed in the gneiss, but it was noticed in limited areas in the pegmatitic granite where a 150 mm  150 mm area was spalled (see detail in Fig. 7). This was the only spalling observation in the whole POSE experiment. Spalling was probably caused by the ‘load carrying’ granite bridge which concentrated the stresses and spalling occurred at the narrowest point. The depth extent of the failure was further

Fig. 7. Structurally controlled crack growth (red dashed line) in the ONK-EH1 experimental hole that occurred sometime after boring of the hole and before boring of the second experimental hole. The zone of visually observable relaxed pegmatitic granite (whitish zone) is surrounded by the green dash–dot line. The further damage (spalling) was observed after the second phase of the POSE experiment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8. Observed damage on the northern wall (two left images) and southern wall (two right images) in the 2nd phase of the POSE experiment for the holes ONK-EH1 and ONK-EH2. Heating induced damage is indicated by the light grey areas (scaled damage areas) and the orange lines (fractures). Boring-induced fractures (pre-heating) are indicated in red and numbered. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. (A) Damage during the 2nd phase in ONK-EH1 towards the pillar centre. (B) Strain readings at 6 m depth, the continuous lines are the measured values and the individual symbols are the predicted values. The fracture propagation windows are marked with colours that correspond to the photograph of the pillar. (C) Predicted stresses at the experimental hole with final scaled damage. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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investigated using water injection and scaling. Wedge and sledge hammering was needed to remove rock associated with the damage surface. The depths of the damaged areas in the gneiss due to heating were less than 100 mm. The second hole was filled with sand to create support pressure and to prevent damage. The support pressure in the second hole was monitored with three pressure gauges installed at 1.4 m, 3.6 m, and 6.7 m depths, with the observed maximum value of 4 kPa in the gauge at the 3.6 m depth. More damage was observed in the second hole, indicating that low (theoretically max. 60 kPa) support pressure is not able to prevent damage. According to the predictions, Phase 2 of the experiment was to reach stress levels well in excess of 130 MPa within the pillar wall, therefore exceeding even the UCS and thus causing definite damage to occur (Fig. 9). The heating sequence did not follow the prediction at the beginning due to a delayed start while making necessary modifications to instrumentation. The final heating sequence was approximately 2000 W for five weeks, 3200 W for three weeks and 6400 for five weeks. The stress evolution was confirmed with back-calculations. 3.3. Outcome of the third POSE phase The third phase of the experiment involved internally heating the third experimental hole (ONK-EH3) of the rock mechanics investigation niche in order to cause a symmetrical thermal stress increase around the hole due to the thermal expansion of rock. ONK-EH3 is located almost completely in pegmatitic granite, see the white texture in Fig. 10 stitched from photographs used to create a photogrammetric 3D model of the hole. Four natural fractures near the top of the hole were mapped after boring ONKEH3, and a more pronounced failure located at the contact between mica-rich gneiss and pegmatitic granite was observed eighteen months after boring, prior to the heating phase. The conditions at the ONK-EH3 location were not monitored during the eighteen-month period; thus environmental changes such as humidity and temperature cannot be ruled out as a cause of the fracture development. Continuous ultrasonic surveys were used to monitor the

change in P-wave and S-wave velocities during the experiment (Fig. 11).41 The wave velocities were calculated at different levels calculated as a mean of the ray paths crossing the region up to different depths to 2.3 m, 3.7 m, 5.4 m and below 5.4 m in depths (last marked as Depth20). The P-wave and S-wave velocities reduce in the region between 1.5 m to 3.7 m to ca. 60 m/s. The AE events were monitored during the experiment and the peak of activity was at the end of the heating period. The AE events were slightly concentrated in the north and south-west direction (Fig. 11). However, it must be noted that on the north side multiple boreholes existed that might have facilitated the triggering of events. Predictions of the state of stress around the ONK-EH3 hole following excavations indicate a magnitude of 46–55 MPa for the maximum principal stress (Fig. 12A). In the experiment, the temperature reached up to ca. 85 °C with the predictions indicating that the Crack Damage threshold of pegmatitic granite had been exceeded (Fig. 12B). The predictions indicate the major principal stress level to exceed 100 MPa at the hole surface. Upon emptying the hole, no clear, visual damage was evident and no spalling had occurred. The fractures that were observed after boring of the hole propagated further during the heating; a new, larger failure was developed parallel to the bottom hole fracture and other minor new fractures initiated (see42 for details). The ground penetrating radar (GPR) method was used to quantify the damage by conducting several radar measurements prior to and after the heating. In addition to the traditional data processing techniques, a new GPR EDZ method (43–47) based on frequency analysis of the reflected signal was used to study the depth of damage. The GPR EDZ response is presented using a black–red–green colour scale where black represents the damaged volume limited by a given threshold value selected based on statistical analysis of the data (see40,48 for details); red to green represents the transition between the background and damaged rock. The threshold values vary between the locations and depend on the surrounding rock, surface roughness and moisture and on the settings used in measurements and processing. Some lithological features cause GPR EDZ method response as these features are reflective on a wide frequency range. As seen in Fig. 10, the

Fig. 10. GPR EDZ and GPR responses in vertical lines in directions to north and south (A) prior to heating and (B) after heating. The GPR EDZ response is presented externally (in black–red–green palette) and GPR response internally (in purple-cyan palette). The image of the bottom of the hole prior to the experiment is distorted. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 11. (A) The P-wave velocity change where horizontal lines above and below each data point indicate the error margin and vertical lines indicate when heaters were switched on and off. (B) The location of AE events recorded during the experiment.41

Fig. 12. (A) Differential stress before the heating. (B) A horizontal cutting plane of the maximum principal stress after 12 weeks of heating.12.

depth extent of the GPR EDZ responses (black area) prior to heating are generally in the range of 6 to 120 mm deep and exhibit clearly discontinuous behaviour. In some locations, the EDZ extends to a greater depth of 250–300 mm, especially in the upper part of the rock in the north and south. The most significant EDZ responses before heating were seen in the north and south. The GPR EDZ response after heating indicates an estimated depth extent of the damage from 120 to 180 mm in general with extension of damage to greater depth in the north and south. These locations have already been damaged during the boring process and may be related to stress-induced fracturing and rock texture.

4. The in situ rock mass strength at Olkiluoto The site geology of Olkiluoto is rather complicated, being anisotropic and heterogeneous, thus linking the rock stress and rock strength strongly to the site lithology. Based on the POSE experiment, two different damage mechanisms exist. In the migmatitic gneiss, the dominant damage mechanism is structurally controlled failure at lithological borders. The damage is caused by the subcritical fracture growth and shearing at contacts between the mica-rich gneiss and pegmatitic granite—which can be considered as the weakest link in the rock. This also relaxes rock stresses, thus preventing further brittle failure (spalling). In pegmatitic granite, no visible failure occurs when stresses are increased and very rarely is spalling or surface damage observed. 4.1. Spalling potential Earlier the rock spalling potential of Olkiluoto rocks has been

studied49 using for example, the relation of the UCS and UCS/T in Ref.20 In the 2011 spalling assessment,49 the Olkiluoto spalling potential was described as low for migmatitic gneiss and medium for pegmatitic granite, which is not fully in line with the POSE experiment. If the fact that T results have not been obtained from UCS result locations is disregarded, it can be investigated if it is possible to have samples with UCS and T values that could host spalling at all. The result is that, using the mean values, and plus and minus a standard deviation, the spalling potential of the Olkiluoto rocks can be characterised for gneissic rocks as negligible, and for pegmatitic granite as negligible or low (Fig. 13). However, the deviation is large, indicating that some locations could be more vulnerable for spalling than others. Many other well-known relations8,50 are based on rock stress (either as in situ or tangential) and rock strength, and they provide a more pessimistic picture (applied for Olkiluoto in51). 4.2. Rock damage mechanism When assessing the failure observed in the POSE experiment, the failure modes studied in19 and21 need to be expanded to include structural failure while taking into account new failure modes included in recent literature. In Failure mode I, the mean stress is near to the rock strength and the differential stress is high because of the high tangential stresses. However, based on recent research,52 Failure mode I is divided in this article into three submodes taking in account the different brittle modes of behaviour. Diederichs52 states that the brittle failure behaviour is governed by the ratio between the uniaxial compressive strength and tensile strength (T) of rock masses and that jointed rock masses may be less prone to spalling, but may suffer a shear block failure. The

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Fig. 13. The mean spalling potential (points) and the approximate predicted deviation of the spalling potential (areas) of the ONKALO rocks for the migmatitic gneiss (VGN, DGN, MGN, TGG) and the pegmatitic granite (PGR). The UCS and T values are not matched for individual samples.

compared to the mean stresses, or if the vertical stress component is high, the excavation can be surrounded by a failure zone (Failure mode IIa).19 However in similar conditions, if the rock mass is relatively strong, a zonal disintegration failure (Failure mode IIb) may occur, as reported in Refs.53–58 When the major principal stress is over 2.5 times the minor principal stress, the failure zone will develop to a butterfly shaped shear failure zone (Failure mode III), more closely studied, i.e., by Shen and Barton.59 In the MBE experiment, the stress ratio was 6:1 and a butterfly shaped zone of tension occurred in the tunnel sidewall.3 Thus the butterfly shaped failure can be caused in a jointed rock mass or with high enough stress ratio compared to the tensile strength of the rock. The rock Failure modes are (Fig. 14) as follows: Ia: V-shaped notch (spalling) is formed in the direction of minor principal stress; Ib: block shear failure of the rock mass (added based on recent literature52); Ic: structurally controlled failure (added based on this work); IIa: the tunnel is surrounded by a failure zone; IIb: the tunnel is surrounded by a zonal disintegration zone (added based on recent literature53–58); III: the tunnel is surrounded by a butterfly-shaped failure zone. 4.3. The onset of the in situ damage - the serviceability limit

fairly intact rock masses with a high UCS/T ratio will be prone to spall (Failure mode Ia) and the rocks with low UCS/T ratio will tend to fail in shear (Failure mode Ib).52 Based on the experiences from the ONKALO, the foliated, and fairly unfractured, anisotropic rock masses will be prone to structurally controlled failure at the lithological borders (Failure mode Ic). In Failure mode Ic the weakest plane will fail, thus relaxing the stresses and restricting the progressive failure. In high stresses and with the rock mass strength being weak

The POSE damage observations after boring of the holes and other experience from the ONKALO facility indicates that the first damage occurs at 40 MPa stress (0.38UCS) with some time dependency. This can be considered as the onset of the in situ damage and it roughly corresponds to the lowest tested CD values of Olkiluoto rock, mostly being samples from mica rich rock types (MGN and VGN). Also the crack damage strength interpretation is the onset of crack coalescence.60 It can be also concluded that the

Fig. 14. The rock mass failure mechanisms; partly based on Refs.19,52

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observed stress level is close to the CDk and CIm values (respectively 51 MPa and 50 MPa; see Table 2). The extent of these failures is very limited and therefore it is not regarded as the rock mass strength. As a failure occurs, it relaxes the stresses in the vicinity of the fracture, which causes the rock mass to stay intact and stable: thus, this is regarded as the serviceability limit. Then, according to the thermoelastic modelling, the further damage in the second phase of the POSE experiment (with heating) started at around 55 MPa. After the fracture initiation in mica rich layers, for a rock mass failure the failure propagated through other rock types as well. This also corresponds approximately to the CDk (51 MPa).

145

calculated using

σ D = σ1–σ 3

(2)

To study at what depth the onset of in situ damage strength occurs, the Examine3D boundary element method programme was used to model the disposal tunnel and hole to accurately estimate the stresses at the disposal hole surface in both KBS-3V and KBS-3H concepts.

where s1 is the major principal stress and s3 is the minor principal stress. In order to calculate the factor of safety, the differential stresses at the disposal hole surface in a random direction and at a random hole depth were compared to the randomly picked rock CD strength, also and for comparison to randomly picked rock CI and UCS strengths. Comparing the in situ experiment results to the 5% fractile and mean values of the CD strength distribution in ONKALO, the CD is considered as the relevant material property to study the rock mass strength. If the failure probability is considered at the time of disposal hole boring, the CD strength of Olkiluoto rocks is exceeded in 3.1% of the cases for the vertical KBS-3 V disposal concept and 0.25% of the cases for the horizontal KBS-3H disposal concept (see Table 5) at  430 m depth. The failure in the KBS-3BV concept is not preventable with minimum support pressure (60 kPa) according to the results from the second phase of POSE; however for higher support pressures this might not be true. The KSB-3H concept is expected to be almost completely free of damage. The results are supported by observations from seven relevant scale vertical experimental holes ONK-EH1…3 and ONK-EH6…9 in demonstration tunnels at a depth of  420 m (accessible in November 2014) that have been bored in ONKALO. There are some or limited damage observations from five out of the seven holes, sometimes after the excavation. This indicates that the rock is close to the failure state. With an approximate mean damage length of 250 mm, it can be calculated that ca. 2.4% of the total experimental hole depth suffered failure. This observation is in line with the calculated percentage of the factor of safety (FOS) below unity. At Olkiluoto, the rock mass strength is believed to be dominated by the complex lithology, rock heterogeneity and variability of the rock strength values.

5.1. Statistical assessment of rock mass stability

5.2. The deterministic mean factor of safety

Comparison of the laboratory strength data and the in situ stresses is an established method of assessing the stability of excavations. A Monte Carlo simulation is used to calculate the factor of safety for Olkiluoto using the weighted rock strength distribution at Olkiluoto and a triangular stress distribution of the stress domain—that are independent of each other. The simulation approach is presented in the flowchart in Fig. 15. If the tangential stress reaches the rock mass strength at the disposal hole surface, the factor of safety can be expressed as

To study at what depth the onset of in situ damage strength is reached without the simulated thermal heating by the spent nuclear fuel, the Examine3D boundary element method programme was used to model the disposal tunnel and hole to accurately determine the mean stresses at the disposal hole surface in both the KBS-3V and KBS-3H concepts (Fig. 16). The disposal tunnel was oriented in the favourable direction, i.e. the trend of the major principal stress direction 0° and 30° degrees from it, and the calculated mean stress at the hole surface compared to the onset of in situ damage strength. The factor of safety for two stress interpretations12 from the niche at  345 m depth was calculated similarly using the 3D model and onset of in situ damage strength. If the simulated thermal stress increase (see Fig. 6) due to

4.4. The rock mass strength Based on the in situ experiment – not observed naturally as yet – the major damage was observed as a strain drop in Fig. 9 at 90 MPa (0.86UCS) level, which can be considered as the characteristic rock mass strength. This corresponds roughly to the CDm (88 MPa). This failure is very strongly dominated by the lithology and may occur in unexpected modes and directions. However, the failure depth is still shallow and not continuous, a phenomenon which could be interpreted to be case also in the future disposal holes. In pegmatitic granite, limited spalling/damage may occur; however, the failure in gneiss may release the stresses and the state of spalling is rarely reached.

5. The KBS-3v and KBS-3h disposal Concepts

factor of safety = σrm/σ D

(1)

where srm is either the rock mass strength or the onset of fracture initiation, whichever is appropriate, and sD is the differential stress

Fig. 15. Flowchart for calculating the rock strength distribution, the mean factor of safety and the failure probability for disposal holes at Olkiluoto.

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Table 5 The in situ rock failure observations and calculated factors of safety for the horizontal KBS-3H (upper row) and vertical KSB-3V (lower row) disposal concepts at  430 m depth. Note that a FOS below 1 is unstable.

Disposal concept

Observations

Factor of safety 100% 90% 80%

FOSCD < 1 = 0.1 % Lowest FOSCD = 0.8 FOSCD,m = 3.2 FOSCD,k = 1.6

70% 60% 50%

No data

40%

FOS (CI)

30%

FOS (CD)

20%

KBS-3H

FOS (UCS) 10% 0% 0

1

2

3

4

5

6

7

8

9

10

100% 90% 80%

FOSCD < 1 = 3.1 % Lowest FOSCD = 0.6 FOSCD,m = 2.2 FOSCD,k = 1.1

70% 60% 50% 40%

FOS (CI)

30%

5 out of 7 experimental holes suffered

FOS (CD)

20%

FOS (UCS)

KBS-3V

failure (71 %). Ca. 2.4 % of the total

10% 0%

depth of the holes affected

0

1

2

3

4

5

6

7

8

9

10

(Figure above is illustrative only) radioactive decay is taken in account, the mean factor of safety dramatically changes during the disposal time (Fig. 17). The thermal stress maximums from Hakala et al.38 are used as the evolution of the stress field (see the maximums in Fig. 6). After disposal, the disposal tunnels are backfilled with bentonite backfill, which will expand and provide support pressure when saturated with groundwater. The tunnel backfill pressure is conservatively not taken in account in the calculations as a force supressing the damage, because no significant swelling pressure is expected during the first decade.

A

6. Conclusions Based on the rock strength laboratory samples, the mean uniaxial compressive strength of Olkiluoto rock is around 104 MPa. The POSE in situ experiments resulted in the conclusion that the damage tends to be structurally controlled, the onset of the fracture initiation is 40 MPa and the rock mass strength is 90 MPa. Although caused by the lithology, the structurally controlled failure may occur in unexpected modes and directions, thus it is hard to predict the stability based on the rock structure, which is in line with observations from AECL’s URL.25 The approach by the European Standard on Geotechnical

B

Fig. 16. (A) The mean factor of safety for the onset of in situ damage (40 MPa) and (B) The mean factor of safety for the rock mass damage (90 MPa) for the horizontal KBS-3H and vertical KBS-3V disposal systems oriented with the maximum principal stress favourable directions, 0° and 30° degrees from the horizontal tunnel trend, and with the two stress interpretations from the rock mechanics investigation niche.

T. Siren et al. / International Journal of Rock Mechanics & Mining Sciences 79 (2015) 135–148

A

147

B

Fig. 17. (A) The mean factor of safety for the onset of in situ damage and the rock mass damage with thermally induced evolution of the in situ stress for the horizontal KBS3H and vertical KBS-3V disposal systems oriented in the favourable principal stress direction. (B) The mean factor of safety after 80 years after the start of disposal with the orientation being the favourable direction in degrees.

design24 is well suited for Olkiluoto rock, because the site has large rock strength deviation and the failure is still governed by the locations with the weakest rock strengths; thus 5% fractions were used. The lowest laboratory CD values are close to the observed fracture initiation onset and the rock mass CDm (88 MPa) corresponds roughly to the observed characteristic rock mass strength (90 MPa). The GPR EDZ results indicate an increase of Thermally Disturbed Zone (TDZ) type damage in the rock due to the experiment, reaching a maximum depth of 120–180 mm in comparison to 60– 120 mm prior to heating. Damage depths in the APSE experiment in Sweden (50–140 mm)10 and in the HFT experiment (20– 70 mm)7 were similar. The ultrasonic surveys showed a clear P-wave velocity drop induced by the experiment. The second Phase of the experiment indicated that low support pressures were not able to prevent the damage. Two other primary goals of the POSE experiment were not successful as the failures were not concentrated in the expected modes and directions. Therefore, the local in situ stress state could not be confirmed with the experiment. Also the predictive modelling did not capture the new failure mechanism. Based on the results from the Olkiluoto site, the Failure mode Ic, “lithogically controlled failure”, is added to the Failure modes originally introduced in Ref.19 Fracture initiation has been confirmed by observations from the seven nearly full-scale deposition holes at the anticipated disposal depth. Thus, the vertical disposal concept could suffer from initiation of new fractures sometime after excavation. However, according to the calculations, the horizontal disposal concept will not suffer from any fracture initiation when the tunnels are excavated; however, 60 years after commencement of the disposal, both concepts will be prone to fracture initiation. Neither of the disposal concepts is expected to suffer any major rock mass failure. This is due to the fact that the secondary stresses are well below the rock mass strength. In particular, the vertical disposal concept is not particularly sensitive if the trend of the tunnels is within 30° of the major principal stress direction.

Acknowledgements Posiva Oy financed and executed the experiments and the Aalto University School of Engineering together with Doctoral Programme for Nuclear Engineering and Radiochemistry funded the writing of this article. The authors wish to thank everyone who contributed to the POSE experiment over the years especially Kimmo Kemppainen, Harri Kuula, Eero Heikkinen, Jussi Mattila,

Juan Reyes-Montes, Junwei Huang, William Flynn, Toivo Wanne, Vesa Järvinen, Rolf Christiansson and Derek Martin. Special thanks to Juha Heine for executing most of the field work. The full list of people who participated in the experiment is given in Ref.40 The views expressed are those of the authors and are not necessarily those of Posiva.

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