Overcoring rock stress measurements in borehole KR6 at Hiistholmen. Finland

Transcription

1 Working Report Overcoring rock stress measurements in borehole KR6 at Hiistholmen. Finland Christer Ljunggren November 1998 POSIVA OY Mikonkatu 15 A, FIN HELSINKI, FINLAND Tel Fax

2 Working Report Overcoring rock stress measurements in borehole KR6 at Hiistholmen. Finland Christer Ljunggren November 1998

3 POSIVAOY WORK REPORT OVERCORING ROCK STRESS MEASUREMENTS IN BOREHOLE KR6 AT IIASTHOLMEN, FINLAND CHRISTER LJUNGGREN VATTENFALL HYDROPOWER AB November 1998 APPROVED: 1 (/I i~o.-.. a_.;t6(..( f~ arl-erik Aim en, SKB

4 Working Report Overcoring rock stress measurements in borehole KR6 at Hiistholmen, Finland Christer Ljunggren Vattenfall Hydropovver AB Svveden November 1998 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.

5 2 (26) ABSTRACT Finnish legislation proclaims that each utility generating nuclear energy is responsible for the management of the radioactive waste resulting from the energy production. Since the turn of the year '95/'96, Teollisuuden Voima Oy (TVO) and Imatran Voima Oy (IVO), both operating two nuclear reactors in Olkiluoto and Loviisa respectively, have established the company Posiva Oy, thereby authorising a joint program with the aim to assure safe management of the high level nuclear waste (HL W) produced in Finland. For disposal of HLW, it is planned to locate a repository at depth in the Finnish bedrock. At first, five locations in Finland were chosen by TVO as investigation sites for the HL W repository. Preliminary investigations at these sites took place between 1987 and Based on the results from the preliminary investigation phase, the number of sites were reduced to three at which detailed investigations started in The investigation areas selected for detailed investigations were: Aanekoski Kivetty, Kuhmo Romuvaara, Eurajoki Olkiluoto. As a result of a feasibility study in 1996, Hastholmen in the Loviisa community was selected as a fourth investigation site for the disposal of high level nuclear waste. The final site for the HL W repository will be decided in the year As part of the site specific investigation programs, rock stress measurements by means of three-dimensional overcoring have been conducted in one borehole at Hastholmen, borehole HH-KR6. The prime objective was to obtain the magnitudes and the directions of the (3-D) principal-and (2-D) horisontal stress field in the rock mass. Primary test levels for 3-D measurements were at 300 m, 450 m and at 600 m including five successful tests per level. Due to rock mechanical problems in form of core discing the program, however, had to be rearranged. The field work started May 6, 1998 and was stopped June 25. Partly successful measurements were only obtained between 297 m and 3 60 m depth. Below that core discing prevented successful tests although several attempts were made. Obtained results at the depth of 3 00 m indicate a magnitude of the maximum horisontal stress at the order of 20 MP a and a minimum horisontal stress of approximately 10 MP a. The obtained results on the vertical stress are not considered reliable. From the measurements an orientation of the maximum horisontal stress in the E-W direction is indicated. Keywords: Hastholmen, stress measurements, overcoring, borehole HH-KR6.

6 3 (26) TIIVISTELMA Suomen ydinenergialain perusteella ydinenergian tuottaja on vastuussa voimalaitostensa tuottamien ydinjatteiden huollosta. Teollisuuden Voima Oy:n (TVO) ja Imatran Voima Oy:n (IVO) yhdessa perustama yhtio, Posiva Oy, aloitti toimintansa vuoden 1996 alussa. Posivan tehtavana on huolehtia kaytetyn polttoaineen loppusijoituksesta Suomessa. Kaytetty polttoaine on tarkoitus loppusijoittaa syvalle suomalaiseen peruskallioon. TVO valitsi alunperin loppusijoitustutkimuksiin viisi tutkimusaluetta, joilla alustavat kenttatutkimukset suoritettiin vuosina Alustavien tutkimustulosten perusteella tutkimuspaikkojen lukumaara pienennettiin kolmeen, joilla yksityis-kohtaiset paikkatutkimukset aloitettiin vuonna Valitut tutkimuspaikat olivat: Aanekoski Kivetty, Kuhmo Romuvaara ja Eurajoki Olkiluoto. Tehdyn erillisselvityksen perusteella Loviisan kunnassa sijaitseva Hastholmenin alue valittiin neljanneksi tutkimusalueeksi. Lopullinen loppusijoituspaikka valitaan vuonna Osana paikkakohtaista tutkimusohjelmaa tehtiin Hastholmenin tutkimusalueen kairanreiassa KR6 in situ jannitystilamittauksia irtikairausmenetelmalla. Tutkimuksen paatavoitteena oli selvittaa paajannitysten (3D) ja vaakajannitysten (2D) suuruudet ja suunnat kalliossa. Mittausohjelman mukaisesti oli tarkoitus mitata jannitykset tasosyvyyksilta 300 m, 450 m ja 600 m ja saada kustakin tasosta viisi onnistunutta mittausta. Kalliomekaaniset vaikeudet, johtuen kairasydamen hairiintyneisyydesta ja osin rikkoutumisesta eli ns. core-discing ilmiosta, aiheuttivat kuitenkin muutoksia mittausohjelmaan. Kenttatyot alkoivat ja paattyivat Kayttokelpoisia mittaustuloksia saatiin ainoastaan syvyysvalilta 297 m- 360 m. Syvemmalla ei onnistuttu tekemaan mittauksia useista yrityksista huolimatta core discing ilmion takia. Saatujen tulosten perusteella suurimman vaakajannityksen suuruus tasolla 300 m on luokkaa 20 MP a ja pienemman vaakajannityksen suuruus luokkaa 10 MP a. Saatuja tuloksia pystyjannitysten suhteen ei voida pitaa luotettavina. Mittausten mukaan suurimman vaakajannityksen suunta on noin ita-lansi. Avainsanat: Hastholmen, jannitystilamittaus, irtikairaus, kairanreika HH-KR6

7 4 (26) FOREWORD The work presented in this report is related to the assessment of the stress field and the mechanical stability at the investigation sites for disposal of high level nuclear waste. The work has been commissioned by Posiva Oy. The contract was signed by the Swedish Nuclear Fuel and Waste Management Co. (SKB) using Vattenfall Hydropower AB as a subcontractor to conduct the overcoring rock stress measurements. The following organisations and persons have participated in the project: Posiva Oy: Saanio & Riekkola Oy: Swedish Nuclear Fuel and Waste Management Co: Vattenfall Hydropower AB: Heikki Hinkkanen J ohanna Hansen Antti Ohberg Erik Johansson Pasi Tolppanen Bo Gustavsson Karl-Erik Almen Christer Ljunggren Hans Klasson Mats Persson Anders Wikman Stig Andersson Kent Lindblad

8 5 (26) TABLE OF CONTENTS Page ABSTRACT 2 TIIVISTELMA 3 FOREWORD 4 1 INTRODUCTION SCOPE LOCATION 6 2 EXPERIMENTAL OVERCORINGPROCEDURE THE BORRE PROBE Strain gauges Adhesive Datalogger Installation tool OVERCORING-TESTPROCEDURE Drilling the pilot borehole Installation of the probe Over coring Biaxial testing OVERCORING - STRESS CALCULATION 15 3 MEASUREMENTS GENERAL GEOLOGY AND SITE CONDITIONS FIELDWORK RESULTS COMMENTS AND CONCLUSIONS 22 4 SOURCES OF ERRORS AND DATA CONFIDENCE 25 5 REFERENCES 26 APPENDIX A APPENDIX B Overcoring graphs Overcoring graphs, rejected tests

9 6 (26) 1 INTRODUCTION 1.1 SCOPE This report constitutes the borehole HH-KR6 fulfilment ofpurchase Order 9607/98/HH placed with the Swedish Nuclear Fuel and Waste Management Company (SKB) by Posiva Oy. The order states that SKB shall conduct rock stress measurements using the overcoring method in borehole HH-KR6 at Hastholmen, Finland. The borehole is located within the rock volume selected as an investigation area for the disposal of high level nuclear waste. The rock volume also includes the VLJ repository for low- and medium radioactive nuclear waste from the Loviisa power plant. The field work in borehole HH-KR6 commenced May 6, 1998 and was terminated June 25, The order primarily stated that measurements should be conducted in one vertical borehole (HH-KR6) at depth horizons of300 m, 450 m and 600 m. The order also stated that five successful tests should be performed at each depth horizon. The purpose of the stress measurements was to determine the magnitudes and directions of the (3D) principal stress field and the (2D) horisontal stress field. The report provides a comprehensive presentation of the stress measurements conducted. Chapter 2 summarises the instrumentation and experimental procedures employed. Chapter 3 presents in detail all results obtained. Sources of error and data confidence are discussed in chapter 4. Raw data is reported separately in appendices A and B. It should be noted that the presentation here is restricted to the work done and the results obtained, as such. It is neither attempted to put the data into a geologicavtectonic context, nor to discuss the implications of the results on the siting or performance of the repository for nuclear waste. 1.2 LOCATION The geographical location of the site is shown in Figure 1.1. The borehole is located on the the isle ofhastholmen close to the Loviisa town in the Loviisa community. Hastholmen is located some 120 km east ofhelsinki at the shore of the Finnish Gulf.

10 7 (26) I i j 1 l.i (' ) I (.-.) SWEDEN \., / { RUSSIA 1.,..., i,.) I / / / \\ LOVIISA POWER PLANT \HASTHOLMEN Figure 1.1 Geographical location ofhastholmen where the rock stress measurement program was undertaken.

11 8 (26) 2 EXPERIMENTAL 2.1 OVERCORING- PRINCIPLE The overcoring technique to determine in situ stresses utilises the principle of stress relief. The method involves measurements of the displacements in a piece of rock when it is released from the rock mass. The in situ stresses are calculated using the measured strains and the elastic properties of the rock according to classical theory (Leeman and Ha yes, 1966). The overcoring method used in the present case consists of coring a borehole at large diameter (76 mm) over a coaxial small-diameter (36 mm) pilot hole in which a strainmeasuring instrument is located. Thus, the cylindrical core sample is isolated from the stress field in the rock mass and the initial state of stress can be back -calculated from the deformations or strains occuring in the sample during overcoring. The calculation of stresses utilises the elastic theory which assumes that the rock behaves in a linearly, isotropically elastic manner, implying that the deformation of the core sample during stress relief is identical in magnitude to that produced by the in situ stress field but opposite in sign. It is further assumed that the rock volume is both continuous and homogenous. Application of the elastic theory also requires knowledge of the elastic parameters ofthe rock material, E (Young's modulus) and v ( Poisson's ratio). 2.2 THE BORRE PROBE The Borre Probe employed by Vattenfall Hydropower AB is a triaxial strain-measuring instrument which allows for the derivation of the complete state of stress tensor in three dimensions from one successful measurement. The Borre Probe has been developed in Sweden by Vattenfall over the last 25 years to perform stress measurements in deep, water-filled boreholes drilled from surface by conventional drilling techniques. Development started in the late 1960s by Hiltscher and Vattenfall, the Swedish State Power Board (Hiltscher et al. 1979). The equipment was developed for use in hard, competent rocks and the normal installation system was tailored for use in <1>76 mm holes drilled by a Craelius T2-76 core barrel. More recent development and commercial operation of the equipment has been carried out by Vattenfall Hydropower. The Borre Probe, described fully in Christiansson et al. (1989) and in Hallbjorn et al. (1990), has been used in Finland on several occasions in the 1980s and also in Loviisa, Hallbjom and Strindell, The evaluation of the complete stress tensor from measurements with a triaxial device such as the Borre Probe requires only the strains induced by overcoring, the orientation of the Probe (generally to magnetic north) and the elastic properties of the rock material. Since the stiffness of the probe's gauges are negligible in comparison to the stiffness of the rock, the overcoring strains represent a complete relaxation of the core. Hence, the core dimensions do not enter into calculations, except in the evaluation of the biaxial test results.

12 9 (26) The probe is cylindrical with a maximum diameter of approximately 54 mm and a length of about 550 mm, Figure 2.1. It is lowered into the borehole on a wireline in a combined installation tool and weight. A brief summary of the component parts of the instrument follows Strain gauges The instrument carries nine electrical resistance strain gauges mounted in three rosettes. Each rosette comprise three strain gauges oriented parallel, at 45 and perpendicular to the borehole axis. Strain gauge configuration is summarized in Table 1. Table 1.1 Orientation of the strain gauges in each rosette on the Borre Probe. Rosette No Gauge No Orientation of gauge within rosette Axial I Longitudinal Circumferential I Transverse 45 Axial I Longitudinal Circumferential I Transverse 45 Axial I Longitudinal Circumferential I Transverse 45 The strain gauge rosettes are bonded to three plastic cantilever arms at the lower end of the probe which is the only part of the instrument that enters into the pilot hole. The arms are located 120 apart at a known orientation to the main body of the instrument, Figure 2.1. Thus, the nine strain gauges of the Borre Probe form an array representing seven spatially different directions. As strain measurements in six independent directions are required to determine the complete stress tensor, the Borre Probe provides redundant strain data. Hence, up to one non-parallel gauge and two parallel gauges may be rejected or malfunction during the overcoring procedure without impairing complete calculation of the stress tensor. The strain gauges are connected to a data logger up the inside of the probe Adhesive The strain gauge rosettes are bonded to the pilot hole wall using an epoxy or resin adhesive depending on the rock and/or water temperature and depth of the measurement location from the borehole collar. The composition of the adhesive is vital to the success of the measurement. Hydro power use an adhesive capable of providing acceptable bonding underwater. The time available for the installation of the probe is limited by the pot life of the adhesive and, therefore the adhesive used for any measurements is engineered carefully to provide adequate installation time as well as a good bond between the gauge and pilot-

13 10 (26) hole wall. The estimated time for hardening of the glue in waterfilled boreholes is 8-10 hours Data logger Besides the nine strain gauges, the Borre Probe also contains a thermistor and one dummy gauge to assess the environmental effects on the readings during the overcoring phase. The downhole data logger that is located in the main body of the probe, Figure 2.1, records eleven channels of data at preset intervals from a preset start time. The logger, powered by a battery also located in the probe's main body, is capable of storing 8 h of data recorded at 60 s intervals. Prior to the installation of the probe, the data logger is connected to a portable computer and programmed with the measurement start time and recording interval. No further connection to the ground surface is required after this programming. After overcoring, the probe is recovered with the overcore sample inside the core barrel. Before removal of the sample and disconnection of the strain gauges, the probe is again connected to the portable computer and the data recorded is retrieved using communication software Installation tool Connected to a wireline, the installation tool with a weight on top carries the probe down the hole and releases it into the pilot hole. The tool contains a mechanical latch that is triggered when the base of the tool lands on the base of the main borehole. Triggering the latch releases the probe from the tool and forces the cantilever arms and strain gauges against the pilot hole wall. The installation tool also contains a magnetic compass, connected to the latch and mechanically fixed in its orientation when the latch is triggered. This effectively records the orientation of the probe as it can only be set in and released from the tool in one orientation. 2.3 OVERCORING- TEST PROCEDURE The procedure for one measurement, from the commencement of drilling the pilot hole to recovery of the overcored sample carrying the Borre Probe, is illustrated in Figure 2.1.

14 11 (26) Advance Q>76 mm main borehole to measurement depth Drill Q>36 mm pilot hole and recover core for appraisal Lower Borre Probe in installation tool down hole Probe releases from installation tool. Gauges bonded to pilot-hole wall under pressure from the nose cone Raise installation tool. Probe bonded in place Overcore the Borre Probe and recover to surface in core barrel. Figure 2.1 Operation for installing and measuring with the Borre Probe Drilling the pilot hole Stress meaurements using the Borre Probe requires a centrally located, coaxial, clean 3 6 mm pilot hole drilled from the bottom of the main 76 mm borehole. Diametral accuracy is necessary as the probe has a limited operating range during installation. The pilot hole needs to be centrally located in the hole base to enable overcoring and biaxial testing of the recovered cylindrical rock sample. At the position of the strain gauges, the pilot should be clear of drill cuttings to ensure a good bond between the gauges and the pilot hole wall. It is preferable also to recover an intact core from the pilot hole. This core should be of sufficient quality to enable the quality of the rock in the large-diameter overcore sample to be anticipated so that the gauges are not located in a position over or adjacent to preexisting discontinuities - normally, such locations will yield poor measurements. If the core break in the main borehole is not flat enough to ensure centric location of the pilot hole, a specially manufactured planing tool run on the 76 mm core barrel is used to

15 12 (26) prepare the base of the 76 mm borehole prior to drilling of the pilot hole. The core recovered from the pilot hole is examined for fractures or other discontinuities adjacent to the gauge position. If such fractures are evident, the pilot hole is abandoned and overcored and another pilot hole is attempted. On the other hand, if the pilot core indicates suitable measurement conditions the Borre Probe is prepared for installation. In the present case, the pilot holes were drilled to a length of approximately 70 cm. Flushing continued until return water was clear before recovering the pilot core in order to improve the conditions for a successful measurement by supplying a clear hole Installation of the Borre Probe Prior to installation of the Borre Probe, a dummy probe is lowered down the borehole and into the pilot to check that the hole is clear of obstructions such as drill cuttings. The dummy weights the same as the probe and installation tool together and has the same external dimensions. Similar to the probe, the dummy is lowered downhole on the wireline attached to the winch. The connection of the strain gauges to the logger and other preparation and testing of the probe and installation tool are generally carried out whilst drilling the pilot hole. If the pilot core recovered displays a rock quality suitable for testing, the data logger is programmed and the compass and the probe are attached to the installation tool. The latching mechanism in the tool is armed and the tool is attached to a weight carried on the wireline. Finally, the adhesive is mixed and applied to the strain-gauge rosettes and the gauges are then covered with a protective cone before the whole assembly is lowered down the borehole on the wireline. As the installation tool reaches the measurement level, the rate of descent is reduced and then stopped (at a position about m above the test level) for about 30 sin order to let any twist that may have occured in the wireline fade out as a spin in the wire could inflict on the compass reading. Lowering then continues and the realease mechanism is activated as the latch touches the ledge formed between the main borehole and the pilot hole. As the probe is installed, the protective cone preventing the adhesive from being removed when lowering the probe down the hole is pushed away further into the hole allowing the gauges to contact the pilot hole wall. The tool is left in the hole until the adhesive has set completely, which is within 8-10 hours depending on the glue mix and the water/borehole temperature Overcoring The requirements for the overcoring of the Borre Probe are that the over core is concentric to the pilot hole, drilled at a constant and steady rate, and that a suitable length of solid core is recovered at the position of the strain gauges. Concentricity of the overcore is both an operational requirement, to ensure the safety of the downhole equipment, and necessary for the calculations of the stress distribution and material properties from biaxial testing of the core sample. A constant drilling rate during overcoring ensures that stress relief on the core occurs in a controlled manner and that the strain gauge response is not unduly affected by

16 13 (26) the drilling. Controlling the rate of penetration reduces the possibility of the overcore sample to crack in weaker strata. The recovery of solid core of suitable length at the gauge position is required for subsequent biaxial testing. Cracking of the core in the proximity of the gauges can also influence the strain gauge response and possibly render the measurement incalculable. After the epoxy setting period and as the downhole data logger is due to commence strain readings, the installation tool is recovered to the surface on the wireline. The compass is removed from the tool and the compass reading is noted. The drill string, now carrying the Craelius T2-76 mm drill bit is then lowered. While holding the drill bit about 10 cm above the bottom of the main borehole, flush water is circulated for a min period to stabilise temperatures before overcore drilling commences. Overcoring is then performed to a depth beyond the bottom of the pilot hole in order to recover the protective cone. After breaking the core loose, the drill string is lifted and the overcore sample and the probe are recovered to surface. If the next pilot hole is planned to be drilled from the bottom of the overcored section, the main hole should be thoroughly flushed before lifting the string. In the present case, overcoring was carried out to a length of about 80 cm. When the overcore sample has been recovered, strain gauge data recordings are immediately transferred from the logger to the laptop computer. The Borre Probe, but not the strain gauges and theri respective connecting cables, is dismounted from the overcore sample. Then, if solid core recovery is of suitable length, e.g. some 25 cm, the overcore sample is subjected to biaxial testing in order to determine the elastic properties of the rock Biaxial testing The biaxial testing of the overcored specimens has two purposes; Firstly, it allows the elastic constants of the rock to be determined and secondly, it provides a check of the performance of the individual strain gauges 1-9, Table 1. The former is required for the subsequent stress computations, and the latter provides input to the examination of overcoring strains as well as to the overall judgement of the validity of the test. All suitable overcore samples are tested in a biaxial test chamber to determine the elastic properties of the rock. During testing the strains induced in the core sample are monitored by the strain gauges installed by the Borre Probe connected to a digital strain readout, see Figure 2.2 for schematic test set up. The test sequence comprise both loading and unloading in order to study possible inelastic behaviour of the rock. The maximum load applied to the core specimens is 10 MP a in load increments of 1 MPa. The core is then unloaded stepwise by 1 MPa increments. The results from the tests are visualized in the form of diagrams of recorded strains, plotted against applied pressure. An example plot is shown in Figure 2.3. Since geometry of the test is axisymmetric, the array of strain gauges of the Borre Probe represents three groups with respect to orientation, axial (parallel), circumferential (perpendicular) and at a 45 angle (to the hole axis). Theoretically, the gauges within each group should respond identically to loading/unloading.

17 14 (26) overcore sample ~ ~ appr. 230 mm : Figure 2.2 Schematic figure of the test set up for biaxial testing. CIRCUMFERENTIAL GAUGES BIAXIAL LOAD [MPa] 45 GAUGES AXIAL GAUGES STRAIN [ J.!& ] Figure 2.3 Example of biaxial testing results, schematic plot. To derive the elastic properties the theory for an infinitely long, thick-walled circular cylinder subjected to uniform external pressure is considered. The assumption of plain stress applies as shown by Obert and Duvall (1967). In the calculation of Young's modulus (E) and Poisson' s ratio (v), the parameters that have to be known are the core dimensions and the axial and circumferential strain readings at different pressure loads. Since the Borre Probe incorporates three pairs of circumferential and axial strain gauges, three pairs of elastic property-values are obtained from each biaxial test. The ambition should be to obtain rock parameters that applies to the relaxation experienced by the rock

18 15 (26) during overcoring. Therefore, the values ofe and v should be taken as secant values, calculated from strain data obtained during unloading of the core specimen. 2.4 OVERCORING- STRESS CALCULATION The equations relating the strains occuring at the pilot hole wall as a consequence of overcoring, to the virgin stress field at the point of measurement are obtained from the classical Kirsch solution the derivation of which can be found in Jaeger and Cook (1979). For isotropic material, the appropriate set of equations were presented by Leeman (1968). Equations applicable to the case of general anisotropic material have been given by Amadei and Goodman (1982), and the influence ofinhomogenities and inelastic behaviour have been described by authors including Martin and Christiansson ( 1991). To calculate the stress tensor from overcoring data hydropower use a computer program written by personnel of the former Swedish State Power Board (now Vattenfall AB). The program is based on the equations given by Leeman (1968). To calculate the threedimensional stress at a measuring point the program requires strain data from at least six independent directions (as described above), the orientation of the borehole, the orientation (magnetic bearing) ofthe Borre Probe, the elastic constants of the rock (Young's modulus and Poisson' s ratio) and finally the gauge -and resistance factors. The gauge factor (strain gauge sensitivity) is given by the strain gauge manufacturer and describes the relationship between strain and the change of resistance in the gauge. The resistance factor is an internal data-logger factor which has to be given to account for the set up of the Borre Probe. When all nine gauges function properly during a measurement, redundant strain data are obtained. The stress calculation program uses a least square regression procedure to find the solution best fitting all the strain data. From this solution, the program calculates the stress field in the horizontal plane and the magnitude and orientation of each of the three principal stresses. If more than one measurement has been carried out at the same level in a borehole, the program defines all measurements in one and the same coordinate system and calculates mean values of the three dimensional stress field. As described in section 2.3.4, Young's modulus and Poisson's ratio of the rock material will be determined from biaxial testing of the overcore samples obtained from the measurements. In the calculation of the Young's modulus the pressure applied to the overcore sample is increased by a concentration factor given by the formula CF = 2D 2 I (D 2 - d 2 ), D being the outer diameter and d the inner diameter of the hollow rock cylinder, in order to account for the increase in applied stress at the centre of the hollow cylinder. The elastic constant used in the computation of the three-dimensional stress tensor are taken as mean values from B and v-values given by the secant method over those sections of the stress/strain response curves that show stable and linear behaviour during unloading.

19 16 (26) 3 MEASUREMENTS 3.1 GENERAL GEOLOGY AND SITE CONDITIONS The bedrock at the Hastholmen site belongs to the Wiborg rapakivi batholith. The rapakivi granite is an intrusive igneous rock which intruded into older bedrock some Ma ago (Anttila, 1988). Three main varieties of the rapakivi granite are represented in the bedrock at Hastholmen; pyterlite, wiborgite and even-grained rapakivi. Pyterlite is the dominating rock type. Rapakivi granite displays regular jointing, characterized by horisontal or gently dipping fractures intersecting two mutually perpendicular vertical joint sets. Fractures close to the surface tend to be open or partially open, although closed fractures are also present. Overcoring stress measurements were performed in one vertical 76 mm OD diameter coredrilled borehole, drilled to the approximate depth of 600 m below ground surface. Figure 3.1 shows the Hastholmen isle and some of the boreholes drilled, including borehole HH-KR6... ~ l:l. ~'!f.dhlll ~..L ' ±.. ""' K lvi'lolm~ n.,~.'..~.~:.\ ~w K fv,flplm n\'''~u.,;_,,,,~;rt ~ ' ~~ - Hastholmen Site Location of Boreholes Projection: Old Finnish Coordinate System, zone 3 Saanio & Riekkola Oy I A. Ohberg, H. Malmlund LEGEND ~ Core drilled borehole j_ j_ l..!. J T.l + Figure 3.1 Hastholmen isle and some of the coredrilled boreholes.

20 --~~ (26) 3.2 FIELD WORK The field period at Hastholmen for measurements in borehole HH-KR6 started on May 6,1998 and was stopped June 25, Simultaneously with the measurements in borehole HH-KR6, rock stress measurements were also conducted in borehole Y26 at Hastholmen. Results from the latter borehole are reported by Ljunggren (1998) and will not be refered here. Core drillings connected with the overcoring measurements were commissioned to a drilling company, Suomen Malmi Oy. The work is reported by Niimimaki (1998) in a separate report. During drilling of the borehole to the 300 m test horizon rock with poorer quality was encountered between approximately 270 m and 295 m depth including a mylonite zone. The zone of poor rock quality caused immediate problems to obtain a pilot borehole free from rock cuttings, and the mylonite zone caused problems in form of poor glue bondings due to the greasy composition of the mylonite. After a few measurement attempts the borehole was cemented between 268 m and 295 m depth. The hole had to be cemented three times to obtain a stable condition. The cause of the poor bondings encountered was identified after a number of bonding tests at surface. Bonding tests were conducted in clean water, in drilling flush water on a clean piece of core, and in drilling flush water on a piece of core smeared with the lubricant from the mylonite zone. From these tests it became evident that acceptable bonding could not be obtained on the mylonite lubricant. Further problems found already at the 3 00 m test horizon was the beginning/tendency of core discing, which caused a number of unsuccessful tests. Despite the fact that five successful tests not had been obtained at the 3 00 m horizon it was decided to deepen the hole to 3 60 m depth as the core log and geological investigations indicated rock conditions better suited for overcoring rock stress measurements at this depth. After one partly successful test at the 3 60 m horizon core discing once again appeared. A large number of pilot hole drillings and a few measurements attempts were conducted until it was finally decided to leave the level and deepen the hole to 450 m depth. Nine pilot hole drillings including one measurement attempt were completed between 44 7 m and 4 78 m, but all showed clear evidence of core discing. The results from the two last attempts were most discouraging, and at depth 3 92,69 m the discing, that initiated during the overcoring procedure, damaged the measurement equipment due to rotating discs. The borehole was then sunk to 504 m depth and pilot drillings to investigate eventual core discing was done every 1 0 m down to 600 m depth. The results from these pilot hole drillings all showed the presence of intense core discing preventing from any overcoring rock stress measurements. 3.3 RESULTS The overcoring rock stress measurements in borehole HH-KR6, Hastholmen included a total of 4 7 pilot hole drillings of which 11 were used for measurement attempts. In total, four measurements gave usable results. Overcoring strain graphs from the accepted measurements are presented in Appendix A, whereas graphs from some of the rejected tests

21 18 (26) can be found in Appendix B. General test data on all measurements are presented in Table 3.1. It shall be noted that the four accepted tests have been obtained in an environment where the combination of present stress conditions, rock strength and the geometrical relations has placed the measurement volume just at the limit of core discing initiation. It might even be so that discing already is present on a micro scale. Table 3.1 General test data, overcoring measurements in borehole HH-KR6, Hastholmen. Test No Depth (m) 295,19 297,18 299,03 299,83 301,18 304,98 305,83 360,19 392,69 392,69 447,00 Comments Rejected due to 20 cm of cuttings in the pilot hole Accepted Accepted Rejected, loose rock fragments in hole, incomplete probe release Rejected, irregular strain readings due to bonding problems Accepted Rejected, irregular strain readings due to core discing Accepted Rejected due to uncomplete release of the measurement probe Rejected due to core discing Rejected due to core discing Note that measurements 1, 4, 9, 10 and 11 did not yield any strain data and, consequently, no overcoring strain graphs could be obtained. It shall also be noted that tests 9 and 10 were both attempted at the same depth, i.e. in the same pilot borehole. Biaxial testing Due to the presence of core discing no successful biaxial testing were obtained when testing the cores from borehole HH-KR6. During pressurisation in the biaxial test chamber, Figure 2.1, the cores started to split up into discs resulting in erroneous gauge readings. Since no elastic values were obtained from the successful tests, elastic parameters from laboratory testing on cores from other boreholes at Hastholmen had to be used. Mean values on the elastic parameters from a total of 57 laboratory, mainly uniaxial laboratory testing, yielded the following results: E = 64 GPa ± 6,2 GPa V= 0,20 ± 0,04 These results were, however, evaluated using the so-called tangent method at stress levels of MPa, whereas the overcoring procedure covers the stress area from some MPa down to zero stress level. Hence, the laboratory tests are not fully as input data for the overcoring stress calculations. Instead, the elastic parameters should be taken as secant

22 19 (26) values from the unloading sequence of a test. The latter normally gives lower elastic parameters as compared to tangent values from a uniaxial compresion test. If we study the few successful biaxial tests on overcores from borehole Y26, Hastholmen (Ljunggren, 1998) we find values on Young's modulus between GP a and a Poisson' s ratio between 0, 18-0,23. Based on the above it was judged to use a Young's modulus of 55 GP a and a Poisson' s ratio of 0,20 for the overcoring stress calculations in borehole HH-KR6. A sensitivity analysis on the influence of the elastic parameters on the calculated rock stresses is presented in Ljunggren and Klasson, From that study it can be seen that a 10 GPa change of the Young's modulus will change the stress magnitudes at the order of 1 MP a. Principal stresses The magnitudes of the principal stress field ( cr1, cr2, cr3) given by the overcoring measurements in borehole HH-KR6 are listed in Table 3.2. A graphical presentation of determined magnitudes is found in Figure 3.2. Table 3.2 Test No Mean: 8 Principal stress magnitudes as determined by overcoring in borehole HH-KR6, Hastholmen. Depth Ot cr2 <JJ (m) (MP a) (MPa) (MP a) 297,18 12,3 5,7 1,3 299,03 22,5 15,2 5,8 304,98 19,5 3,0-1,3 15,4 8,5 4,1 360,19 21,4 8,2 0,1 Table 3.3 gives the orientations of the principal stresses as evaluated from the overcoring data. A stereo graphic plot of the stress directions can be found in Figure Table 3.3 Test No Mean: 8 Principal stress orientations as determined by overcoring in borehole HH-KR6, Hastholmen. Orientations are given as strike/dip of the stress vectors cr~, crz and cr3. Depth <Jt (Jl <JJ (m) e> e> e> 297,18 077/22 339/18 214/61 299,03 139/06 230/12 022/76 304,98 245/02 342/75 154/15 076/03 166/01 278/86 360,19 088/07 179/10 326/78 Note that strike in Table 3.3 is counted clockwise with reference from magnetic north and dip is counted positive downwards with horisontal being equal to zero.

23 20 (26) ~ :::~j ASigma 3 Magnitude (MPa) --~ t?rrt77tt :!.1:1 Q. u "Cl -; Col "f u > Figure 3.2 Principal stresses as function of depth, borehole llli-kr6 Hastholmen. EQUAL ANGLE LOWER HEMISPHERE (MAGNETIC) N LEGEND 0 r + I o' I l BOREHOLE HH-KR6 Sigma 1 0 Sigma2 E SigmaJ s Figure 3.3 Principal stress directions from overcoring measurements in borehole Illi-KR6, Hastholmen.

24 21 (26) Horisontal- and vertical stresses Calculated from the principal stress data, the magnitudes of the horisontal- and vertical stresses are presented in Table 3.4 Table 3.4 Test No Mean: 8 The horisontal- and vertical stress state as determined from overcoring rock stress measurements in borehole HH-KR6, Hastholmen. Depth crh crh crv crh (m) (MP a) (MP a) (MP a) eo> 297,18 10,8 5,2 3, ,03 22,3 14,8 6, ,98 19,5-1,0 2, ,3 8,5 4, ,19 21,1 8,0 0,6 88 Figure 3. 4 through 3. 5 illustrate the horisontal- and vertical stress field as determined from overcoring in borehole HH-KR6..Sigma H 8Sigma h ASigmav Magnitude (MPa) :! -=... c. u 'e ea u t: u > Figure 3.4 The horisontal- and vertical stresses as function of depth, borehole HH-KR6 Hastholmen.

25 22 (26) N 304,98 w 360,19 E s Figure 3.5 Orientations of the maximum horisontal stress as determined from overcoring measurements in borehole HH-KR6, Hastholmen. 3.4 COMMENTS AND CONCLUSIONS The overcoring rock stress measurements in borehole HH-KR6 at Hastholmen were only partially successful. Core discing, evident already at the 3 00 m level, allowed only a few acceptable measurements between 3 00 m and 400 m depth, whereas no successful tests at all could be obtained at deeper levels. In total, 4 7 pilot hole drillings were conducted. 11 of these were used for measurement attempts but only four gave strain data usable for stress analysis. The four results obtained are, however, of poor quality and can only be used to confirm stress levels and directions of the stresses. Despite the poor outcome in success rate, the four results indicate some most important data on the virgin stress field in the area: A high anisotropy in the horisontal plane is observed, up to 2,5. A high maximum horisontal stress ( MPa) at 300 m depth as compared to the general trend of magnitudes in Finland. An extremely low vertical stress, at the order of some 4 MP a. This low measured magnitude is, however, strongly believed to be an artifact caused by the fact that a very high horisontal stress field does exist which resulted in core discing. It is suggested that the vertical stress is higher than 4 MPa but lower than the theoretical vertical stress, pgz (~ 8 MPa). Principal stresses are mainly oriented in the horisontal- and vertical plane.

26 ~~-- 23 (26) An orientation of the maximum horisontal stress in the E-W direction. The orientation coincide with the results from other rock stress measurements at Hastholmen (Ljunggren and Klasson, 1992). Except for the measured magnitude of the vertical stress, it is suggested the remaining other qualitative results above are valid for the in-situ stresses at Hastholmen. In 1987 overcoring rock stress measurements were performed at the 50 m and 110 m levels in borehole Y20 at Hastholmen, Hallbjom and Strindell (1987). Those measurements gave the same picture of the stress field as obtained from the present tests, i.e. a high maximum horisontal stress as compared to the general trend, a high anisotropy in the horisontal plane, a small vertical stress compared to the weight of the overburden and a direction of the maximum horisontal stress in the E-W direction at the 110 m depth level. The hydrofracturing measurements conducted by Ljunggren and Klasson, 1992 indicate a similar pattern, i.e. a high anisotropy in the horisontal plane and ane-w orientation of the maximum horisontal stress. Despite the numerous attempts to measure at depths between 400 m and 600 m no usable data could be obtained. The core discs became thinner and thinner with depth and at some test points the rock even split up into small rock pieces due to the quite extreme conditions. Figure 3.6 below illustrates the interpreted stress field in the horisontal- and vertical plane. The results from Ljunggren and Klasson, 1992 are included in the figure. From Figure 3.6 it can be seen that, although the overcoring results are not of a good quality, there is a reasonable good correlation with the shallower hydrofracturing results. The solid lines in the figure are linear regression lines to present a rough estimate of the stress change with depth.

27 24 (26) Slgma H Sigma h Magnitude (MPa) Figure 3.6 Interpreted in-situ stress field at Hastholmen.

28 25 (26) 4 SOURCES OF ERROR AND DATA CONFIDENCE By experience we know that the instrument errors (that is; the difference between the actual strain subjected to a discrete gauge and the corresponding read out value) can be neglected for given circumstances. The instrumentation used were tested for accuracy using an aluminium hollow cylinder with known properties. The back calulated elastic properties from the test in the aluminium cylinder gave exactly the same values as delivered by the manufacturer of the aluminium. All test data from this test has been given to the client. The problem is to estimate the errors introduced by the fact that the rock does not fulfill the assumption of homogenity and linear elastic behaviour. The biaxial testing is normally a good indicator of eventual material defects such as heterogenity, non-linear behaviour and unelasticity. All these factors do, of course, introduce errors. Here, biaxial testing was not possible as the cores were too fractured due to the presence of discing and, hence, this could not be checked.

29 26 (26) 5 REFERENCES Amadei, B. & Goodman, R.E., The influence of rock anistropy on stress measurements by overcoring techniques. Rock Mechanics, 15 (4), Anttila, P, Engineering geological conditions of the Loviisa power plant area relating to the final disposal of reactor waste. Report YJT , Nuclear Waste Commsion of Finnish Power Companies, 130 p. Bjamason, B., Hydrofracturing Rock Stress Measurements in the Baltic Shield. Licentiate thesis 1986: 12L, Lulea University of Technology, 122 p. Hallbjorn L, Ingevald K, Martna J, Strindell L, New automatic probe for measuring triaxial stresses in deep boreholes. Tunneling Underground Space Technology, Vol. 5, No. Yl, Hubbert, M.K. and Willis, D. G., Mechanics ofhydraulic Fracturing. Trans. A.I.M.E., 210, pp Jaeger, J.C. and Cook, N.G.W., Fundamentals of Rock Mechanics. 3rd Edition, Chapman and Hall, London, 593 p. Leeman E.R. and Hayes, D.J., A Technique for Determining the Complete State of Stress in Rock using a Single Borehole. 1st Int. Congr. Soc. Rock Mech., Lisbon, Vol. 2, pp Leeman, E.R., The Determination of the Complete State of Stress in Rock in a Single Borehole-Laboratory and Underground measurements. Int. J. Rock Mech. Min. Sci., Vol. 5, pp Ljunggren, C. and Klasson, K., Hydraulic fracturing rock stress measurements at Hastholmen, Finland. Report YJT-92-26, Commission offinnish Power Companies., 47 p. Ljunggren, C. And Klasson, K., Rock stress measurements at the three investigation sites, Kivetty, Romuvaara and Olkiluoto, Finland, Volume 1. Work Report PATU-96-26e, Posiva Oy, Finland, 99 p. Ljunggren, C., Overcoring rock stress measurements in borehole Y26 at Hastholmen, Finland. R&D Report 98-09, Posiva Oy, Finland, 28 p. Niimimaki, R., Core drilling of deep borehole HH-KR6 at Hastholmen in Loviisa. Posiva Oy work report (in Finnish).

30 Al APPENDIX A OVERCORING GRAPHS -ACCEPTED TESTS

31 MEASURED STRAINS DURING OVERCORING,.,., \ ' ' '. ' 'Jj--, > (\' ' ' '. ' \ I \ ~ I ' J.~,-,_ ; ' ~~ ~ ~ ooo - ooo ~ ~ 0000 ~..., ~ ~ oooo A ~ ~ ~ - ~ooo -~ ~ _. ~ ~ ' - ~ ~ ~ ~ ~ ~ w ~ w ~ ~ " -~ w w w ~ 1/i /"V-~----- ': \_{. /1.\N ~~ ~ ~.... V I ~-\ ~ "w i~~- ~- --~ _i1 ~~~ _ -~ _ / \ rs:.. L "'.... t,\, '... -~,-1~--:-:.e.-::::_7=~:;, ;; ~...,., - ~ ~-~: :: ~::... ~ : ::: :., ::. ;..... >.. "' :: :: :: :,"' ---Gauge 1 ~-~Gauge 2 Gauge Gauge4 Gauge 5 Gauge 6 --Gauge7 ---Gauge 8... Gauge r- 0 C Start ~16cm -o---0 C Stopp -x-core break ~ -. ~' ~. ~ ---.~ r :16 09:21 09:26 09:31 09:36 Time HASTHOLMEN, BOREHOLE HH-KR6, TEST 2, HOLE DEPTH: m 5463/ChLj

32 MEASURED STRAINS DURING OVERCORING Microstrain ~ / ~ I _l 1100! l ~ 1000 I 900 /'1\ f t! j BOO ~y j 100 L -J -!-r-_-_- V/\ 1 r,l_-_..._----'--~ ~-~... ~ ~ ~ :~~ ~~ 1 f i... 1 // ~ J i J t I r 300 : "" -... h "' i 2oo I' I ',_ '" If l j\ ~._; 100 / :i,/.\ ' J----&:ill!!i!:l::e~l!.\'.li: -c"'-:~,.:lr~!!!! c:.!:;:;>~4-o~. :\ -=--cc... ~~::....._?:'_:.: :... "(':~.' \... :: :: :: :: ~. ::....::. 0 \... -;...! \ -200 ::~~ ~. ~ ~=~~~~ f l - /V/ / ~~. ' -600 : 800 I I -700 I, :';...: +----, ~~r---~ ~~~~ ~~ ~~ ~~~- ~--- 09:46 o.' 09:51 09:56 10:01 ~ / Time I I I 10:06 10:11 J --Gauge 1... Gauge Gauge Gauge 4 ---Gauge Gauge 6 --Gauge7... Gauge 8 y Gauge 9 --1r- 0 C Start ~16cm ~OCStopp -)le- Core break ~ HASTHOLMEN, BOREHOLE HH-KR6, TEST 3, HOLE DEPTH: m 5463/ChLj

33 MEASURED STRAINS DURING OVERCORING Microstrain ~ JP~~ 1100 /'..., I..., I... ~ I 1000 I I """..., i : i _ I I 700 t f l 600 i 100 ±~~~~~ ~~~~~. 7.~. -:~ ~~.!?\.- 0 I -...,...~ ? -~.1'... J>,r "'",., " '"'.., " I Gauge 1 ~~ ~ Gauge2 Gauge Gauge4 ~~~Gauge 5 Gauge 6 --Gauge7 ~~~Gauge 8 h h h n n h Gauge 9 --A- 0 C Start ~16cm -o-- 0 C Stopp --JC-- Core break ~ r-----f ,.,,...; --1, ~ ':~~-A--~--~~~~--~--~~---r--~~--~--~--~~--~--~--~-+--~- 09: 06 ~-' :11 09:16 09:21 09:26 Time HASTHOLMEN, BOREHOLE HH-KR6, TEST 6, HOLE DEPTH: m 5463/ChLj

34 Microstrain MEASURED STRAINS DURING OVERCORING 1100~------~ ~ ~ Vattenfall Hydropower AB Project number: 5463 Client: Posiva Oy 1000 I 1 :1 / ~~~=+==-'<:...., \ 0...,. --Gauge 1 ---Gauge Gauge 3 --Gauge4 ~~~Gauge 5 Gauge 6 ---Gauge7 ---Gauge 8 Gauge 9 --&-- 0 C Start -<>--16cm ---o- 0 C Stopp -:1:- Core break > VI ~~~--~~~--~~-o--~~~--~~~--r-~~--~~~~--~~~--+-~~--~~~ 09:40 09:45 09:50 09:55 10:00 10:05 Time HASTHOLMEN, BOREHOLE HH-KR6, TEST 8, HOLE DEPTH: m

35 Bl APPENDIXB OVERCORING GRAPHS -REJECTED TESTS

36 MEASURED STRAINS DURING OVERCORING Microstrain 2000 ~ A 15oo I :' - u Hr 1000 I H. I..,_ ,.. ' j ---, I '). " ' ! 500 ii J"""' l ~- ici ~/._...J. _ I, l,. ~' l I '-- I ~... I J I w E "'<._ _..;.1-r \ I'L /I,/ M-1 _,.,. 7.,...,... ( Gauge 1 ~~~Gauge 2 w w w w w Gauge 3 ---Gauge4 r ~-.., j--- Gauge 5 I YGauge 6 Gauge 7 ~-~Gauge 8 Gauge 9 ~ I -A- 0 C Start I to N --<>---16cm o-0 c Stopp ' ' le " \ l,,.~"" ;>',J,..-..-~ I. I ' -JC- Core break :01 09:06 09:11 09:16 09:21 09:26 09:31 09:36 Time HASTHOLMEN, BOREHOLE HH-KR6, TEST 5, HOLE DEPTH m 5463/ChLj

37 MEASURED STRAINS DURING OVERCORING Microstrain ~~ ~ ~~------~~ ~~ o ~---=~ l ~ I ' ~---~ ~ ~~-----~ : ===~L-f-i-----,(" "'--'--! /. -- "' ~:~ ~ :.:.;~~.. ~~~ ---Gauge 1 ~ -- - Gauge Gauge 3 --Gauge4 -- ~ -- Gauge Gauge 6 --Gauge Gauge Gauge C Start --o--16cm ---o- 0 C Stopp -:.:- Core break to w U >#' A... o, ""'... ~ :: 09: :24 09:29 09:34 09:39 Time HASTHOLMEN, BOREHOLE HH-KR6, TEST 7, HOLE DEPTH: m 5463/ChLj