Christiansson, Rolf (Swedish Nuclear and Fuel Waste Management Company) | Olofsson, Isabelle (Swedish Nuclear and Fuel Waste Management Company) | Martin, C. Derek (University of Alberta) | Holmberg, Mats (Tunnel Engineering AB) | Carlsson, Anders (Carlson GeoEngineering AB)
The Äspö Expansion project required the excavation of approximately 300mof new experimental tunnels on the 410 Level. At this depth there is a hydraulic head of approximately 365 m. The project design required that the maximum allowable draw down not exceed 50mrelative to the initial conditions. Borehole inflow tests carried out in the pilot holes before construction commenced provided a model for relating the inflows and draw downs. Based on previous experience at Äspö HRL, grouting was selected as a cost effective solution. The grouting strategy used the methodology developed for Real Time Grouting Control (RTGC). The principles of the Observational Method as specified in Eurocode 7, in conjunction with the RTGC were successfully used in the design and construction of the Äspö Expansion project that limited the drawdown to approximately 25 m.
New demonstration experiments supporting the technology required for the implementation of the design of the KBS-3 geological nuclear waste repository required construction of two main tunnels and several large niches at the 410-m level of the Äspö Hard Rock Laboratory (HRL), Figure 1.The construction of these facilities complete with the installation of infrastructure and furnishings suitable for the experimental purposes is referred to as the Äspö Expansion Project. One of the design constraints for the project was that the water pressure (expressed in metres) above the experimental area should be kept within 50mof its initial level prior to construction. Past experience at the HRL demonstrated that cement grouting technology at the 450 m-Level was a viable method for limiting inflows to small single heading tunnels. However it was not known if the technology would be suitable for controlling the head loss with more complex geometry and large caverns.
A pilot hole was drilled along the axis of each tunnel and used to characterize the groundwater flow system. Amonitoring system and short duration flowtests demonstrated a rock mass permeability with good connectivity. It became obvious that the 50m drawdown constraint would easily be exceeded if the tunnel pregrouting was not successful. In order to reduce the project technical risks and costs to acceptable levels, it was proposed to design and construct the tunnels and caverns using the Observational Method as outlined in EUROCODE-7.
Developing resource shale and/or tight plays can be extensive and demanding, particularly when determining an optimal multi-stage fracture stimulation design. A common approach has been to duplicate the so-called Barnett design, such as using a slick water fracturing fluid with a low concentration of proppant. However, it has been proven that the Barnett design was inefficient in many other fields, such as the Haynesville, Bakken, and Eagle Ford. A recent trend for developing resource shale and tight plays has been to attain an analog field, duplicate the design optimized in the analog field, and further optimize its design by trial and error. However, even this approach requires a considerable learning curve and associated costs to determine the optimal multi-stage fracturing design. Shale geomechanics can help minimize this learning curve and provide optimal fracture design recommendations based on geomechanical analysis combined with geological, geophysical, and petrophysical knowledge.
After successful development of the Barnett shale, other resource shale and tight plays have been commercialized all over North America, and now extending to Central/South America, Europe, China, Australia, and Russia. The success of resource shale and tight plays has mainly derived from technological advancements during the past 10 years, including large volume multistage hydraulic fracturing in horizontal completions, passive microsiesmic monitoring, and expanded use of 3D seismic of the fields.
These recent technological advancements in resource shale and tight plays present unique engineering challenges with respect to geomechanics, such as long, horizontal well drilling and completion methods that allowcomplex multi-stage hydraulic fracture stimulation design. Exceptionally long horizontal drilling can create significant wellbore stability issues, which are stress-induced and time-dependent, from fluid-formation interaction. Complex multistage hydraulic fracture designs require extensive geomechanical information for optimization, such as selection of well placement, horizontal well direction, stage isolation method, stage interval selection, perforation locations, and fracturing fluid system/proppant selection.
Such demanding challenges can be addressed, fully or partially, by integrating all geomechanical information obtained from geological, geophysical, petrophysical, and laboratory data. Geophysical and petrophysical analyses of natural fractures and faults can also be used during the final multi-stage hydraulic fracture design.
In this paper, a rigorous analysis by a 2D finite element code of the influence of rock mass fracturing and the influence of the water seepage in the design of a tunnel support, excavated by N.A.T.M. are introduced. Also, the existence of longitudinal joints in the rock mass is their influence in the variation of the safety factor of the primary support is taken into account. The rock mass formation is constituted by schists and shales. The field case of reference is the Prado Tunnel in the High Speed Line between Lubián and Ourense (North of the Spain). The rock characterization has been done by the Bieniawski’s rock index RMR and Barton’s index Q. Also the definition of the Hoek and Marinos GSI index is employed. The range of corrections of the RMR and Q index, originated by the influence of the analyzed factors, is determinated by the numerical simulation.
1. Introduction and Background
Two factors considered at geomechanical classifications that include direct support application criteria in tunnels are the water inflow existence and the joint disposition towards the tunnel axis alignment.
Although according to Hoek “water pressures are generally not too serious a problem in underground excavation engineering”, it must be considered to get the support in underground excavation. Equally, the orientation of joints must be considered over all when direction of discontinuities coincides with tunnel axis.
With regards the water inflow, Hoek & Marinos published in 2004, that when assigning support not only the flow net but also the joint strength reduction should be considered (GSI Index) if the joint fill or the formation are sensible to water as slate, schist, phyllite etc. Likewise the joint disposition, which is taken into account on Beniawski RMR, as well as Barton’s Q index application criteria, as reflected on 2013 update performed by N.G.I.
In offshore Iran, wellbore instability is quite common and the main cause for most of problems during drilling operations. In this study, the existing relevant logs, drilling and other data from offset well were analyzed and integrated to construct a precise Mechanical Earth Model (MEM) describing pore pressure, stress magnitudes and orientation, and formation mechanical properties of the South Pars Gas field. Then, the constructed MEM was refined and calibrated using the existing caliper, image logs, rock mechanical core test and drilling data and through history matching to constrain and reduce the uncertainties associated with limitations and availability of the existing data. Using the validated MEM built for offset well, a sensitivity analysis by formation was performed to evaluate optimum drilling parameters and to evaluate the stability of future deviated or horizontal wells in the field. Finally, a quantitative risk study was conducted to assess the sensitivity of breakout and fracturing mud weights to various input parameters. Results indicate that the stable mud weight window decreases as the borehole deviation increases in the most of formations except reservoir section. It was shown that well azimuth has very little impact on the breakout mud weight, while the breakdown mud weight is significantly higher in wells drilled in the minimum horizontal stress direction. Moreover, It would be better to keep the well deviation as low as possible and not more than 45 and 60 degrees in the Kazhdumi and Fahlyan-Surmeh formations, respectively.
Wellbore stability maintenance is a crucial step during drilling a well for oil and gas production. Wellbore instability problems may cause increasing wellbore’s drilling costs, delay in drilling of the wellbore and in severe conditions wellbore abandonment. It is estimated that at least 10% of the average well budget is used on unplanned operations resulting from wellbore instability. These costs may approach one billion dollars per year worldwide (Aadnoy 2003). The simplest constitutive model for describing the behavior of rocks is linear elastic, which is the foundation for all aspects of rock mechanics. This theory is based on the concepts of stress and strain, which are related according to simple Hooke’s law.Two parameters are required for describing the elastic response of a material: Young’s modulus and Poisson’s ratio. The solution of a given problem considering the elasticity theory consists on the determination of the stress, strain, and displacement components. Bradley (1979a) and Fjaer et al. (1992) among others presented analytical equations to compute stresses around boreholes. They assumed a state of plane strain. The derivation of the stress solution is in the study presented by Jaeger and Cook (1979), and the final equations are given by Bradley (1979a) and Fjaer et al. (1992).
Various physico-mechanical and chemical properties of Cement and Resin capsules used in underground mines of different manufacturer of India have been tested as per Indian Standards (IS) code and Director General of Mines Safety (DGMS), India, guide lines mentioned in Technical Circulars. Correlations which have come out between these properties have been analysed and highlighted in this paper. From the tests carried out on cement capsules it has been observed that the compressive strength is inversely proportional with the percentage of Sulphuric Anhydrite and also up to some extent it is inversely proportional to the percentage of Chloride. For resin capsule it has been established that both the compressive strength as well as the shear strength is inversely proportional to the percentage of filler material present in the resin capsule. In case of fast setting resin capsule, for which the compressive strength measurement is difficult, the shear strength of resin capsule will be a broad indicator for its compressive strength as well.
Accidents due to movement of strata in underground coal mines had been a major concern for the mining community from the very beginning. Over the years compiled statistics of accidents in Indian coal mines identified “Fall of Roof” as a major causes of mine accidents. Various efforts were made by all concerned to reduce the hazard of strata movement by mining companies, research institutions, academicians and DGMS, India. One of the well-known efforts is use of rock bolts which not only enhance the inherent strength of the rock mass stabilization but also shows a downward trend in roof fall accidents. During the last two decades, roof bolting has emerged as a safer and more effective alternative to the timber/steel support used earlier in the Bord & Pillar method of mining. Cement and resin capsules are the integral part of roof bolting. Various manufacturers are there in the market to supply resin and cement capsules for these roof bolts. There are DGMS guidelines for testing these cement and resin capsules giving therein the specified range of physico-mechanical and chemical properties that these capsules are required to maintain. However, only fewco-relations are available till date between the specified wide range of different properties and the ultimate bonding strength of resin/cement capsules. Such co-relation once established, will help in optimizing the composition of cement/resin capsules to ensure maximum bonding strength.
This paper highlights some of the co-relations between physico-mechanical properties and chemical properties both for cement and resin capsules. Corelations have been established by the results obtained by testing the various physico-mechanical properties and chemical properties of cement and resin capsules. Testing was carried out for six different manufacturers of cement capsules and eleven different manufacturers of resin capsules.
In this paper we briefly review the rock mechanics works carried out to analyse the stability of an old room and pillar hematite mine. Field work was initially carried out to recover a good number of discontinuity data, also a number of rock samples for lab testing were collected. UCS and triaxial tests were performed and rock masses were characterized in order to estimate rock, pillar and rock mass properties. Room stability and pillar strength and stability studies were carried out in all the relevant areas of the mine to find out a general good level of stability. Only in a location of the mine incipient instability problems were detected. Simplified stabilization methods were proposed, analysed and implemented, including the construction of a timber crib and the perimeter cabling of not-so-stable pillars in order to avoid rock-fall and progressive failure and to slightly increase pillar strength, as demonstrated by means of numerical approaches.
1 Mine Characterization
The studied case involves an old underground room and pillar mine where a hematite rich bed of Cambrian age is exploited. The mine follows the stratum dip of around 15° and rooms are located around the main gallery. The ore rock mass present bedding and other four joint sets- which are perpendicular two by two (Fig. 1). Once largely fractured, the rock suffered a near to metamorphism process that reassembled the rock mass and re-glued faults and joints, producing a good quality rock mass (RMR=70).
An exhaustive laboratory program which involved UCS and triaxial strength tests with strain measurement was planned to characterize the rocks that exist in the mine (hematite ore and dolomite on the walls) obtaining the most relevant parameters of strength and deformability as well as other important index properties (density, joint friction angle…). All these parameters of the intact rock are presented in Table 1.
Using the above mentioned RMR, the characterization of the intact rock performed in the laboratory and the software RocLab (Rocscience, 2011), a characterization of the rock masses in terms of strength and deformability was also performed. But the emphasis should be placed in the pillar scale, once the failure criterion was obtained for both the rock (upper limit) and the rock mass (lower limit), one can interpolate between these limits and obtain the failure criterion for the pillars.
Cobre Las Cruces is an open pit mine that extracts copper sulphides from the same volcanosedimentary Paleozoic deposit as the mines of Rio Tinto, in the SW of Spain. The pit measures 1600m Long×900m wide×250m deep.
The ore is overlain by 150 metres of the tertiary marly formation which behaves as overconsolidated clay, locally known as “Guadalquivir Blue Marls”. These marls are structured with bedding at approximately 5m vertical intervals with an average dip of 3° to the South. A detritical aquifer between the ore and the marls exists. The water table is located 30m below the surface and pore pressure has been shown to play a dominant role on the slope stability, particularly in the marls benches. Mineralization is embedded in volcanic and other metamorphic rocks, including some soft tuffs and clayey slates.
To provide accurate data for these calculations, a comprehensive geological and geomechanical characterization has been undertaken. The geological work includes the elaboration of structural maps every 10m (eg, every bench) based on the geological mapping of the existing pit as well as in the analysis of the borehole data that includes over 500 boreholes and 100,000m of cores.
The geomechanical works consist in the construction of RMR quality maps for each bench while the characterization is based on lab and in situ tests (dilatometer and borehole televiewer).
With all these characterization an advanced 3D model solved with FLAC has been undertaken. This work forms a decisive component of the pit optimization after the first seven years of exploitation of the ore body.
Cobre Las Cruces is an open pit mine that extracts copper sulfhides from the same volcano-sedimentary paleozoic deposit as the Rio Tinto and Aznal collar mines. The ore is overlain by 150 metres of the tertiary soft marls known as “Guadalquivir Blue Marls”. Below these marls there is a sandy formation that constitutes, jointly with the weathered top part of the Paleozoic, a regional aquifer known as “Niebla- Posadas”. The water table is located 30 metres below the surface.
Finally the Paleozoic in which the mineralization is embedded, is constitute by slates, tuffs and porphyric rocks.
The Matinkylä station is the end station of the West Metro. The bedrock in the Matinkylä area is mainly densely jointed granite with open horizontal fractures. The station hall is 23m wide, 9m high and 230m long rock cavern. The span of the track switching halls is 21 m, the height 9m and the length 100 m. Due to the numerous weakness zones, the large span, the thin rock roof and the horizontal rock stress field, the numerical analysis was carried out consisting of both 2D and 3D numerical analysis. The analysis showed that the maximum displacement is expected to be about 20 mm. An extensive monitoring system was installed. During the construction phase it was noticed that the closure of the walls was about 8 mm, the maximum rising of the ground surface was about 15 mm, cracks appeared in the shotcrete at the roof arc and leakages appeared due to the rock displacements.
1. Description of the West Metro
The West metro is an extension of the Helsinki metro line to the city of Espoo. The length of the tunnel line is 13.9 km with two parallel tunnels excavated in rock (Fig. 1). The rail tunnels are connected to each other at the intervals of 100m to 150 m.
Nine access tunnels have been made for construction and maintenance purposes and fifteen vertical shafts for smoke exhaust, pressure equalization, emergency exits and technical installations. All tunnels shafts and stations are excavated using the drill and blast method, and reinforced with rock bolts and shotcrete.
The Matinkylä station is the western end terminal station. The Matinkylä contracts covered the excavation of two 1.8 km long metro tunnels, a service tunnel, two track switching halls (located before and after the station hall) as well as a station including passenger and technical shafts of the station, two shafts along the line, and open excavations for the station.
The span of the station and track switching halls is about 23 m, the height of walls is 6.3m and the total height varies from 9m (track switching hall) to 12m (station hall). Each one of these halls is 200m long. The excavation was made in three stages. The width of each stage rage from 7 to 8mand the maximum length of each blasting round was 5 m.
The platform will be approximately 30m below the ground level, and there are escalator shafts located in both east and west end of the station.
This paper presents a study of the deformational behaviour of the rock mass during the excavation of the underground powerhouse cavern of the Salamonde II re-powering scheme, located in a mountainous region. The rock mass parameters considered in the three-dimensional numerical model and the initial state of stress resulted from previous studies, which included stress measurements and a global analysis of the results. The calculated and measured displacements along the complete excavation sequence were compared and, by using a back analysis process, the elastic moduli of the rock mass that lead to the best approximation were identified. Considerations regarding the results obtained, namely its dispersion, are presented.
For the design of large underground caverns, once the geological investigation helped defining their location, the most important tools used for their geometric definition and for the design of the support system are in situ and laboratory tests, which provide information on the rock mass strength and deformation and on the stress field, as well as the numerical models that make use of this information.
The Salamonde II hydroelectric scheme, promoted by Energies of Portugal (EDP), is currently under construction and corresponds to the re-powering of the 50 years old Salamonde scheme, built in a mountainous zone in the North of Portugal. It includes a concrete dam, a hydraulic conduit and a powerhouse cavern. Salamonde II has a new, 2 km long, underground hydraulic conduit and a much larger powerhouse cavern, 66m long, 26.5m wide and 56m high at the turbine hall, located 150mbelowthe ground surface (COBA 2009). The rock mass is a coarse grained granite, mostly of good quality, with several fault zones intersecting the hydraulic conduit. Figure 1 shows the location of the new and old powerhouses, the Cávado river where the dam is located and the Mau river, a tributary.A perspective of the powerhouse cavern and of the excavations in its vicinity is also represented in Figure 1.
For the design of the Salamonde II hydroelectric scheme a complete site characterisation programme was carried out. As part of it, LNEC carried out an in situ stress measurements programme and obtained the most probable state of stress in the rock mass (LNEC 2012).
This paper describes the analysis performed in order to obtain the relationship between the static and the dynamic modulus of one sedimentary rock (the San Julián’s stone) heated at different temperatures. The rocks have been subjected to heating processes at different temperatures (reaching up to 600°C in steps of 100°C), and two cooling methods for each temperature, to produce different levels of weathering on 24 cylindrical samples. The static and dynamic modulus has been measured for every specimen. Two analytic formulae are proposed for the relationship between the static and the dynamic modulus for this stone. The results have been compared with some relationships proposed by different researchers for various types of rocks. Generally low elastic modules imply highly fissured or damaged rocks. The mechanical properties, including static modulus, are highly dependent of the cracks size, orientation, and spatial distribution of these cracks. The ability to adequately detect the physical changes that affect rock mechanical capabilities by studying the propagation of ultrasonic waves has been widely discussed in many scientific papers. In this work, a high correlation between static and dynamic modules has been observed. It is concluded that in the studied range (i.e. Edyn values lower than 50 GPa) and for the soft rocks, static modulus can be obtained from dynamic tests, being the dynamic modulus (i.e. ultrasonic waves propagation velocities) a good indicator of the material degree of deterioration. The obtained relationships will allow the computation of the static modulus of elements of cultural heritage of Alicante city made of San Julián’s stone, from non-destructive field tests, for the analysis of the integrity level of historical constructions affected by high temperatures.
Young’s modulus, also called elastic modulus, is one of the most important mechanical characteristic parameters of the rocks in relation to its use as a construction material. The dynamically determined elastic modulus (Edyn) is generally higher than that statically determined, and both methods provide high divergent results for low elasticity modulus rocks (Ide, 1936). Several studies (Ide, 1936, Vanheerden, 1987, Al-Shayea, 2004, Kolesnikov, 2009) explain these differences by means of the nonlinear elastic response at different ranges of amplitude of the strains (ε:) involved in the distinct techniques. Other authors (Kolesnikov, 2009, Ciccotti & Mulargia, 2004) consider that the static test is a dynamic test at a very low frequency, and they highlight the nonlinear elastic response to different associated frequencies (f).Kolesnikov (2009) uses the Kjartansson constant Q-model (Kjartansson, 1979) to analyse the effects of intrinsic dispersion of pressure waves velocities in absorbing media (and it is well known that all rocks absorb energy of elastic waves to a greater or lesser extent).