Poor Rock Mass Research Articles

Application of a new seismic reflection imaging system TSP-SK in tunnel advanced geological prediction

When there are water rich-karst caves or large faults in front of the tunnel face, blind construction may easily cause safety accidents such as water inrush, mud outburst and collapse. Using advanced geological prediction methods to predict the spatial distribution of unfavorable geological bodies can greatly reduce construction risks. This paper introduces a new seismic reflection imaging system TSP-SK, and discusses the features, working mode and data processing methods of the prediction system, and verifies the reliability of TSP-SK through practical tunnel engineering prediction case. The result shows that TSP-SK can collect high signal-to-noise ratio seismic reflection data with little interference from clean first arrival when the water sealing effect of explosives in the borehole is good. Good reflection wave pickup effect can be obtained when the maximum gain of inverse Q filter and Q value are taken in the range of 15 to 30. The prediction results of TSP-SK are in good agreement with the actual excavation results, that is, the areas with low P-wave velocity indicate poor rock mass quality, and the area of low S-wave velocity is more likely to produce water from construction.

Engineering Gology Model of Bener Dam Diversion Tunnels in Geological Risk Disaster Mitigation

The Bener Dam Diversion Tunnel Plan is located in Bener District, Purworejo Regency. Engineering geology mapping data, drillimg data and laboratory data used as primary data. Surface and subsurface analysis show that each rock unit has different index and mechanical properties. Generally, the rock mass quality conditions in the dam belonged to good Rock (80%) in the Rock Mass Rating (RMR) system (Bieniawski, 1989). The other rock mass quality type also found among them fair rock (5%), poor rock (5%), and very poor rock (10%). Poor rock mass quality conditions were controlled by geological structures, especially faults that partially cut through the tunnel geometry. The very poor quality of rock mass was in the volcanic lens (loose sand material) did not cut through the tunnel path. The difference stand-up time of the rock on the tunnel requires proper mitigation (Nguyen Nguyen, 2015). The stand-up time belonged to the dangerous condition was in the fault zone with poor rock mass quality, while the lens with very bad rock mass quality did not affect the stability of the excavation of the tunnel.

The Influence of Fresh and Weathered Rock Foliation on the Stability of the Monte Seco Tunnel

The Monte Seco tunnel (MST) is one of several old unlined tunnels constructed for Brazilian railways in the 1950s, and the MST still receives intense train traffic (primarily for iron ore transportation). The MST was constructed in a fractured gneissic rock mass, which has potential instability problems associated with gravity-driven block falls (at low stress confinements) defined by three fracture sets and the metamorphic foliation. Previous geological and geotechnical surveys at the MST site have included basic geomechanical classifications from outcrops and drill core specimens, detailed terrestrial laser scanning (TLS) mapping of foliation and fracture planes, geoelectrical imaging and laboratory tests. These surveys discovered two sections of the tunnel (referred to herein as MS1 and MS2) with major potential for instability problems (i.e., poor rock mass qualities). In this paper, the 3D discrete element method (DEM) was used to assess the potential stability problems in these sections with particular interest on the role of the rock foliation strength on the current stability state of these sections. Three instability indicators were developed and monitored during and after the DEM simulations: the total failed block volume (FBV), the average displacement of the tunnel face (AD), and the percentage of slipping contacts near the tunnel face (SC). The weathering effects were considered in two different scenarios: the occurrence of isolated weathered foliation planes (WFPs) and the general weathering advance through potentially weathered regions. In the latter scenario, the percentage of weathered contacts (WC) increased progressively. The overall results indicated that fresh foliation planes ensured the current tunnel stability due to the moderate mica content (Ma) in such geological structures. Thus, underestimating the strength of foliation planes causes unrealistic instability conditions in the tunnel. However, the weathering advance on fractures and foliation planes can trigger important instability problems over time.

Characteristics of failure area and failure mechanism of a landslide in Yingjiang County, Yunnan, China

Due to excavations and continuous rainfall, two landslides occurred on February 9, 2018, and July 28, 2019, respectively, in the quarry of the Changdifang Reservoir engineering area, in Yingjiang County, Yunnan Province, China. The volume of the second landslide was approximately 2.4 × 105 m3, threatening the safety of construction workers and the hydropower station project construction severely. In order to explore the causes of these two landslides, the geological and hydrological conditions and the current excavation status of the landslide area were investigated, the relationship between rainfall and the displacement of the landslide body was compared and analyzed, and the landslide process and failure characteristics were described in detail. The failure mechanism of these two landslides were distinguished: (1) the poor rock mass properties, due to the strong weathering and tectonic joint of the rock mass; (2) the steep slope (of 63° in largest dip angle) induced by the slope excavation, as well as (3) the continuous heavy rainfall. The unfavorable geological condition was a determinant factor, and the combined effects of excavation and continuous rainfall were triggering factors that induced the landslide. The research results show that the displacement of the landslide body has significant seasonal characteristics. In the rainy season, there is a significant acceleration of displacement, and the slow displacement will remain for a long time after the rainy season. Consequently, it is very necessary to take reasonable slope treatment measures and continue to monitor the slope.

Rock Mass Characterization for Assessment of Safe Cut Slope and Rock Bearing Capacity at Gondang Dam Site, Karanganyar, Indonesia

This paper presents results of surface rock mass characterization for assessment of safe cut slopes and allowable bearing capacity of foundation rocks at the construction area of Gondang Dam. The rock mass characterization involved determination of intact rock engineering properties and rock mass quality based on the Geological Strength Index. The rock mass characterization results showed that the research area consisted of moderately to highly weathered and very weak to weak andesite breccia and andesite tuff breccia. The andesite breccia had very poor to fair rock mass quality, while the andesite tuff breccia had poor to fair rock mass quality. The research area was divided into three zones of safe cut slope and allowable bearing capacity. Landslides occurred at natural slopes having poor to very poor rock mass quality and inclinations greater than the determined safe cut slopes.The foundation rock of the embankment dam had fair rock massquality and 135–280 T/m2 allowable bearing capacity

Rock Mass Analysis of -915m in a Gold Mine in Northwest Jiaodong

According to the requirements of ore mining, this paper comprehensively evaluates the quality of rock mass below 915m in a gold mine in northwest Jiaodong by rock mass engineering geological survey and comprehensive quality evaluation. The results show that the rock mass near the ore body or the F1 fault area is gradually deteriorated, and the rock mass of the ore body or the F1 fault is still poor. The RMR index of the rock mass is in the range of 35∼50, mostly the grade III poor rock mass.

Analysis and Evaluation of Tunnel Contour Quality Index

Abstract Overbreak, which is inevitably caused by geological and operational conditions in a drill & blast tunnel and is one of elements consisting of tunnel contour quality, has been a major cause to increase construction time and costs. Thus, many attempts have been made to reduce it worldwide. Against this backdrop, this paper suggests Tunnel Contour Quality Index (TCI) established based upon laser profiling technique for the effective management of tunnel contour quality, and analyzes TCI as well as the factors on TCI through field investigations using laser profilers in Norway and Korea. TCI is defined as a fraction form consisting of three elements (overbreak depth, contour roughness, longitudinal overbreak variation) that are calculated from surveyed tunnel profiles, three weights for the elements and one constant for range adjustment. The result shows a large difference in TCI, i.e., TCI for the Korean tunnels is average 63.4 by overbreak depth of 21.4 cm, contour roughness of 1.066 (meaning a 6.6% increase than the planned contour area by blasting) and longitudinal overbreak variation of 5.7 cm, while TCI for the Norwegian tunnels is average 45.9 by 64.2 cm, 1.120 and 11.1 cm respectively. The main causes for the large difference are the different philosophy in tunnel design between the two countries, as well as the hard rock having distinct joints of Norway that makes it easier for blasting pressure to break off rock masses along existing joints than to develop new cracks in the hard rock. In addition, it is found that the major element affecting TCI is overbreak depth because large overbreak depth increases the possibility of high contour roughness. The results show that most overbreak primarily results from large look-out and poor rock masses, and major causes resulting in high contour roughness are heterogeneous geological conditions in the tunnel face and big deviation in drilling position of contour holes. Finally, it is thought that TCI established based upon laser profiling technique can be used for the more effective management of tunnel contour quality.

Gateway stability analysis by global-local modeling approach

Longwall mining is the most widely used mining method in underground coal mining. The stability of gateways is crucial for longwall mining as they provide many essential services such as transportation of excavated coal, ventilation of longwall face. High in situ stresses, mining induced stresses due to advancing longwall face, and poor rock mass quality leads to large deformation jeopardizing the stability of the gateway in Guobei coal mine. In this paper, the global-local modeling approach is used for analysing the stability of the gateway in the mine. In this approach, a mine scale 3D global model was constructed to extract the stresses acting on the gateway. The extracted stresses from global model were implemented into 2D local model of the gateway for detailed analysis. The used approach was verified by the field measurements taken from the modeled gateway. In an attempt to deal with large mine induced displacements, a support system composed of rock bolt, cable bolt and shotcrete was proposed. The verified model was used to evaluate the performance of proposed support system. It was concluded that the proposed support system can significantly reduce the displacements. The verified model facilitates the performance evolution of other alternative support systems considered by the mine management. The presented approach can be applicable for other deep underground coal mines, experiencing large gateway convergence.

Application of the Cohesion Softening–Friction Softening and the Cohesion Softening–Friction Hardening Models of Rock Mass Behavior to Estimate the Specific Energy of TBM, Case Study: Amir–Kabir Water Conveyance Tunnel in Iran

The specific energy (SE) of an excavation is an important factor to consider in economic and technological investigations of mechanical excavation projects using a tunnel-boring machine (TBM). SE is defined as the energy consumed during excavation of per unit volume of rock mass, and it can be determined in real time from the data recording the performance of a TBM. Several experimental, empirical, and analytical methods have been developed to predict SE based on rock mass and machine parameters. In this study, a new empirical method is proposed to predict SE based on the strain energy ratio of rock mass (Ψ). This is defined as the ratio of the residual post peak strain energy to the stored pre peak strain energy of the rock mass. It depends on three important parameters, namely rock mass properties, intact rock parameters, and rock mass behavior models. In this study, to estimate the strain energy ratio of rock mass, two post peak rock mass behavior models—cohesion softening–friction softening (CSFS) and cohesion softening–friction hardening (CSFH)—were used. Based on actual data from the Amir–Kabir water conveyance tunnel project, the relationships between the SE of TBM and the strain energy ratio were investigated. Due to different rock mass qualities in the tunnel route, classification of rock mass according to Hoek and Brown’s proposal was carried out, and the correlation between the mentioned parameters in each class was studied. The results showed a direct relationship between the parameters, and the best relationships in poor and moderate rock mass (geological strength index [GSI] 70) because of its ability to modify the brittle behavior of brittle rock mass.

Revisiting crown stability of tunnels deeply buried in non-uniform rock surrounds

A kinematic analysis procedure is presented to investigate the crown stability of deep-buried tunnels in non-uniform rock strata. The non-uniformity of rock properties is considered using the discretized approach with finite thickness layers. In the realm of upper bound theory, kinematic analysis is briefly presented for a single layer so as to provide basis for multi-layer analysis. A 2D collapse mechanism is postulated to depict the potential falling block with an arbitrary tunnel cross-section, although rectangular tunnels are considered in the 3D kinematic analysis for ease of calculation. An impending collapse is likely to occur, provided that the width of the excavation is no less than the collapse width. In order to reflect the nonlinear characteristics of rock materials at failure, the Hoek-Brown criterion is employed for tightly interlocked and very poor rock masses, with its form expressed by normal and shear stresses to facilitate calculations of internal energy dissipation rate. Based on the variational principle and power-based balance equation, the effective collapse mechanism is derived in closed form. Comparison is carried out to verify the robustness of the proposed approach. Sensitivity analysis is performed to depict the influence of each non-uniform rock strength parameter on crown stability. As expected, such a discretized approach incorporated into kinematic analysis enables one to account for non-uniformity of rock surrounds, which would fill the knowledge and design gap of similar tunnelling issues much needed at present state of practice.

Triaxial strength behaviour of rockmass satisfying Modified Mohr-Coulomb and Generalized Hoek-Brown criteria

The triaxial strength of twenty rockmass types was predicted using two non-linear triaxial strength criteria for rockmass i.e. Modified Mohr-Coulomb (MMC) criterion and Generalized Hoek-Brown (GHB) criterion. Four different rockmass classification systems were used for the calculation of MMC criterion parameters while only GSI classification system has been used for calculation of GHB parameters. The representative value of the uniaxial compressive strength and elastic modulus of rockmass have been estimated using probabilistic approach. A hypothetical case of an unsupported tunnel has been analyzed considering both MMC and GHB criteria. The analysis was done using the convergence-confinement method with two different approaches. The first approach predicts the tunnel response using GHB criterion directly. The second approach predicts the tunnel response using equivalent Mohr-Coulomb parameters obtained by linearization of triaxial data points obtained from MMC and GHB criteria. The tunnel response has been estimated in terms of radius of plastic zone, tunnel convergence and tunnel convergence strain. For very poor rockmasses the tunnel response predicted by MMC criterion is less than that predicted by GHB criterion. For poor and fair rockmass, the tunnel response estimated considering both the criteria are comparable except for few cases. Squeezing condition in rockmass has been also evaluated.

Qualitative stability assessment of cut slopes along the National Highway-05 around Jhakri area, Himachal Pradesh, India

Several deformation phases in tectonically active Himalayas have rendered the rock masses very complex in terms of structure, lithology and degree of metamorphism. Again, anthropogenic activities such as roads, tunnels and other civil engineering constructions have led to a state of disequilibrium which in many cases, results in failure of rock masses. National Highway-05 around Jhakri area in India is a major connecting route to the China border in the hilly terrains of the state Himachal Pradesh. It cuts through the Himalayan rocks and has a hazardous history of landslides destroying human lives and interrupting communication very frequently. As a contribution towards the mitigation process, a study has been carried out along the highway to analyse kinematic stability and qualitative estimation of rock mass condition through rock mass classification systems. The kinematic analysis shows that the rock slopes are prone to planar and wedge failure. Rock mass rating for most of the locations lies between 7 and 34, representing a poor rock mass quality (Class IV), whereas slope mass rating is more disperse and ranges from 11 to 52 for most of the slopes (Class III, IV and V).

Classification of geological and engineering properties in weak rock: a case study of a tunnel in a fault zone in southeastern Korea

Given that the rock mass classifications of weak rocks observed in tunnels are evaluated only as ‘poor rock mass’ or grades IV–V using existing rock mass classification methods, a new scheme is needed that would better distinguish the various geological properties of weak rocks. In this study, geological and engineering properties of weak rocks were classified based on the analysis results of a total of 55 faces in a tunnel that passes through a fault zone in southeastern South Korea. Geological observations, point load tests, and Schmidt hammer tests were conducted to analyze the properties of the weak rocks. Through this analysis, the weak rocks were classified into two weathering types: ‘disintegrated rocks’ and ‘decomposed rocks’. This division is based on the tectonic mechanisms and parameters necessary to determine the engineering geological properties of individual rock-weathering types according to their geological properties. The ‘disintegrated rocks’ have been physically weathered by brittle deformation and are classified into three subtypes using the rock quality designation (RQD) and joint set number (Jn), which can characterize the discontinuity properties of the rocks. Point load tests were conducted on these individual subtypes to calculate the point load index (Is(50)). The resultant values are clearly related to the assigned three subtypes. The ‘decomposed rocks’ have been chemically weathered by ductile deformation and are classified into five subtypes based on the results of Schmidt hammer tests conducted on faces and Is(50) values calculated using the average of the Schmidt hammer values. The geological properties of the individual subtypes of the ‘disintegrated rocks’ and ‘decomposed rocks’ are clearly related to their engineering properties. Based on these results, rock mass classification diagrams are proposed that can be simply and easily applied to weak rocks.

Event tree analysis for rockfall risk assessment along a strategic mountainous transportation route

A quantitative risk analysis has been performed in a tourist area of Sicily (Italy) with the aim of assessing the rockfall risk along an important transportation corridor, crossing a geologically complex area and poor rock masses. The procedure followed herein is based on an event tree analysis, which was properly customized to take into account the peculiarity of the area and of the road path. Rock mass surveys, trajectory simulations and probabilistic models are proposed with the aim of calculating the probability related to possible scenarios in case of rockfalls. Achieved outcomes demonstrate that such procedure, resulting from a multifaceted study, is a reliable tool, which can be taken as reference to calibrate further risk models in comparable contexts of the world, where rockfall threaten communication routes. This would represent a helpful instrument to the scientific community and to local authorities dealing with one of the most troublesome natural phenomena affecting the public safety.

Weathering characterization for landslides modeling in granitoid rock masses of the Capo Vaticano promontory (Calabria, Italy)

This work focuses on developing multidisciplinary researches concerning weathering profiles related to landscape evolution of the Capo Vaticano promontory on the Calabria Tyrrhenian side (southern Italy). In this area, the tectonic uplift, occurred at least since Pleistocene, together with the Mediterranean climatic conditions, is the main cause of deep weathering and denudation processes. The latter occurred on the outcropping rocks of the crystalline-metamorphic basement, made up of weathered granitoids, in turn belonging to the Monte Poro granitoid complex (intermediate to felsic plutonic rocks covered by Cenozoic sedimentary successions). Field observations coupled to borehole explorations, geophysical surveys, and minero-petrographical analyses allowed the characterization of the granitoid outcrops typical of the studied area in terms of kind and degree of slope instability. This characterization was based on suitable correlations verified between several factors as weathering degree, elastic properties of rocks, and discontinuity features. Weathering profiles are mainly composed by rock masses varying from completely weathered rock with corestones of highly weathered rock (classes IV–V) to slightly weathered rocks (class II). The weathered rocks are involved in several landslide typologies such as debris flow (frequency 48.5%), translational slide (frequency 33.3%), and minor rock fall and rotational slide (frequency 9%). The achieved data allowed the establishment of a general correlation between weathering degree and type of slope instability. Debris flow-type instabilities are predominant on the steeper slopes, involving very poor rock masses ascribed to the shallowest portions of the weathering class IV. Translational slides are less widespread than the previous ones and often involve a mixture of soil and highly weathered rocks. Rotational slides are more frequently close to the top of the slopes, where the thicknesses of more weathered rocks increase, and involve mainly rock masses belonging to the weathering classes IV and V. Rock falls mostly occur on the vertical escarpments of the road cuts and are controlled by the characteristics of the main discontinuities. The assessment of rock mass rating and slope mass rating, based on the application of the discontinuity data, allowed respectively an evaluation of the quality of rock masses and of the susceptibility of rock slopes to failure. The comparison between the last one and the real stability conditions along the cut slopes shows a good correspondence. Finally, the geological strength index system was also applied for the estimation of rock mass properties. The achieved results give a worthy support for a better understanding of the relationship between the distribution of landslides and the geological features related to different weathering degrees. Therefore, they can provide a reliable tool to evaluate the potential stability conditions of the rock slopes in the studied area and a general reference framework for the study of weathering processes in other regions with similar geological features.

In Situ Observation of Failure Mechanisms Controlled by Rock Masses with Weak Interlayer Zones in Large Underground Cavern Excavations Under High Geostress

A weak interlayer zone (WIZ) is a poor rock mass system with loose structure, weak mechanical properties, variable thickness, random distribution, strong extension, and high risk due to the shear motion of rock masses under the action of tectonism, bringing many stability problems and geological hazards, especially representing a potential threat to the overall stability of rock masses with WIZs in large underground cavern excavations. Focusing on the deformation and failure problems encountered in the process of excavation unloading, this research proposes comprehensive in situ observation schemes for rock masses with WIZs in large underground cavern on the basis of the collection of geological, construction, monitoring, and testing data. The schemes have been fully applied in two valuable project cases of an underground cavern group under construction in the southwest of China, including the plastic squeezing-out tensile failure and the structural stress-induced collapse of rock masses with WIZs. In this way, the development of rock mass failure, affected by the step-by-step excavations along the cavern’s axis and the subsequent excavation downward, could be observed thoroughly. Furthermore, this paper reveals the preliminary analyses of failure mechanism of rock masses with WIZs from several aspects, including rock mass structure, strength, high stress, ground water effects, and microfracture mechanisms. Finally, the failure particularities of rock masses with WIZs and rethink on prevention and control of failures are discussed. The research results could provide important guiding reference value for stability analysis, as well as for rethinking the excavation and support optimization of rock masses with WIZs in similar large underground cavern under high geostress.

Theoretical solutions for displacement and stress of a circular opening reinforced by grouted rock bolt

This paper presented solutions of displacement and stress for a circular opening which is reinforced with grouted rock bolt. It satisfies the Mohr-Coulomb (M-C) or generalized Hoek-Brown (H-B) failure criterion, and exhibits elastic-brittle-plastic or strain-softening behavior. The numerical stepwise produce for strain-softening rock mass reinforced with grouted rock bolt was developed with non-associative flow rules and two segments piecewise linear functions related to a principle strain-dependent plastic parameter, to model the transition from peak to residual strength. Three models of the interaction mechanism between grouted rock bolt and surrounding rock proposed by Fahimifar and Soroush (2005) were adopted. Based on the axial symmetrical plane strain assumption, the theoretical solution of the displacement and stress were proposed for a circular tunnel excavated in elastic-brittle-plastic and strain-softening rock mass compatible with M-C or generalized H-B failure criterion, which is reinforced with grouted rock bolt. It showed that Fahimifar and Soroush's (2005) solution is a special case of the proposed solution for n = 0.5. Further, the proposed method is validated through example comparison calculated by MATLAB programming. Meanwhile, some particular examples for M-C or generalized H-B failure criterion have been conducted, and parametric studies were carried out to highlight the influence of different parameters (e.g., the very good, average and very poor rock mass). The results showed that, stress field in plastic region of surrounding rock with considering the supporting effectiveness of the grouted rock bolt is more than that without considering the effectiveness of the grouted rock bolt, and the convergence and plastic radius are reduced.

Stress characterization and support measures estimation around a coalmine tunnel passing through jointed rock masses: constraints from BEM simulation

In the present research, the stress characterization around an unsupported coalmine tunnel passing through jointed rock masses was ana-lyzed and effective support system was calculated by BEM numerical simulations. The distribution and magnitudes of major and minor principal stress contours, mean stress, differential stress, total displacement, maximum shear strain, maximum shear stress contours around the tunnel are simulated by using the examine2D software. It is reasonable to mention that examine2D is a plane strain boundary element program for calculation of stresses and displacements around underground and surface excavation in rock. Modeling results reveal that the major principal stress (σ1) was about 13 MPa at the immediate roof of the tunnel that ultimately increased to 20 MPa toward the left side and right side. Mean stress contour value was 12 MPa at the immediate roof and 15 MPa toward the both rib sides. The distribution contour value of differential stress at the roof and rib sides were 16 MPa and 23 MPa, respectively. The contour values of the strength factor around the tunnel ranged from 0.51 to 1.02, which specify that the loosening zone would be extended up to 1.53 m towards the roof and 1.25 m at the sidewalls. The thickness (1.53 m) of loosening zone can be classified as soft or poor rock mass. In the immediate roof, floor and the both rib sides of the tunnel, the Spalling Criterion values ranged from 2.7 to 8.0 MPa, which indicate no potential for rock-burst around the tunnel. However, flexible support would be required to accommodate the dilatancy deformation during development period. Finally, the stiff support would be required to provide a strong supporting reaction and to maintain the long-term stability of the tunnel.

Empirical rock mechanical site-descriptive modeling (RMSDM) for the Korea Atomic Energy Research Institute Underground Research Tunnel (KURT)

As part of master plan, when preparing to construct the second stage of the Korea Atomic Energy Research Institute Underground Research Tunnel (KURT-2), site investigations were carried out to characterize the rock mass and its related geophysical nature. The rock mass at the study site was divided into rock units depending on a rock type and fracture developments in rock. The rock mass quality for each rock unit was evaluated, using rock mass classification systems, such as Q, rock mass rating (RMR), and rock mass index (RMi). The mechanical properties of rock units were empirically estimated with associated rock mass classification systems, including deformation modulus, uniaxial compressive strength, and cohesion and friction angles. Ultimately, a rock mechanical site-descriptive model (RMSDM) covering the block size of 85 m × 120 m × 80 m was developed by combining the analysis of rock units. Rock block consists of granite, dike, and fault, being divided into six rock units, such as G1, G2, G3, D1, D3, and F3. G1 and D1 rock units were classified as good rock masses, and the rock mass quality of G2 is fair. G3 and D3 rock units were classified as poor rock masses, and the F3 rock unit was very poor. The mechanical properties of rock unit G1 and D1 were almost considered similar and best, and those of G2 were considered middle class. G3 and D3 rock units had poor mechanical properties and those of F3 were the worst. G1 occupies most of block volume, but other rock units were distributed as small portions. Two-dimensional distributions of rock mass from modeling and face mapping at the elevation of the tunnel excavation exhibited that predicted distributions well fit to those observed. Therefore, RMSDM will be of help with the geoscientific understanding of site investigation and designing of a site and construction of a site, similar to KURT-2, and ultimately, it will become a scientific base to future-related research and development projects of the Korea Atomic Energy Research Institute (KAERI).

Main challenges for deep subsea tunnels based on norwegian experience

For hard rock subsea tunnels the most challenging rock mass conditions are in most cases represented by major faults/weakness zones. Poor stability weakness zones with large water inflow can be particularly problematic. At the pre-construction investigation stage, geological and engineering geological mapping, refraction seismic investigation and core drilling are the most important methods for identifying potentially adverse rock mass conditions. During excavation, continuous engineering geological mapping and probe drilling ahead of the face are carried out, and for the most recent Norwegian subsea tunnel projects, MWD (Measurement While Drilling) has also been used. During excavation, grouting ahead of the tunnel face is carried out whenever required according to the results from probe drilling. Sealing of water inflow by pre-grouting is particularly important before tunnelling into a section of poor rock mass quality. When excavating through weakness zones, a special methodology is normally applied, including spiling bolts, short blast round lengths and installation of reinforced sprayed concrete arches close to the face. The basic aspects of investigation, support and tunnelling for major weakness zones are discussed in this paper and illustrated by cases representing two very challenging projects which were recently completed (Atlantic Ocean tunnel and T-connection), one which is under construction (Ryfast) and one which is planned to be built in the near future (Rogfast).