A Pen206 Borehole Jack Suitability Assessment for Rock Mass Deformability Determination

Microcracks and joints, two types of flaws that appear in a rock mass, affect both the rock mass strength and deformability. A model that can simultaneously reflect the effect of these two types of flaws on the mechanical behavior of a rock mass with persistent joints is not yet available. This study focusses on a microcracked rock mass with persistent joints and establishes a mechanical model, accounting for the anisotropy in the rock mass strength and deformability induced by the existence of the joints. Firstly, the compound damage variable from the coupling macroscopic and mesoscopic flaws is deduced based on the Lemaitre strain equivalence hypothesis. Secondly, the corresponding damage constitutive model for a jointed rock mass is set up. Thirdly, the joint shear failure criterion is incorporated into the constitutive model to extend the model. Finally, the results of the calculation examples show that the existence of the joint will reduce the strength, enlarge the deformability, and lead to anisotropy of the rock mass. A series of calculation examples and comparisons validate that the proposed model is capable of presenting the joint-induced anisotropy in rock mass strength and deformability, determining its possible failure modes, and reasonably simulating its complete stress–strain relationship.

The mathematical model of structurally inhomogeneous rock mass, describing the property of rocks to accumulate and release elastic energy is used in the study. The finite element algorithm and programs for solving plane boundary-value problems are developed. The authors solve a problem on deformation of rock mass with a disjunction. It is shown that driving of horizontal tunnel nearby the disjunction can provoke the accumulated elastic energy release, which affects stress state of rock mass around the tunnel.

In this paper, an improved rock mass characterization method is presented. This method used a quantified geological strength index (GSI) and synthetic rock mass (SRM) model based on a discrete fracture network (DFN) and bonded particle model. A quantitative method is initially introduced to assist in the use of the conventional GSI chart, employing the rock block volume as the quantitative factor. The block volume for a given DFN model is calculated by a volumetric joint count, which is defined as the number of fractures intersecting a cube with unit length. The peak strength and deformation modulus of the rock mass are estimated based on the derived GSI, and then selected as references to constrain mechanical parameters of joints in the SRM model. A parametric study shows a positive correlation between the joint stiffness/friction coefficient and both the peak strength and deformation modulus. The joint friction coefficients of 0.5, 1.0, and 1.5 are suggested to correspond to the surface quality of ‘FAIR’, ‘GOOD’, and ‘VERY GOOD’ in the GSI chart. An advantage of the SRM over the GSI lies in the characterization of the post-peak strain softening behavior. The numerical results show that lower joint stiffness and friction coefficient result in a more ductile post-peak behavior. Additional SRM models with varied joint distributions show the complexities of rock mass characterization, where the rock mass strength, modulus, and post-peak strain softening rate may vary significantly with only one aspect of distributions of the joints, such as intensity or diameter. Triaxial testing is also conducted, and the SRM peak strengths are compared with the Hoek–Brown strength criterion based on the GSI system. The strengths of the regular triaxial models show general convergence with the Hoek–Brown criterion, but the strengths of the true triaxial models are less than those of the regular triaxial models due to deviatoric confining stresses. Meanwhile, ductile post-peak behavior is generally simulated in the triaxial testing. Cracking models are finally investigated and presented, behaving as additional information to explain the changes in the rock mass strength and deformability with different confining stresses. The method presented in this paper is meant to help geologists and engineers to quantify the GSI for jointed rock mass, and characterize the rock mass modulus and strength. Meanwhile, the synthetic rock mass method provides an alternative for rock mass characterization in addition to the traditional GSI method.

To improve the reliability of ground support systems and the understanding of deformation mechanisms in weak rock masses, an integrated in situ monitoring system combining multipoint displacement meters (MPDMs) and bolt axial load gauges was deployed in a typical underground metal mine roadway. A wireless monitoring network was adopted to enable long-term, multi-channel acquisition of surrounding rock mass deformation and bolt axial load evolution under complex underground conditions. Based on continuous field monitoring data, the spatial characteristics of rock mass deformation and the coupled response between deformation and bolt axial load were systematically analyzed. The results indicate that rock mass deformation exhibits pronounced depth-dependent stratification with strong directional anisotropy. In the horizontal direction, a significant deformation gradient is observed between depths of 2 m and 3 m, indicating that the loosened zone extends beyond 2 m. In contrast, deformation differences among depths of 2 m, 3 m, and 4 m in the inclined upward direction are relatively minor, suggesting a shallower deformation zone in the roof, predominantly confined within 2 m. A clear positive correlation is identified between rock mass displacement and bolt axial load, with synchronous increases observed in most monitoring sections, reflecting a strong rock mass–bolt interaction during time-dependent deformation. The proposed monitoring framework provides reliable field evidence for characterizing deformation zones and support response mechanisms in soft rock roadways, offering practical guidance for support optimization and stability assessment in underground metal mines.

The rock fracture data provided by Swedish Nuclear Fuel and Waste Management Company were used to develop a 3-D stochastic fracture network model for a 30 m cube of Aspo diorite located at a depth of 485 m at Aspo Hard Rock Laboratory, Sweden. This fracture network model was validated. A new procedure is developed to estimate rock block strength and deformability in three-dimensions allowing for the anisotropy and incorporating the inherently statistical fracture geometry for the selected cube. The mean rock mass strength was found to be 47% of the mean intact rock strength of 297 MPa at 485 m depth. The mean rock mass modulus was found to be 51% of the intact rock Young's modulus of 73 GPa. The rock mass Poisson's ratio was found to be 21% higher than the intact rock Poisson's ratio of 0.28. These percentages indicate the level of weakening of the rock mass due to the presence of fractures. The ratio of mean major principal rock mass strength/mean minor principal rock mass strength turned out to be 1.28. The ratio of mean major principal rock mass modulus/mean minor principal rock mass modulus turned out to be 1.21.

The Geological Strength Index (GSI) is a system of rock mass characterization that has been developed in engineering rock mechanics to meet the need for reliable input data related to rock mass properties required as input for numerical analysis or closed-form solutions for designing tunnels, slopes, or foundations in rocks. The geological character of the rock material, together with the visual assessment of the mass it forms, is used as a direct input for the selection of parameters for predicting rock mass strength and deformability. This approach enables a rock mass to be considered as a mechanical continuum without losing the influence that geology has on its mechanical properties. It also provides a field method for characterizing difficult-to-describe rock masses. Recommendations on the use of GSI are given and, in addition, cases where the GSI is not applicable are discussed.

<p>Rock mass damaging has become a topic of great interest in the engineering-geology research community during the last decades as it can significantly influence slopes stability. In this sense, the study of mechanics and dynamics of jointed rock masses represents a challenge because it will allow to better understand how external continuous and transient stressors can influence the short- to long-term stability controlling their pre-failure behavior. Consequently, the detection of permanent changes in physical and mechanical parameters, due to periodic or transient stressors, is an important target to mitigate the related geological risk as it can potentially lead rock masses to failure, especially when infrastructures and natural or cultural heritages are exposed elements. In this framework, the Acuto field laboratory (Central Italy) has been designed and implemented in 2016 within an abandoned quarry by employing an integrated geotechnical and geophysical monitoring system, with the aim of investigating how natural and anthropic conditioning factors could lead fractured rock masses to failure. The integrated monitoring system, which is installed on a potentially unstable 20-m<sup>3</sup> jointed rock block, is composed of several strain devices (i.e., strain gauges -SG- and jointmeters -JM-), one fully equipped weather station, one rock thermometer, eight high-sensitivity microseismic uniaxial accelerometers and optical and InfraRed Thermal cameras. The acquisition of long-term monitoring time-series, coupling multimethodological approaches, allowed to establish cause-to-effect relationships among different environmental stressors and induced strain effects, highlighting the continuous action of thermal stresses on rock mass deformations both at the daily and seasonal timescales. In fact, while the analysis of thermal and strain monitoring data allowed to characterize the cyclic contraction and relaxation response of major rock fractures and microcracks to temperature fluctuations, the microseismic monitoring array was able to detect during thermal transient (i.e., freezing conditions) the occurrence of microseismic emissions potentially related to the genesis or progressive growth of pre-existing cracks.</p><p>Starting from 2018, experimental activities at the Acuto field lab are supported by the “Dipartimento di Eccellenza” project of the Italian Ministry of Education Universities and Research funds attributed to the Department of Earth Sciences of the University of Rome “Sapienza”.  In this framework, the Acuto filed laboratory will undergo a structural upgrade that will be aimed at the investigation of two new sectors of the abandoned quarry. These new sectors will be instrumented with innovative thermal profiles probe, fiber Brag grating sensors and traditional SG and JM for detailed stress-strain monitoring, acoustic emission sensors and high-frequency and low-frequency geophones for ambient seismic noise monitoring and microseismic events detection as well as accelerometers for evaluating the rock mass response in the case of seismic shaking. The main goal of such an improvement will be both technical and methodological, and will shed light on the application of integrated geophysical and geotechnical monitoring approaches in investigating the multiscale rock mass damaging process as well as the detection of rock mass failure precursors by using non-conventional combinations and configurations of geotechnical and broad-band geophysical devices.</p>

The paper presents some experiences during the reinforced cut construction in the flysch deposits on the Adriatic motorway near city of Rijeka, Croatia. The major part of the motorway was constructed by cutting in flysch rock mass. Stabilities of the cuts were ensured by reinforcing of rock mass with rock bolts and appropriate supporting systems. The self boring rock bolts in combination of multilayered sprayed concrete or reinforced mat construction were used. The shallow boring drains were installed for dissipation of ground water collected at contact of permeable cover and impermeable flysch rock mass. Interactive rock mass cutting design, based on the observational methods, was introduced during the construction. The active design approach has allowed the designer, based on the rock mass conditions and monitoring results, changing the support system at some unfavourable locations. The measured values and the back analysis enabled establishing real rock mass strength parameters and deformability modulus.

Characterizing the surface deformation during the inter-survey period could assist in understanding rock mass progressive failure processes. Moreover, 3D reconstruction of rock mass surface is a crucial step in surface deformation detection. This study presents a method to reconstruct the rock mass surface at close range in a fast way using the improved structure from motion—multi view stereo (SfM) algorithm for surface deformation detection. To adapt the unique feature of rock mass surface, the AKAZE algorithm with the best performance in rock mass feature detection is introduced to improve SfM. The surface reconstructing procedure mainly consists of image acquisition, feature point detection, sparse reconstruction, and dense reconstruction. Hereafter, the proposed method was verified by three experiments. Experiment 1 showed that this method effectively reconstructed the rock mass model. Experiment 2 proved the advanced accuracy of the improved SfM compared with the traditional one in reconstructing the rock mass surface. Eventually, in Experiment 3, the surface deformation of rock mass was quantified through reconstructing images before and after the disturbance. All results have shown that the proposed method could provide reliable information in rock mass surface reconstruction and deformation detection.

A closed-form solution is presented in this paper for the prediction of stress field, strain field, support force and radius of plastic division in the rock mass subjected to axisymmetric loads around circular openings, which has an important role when dealing with the deformation of tunnel rock mass. The rock mass is assumed to be governed by three-parameter unified strength theory proposed by YU Maohong. Some important facts were considered, such as the in-situ 3D stress condition and the characteristic deformation of rock mass, the correct choice of strength theory which taking the intermediate principal stress into consideration. The results were found to be more practical and worthy of reference when the support design was performed.

Arch Dam and Skewback and Dam Abutment Rock Mass Deformation under Seepage Field and Stress Field Coupling

Rock mass deformability modulus is one of the most important parameters for underground structures and dam site design. The current methods for determining this parameter are in situ tests and empirical equations. In situ tests are the best and most accurate methods and include plate bearing, flat jack and dilatometer tests. However, these in situ tests are difficult, time-consuming, expensive and sometimes even impossible. Therefore, many empirical equations have been developed to determine the rock mass deformability indirectly based on various parameters. In this study, the correlation between the deformation modulus values, rock mass rating (RMR) and tunnelling quality index ( Q ) classification has been studied for the Ghezel Ozan dam site in NW Iran. Then, empirical equations are used to calculate the rock mass deformation modulus ( E m ) based on dilatometer in situ tests and rock mass classifications as mentioned above are defined. Finally, the correlation between the equations given in this study and the empirical equations presented by other researchers based on RMR and TQI ( Q ) are also investigated using statistical indicators. According to this analysis, the equation estimated based on RMR has a good compliance with the equations presented by earlier researchers. Supplementary material: Supplementary datasets and figures are available at https://doi.org/10.6084/m9.figshare.c.6139108

The groundwater drawdown caused by tunnel excavation may induce large-scale deformation, including uneven settlement deformation of rock mass and local deformation of faults, which poses a great risk to engineering safety, and requires the constraint of deformation stability. Based on the theory of deformation reinforcement and the effective stress principle, we study the nonuniform deformation and unbalanced force distribution of the rock mass and faults caused by the groundwater drawdown after the excavation and drainage of the deep tunnel. The results suggest that the groundwater level drawdown can increase the effective stress in a wide range, and significantly disturb the stress and deformation of the rock mass. It is noted that the fault crossing the tunnel is relatively dangerous under these disturbances, and deserves special attention. Moreover, the unbalanced force and the plastic zone can be transmitted to further distances along the fault, and reinforcement and support should be carried out in time. In addition, the greater the Biot coefficient can lead to the greater impact of groundwater drawdown on stress changes and deformation of the rock mass and faults.

A multimethodological method based on environmental, stress–strain, microseismic, and ambient seismic noise monitoring is here presented, with a view to identifying non-linearity of thermally-induced deformation of jointed rock masses at different dimensional scales. Rock masses experience non-negligible deformation cycles due to the continuous fluctuations of their surficial temperatures. However, the interpretation of such strain effects, in terms of the ratio between elastic and inelastic percentages, is still debated. In particular, the relation between microseismic emissions, considered as primary indicators of crack-growth related energy release, and resonant frequencies fluctuations of rock structures, witnesses of the thermally-induced effect at the macro- or structure-scale, have not been yet studied within a coupled framework. The combination of different approaches able to investigate the behavior of rock masses from micro- to macro-scale, then from fracture-scale to joint-isolated rock blocks up to rock structures, could provide new insights and perspectives on the effects related to shallow thermal stresses fluctuations. This paper presents the preliminary outcomes from two case studies, the Acuto experimental test-site (Italy) and the Wied Il-Mielaħ sea arch (Malta), where multiparametric monitoring surveys were conducted and are still ongoing, aiming at the assessment of the cause-to-effect relation between near-surface thermal stresses and induced strains. Data analysis was carried out following different approaches, with a particular emphasis on the Acuto test-site dataset recorded so far, allowing to establish a well-constrained correlation among temperature fluctuations and rock mass deformation both at the daily and seasonal scale.KeywordsMultiparametric monitoringThermal stressRock masses

Sea-wave impacts and near-surface thermal fluctuations are periodic stressors capable of exerting long-term mechanical effects that drive progressive rock mass failure and are regarded as a preparatory process for landslides. These processes pose significant hazards in coastal areas, which are expected to increase due to climate change. Despite scientific consensus identifying sea waves as a causative factor for slope instability, the mechanisms that govern subcritical crack growth and the transition to critical failures remain poorly understood. Failures often occur without any precursory signs and may happen even in the absence of major destabilizing forces.To investigate the fatigue processes experienced by rock masses under periodic loading and to understand the mechanisms driving coastal cliff failure with a goal in hazard assessment, a coastal sector was instrumented, monitoring both subaerial and underwater environments. This monitoring focuses on the effects of stressors and corresponding rock mass deformations. As part of the TRIQUETRA Horizon EU project, the Ventotene Field Laboratory (VFL) was established and inaugurated in May 2024. The VFL was located in a portion of the tuffaceous sea cliff of the Punta Eolo promontory, where a thick succession of ignimbrite deposits (ascribable to the Parata Grande geological unit) experienced large instabilities in the past tens of years.  The cliff is still evolving with rockfall and rock toppling mechanisms, that are threatening the archaeological excavation of the Roman Villa of Giulia (of the 1st century A.D.) and the Cemetery where Altiero Spinelli, the father of European thought, lies.The monitoring system includes a fully equipped weather station, conventional geotechnical sensors, and specialized devices for measuring sea-wave characteristics. These devices include a sea-wave and currents Doppler profiler, dynamic titanium water-tight pressure gauges to assess wave impacts at the cliff base, and instruments to measure elastic and plastic deformation of the fractured rock mass, such as crack meters, a biaxial tiltmeter, thermocouples, and load cells.Additionally, laboratory mechanical investigations were carried out to evaluate the strength and stiffness of the intact rock while examining the roles of water saturation and salt crystallization in rock weathering.Findings from the first year of monitoring revealed notable responses of the fractured rock mass to intense rainfall events, which caused sharp and partially reversible fracture openings. Cyclical deformation, including dilation and block tilting, was observed in response to daily and seasonal temperature fluctuations. Data collected from the monitoring system have been used to inform a stress-strain finite difference numerical model to analyse the static influence of basal notches on slope predisposition, as well as the preparatory effects on slope stability of combined thermal and marine actions. Ongoing numerical and laboratory geomechanical analyses aim to provide a more comprehensive understanding of the progressive rock failure processes driving the evolution of these complex systems.