bluuebunny/crossref_metadata_2025 huggingface dataset:
Variation of Apparent Cohesion and Friction Angle Under Polyaxial Stress Conditions in Concrete
Authors: Mutaz E , Serati M , Rimmelin R , Williams D J
Published: September 2021

Amongst many mechanical properties, cohesion (c) and angle of internal friction (φp) are probably the most widely used rock and rock-like material strength design parameters. However, unlike Mohr-Coulomb (MC) failure criterion that assumes cohesion and friction angle are intrinsic material properties and are not affected by the applied stress level, the Hoek-Brown (HB) criterion predicts a continuous change of apparent cohesion and friction angle if the induced normal stress on the fracture plane changes. That is, at low values of normal stresses, the instantaneous angle of friction will be relatively large, whereas cohesion will be a small value. As the applied normal stress value on the fracture plane increases (moving ‘up’ the non-linear Hoek-Brown strength envelope), the angle of friction reduces, and the cohesion increases. This is an important result from the HB failure model and allows a more realistic estimate of shear strength to be made at low values of normal stress, preventing potential over-design problems. Nevertheless, the HB model neglects the effect of the intermediate principal stress on material properties. Limited studies on the variation of apparent cohesion and friction angle under polyaxial stresses in concrete are available in the literature. Therefore, this paper aims to investigate the effect of true triaxial stresses on concrete cohesion and friction angle using a polyaxial strength criterion developed by Mogi. The results of concrete show that the intermediate principal stress has a pronounced effect on cohesion degradation and the mobilization of internal friction angle as the ratio of the intermediate to the minor principal stress changes. The results are expected to provide a framework for a more realistic design of underground concrete structures at depth.
The effect of impact angle and bond strength on fragmentation in laminated materials
Authors: Butcher C , Fityus S , Giacomini A , Thoeni K , Guccione D E , Buzzi O
Published: September 2021

An experimental study was undertaken to explore the effects of weak laminations on the fragmentation of impacting rocks. In this work, the fragmentation behavior of manufactured disc specimens of laminated travertine marble is described according to variations in the lamination bond strength, the orientation of laminations at impact, and the impact energy. An experimental apparatus and procedure were developed so that the discs, 96mm in diameter and 25mm thick, could be impacted neatly on their circumferential edge, at a selected lamination impact angle, and subject to filming with a high speed camera. The results of the study generally indicated that weaker bonds cause fragmentation from lower drop heights, as expected. However, the results also indicate that the critical height for breakage is relatively lower and insensitive to the impact angle when the laminations are between 45 and 90 degrees relative to the impacting surface, but the critical height for breakage increases significantly (by a factor of between 2 and 4) as the orientation of the laminations becomes parallel to the impact surface. The presence of laminations and their orientation to the impact surface, directly influenced the failure mechanism of specimens upon impact. At low lamination impact angles, failure was by parting along laminations in direct tension (reflected compression). At intermediate impact angles (approximately 45 degrees), the fragmentation of the specimens was more explosive and fractures were a combination of steps across stronger sheets, and splits along weaker bonded laminations. For high impact angles, fragmentation occurred in indirect tension, along weak laminations.
Mechanical investigation of rock-proppant interaction with focus on geothermal applications
Authors: Witte L , Pollmann N , Backers T
Published: September 2021

Since decades proppants have been used to enhance hydraulic pathways for the exploitation of hydrocarbon reservoirs. These technologies may also be adapted for geothermal applications, but geothermal reservoirs rocks strongly differ from those typically targeted by hydrocarbon industries. Therefore, the mechanical interaction between conventional proppants and geothermal reservoir rocks and their effect on hydraulic properties need to be investigated in more detail to maximize the success of such method adaption. In principle, two mechanical requirements must be fulfilled: (1) proppants must withstand the stresses that are present in a reservoir, and (2) proppants must not penetrate the rock formation of the reservoir, because this would reduce the fracture conductivity. Fine mobilized rock- or proppant particles can also clog the fracture. For the mechanical suitability of proppants, uniaxial compressive strength tests and long-term creep tests were carried out in the laboratory. The test setup consists of a proppant monolayer placed between two cylindrical rock samples that were axially loaded. We determined Young’s moduli and creep rates for sample stacks with various proppants under different stress conditions. The laboratory tests are part of the ZoKrateS Project, which aims at showing the feasibility of enhancing fractured carbonate rock mass by proppant placement for geothermal applications.
Experimental analysis of the pseudo-compact tension (pCT) testing configuration using two alternative sample geometries
Authors: Herbón-Penabad M , Muñoz-Ibáñez A , Delgado-Martín J
Published: September 2021

The pseudo-compact tension (pCT) method recently proposed by Muñoz-Ibáñez et al. (2020) is a satisfactory approach to measure mode I fracture toughness (KIC) in rocks and other materials using disc-shaped samples loaded under pure tensile conditions. In contrast to other methods, such as the semi-circular bend (SCB) suggested by the ISRM (2014), the pCT test provides with good control after peak load, making it possible to further characterize the processes involved in fracture propagation. In this work we assess the influence of the testing configuration at the onset of unstable crack propagation. In order to extend the pCT concept to complementary geometries with potential interest we studied an alternative to the SCB specimen, which we call pseudo-SCB (pSCB). To compute KIC
in this configuration we have derived the corresponding dimensionless stress intensity factor function (Y’) based on the finite element method. The results show that the pSCB test provides with consistent values of KIC
and it also allows to control the propagation of the crack beyond peak load, which reinforces the idea that the loading conditions may be a more determinant factor than the sample geometry in controlling post-peak behaviour. In addition, an expression of Y’ is presented for cubic samples tested using the pCT approach. This configuration may be useful for testing other materials amenable of moulding such as mortar, concrete, ceramics, etc.
The influence of extremely abrasive rock on the drilling velocity and tool wear in bored piles
Authors: Sattler T , Loibl A , Festl J , Zweininger T , Eckl W , Thuro K
Published: September 2021

The significance of the excavatability for the successful completion of constructional projects has been recognized in recent years, but little attention has been paid specifically to rotary pile boring. Given the quantity of geological parameters alone which need to be taken into account, it is apparent that the estimation of the drilling rate is difficult. Based on a case study in extremely abrasive tertiary silcretes (quartzite), the engineering geological influences on the drillability are examined. A minutely detailed documentation of the drilling process was produced, and laboratory testing was conducted to gain a better understanding of the correlation between significant rock properties and wear on drilling tools. The examined silcretes (quartzite) show CERCHAR abrasivity indices (CAI) up to a value of more than 6. It comes as no surprise that this highly abrasive rock causes extensive wear on piling equipment and an extremely high consumption of wear parts. In combination with the drilling velocity of both the drill tools and the casing shoe remaining well below 10 cm/min, it is obvious that the drilling performance is significantly impacted in more than one way. A correlation between the lifespan of the wear parts and the net drilling velocity is developed, allowing for a more reliable performance prediction for bored piles.
Extraction of discontinuity sets of rocky slopes using iPhone-12 derived 3DPC and comparison to TLS and SfM datasets
Authors: Riquelme A , Tomás R , Cano M , Pastor J L , Jordá-Bordehore L
Published: September 2021

Characterisation of a rock mass requires data from the intact rock along with the discontinuities. Assuming that the discontinuities are planar, its characterisation requires its number and orientation. This leads to the analysis of the normal spacing, the persistence and the roughness, among others. The geometrical analysis of the surface enables the calculation of the parameters to characterise the discontinuities, and the use of digital datasets enhance them. Remote sensing techniques, such as the Terrestrial Laser Scanning (TLS) instruments of Structure from Motion (SfM) technique, provide 3D point clouds that enable the geometrical analysis. The scientific community has been testing both techniques since the 2000s, and companies are introducing their use in their workflows. However, the cost of the TLS instrument could still be a barrier to its use to most scholars. Because of this, the community shows a growing interest in Remotely Piloted Aircraft Systems (RPAS) equipped with digital cameras or in smartphones equipped with high-quality cameras to capture digital datasets of rocky slopes. The SfM workflow processes the captured images, reconstructing the rocky slope through a 3D point cloud and textured meshes. Although previous studies show that the SfM-derived point clouds present less quality than TLS-derived datasets in terms of accuracy, the use of SfM is still of interest because of its cost. In 2020 Apple launched the Iphone-12 device, which is equipped with a LiDAR sensor that is not used to capture the surface coordinates but to enhance the photo’s quality. Since then, the community has developed several applications to reconstruct 3D surfaces using this device. This leads to consider this device as an intermediate option between the TLS and SfM to characterise rocky slopes and their discontinuities. In this communication we explore the digitalisation of a rocky slope via TLS instruments, SfM technique and using the Iphone-12 device. It comprises a 26 meter high mechanically excavated rocky slope in Cretaceous marlstones and limestones. To capture the surface, we used three configurations, and we found that to scan ground surface the distance device-surface had to be less than 3 meters. The discontinuities are characterised using the three sources of information using the DSE software. The results show a promising match compared to the TLS or SfM. This evidences that these devices will soon be widely employed for evaluating rocky slopes.
A fractal analysis method to characterise rock joint morphology
Authors: Guida G. , Casini F. , Viggiani G.M.B.
Published: September 2021

Rock joints morphological characteristics strongly influence the shear strength dilatancy response of the discontinuity. Morphological features of rock joints are commonly identified with a roughness descriptor along standard 100 mm-length profile detected by instrumentation such as profilometers. This work extends a method proposed for fractal analysis of particle contours to describe rock joint profiles in terms of quantitative descriptors of their roughness. It is well-established that natural surfaces have a fractal nature, self similar over a wide range of scales. This implies that the measured length of their outline is a function of the measurement scale: the smaller the measurement scale, the longer the profile length. Based on the interpretation of the fractal analysis of rock joint profiles, relating the length of the profile to the measurement scale, descriptors identifying the roughness and its characteristic scale are proposed. The method is first applied to some artificial profiles, and later to real rock joint profiles.
Simulation of temperature and chemical weathering effect on marble rocks
Authors: Vagnon F , Costanzo D , Ferrero A M , Migliazza MR , Pastero L , Umili G
Published: September 2021

Physical and mechanical properties of building stones can vary due to different degradation mechanisms caused by temperature and chemical agents. The problem of chemical and thermal weathering on marble rocks is an important issue to consider for designing building façades since it may cause sugaring, bowing, cracking and spalling. Moreover, an accurate comprehension of induced damages is required for restoration and conservation of heritage monument purposes. While thermal weathering has been widely studied in the last years, the combined effect of thermal and chemical weathering (that here is intended as the combined action of rainwater and atmospheric pollutants) is still poorly understood. In this study, no-destructive (ultrasonic pulse velocities) and destructive tests (bending tests) were performed on Carrara marble slabs in natural and after thermal (with target temperatures respectively of 50 and 90°C) and thermo-chemical treatment. Thermo-chemical treatments were performed by soaking the specimens in a 5×l0−6 mol/l solution of sulphuric acid at pH=5 to simulate the acid rain behavior, at constant target temperatures, for one week. In general, for each weathering mechanism, progressive degradation of the physical and mechanical properties of marble specimens was observed. In particular, a marked drop in flexural strength, mirrored by a wide variation in P- and S-wave velocity, was found in specimens chemically treated at target temperature equal to 90 °C.
Investigations and stability analysis of San Marcellino cavities in the historical centre of Naples
Authors: Fasano G , Allocca V , Bilotta E , Nicotera M V , Ramondini M , Russo G , Russo M , Calcaterra D
Published: September 2021

The historical centre of the city of Naples is characterised by a network of underground anthropogenic cavities. Most of these cavities are stable and safe but sometimes for several reasons they are at least partially responsible of the occurrence of instability phenomena such as sinkholes and collapses. The opening of a sinkhole in the cloister of the Ancient Royal Boarding School (known as Educandato Reale of San Marcellino) led to fears of instability in the known underlying cavities. Therefore, a site investigation program was carried out to ascertain the causes of the event. The investigation concluded that the sinkhole was not related to the underneath cavities, located in tuff between 12 and 20 m beneath the main building. Nevertheless, due to the interest raised by the preliminary safety assessment, such a cavity system was selected within the MOSCAS project for further studies. The results of the site investigations and the stability of the cavities determined via FEM code Plaxis 2D are presented and discussed in this paper.
Investigation, design and mapping of Shaft O1 on Phase 2 of the Deep Tunnel Sewage System project in Singapore
Authors: Jack C D , Debroy P , Lyall R J W , Jinrong Y
Published: September 2021

The Public Utilities Board (PUB) of Singapore is currently progressing Phase 2 of the Deep Tunnel Sewage System (DTSS) project. Leighton Contractors Asia Limited (LCAL) have been awarded Contract T-09 of the project which runs approximately 8 km from the intersection of Clementi Road and the Ayer Rajah Expressway (AYE) to the intersection at Jurong Pier Road and AYE. LCAL have appointed COWI to act as their designer. Contract T-09 includes eight shafts, including Shaft O1 located between the Jurong River to the west and Penjuru Road to the east in the Jurong region of south-west Singapore. This paper discusses the rock engineering aspects of Shaft O1, including the engineering geological aspects and the methods of analysis used to design the reinforcement and support of the section of the shaft excavated in rock. Finally, the conditions encountered and the support installed during the engineering geological mapping of the shaft are described. The geology at Shaft O1 comprises made ground overlying Kallang Formation deposits and then weathered rocks of the Jurong Formation. Development of the ground model included several stages of desk study and ground investigation and ultimately six boreholes and eight probeholes were carried out at and around the shaft location. The analysis methods used included the Q-system, analytical closed-form calculations, use of the Unwedge software package, rock bolt and sprayed concrete design and a stand-up time assessment. The design was prepared during 2018-19 and the shaft was successfully constructed during 2019.
Rock characterisation for a road tunnel in Mumbai (India)
Authors: Battaglia D , Redaelli M
Published: September 2021

The Mumbai Coastal Road is a freeway currently being constructed along the western coast of Mumbai. A key element of this route is a twin-bore tunnel with total length of about 3km (a 2km TBM-excavated central section, plus cut-and-cover ramps at both ends). The tunnel alignment underpasses Malabar Hill and then runs under Chowpatty Beach for most of its length. The central section is located below the sea level with ground covers ranging from a maximum of 70m at Malabar Hill to less than 15m under Chowpatty Beach. Each bore, excavated by Tunnel Boring Machine (TBM), has an internal dimeter of 11m and includes a section with a horizontal radius of curvature tighter than 400m. This paper describes (i) the desk studies and the geological survey which provided the basis for an additional ground investigation plan, (ii) the results of the additional boreholes, in situ test and laboratory test, (iii) the overall characterisation of the rock mass (and of the overlying soils) based on the entire set of old and new data. The rock characterisation provided the fundamental information for the TBM type selection, and for the design of all the underground works. The launching chamber and the first part of the TBM drive are located within a Basaltic lava flow, while the second part of the alignment is set in Volcanic Breccia underlying the fine-grained soils and made ground of Chowpatty Beach.
EARTH RETAINING STRUCTURES THAT ALLOW ROAD INFRASTRUCTURE PROJECTS COASTING A FLYSCHIOID ROCK VALLEY
Authors: Mangiola A , Malgeri G D , Mittiga E
Published: September 2021

This note concerns the main geotechnical features of two road infrastructures, located in northwestern Italy. They are adjoining and situated from km 31+500 to km 35+600 of the national road “Strada Statale n. 45 di Val Trebbia”, along Trebbia river valley, across the Ligurian Apennines. The road, mostly in the half-way section, coasts a valley featured by orographic slopes with an average slope of 25°-30°, which rise up to 1100m above sea level. The stratigraphy, starting from the top level to the deepest, shows: a thin layer of eluvial-colluvial soil; a layer, 3-5m thick, of fractured rock, and underneath the hard marl substrate that belong to the flyschioid formation of Ronco. In this context, to widen the carriageway, improve the design speed of the road section and carry out the works under traffic, a systematic use of earth retaining structures has been conceived. Thanks to the experience taken from the works of the previous project, a more conservative geotechnical characterization has been adopted in the second one. This has meaning for the lithotype called intact rock, in the equivalent Mohr-Coulomb model, a significant reduction in cohesion against a slight increase of friction angle; furthermore, a correspondent variation resulted for the fractured rock.
Effect of rear walls on the rocking response of rock blocks under seismic excitations
Authors: Mennitti M , Maiorano R M S , Aversa S
Published: September 2021

The rocking response of rigid free standing bodies subjected to seismic excitation has been studied by many researchers interested in different slender elements such as ancient stone columns, tombstones, rigid building structures. The extension of this model to rock mechanics has been proposed by a few authors. The rocking response of rectangular free standing bodies subjected to horizontal accelerations of natural recorded motions showed that the pseudo-static approach, based on Peak Ground Acceleration (PGA), permits only the determination of the uplift conditions and the beginning of rocking. It does not permit to evaluate the overturning of the blocks. The combined effect of vertical and horizontal seismic motions is negligible and, in some cases, beneficial. This paper presents a new mechanical model, called “one-sided rocking”, that takes into account the presence of a rear rigid wall, that is a typical scenario for the rock blocks completely detached from the cliff but close to it. The dynamic response of a great number of rectangular rigid blocks, subjected to 62 recorded earthquake motions on rock soil (from US, Europe and Asia), has been analysed considering only the horizontal acceleration. The results show that the presence of the wall is detrimental for the rocking stability. However, there is still a safety reserve more significant for large blocks and rich frequency content time histories. This reserve could be taken into account in simplified (pseudo-static) analyses through reductive coefficient of PGA.
The assessment of possible deformations in the fault zone
Authors: Barykina O , Kalinin E , Zerkal O
Published: September 2021

One of the most relevant aspects for engineering geology regarding faults is the characterization of the changes induced by a fault and the assessment of possible fault movements. This zone cannot be clearly and well observed in field studies. However, it plays a negative role in the construction and operation of structures. In this zone, unfavorable phenomena, such as rock collapses, displacements, etc., may occur. The purpose of the present study is to estimate the size of fault changes, to reveal the peculiarities of stress distribution within the fault influence zone and to characterize free surface movements. The site of the Rogun HPP was chosen as the object of the study. Rogun HPP is located on Vakhsh River (Tajikistan) in a gorge of 400 - 500 m deep. The site of the dam, which will be about 350 m high, is located in a single tectonic block bounded by sub-parallel Ionakhsh and Gulizindan Regional Faults. The complex of Jurassic-Cretaceous sediments in this block lies monoclinically. According to the results of complex inclinometer-deformographic observations at the Rogun HPP site, the faults are tectonically active, with vertical displacements in 1-3 mm per year. In the scheme of numerical simulation, the width of the Ionakhsh Fault together with the crushing zone was taken as 20 m. For all rocks it was accepted: Young modulus – 3000 MPa; Poisson’s ratio - 0.3; density - 2.6 g/cm3. Horizontal compressive tectonic force was given as 12 MPa. The simulation showed that the dynamic influence zone is limited to 200 m on the side of the lying wing and 70 m on the side of the hanging wing, and the concentration zones are located at the mouth and in the middle part of the fault on the side of the lying wing and between the hanging side of the fault and the bottom of the valley. The analysis of the movements of eight points of the surface showed that the direction of movement corresponds to observed in nature. The total amplitude of vertical displacements of two points at the valley is 0.51 m. The values of the resulting displacements on the different sides of the fault are 0.22 and 0.49 m. According to geological data, the width of tectonic breccia zone of the Ionakhsh Fault is 120 m. However, according to numerical simulation the influence zone is about 270 m. Thus, the simulation showed that the size of real fault influence zone is more than twice as large as that measured in nature.
Rainfall-triggered debris flows: triggering-propagation modelling and application to an event in Southern Italy
Authors: Porta G La , Leonardi A , Pirulli M , Castelli F , Lentini V
Published: September 2021

Debris flows are high-speed and unpredictable phenomena, considered among the main sources of hazard worldwide, since they can affect structures, the economy, and human lives. Rainfall typically triggers these events, causing the flowing of the unconsolidated soil downslope. This work focuses on debris-flow events characterized by multiple triggering areas, which are extremely complex since they involve a spatial sequence of numerous triggers in a relatively small portion of the slope. Numerical modelling of this type of phenomenon can contribute to hazard and risk assessment, which is key to designing effective mitigation structures. In this article, two different models are applied for triggering and propagation, respectively. The former computes the transient pore-pressure changes and the consequent factor of safety variation caused by rainfall infiltration, inducing the triggering of the event. The latter is a depth-averaged numerical model that simulates the event runout, and whose parameters are calibrated through back-analysis. The applicability of the two combined approaches is tested through modelling of an historical event in Southern Italy, which was characterized by large mass releases from multiple triggering zones. Residential areas were hit, suffering serious consequences. Two rheologies are compared to individuate the most suitable propagation model for the study case and obtained results are commented.
Modelling the behaviour of a soft rock using a kinematic hardening constitutive model
Authors: Laera A , Rouainia M , Elia G
Published: September 2021

Soft rocks are of great geological and geotechnical interest, due to their mechanical and structural characteristics falling between those of a soil and those of a rock. This presents several challenges for constitutive and numerical modelling. Here we propose a calibration strategy of an advanced kinematic hardening model with structure degradation against experimental data on soft rocks, accounting for their features at the micro- and macro-scales. The experimental results show a highly structured material, with strength and stiffness being controlled by inter-granular bonding as well as by mean effective stress. The clay content of soft rock samples seems to be a dominant factor: the specimens show lower stiffness and strength when the content is higher. Accordingly, the samples have been classified into two different types (A and B). The constitutive model reproduces well both the mechanically strong and stiff behaviour of Type A as well as the more ductile mechanical response of Type B specimens observed during drained triaxial compression tests. With increasing confining pressures, the transition from brittle and dilatant to more ductile and contractant behaviour observed for both types of rocks is well captured by the adopted constitutive model.
Analysis of the techniques for assessing the features of blast-induced fragmentation in an open pit quarry
Authors: Cardu M , Calzamiglia A
Published: September 2021

The paper deals with the study of empirical models for the analysis of blasts-induced fragmentation. To verify the reliability of the method, the results obtained a priori through the application of the models are compared with the analyses of muck piles photos taken from a series of blasts in an open pit quarry, thanks to the Split-desktop software (Split Engineering, LLC, Tucson, USA). The study of the particle size distribution is very important when sizing an excavation site, as the average size of the pile must be compatible with the equipment used for clearing and conveyance, and with the first phase of the processing plant, i.e. primary crushing. After a description of the excavation technique in use at the quarry site, its main characteristics are outlined. The parameters used to create the empirical models refer to the geomechanical characteristics of the rock, the geometry of the blasts, the characteristics of the explosive and its interaction with the rock mass, paying attention to the correlation between initiation timing and induced fragmentation. A detailed description of some empirical models together with the Split-desktop software were therefore provided. Thanks to both data and photos of the muck-piles from the exploitation at the quarry, the particle size curves resulting from each model were compared with those obtained from the software. This allowed to validate the empirical method in a preliminary design phase.
Advancing of the geomechanical forecasting system and substantiation of mining at rockburst hazardous deposits
Authors: Semenova I E , Avetisian I M
Published: September 2021

The paper presents the main approaches and their advancing for the systems of rock mass stress-strain state forecasting during mining of adjacent rockburst hazardous deposits. Such systems based on the original Sigma GT software are used at several Russian mining companies which develop solid rock deposits, prone to brittle fracturing and undergoing the gravitational-tectonic stress-strain state. At that, the mining-induced impact on the rock mass is accompanied by occurrence of dynamic rock pressure. The geomechanical situation forecast systems have been applied directly at mining companies for more than 10 years and have shown their high efficiency. The efficiency is associated primarily with the operative calculation of stress-strain state in case of exacerbations of the geodynamic situation and the potential consideration of various mining options in order to reduce stress concentration and give recommendations for supporting and unloading measures in mine workings. In addition, the Sigma GT software is used in annual and long-term mining planning. Initially, such systems were developed for individual rockburst hazardous mining blocks, and their efficient operation time was limited to a 3-5 year period, that is, the block mining time. Further on, taking into account the needs of companies, the systems began to include a whole deposit or even several deposits. Being applied, the system is constantly developed and advanced and adapted to specific conditions and tasks. The most actual tasks are combination of the results of stress-strain state modeling with seismic monitoring data; creation of a hierarchical system of interconnected multiscale volume models; more complete consideration of structural heterogeneities of the rock mass and development of a recommendation block for safe mining operations, taking into account the geomechanical situation forecast data. The solution of these problems is considered on the example of a section of the Khibiny apatite arc, within which underground and open mining is carried out in the zone of mutual influence.
Evaluating the behaviour of stiff polymer-modified cementitious thin spray-on liners from small and large scale testing
Authors: Plessis M du
Published: September 2021

The application of permanent areal support is recognised as a measure to either retain or prevent the mobilization of potentially unstable keyblocks between installed support elements. Various areal support methods have been developed to provide support to the freshly exposed area created by the blast and to provide support between existing support elements. These methods consist of either shotcrete, thin spray-on liners (TSL), blast resistant mesh or temporary (removable) nets. This paper focuses on the application of polymer-modified cementitious TSLs which is the type predominantly applied in South-African underground mines. Significant amounts of research has been conducted on TSLs. The majority of the research consisted of predominantly laboratory testing. The results of the tests only provide an output which can be used to measure a specific characteristic (i.e. tensile strength) against other products. The output obtained from laboratory tests are not understood, cannot be benchmarked against good or poor performing products or quantitatively related to the performance of the product when applied in-situ on a large scale. This situation is problematic as different support suppliers make claims in terms of the performance of their products based on a material property or small scale testing. The behavioural mechanism of a rockmass supported by TSLs is not understood. It is therefore not clear what critical TSL parameters will affect rockmass stability. Very little work has been conducted locally to test the in-situ behaviour of TSLs once applied. An extensive testing program was developed to establish how the material properties of a TSL as tested in a laboratory would relate to small and large scale in-situ behaviour. This paper discusses the testing conducted and the high level findings.
Propagation analysis and risk assessment of an active complex landslide using a Monte Carlo statistical approach
Authors: Brezzi L , Carraro E , Gabrieli F , Dalla Santa G , Cola S , Galgaro A
Published: September 2021

The risk assessment of a rapid landslide is a difficult topic, even if based on the results of numerical analyses. The hypotheses on which every model is developed, the choice of rheological laws to be adopted, and the selection of soil parameters make the simulation results highly dependent on the user. This is particularly evident when there is no model calibration for the specific site or reliable information on soil properties. The paper presents a forecasting process obtained using a Monte Carlo approach in coupling with a propagation model developed with the SPH integration technique. The Monte Carlo analysis allows automatically carrying out a large number of simulations, each performed using an independent parameter set randomly selected within a priori assigned statistical distributions. The numerical results are then analysed with statistical tools to create a risk map based of the frequency of the unstable mass runouts. In this way, it is possible to reduce the user dependence of results and increase the examined potential scenarios. The procedure is here applied to the case study of the Sant’Andrea landslide, a slope movement active since several decades in the municipality of Perarolo di Cadore (Belluno, Italy). This complex slide involves an about 30 m-thick deposit of calcareous debris overlying anhydrite-gypsum rocks. Depending on the intensity and duration of rain, the slope alternates phases characterized by slow displacements and significant accelerations, then followed by a long relaxation period in which the displacement rate slowly regresses, without returning to the previous condition of movement. In recent years, the landslide activity has caused a progressive enlargement of the unstable area and a gradual increase of the basal rate, thus increasing the risk that the landslide may suddenly undergo to the collapse. Moving from the knowledge of the unstable volume, an SPH propagation model is used to study the area affected by the debris-flow runout. In particular, the analysis aims to define a statistical strategy to perform and interpret a large number of simulations and to create the consequent risk map. The analyses carried out lead to a satisfactory interpretation of the spatial variability of the deposit heights referred to the post-failure conditions, useful for the development of a risk analysis, from which a site risk map can be obtained.
Contribution to the geomechanical stability of marble underground openings using backfill
Authors: Franqueira M Formoso , Paneiro G A
Published: September 2021

A stability analysis on the possibility of using room and pillar underground mining method, in a marble quarry in Alentejo (Portugal) with backfill, is presented. Given the data provided by the laboratory tests over marble samples and in situ tests as well as a thoroughly geology acknowledgment of the area, a new excavation design was presented for the underground phase, using Finite Element analysis, where global and local stability is assessed. Moreover, the innovative idea, for marble underground quarries, was to contribute to the circular economy and the current environmental policies by making a use of the quarry’s waste rock, as the principal aggregate for a backfill material, that not only reduces the impact of wastelands in the surface but also acts as support for the newly excavated area leading to residual pillars recovery. Given this, an overview of the stress displacement is presented, and the main potential instability zones are assessed. The results are a quantitative contribution to the characterization of the rock mass for alternative ways of ornamental marble production in this region.
Sustainable development for underground mining of strategic industrial minerals: the geological and geotechnical modelling as a key factor. The case of the “Marmorino” mineral deposit (Sacile - NE Italy)
Authors: Gabriele P , Damiano A , Varrone D , Piovano V , Cravero M , Castelli E , Anselmi B
Published: September 2021

The growing industrial minerals demand, in the perspective of a sustainable development of mining activities, implies a strong orientation to underground operations in the near future. In complex geological areas, for the definition of a reliable 3D ore body model a methodological approach, aimed to improve geological and geotechnical model reliability allows a confident mineral deposit evaluation and a reliable mine design, or, in other terms, costs reduction, safety increase and mining project sustainability. The described method finds its maximum application in complex geological and structural contexts, such as the Valmadonna-Pedemonte “Marmorino” quarry.
Effect of thermal cycles on rock cliff deformation. Monitoring and interpretation
Authors: Villarraga C. , Vaunat J. , Virely D. , Gasc M.
Published: September 2021

Climatic actions are one of the factors controlling the evolution of slopes, this paper is devoted to a specific effect, relatively little studied, related to the effect of climate-driven temperature changes on rock massif deformation. The particularity of the study is to focus on permeable rocks and Temperatures varying in a range which discards freeze/thaw effects. Research has been carried out in relation with the analysis of the real case of a limestone cliff located in the Périgord region, the massif was highly instrumented, results show a slow cyclic accumulation of deformations with time, essentially synchronic with thermal cycles. An advanced constitutive model, specifically developed to capture rock degradation due to the differential expansion of the main minerals composing the rock, has been developed. It has been calibrated on experimental results obtained in the laboratory on block samples tested in a climatic chamber for a long series (several months) of daily thermal cycles. Deformation and shear wave velocity were monitored during the test. Model shows a good agreement with laboratory measurements.
A quantitative approach to predict tunnel overbreak based on the Q-system
Authors: Ekeberg R , Holmes H , Paraskevopoulou C
Published: September 2021

Overbreak can be a significant challenge in tunnelling and is usually assessed on a project-by-project basis. This paper attempts to quantify tunnel overbreak in jointed rock masses by producing a predictive model that practitioners can utilize at the preliminary design stage. Discontinuum numerical analysis of a circular-shaped tunnel is performed using the combined finite difference – discrete element method to generate a database of rock masses prone to overbreak. The Q-system, rock mass quality index, is used to classify the rock masses that are expected to overbreak in various tunnelling environments. The results are scrutinized to identify trends between overbreak and: i) Q, ii) average joint spacing and, iii) in-situ stress conditions. It is observed that overbreak increases when Q < 4 and is uniform when Q ≤ 4. A predictive model is generated through multiple regression analysis and can be used to identify and highlight the maximum likely overbreak or scenarios where overbreak is expected to occur.
Practice of the Doorstopper stress measurement method during the last 30 years in Italy
Authors: Guido S , Acerbis R , Sossi G
Published: September 2021

In Italy, during the last decades, most of the in situ stress measurements were carried a shallow depth, in existing geotechnical structures. In these cases, the stress measurements involves concrete or low disturbed rocks material; thus linear, elastic, non-fractured materials. For this reason the stress tests interpretation is straightforward and reliable. In these situations, since the early 90s, the Doorstopper technique became the most used stress measurement technique. It has proven to be effective and cheap. A collection of punctual tests can be interpreted with a suitable numerical model. The numerical model provide the interpolation of the punctual measurements and can give important information about the virgin stress field. After recalling the main features and improvements of the doorstopper method in the last 30 years, this paper illustrates two Italian cases. The first case is the Sparvo Tunnel (Bologna-Firenze highway) where was performed a massive measurement campaign made up of more than six hundred Doorstopper tests in two adjacent tunnels; the results of the two tunnel are compared; the 2d FEM model interpretation is discussed. The second case is a marble quarry in Alta Carnia (Udine): the Doorstopper and stress variation measurements were carried out and interpreted by means of a 2d FEM model, resulting in the definition of the natural stress field.
Effect of the lay distance of a stiff support on the applicability of the Convergence-Confinement method
Authors: De La Fuente M , Berro F , Khodr S , Taherzadeh R , Sulem J
Published: September 2021

The use of the classical ConVergence ConFinement (CV-CF) method which is a simplified tunnel design tool is very extended nowadays. A major limitation of this plane-strain approach is encountered when it is applied to the study of tunnels with a stiff lining near the tunnel face. Under this configuration, the pressure exerted by the ground over the lining can be underestimated. The reason stems from the fact that this method is not able to simulate the tridimensional arch effect taking place between the ground and the lining. To account for this ground-support interaction, some authors have proposed to enhance the classical CV-CF method by resorting the so-called implicit methods. However, these approaches still show some limits for very stiff lining. In this paper, the applicability domain of the existing CV-CF methods when applied to the design of full-face excavated circular tunnels with a stiff support system is discussed. The results of the ground-lining equilibrium state obtained with the plane-strain approaches are compared with the equilibrium state obtained with an axisymmetric numerical model which can properly capture the arch effect taking place at the vicinity of the tunnel face. A sensibility analysis with a focus on the support lay distance has been carried out. Finally, a simple chart that clarifies which CV-CF method is more adapted to each tunnel configuration is proposed.
Preliminary Investigations and the Financial Sustainability of Underground Project
Authors: Coli M , Martelli L
Published: September 2021

Tunnels upgrade connectivity and mobility, with an all-out venture cost that mostly depends on the financial support given by significant lenders. Financial sustainability is related to Risk and Sensitivity Analysis, which in turn depends the level of knowledge of the geology of the project site. The target of this paper is twofold: depict the cycle of i) a well define comprehension of the geo-related model and ii) represent the impact of geo-related uncertainties into the Risk and Sensitivity Analysis. Key elements for assessing the economic sustainability of the investment are here outlined. The undeniable degree of financing preliminary investigations because of high forthright expenses is regular for the vast majority of the underground and tunnel projects. However, there are additional risks such as the geological uncertainty mainly related to the robustness of the preliminary investigations. Significant investment is needed prior to arriving at solid comprehension of the geological model and the connected hazards. Geological uncertainties can be characterized into two significant classes: epistemic and aleatory. Fundamental investigations are normally scant and don’t cover the epistemic vulnerability related to the chosen tunnel alignment or underground facility location. For an underground infrastructure project, the geological uncertainty exerts the highest impact on the project and financial sustainability. Variables must be surveyed and contrasted with the assessed cost in order the project being eligible to some kind of financing. Depending on the result of the risk analysis, solutions can differ: i) withdrawal from the project or restructuration, ii) chances ensures, iii) dividing the risks between parties and phases, and more significant, iv) a powerful undertaking readiness. Last option being the most efficient method for risk mitigation however depends on fundamental examination by specialists. Blended finance presents a risk sharing approach.
Study of damage to rockbolts used in the foundations of protection structures against gravitational hazards
Authors: Bost M , Gaillet L , Fargier Y , Martin R
Published: September 2021

Fully grouted rockbolts are used to anchor protection structures against natural gravitational hazards. In recent years, contractors have observed a premature degradation of the steel bars of these rockbolts. In the dimensioning of a rockbolt, the choice of the steel bar is currently made only on its pure tensile strength. The objective of this study is to identify whether the degradations observed in situ are related to the metallurgical nature of the bar steel and/or the load conditions. 16 damaged rockbolts were taken from four different sites in the Alpine area. They were subjected to a metallurgical analysis and a supplementary analysis of their environment. In conclusion, it was underlined that the damages were due to a lack of knowledge of the effective loads transmitted by the snow-filled structures, but also to a lack of consideration of the deformation behaviour of the foundation. The loads applied to these rockbolt bars generated a bending depending on the nature of the surrounding ground. It has been shown that these loads can occur in successive jolts and with modification of their orientation in relation to the number of links between the rockbolt and the protection structure.