Weathered Rock Mass Research Articles

Optimizing the Support System of a Shallow Buried Tunnel under Unsymmetrical Pressure

In the construction process of tunnel inlet sections, the rock mass can sustain unsymmetrical pressure due to asymmetrical terrain on the two sides of the tunnel. The fact that the inlet sections are usually under shallow buried conditions with strongly weathered rock mass exacerbates the issue. This paper discusses optimization strategies of the initial support of a shallow buried tunnel based on the analytical results of asymmetrical loading characteristics. Numerical simulation is performed with particle flow code (PFC) using the Jianshanji tunnel project as an example. The simulation results show that the bench excavation has slightly less total deformation than the full-section excavation but the deformation range is wider, especially in the tunnel arch. Both lining support and slope reduction treatments can effectively improve rock deformation, with lining support demonstrating better performance in controlling deformation and adjusting stress distribution. Based on the simulation results, the bench excavation and lining support are used in the actual project, and the corresponding optimization control measures were adopted to address deformation issues, including crushed-stone backfilling for compression resistance, advanced grouting reinforcement, and grouting. The field data show that the tunnel stability is effectively improved by adopting the optimization schemes, which further validates the effectiveness of the proposed unsymmetrical control method.

Creeping slopes in NW Himalaya and Joshimath slide: constraints from GPS measurements

Slow slope movement is very common in the Himalayan region. In most cases, the topsoil, weathered rock mass, debris of paleo-landslide or of glacial origin, etc. slowly slides until it stabilizes or fails. In some cases, even the bedrock may be involved in sliding. Here we report four such sliding slopes on which GPS were actually installed for tectonic deformation monitoring in the Himalayan region after a careful investigation but they recorded slow slope sliding. The slow sliding motion on these slopes varied from 10-15 mm/year to 5-6 m/year. The slow slides did not show any anomalous seasonal variation due to hydrological loading. Amongst the four, the Joshimath slide exhibited a large variation in sliding motion which varied from less than a mm/day to 15 mm/day during 2022-2023, causing cracks on the ground and buildings and panic amongst the residents. Even the highly non-linear motion with a sudden change in sliding motion at Joshimath appears to be of viscous creep type. The exact time of initiation of this episode of slide is not known but the available InSAR analyses imply that it is moving at least since 2018. Its motion does not seem to be affected by the February 7, 2021 floods, however, whether the anthropogenic activities around the sliding zone influenced its motion and initiation, remains to be debated.

New analytical approach to simulate the longitudinal fiberglass dowels performance installed at the face of a tunnel embedded in weak and weathered rock masses

A new theoretical method is proposed to simulate the fiberglass dowels performance in the confinement of the tunnel face in weak rock masses. In this method, to find the stress distribution ahead of the tunnel face, the spherical symmetry hypothesis is used. Thus, the tunnel's cylindrical shape is approximated by a hemispherical void with the same radius. Considering the relative shear displacements between the reinforcing elements and rock mass, the distribution of the axial forces and shear stresses along the reinforcements are obtained. Some examples are solved, and the results are compared with those derived from FLAC3D simulations. A good agreement is observed between the results obtained with both methods.

Challenges in Tunneling in the Himalayas: A Survey of Several Prominent Excavation Projects in the Himalayan Mountain Range of South Asia

The Himalayas are one of the most unstable regions in the world. Underground excavations are very challenging in this region due to the fragile geology, tectonic activities and complex geological structures. In the present study, three hydropower head race tunnel projects from Nepal, as well as six additional tunnel projects from Bhutan, India and Pakistan located at different parts of the Himalayas, are reviewed with a focus on the common problems encountered during the tunneling and their possible causes and remedies. It is found that rock bursts often occurred around the overburden of 1000 m or higher, while tunnel squeezing problems were commonly observed in a wide range of overburden. Most of the rock failures occurred in sheared zones, thrusts and heavily weathered rock mass. The geological conditions around these projects are often characterized by significant presence of folds, faults, joints and interbedding of different types of rock strata; they are so complex and diverse that it is often difficult to accurately predict the field condition from conventional geotechnical site investigation. In many cases, the presence of shear zones, thrusts and tectonic activities has a strong influence on the in situ stress. Review of these prominent projects suggests that the complicated dynamics between a wide range of geological and geotechnical factors play a critical role in the tunneling in the Himalayas.

Use of electrodeposition to realise the crack-healing and pore-filling of weathered rock: A small specimen case

Medium- to long-term weathering phenomena is one of the major causes of rock instability. Although many geo-protection technologies have been developed to mitigate the hazards such as slope failures of natural rock weathering, these techniques provide no self-healing function of the rock around the facilities. The status quo in rock instability problems is to provide rock maintenance and to constantly monitor the situation depending on the particular conditions. Improper rock maintenance comes with the risk of critical accidents which may result in human casualties as well as significant environmental destruction. It is therefore necessary to develop novel techniques to realise the continuous healing of weathered rock in the medium- and long-term. In this study, the electrodeposition method was applied for the first time to heal weathered rock mass in a series of fundamental experiments, using volcanic tuff sampled in Japan as the weathered rock model. It was noted that a reduction reaction occurs around the cathode side, and the electrodeposits fill the pores and cracks in the rocks selectively, thereby improving the surface hardness in a small specimen scale. Moreover, the application of a weak current over a lengthy period was shown to facilitate the deposition of stable crystals, such as calcite and magnesian calcite. The results of this study on electrodeposition on unstable rock formations show the potential of crack-healing and pore-filling in a self-organised manner.

Evaluation of Slope Stability Analysis using Kinematic and Chart Method: A Case Study

Abstract The variability and unpredictability of climatic conditions are adding to the complexity in the prediction of the natural hazards. The Himalayan region is having a very complex geological and tectonic setting. Besides, climatic conditions and geological setting of the area, human encroachment is adding to the vulnerability of mountain slopes. The landslide, particularly rockfall, became a big problem for commuters and inhabitants in the hilly region. The landslides are mostly occurring either in the summer season or in the rainy season due to the percolation of water along the jointed weathered rock mass slopes. The researchers/scientists have investigated various reasons for the slope failure using conventional methods like kinematic and limit equilibrium method or using numerical methods like finite difference method (FDM), finite element method (FEM), discrete element method (DEM). The numerical methods FEM, FDM, and DEM has wide application but these are very costly and requires high end computation system. On the other hand, the conventional methods, which were used in the present study, is economical and requires geological experiences only. In the present study, slope stability analysis was performed for roadcut slopes of National Highway -109 (NH-109) from Bhowali to Almora (53 km) in the district of Nainital and Almora, Uttarakhand, India. A total of fourteen vulnerable locations were identified and their instability was evaluated. The types of failure were determined using the kinematic method, stability and classification of the slope were examined using slope mass rating (SMR). In addition, the chart method was used to calculate the factor of safety (FoS) for only wedge type of failure. A new graph has been proposed to find out vulnerability of slope considering only field-based approach.

A novel method for modelling the existence of fault fracture zones within 3D weathered rock slopes

High-steep weathered rock slopes are often encountered in vicinity of the large-scale hydropower engineering projects. A variety of fault fracture zones may exist inside the weathered rock mass due to the complexity of geological activities. However, few publications have considered the state of fault fracture zones within a rock slope in a realistic way. For this goal, this study proposes a new approach to modelling the existence of fault fracture zones inside rock slopes based on 3D Gaussian random field, in which the basic features of fault fracture zones can be well established in terms of the need of site conditions. The results demonstrate that the inclination of week fractured zones has pronounced effects on the development of the failure mode of a rock slope. The degree of connectivity of the resultant irregular slip surfaces at each cross section of the 3D model significantly affects the system stability of a weathered slope. In addition, using a 3D numerical model for the analysis of a fractured slope gives a more reasonable and reliable estimation of the slope stability than the 2D one. The presented results can offer a new insight for the stability investigations of weathered rock slopes with fault fracture zones, aiming to reduce the risk of landslides.

Methodology for Resloping of Rock Slope Using 3D Models from UAV-CRP Technology

Assessing the stability of rock slopes is essential to ensuring the typical performance of adjacent transportation infrastructures but simulating the existing field topography and boundary conditions for stability analysis is challenging. This paper describes a case study in which highly weathered rock slopes adjacent to a railroad track in Texas were analyzed using close-range photogrammetry data obtained from an unmanned aerial vehicle with close-range photogrammetry (UAV-CRP). A highly weathered rock mass with low intact strength was considered as a single continuum that undergoes circular failure and was analyzed using Spencer’s two-dimensional (2D) and three-dimensional (3D) limit equilibrium method (LEM) analyses. A 3D critical slip surface was obtained to identify the critical sections on which to conduct the individual 2D stability analysis and obtain a safe resloping angle. These angles were further used to reslope the whole rock slopes and conduct a 3D analysis to arrive at a safe and economic slope angle for each side of the rock-cut. The realistic geometrical slopes developed from the aerial imagery were used to develop a framework for comprehensively analyzing the stability of a rock slope undergoing circular failure.

Compositional Features and Swelling Potential of Two Weak Rock Types Affecting Their Slake Durability

Weak and weathered rocks are well known for their sensitivity to changes in moisture content. Degrading behavior is common in weak rocks with moisture-sensitive mineral components and present numerous stability problems. The slake durability is a measure of the resistance to weakening and disintegration of rock materials which quantitatively distinguishes durable from non-durable rock materials. Several rock material parameters interact on the process of disintegration when exposed to cyclic moisture changes, whereby the content of clay is believed to play a major role. This manuscript evaluates the overall material composition of flysch and serpentinite rocks cored from the wall of the shotcrete-lined headrace tunnel of a hydropower project, including minerals, structure, porosity, the presence of micro-discontinuities, and swelling potential, and links these properties to the slake durability. Further, the different methods used to assess compositional features affecting the durability of weak rocks are evaluated and discussed. The manuscript argues that the mineralogical composition and microstructures present in the intact rock and the content of moisture-sensitive constituents, as swelling clays, control the long-term durability of weak rock material. It is demonstrated that XRD assessments are not sufficient to detect the content of brucite and swelling components, and that methods as thin section and SEM analyses should be carried out in the assessment of weak and weathered rock mass.

Evaluation of Weathered Rock Mass Strength and Deformation Using Weathering Indices

Rock weathering and the resulting effects on geotechnical properties of weathered rock mass has long been a concern for geotechnical engineering practicing since the strength and deformational properties of weathered rock masses is needed for analysis and design of underground openings, opens pits and excavations for foundations of dams, bridges and high-rise buildings in and on weathered rock mass. It is a very challenging task to obtain required core samples to determine the strength and deformation behavior for the assessment of weathered rock mass performance. This is because of weathering which causes change in fabric of rock by creating voids and micro cracks after dissolving grain boundaries particularly, at higher stage of weathering. This paper attempt to present a method of sampling and testing of weathered rocks and proposed a weathered rock mass indices chart for estimating rock mass strength and deformation properties. To investigate the applicability of the proposed weathered rock mass indices chart for other regions, available data from literature for same rock has been used along with the extensive testing data generated in the present investigation Deccan plateau basaltic rocks.

Effects of rock weathering on guided wave propagation in rock bolts

Rock quality is a critical design parameter of rock bolts. However, rock weathering deteriorates rock quality and adversely affects the performance of rock bolts. The objective of this study was to investigate variations in the velocity and predominant frequency of guided waves in rock bolts subjected to rock weathering. In experiments, chemical and mechanical weathering on 10 rock specimens was achieved with salty water and a slake durability test, respectively. The measured properties of weathered rock specimens were used in a numerical simulation model. Numerical studies on rock bolt surrounded by weathered rock mass were performed to simulate guided waves in the rock bolt. The results show that the group velocity and predominant frequency of the rock bolt decreased the weathering progressed. The grey relational grade value and importance score based on the random forest algorithm indicate that the group velocity and predominant frequency are significant factors sensitive to rock weathering. This study demonstrates that the group velocity and predominant frequency of guided waves can be useful tools for predicting rock weathering surrounding rock bolts.

Deformation and Stability Characteristics of Layered Rock Slope Affected by Rainfall Based on Anisotropy of Strength and Hydraulic Conductivity

The strength and hydraulic conductivity anisotropy of rock slopes have a great impact on the slope stability. This study took a layered rock slope in Pulang, Southwestern China as a case study. The strength conversion equations of the seriously weathered rock mass were proposed. Then, considering the anisotropy ratio and anisotropy angle (dip angle of bedding plane) of strength and hydraulic conductivity, the deformation and stability characteristics of rock slope were calculated and compared with field monitoring data. The results showed that the sensitivity analysis of strength and hydraulic conductivity anisotropy could successfully predict the occurrence time, horizontal displacement (HD), and the scope of the rock landslide. When the anisotropy ratio was 0.01 and the dip angle was 30°, the calculated HD and scope of the landslide were consistent with the field monitoring data, which verified the feasibility of the strength conversion equations. The maximum horizontal displacement (MHD) reached the maximum value at the dip angle of 30°, and the MHD reached the minimum value at the dip angle of 60°. When the dip angle was 30°, the overall factor of safety (FS) and the minimum factor of safety (MFS) of the rock slope were the smallest. By assuming that the layered rock slope was homogeneous, the HD and MHD would be underestimated and FS and MFS would be overestimated. The obtained results are likely to provide a theoretical basis for the prediction and monitoring of layered rock landslides.

A Coupled Nonlinear Flow Model for Particle Migration and Seepage Properties of Water Inrush through Broken Rock Mass

A large number of statistics indicate that broken rock mass always transforms into a flowing channel and leads to water inrush disasters in mining engineering, such as fault, karst, and strongly weathered rock mass. During the process of water inrush, the structure of the broken rock mass is constantly changing due to seepage erosion under high-velocity flow. Therefore, it is of vital importance to quantitatively evaluate the flow behavior of the water inrush related to the seepage erosion in order to prevent or reduce the risks. This study described a coupled nonlinear flow model, which couples the high-velocity seepage, the small particle migration, and the evolution of the broken rock mass structure. The model was verified firstly for simulation of nonlinear flow behavior by comparing with the traditional one. Then, the proposed model was used to simulate the evolution of particle migration and seepage properties of the water inrush through broken rock mass by a numerical case. The simulation results generally agree well with the existing experimental results. The simulations indicate that small particle migration causes the unstable characteristics of the seepage and the heterogeneity properties of the broken rock mass, which lead to the nonlinear flow behavior of the water inrush in both time and space. From a different perspective, it also indicates that the proposed model is capable of simulating the interaction of high-velocity seepage, small particle migration, and evolution of broken rock mass structure in the process of water inrush.

Proposed adjustments and validation of different RMR89 geomechanical classification for Quadrilátero Ferrífero lithologies

Geomechanical classifications of rock masses are of great interest and importance for the design of civil and mining works, as slopes, open pits, and underground excavations stability are directly related to the mechanical properties of the rock mass classes. Some of the most common geomechanical rock mass classifications currently used are the RMR (Rock Mass Rating), the Q system, and the GSI (Geological Strength Index). Recently (2011), it was presented a new approach for RMR in which the author had pointed an erroneous understanding by some of the RMR users related to the weights adopted for some parameters. Due to the great difficulty found by some professionals to classify highly weathered rock masses by using RMR89, various authors have proposed a correction of for RMR89 values by considering an adjustment index. On this context, the present work has the aim of verifying, for several rock masses from Quadrilatero Ferrifero, Minas Gerais, Brazil, the effectiveness of these three proposed adjustments on RMR89. The results has shown that the corrections proposed better represent the weathered rock masses usually found on open pit on Quadrilatero Ferrifero.

Characteristics and dynamic process analysis of the 2018 Mabian consequent landslide in Sichuan Province, China

A consequent slope, comprising weak planes in the same dip direction along the slope face, is the most dangerous type of rock slope. Herein, a consequent landslide that occurred on May 5, 2018, in Mabian County, Southwest China, was investigated. This landslide occurred in a region with a complex geological background and strong geological activity and is considered a typical consequent landslide. Field investigations and aerial photographic interpretations revealed that the rock mass slipped along the contact surface of the strongly and moderately weathered rock mass, triggered by continuous rainfall. A depth-integrated continuum method was used to study the dynamic process of the landslide. The applicability of the depth-integrated method was verified by comparing the moving boundary of the landslide extracted from a video with the simulation results. The results show that the velocity of the landslide first increased and then decreased from the top to the bottom of the landslide and reached a maximum of 21 m/s in the middle of the landslide. Based on this method, we predicted the potential danger zones and velocity distribution of unstable masses next to the Mabian landslide. On comparing the calculated results of a depth-integrated continuum method with the Scheidegger method, in the case of the Mabian landslide, a reduction factor λ was added to the Scheidegger formula to achieve a modified formula that determines the velocity of such consequent landslides relatively accurately. This work could provide reference for the study of landslide velocity and disaster prevention.

Characterization of Aquifers in Metamorphic Rocks by Combined Use of Electrical Resistivity Tomography and Monitoring of Spring Hydrodynamics

Crystalline rocks are generally characterized by negligible porosity and permeability in terms of groundwater exploitability. However, alteration processes can greatly increase their fracture permeability and induce formation of modest, but locally important aquifers. Therefore, subsurface characteristics of alteration zones are of major importance for hydrogeological evaluation of crystalline terrains. Alteration processes greatly affect rock total porosity and water content, causing contrasting electrical resistivity of rocks affected by varying degrees of weathering. This makes electrical resistivity tomography (ERT) a preferable geophysical method for the exploration of alteration zones in crystalline rocks. In our research, we used an integrated approach, combining the ERT method with monitoring of spring discharge and hydrochemistry to characterize metamorphic aquifers on slopes of the Medvednica Mountain (Croatia). Significant fracture flow aquifers are found to be formed in intensely fractured but not highly weathered rock masses (medium to high resistivity values), while highly weathered masses (low resistivity values) form local barriers for fracture flows. Subsurface structure of the alteration zone proved to be highly irregular, with sharp contacts between more and less weathered rocks. Decrease of permeability below the alteration zone keeps the water level near the surface and enables spring occurrence on the mountain slopes. Studied aquifers have relatively limited extent, resulting in typical capacity of major springs of a few l/s. More frequent but less productive springs are attributed to the draining of the shallow part of the alteration zone (mostly saprolite). Combination of the ERT method with spring monitoring proved to be very effective as a first and relatively inexpensive methodology for hydrogeological characterization of crystalline terrains, both in local and catchment scales.

Evaluation of rock slopes susceptible to circular failures using logistic and multiple regression models

The analysis of stability of slopes is a classical problem for geotechnical engineers. In practice, many methods are available for the desired purpose, from basic kinematic analysis to two- and three-dimensional limit equilibrium analyses and numerical modeling with various user-friendly softwares. However, additional techniques are also required to provide knowledge necessary for decision-making. In this research, a reliable dataset provided by Sah et al. (1994) was used to analyze the stability evaluation of rock slopes subjected to circular failures. For this purpose, using Slide 2018 program, 44 separate limit equilibrium slope models were built for each case given in the original work. The provided material properties and slope geometries in heavily fractured and/or very weak or highly weathered rock masses were considered during the model building stage. It was found that Slide 2018 program generated dissimilar safety factors compared to those given by Sah et al. (1994) for the investigated slope cases. Binary logistic and multiple linear regression techniques were implemented in the study to promote alternative approaches for the prediction of the stability condition and safety factor (SF) of slopes excavated in heavily fractured/highly weathered rock masses. The condition of slopes (stable or failed) was predicted by binary logistic regression model with 90.9% accuracy. The SF of the slopes was estimated by a multiple linear regression model with 95.6% accuracy. It was concluded that both statistical techniques could be sufficiently used as alternative approaches to predict the stability condition and SF of rock slopes prone to circular (rotational) failures.

Comprehensive Ahead Prospecting of Tunnels in Severely Weathered Rock Mass Environments with High Water Inrush Risk: A Case Study in Shaanxi Province

The ahead geological prospecting of severely weathered surrounding rock in a tunnel is challenging due to complex geological conditions. Hence, the best approach is to use a variety of geophysical exploration methods. In this study, we have applied seismic, electrical resistivity (ER), and transient electromagnetic (TEM) methodologies for ahead prospecting of the highly weathered and complex surrounding rock at the diversion tunnel of the Hongyan River to Stone River Water Transfer Project in Shaanxi Province, China. The seismic method was employed to detect structural information for an area of 100 m from the front of the tunnel, the TEM method was used to obtain the resistivity information within 60 m, and the ER method was conducted to obtain detailed resistivity information over a range of 30 m. The integrated results generated a comprehensive interpretation of the geological body located within 100 m of the front of the tunnel. We divided this geological area into intact, severely weathered, and slightly weathered sections. Information on the water content of each section was also produced. The results of subsequent excavations are consistent with our results, proving the effectiveness and feasibility of a comprehensive approach for analyzing highly weathered and complex surrounding rock.

A multi-approach rockfall hazard assessment on a weathered granite natural rock slope

After a recent forest fire, a weathered granite rock slope located in the northwest of Spain may become increasingly susceptible to rockfalls. This study presents a multi-approach assessment of rockfall hazard, with some features deserving particular attention. First, the geomorphological context represented by a weathered rock mass with multiple dispersed blocks presenting various potential instability mechanisms. Secondly, the presence of a hillfort behind the slope and a small village at its toe, limiting the available solutions for rockfall protection. Finally, the combination of different remote sensing techniques (unmanned aircraft system topography and light detection and ranging) with a semi-automatic geostructural analysis has been successfully applied to obtain both a 3D point cloud of the wide area under study in addition to an estimation of mean block volume for rockfall simulations. Additionally, the design and implementation of a protective measure (rock dynamic barrier) have been addressed.

Composite Anchors for Slope Stabilisation: Monitoring of their In-Situ Behaviour with Optical Fibre

Composite anchors are special passive sub-horizontal reinforcements recently developed for remediation of unstable slopes. They are composed of a hollow steel bar, installed by a self-drilling technique in the soil, coupled with tendons cemented in the inner hole to increase the global anchor tensile strength. The anchors are primarily intended to stabilise medium to deep landslides, both in soils or weathered rock masses. Among the valuable advantages of composite anchors are their low cost, ease of installation, and flexibility in execution, as testified by a rapid increase in their use in recent years. The bond strength at the soil-anchor interface is the main parameter for both the design of these reinforcements and the evaluation of their long-term effects for landslide stabilisation. After a brief description of the composite anchor technology, this paper presents a novel methodology for monitoring the strain and stress accumulated in the anchors over time when installed in an unstable slope. The new monitoring system is composed of a distributed fibre optic sensing system, exploiting the optical frequency domain reflectometry (OFDR) technique, to measure the strain exerted on the optical fibre cable embedded with the tendons inside the bar. The system permits an evaluation of the axial force distribution in the anchor and the soil-anchor interface actions with a spatial resolution of up to some millimetres. Therefore, it allows determination of the stabilising capability associated with the specific hydrogeological conditions of the site. Furthermore, upon an extensive validation, the system may become part of a standard practice to be applied in this type of intervention, aimed at evaluating the effectiveness of the anchor installation and its evolution over time.