Multi-point Extensometers Research Articles

Damage assessment of High-Stress brittle surrounding rock masses in a deep Large-Span High-Sidewall underground Cavern: In situ measurement and numerical simulation

Excavation within high-stress brittle rock masses often encounters severe problem of structural damage in deep underground engineering, necessitating a significant scientific demand for accurately assessing the damage to surrounding rock masses through in-situ measurement and numerical simulations. This study reports detailed findings from an exhaustive investigation on the damage characteristics and underlying mechanisms in the high-stress brittle surrounding rock masses during excavation at the Shuangjiangkou (SJK) underground caverns. These findings were obtained through a comprehensive long-term in situ measurement using multi-point extensometer monitoring, borehole digital optical televiewer observing, and acoustic testing. Based on a deep understanding of the damage and failure mechanisms peculiar to the SJK high-stress brittle surrounding rock masses, a new damage index specifically designed for high-stress brittle rock masses was developed. This index is formulated by incorporating the failure approach concept into a modified Hoek−Brown criterion adapted for high-stress brittle rock masses. Comparison between the numerical predictions and in situ measuring results of the excavation damage zones during layered excavation of the SJK underground powerhouse revealed remarkable concordance. This consistency validates that the newly proposed damage index has good applicability and reliability for quantifying excavation-induced damage in high-stress brittle surrounding rock masses. These research findings are poised to provide valuable insights into the damage identification of brittle surrounding rock masses subjected to high-stress excavation, contributing to optimize excavation design and enhance the safety and sustainability of underground engineering projects.

Rock fracturing observation based on microseismic monitoring and borehole imaging: In situ investigation in a large underground cavern under high geostress

During the excavation of deep rock engineering projects under high geostress, the brittle failure of hard rock often occurs. To understand the excavation-induced fracturing process of the surrounding rock of a large underground powerhouse, in situ observation schemes have been proposed, including microseismic (MS) monitoring, imaging with a digital panoramic borehole camera, multipoint extensometer measurements and on-site surrounding rock failure investigation. First, a MS system is used to capture the microfractures inside the rock mass, revealing the potential high-risk failure regions. The spatial distribution of MS events shows that the excavation-induced MS events are progressively clustered in the upstream spandrel and downstream foot of the powerhouse, approximately parallel to the direction of the maximum principal stress. Second, the digital panoramic borehole camera and multipoint extensometer are arranged in the high-risk region to further reflect the rock mass fracture evolution and deformation response characteristics in the monitored section. The borehole camera observations of the top arch and sidewall indicate that the excavation unloading not only causes surrounding rock fracturing near the periphery of the cavern but also induces deep fracturing at a certain depth from the periphery of the cavern. From these comprehensive observation results, the mechanism of rock mass mechanical behaviour is clarified; that is, the macroscopic deformation or failure characteristics of the surrounding rock are directly related to the evolution of microfractures. The research results of this paper are valuable for the stability analysis and excavation optimization of similar underground caverns.

The field monitoring experiment of the high-level key stratum movement in coal mining based on collaborative DOFS and MPBX

The deformation and movement characteristics of high-level key stratums in overlying strata are important for estimating ground subsidence and understanding failure characteristics of ultrathick strata during mining. In this study, a distributed optical fiber sensor (DOFS) and multipoint borehole extensometers (MPBXs) were collaboratively employed to monitor the deformation of high-level key stratums in situ during the mining process at working face 130,604 of the Maiduoshan Coal Mine. DOFS monitoring results showed that the distance from advance influence of mining on the ground surface is 219.2 m. The deformation of the shallow stratums were greater and was affected earlier than that of the deep stratums. The deformation in the strata did not occur continuously and the boundary curve of the impact from advance mining was not a straight line with the advancement of the working face. By the MPBX technology, we measured the strata movement and obtained four-stage characteristics of high-level key stratum movement. The subsidence of the primary key stratum and the sub key stratum were monitored to reach 1389 and 1437 mm; their final relative displacement differed by 48 mm. No bed separation was observed in between the strata, and the key stratums tended to sink as a whole with the advancement of the working face. This research guides the analysis the movement of thick high-level key stratums.

Effects of groundwater exploitation and recharge on land subsidence and infrastructure settlement patterns in Shanghai

Abstract Land subsidence is a severe geo-hazard in coastal regions and represents a great threat to infrastructure. Based on long-term measurement data, including that for deep compaction from vertical multipoint extensometers, land subsidence and infrastructural settlement from leveling surveys, groundwater levels in multiple confined aquifers, and hydrogeological conditions, this study investigates the effects of groundwater exploitation and recharge on land subsidence and the settlement of infrastructure in Shanghai, China. Further, groundwater exploitation and the consequent responses of piezometric heads in the multiple confined aquifers of Shanghai soft deposits are discussed. Subsequently, regional land subsidence, local deep-soil deformation, and infrastructural settlement are analyzed in terms of groundwater exploitation. It is found that the subsidence pattern is related to the trend of groundwater exploitation and the variation of piezometric heads in the aquifers, which can be roughly divided into three phases. Subsidence funnels could occur, and could be interpreted according to the differences in groundwater exploitation intensity and the spatial variability of lithology. Since the towers of Nanpu Bridge are located within the area of the subsidence funnel, this could be the main reason for the differential settlement of the bridge.

Reinforcement effects and safety monitoring index for high steep slopes: A case study in China

High and steep slopes which have developed fractures and intercalations are a great threat to the operation of dams and reservoirs. In this work, the geological conditions and potential modes of failure of the slope found in the right bank of Suofengying hydropower station are investigated for the slope stability and the results are presented. In order to strengthen the slope, an innovative stabilization scheme is employed. The stabilization techniques include development of anti-shear tunnels, anti-slide piles, anchor cables, concrete support structure, etc. Further, the slope stability and reinforcement effects using various stabilization techniques are studied by using finite element strength reduction method. Moreover, in situ monitoring is carried out and the data obtained is analyzed. From the results, it is observed that the deformations that are detected using multipoint extensometers have decreased after the installation of remedial reinforcements. From the analysis of remedial reinforcement methods, it is found that the most critical reinforcement is the development of anti-shear tunnels. In order to monitor the stresses in stirrups and the propagation of cracks in the anti-shear tunnels, three levels of safety monitoring index are proposed. The safety monitoring index is developed based on the results obtained by the simulation of the process of failure of the reinforced slope. The developed safety monitoring index is further applied to the Suofengying project in order to evaluate the overall stability of the slope. The results obtained by monitoring indicate that the performance of the reinforcement structures is satisfactory and the slope has better stability. The methodology proposed in this work shall be useful for similar projects to obtain stability of slopes.

Experimental Study on Deformation Law of Soil Layer Induced by Undercut between Existing Twin Tunnels based on 3D Geomechanical Model

The underground excavation of subway station spaces between existing twin tunnels is vital for ensuring the normal flow of ground traffic during excavation, reducing costs, and protecting the environment. However, such excavation would induce the deviation of the existing tunnel segment due to the collapse upon soil layers. Based on model test theory, a large-scale geomechanical model test frame was conducted to investigate the soil layer deformation law. A high-precision multi-point extensometer and an earth pressure cell were embedded in the design position during the manufacture of the model stratum. All of the deformation and earth pressure data at each monitoring point were obtained during the whole test process, and these data were used to analyze the deformation law in three test phases: excavation and support of the station space, transverse tunnel excavation, and overload pressure exertion on the model top surface. Results showed that the deformation induced by the underground excavation space was small, and the excavation of the transverse tunnel and overload pressure on the top surface of the model were the main factors inducing the soil deformation. The maximum vertical displacement and horizontal displacement were 1.29 mm and 0.58 mm, respectively; as indicated by the geometric similarity ratio, the displacements of the corresponding points were 12.9 mm and 5.8 mm, respectively. The test also revealed that a pre-reinforced pile outside the shield tunnel could effectively limit the segment deviation and reduce the deformation of the outside soil mass. This method might be used in the excavation of underground station spaces under good geological conditions.

Comprehensive analysis of the surrounding rock mass stability in the underground caverns of Jinping I hydropower station in Southwest China

The geological conditions of the underground caverns in the Jinping I hydropower station are particularly complicated, with high ground stress (the extreme value of the measuring point is 35.7 MPa), complicated deformation and failure mechanisms, and large unloading depth. The large deformation of the surrounding rock mass was beyond current engineering and academic understanding. During the construction period, the deformation and failure mechanisms were complex, and the unloading depth was large. The maximum deformation of the sidewall of the main powerhouse reached 246 mm. The monitoring results of multi-point extensometers basically converged two years after the completion of cavern excavation. Regarding the deformation and failure mechanism of the large underground caverns, the energy dissipation theory that can better describe the complex mechanical response analysis was proposed. The deformation failure mechanism, mechanical behaviour, supporting effect and stability evaluation of the large underground caverns were investigated from the perspectives of excavation unloading and energy dissipation and release. The excavation unloading-induced relaxation characteristics of the surrounding rock mass were analysed using a comprehensive approach incorporating the results of borehole television, acoustic wave testing, multipoint extensometer and numerical simulations. A criterion of the surrounding rock mass delineation incorporating the energy dissipation ratio, acoustic wave testing, wave velocity variation and elastic modulus variation was also proposed. Furthermore, the phenomena and law of deformation and failure of surrounding rock mass in the large underground caverns were revealed. The stability of surrounding rock mass in the underground caverns was evaluated. The related achievements can provide some references for similar deep underground rock engineering.

Monitoring of Forces in the Active Anchorage System Installed in the Underground Structures According to Extensometer Data

The paper presents the method developed to evaluate acting forces in the active anchorage system based on the data provided by multipoint borehole extensometers installed in the chamber-type underground power house cavern of Rogun HPP (Republic of Tajikistan) and the results of its practical application. This method allows for quickly and reliably determination of the actual values of forces in uninstrumented anchors and evaluation of their remaining service life.

Deformation Features of Rock Mass Enclosing Underground Machine Hall of Rogun HPP at Final Construction Stage

This paper presents an analysis of observations of the displacement of the rock mass at the site of the underground machine hall of the Rogun hydropower plant during its completion. The main features of the displacement of the rock mass are considered and the advantages of using multipoint extensometers to observe the state of the rock mass are noted.

A method to identify the position of fracture surface in a rock slope based on multi-point extensometer monitoring

The multi-point extensometer is widely used to measure the displacement of geological bodies. In this work, we proposed a simple mathematical approach to directly identify the position of the major fracture surface in a slope based on the data from multi-point extensometers. By comparing the cumulative absolute displacement per unit length between two adjacent gauging points of each multi-point extensometer, the most likely zone of the fracture surface is determined preliminarily. Then, selecting any three adjacent multi-point extensometers across a fracture zone as the reference fracture surface, and assuming there is a same displacement catastrophe, thus the equations relating to the coordinates of the three intersection points between the multi-point extensometers and the fracture surface can be obtained according to the characteristic of displacement distribution. On the basis of the solved coordinates of the two endpoints of the reference surface, the coordinates of the intersection point between the fracture surface and the displacement extensometer are determined step by step. Finally, the entire fracture surface can be deduced by connecting all the solved intersection points. In addition, a real slope monitored by multi-point extensometers and simulated by the finite element software ABAQUS to verify the feasibility and rationality of the proposed method. The adaptations and limitations of this method are also discussed. The results show that the proposed method can effectively detect the position of the internal fracture surfaces in a slope.

A novel method for measuring traffic load-induced settlement at different layers of embankment in highway

A large number of high embankments have been constructed in highway engineering with the fast development of highway. The measurement and treatment of traffic loading-induced settlement is a research focus in recent years. This paper focuses on the measurement of the layered settlement, induced by the traffic loading, in the embankment of highway. A novel method to measure the traffic loading-induced settlement at different depths in the embankment is proposed. The practical measurement equipment, including a multipoint extensometer, rigid base, settlement detecting plate, protector tube and connecting rod, is developed. The field tests were carried out in the filling and surcharge period to verify the stability and accuracy of measurement equipment. The test results indicate that the developed equipment and the presented installation method can be capable of measuring traffic-induced settlement at different depths in the embankment of the operating highway.

Analysis of a Complex Flexural Toppling Failure of Large Underground Caverns in Layered Rock Masses

Flexural toppling failure is one of the most common instabilities in large underground caverns and occurs when the steeply inclined rock layers form a small angle to the cavern axis. This study investigates a typical flexural toppling failure in the underground powerhouse at the Wudongde hydropower station, Southwest China, using in situ tests and numerical tools. The characteristics of flexural toppling failure in the large underground caverns were initially analysed by field observations and conventional testing methods, including the use of multipoint extensometers, acoustic wave tests and borehole television images. A high-resolution microseismic monitoring system was installed in the right main powerhouse to reveal new and important insights into the mechanisms of typical failures that occur in bedded rock masses. The fracturing processes of the rock mass associated with the flexural toppling failure were analysed based on microseismic monitoring data, and the development of flexural toppling failure was related to shear failure. Then, the discrete element method was used to further investigate the effect of geological planes on the formation of flexural toppling failure. Finally, the complex mechanism of flexural toppling failure was evaluated by incorporating field observations, in situ monitoring and numerical modelling. An effective method of preventing this type of failure in underground caverns was analysed. The results can improve our understanding of flexural toppling failure in the underground caverns of the Wudongde hydropower station, and the proposed method can be adopted to study similar large underground caverns.

Large deformation evolution and failure mechanism analysis of the multi-freeface surrounding rock mass in the Baihetan underground powerhouse

In a large underground powerhouse, the surrounding rock mass at the intersection of the caverns has multiple free faces. This rock mass is more susceptible to stability issues because of its peculiar structure. This paper takes the Baihetan underground powerhouse as a case study example, wherein the failure issue of multi-freeface surrounding rock mass appears prominent. Through analysis of monitoring data from multi-point extensometers, it has been concluded that the average daily deformation of the main powerhouse sidewall on downstream side in the presence of multi-freeface rock mass proves larger than that on upstream side; the maximum deformation reaches 0.48 mm/d, overshadowing that at the rock anchor beam as well. The multi-freeface surrounding rock mass is not only more subject to large deformation but also shows more sensitivity to positional change in the working face. Moreover, the sidewall and rock anchor beam witness greater spatial deformation compared with the arch area from a spatial perspective, and from the point of time the time-related deformation presents a step growth, which tends to eventually converge. The failure mechanism is also discussed, where fiercer stress redistribution and denser cracks account for the greater deformation of the multi-freeface surrounding rock mass. In general, a wide range of underground powerhouse stability issues has arisen with respect to huge-scale and adverse geological conditions since the construction of the Baihetan Hydropower Station, and settling these issues will undoubtedly have milestone significance with regard to the construction and development of hydropower engineering projects among the worldwide.

Measurement and analysis of the internal displacement and spatial effect due to tunnel excavation in hard rock

It is very difficult to observe the deformation of the surrounding rock within a very short time after excavation in hard rock such as granite. This paper reports an in situ monitoring test of the internal displacement of the surrounding rock conducted using multi-point extensometers that were pre-embedded before excavation in the Beishan Exploration Tunnel (BET) in China's Gansu Province. The internal deformation characteristics of the surrounding rock throughout the whole tunnel excavation process were analyzed, and an analysis of the spatial effect of the tunnel excavation face and an estimation of the loss displacement were carried out with 3 typical longitudinal deformation profile (LDP) curve equations: the Lee equation, the Hoek equation and the Vlachopoulos and Diederichs (V-D) equation. The results show that under the geological conditions of Beishan granite, the internal displacement of the surrounding rock throughout the whole process of blasting excavation passes through 3 stages, including initial growth, rapid growth and stable convergence. The temporal curve of the internal displacement of the surrounding rock near a measuring section shows stepwise characteristics, which are similar to the morphological characteristics of a creep test curve for rock subjected to graded loading. Measuring points farther from the side wall of the tunnel are less affected by the spatial effect of the tunnel excavation face and experience a larger displacement release ahead of the tunnel face relative to the total displacement. The loss displacement calculated with the Hoek equation is closest to the measured value; thus, the Hoek equation is the optimal estimation equation. When the measuring section is located 2 m from the tunnel face, the loss displacement accounts for more than 50% of the total displacement.

Analysis of the stabilising effect of ribs of reinforced sprayed concrete (RRS) in the L\xf8ren road tunnel

The Løren tunnel is a road tunnel at Ring road 3 in Oslo, Norway. The tunnel has a length of 915 m in rock, has two tubes with three lanes and breakdown fields, and was first opened in 2013. For rock support in the case of weak rock masses, ribs of reinforced sprayed concrete (RRS) were used. The scope of this article is to present and analyse the results of a measurement programme carried out on three of these ribs. This is done by focusing on deformations in the rock and the support function of the ribs due to these deformations. The instrumented RRS had strain meters installed in the reinforcement and the concrete. From the surface above the RRS, multipoint borehole extensometers were placed to survey the soil and rock mass deformations caused by tunnel advancement. In addition, 2D and 3D rock stress measurements and rock property testing were conducted. The measurements and numerical modelling show that the deformations are too small to cause a considerable load on the installed support construction and that the 2D stress measurements seem to best fit the in-situ stress conditions. The rock mass quality in the area of this study is on the verge of where one usually starts using reinforced ribs. It is concluded that the RRS are not required because of deformations in the rock but, rather, because of the need to lock blocks, increase the friction in joints and prevent movement in larger filled joints. For this purpose, the RRS should probably be designed differently to get the most out of the materials used.

Design of a Displacement Monitoring System Based on Optical Grating and Numerical Verification in Geomechanical Model Test of Water Leakage of Tunnel

The displacement monitoring of surrounding rock is necessary in geomechanical model test. However, traditional monitoring technology is difficult to meet the needs of displacement monitoring in small geological model tests. To solve the problems mentioned above, the authors developed a multi-point displacement monitoring system based on optical grating including multi-point extensometer, grating scale and multi-channel data acquisition system. Firstly, 3D anchor head with six barbs is designed and connected to the grating ruler by the steel wire, which proved to be rather sensitive to the rock deformation. Additionally, the displacement data collected can be transformed into electrical signal, which can be obtained by multi-channel acquisition system. Finally, the system was used in the model test of tunnel water leakage. The designed anchors were pre-embedded in some key monitoring points around the section in order to investigate measurement of displacements in the lining during the loading of geostress and hydraulic pressure. Afterward, FLAC3D, the finite-difference method, is adopted to simulate the whole process of physical model test and to compare with the experimental results. The results show that the experimental data was in good agreement with the numerical simulation results. In conclusion, the multi-point displacement monitoring system based on optical grating has higher precision and can be widely used in the physical model test of geotechnical engineering.

Deformation Forecasting of Surrounding Rock Mass Based on Correlation between Frequency and Fracture Scale of Microseismicity

The macroscopic deformation and failure of engineering rock mass may occur as a result of evolution and breakdown of its internal microfracture. Therefore, the macroscopic state of rock mass can be obtained from fracture scale of microfracture in real time. To assess instability and predict macroscopic deformation and failure of engineering rock mass, a time‐frequency analysis technique based on S transform was proposed to investigate microseismic waveform and reveal the correlation between macroscopic deformation failure and microseismic frequency characteristics of engineering rock mass in combination with fracture scale. To minimize the influence of external factors on parameters calculated, a significant amount of microseismic data from three large‐scale hydropower projects in southwestern China was collected as the statistical sample. The analysis of correlation between fracture scale and frequency characteristics of microseismic events was carried out based on the statistical sample. Combining with microseismic data and multipoint extensometers in the underground powerhouse of the Houziyan hydropower station, engineering verification was conducted. The result shows that the high‐frequency components decrease and microseismic signals display low‐frequency characteristic as the fracture scale increases; the microseismic high‐frequency components decreased at first and then increased during the deformation process of surrounding rock mass, and the frequency of microseismic events shifts from high band to a lower one before deformation.

In Situ Observation of Hard Surrounding Rock Displacement at 2400-m-Deep Tunnels

This paper presents the results of in situ investigation of the internal displacement of hard surrounding rock masses within deep tunnels at China’s Jinping Underground Laboratory Phase II. The displacement evolution of the surrounding rock during the entire excavation processes was monitored continuously using pre-installed continuous-recording multi-point extensometers. The evolution of excavation-damaged zones and fractures in rock masses were also observed using acoustic velocity testing and digital borehole cameras, respectively. The results show four kinds of displacement behaviours of the hard surrounding rock masses during the excavation process. The displacement in the inner region of the surrounding rock was found to be greater than that of the rock masses near the tunnel’s side walls in some excavation stages. This leads to a multi-modal distribution characteristic of internal displacement for hard surrounding rock masses within deep tunnels. A further analysis of the evolution information on the damages and fractures inside the surrounding rock masses reveals the effects of excavation disturbances and local geological conditions. This recognition can be used as the reference for excavation and supporting design and stability evaluations of hard-rock tunnels under high-stress conditions.

Unloading performances and stabilizing practices for columnar jointed basalt: A case study of Baihetan hydropower station

The columnar jointed rock mass (CJR), composed of polygonal cross-sectional columns cut by several groups of joints in various directions, was exposed during the excavations of the Baihetan hydropower station, China. In order to investigate the unloading performances and the stability conditions during excavation of the columns, an experimental field study was performed. Firstly, on-site investigations indicated that the geotechnical problems, including rock relaxation, cracking and collapse, were the most prominent for the CJR Class I that contains intensive joint network and the smallest column sizes. Comprehensive field tests, including deformation measurement by multi-point extensometers, ultrasonic wave testing, borehole television observation and stress monitoring of rock anchors, revealed that the time-dependent relaxation of the CJRs was marked. The practical excavation experiences for the Baihetan columnar jointed rock masses, such as blasting scheme, supporting time of shotcrete and rock bolts, were presented in the excavations of the diversion tunnels. These detailed investigations and practical construction experiences can provide helpful information for similar geotechnical works in jointed rock mass.

A new mini-grating absolute displacement measuring system for static and dynamic geomechanical model tests

Abstract In this paper, a mini-grating displacement measuring system is developed for geomechanical physical model tests. This system consists of a portable multi-channel grating data acquisition instrument, a grating ruler, a micro multi-point extensometer, and other related accessories. The motherboard of the portable multi-channel grating data acquisition instrument uses a self-developed circuit board and all channels can collect high-speed and high-precision grating displacement signals. The micro multi-point extensometer has advantages of micro-flexibility and small interference in physical model tests. Embedded measuring points in tunnel surrounding rock connect with the grating ruler which is fixed on an independent frame outside of steel wires and connects to the data acquisition system with wire lines. The system is applied to a static and dynamic geomechanical model test on a tunnel. Testing results prove that the system can be satisfactorily applied in geomechanical model tests.