Rock Anchors Research Articles

Lateral damage mechanism of the double-track tunnel in the mountainous layered soft rock and NPR anchor cable control

Due to complex geological formations, lateral damage often occurs during excavation in mountainous layered soft rock double-track tunnels. This paper discusses the stresses and the damage characteristics of the surrounding rock under overload in a mountainous layered soft rock double-track tunnel through indoor model experiments to provide a basis for the effective control of lateral damage. The experiments show that the conventional support method cannot effectively control the lateral damage due to interlayer sliding. Therefore, the negative Poisson’s ratio (NPR) anchor/cable control method is proposed. And the scientificity and feasibility of the NPR anchor/cable control technology are proved by the field application and monitoring data of the Minxian Tunnel and Changning Tunnel. It is further demonstrated that high preload is the most effective way to control the lateral damage in layered soft rock tunnels.

Mechanical Behavior of Fully Grouted Rock Bolts in Hydraulic Tunnels Subjected to Elevated Ground Temperatures

In this study, the mechanical behavior of fully grouted rock bolts in hydraulic tunnels subjected to elevated ground temperatures was investigated. A differential equation for axial displacement of the rock bolt was formulated, which considers the force equilibrium of infinitesimal bolt segments and the stress transfer mechanism at the anchor–rock interface. The distribution functions for axial stress within the bolt and the interfacial shear stress were obtained by solving the differential equation, which incorporated the displacement of the surrounding rock mass as a parameter. This study showed that the effectiveness of the bolt–shotcrete support system decreases over time, considering the displacement relaxation rate of the surrounding rock mass. The mechanical model’s variation laws at 20 °C, 50 °C, and 80 °C were summarized by integrating the thermal deformation equation for material parameters, and the numerical simulation results were compared and analyzed. The findings revealed that the bond strength between the rock bolt and the rock mass diminishes as the temperature of the surrounding rock increases, leading to a reduction of interfacial shear stress at both extremities of the bolt. Moreover, the maximum axial force within the bolt escalates as the neutral point migrates farther from the tunnel wall.

Provenance of Bronze Age stone anchors, a case study from the Carmel coast, Israel

Many stone anchors from the Bronze Age period have been found throughout the Eastern Mediterranean. Previous research found that most stone anchors of the Middle Bronze Age (a.k.a. Byblian type) and of the Late Bronze Age (a.k.a. Uluburun type) were made from carbonate rocks and aeolianites (kurkar, in Hebrew; quartz sand cemented by calcite). These rocks are common along the Eastern Mediterranean coasts and cannot be easily distinguished into quarry sites on a micro-regional scale. While previous research utilized rock petrography to propose provenance of such anchors, we present a multi-disciplinary methodology that utilizes several lines of mineralogical and geochemical evidence. In this study, we report on preliminary results obtained from mineralogical, isotopic, and petrographic analyses conducted on 17 Bronze Age stone anchors from well-dated underwater assemblages found along the Carmel coast, northern Israel, as well as on an initial rock reference collection of carbonate rocks, kurkar, and beachrock outcrops from various coastal regions of Israel. Comparing the data of the reference rocks with stone anchors we show that all carbonate rock anchors were prepared from chalky limestone (rather than limestone or dolomite), with high likelihood that nine out of the 12 such anchors were produced from the Arqan Formation chalky limestone, from the Carmel ridge. One carbonate rock anchor may originate from a geological formation we have either not sampled along the Israeli coast or from a foreign origin. Other stone anchors were produced from kurkar or beachrock. Our findings suggest that these were produced from rocks from the Carmel coast and further north. We also identify-two procurement strategies for stone anchors: either as a unified set from a production center, or mixed and matched from different rock types and sources. This pilot study opens the possibility to analyze stone anchor provenance in the Eastern Mediterranean in more detail than before. Future research to enlarge the rock reference collection beyond the Israeli coast will provide further archaeological and anthropological insight into maritime activity during the Bronze Age.

Laboratory block model tests simulating rock anchoring in rock mass

Rock anchors are used to stabilise large scale infrastructures. Failure of rock anchors could threaten the stability and lead to severe economic and social consequences. The design of rock anchors is based on four principal failure modes, which are breakage of the steel tendon, tendon-grout interface failure, grout-rock interface failure and rock mass failure around the anchor. The current dimensioning methods are uncertain due to limited knowledge of the grout-rock bond strength and the rock mass failure. The dimensioning method against rock mass failure is based upon the weight of the overlying rock mass in an inverted cone and the shear strength of the rock mass along the inverted cone, which results in conservative design of rock anchors. A small laboratory model was developed to evaluate the rock mass failure of a rock anchor at maximum pullout load. The model contained small concrete blocks set in a staggered pattern representing the rock mass. The loading of the model showed that the load was transferred to the sides by rotation and lifting of the blocks, which created a small load bearing arch in each layer during loading. This resulted in the model being able to carry much higher loads than what was calculated with the dimensioning methods. Back calculation of the results from the physical tests in UDEC showed the same arching effect in each layer during loading. The testing has improved the knowledge on the concept of how the load transfer from a rock anchor to the rock mass.

Numerical modelling and field monitoring study on large-span tunnelling using pretensioned bolt–cable combined support system

This study focused on a new support method, a pretensioned bolt–cable combined support system, for large-span tunnelling. The Badaling tunnel is a high-speed railway station tunnel that has a span of 32.7 m, a cross-sectional area of 494 m2, and a maximum depth of 102 m. The super large cross-section and unfavorable geological conditions around the tunnel resulted in considerable difficulties for tunnel construction. Because this tunnel was successfully excavated using the pretensioned bolt–cable support, the mechanical characteristics of this new support system are analyzed in this study. First, the necessity of a bolt–cable support for large-span tunnelling was analyzed using a numerical model. The deformation characteristics of rock mass in the large-span tunnel are discussed based on the modelling results of the tunnelling process. Second, based on field monitoring results, the mechanical behaviors of pretensioned rock bolts and anchor cables were analyzed. The anchoring forces of the rock bolts and anchor cables were determined by the pretension forces, loss of pretension forces, and passive support effects. The development of the anchoring force can be divided into three phases: rapid loss of the pretension force, a fluctuation phase, and a stable phase. Finally, The loading effect and mechanical responses of the primary lining were analyzed according to the monitoring results of the primary lining, which indicated that the rock mass was effectively reinforced by the pretensioned bolt–cable system and that the primary lining was safe.

Large Deformation Mechanism of Foliated Rock and NPR Anchor Cable Support Technology in the Changning Tunnel: A Case Study

Large Deformation Mechanism of Foliated Rock and NPR Anchor Cable Support Technology in the Changning Tunnel: A Case Study

Geochemical provenance of an Indo-Arabian stone anchor from Manikapatna highlights the medieval maritime trade of India

India is one of the oldest maritime nations in the world, and the overseas contacts date back to the third millennium BCE. Besides several archaeological vestiges, numerous stone anchors of various types have been documented during maritime archaeological explorations along the Indian littoral. During a recent maritime archaeological exploration, a broken Indo-Arabian stone anchor, of the Medieval period, was discovered along the Manikapatna coast of Odisha, Indian eastern littoral. In an attempt to determine the provenance of the anchor, we carried out a detailed petrographic, geochemical (major/trace elements) and Sr–Nd isotopic investigation. The results of our study reveal that the stone of the anchor had been cut out of a geologically young, vesicular, subalkalic basalt lava flow. Source fingerprinting done using petrographic, geochemical and isotopic data, suggests that contrary to the general perception, the anchor rock did not come from any local rock formations. All data point to the most likely scenario that the anchor rock was sourced from one of the lava flows of the Deccan Traps at Palitana in the Saurashtra region of Gujarat, western India. This result confirms the existence of Medieval maritime trading between western and eastern Indian states.

Small Block Model Tests for the Behaviour of a Blocky Rock Mass Under a Concentrated Rock Anchor Load

Rock anchors are used to stabilise large-scale infrastructure such as the foundations of high buildings or road cuts, and their failure could lead to severe economic and social consequences. A rock anchor can fail in one or more of the following four modes; breakage of the tendon steel, tendon-grout bond failure, grout-rock bond failure and rock mass failure. In current rock anchor design, the knowledge for the first two failure modes is satisfactorily known and is relatively easy to test. The scientific background for the last two failure modes still has a potential of improvement and further development of the design criteria is needed. To help fill this gap, a series of block model tests were conducted on a small-scale test apparatus to evaluate the load capacity of a blocky mass under a rock anchor load. Digital image correlation (DIC) was used to monitor the full-field displacements in the block models. The tests showed that load arches were formed in the block layers and the anchor load was transferred to the side frames of the apparatus through these arches. The load capacity of the mass increased with the number of the block layers and the confinement. The observed load arches and the effect of the confinement were also captured in a numerical model using UDEC software. The DIC monitoring showed that the displacements in the block model were identical along a vertical line. At a given depth, the displacements decrease with the distance to the centre line of the anchor.

Investigation of the Bearing Characteristics of Bolts on a Coal–Rock Combined Anchor Body under Different Pull-Out Rates

In order to reveal the influence of the pull-out rate on the load-bearing properties of the coal–rock combined anchor body, the mechanical properties and failure characteristics of a coal–rock combined anchor body under different pull-out rates (10, 20, 30, 40, 50 mm/min) were studied using the pull-out test and theoretical analysis. The results show that the bearing capacity of the bolt on the coal–rock combined anchor body improves under a dynamic load, but the load-bearing properties of the coal–rock combined anchor body are different from those of the full rock (coal) anchor body. With the increase in the pull-out rate, the maximum pull-out load of the bolt on the coal–rock combined anchor body increases first, then decreases, and finally tends to be stable. Under the condition of a low drawing rate, the bearing capacity of the coal–rock combined anchor system can be greatly improved, but when the pull-out rate exceeds 20 mm/min, the bearing capacity of the anchor system is reduced. The debonding process of the anchoring section of the coal–rock combined anchor body gradually expands from the beginning section of the anchor to the bottom of the borehole. The coal–rock combined anchor body undergoes time differential development of cracks, and the failure of the coal and rock mass occurs at different times. Its failure process can be divided into three stages: (1) the coal anchor and rock anchor act together; (2) the rock anchor acts alone; and (3) the coal anchor and rock anchor have residual action.

Going Deep to Anchor a Pump Station

A geotechnical contractor addressed challenges and finished a construction project on schedule by being prepared with the right equipment, the right strategy, and the right team. The company also thoroughly tested its work—rock anchors, in this case—to ensure the job was a success.

Mechanical Behaviour of Anchored Rock Containing Weak Interlayer under Uniaxial Compression: Laboratory Test and Coupled DEM–FEM Simulation

The reason for instability in a rock mass with a weak interlayer is not only the sliding failure of the rock interlayer structural plane but also the tip crack propagation caused by the stress concentration at the tip of the interlayer. In this study, a uniaxial compression test of an anchored rock mass with a weak interlayer was carried out to determine the influence of the anchorage on the failure pattern and the strength of samples with different interlayer dip angles. In addition, the coupled DEM–FEM numerical simulation method was used to study the internal stress evolution of the sample and the stress distribution of the anchor under the anchorage effect. The results showed that the anchorage effect on reinforcement and strength enhancement was greatest for the sample with an interlayer dip angle of 30°. Under the anchorage effect, crack initiation was limited and there was more shear failure in the samples. The reinforcement range of the anchorage effect for anchors with restrained ends was larger than for anchors with free ends. When the rock–anchor interface was unbonded, the effect of the free-ends anchor reflected the residual friction, but the restrained-ends anchor still worked by limiting the lateral expansion of the rocks. The stress values and deformation of the anchors decreased gradually with an increase in dip angle.

Corrosion Investigation of Rock Anchors Served over 10 Years in Underground Powerhouse of a Hydropower Station

With the wide application of prestressed anchors in high slopes, underground structures, and bridge projects, the corrosion problems of the anchor structure during the long-term service have demanded more attention. Because of the environmental impact and structural characteristics, anchor cable corrosion is almost inevitable. To obtain the corrosion status of the anchors that have served over 10 years in the underground chambers of a power station, a field investigation was performed. The environment aggressivity was evaluated, and the anchor corrosion was inspected during the investigation. The results show that the effect of groundwater and stray current on anchor corrosion is slight. The main cause of the severe corrosion of the anchor heads is the high temperature and humidity in the chamber. To better evaluate the corrosion tendency and corrosion degree of the anchor head, a corrosion rating method was proposed based on the principles of existing investigation methods and the actual corrosion situation of anchors observed on-site. It was proved that the rating method can effectively reflect the corrosion characteristics and determine the areas prone to corrosion. This field investigation suggests that a regularly scheduled investigation is of great significance and meaning for accurately obtaining the corrosion status, assessing the failure risk, and issuing an early warning.

Seismic anisotropy of Opalinus Clay: tomographic investigations using the infrastructure of an underground rock laboratory (URL)

Seismic anisotropy and attenuation make claystone formations difficult to characterize. On the other hand, in many geotechnical environments, precise knowledge of structure and elastic properties of clay formations is needed. In crystalline and rock salt underground structures, high-resolution seismic tomography and reflection imaging have proven a useful tool for structural and mechanical characterization at the scale of underground infrastructure (several deca- to hundreds of meters). This study investigates the applicability of seismic tomography for the characterization of claystone formations from an underground rock laboratory under challenging on-site conditions including anisotropy, strong attenuation and restricted acquisition geometry. The seismic tomographic survey was part of a pilot experiment in the Opalinus Clay of the Mont Terri Rock Laboratory, using 3-component geophones and rock anchors, which are installed 2 m within the rock on two levels, thus suppressing effects caused by the excavation damage zone. As a source, a pneumatic impact source was used. The survey covers two different facies types (shaly and carbonate-rich sandy), for which the elliptical anisotropy is calculated for direct ray paths by fitting an ellipse to the separated data for each facies. The tomographic inversion was done with a code providing a good grid control and enabling to take the seismic anisotropy into account. A-priori anisotropy can be attributed to the grid points, taking various facies types or other heterogeneities into account. Tomographic results, compared to computations using an isotropic velocity model, show that results are significantly enhanced by considering the anisotropy and demonstrate the ability of the approach to characterize heterogeneities of geological structures between the galleries of the rock laboratory.

Study on timing sequence control fracture blasting excavation of deep rock masses with filled joints

During the blasting excavation of deep underground caverns, the effects of the structural surface on crack propagation are usually considered in addition to the clamping effects of high in situ stress. Based on the notched borehole and timing sequence control (TSC) fracture blasting method, this paper studies the effects of different borehole shapes on the degree of damage of the surrounding rock and profile flatness of the rock anchor beams and the effects of different filled joint characteristics on the blasting crack propagation rules. The results show that the damage depth of the surrounding rocks by round hole smooth blasting is approximately twice that by notched hole smooth blasting, by which the profile formed is flatter. The notched primary borehole (PBH) remains a strong guidance for crack propagation in a rock mass with filled joints, while the stress concentration effects of the round target borehole (TBH) cannot fully guide the cracks until they fall within a certain distance between the PBH and TBH. It is favourable for cracks to propagate along the lines between boreholes with larger filled joint strengths and larger angles between boreholes.

New system for investigating the corrosion of existing rock anchors

In this study, the characteristics of corrosion were described on the basis of previous findings on the corrosion of rock anchors, and various factors causing corrosion were explored. A corrosion investigation system of rock anchors was established by summarizing various methods for the detection of corrosion and influencing factors. In this system, corrosion was investigated by detecting and assessing the environmental influencing factors, analyzing the corrosion rate and morphological characteristics, testing the anchoring structure, and evaluating the causes, development, and prediction of corrosion. On-site corrosion cases were also examined. Results verified that this system is reasonable, effective, and practical.

Study on unloading of overloaded prestressed anchors in an underground cavern complex

The influence of unloading of overloaded prestressed anchor cables in underground caverns on overall stability is discussed in this paper, including the theoretical background on the reinforcement effect of anchor cables, and quantitative estimates from numerical modeling of the influences of anchor cable force unloading on rock mass deformation and forces in the other adjacent support systems. The numerical model shows the same results as the recording during the site unloading test: unloading the force of overloaded anchors by about 50-150 tonnes has limited influence on rock deformation, stress adjustment, the depth of the plastic zone in the rock mass, and the forces on the adjacent rock bolts and anchors. From the perspective of the overall stability of the underground caverns, unloading the force of the overloaded anchors can be another solution in addition to replacing the overloaded anchors with new ones to meet the requirements of the design standard.

Design and Economy of Rock Anchor Foundation for Transmission Line

Design and Economy of Rock Anchor Foundation for Transmission Line

The velocity of roof deflection as an indicator of underground workings stability – Case study from polish deep copper mines

The paper presents the research results of a study concerning the recognition of roof instability mechanism in the conditions of the Legnica-Głogów Copper Belt (LGCB), in which the final mining phase encompasses roof collapse. Due to the high potential risk of this type of phenomenon to the health and safety of mine operators it was decided to launch a roof stability assessment study in order to identify timely preventive measures adequate for this hazard.The approach is based on taking measurements of the changes of roof angles of inclination near the sidewalls of the mine working using high-sensitivity inclinometers fixed to the end of rock anchors in the roof. In this paper, it is shown that this method works well as it captures the natural bending phenomena of the roof prior to its collapse. It is found that the observed roof bending deformation is predominantly the result of combined gravity force and in-situ horizontal stress, regardless of the presence of other factors.Field measurements indicate that the roof collapse is preceded by a characteristic process covering three specific phases of development: these are described in detail. A new criterion for the recognition of imminent roof destabilization was developed based on the analysis of collected field measurements.

Analysis on Deformation and Stability of Tunnel Anchor Excavation under Broken Rock Mass

Based on the concrete engineering support of the tunnel anchor of the Dadu River Bridge on the Sichuan-Tibet Railway, the geological background of broken rock mass, extremely developed joints and fissures, and poor self-stability of surrounding rock within the range of side anchors in Chengdu, a three-dimensional elastoplastic numerical calculation method is used. The current engineering geological conditions and the tunnel anchor design plan are to establish a three-dimensional fine numerical simulation model of the Chengdu side tunnel anchor that comprehensively considers rock mass unloading, stratum classification, and anchor tunnel excavation and support structure. The “excavation-support” process in each construction footage is simulated to realize the three-dimensional calculation of the dynamic construction and support of anchor tunnels, and to study the surrounding rock deformation, stress and plastic zone distribution changes during the excavation process. Comprehensive analysis found that the deformation of surrounding rock during excavation shows that it develops toward the empty surface, and the displacement distribution is obviously affected by the distribution of ground stress and rock formation. The greater the buried depth, the greater the deformation of the surrounding rock.

Research on Cumulative Damage Characteristics of Rock Anchor Beam Concrete Supporting Structure by Blasting Vibration of Underground Powerhouse

Aiming at the dynamic disturbance problem of the upper rock anchor beam concrete support structure caused by the blasting excavation of the lower rock mass of the underground powerhouse of the hydropower station, the dynamic finite element software was used to numerically simulate the cumulative damage characteristics of the rock anchor beam concrete structure under multiple blasting, which aims to study the effect of blasting times and blast center distance on the cumulative damage characteristics of rock anchor beam concrete structures in two directions at different ages. The results show that 7 days after the completion of the concrete pouring of rock anchor beam, the damage and destruction effects are produced under the single blasting action. 28 days after the completion of the concrete pouring, there is basically no damage and destruction under the action of five times of blasting. 14 days after the completion of the concrete pouring, the growth process of the cumulative damage effect of the rock anchor beam concrete structure under the disturbance of the blasting excavation of the underground powerhouse shows nonlinear increasing characteristics with the increase of blasting times and nonlinear decreasing characteristics with the increase of blasting center distance. The cumulative damage of rock anchor beam along and perpendicular to the axis of powerhouse conforms to the relationship of cubic polynomials with the blasting times, which also conform to the relationship of quartic polynomial and quadratic polynomial, respectively, with the blast center distance.