Rock Anchors Research Articles

Open pit slope deformation monitoring by fiber Bragg grating sensors

With microstrain resolution and the capability to sample at rates of 2000 Hz or higher, fiber Bragg grating (FBG) strain sensor offers exciting new possibilities for in situ deformation monitoring induced by blasting load in an open pit slope. Here, we are developing a new technology for measuring deformation in real time on the microstrain in an open pit slope during the blasting. A fiber optically instrumented rock mass strain sensor measured strain at 100-cm intervals along a two anchor rock bolt grouted in the slope intact rock mass. In field testing, a number of transient signals have been observed, which in some cases were large enough to trigger rapid sampling. The combination of short- and long-term observation offers new insight into the slope stability and blasting cumulative effects. Therefore, FBG sensors are a useful tool for measuring in situ strain in intact rock masses.

Experiment Research of Rock Anchor Foundation on Sandstone Ground in Transmission Line

Rock anchor foundation can be widely used for the practical transmission line engineering in mountain areas. On strong-weathered sandstone ground, ten sets of uplift static load test of single rock anchor foundation were carried out. Based on the testing results, the typical failure modes and the ultimate uplift bearing capacity of foundations were obtained. Internal force distribution and effective anchorage depth were investigated through stain measurements. Design parameters of rock anchor foundation on sandstone ground were proposed, which can provide a reference for transmission line engineering design.

Failure Mechanism of Unbonded Prestressed Thru-Anchor Cables: In Situ Investigation in Large Underground Caverns

Prestressed rock anchor cables are commonly utilized in geotechnical and mining engineering (Peliua et al. 2000; Tezuka and Seoka 2003; Koca et al. 2011) because their installation increases the effective strength and stability of the reinforced rock (Maejima et al. 2003; Li et al. 2012). Nevertheless, in some cases, prestressed cables have failed in large underground caverns or tunnels because of excessive deformation of the anchored rock mass or inadequate assumptions used when designing the cables (Li 2004; Lu et al. 2011; Gong et al. 2011). During the failure process of anchor cable, the stress redistribution and progressive deterioration of the reinforced mass rock can adversely affect the overall stability of the free surface, manifested as large deformation or indeed collapse of rock mass (Galvez et al. 2006; Zhu et al. 2010). Therefore, a deeper understanding of the mode and mechanism of prestressed anchor cable failure will better inform the design process of anchor cables to mitigate future failures. Prestressed anchor cables can be categorized into three general groups based on the anchoring method (Jarred and Haberfield 1997; Chen and Yang 2004): (1) tip-grouted anchor cables that have grout-bonding segments and free segments; (2) fully grouted anchor cables with anchor wires that are fully bonded with the rock; (3) prestressed thru-anchor cables that have two anchor bases without grout-bonding segments. The tip-grouted and fully grouted prestressed cables have been the subject of research more than the thru-anchor cables because they have more extensive applications (Spang and Egger 1990; Hyett et al. 1995; Serrano and Olalla 1999; Huang et al. 2002; Cai et al. 2004; Ugur et al. 2011). Even less attention has been paid to the unbonded prestressed thru-anchor cables (UPTACs) based on the limited information available in the academic and professional literature. The mechanical interactions of UPTACs are different from those of groutbonding cables because the UPTAC does not have a groutbonding section, but has two anchor bases. Under loading, the prestressed cable can restrain the deformations of the anchored rock, and the anchored rock can also transfer the rock stress to the cable via the bridge of anchor bases. This paper focuses on evaluating the failure mechanism of UPTACs based on a case study of underground caverns in Sichuan Province, China. Several external failure modes of the UPTACs observed in this project are first presented and special design techniques for the UPTACs in the large underground caverns are summarized based on in situ investigation: failure depth of disabled UPTACs, break face, measured working load and installation method.

Beam–solid contact formulation for finite element analysis of pile–soil interaction with arbitrary discretization

SUMMARYThis paper presents an embedded beam formulation for discretization independent finite element (FE) analyses of interactions between pile foundations or rock anchors and the surrounding soil in geotechnical and tunneling engineering. Piles are represented by means of finite beam elements embedded within FEs for the soil represented by 3D solid elements. The proposed formulation allows consideration of piles and pile groups with arbitrary orientation independently from the FE discretization of the surrounding soil. The interface behavior between piles and the surrounding soil is represented numerically by means of a contact formulation considering skin friction as well as pile tip resistance. The pile–soil interaction along the pile skin is considered by means of a 3D frictional point‐to‐point contact formulation using the integration points of the beam elements and reference points arbitrarily located within the solid elements as control points. The ability of the proposed embedded pile model to represent groups of piles objected to combined axial and shear loading and their interactions with the surrounding soil is demonstrated by selected benchmark examples. The pile model is applied to the numerical simulation of shield driven tunnel construction in the vicinity of an existing building resting upon pile foundation to demonstrate the performance of the proposed model in complex simulation environments. Copyright © 2014 John Wiley & Sons, Ltd.

Improved Geometric Design of Earthquake-Resistant RC Slender Structural Walls. I: Parametric Study

AbstractIn multistoried RC wall-frame buildings, properly designed and detailed RC slender structural walls significantly improve earthquake resistance. In walls on isolated spread footings with marginal taper, severe stress concentration is observed at the wall-footing junction during earthquake shaking. In this paper, new tapered configurations are proposed in the bottom portion of walls with and without enlarged boundary elements. Analytical correlations are derived among salient structural and soil parameters of the tapered wall-footing. An extensive parametric study is carried out through linear-elastic finite-element analysis of an isolated wall-footing system under estimated actual vertical and lateral forces. Under the estimated forces, significant loss of contact is observed at the bottom of the wall-footing; thus, soil or rock anchors need to be provided to ensure stability of the wall-footings during strong shaking. Force flow from wall to footing improves significantly in the proposed integrat...

Stress State Analysis on a Rock Anchor RC Foundation of Wind Driven Generator

The stress state of a rock anchor reinforced concrete (RC) foundation of a wind driven generator located in the new constructed wind power station in Guodian Tieling Development Zone of China is numerically analyzed in this paper by using the large general-purpose finite element analysis (FEA) software ABAQUS, to discuss the mechanical characteristics of the rock anchor RC foundation, and validate the safety of normal use of the rock anchor foundation, and compare the developed rock anchor foundation with the traditional gravity one. The results show that the new-type RC foundation has greater advantage than the traditional one and it can completely replace the traditional gravity foundation in the condition of rock soil environment in the future.

터널출입구 시공에 따른 석회암 사면의 안정성 및 거동 분석

석회암 사면에서 터널 출입구를 시공하기 위하여 진입로를 개설하던 중 사면붕괴가 발생되었다. 사면의 붕괴원인을 조사하기 위하여 현장조사, 실내시험 및 수치해석을 수행하였다. 수치해석결과에 따르면 터널 출입구 진입로 개설이전 사면안전율은 1.0이하이며, 진입로 개설이후 사면안전율은 더 저하되는 것으로 나타났다. 그리고 진입로 개설이후 전단변형률 및 소성영역은 사면상부에서 하부로 전이되므로 사면활동영역이 증가되어 사면안정성을 저하시키는 것으로 해석되었다. 터널 출입구 시공을 위해서는 해당사면에 대한 안정성을 확보하여야 하므로, 대상현장의 조건을 고려하여 록볼트, 록앵커 및 FRP 그라우팅을 사면에 보강하는 것으로 결정하였다. 적용된 사면보강공법의 효과를 확인하기 위하여 현장계측을 수행하였다. 현장계측결과 사면지반의 수평변위는 매우 미소하며, 대부분 발생된 이후 다시 회복되는 탄성거동을 보였다. 그리고 록볼트 및 앵커축력은 터널굴착작업 및 장마기간에 의한 영향보다는 사면굴착작업시 영향을 가장 크게 받으며, 터널굴착작업이후 수렴하는 경향을 보이므로 대상사면은 안정한 상태임을 확인할 수 있다. A calcareous rock slope failed during excavation of the slope for construction of a tunnel entrance. The slope is located at the construction site for widening highway in Yeongwol, Korea. Field surveys, laboratory tests, and numerical analyses were performed to determine the reason for the slope failure. The numerical analysis revealed that the safety factor of the slope before construction of the entrance was less than 1, and that this decreased after construction. After construction of the entrance, the sliding zone of the slope increased and slope stability decreased because the shear strain and plastic zone in the slope over the tunnel entrance showed an increase relative to the lower part of the slope. To enhance the stability of the slope for construction of the tunnel entrance, countermeasures such as rock bolts, rock anchors, and FRP (Fiber glass Reinforced Plastic) grouting were adopted in light of the field conditions. Serial field monitoring performed to confirm the reinforcing effects of the adopted countermeasures revealed a small amount of horizontal deformation of the slope soils, most of the elastic deformation that can regain its former value. In addition, the axial forces of the rock bolt and anchor were more strongly affected by slope excavation during construction of the tunnel entrance than by tunnel excavation or the rainy season, and the axial forces tended to converge after excavation of the tunnel. Therefore, we can confirm that the slope is currently safe.

Stabilization of the limestone escarpment of the Skete of Osios Nikanoras under wet conditions due to future filling of the Ilarion dam's reservoir.

The Skete of Osios Nikanoras is located in a cavern of a vertical limestone escarpment 80m high, over Aliakmon River in West Macedonia. The vertical slope is structured predominantly of thick-bedded limestone, dipping moderately towards NW and only locally of thin-bedded limestone. It is crossed by three transverse and four normal faults. Rock mass was divided in sections, demarcated by bedding planes and major discontinuities. Each section was subdivided into rock blocks. Rock mass description was carried out in areas below the entrance level and the surrounding areas of the Skete as well. The future water level rise, due to the filling of the Ilarion Dam’s reservoir, followed by water level fluctuations, will impose an initial increase and a subsequent variation of water pressures in the rock mass discontinuities. The evaluation of the rock mass stability, due to water induced loads, was therefore required. Stability analysis has been performed in order to determine potential wedge, plane and toppling failure modes. The measures that were adopted for the conservation of the monument were designed taking into account the restrictions regarding the monuments preservation. The proposed solution comprised three stages of installation of support measures, namely, installation of temporary rock bolts for the pre-strengthening of the rock mass, followed by the execution of grouting and finally installation of permanent rock anchors, thus ensuring adequate factors of safety.

Bearing Capacity Predicting Method on Mortar-Rock Interface of Compression Type Rock Anchor

To predict bearing capacity on mortar-rock interface of compression type rock anchor, the failure model of fixed anchor length is established. The maximal shear stresses on the interface in front of the bearing plate are determined based on tests. The relation of maximal shear stress and fixed anchor length is assumed. Double exponential function curves are adopted to simulate the ultimate state of the fixed anchor length. Two parameters are introduced to adjust the curve shape. According to the results of the field test, the values range of the parameters in soft rock is determined. After comparing ultimate bearing capacity measured in test with predicting value, it is shown that the empirical formula can calculate the bearing capacity of fixed anchor length reasonably. Compression type rock anchors have more capacity than ordinary tension type anchors and the capacity is convergent gradually with fixed anchor length increase.

Field and Laboratory Investigation of Pullout Resistance of Steel Anchors in Rock

AbstractThis paper presents a field and laboratory investigation of pullout resistance of steel anchors in rock. The field testing involved pullout tests of six steel anchors installed in rock by gravity grouting and pressure grouting. The laboratory testing involved unconfined axial compression tests of six grout specimens prepared in custom-made molds using the two grouting methods to investigate the effect of grouting method on the mechanical properties of grout. The pressure-grouted specimens had a higher density and compressive strength, which was a result of their denser microstructure with significantly fewer and smaller voids from air bubble inclusions than the gravity-grouted specimens. The field pullout tests suggest a progressive failure mechanism, which was manifested through the progressive elongation of the bond zone. The pressure-grouted anchors yielded higher pullout resistance than did the gravity-grouted anchors. The increase in rock-grout bond strength from pressure grouting is likely d...

Estimating Pullout Loads for Hill-Embedded Rock Anchors

AbstractBased on classical mechanics, a mathematical model is formulated to predict the pullout load of an expanding steel anchor embedded in solid rock on the side of a hill. Thirteen anchor/rock system parameters are identified. Results are illustrated with a case study involving a rock anchor that failed while giving partial support to a line of utility poles subjected to high winds.

Coupled Supporting of Rock Bolt and Anchor Cable in Large Cross-Section Roadway

With development of coal mining technology and equipment, the coal roadway section size increases gradually, making roadway more difficult to support. In the paper, the supporting of large cross-section roadway in Hedong mine is studied and roadway excavation supporting process is simulated by FLAC3D. Coal roadway deformation and the problem of original supporting scheme are analyzed including displacement field, stress filed and plastic zone. Several supporting optimizations are proposed combined with site investigation and simulation analysis. And the optimizations are simulated and calculated. By comprehensive comparison, the optimal supporting scheme is obtained and some supporting law in large cross-section roadway is concluded. These are helpful for supporting design in future.

Stability Analysis of Underground Caverns by Jointed Finite Element Method

The jointed finite element method (JFEM) is used to analyze the deformation and failure characteristic of fractured rock mass and anchor reinforcement effect for the water collection shaft of the main power house of Dagangshan Hydropower Station. The results show that the JFEM not only simulates the actual rock mass structure very well, but also gives the reasonable simulation results for the common unstable rock mass. The JFEM may accurately simulate the major deformation characteristic and collapse mechanism, which is another effective way to analyze the stability of fractured rock mass. The on-site monitoring results show that the anchor reinforcement is effective for the water collection shaft

The Numerical Analysis on Stability of Deep Roadway Support System

The effect of the roadway with no supporting and roadway that was reinforcement by rock bolts and anchor robs, anchor rod combined bolting shoring, anchor rod combined anchor rope and lining timbering was simulated by using numerical simulation software FLAC3D. The results showed that the deformation of no supporting roadway is maximal, and the maximal position is vault. The displacement of top floor and two side decreases after supported, which can reinforce surrounding rock. The reinforcement effect of the anchor rod combined anchor rope and lining timbering is best.

Improved Geometric Design of Earthquake-Resistant RC Slender Structural Walls. II: Design Implications

AbstractIn a companion paper, new tapered configurations are proposed of slender RC structural walls with and without enlarged boundary elements at the wall-footing junction region. On the basis of identified parametric limits and the wall response in linear-elastic finite-element analyses, a stepwise seismic design procedure is proposed of tapered integrated wall-footing systems with soil or rock anchors at the bottom. This incorporates a capacity design of the plastic hinge region above the tapered portion and an elastic design of the tapered portion. The location of the region of seismic damage and energy dissipation in the wall is controlled by proportioning of the tapered wall-footing as per the new design procedure.

Experimental Research on Interfacial Bond Performance in Rock Anchor with High-Performance Grouting Medium

There exist the problems such as low bond strength and bad durability in the ordinary grouting slurry of the ground anchor system at present. The high-performance grouting mediums RPC (Reactive Powder Concrete) and DSP (Densified Systems containing homogeneously arranged ultrafine Particles) would become the potential replacement of grouting medium in ground anchor resulting from their high compressive strength, durability and toughness. Based on a series of pull-out tests on ground anchors with different high-performance grouting medium of RPC and DSP , different bond length in the construction field, the bond performance on the interfaces between anchor bolt (deformed steel bar) and grouted medium as well as between grouted medium and rock mass was studied. The results indicate that the interfacial bond strength between RPC or DSP and deformed steel bolt ranges within 23-31Mpa, far greater than that (about 2-3MPa) between the ordinary cementitious grout and deformed steel bar. Even though the interfacial bond strength between the grouted medium and rock mass of limestone was not obtained in the test since the failure mode was pull-out of those steel bar rather than the interface shear failure between grouted medium and rock mass, the bond stress on the interface reached 6.2-8.38 MPa, also far greater than the bond strength (about 0.1-3MPa) between the ordinary cementitious slurry and rocks.

High-Performance Grouts for Rock Anchor

The mix proportion of DSP for the sake of rock anchor grouting has been studied, and the effects of water-binder ratio, volume of nanometer silica added in and various curing methods on the behaviour of the grout have been investigated. Based on the test results, a appropriate proportion of DSP grout with good working and mechanical performance was successfully found.

The investigation and design for a unique architectural space – The Chillida Cavern, Mount Tindaya, Fuerteventura

The late Spanish artist, Eduardo Chillida, proposed the creation of a large subterranean sculpture within Tindaya Mountain, on the island of Fuerteventura, Canary Islands. The sculpture consists of a 15m span entrance tunnel to a cavern (65m long, 49m wide and 45m high) and two rectangular light shafts (20m×20m and 21m×30m). The two light shafts are orientated to capture the rotation of the sun and moon as they cross the mountain and provide the only light in the cavern. The entrance tunnel would provide a view to the sea and western sky from within the cavern.The sculpture would have a flat roof and slightly inclined walls. The excavated surfaces are to be formed with flat, high quality finishes, with no visible support. This results in a unique engineering challenge, whose construction would push the limits of cavern design and construction.In order to realise the artist’s concept, the design comprises the formation of a series of reinforced rock arches within the rock mass above the cavern roof and pre-support of the cavern roof with rock anchors fixed to the arches. The arches would be created with rock anchors radiating around the tunnels which would also provide access for long term maintenance, monitoring instrumentation and drainage. The reinforced rock arch tunnels would be formed from the light shafts prior to the main cavern excavation. The rock anchors supporting the cavern roof would be installed in predrilled holes as they are exposed during cavern excavation.The cavern would be constructed within the trachyte rock mass making up the majority of the mountain and would also encounter highly persistent master joints and basalt dykes. Specific local treatment and support is expected to be required to stabilise these features. There are also durability issues associated with the particular micro-structure and porosity characteristics of the predominant trachyte rock itself that have been addressed to meet the long-term design requirements.This paper describes the phased investigation and development of the design concept. The project is currently under consideration by the Canary Islands Government.

Study on Surrounding Rock Pre-Stressed Crossed Anchor Control Technology in Roadway with Large Mining Depth

Based on the deep soft rock creep mechanism, according to the physical characteristics of bauxitic mudstone in ingate at the level of -1015 m of Kong Zhuang colliery, we solved the technical difficult problem of choosing anchor points of soft rock supporting roadway in ingate. Using high strength and low relaxation steel strand to reinforce by crossed anchor, in combination with the depth hole grouting technology, improve pressure-bearing capacity of wall rock in the ingate, safe passage and drainage pipe area. We analyze the stress distribution characteristics of rock mass, contrasting on the stress distribution at the end of partially grouted bolt and the rock mass. Practice shows that crossed anchor rock mass has triaxial compression over all, the bunch body has a single structure, and it is easily to be analyzed. We not only control the roof settling in the area of ingate is, but also solve the floor heave problem in safe passage and drainage pipe area.

Monitoring Analysis of the Underground Cavities in Guandi Hydropower Station during the Construction Period

The underground cavities of the Guandi Hydropower Station comprise four pressure division tunnels, a generator chamber, a main transformer chamber, a tail water control chamber and two tail water tunnels and arrange in the basalt mountain on the right bank of the dam. Based on the brief introduction of cavity arrangement, geological conditions, monitoring design and execution, the deformation magnitude and deformation characteristics of the three major cavities are focused on analyzing. The stability of the cavities is evaluated. The quality of the surrounding rock masses of the underground cavities of the Guandi Hydropower Station is good and the deformation is normally less than 30mm. However, some parts have large deformation due to the influence of the structure surfaces and the maximum deformation is 61.49 mm. Most of the parts with larger deformation are the middle and upper positions of the side walls and rock anchor beam positions. The structure surfaces have noticeable action for controlling the surrounding rock mass deformation. The stability of the cavities is good.