Rock Excavation Research Articles

Excavation based enhanced geothermal system (EGS-E): introduction to a new concept

The current technology for enhanced geothermal system (EGS) typically involves drilling of deep injection and production wells to inject cold water into the injection well to the EGS reservoir depth, to extract heat by permeating through the fractured hot rock masses, collected by the production well and return to the surface as heated water. In this article, a new approach is proposed to develop EGS based excavation technology (EGS-E). This method consists of (1) excavation to deep rocks, including a deep shaft from surface to the EGS depth, a room-and-pillar mined cavern complex to be filled with water to become a large underground heated water reservoir in the hot rocks; (2) enhanced heat extraction from rock, aided by additional drillholes spreading out and down from the caverns, and induced fracturing of the surrounding rocks for enhanced rock–water heat transfer; (3) enclosed heat transmission from heated water reservoir to the power plant by using independent heat conducting pipes running between them, continuously supplying the heat for power generation, without water movement in the reservoir. EGS-E offers the following advantages: (1) utilizing mechanized rock excavation technology for large volume energy production; (2) greatly increasing heat extraction volume by easily extending vertically and horizontally; (3) maximize the energy transfer from hot rock to the water reservoir; and (4) separated heat transmission from heated water reservoir in EGS to power plant to minimize environmental pollution.

About the use of 110G13L steel as a material for the excavator bucket teeth

The causes of failure of the bucket teeth of the excavator during excavations in rocks are analyzed in the article. It is shown that the most typical cause of failure of bucket teeth is wear of teeth. The wear with soft rocks (less than 1100 HV) which hardness is lower than the hardness of steel in the cold-work hardened state, a preliminary cold-work hardening can essentially (up to 10 times) increase the material wear resistance. The wear with rocks, having greater hardness with hardness of ~1250 HV, the cold-work hardening do not lead to increase of the Hadfield steel wear resistance.

The surface wave attenuation as the effect of vibratory compaction of building embankments

There are two different sources of dynamic effects observed on buildings and structures - natural phenomena and human activities. Both may be related to the mining production or civil engineering construction. Ground vibrations induced by human activities characterize much lower intensity compared to the natural phenomena like earthquakes. However, due to the high urban development and sustainable development ideas, preventing the destructive effect of man-made vibrations belongs to the most important problems considering the structure and soil dynamics. Construction activities, road and railway traffic, and operation of machines, including crushing of spoil material (debris) from demolition works are the main categories in the classification of human activities which can generate ground vibrations. This paper takes a special interest in the first of the factors mentioned above. It is evident that the main construction or demolition activities causing dynamic problems in close proximity of structures are caused by pile or sheet pile driving and dynamic soil compaction by means of vibratory rollers or impulse compactors; as well as by demolition of structures, rock excavation and soil deep compaction by explosives. The vibration monitoring process needed to prevent structures from damage is described by various standards. They give particular criteria depending on various technologies and construction materials. The reduction of man-made ground vibrations is now the crucial scope of interest due to the need for sustainable technologies implementation. The negative environmental impact may be reduced at the source. Some new specialized methods are also proposed for minimizing the vibration transmission with the use of an active generator. The aim of the presented paper is to describe the modelling of dynamic vibratory soil compaction, using finite element method. The presented solution is addressed to short time vibration, generated by geotechnical works. This conclusion can be helpful by panning geotechnical works in the neighbourhood of vibration sensitive buildings/structures.

Development of a model for predicting the dynamic effect on the stability of rock excavation

The present work is devoted to the prediction of the excavation stability and analysis of the geo-mechanical processes near the excavation under dynamic impact. The development of a numerical model of the forecast of the development of a stress-strain state with allowance for the dynamic effect taking into account the geometric parameters of the mining development, the speed and nature of the application of the load, and the formation of zones of intense fracturing in its vicinity are performed. The influence of the intensity of the dynamic effect on the nature of the manifestation of geo-mechanical and geodynamic processes in the vicinity, the movement of the contour of the rock excavation, and the probability of ejection of the rock mass are determined.

Recommendations for the minimum number of laboratory tests for intact rock

The design of mining excavations in rock requires access to a representative geotechnical model that includes the mechanical properties of the rock mass. The available geotechnical data provide the necessary input to analytical, numerical, and empirical design tools. Consequently, any geotechnical analysis is influenced by the quality of the input data. Therefore, to understand and mitigate the design risk caused by data uncertainty, it is critical to evaluate the level of confidence in the collected geotechnical data. A practical limitation of current mine design practice is the absence of quantitative guidelines to select the number of laboratory tests required. This investigation employs small-sampling theory to determine the minimum number of tests necessary to obtain predefined confidence intervals in intact rock estimates at South African mines. A key element of this work is the introduction of geotechnical domain complexity as a significant factor in establishing quantitative recommendations for the required minimum number of laboratory tests. A tangible contribution of this work is the development of an original methodology for planning laboratory testing campaigns for a new mining project or for updating the geotechnical database of operating mines. The proposed quantitative methods can eventually replace subjective assessments in addressing data collection requirements.

Analysis of the effects of blast-induced damage zone with attenuating disturbance factor on the ground support interaction

ABSTRACT The rock mass properties are typically influenced by the excavation technique and the changes in the state of stress due to the rock excavation. The amount and extent of damage introduced in the rock mass depends on the excavation technique and practice quality. The influence of blasting in the rock mass near the tunnel periphery is far more significant due to the energy of waves and redistribution of stresses and the severity of the damage diminishes as the radial distance from the tunnel opening increases. Therefore, it is important to consider the effect of the damaged zone when analysing the stresses and deformations around a tunnel. This study aimed at providing a new numerical solution for the determination of the ground response (reaction) curve with the consideration of the non-uniform damage zone around the tunnel periphery. A deep circular tunnel subjected to hydrostatic stress condition and excavated in rock materials obeying the Hoek-Brown failure criterion is considered. A solution for the determination of stresses, strains, and deformations around the circular deep tunnel is presented in order to correctly assess the attenuation of damaged rock as the distance from the tunnel perimeter increases considering the loads applied to the supporting structure.

The use of acoustic emissions technique in the monitoring of fracturing in concrete using soundless chemical demolition agent

Soundless chemical demolition agents (SCDAs) have been used during the last decades in the demolition of boulders and concrete structures as well as in open-surface and sub-surface rock excavation, as an alternative to the use of explosives posing safety risks. However, the knowledge of the governing fracture mechanisms in brittle materials is rather limited. In the present work, we thoroughly investigate the use of the acoustic emission technique to study the SCDA-induced fracture process in concrete blocks. Energy-related features and waveform parameters of the recorded AE activity are correlated to the fracture mode of the concrete where a quasi-static behavior is observed. Monitoring of the progressive fracture is also achieved by the 3D localization of the AE sources. The distribution of the inter-event times of the recorded hits is further analyzed in the context of non-extensive statistical physics.

Influence of Rock Quality and Excavation Mode on Horizontal Displacement of Crushing-zone Tunnel

In order to simulate the tunnel excavation of super-long broken rock zone, finite element analyses using Plaxis3D are carried out along with the construction, in which the influence of grade of surrounding rock, are excavation method on the deformation of tunnel are specially considered. Before numerical calculation, the parameters are reasonably simplified. The calculation results show that the CD method can efficiently reduce the deformation of rock in the broken zone to keep the stability. In the practical construction, it is recommended that the CD method is preferred when entering the broken zone.

Triaxial Experimental Study on Changes in the Mechanical Properties of Rocks Under Different Rates of Confining Pressures Unloading

To determine the relationship between the unloading rate and the rock's mechanical properties, an experimental study on specimens of granite at different unloading rates was conducted. The results of the study show that during unloading, the axial strain increases slowly and the lateral expansion of the rock sample is considerable. The slower the confining pressure unloading rate, the more thoroughly the fissures propagate through the rock and the more complete the stress transfer. The result is that more fracture surfaces are produced and the rock is broken more fully. Conversely, if the confining pressure is unloaded more quickly, the crack propagation and stress transfer are terminated abruptly and the rock specimen can only generate a few rupture surfaces along the initial rupture direction. Rock samples are easier to break when unloaded at a greater rate, and the stress difference at break is smaller. Poisson's ratio increases with decreasing confining pressure. The research results provide a theoretical basis for the prevention and control of geological disasters in deep rock excavations.

Microbiologically Influenced Corrosion (MIC) of Ground Support

Ground support is an integral part of strategies to ensure the stability of underground excavations in rock. In a mining environment the long term performance of ground support is influenced by several factors including ground conditions and mining induced stress changes. In corrosive underground environments ground support can be compromised leading to loss of performance and potentially falls of ground that may compromise the safety of workers and damage to equipment. An improved understanding of the role of atmospheric and aqueous corrosion on the long term performance can facilitate the selection of ground support. The contribution of microbacterial influenced corrosion (MIC) on the corrosion rate and degradation of ground support has received minimal attention in the past. This paper confirms the presence of MIC of ground support in underground hard rock mines which resulted in considerable higher corrosion rates in otherwise similar environments. This can have significant implications in establishing long term ground support strategies.

Impact of Steel Properties on the Corrosion of Expandable Rock Bolts

The selection of ground support for underground excavations typically focuses on matching a system to the anticipated ground conditions. In this context, the emphasis on materials selection involves picking a rock bolt type that meets a set of predefined capacity and deformation properties to manage the applied loads. Exposure of ground support to corrosive environments in underground hard rock mines can result in significant material degradation and loss of mechanical performance. This can have significant repercussions on the structural integrity of excavations in rock and potential falls of ground that may endanger the safety of workers and equipment. Consequently, there is a need to ensure that the selected rock bolt provides sufficient resistance to corrosion for the working life of the excavation. Current design practice is limited in selecting rock bolt types perceived as more resistant to corrosion, e.g., grouted versus friction rock stabilizer bolts. This investigation suggests, however, that there are significant variations in corrosion susceptibility of rock bolts that may otherwise display similar mechanical behavior. This has significant implications for mines operating in very corrosive environments.

Experimental study on modified specific energy, temperature field and mechanical properties of Xuzhou limestone irradiated by fiber laser

This paper reports the experimental investigation of the modified specific energy, mechanical properties and temperature field of limestone rock that was irradiated by continuous wave fiber laser at different irradiation powers. Results show that the specific energy decreases nonlinearly from 17 kJ/cm3 to 11 kJ/cm3 and rate of perforation increases from 0.2 mm/s to 0.65 mm/s with laser power increasing from 600 W to 1000 W. The modified specific energy which is used to evaluate rock excavation efficiency is observed to decrease monotonously from 0.27 kJ/cm3 to 0.068 kJ/cm3 with increasing laser power. Higher laser power results in larger surface temperature and temperature gradient. The rate of rock surface temperature irradiated by laser with power of 1000 W reaches 14 °C/ms within 140 ms. The temperature gradient, with largest value of 1400 °C/mm near to the sample axis, decreases sharply with radius. The high solution 3D X-ray microanalyser (XRM) images show that the rock perforation is a V-shaped hole and obvious cracks penetrate the whole rock sample. The extremely rough surface and well-developed cracks for irradiated samples are observed through scanning electron microscope (SEM). X-ray diffraction (XRD) results also show that the main minerals of rock after irradiation are quite different from that of original sample. The maximum reduction rate of compressive strength of irradiated rock with laser power of 1000 W is 60%, and the maximum increasing rate of peak strain of irradiated rock with laser power of 800 W is 65%.

Excavation-induced fault instability: Possible causes and implications for seismicity

Fault rupture and associated seismicity have been reported frequently in underground excavation. The mechanism that drives the human-induced geohazards still eludes explanation, and uncovering this mechanism relies heavily on our understanding of the failure characteristics of fractured rock. We carried out the triaxial compression testing and discrete element modelling on intact and fractured granite samples to consider the controllable change of the angle β between the maximum principal stress orientation and pre-existing fracture orientation and to interpret the complicated mechanism of fault rupture adjacent to rock excavation. Our results show that the reduction of the angle β leads to the failure pattern of fractured samples changing from the tensile failure of rock matrix to the shear failure along pre-existing fractures. This transition results in relatively small b value, as small magnitude AE events decrease in the rock matrix and large magnitude AE events concentrate along pre-existing fractures. The change of b value in the pre-peak stage is independent to the angle β, implying the challenge in rock failure prediction. Our study also indicates that fault rupture adjacent to rock excavation is due to the changes of magnitude and orientation of principal stresses, and the associated seismicity is likely dependent on the portions of rock matrix and fractures involved in the failure process.

A new classification of failure mechanisms at tunnels in stratified rock masses through physical and numerical modeling

Stability of tunnels in stratified rock masses under low stress conditions is governed by the rock mass structure (blocks) and excavation geometry and dimension. A physical model was built and the effects of bedding dip, discontinuity spacing, and tunnel dimensions on tunnel failure mechanisms were examined. Based on the image analysis, three zones were recognized around the excavation consisting: stationary, collapsed and buckling zones. The buckling zone was more affected by layer dip angle. The collapsed zone occurred in three modes of block falls and sliding, and toppling. Also, numerical simulation using distinct element method was performed and verified. After model calibration, numerical simulations were extended to a wide range of block size and discontinuity gradients. Finally, considering physical test results, image analysis, and numerical simulation results, failure mechanisms in stratified rocks were classified using “bedding dip” and so called “dimensional ratio”. Results lead us to a deep insight of ground behavior around the shallow underground excavations in stratified blocky rock masses under low stress condition. Additionally the deformation mechanisms of blocks such as buckling, shearing, sliding are discussed in detail.

Grinding Test on Tremolite with Fibrous and Prismatic Habit

The main objective of this work is the evaluation of the morphology change in tremolite particles before and after a grinding process. The crushing action simulates anthropic alteration of the rock, such as excavation in rocks containing tremolite during a tunneling operation. The crystallization habit of these amphibolic minerals can exert hazardous effects on humans. The investigated amphibolic minerals are four tremolite samples, from the Piedmont and Aosta Valley regions, with different crystallization habits. The habits can be described as asbestiform (fibrous) for longer and thinner fibers and non-asbestiform (prismatic) for prismatic fragments, also known as “cleavage” fragments. In order to identify the morphological variation before and after the grinding, both a phase contrast optical microscope (PCOM) and a scanning electron microscope (SEM) were used. The identification procedure for fibrous and prismatic elements is related to a dimensional parameter (length–diameter ratio) defined by the Health and Safety Executive. The results highlight how mineral comminution leads to a rise of prismatic fragments and, therefore, to a potentially safer situation for worker and inhabitants.

On the Issue of Application of Mining Machines in the Arctic Conditions

The question is the experience of studying the wear resistance of mining machines in arctic conditions. Nikolay Tarasov investigated the wear during the penetration of excavations in rocks with negative temperatures. The importance and prospects of similar studies are discussed, taking into account the plans for the exploration of outer space. According to the corporation “ROSCOSMOS” not frozen rocks on the planets of the solar system are found only in the equatorial zone. An analysis is made of the wear characteristics of working tools of mining machines in the development of rocks with temperatures ranging from -9 to -110 °C. Made suggestions: to reduce the abrasive wear of the working tool, new approaches to the solution of the problems of wear resistance must be applied; to make an analysis of the possibility of using nonmaterial to increase the wear resistance of working tools of mining machines; taking into account the prospects of cosmonautics, to use the permafrost rocks of the Arctic zone as a testing ground for studying the wear resistance of rolling machines.

Gas permeability evolution of granite under confining pressure unloading tests

Tunnel excavations in rock usually cause initial stress release, which is different from the stress path in classical triaxial compression tests. To analyze the permeability variations in excavation disturbed zone, two different stress paths, e.g. classical triaxial compression and confining pressure unloading, are applied on granite, and the permeability at different damage levels is measured using gas as transport medium. The differences in deformation and failure mechanism and their impacts on gas permeability variation are discussed. The test results show that lateral deformation of samples in confining pressure unloading tests is obviously larger than that in classical triaxial compression tests. Unlike the permeability decrease-increase transition in classical triaxial compression tests, the permeability in confining pressure unloading tests shows an overall progressively increase trend. The permeability in both tests increases significantly near failure and achieves the same order of magnitude. The differences in permeability evolutions under the two stress paths are directly attributed to their different deformation mechanisms. The test results could provide a new understanding of permeability properties during the underground excavation.

Damage threat to sensitive structures of a thermal power plant from hard rock blasting operations in track hopper area: A case study

The article deals with the development of controlled blast design patterns and methodologies for excavation of hard rock for foundation work of a track hopper in a running thermal power plant in close proximity of many sensitive structures. Test blasts were conducted with different blast geometries and charge loading patterns at selected locations. The results of the test blasts were analysed to evolve safe controlled blast design patterns and methodologies for completing the total excavation work. The threshold value of vibration for the safety of various structures/foundations present has been taken as 25 mm/s based on the dominant frequency content as well as guidelines from the regulatory authority. The dominant frequency content of the ground vibration waves was in the range of 10–40 Hz. Controlled blast design parameters, namely maximum explosive per delay, size of the blasts and blast geometry were formulated based on established ground vibration predictor equation and nearness of structures concerned from the blasting point. Two blasting zones were classified for safe excavation work. For the first zone within 20 m from the structures, controlled blasting using 32-mm diameter blast holes with small blast geometry was suggested, whereas for the second zone beyond 20 m, controlled blasting with 100-mm diameter holes with lesser hole depth and limited number of holes was suggested. The whole excavation work in the track hopper area was completed safely without causing any damage to the nearby sensitive structures of the running thermal power plant.

The Glendoe Tunnel Collapse in Scotland

For Glendoe hydro-electric scheme in Scotland, the tunnelling contract was design–build. After a year, the tunnel collapsed, blocking a 71-m length, which necessitated construction of a by-pass tunnel. The responsibility for the collapse was examined in court and the findings were later appealed by a further three judges. The judge of the first case found that the cause of the failure was “erodible rock” and the other three judges agreed, but, it is argued, that there is evidence for a complex wedge failure at a scale larger than the geological mapping. It is considered that the Owner of the tunnel, Scottish Southern Electricity took on the risk by agreeing to a TBM-constructed, mostly unlined tunnel rather than a drill and blasted, fully lined tunnel as had been postulated at tender. The Contractor, Hochtief, constructed the tunnel and lined it in accordance with a Rock Excavation Classification design sheet that was agreed by all parties and approved by the Engineer. A clause, ‘Option M’ limited the responsibility of the Contractor “for defects in the works due to his design so far as he proves that he used reasonable skill and care to ensure that it complied with the works information”. The question is posed as to whether or not the current judicial process may be an impediment to valid decision-making on responsibility for dealing with complex geotechnical problems. This paper suggests that these issues require a technical assessment and engineering judgement decision, rather than a legal opinion, adjudicated on the basis of opined contractual responsibility. Perhaps, an erudite technical panel deliberation rather than a judicial process should be given the final authority in such cases.

Physical and Mechanical Characteristics of Soft Rock Tunnel and the Effect of Excavation on Supporting Structure

The problem of large deformation is very prominent in deep-buried tunnel excavation in soft rock, which brings serious potential safety hazards and economic losses to projects. The knowledge of deformation law and support measures is the key to ensure the rational design and safe construction in a large deformation tunnel of soft rock. This paper describes rock physical and mechanical tests and field monitoring is employed to investigate the cause and development process of large deformation in Dongsong hydropower station in Sichuan Province, China. The results show that the free expansion rate of the rock sample is 20.0%, the average expansion stress of the rock sample is 11.0 kPa, and the expansibility of the rock is low. Large deformation of surrounding rock mainly comes from the dilatancy effect with high geostress and relaxation deformation with weak support. Shotcrete sealing exposed surrounding rock, and early strength support avoiding water immersion are useful to deal with the three main factors (weathering, water and confining pressure) that affect the strength of surrounding rocks. The second lining applied in time can effectively limit the further development of stress and deformation of initial support, and prevent the cracking and large deformation of concrete. Clearance convergence is suggested to be the main monitoring work in construction, because of its advantages of intuitive results, easy quality assurance of instrument installation and high accuracy.