Hard Strata Research Articles

Development of Specialized PDC Drill Bits for the Interaction of Soft and Hard Strata in Igneous Rocks

In response to the technical challenges of low mechanical drilling speed and severe drill bit wear caused by the development of some igneous rocks in the Dongying and Shahejie formations of the Bozhong 34-9 oilfield, it is urgent to develop a specialized PDC drill bit suitable for the soft hard interaction formation of the BZ-34-9 oilfield. The PDC drill bit is required to have strong aggressiveness, as well as good impact resistance and abrasion resistance, in order to meet the drilling requirements of the soft hard interaction formation of the BZ34-9 oilfield. Through on-site use, the mechanical drilling speed has been significantly improved.

Vibration response of piles at different distances induced by shield tunneling in hard rock strata

Excavation of subway tunnels in hard rock generates strong vibration waves that pose potential risks to the stability of surrounding structures. In this study, the discrete element method-finite difference method (DEM-FDM) coupling was adopted to build the model of tunnel structure-rock-pile, which was validated by field monitoring data. Then, the vibration response of piles under various pile-tunnel spacings was analyzed, revealing the occurrence of vibration peak rebound phenomena within certain distance ranges. The range of vibration effects was categorized. Furthermore, in shield tunneling construction, the energy induced by vibrations was mainly concentrated within the 50 Hz range. Low-frequency vibrations result in a wider effect range. The study also demonstrated that within a 1d (tunnel diameter) range of the pile-tunnel spacing, the vibration induced by shield tunneling construction had a more significant effect. As the pile-tunnel spacing increased, the piles transitioned from being subjected to bending forces to experiencing bending-shear forces. Finally, the vibration effects on the existing piles were evaluated under field working conditions. It also provided suggestions for construction based on the effects and laws of the pile dynamic response.

Failure Mechanism and Structural Optimization of the Primary Support Structure for Expressway Tunnel in Soft Rock: A Case Study

When the highway tunnel crosses the soft rock, the primary support structure often fails and damages because it cannot withstand the excessive surrounding rock pressure. For the Magu tunnel in Yunnan Province, which is in the upper hard and lower soft stratum, the surrounding rock deformation is large, and the primary support structure occurs spray concrete cracking, steel arch frame twisting deformation. According to the convergence constraint curve, this paper proposes to adopt the primary support optimization design scheme of "shortening the bench length + optimizing the length and arrangement of anchor rods". According to the numerical simulation results of FLAC3D, the important indexes such as deformation convergence, surrounding rock stress, anchor axial force and thickness of plastic zone in different excavation support conditions are analyzed. The conclusions are as follows: 1. The soft rock at the foot of the benches does not provide sufficient support resistance, resulting in large settlement and convergence deformation that can cause the primary support structure to fail and become damaged. 2. The length of the middle bench and the primary support strength and stiffness contribute most to the deformation. 3. The deformation control effect of the short anchor rods on the surrounding rock is relatively weak. Increasing the length of anchor can better play the role of anchor suspension enhancement and give full play to the surrounding rock's own bearing capacity. Finally, the deformation of the tunnel is under control after adopting the optimized primary support structure, which ensures the smooth construction.

Controlled reconstruction and application research of lateral overlying strata structure in high-gas mines with thick-hard roofs for a fully-mechanized top-caving face

Due to the large mining area, the fully-mechanized top-caving mining with thick-hard roof is easy to form cantilever structure on the lateral roof of the working face, which on the one hand causes high stress level of adjacent roadway and serious deformation of roadway, on the other hand causes gas accumulation in corners, which brings severe challenges to safe and efficient mining of the mine. In this study, a mine facing such problems in Jincheng, China was taken as the research object. Based on the mechanical characteristics of coal and rock, the characteristics of overlying strata activity in the mining process of working face are mastered, and the dual effects of controlled transformation of lateral overlying strata structure on stress field and gas field were revealed. On this basis, roadway reinforcement and gas drainage schemes were put forward and applied. The results showed that the strength of the hard rock stratum was high in the triaxial stress environment, and it was not easily destroyed. However, once the strata exceed their strength threshold, they break down. In addition, the strength of coal is relatively low, and it is continuously deformed when the force exceeds its strength. The overlying strata structure after thick-hard roof fully-mechanized top-caving mining evolves in the following manner: “long cantilever length formed by the main roof being broken in the initial stage, voussoir beam formed by the upper hard roof being broken in the middle stage, and double cantilever beam formed by overlying strata compaction.” The stress carried by upper hard rock stratum is transferred to coal pillars, which is the main reason for the high stress environment of multi-purpose roadway with large coal pillars. The controlled transformation of lateral overlying strata structure by pre-splitting and roof cutting can realize the "transfer-unloading" of coal pillar stress and the "plugging and driving" of corner gas. Based on the hydraulic fracturing reconstruction of lateral overburden structure, the grouting reinforcement scheme of roadway and dynamic gas drainage scheme were put forward. The results demonstrated that after roof cutting, the maximum deformation of the surrounding rock in the multi-purpose roadway was reduced by approximately 90 %, and the maximum concentration of corner gas was decreased by 15.28 %. This approach successfully achieved a collaborative control effect on roadway surrounding rock stability and gas emission well within the safety limits.

Peridynamics simulation of TBM cutting process in soft and hard composite strata with a simplified loading scheme

In this study, we address the complex geological conditions encountered during tunnel boring machine (TBM) excavation, particularly the challenges posed by soft and hard composite strata. To overcome limitations of conventional numerical methods for analyzing rock fracture in such strata, we employ Peridynamics to simulate rock fragmentation induced by disc cutters. We propose a virtual disc cutter loading scheme that enhances computational efficiency by replacing traditional contact models. Through numerical examples, we demonstrate the efficacy of this loading scheme in simulating rock fragmentation within soft and hard composite strata. With this method, we conduct numerical studies on kerfs-assisted rock fragmentation in such geological conditions, finding that kerfs can help mitigate differences in cutter forces between soft and hard rocks, thus showing potential for improving TBM performance. Our findings offer insights into optimizing TBM operations in challenging geological environments.

Longitudinal seismic analysis for underground structures passing through soft-hard strata based on periodic stratum deformation

Poor geological conditions, e.g., soft and hard strata, have a significant impact on the seismic response of underground structures. In this study, a series of longitudinal seismic analysis for underground structures passing through soft-hard strata were conducted based on the concept of periodic stratum deformation. Firstly, the analytical expressions of stratum deformation in soft-hard strata at the oblique incidence of plan P-, and S- wave were obtained via decomposing the stratum deformation into the reflected- and transmitted wave fields. Then, the wave propagation history in soft-hard strata in the time domain was transformed into strata deformation in a period via introducing concept of the periodic stratum deformation. Base on the proposed expressions, a self-developed Python program was implemented into finite element software ABAQUS to establish seismic analysis model and to calculate the longitudinal seismic response of underground structures. Finally, example analyses were conducted to study the influence of plane P- and S- wave, wave velocity ratio of soft and hard media, as well as incident wave angle on the seismic response of underground structures located in soft- hard strata. This study may provide a reference for seismic design of underground structures located in soft- hard strata.

Failure characteristics of thick hard roof stratum under hydraulic pre-splitting and its application in a coal mine, Dongsheng mining area

Failure characteristics of thick hard roof stratum under hydraulic pre-splitting and its application in a coal mine, Dongsheng mining area

Simulation Study and Engineering Application of Weakening Mine Pressure Behavior in Stope through Ground Fracturing Thick and Hard Rock Strata

Fracturing hard roofs by ground hydraulic action is an important control technology for the strong mine pressure in the stope. In this paper, a new simulation method, “separate + interface,” is proposed, and two physical simulation experiments are conducted; the phenomenon of increased goaf pressure and decreased front abutment pressure is discovered after fracturing in the key strata, and then the influence of different fractured crack shapes on the front abutment pressure and the goaf stress is revealed. The results are as follows: Firstly, the separation under the high-level hard strata blocks the transmission of overburden load to the goaf, leading to the high-stress concentration of the coal seam, which is the main reason for the large deformation of roadways and the breakage of a single hydraulic prop in the roadway. Secondly, the weakening effect of mine pressure differs when hard rock strata are fractured artificially with different types of cracks. The peak value of abutment pressure is reduced from 24.91 to 20.60 MPa, 17.80 MPa, and 16.13 MPa with the vertical crack spacing of 20 m, 15 m, and 10 m, respectively, and the related goaf pressure is increased from 2.61 to 3.54 MPa, 3.91 MPa, and 4.34 MPa, respectively. The peak value of abutment pressure decreased from 24.79 to 22.08 MPa, 19.88 MPa, and 17.73 MPa. The related goaf pressure increased from 2.61 to 3.39 MPa, 3.81 MPa, and 4.43 MPa, respectively, with the key strata also fractured into two horizontal layers, three horizontal layers, and four horizontal layers with horizontal fractures. Thirdly, after the hard roof is fractured above the No. 8202 working face, the first breaking step distance of the main roof decreased from 112.6 to 90.32 cm, while the first breaking step distances of KS2 and KS3 decreased from 106.3 and 135.8 cm to 93.5 cm and 104.8 cm, respectively, and the goaf pressure also increased. Compared to the adjacent unfractured No. 8203 working face, the mine pressure intensity of the No. 8202 working face is significantly reduced. The research results can provide useful guidance for the treatment of strong mine pressure.

Mechanical Performance of Concrete Segment Lining Structure of Shield Tunneling in Different Strata

There are many problems in the development of urban space in China. Among them, urban tunnels generally pass through many sections with very complicated geological conditions, and the construction will encounter great difficulties, so the mechanical behavior of shield segments in different complex strata is worth discussing. In this paper, the axial force, bending moment and pore water pressure of shield tunnel segments in the soft and hard uneven stratum, clay stratum and fully weathered granite stratum of overlying buildings are studied by establishing a rectangular element mechanical model based on the field test method. The analysis shows that the mechanical properties of shield tunnels in different strata are quite different, but their mechanical properties change stages are the same. The earth pressure on the left and right sides of the test ring is asymmetric in the soft and hard uneven stratum, and the vault pressure is much greater than the vault bottom pressure. The distribution of earth pressure in each position of the segment ring in clay stratum is relatively balanced, and the earth pressure on both sides is relatively small; in the fully weathered granite layer of the overlying building, the segment ring of the test ring is subjected to greater additional stress, and the internal force of the segment is much greater than that without the overlying building. Exploring the similarities and differences of segment stress in these three complex strata can provide an important basis for the design and construction of shield segments in complex strata.

Research on the causal mechanism of a rock burst accident in a longwall roadway and its prevention measures

Based on the disaster characteristics and the geo-conditions at the scene, in this study, the occurrence mechanism of a serious rock burst accident that occurred in the Tangshan Coal Mine, China, was analysed. Ground stress measurements showed that the mine is in a high ground stress area dominated by horizontal tectonic stress around 33 MPa. Laboratory testing revealed that the coal was a hard seam of 8.3 MPa over bedded by a thick and hard roof stratum with uniaxial compressive strength of 66 MPa. The calculation results indicated that the accident occurred in the roof rebounding area. It is proposed that the hard roof and the hard coal seam formed a seesaw structure around the working face. The vertical pressure relief caused the rib coal mass to lose its clamping forces from the roof and floor and rush into the roadway, resulting in a rock burst accident. Based on the causality mechanism of the rock burst disaster developed in this study, pertinent coal bump prevention measures have been undertaken in practice. Large-diameter boreholes were drilled to eliminate the pivot effect of the seam. Roof blasting was undertaken to prevent the roof from forming a seesaw plank. To summarize, a new causality mechanism for rock burst in coal mines under hard roof and hard seam geo-conditions was developed.

Stability analysis of a vertical excavation reinforced with horizontal nails using three-dimensional finite elements limit analysis

Three-dimensional stability analysis of a vertical excavation, reinforced with grouted horizontal nails, has been carried out by using the three dimensional lower and upper bounds based finite elements limit analysis (FELA) using Optum G3. The nail along with the surrounding grouted cementitious material is assumed to behave as a single monolithic cylindrical block which was assumed to be either rigid or an elasto-plastic material following the Von-Mises yield criterion. The excavation is assumed to fail on account of either an interface shear slippage between nails and surrounding soil mass or on an account of axial tensile rupture of the composite nail material for a few typical cases as well. The soil unit weight has been varied to induce the ultimate shear failure of the soil nailed excavation. The unit weight of the soil mass at which the failure of the nailed wall occurs is termed as γcr. Two different types of boundary conditions along the base of the excavation have been considered. In the first case, the in-situ soil mass behind the excavation is extended below the base as well (Case 1) and in the second case, the excavation is assumed to simply lie on a hard stratum (Case 2). The results have been provided in the form of a non-dimensional stability number N=γcrHc as a function of soil friction angle φ, Sv/H, SH/H, d/H, L/H and m; where H refers to height of the wall, d is diameter of the grouted nail, Sv and SH imply centre to centre vertical and horizontal spacing of the nails, and m is the mobilization factor between grouted nails and adjoining soil mass. The results from the analysis have been found to compare well with the data reported in literature. All the results have been presented in the form of non-dimensional stability charts, which will be useful for design for any given values of height H and cohesion c. From the observed failure mechanisms, an analysis has also been carried out to determine the optimized lengths of nails. The optimized lengths of nails for both Cases 1 and 2 have been also provided. The type of failure mechanism, base or face failure, dictates the optimized lengths of the nails.

Novel hard rock breaking technique using ultra-high-frequency particle impact induced by ultrasonic vibration field

With the gradual increase in drilling and mining depth in geothermal, oil, and other fossil energy explorations, complex geological environments (especially hard strata) have forced researchers to develop high-efficiency rock-breaking techniques. Inspired by the dynamic impact theory and the particle jet technique, an ultrasonic vibration field-induced ultra-high-frequency particle impact rock-breaking (UPIRB) technique was proposed to solve these problems. After being subjected to ultrasonic vibration, the small-sized particles generate ultra-high frequency and high impact stress on the rock sample, which can significantly promote rock degradation and improve rock-breaking efficiency. A comparative study between UPIRB and traditional ultrasonic vibration rock-breaking (UVRB) techniques in terms of rock-breaking efficiency was conducted using a series of physical experiments and two-dimensional discrete element numerical simulations. The results showed that owing to the small contact area due to small size particles, UPIRB has incomparable advantages compared with traditional UVRB: its impact stress level is improved by more than ten times, the rock-breaking time is shortened from minutes (2–3 min) to seconds (2–4 s), and the rock-breaking efficiency is improved nearly ten-fold, overcoming the disadvantages of UVRB. This study proposed a novel and efficient rock-breaking technique for energy exploration engineering.

Research on Optimization Design of Tunnel Blasting Scheme Adjacent to Buildings

The section of Jialingjiang Road Station to Xiangjiang Road Station along Qingdao Metro Line 13 is located in Qingdao, China. All of them show obvious characteristics, being soft on the top and hard on the bottom, and the interval tunnel is faced with the problem of existing adjacent buildings. In order to ensure the smooth progress of construction, as well as minimize the damage to the buildings, a new mechanical excavation combined with a blasting construction scheme for the adjacent buildings is proposed. In this scheme, the step method is used for excavation. The upper step is in the weak stratum, and the mechanical method is therefore used for excavation; the lower step is in the hard stratum, and the drilling-and-blasting method is thus used for excavation. Using FLAC3D 5.0 finite difference software and the method based on blasting an equivalent load, the vibration velocity at adjacent buildings caused by the combined mechanical excavation and blasting scheme, as well as the traditional full-section blasting scheme, is compared and analyzed. Further, the construction parameters of the combined mechanical excavation and blasting scheme are compared and selected based on building settlement, the plastic zone of surrounding rocks, building vibration velocity and other factors. The results show that under the mechanical excavation blasting scheme, the peak particle velocity of each monitoring point decreases significantly compared with that under the full-section blasting scheme, with a maximum reduction of 61.1%, which is within the allowable range of the project, demonstrating the rationality of the new scheme. Finally, the mechanical excavation advance in the upper step is determined as 0.5 m. The optimized parameter construction effect is monitored and evaluated, the problems encountered in the project are successfully solved using the combined mechanical excavation and blasting scheme, and the expected construction period is shortened by 3 months, which shows the rationality of the blasting construction scheme proposed and its parameters, as well as the validity of the calculation results. The research results can be used as a reference for the construction scheme design of similar projects.

Study on Mechanism and Regularity of Rock Breaking by Pulsed Water Injection with Particles

In recent years, the drilling technology applied to deep and ultra-deep formations has developed rapidly, but the drilling speed for hard formations is low. Therefore, it is very important to study the drilling methods for deep and hard strata. Particle pulsed jet drilling is a new drilling method based on particle jet impact drilling technology and high-pressure pulsed water jet impact drilling technology. In this paper, the mathematical models of the shear layer amplification coefficient and wave velocity are established based on a similar network theory, and the motion equations of a single particle and particle swarm are established according to the motion of particles in a pulsed jet environment. Then, based on the self-designed particle jet impact rock-breaking experimental platform, the numerical simulation results are compared, analyzed, and verified. The results show that the rock-breaking efficiency increases with the increase in the average velocity of the particle pulsed jet. When the average speed exceeds 160 m/s, the rock-breaking efficiency increases significantly. With the increase in the particle concentration, the rock-breaking efficiency of the particle pulsed jet also increases, but there is an optimal value. When the concentration is too high, the impact of particles on the rock is affected by the collision between particles, and the wear of the drill intensifies. This research is helpful for understanding deep-well drilling-speed-increasing technology and promotes the development and engineering applications of particle jet impact drilling theory.

Prediction of mining-induced seismicity and damage assessment of induced surface buildings in thick and hard key stratum working face: a case study of Liuhuanggou coal mine in China

Thick and hard key stratum working faces are characterized by frequent mine tremors and significant ground tremors during mining which seriously threaten the safety production of the mine. With working face (4-5) 06 of Xinjiang Liuhuanggou Coal Mine as the engineering background, using field investigation, microseismic monitoring, and theoretical analysis, a mining-induced seismicity prediction method and damage assessment of surface buildings for thick and hard key stratum working faces is proposed, which is based on the evolution characteristics of overlying strata spatial structure and the motion state of the key stratum. The results of the study are as follows: 1) The movement law of overlying strata is the basis of mining-induced seismicity prediction for working faces. The magnitude of the risk of mining-induced seismicity occurrence is mainly related to the boundary conditions of the working face, the thickness of the key stratum, the distance from the coal seam to the key stratum, the height of the overlying strata spatial structure, and the fracture step of the key stratum. 2) The mining-induced seismicity energy contains the original accumulation elastic energy of the key stratum, the transfer elastic energy of low rock strata, and the accumulation elastic energy of gravity work. Based on this, a mechanical model of surface building damage induced by the release of mining-induced seismicity energy was established. A ground vibration damage boundary and vibration induction boundary under the action of strong mining-induced seismicity were proposed, and the service life of buildings when they reach the critical damage value under the action of frequent mining-induced seismicity was obtained. 3) The temporal and spatial distribution law of mining-induced seismicity activities in thick and hard key stratum working faces was revealed. According to the results of micro-seismic monitoring, the “zonality” characteristics of the time series and the “transition” law of spatial distribution of mining-induced seismicity verified the reliability of the mining-induced seismicity prediction method. The research results provide a theoretical basis for predicting mining-induced seismicity and assessing the risk of induced disasters during the mining process of thick and hard key stratum working faces, and can provide technical support for mining-induced seismicity prevention and control and safety production in mines with similar conditions.

Static Behaviour of Geo-Cell Confined with Reinforced Soft Clay Soil

Engineers often experience difficulties in starting any construction work on soft clay because there is no hard strata at a depth of 15 m.  The application of a Geo-cell structure provides a comparatively harder stratum at the top of the soft subgrade.  Therefore, experimental investigations have been carried out on the stability of a Geo-cell reinforced soft soil structure, to evaluate the effect of the Geo-cell.

Comparative Study of the Rock-breaking Mechanism of a Disc Cutter and Wedge Tooth Cutter by Discrete Element Modelling

The operation of a shield tunnel boring machine (TBM) in a high-strength hard rock stratum results in significant cutter damage, adversely affecting the thrust and torque of the cutter head. Therefore, it is very important to carry out the research on the stress characteristics and optimize the cutter parameters of cutters break high-strength hard rock. In this paper, the rock-breaking performance of cutters in an andesite stratum in the tunnel of Qingdao Metro Line No. 8 was investigated using the discrete element method and theoretical analysis. The rock-breaking processes of a disc cutter and wedge tooth cutter were simulated by software particle flow code PFC3D, and the rock-breaking degree, stress of the cutter, and rock-breaking specific energy were analyzed. The rock damage caused by the cutter in a specific section was divided into three stages: the advanced influence, crushing, and stabilizing stages. The rock-breaking degree and the tangential and normal forces of the wedge tooth cutter are larger than that of the disc cutter under the same conditions. The disc cutter (wedge tooth cutter) has the highest rock-breaking efficiency at a cutter spacing of 100 mm (110 mm) and a penetration depth of 8 mm (10 mm), and the rock-breaking specific energy is 11.48 MJ/m3 (12.05 MJ/m3). Therefore, two types of cutters with different penetration depths or cutter spacing should be considered. The number of teeth of wedge tooth cutters can be increased in hard strata to improve the rock-breaking efficiency of the shield. The research results provide a reference for shield cutterhead selection and cutter layout in similar projects.

Deformation-Based Basal Heave Reliability Analysis and Selection on Monitoring Points for General Braced Excavations

A framework for evaluating deformation-based basal heave stability is proposed in order to distinguish between the different responses under freely developed and prohibited basal heave failures. In the case of freely developed basal heave failure, the maximum deformation values occur at the center point of pit bottom, whereas this is not the case for the prohibited basal heave failure. The critical thickness of soft soil layer between the end of supporting structures and the top of hard stratum is about 0.3B (B = excavation width), beyond which the freely developed basal heave failure arises. In situations otherwise, the prohibited basal heave failure occurs. The failure probability of basal heave failure at the center point increases significantly as B ranges within a limited value; then, it begins to decrease or to vary slightly at a certain value under a given thickness of soft soil layer. If the thickness of soft soil layer is so sufficiently large that freely developed basal heave failure occurs for any of B, the failure probability of basal heave failure at the center point increases as B increases. The selection of the optimum monitoring points for basal heave stability is recommended to account for the weights in the contribution to the basal heave deformations of the influencing factors such as excavation width and thickness of soft soil layer. The proposed framework is applicable to basal heave reliability analysis for braced excavations where deformation values are focused.

Seismic Response Analysis of a Shield Tunnel in a Coastal Nuclear Power Plant under a Complex Foundation

As a transportation hub connecting sea and land, the seismic safety of an undersea tunnel is very important. Based on the structural and stress characteristics of a shield tunnel, a seismic response analysis of a coastal nuclear power plant under a complex foundation is carried out in this paper. Firstly, adopting the lateral and longitudinal response displacement methods, the authors study the variation and distribution features of the rate of diameter change and joint-stretching value of the shield tunnel segments. On this basis, a three-dimensional refined finite element model of a sinking tube–soil mass under a complex soil foundation is established. Finally, when the joints and segments of the shield tunnel are subjected to ground motion, the deformation and internal force distribution of its lining segments are studied. The findings show that under influence of SL-2-magnitude ground motion, the soft–hard stratum junction is the weak point of the shield tunnel structure’s longitudinal seismic resistance. When the silt layer at the position of the shield segment is thicker and the geological conditions are relatively poorer, the internal force, diameter deformation rate, and joint-stretching value will be larger. The research results can provide a reference for seismic research on shield tunnels in sea areas under complex foundations.

Investigation into Pressure Appearances and Hydraulic Fracturing Roof-Cutting Technology in Mining Working Face under Residual Pillars: A Case Study

Strong mining pressure disasters are prone to happen when the mining working face is under residual pillars (MWFRPs). The purpose of this study was to experimentally investigate and evaluate pressure manifestations and hydraulic fracture roof-cutting technology in the development of a working face under residual pillars using a physical model and numerical modelling tools. A scheme for hydraulic fracturing cutting technology was proposed and carried out on-site at the 31106 working face. The results show that the instability of the overlying residual pillar causes the upper thick, hard strata (THS II) to rupture and form a “T-shaped structure”. The rotation and sinking movement of the structure leads to the transmission of the dynamic load downwards, causing shear failure in the lower thick, hard strata (THS I) along the boundary of the residual pillar. The smaller the length of the THS II fracture block, the smaller the shear damage of THS I, and the lesser the mining pressure in the working face. Field trials proved that hydraulic fracture roof cutting can effectively destroy the integrity of the thick hard strata and promote their collapse, which reduces the strong dynamic load borne by the hydraulic support. This research provides a reference for safe mining at a working face under similar conditions.