Bench Excavation Research Articles

Determination of Excavation Method and Support System for Sultan Alimuddin - Kakap Tunnel Based on Surface Geology Mapping and Excavation Face

Abstract With Samarinda serving as a key buffer to the capital city of Nusantara, which grapples with urbanization and congestion challenges, infrastructure development has become a critical focus, particularly the construction of the Sultan Alimuddin - Kakap Tunnel. Planned for Selili Village, this tunnel will be the first road tunnel in East Kalimantan and is strategically important for alleviating traffic congestion, enhancing the distribution of goods and services, and ensuring safety. However, the geological conditions at the construction site pose significant challenges, mainly due to weak rock with a high potential for landslides. By assessing the rock mass utilizing the Japan Society of Civil Engineers (JSCE) evaluation methodologies and the Rock Mass Rating (RMR), this research seeks to identify the most appropriate excavation techniques and tunnel support systems. To characterize the engineering geology, surface geological mapping, core drill analysis, tunnel excavation face mapping, and laboratory testing were performed. The RMR and JSCE assessment methods were compared to assess the rock mass quality and recommend appropriate excavation techniques and support systems. The findings reveal that the tunnel alignment is dominated by interbedded siltstone and sandstone lithologies. The rock mass quality along the tunnel, as reported by RMR, varies from fair to poor. Based on the JSCE method, the tunnel route falls into the DI, CII, and CI rock categories. The top heading and bench excavation method is recommended based on RMR, while the double bench method is suggested by the JSCE assessment. Although there are slight differences in the recommended support systems, both RMR and JSCE consistently advise using rock bolts and shotcrete as the main method of support for the tunnel.

Analysis of Excavation Methods Against Deformation in Highway Tunnels

Cisumdawu tunnel is a highway tunnelling which composed of very weak rock mass conditions has an average compressive strength less than 1 MPa. It is located below groundwater level. The shape of tunnell is horseshoe, diameter of tunnel section 14,413 m and height 11,083 m. Cisumdawu tunnel is a shallow tunnel which has a maximum overburden of 52,8 m and minimum overburden approximately 14 m. In the case of shallow tunnel which has a large section could be lead an instability during tunnel construction. Tunnel deformation analysis is using an observation and numerical modelling approach with finite element method which is assisted by RS 2019 (Rocscience) software. Based on the research is obtained that excavation method greatly affects the level of deformation that occurs during tunnel construction. As tunnelling advanced, Vertical displacement in the longitudinal direction using three bench seven step excavation method is smaller than full face and bench excavtion method. The maximum displacement using three bench excavation method is -4.23 mm, the full face method is -5.51 mm and the bench method is -4.91 mm. The amount of displacement is affected by the length of excavation and stage in tunnelling. The rate of deformation is increasing as the length of excavation getting deeper, but this is applied in unsupported tunnel.

Optimizing the Support System of a Shallow Buried Tunnel under Unsymmetrical Pressure

In the construction process of tunnel inlet sections, the rock mass can sustain unsymmetrical pressure due to asymmetrical terrain on the two sides of the tunnel. The fact that the inlet sections are usually under shallow buried conditions with strongly weathered rock mass exacerbates the issue. This paper discusses optimization strategies of the initial support of a shallow buried tunnel based on the analytical results of asymmetrical loading characteristics. Numerical simulation is performed with particle flow code (PFC) using the Jianshanji tunnel project as an example. The simulation results show that the bench excavation has slightly less total deformation than the full-section excavation but the deformation range is wider, especially in the tunnel arch. Both lining support and slope reduction treatments can effectively improve rock deformation, with lining support demonstrating better performance in controlling deformation and adjusting stress distribution. Based on the simulation results, the bench excavation and lining support are used in the actual project, and the corresponding optimization control measures were adopted to address deformation issues, including crushed-stone backfilling for compression resistance, advanced grouting reinforcement, and grouting. The field data show that the tunnel stability is effectively improved by adopting the optimization schemes, which further validates the effectiveness of the proposed unsymmetrical control method.

Study on the Two-Step Construction Method of Super Large Cross-Section Tunnels Crossing Karst Cave Areas

To explore the solution of the two-step method applied in the rapid construction of super large cross-section tunnels passing through IV-and V-grade surrounding rock sections in karst cave areas, based on an engineering example of the Lianhuashan Tunnel, we use the numerical calculation method to analyze the stability of surrounding rock and the design parameters of the control measures for super large cross-section tunnels during the construction of the step method. The calculated results show that the working face of IV-grade surrounding rock can be stabilized by an advanced small pipe, and the stability of the supporting structure should be controlled mainly by IV-grade surrounding rock. In order to control the stability of the tunnel face, it is necessary to use an advanced large pipe shed in the surrounding V-grade rock. The reinforcement range of the advanced large pipe shed is 120° and the length is 20 m. This is the most economical design parameter of the advanced large pipe shed, ensuring the deformation control effect. For control of the stability of the supporting structure, under the condition that the working space is suitable for large machinery, the settlement of the arch of the supporting structure can be obviously reduced by shortening the step cycle footage and reducing the step length, and the peripheral convergence of the supporting structure can be obviously reduced by reducing the step height. After comprehensive analysis and considering the development of karst caves, the advanced support measures, design parameters, bench excavation design parameters, initial support measures, karst cave treatment measures, and bench construction process of IV- and V-grade surrounding rock is determined. The application verification shows that the research results have a good control effect on the stability of the surrounding rock and cave and are suitable for large-scale mechanical operations, which can significantly improve the excavation speed of the super large cross-section tunnel passing through the IV- and V-grade surrounding rock sections in the karst cave area.

Simulation of bench stepping and optimization of bolt parameters based on multiple geological information fusion

In the stability analysis and design of rock support for tunnels, the establishment of a refined numerical model relies on reliable and accurate geometrical information of structural planes. To better analyze and optimize the tunnel construction method and support mode, it is crucial to establish a refined numerical model for the tunnel. Based on the developed geotechnical intelligent analysis and multi-interpretation system, the feature extraction and interpretation of the rock mass structural surface are achieved. Meanwhile, coupled with multiple geological information, the comprehensive analysis and judgment of rock mass structural surface information are achieved through continuous dynamic tracking. According to this method, based on grade IV fractured rock data collected from the construction site of Dapuling tunnel, the safe distance of step in bench excavation method has been optimized based on the numerical modelling, and the corresponding bolt support design. The optimized tunnel excavation process is more stable and safer, providing a scientific basis for predicting, identifying, controlling, and preventing rockfall disasters during the excavation process of tunnels and underground caverns in fractured rock masses.

Deformation Response of a Pipeline to Nearby Deep Foundation Pit Excavation: Numerical Simulations and Field Tests

Deep foundation pit excavation (DFPE) for metro stations in soft layers can cause strata deformation, impairing the safety and performance of nearby existing structures. This paper aims to explore the impact of DFPE on an adjacent oil pipeline using numerical simulations and field tests. Using the project of Xinxingcun Station of Tianjin Metro Line 4 in China as a case study, two approaches of layered excavation (LE) and bench excavation (BE) are proposed for DFPE. The deformation induced by the two approaches was analyzed using the FE method. The maximum vertical settlement of the oil pipeline caused by LE was 8.1 mm, compared with the maximum vertical settlement of 12.5 mm caused by BE. In practice, LE was selected as the construction scheme for the DFPE in Xinxingcun Station. Some field measurement work on pipeline deformation was conducted, and the results were in good agreement with the numerical simulation. Lastly, the change pattern in the pipeline-soil gap was studied. The angle of the position with peak gap width increased from 25° to 110°.

Study on stress variation of advance fiberglass anchor bolts during tunnel excavation process

One of the main causes for excessive deformation within a tunnel is due to the instability of the soil or soft rock ahead of the excavation face. Fiberglass bolts have been shown to be a useful advance reinforcement method for the excavation face. In this paper, an improved ADECO-RS (Analysis of controlled deformation in rock and soils) method had been proposed for soft rock mountain tunnels, in terms of the partial (mainly the upper bench) excavation face reinforcement and also for the bench excavation method. Strain gauges were used to test the micro-strain in the fiberglass bolt to investigate how the axial force of the fiberglass bolt varied during the tunnel excavation. In addition, combined with the field tunnel deformation monitoring data, the relationship between the reinforcement parameters of the fiberglass bolts and the tunnel construction phase were discussed. The research results show that: (1) The stress state of the anchor rod is related to the reinforcement length of the anchor rod; (2) Excavation within the lap area of the fiberglass bolt leads to an increase in the axial force of the bolt, while excavation outside the lap area of the fiberglass bolt has no effect on the anchor; (3) Reducing the reinforcement area of rock mass will affect the stability of the excavation. To ensure the stability of the excavation face, the initial support construction loop should be completed as soon as possible; (4) In a future project with similar conditions, the recommended lap length of the fiberglass bolt could be 3 m utilizing the fiberglass bolt grouting face reinforcement method.

Study on ground vibration characteristics induced by liquid CO2 blasting technique in bench excavation

The liquid CO2 phase change fracturing technique has been widely applied in rock engineering for a long time. However, the ground vibration characteristics in bench excavation have not been well demonstrated. In this study, the particle velocities, main frequencies, and displacements of rock induced by liquid CO2 blasting are investigated based on field tests and numerical simulations. Firstly, three field tests of bench excavation using DM83-1.4 type liquid CO2 storage tube were implemented in a specific highway slope project, in which the TC-4850 N vibration monitors were installed at horizontal distances of 4.2 m, 10.6 m, 20.3 m and 32.2 m from the detonation source to detect the rock vibration velocity and frequency. Subsequently, by employing the dynamic finite element program LS-DYNA, a three-dimensional numerical model was established, and then the particle velocities and displacements were analyzed and discussed. The research results indicate that the peak particle velocity (PPV) decays with distance in an exponential pattern, that is, the PPV decays rapidly in the near zone of the blasting source, and it decays more slowly in the far zone. At the position of 20.3 m in the horizontal distance and 15.8 m in elevation, an elevation amplification effect appears with an amplification factor of 1.12. Moreover, the vibration frequency generated by the liquid CO2 phase change fracturing technique is in the range of 0-80 Hz, which is much smaller than that induced by explosive blasting. The main frequency in the near-field of the blasting source is dominated by the radial frequency (Fr), while the vertical frequency (Fv) is dominant in the far-field. The attenuation law of displacement is generally consistent with the vibration velocity, and the difference between radial and tangential displacement increases gradually with distance.

Numerical analysis on the reinforcement design of a granular strongly weathered rock slope based on in-situ direct shear test

It is difficult in the intact sampling of the granular strongly weathered rock of a slope by bench excavation, which will in turn influence the strength test results. In order to provide more accurate design parameters for the slope, its rock shear strength is analyzed by in-situ direct shear test, then the slope reinforced with different anchor frame beams are simulated by finite element method based on the strength parameters obtained from the in-situ direct shear test. The results show that the strength parameters of the granular strongly weathered rock are c=75.69 kPa and φ=35.23°. Reinforced with 1 ~ 4 layers of anchor frame beams, the maximum lateral displacement of the slope can be reduced by 30%, 47.6%, 53.5% and 53.5% compared with that of the unreinforced slope, respectively, which means that the deformation of the slope decreases with the increase of anchor frame beams. However, the fourth-layer anchor frame beam is not efficiency in the reinforcement. The maximum lateral displacement locates near the second excavation bench, which needs to be specially reinforced in the design.

Research on Settlement Control of New and Old Highway Roadbed Based on Different Bench Excavation Dimensions

The reconstruction and expansion of expressways is an important means, and how to control the settlement deformation of the subgrade width is an urgent problem to be solved. Step splicing is an effective means to control settlement. Therefore, it is very important to study the size of the steps. Expansion is based on engineering. Through literature research, numerical simulation and other research methods, combined with theoretical knowledge of soil mechanics, the settlement control of widened roadbed in highway reconstruction and expansion is studied, and the following conclusions are obtained: (1) Three different types are studied. The deformation law of subgrade and foundation top surface of step size; (2) A combined subgrade step excavation method is proposed, which provides a reference for similar projects. (3) The method of excavation and splicing of combined subgrade steps was verified, and the correctness of combined subgrade excavation and splicing was verified.

Longitudinal Deformation Profiles for Sequentially Supported Deep Tunnels in Strain-Softening Hoek–Brown Rock Mass

Numerical methods are commonly applied in tunneling design practice for the prediction of rock mass behavior during tunnel construction and for primary support dimensioning. Although the sequential excavation and primary support installation in phases is a complex three-dimensional problem, in practice it is usually approximated with plane strain simulations. For their correct application, the knowledge of the confinement level prior to the installation of the support is necessary and requires the construction of an accurate longitudinal deformation profile. This study presents a practical methodology for the estimation of the confinement level at the support installation position based on the execution and interpretation of a large number of plane strain, axisymmetric and 3D numerical simulations. All simulations incorporate sequential excavation and support cycles in confinement depended strain-softening Hoek–Brown rock mass. The results indicate the importance of the post-peak rock mass behavior and the influence of the installed support to the confinement level for both top-head and bench excavation and support stages.

Squeezing deformation control during bench excavation for the Jinping deep soft-rock tunnel

Jinping II hydropower station in Sichuan Province, China, is one of the largest tunnel groups in the world. The tunnel was excavated using bench excavation with a top heading and a bench. Severe squeezing problems were reported immediately after the excavation of the top heading when crossing the chlorite schist stratum at Ch.1+537 m. Squeezing deformation could last for a long period during tunnelling, similar to the problem of rockburst reported in the marble stratum distributed in the middle and east of the headrace tunnels. One year after the top heading excavation, the profile of the top heading still violated the profile tolerance and had to be reshaped due to the large convergence. A new support scheme was proposed to ensure the stability of the surrounding rock mass and control the excessive squeezing deformation during the bench excavation. For this, laboratory triaxial tests were first conducted under saturated and dry conditions to acquire the mechanical properties of chlorite schist. Then, the mechanical parameters of intact and damaged surrounding rock mass were determined using Hoek empirical estimation and a displacement inversion method, respectively. Next, initial support design was re-reviewed and evaluated based on the field displacement measurements, and the potential for squeezing in the overstressed rock mass was estimated. Thereafter, 10 supplementary support schemes were proposed using the numerical method in combination with laboratory data and field observations. Finally, the optimised support scheme was selected based on the results of comparative analysis. Field application of the suggested scheme showed that the convergence was controlled to within an acceptable level throughout the entire period of excavation of the lower bench. The experiences of controlling excessive squeezing deformation and optimisation of the support scheme for rock strata in a high geo-stress environment provide a reference for similar projects in the presence of soft rocks.

Study on Large Deformation Control Technology and Engineering Application of Tunnel with High Ground Stress and Weak Broken Surrounding Rock

In order to prevent and control the large deformation of surrounding rock of tunnels, the selection of section type and the determination of excavation method were numerically investigated by using the self-developed code—Realistic Failure Process Analysis (RFPA2D) and taking a specific project as the engineering background—Ruyi tunnel. Then, special measures were employed to deal with the dangerous parts of the section of the tunnel. Finally, the control effect of surrounding rock deformation was checked with displacement monitoring and microseismic monitoring. The results showed that the horseshoe-shaped tunnels with six centers and the bench excavation can prevent the occurrence of large deformation. The “support resistant limiting damper-H230 grid steel frame” and the increased depth of the inverted arch can effectively control the deformation of the vault and the arch bottom. Besides, it was revealed that the propagation and connection of cracks at the top of the tunnel are the most critical cause for the instability failure of the tunnel. Engineering application indicated that the selected section type, excavation method and special support measures for dangerous parts greatly reduce the deformation of surrounding rock and effectively solve the problem of large deformation of the weak broken surrounding rock.

Numerical evaluation of new Austrian tunneling method excavation sequences: A case study

The main aspects that require attention in tunnel design in terms of safety and economy are the precise estimation of probable ground conditions and ground behavior during construction. The variation in rock mass behavior due to tunnel excavation sequence plays an important role during the construction stage. The purpose of this research is to numerically evaluate the effect of excavation sequence on the ground behavior for the Lowari tunnel project, Pakistan. For the tunnel stability, the ground behavior observed during the actual partial face excavation sequence is compared with the top heading and bench excavation sequence. For this purpose, the intact rock parameters are used along with the characterization of rock mass joints related parameters to provide input for numerical modelling via FLAC 2D. The in-situ stresses for the numerical modelling are obtained using empirical equations. From the comparison of the two excavation sequences, it was observed that the actual excavation sequence used for Lowari tunnel construction utilized more support than the top heading and bench method. However, the actual excavation sequence provided good results in terms of stability.

Pre-supporting mechanism and supporting scheme design for advanced small pipes in the silty clay layer

This study presents a design method for the advanced support of subway tunnels for different water content conditions in the silty clay layer. The influence of particle size distribution and water content on the mechanical properties of silty clay is studied through laboratory tests. In addition to the actual construction of the Harbin subway with a silty clay layer, a numerical calculation model is established to study the surface settlement, crown settlement, and supporting structure internal force during the tunnel excavation and support process under different working conditions. Furthermore, the mechanism of action of the advanced small pipe support is clearly defined, and a stability analysis model of the top and bench excavation in a silty clay layer is proposed to evaluate the stability of the tunnel face. The necessity and rationality of the pre-support is quantitatively analysed using the stability coefficient. The laboratory tests illustrate that within a certain range of clay content, the shear strength of silty clay decreases with an increase in water content. The silty clay was further divided into subgrades. The pre-support transfers the loose soil load to the initial support, which improves the early utilization of the lining structure. The advanced small pipe grouting increases the range of the bearing arch, shortens the interval distance of the supporting arch feet, and reduces the vertical load acting on the tunnel face. For the condition of subgrade V in silty clay strata, the stability of the tunnel face can be maintained without the advanced small pipe support. For the condition of subgrade VI1, the advanced small pipe support must be applied to ensure the stability of the tunnel face, but the grouting reinforcement measures are not required. For the condition of subgrade VI2, the advanced small pipe support and grouting reinforcement must be used simultaneously to ensure the stability of the tunnel face.

Dynamic analysis of rock mass deformation in large underground caverns considering microseismic data

The underground powerhouse of a hydropower project is one of the most significant structures that will be permanently in use. Large scale excavation induced rock mass damage may pose a threat to the stability of the underground powerhouse. In the present study, a microseismic (MS) monitoring system was established to capture microcracks induced by excavation of the underground powerhouse caverns at the Houziyan hydropower station. Based on tempo-spatial characteristics of the MS events, the relation between MS activities and field construction was analyzed and the main damage regions were revealed. A damage-driven model considering the effect of MS source sizes is adopted for dynamic analysis of rock mass deformation of the underground caverns. The rock mass damage is simulated by a micro-mechanics-based constitutive model described by the fracture density determined by the MS source radius. A numerical model of a typical profile of the underground powerhouse was established. The MS data monitored in each bench excavation period of the underground powerhouse caverns was imported to the numerical model for dynamic analysis of deformation. The results showed that the effect of rock damage played a significant role in the rock mass deformation of the underground powerhouse during each bench excavation. The displacements of rock mass considering MS data obtained from the numerical analysis can provide better predictions for real conditions on site.

The air-flow structure and gas dispersion behavior in gas tunnel construction through bench cut method

The inability to accurately predict the in rush of gas increases the fear of field workers security and unnecessary investments in tunnel ventilation. In this study, a 3-D numerical model was established to investigate the air-flow structure and gas diffusion characteristics of a gas tunnel during construction. The result showed that the flow inside the bottom bench mainly came from the re-circulation zone of the upper bench. Mutation in the tunnel section due to bottom bench excavation would cause the formation of some air circulation in the bottom bench. The circulation air in the bottom bench came from the re-circulation zone of the upper bench, while the air-flow in the considered zone was a polluted air. Based on the obtained results, there was no accumulation of CH4 inside the bottom bench. Nevertheless, gas gushing will still appear during the construction of the bottom bench surface, which can possibly lead to gas disaster. This study aims to provide some guidance for tunnel engineers on safety monitoring systems.

Support system analysis in building diversion tunnel ST1-ST4 of Leuwikeris dam construction at West Java Province

Construction activities in diversion tunnel of dam need to considere the stability of the tunnel which is very important factor for its continuity. In order to become a safe mining method, it is necessary to analyze the tunnel stability and excavation method using drill and blast method. This analysis is based on empirical method, which is approach using rock mass classification and analytical method based on stresses and deformation of rocks. Based on the analysis, there are three types of rocks passed by the diversion tunnel, i.e breccia, sandstone and claystone, with RMR value ranged from 36 to 51, poor rock class to fair rock class, 2,5 – 3 meters span, top heading bench excavation method, rockbolt, and shotcrete support system with thickness between 256 to 334 mm and steel support.

Alteration characteristics of granite contact zone and treatment measures for inrush hazards during tunnel construction – A case study

Several large-scale mud and water inrush events occurred during excavation of the Denghuazhai Tunnel at a depth of 175 m in the contact zone between geodic granite and tuff. By analyzing the inrush process, the characteristics of the inrush hazards, such as high pressure, abundant water, high mud and sand content and strong destructive power, were revealed. In addition, the inrush hazards were intermittent, uncertain and sudden. An intrusive contact zone between geodic granite and tuff was revealed by regional geological investigation. Large-scale alteration occurred both in geodic granite and tuff near the contact zone. Petrographic analyses indicated that alteration to albite, sericite and kaolinite occurred in geodic granite, and alteration to quartz and montmorillonite took place in tuff gradually toward the contact zone. Furthermore, the changes were gradational within and between various alteration grades. The closer to the contact zone, the higher degree of alteration, the less the primary minerals including quartz, K-feldspar and plagioclase, the more the secondary minerals such as sericite and clay minerals. Tests on porosity, saturated water content and uniaxial compressive strength indicated that alteration led to significant deterioration of physical and mechanical properties of rocks (except for silicified tuff). Especially, for the zonal clay in the central area of contact zone, the physical and mechanical properties were poor and obvious swelling, disintegration and argillation were observed. Tunnel excavation triggered swelling, disintegration and argillation of clay, leading to several large-scale inrush events. In terms of treatment of inrush hazards, first of all, a comprehensive exploration method based on petrographic analyses, geophysical detection and borehole drilling was established according to the alteration characteristics, so as to identify the scale and composition of the altered zone. The principle of “consolidation before removal” was adopted to deal with the inrush deposits for breach sealing and reinforcement. Energy release and stress reduction during tunnel construction and regular drainage during operation were achieved by dewatering measures, namely, “drainage supplemented by sealing, water drainage and solids remaining”. Reinforcement measures, including full-face curtain grouting and double-layer pipe sheds, were adopted to reinforce and seal the highly disturbed rock and soil in advance. Finally, technical measures, including “bench excavation, enhancement of support stiffness and timely installation of secondary support”, were employed to effectively prevent large deformation in disturbed rock masses. With the above measures, the left and right tunnels of the Denghuozhai Tunnel have passed through the altered zone successfully.

Rockburst Generation in Discontinuous Rock Masses

We study rockburst generation in discontinuous rock masses using theoretical and numerical approaches. We begin by developing an analytical solution for the energy change due to tunneling in a continuous rock mass using linear elasticity. We show that the affected zone where most of the increase in elastic strain energy takes place is restricted to an annulus that extends to a distance of three diameters from the tunnel center, regardless of initial tunnel diameter, magnitude of in situ stress, and in situ stress ratio. By considering local elastic strain concentrations, we further delineate the Rockbursting Prone Zone found to be concentrated in an annulus that extends to one diameter from the tunnel center, regardless of original stress ratio, magnitude, and the stiffness of the rock mass. We proceed by arguing that in initially discontinuous rock masses shear stress amplification due to tunneling will inevitably trigger block displacements along preexisting discontinuities much before shear failure of intact rock elements will ensue, because of the lower shear strength of discontinuities with respect to intact rock elements, provided of course that the blocks are removable. We employ the numerical discrete element DDA method to obtain, quantitatively, the kinetic energy, the elastic strain energy, and the dissipated energy in the affected zone in a discontinuous rock due to tunneling. We show that the kinetic energy of ejected blocks due to strain relaxation increases with increasing initial stress and with decreasing frictional resistance of preexisting discontinuities. Finally, we demonstrate how controlled strain energy release by means of top heading and bench excavation methodology can assist in mitigating rockburst hazards due to stain relaxation.