Tunnel Working Face Research Articles

Evaluation of cooling measures performance and thermal-humidity environment for ultra-high geothermal railway tunnel construction by using WBGT

High-geothermal damage is one of the major challenges in high-speed railway tunnel construction. Current studies have shortcomings in the formation mechanism of tunnel environment and the environment evaluation system. This may lead to an over-optimal environmental acceptability evaluation and jeopardize tunnel construction safety. This study established a mathematical model of the tunnel environment formation mechanism, proposed the wet-bulb globe temperature (WBGT) standard (26.49°C) for the tunnel construction, analysed the cooling efficiency of different environmental control measures. The results show that increasing the mechanical air supply volume can reduce tunnel temperature. Fog cannon can reduce the air temperature near the tunnel working face, the cooling range in front of the tunnel working face is 10.8 m. The cooling shield can control the WBGT below the standard. Under the 90°C rock temperature, the environmental acceptability of the tunnel under pure mechanical ventilation is 0%; the single fog cannon and double fog cannon (both cooperate with double-duct ventilation) are 22.85% and 54.29% respectively; the cooling shield with one fog cannon and double duct ventilation is 95.72%. This study improves the tunnel environment evaluation system during the construction stage, and provides design reference for temperature control measures in tunnel construction. Highlights: Tunnel environment under the impact of multi-physical coupling fields was analysed. Thermal radiation is taken into account in the formation of the tunnel environment. A new benchmark based on WBGT for high-geothermal tunnels construction was proposed. A tunnel environment assessment system was proposed. Cooling shield environmental-control measure was proposed for tunnel construction.

Research of wet string grid dust removal vehicle and creation of dust control area on tunnel working face

The spread of blast dust throughout the tunnel becomes a common problem in drill and blast tunneling,the key to breaking through the problem is the creation of a dust control area on the working face.In view of this key problem, a wet string grid dust removal crawler vehicle was developed, the power of the vehicle came from the diesel generator, and further, the air cooler of the diesel generator was used to generate airflow, and the suction process formed by the on-board axial flow fan was coupled to create a dust control area of the working face after blasting.The results show that when the frequency of the axial flow fan is adjusted to 30 Hz, the airflow speed of the wet chord grid section reaches 3.34 m/s, and the dust removal efficiency is the highest, with a value of 94.3%.Compared with the non-use of the dust removal vehicle, when the air outlet of the air cooler is front, horizontal front, horizontal rear, the dust concentration is reduced by 74.37, 92.39 and 50.53%.Finally, the optimized wet grid dust removal crawler was installed in the Dading tunnel, and the actual dust reduction efficiency was about 78.49%. The results obtained provide an important technical way to improve the working environment of the drilling and blasting construction tunnel.

Analytical study and field investigation on the effects of clogging in slurry shield tunneling

The pressurized slurry is the essential part of the slurry pressure balanced shield (SPBS) for stabilizing the working face and transporting the excavated soil. If clogging occurs in case of adverse ground conditions or equipment failure, a series of consequences will happen, such as sudden changes in slurry pressure, massive ground loss, and deformation or even collapse at tunnel workface. The present study reviewed and summarized the clogging mechanism of the slurry shield under different ground conditions. Then, based on the law of conservation of mass and Bernoulli’s Equation, an analytical approach was developed to investigate the effects of clogging on the slurry velocity and pressure along with the deformation of tunnel workface. Furthermore, a case study and field measurements were conducted to validate the analytical method. The significant findings drawn from the study can be summarized as follows: (1) The clogging of a slurry shield imposes severe effects on the slurry pressure balancing mechanism of shield tunneling. It can occur in both cohesive and non-cohesive ground with different formation mechanisms. (2) During the initial clogging, the slurry pressure above the spring line increases, and that below the spring line decreases. With the clogging deteriorating, the slurry velocity drops considerably at the top of the deposit area and escalates near the discharging port. The presence and increase of deposit lead to a sharp rise of slurry pressure in the slurry chamber, especially in the clogging region. (3) The reverse circulation of the slurry system (RCSS) is useful to clear the clogging. Still, the slurry deposits at the bottom will obstruct the slurry reverse flow, resulting in inward deformation of the tunnel face and a drastic increase in overcutting volume. (4) The ultimate volume deviation ratio of the slurry was proposed as a useful index to alarm the potential risk of collapse induced by the RCSS. (5) The timely adoption of RCSS is strongly suggested when any abnormal changes in the slurry pressure or the ultimate volume deviation ratio are observed for the first time.

Numerical investigation on temperature–humidity field under mechanical ventilation in the construction period of hot-humid tunnel along the Sichuan–Tibet Railway

High temperature–humidity environment is one of the severe challenges in constructing Sichuan–Tibet railway tunnels. For enhancing construction efficiency and guaranteeing tunnel worker safety, addressing the high temperature–humidity environment has irreplaceable significance. This study innovatively investigated the distribution law and evolution process of the thermal–humidity coupling field during tunnel construction. Besides, the corresponding ventilation measures were proposed based on the evaluation heat index (HI). If the initial temperature of the surrounding rock is constant, the distance between the ventilation duct outlet and the tunnel working face should not be greater than 30 m. Otherwise, the cooling performance of ventilation will be significantly weakened; If this temperature ranges in 65–85 °C, the recommended ventilation velocity is 10–25 m/s, and the ventilation distance should not exceed 20 m. The conclusions in this study are beneficial to better understand the high temperature–humidity environment in tunnel construction and can be a guideline for practical engineering application.

Risk Assessment of Tunnel Face Instability under Multi Factor Coupling Based on Conditional Probability and Tunnel Construction Mechanics

The factors leading to the collapse of tunnel face are diverse, and there is an interaction relationship between multiple influencing factors at the physical and mechanical level. By analyzing the coupling relationship between the factors affecting the instability of tunnel working face, this paper deduces the calculation method of the coupling effect of tunnel construction risk factors for a certain risk event based on conditional probability theory, and makes reasonable assumptions on the monitoring data and risk probability. Finally, it is verified by a model test. The results show that the tunnel construction risk factors have a coupling effect by affecting other risk factors or changing the physical and mechanical parameters of surrounding rock, in which the coupling amplification effect will significantly change the failure probability of the risk carrier, so it is unfavorable to the stability of the tunnel face. The coupling amplification effect of risk factors will cause the increase of tunnel face extrusion deformation. Even if it causes a small deformation increment, it can also make the transition of risk probability and grade of tunnel face unstable, and cause tunnel face instability collapse. The research results of this paper can quantitatively evaluate and predict the stability of the working face in the process of tunnel construction.

Analysis and Application of Vibration Damping Hole in Tunnel Working Face Based on Grey Correlation Analysis

In the process of tunnel blasting construction, certain vibration hazards will be caused to adjacent buildings. Therefore, it is necessary to explore feasible vibration reduction measures in the construction process. Combined with field measurement and numerical calculation, the law of vibration reduction effect of vibration reduction holes in the blasting construction of subway tunnels is analyzed. The gray correlation theory is used to determine the influence degree of vibration reduction hole parameters on the vibration reduction effect; each parameter is clarified, and the primary and secondary relationship of each parameter in the vibration reduction effect is clarified. Among them, the effect of damping hole spacing is particularly obvious. Combined with numerical calculation and comprehensive analysis of on-site construction effects, it is found that during the construction process, the distance between the vibration damping holes is 120 mm, the distance between the vibration damping holes and the explosion source is 2000 mm, the diameter of the vibration damping holes is more than 120 mm, the depth of the vibration damping holes is 1200–1500 mm, and double-row vibration-damping hole spacing is 50–100 mm, which can achieve a better vibration-damping effect. The research results will provide a reference vibration reduction technology for the blasting construction of urban shallow tunnels and have certain reference value and guiding significance for the blasting scheme design and vibration control of similar projects.

Parametric Study of Water Inrush in a Tunnel Crossing a Fault Based on the “Three Zones” Fault Structure

As tunnelling progresses into the complex geological environment such as fault zones, water inrush has become one of the main geological hazards during tunnel construction. Consequently, understanding the evolution of pore pressure and flow velocity when a tunnel is excavated in a fault zone is crucial to ensure safe working conditions and reduce construction risks. In this work, based on the concept of “Three Zones” fault structure, we simulate the nonlinear water inrush process by solving the Darcy-Brinkman flow equation for the host rock and the fault zone. We examine the impacts of 1) the angle between the tunnelling direction and the fault and 2) the relative position from the tunnel face to the fault on the evolution of pore pressure and flow velocity near the tunnel face. The results show that within 5 m to 20 m ahead of the working face, pore pressure, flow velocity, and water inrush rate are the smallest when the angle is 90°. As the angle decreases, both pore pressure and flow velocity ahead of the working face increase. The pore pressure is larger when the tunnel has not reached the fault zone than when the tunnel has crossed the fault zone. Flow velocity also exhibits similar behaviour as pore pressure. With different relative positions from the tunnel working face to the fault, the closer the tunnel face to the fault, the lower the pore pressure and the larger the flow velocity ahead of the tunnel face. The largest water inrush rate occurs when the tunnel face is excavated to the center of the fault core, and the water inrush rate declines as the distance away from the fault increases. The simulation results provided a new method for simulating water inrush when a tunnel crosses a fault and could provide valuable references for the prediction of water inrush for underground projects.

Failure analysis and control measures for tunnel faces in water-rich sandy dolomite formations

Revealing the mechanism of sand gushing triggered by failure of tunnel face in water-rich sandy dolomite strata contributes to the safe construction of tunnels. In this paper, we find that sandy dolomite and the water environment are the key factors causing sand gushing through geophysical prospecting at the tunnel face where 12,000 m3 sand surge occurred. Through the flow-solid coupling model, the extrusion deformation at the tunnel face is the main deformation form in the tunnel of water-rich sandy dolomite strata. The influence of different factors on the deformation value of the tunnel face is from strong to weak: sandification degree, head height, and tunnel depth. Combined with the study of pressure arch characteristics of the model, the limit equilibrium theory considering seepage effects and pressure arch characteristics is proposed. We get that sand gushing will occur in the tunnel in the intensively sandy dolomite strata where the head exceeds 80 m. For the intensively sandy dolomite strata with head below 80 m, the stability of the tunnel face is controlled by the degree of seepage damage. According to the reason of sand gushing, the comprehensive control measures consisting of risk identification by over-detection, double-layer close-packed pipe shed, grouting sealing technology and dewatering technology, and Milling and blasting combined construction method are proposed, and the effective control of tunnel working face is realized in practical application.

Surface Settlement Analysis of Double-Line Shield Passing Through a Power Tunnel and Directly Cutting Pile Foundation

Three-dimensional numerical simulation was conducted to investigate the influence of shield directly cutting the pile foundation of surface settlements using the Hangzhou Metro Line 4 shield passing through a power tunnel. The thrust force of the tunnel working face and grouting pressure are used as the tunneling parameters for the numerical simulation, from which field monitoring data is obtained and analyzed. The simulation results are verified to a certain extent by comparing them with the monitoring data. The power tunnel with high stiffness acts as a beam in the soil and can effectively reduce soil settlement, making the settlement curve affected by the power tunnel no longer axisymmetric. The final surface settlement value decreases with the increase of thrust force of the tunnel working faceand grouting pressure. In the influence range of the power tunnel, the thrust of the tunnel working face has a significant effect on the width of the settlement trough, while the effect of grouting pressure on the width of the settlement trough is not as significant as with the thrust.

Effects of slurry viscosity and particle additive size on filter cake formation in highly permeable sand

Filter cake is critical to maintaining the stability of the excavation face of an underwater shield tunnel in a high-permeability stratum. In a high-permeability formation, generating an effective filter cake on the excavation face is difficult with a pure bentonite slurry, which penetrates the ground and may not achieve the required suspension pressure. Determining how to efficiently and quickly form a thin and low-permeability filter cake on the tunnel working face has become a key engineering problem in the construction of slurry shield tunnels in high-permeability strata. In this study, the relationship between slurry viscosity and the slurry pressure gradient of pure bentonite was established by performing slurry permeability experiments. The influence of slurry viscosity on the formation of the filter cake in a high-permeability formation was studied under different pressure gradients. In addition, the effect of additive particle size on the slurry filter cake formation was analyzed by introducing additives with different particle sizes to pure bentonite slurries with different viscosities. The test results indicate that (1) for the pure bentonite slurry, the critical initial pressure gradient can be used as a rough indicator of the formation of the filter cake, and the relationship between the critical maximum pore diameter and the average pore diameter of the formation can be compared to establish and analyze the formation law of the slurry filter cake; (2) adding particles to the slurry can enhance the effect of the pure bentonite slurry; and (3) adding coarse-grained materials can effectively improve the film-forming effect. The slurry is more compact when the particle size is close to the average pore size of the formation.

Case study on ventilation and cooling control technology of multi heat source coupling in long distance subsea tunnel construction

To solve the high-temperature environmental hazards caused by concrete hydration heat release in tunnel construction, this paper studies the heat generation mechanism of heat source in the tunnel using theoretical analysis and numerical simulation, and analyzes the temperature control in the tunnel under different ventilation wind speed (1 m/s, 2 m/s, 3 m/s, 4 m/s, 5 m/s) and ventilation temperature (298 K, 293 K, 288 K, 283 K, 278 K). The results show that: increasing the ventilation speed can effectively reduce the environmental temperature in the tunnel, and the ventilation cooling effect is the best when the air duct outlet speed is 3 m/s. Reducing the ventilation temperature can further reduce the tunnel temperature based on increasing the ventilation speed. When the ventilation temperature is set at 293 K, it can meet the needs of ventilation and cooling at the tunnel face. The simulation results show that when the tunnel ventilation speed is 3 m/s and the ventilation temperature is 293 k, the maximum cooling value of the working face can reach 6.66 k, and the maximum cooling rate is 2.18%. Within 5 m from the tunnel working face, the cooling effect of the enclosure structure is the most obvious, which can effectively ensure the comfort of the working environment.

Prediction of Bench Blasting Vibration on Slope and Safety Threshold of Blasting Vibration Velocity to Undercrossing Tunnel

The reconstruction and expansion project of oil reserve base often faces the excavation and blasting of the slope and undercrossing tunnel at the same time. Due to the flammable and explosive liquid storage nearby, the tight construction period, and the high requirements of collaborative construction, once the blasting accident occurs, the consequences are unimaginable. To facilitate safe and timely cooperative blasting construction of the slope and undercrossing tunnel, a vibration monitoring test of the slope and tunnel surrounding rock is conducted. The vibration response characteristics of the rock surrounding the slope and tunnel are analyzed, and a mathematical prediction model for the peak particle velocity (PPV) with consideration of the influence of the relative slope gradient (H/D) is established based on dimension analysis theory, which improves the prediction accuracy of PPV at the slope surface. ANSYS/LS‐DYNA is used to establish a 3D finite element model for the slope and tunnel, and the dynamic response of the tunnel surrounding rock under blasting load is verified through field monitoring data. A linear statistical relationship between PPV and effective tensile stress (ETS) of the tunnel surrounding rock is established. The PPV safety criterion of the tunnel surrounding rock under blasting load is proposed to be 10 cm/s according to the first strength theory, and hence, the minimum safety distance from the tunnel working face to the slope surface is calculated to be 36 m. Finally, the excavation timing arrangement of the slope and tunnel is proposed, which has been successfully applied to the expansion project, and the construction period has been effectively shortened by 45 days while ensuring construction safety. The research results have great guiding significance to similar cooperative blasting excavation engineering for high slope and adjacent tunnel with safety and efficiency.

Progressive Deformation Failure Characteristics of Highway Tunnel Working Face in Sand Pebble Stratum

The sand pebble stratum is typically unstable with discrete particles. The most important constituent elements are sand and pebbles. The surrounding rock of tunnels in the sand and pebble stratum has prominent engineering problems, such as being loose, easy to collapse, and unstable when exposed to water. Therefore, it is critical to ensure the safety of tunnel construction in this stratum. This paper studies the failure modes of surrounding rock on sand and pebble tunnel working face of different excavation footage, and the formation of subsidence law starting from the micro-mechanical and motion characteristics of particulate matter. This study is based on the Bai Heqiao tunnel of the Wudu expressway in Gansu Province. The results show that the instability failure of the surrounding rock mainly occurs in the upper part and sliding area in front of the tunnel working face. The sliding area is a curved body after the failure of the tunnel working face. The deformation and failure of the surrounding rock in the lower half of the tunnel working face are relatively small, and are gradually transferred from the vault to the surface, resulting in the subsidence of the surface and collapse. The deformation degree of the surrounding rock in the region closer to the tunnel vault is more obvious. After the formation is stabilized, the maximum subsidence deformation occurs above the tunnel vault. The final settlement amount of the stratum is also different with different footage. The smaller the excavation footage, the smaller the stratum settlement amount. Therefore, the appropriate excavation footage is selected as the principal safety control index during the excavation with the short-step method. The research results provide the necessary reference for the fine design and construction of tunnel engineering in the sand pebble stratum.

Analysis on microseismic characteristics and stability of the access tunnel in the main powerhouse, Shuangjiangkou hydropower station, under high in situ stress

Access tunnel in the main powerhouse of Shuangjiangkou hydropower station was deep buried with high in situ stress and complex geological conditions. Microseismic monitoring technology was established to monitor microcrack evolution process inside the surrounding rock in early excavation stage. Serious falling blocks in the left spandrel of the tunnel were predicted in a timely manner by delimiting major damage areas in the tunnel. Based on comparative analysis on microseismic activity law and field failure characteristics of the access tunnel, a quantitative index was supposed between slight rockburst like falling blocks and microseismic events. Moreover, the change law of daily average apparent stress difference and b value were analyzed based on microseismic event data. In addition, a three-dimensional numerical simulation software (RFPA3D) was used to simulate the damage distribution around the tunnel, and a relationship between spatial position of tunnel damage and direction of the maximum principal stress was qualitatively analyzed. The study results showed that advance speed of the tunnel working face was an important factor affecting the state of stress redistribution in surrounding rock mass, and the change law of b values of microseismic events could be used to predict activity state inside the surrounding rock effectively, which reflected mechanical properties and stress state of surrounding rock. In particular, field falling blocks became more serious with increasing b value, and field surrounding rock was relatively stable with minor b value. A risk of surrounding rock instability was relatively high with small b values. It provided an efficient method of predicting and assessing slight rockburst like falling blocks. The study results can provide significant guidance for field construction and later construction planning.

Field Experimental Study on Mechanical Characteristics of Primary Supporting in Weak Surrounding Rock Tunnel

Since the poor self-stability of weak surrounding rock when applied the full-face excavation in a tunnel construction, a large surrounding rock deformation are tended to occur, which causes a big security risk. Under the engineering background of gaojiaping tunnel of zheng-wan high-speed railway, the stress of surrounding rock, steel frame, initial shotcrete stress, bolt axial force and extrusion deformation of working face of the tunnel are monitored and analyzed respectively. The results show that the surrounding rock stress, steel frame stress and initial shotcrete stress of the tunnel are significantly affected by surrounding rock lithology and construction disturbance. The plastic zone of surrounding rock can be preliminarily determined as 3m according to the peak value of bolt axial force. The extrusion deformation of the tunnel working face shows that the surrounding rock integrity of the test section is good. The experimental study is helpful to collect the deformation of weak rock mass and provide reference for establishing scientific and reasonable support system.

Force and Deformation Characteristics during the Reconstruction and Expansion of Shallow Single-Tube Tunnels into Large-Span Multiarch Tunnels

At present, there are an ever-increasing number of tunnel expansion projects in China. Studying the mechanical properties of the expanded tunnels is of great significance for guiding their safe construction. Through model testing and numerical simulation, the mechanical properties of a double-arch tunnel constructed through the expansion of the middle pilot heading from an existing single-tube tunnel were studied. The variation characteristics of the surface subsidence, surrounding rock stress, and stress and strain of the middle partition wall and lining during the tunnel reconstruction and expansion were investigated. The mechanism for transferring stress and strain between the left and right tunnel tubes was studied by a numerical simulation method. The results showed that the surface subsidence caused by the excavation of the left (i.e., the subsequent) tunnel tube was larger, and the maximum surface subsidence occurred at the right (i.e., the first) tunnel tube. The surrounding rock on the middle wall was the sensitive part of the tunnel excavation, the stress of the surrounding rock at the left spandrel of the right tunnel tube fluctuated and exhibited the most complex variation, and the stress of the surrounding rock at the right spandrel of the left tunnel tube exhibited the largest variation. The excavation of the left tunnel tube had a great influence on the forces of the middle partition wall and the lining structure of the right tunnel tube, the middle partition wall was subjected to eccentric compression towards the left tunnel tube, and the stress at the left spandrel under the initial support of the right tunnel tube exhibited complex variations. The excavation of the left and right tunnel tubes had a great influence on the stability of the surrounding rock, as well as on the force-induced deformation of the middle partition wall and the support structure, within the width of the single tunnel tube span behind the tunnel working face. Due to the different construction sequences, the stress and strain at the symmetric measurement points of the middle partition wall, as well as the left and right tunnel support structures, were very different.

Stability Analysis and Case Study of Shallow Tunnel Using Pipe Roof Support

Pipe roof system is widely used in the shallow tunnel as the support system. Although the diameter of pipe roof is diversity, the pipe roof system of small diameter pipe roof and small spacing steel grating is widely adopted in China. The stability mechanism of that pipe roof system is to form shed-scaffold system. In that shed-scaffold system, the function of pipe roof is to form micro-soil-arch in overlying soils between the pipes and to transmit the excavation loads. The steel gratings are the main bearing structure. To form the shed-scaffold system effect, three stabilities, stability of soils between pipes, stability of tunnel working face and stability of tunnel foundation, should be satisfied simultaneously. On that basis, the collapse accidents of Tianhengshan highway tunnel and Haide highway tunnel in China are analyzed. It shows that the stability of tunnel foundation would be overlooked easily and causes even overall collapse. At last, the measures to improve the stability are proposed.

Rock Cracking Indices for Improved Tunnel Support Design: A Case Study for Columnar Jointed Rock Masses

Measurements indicate that the development of cracking is a key feature relating to the strength and collapse of a columnar jointed rock mass. In this context, a new support design method utilising rock cracking indices for columnar jointed rock mass under high stress is proposed to restrain the development of cracking in the surrounding rock mass. The method involves limiting the cracking evolution of the surrounding rock mass by designing the appropriate parameters and time of installation of the support system. Two indices are suggested: the allowable depth of the excavation damaged zone (EDZ); and the allowable damage extent of the rock mass in the EDZ. The method involves limiting the evolution of cracking in the surrounding rock mass by designing the parameters and time of installation of the support system. The support system should have a suitable stiffness and installation time so as to restrain the evolution of the depth and damage extent of the EDZ within the surrounding rock. Therefore, the depth and damage extent of the EDZ, as well as the axial stress in the anchor bolts, are calculated at different distances between the support location and the tunnel working face to find the appropriate stiffness and installation time of the support system. The method has been successfully adopted to determine the thickness of shotcrete, the arrangement and installation time of rockbolts, and other parameters, for five large diversion tunnels at the Baihetan hydropower station, China, which were excavated in columnar jointed rock masses.

The Pre-Evaluation of Stability during Tunnel Excavation using Unconfined Compression Strength of Intact Rock or Rock Mass and Crown Settlement Data

PURPOSES : It is difficult to estimate tunnel stability because of lack of timely information during tunnel excavation. Tunnel deformability refers to the capacity of rock to strain under applied loads or unloads during tunnel excavation. This study was conducted to analyze a methods of pre-evaluation of stability during tunnel construction using the critical strain concept, which is applied to the results of tunnel settlement data and unconfined compression strength of intact rock or rock mass at the tunnel construction site. METHODS : Based on the critical strain concept, the pre-evaluation of stability of a tunnel was performed in the Daegu region, at a tunnel through andesite and granite rock. The critical strain concept is a method of predicting tunnel behavior from tunnel crown settlement data using the critical strain chart that is obtained from the relationship between strain and the unconfined compression strength of intact rock in a laboratory. RESULTS : In a pre-evaluation of stability of a tunnel, only actually measured crown settlement data is plotted on the lower position of the critical strain chart, to be compared with the total displacement of crown settlement, including precedent settlement and displacement data from before the settlement measurement. However, both cases show almost the same tunnel behavior. In an evaluation using rock mass instead of intact rock, the data for the rock mass strength is plotted on the lower portion of the critical strain chart, as a way to compare to the data for intact rock strength. CONCLUSIONS : From the results of the pre-evaluation of stability of the tunnel using the critical strain chart, we reaffirmed that it is possible to promptly evaluate the stability of a tunnel under construction. Moreover, this research shows that a safety evaluation using the actual instrumented crown settlement data with the unconfined compression strength of intact rock, rather than with the unconfined compression strength of a rock mass in the tunnel working face, is more conservative.

Schedule compression in tunneling projects by adjusting workflow variability

Usually, the tunneling operation is on a critical path in road or railroad construction projects. For minimizing project duration, this study identifies feasible work scenarios, builds simulation models for each scenario, and then analyzes the average duration per unit length of driving distance and efficiency of tunneling equipment. The tunnel work face is usually advanced as far as the ground condition allows the intention to finish the work earlier. As the rock type changes, however, the length per cycle advances, and the work cycle time varies accordingly. Therefore, workflow variability occurs. Workflow variability in tunnel construction operations brings with it waiting time for each work crew and results in delays in the work process. This study evaluates the effects of workflow variability and suggests an approach for shortening the duration of tunneling work by adjusting the driving length of each excavation in the case of two parallel tunnels, not a single tunnel.