Loess Tunnel Research Articles

Research on the Experimental System of Reinforcing the Base of Shallow Buried and Wet Collapsible Loess Tunnels

Due to the complexity of the surrounding rock structure and the geological environment of tunnel excavations, traditional analytical methods are insufficient in effectively dealing with the complex nonlinear deformation problems arising from tunnel excavation. In contrast, geomechanical model tests can comprehensively simulate the excavation construction process of tunnels and the mode and time effects of loads, providing a more realistic reflection of the complete process of engineering stress and deformation. Therefore, this study conducted a model test on reinforcing the loess tunnel base, building upon the first tunnel of the Lanqing Expressway located on the north bank of the Yellow River in Lanzhou City. The study utilized similarity theory to explore the theoretical design of the model and established a specialized model test platform to design the experiments with the goal of obtaining more scientific and effective experimental schemes to ensure the safety of soil reinforcement in tunnel bases during construction. This research will contribute to improving the safety, reliability, and economy of loess tunnel base reinforcement projects, and has a certain reference value for research in this field.

Calculation of Rock Pressure in Loess Tunnels Based on Limit Equilibrium Theory and Analysis of Influencing Factors

The research on the calculation method of tunnel envelope pressure is a key issue in the design of tunnel engineering support structure. Based on the limit equilibrium theory, this paper proposes a method to calculate the surrounding rock pressure in shallow buried loess tunnels. Firstly, based on the investigation of the damage mode of the loess tunnel surrounding rock and the field measurement results of the surrounding rock pressure, the damage mode of the loess tunnel is proposed, and then a method of calculating the surrounding rock pressure applicable to the shallow buried loess tunnel is derived according to the limit equilibrium condition of the tunnel square soil body and the side wedge; the basic mechanical parameters are known in this method, so only the rupture angle β needs to be determined, and the rupture angle calculation model in the shallow buried loess tunnel is proposed Three assumptions are made in the rupture angle calculation model, and the rupture angle calculation formula is derived according to the stress state on the slip surface of the surrounding rock; the pressure of the surrounding rock in the loess tunnel obtained by this method is compared with four methods, namely, the pressure theory of the surrounding rock in the existing loose body of Taishaki, the pressure formula of the deeply buried surrounding rock in the railroad tunnel design code, the Beer Baumann method, and the Xie Jiayi method, in order to verify the correctness and validity of the calculation method used, and to analyze the influence of different parameters on the surrounding rock pressure. The innovation of this paper lies in the derivation of a method for calculating the pressure in the surrounding rock of a shallow buried loess tunnel using the limit equilibrium theory, and also further proposes a formula for calculating the rupture angle. The pressure of surrounding rock decreases with the increase of static earth pressure coefficient, lateral pressure coefficient, friction angle and cohesion in soil, but the static earth pressure coefficient has a greater influence on the surrounding rock pressure. With the increase of sagittal span ratio, tunnel burial depth and soil weight, the surrounding rock pressure peaked with the increase of tunnel burial depth, and the surrounding rock pressure curve increased first and then decreased.

Study on Stress and Displacement of Axisymmetric Circular Loess Tunnel Surrounding Rock Based on Joint Strength

The development of an effective evaluation method suitable for loess-tunnel excavation is necessary to avoid the collapse accidents caused by tunnel excavation and any secondary disasters. Although the Fenner formulas and the modified Fenner formulas are widely used in tunnel engineering, a defect still exists in these formulas because the Mohr–Coulomb (M–C) criterion exaggerates the tensile strength of the surrounding rock of the loess tunnel. A newly modified Fenner formula was derived based on joint strength to overcome this deficiency. First, the expressions of stress and the radius of the plastic zone of the surrounding rock of the loess tunnel and the expressions of radial displacement were derived based on the stress-equilibrium equation of the axisymmetric plane and the joint strength. Then, the difference in the modified Fenner formulas based on the two kinds of strength criteria for the loess tunnel were compared. The results showed that the radius of the plastic zone and the radial displacement of the loess tunnel determined by the modified Fenner formula based on joint strength were larger than those determined by the modified Fenner formula based on M–C strength. However, the plastic stress of the plastic zone determined by the modified Fenner formula based on joint strength was smaller. The comparative analysis reveals that the modified Fenner formula based on joint strength can evaluate the stress and plastic-displacement field of the surrounding rock of a loess tunnel more reasonably.

Impact of Evaluation of Freeze–Thaw Cycles on Collapse Zone at Entrance and Exit of Loess Tunnel

The entrance and exit of loess tunnel are easily affected by freezing and thawing, which leads to collapse in cold regions. Based on the slip line network method, this paper proposed a graphical method and analyzed the impact of evaluation of freeze–thaw cycles on the collapse zone at the entrance and exit of a loess tunnel. Firstly, the slip line network method was improved for a graphical calculation program, and the program was validated by monitoring the data of collapse evolution in an unlined loess tunnel in situ. Then, some triaxial tests of freeze–thaw cycles were carried out on loess, and the attenuation law of loess strength was summarized after freeze–thaw cycles in cold regions. Finally, the graphical calculation program was applied to evaluate the impact of freeze–thaw cycles on collapse range in the unlined loess tunnel. The results show that introducing the classic slip line field into the analysis process of a tunnel collapse zone can provide a feasible and efficient analysis method for a tunnel surrounding rock collapse arch for tunnel engineering, and the collapse zone increases with the growing number of freeze–thaw cycles. Moreover, the attenuation of loess strength which suffered from the freeze–thaw cycles would cause the collapse zone area to increase by a maximum of 16.13%.

Stability Analysis Method for Initial Support Structure of Tunnel in Swelling Loess

The stability of tunnel is greatly affected by the changes in volume and strength of swelling loess caused by humidification. Quantitative reflection of the influence of humidification on stability in mechanical model has important engineering significance for the design of support structures. Based on the fact that parameters related to swelling and strength are functions of water content, the water content was introduced into the classical surrounding rock–support model, and a safety factor was employed as the index of stability. Thus, a stability analysis framework of swelling loess tunnel was established by considering the swelling and softening of swelling loess. In an engineering case, the theoretical models presented in this paper were introduced to investigate the change rules of surrounding rock deformation and tunnel safety factor, as well as the sensitivity of the safety factor of tunneling in swelling loess. Analysis results showed that the change rules of surrounding rock response curves and safety factors obtained by the models were consistent with the actual situation, and the predicted maximum radial displacement of free face was close to the measured value. These indicate that the models are applicable to the stability analysis of initial support of swelling loess tunnel.

Effects of Jet-Grouting Piles on Loess Tunnel Foundation with Centrifugal Model Tests

Loess is a common soil layer of geological movement. It has macropores, vertical joints, and tubular pores and also low strength and strong water sensitivity. When building a tunnel in a weak loess stratum (Q3 loess or Q4 loess), the poor geological characteristics of loess usually bring great difficulties to the construction of tunnel invert, and it is very easy to cause subsidence at the tunnel base, uneven subsidence at the tunnel bottom, and cracks in the road and lining. The use of jet-grouting piles to strengthen the weak loess foundation is an effective method to improve the geological characteristics of loess. To investigate the effect of jet-grouting piles on the reinforcement of the tunnel foundation in weak loess areas, centrifugal model tests were used to analyze the subsidence and structural mechanics response characteristics of the tunnel base under the influence of jet-grouting pile length and pile diameter. Research shows that subsidence at the tunnel bottom can be reduced effectively and the adjustment time of the surrounding rock stress can be shorten significantly when using jet-grouting piles to reinforce the tunnel foundation. The length of the jet-grouting pile is better than the diameter in improving the strength of the foundation. However, with the improvement in the strength of jet-grouting piles, the efficiency of foundation reinforcement is reduced, the surrounding rock pressure and lining stress gradually increase, and the construction economy and structural safety are gradually reduced. When selecting parameters of jet-grouting pile, consideration should be given to safety, economy, and ease of construction.

Stability of the loess tunnel foundation reinforced by jet grouting piles and the influence of reinforcement parameters

The special geological characteristics of loess, such as collapsibility and vertical joint development, usually bring great difficulties to the construction of tunnel inverts. Effectively resolving the subsidence of the tunnel bottom, shortage of the bearing capacity, and uneven subsidence caused by weak loess has been a key challenge during the construction and operation of loess tunnels. Jet grouting piles are widely used because they can effectively improve the geological characteristics of loess and enhance the stability of the foundation. To investigate the reinforcement effect of jet grouting piles in loess tunnel foundations and the effect of changes in the geometric parameters of jet grouting piles on the stability of tunnel foundations, centrifugal model tests and numerical simulations were used to analyse the mechanical properties and the deformation characteristics of the tunnel lining. The results show that the deformation of the loess tunnel bottom caused by the operation and construction can be reduced effectively by using jet grouting piles, but the surrounding rock pressure and settlement of the tunnel vault will be increased at the same time. The length of the jet grouting piles has a better impact than the diameter to improve the reinforcement effect on the stability of the foundation, and the deformation of the tunnel bottom and the stress of the surrounding rock at depth are reduced greatly. Based on the relationship between the length and the reinforcement efficiency of jet grouting piles, the length of jet grouting piles is recommended to be 5–12 m.

A Study on the Formation Mechanism and Calculation Method of Surrounding Rock Pressure in Shallow-buried Loess Tunnel Considering the Influence of Vertical Joints

Vertical joints have significant effects on the stability of shallow-buried loess tunnel. Due to the presence of vertical joints distributed widely in loess, the formation mechanisms of surrounding rock pressure on shallow-buried loess tunnel and its computational method are far different from those in other types of soil tunnel. In this study, the distribution characteristics of surrounding rock pressure in shallow-buried loess tunnel were investigated, and discrete element software 3DEC was used to analyze the formation mechanism and development process of surrounding rock pressure in loess tunnel under the influence of joints. We correlated the stratum deformation characteristic with the frictional force caused by the mutual dislocation between joints, and revised the semi-empirical method (Chinese Code methods) in China. The results show that the measured data of radial surrounding rock pressure of shallow-buried loess tunnel are generally between 0–600 kPa, and 80% in the range of 0–200 kPa. The surrounding rock pressure increases with the burial depth, while the distribution of lateral pressure coefficient is relatively discrete. The existence of loess vertical joints (LVJs) aggravates the uneven distribution of the vertical pressure above the tunnel, and the reduction of the lateral earth pressure coefficient at rest under the influence of joints leads to an increase in pressure near the arch shoulder, these two constitute the essence of the joints action on the surrounding rock pressure. The surrounding rock pressure of the shallow-buried loess tunnel reaches the peak stress at about 0.5D from the tunnel face (D is the tunnel span), which is about 1.04–1.32 times of the initial stress. After the excavation of the upper step, the stress of the tunnel crown monitoring point decreased to about 60% of the initial stress. Compared with ignoring the influence of joints, the time of reaching the peak stress at the arch shoulder monitoring point is earlier, and the final stress release degree is smaller. The comparison of the analytical results (proposed method) with the numerical results and the field data exhibits good agreement, proving the proposed method's correctness.

Adaptability Evaluation of Rotary Jet Grouting Pile Composite Foundation for Shallow Buried Collapsible Loess Tunnel

The deformation control effect of loess tunnel composite foundation plays an important role in optimization design and reinforcement effect evaluation. Systematically evaluate the adaptability of the composite foundation of jet grouting pile in shallow collapsible loess tunnel. Taking the shallow buried section of Fujiyao Tunnel with a buried depth of 20 m as an example, using MIDAS finite element numerical simulation software, the foundation deformation control during construction and settlement control after construction are systematically studied, the differential deformation control is analyzed, and the reinforcement effect of the tunnel bottom is evaluated. The results show that the uplift displacement can be controlled by changing the pile length and increasing the replacement ratio. The combination of long and short piles can significantly reduce the uneven settlement and plastic zone of the foundation. The uneven settlement of 9 mm can be used as the evaluation index of the composite foundation reinforcement effect in a shallow buried section of the loess tunnel.

Study on the construction deformation of a slotted shield in loess tunnels with different buried depths and large sections

Since there is no precedent for the use of slotted shield tunneling in the large section of high-speed railways in China, the relevant technological accumulation and systematic research achievements are few. Therefore, this paper provides theoretical support for loess tunnel construction decision-making through the study of slotted shields and is expected to promote the mechanization and even intelligent construction of a high-speed iron-loess tunnel. Taking the Luochuan tunnel of the Xiyan high-speed railway as the engineering background, this paper uses the numerical simulation software packages of ANSYS and FLAC3D to study the tunnel deformation (surface settlement, vault settlement, tunnel bottom uplift, and horizontal convergence) caused by the slotted shield construction in three different buried depths of 30, 40, and 50 m surrounding rock. The deformation law and mechanical characteristics of a cutter shield construction of large cross-section loess tunnels under the influence of different buried depths are put forward. Results showed that 1) the mutual interference between the working procedures can be significantly reduced by inserting the cutting tool into the soil instead of the advanced tubule before excavation; 2) the settlement in the upper part of the longitudinal axis of the tunnel is the largest; the greater the depth of the tunnel is, the smaller the surface settlement is; and 3) the horizontal deformation of the arch waist and foot of the tunnel under different buried depths is symmetrically distributed into the tunnel during the whole process of slotted shield tunneling.

Failure analysis and treatments of collapse accidents in loess tunnels

Statistical analysis of 44 loess tunnel collapse accidents in the past 15 years as related to causes and characteristics are discussed in this study. First, several statistical indicators suitable for loess tunnel collapse including loess types, causes, the burial depth, collapse form, original designs and the excavation method are proposed. Then, typical types of collapse failure in the loess tunnel are divided according to the hydrogeology factors and collapse causes: collapse failure of sandy loess: collapse failure of natural water-rich loess; collapse failure of excavation disturbance-induced water-rich loess; and collapse failure of surface infiltration water-rich loess. Finally, the causes, characteristics and mechanisms of collapse accidents in different types are analyzed and treatment schemes for different types are proposed of collapse failure. This paper aims to share practical experiences about collapse failure causes, the treatment of instability zones, and key elements of passing through the loess in tunnelling.

Numerical investigation of characteristics and mechanism of tunnel erosion of loess with coupled CFD and DEM method

Tunnel erosion of loess can cause geological disasters such as landslide and debris flow, as well as threaten the safety of civil engineering structures. Understanding the mechanism and evolution law of tunnel erosion is not only helpful to analyze the occurrence and development of gully erosion, but also conducive to the guidance of engineering practice. Although many scholars have carried out statistical analysis on loess tunnel erosion based on field investigation, quantitative analysis and visualization at the micro level are lacking. Because the combination of computational fluid dynamics (CFD) and discrete element method (DEM) has great advantages in studying the seepage erosion process. This manuscript adopts the CFD-DEM coupling method, and the virtual cement method is used to transform the failure of cemented matrix into the reduction law of bonding strength. The micro mechanism of tunnel erosion of three areas is discussed from the aspects of particle erosion rate, cement evolution and micro mechanical field. The results show that the more the fine particle content is, the greater the cementation is, the stronger the erosion resistance is, the less developed the erosion rate of the particles is, and the dissolution failure rate of the cement is usually earlier than the erosion rate of the particles, and the variation range of the anisotropy of the contact force is also the smallest. However, under high water pressure, their erosion rates are almost synchronous and reach the slow erosion stage quickly. The anisotropic parameters of the contact force of the three samples virtually decreased gradually in the process of erosion, and finally tended to be stable.

Stability classification probability model of loess deposits based on MCS-Cloud.

The stability classification of loess deposits around tunnels is a vital prerequisite for safe construction in underground environment. Due to the fuzziness and randomness of loess physical and mechanical parameters, the stability prediction of loess deposits shows uncertainty. Existing loess deposit stability classification models rarely consider the uncertainty of influencing factors. A novel classification probability model of loess deposits is proposed for the above problems based on Monte Carlo simulation and multi-dimensional normal cloud (MCS-Cloud). Specifically, five loess parameters, including water content, cohesion, internal friction angle, elastic modulus, and Poisson ratio, were selected as predictors for the stability level of loess deposits. The weights of the predictors were obtained through 50 test samples. After acquiring the numerical characteristics of the normal cloud, the stability level can be comprehensively evaluated with the weighted multi-dimensional normal cloud model. The classification model was applied to the loess tunnel in Yan'an, China. The prediction results are in good agreement with practical engineering, denoting the rationality of the weighted multi-dimensional normal cloud. Finally, the stability classification of loess deposits was discussed from the perspective of uncertainty analysis with the application of MCS. Results proved that the MCS-Cloud model is feasible for classifying the stability of loess deposits surrounding tunnels. The obtained classification probability can be used for quantitative risk assessment of loess tunnels.

Active Bearing Technology of Foot Steel Pipe Applied in Controlling the Large Deformation of Tunnels: A Case Study

Foot steel pipe was the main arch foot supporting structure to control large deformation of loess tunnels, but the supporting effect was not ideal. Taking Yulinzi Tunnel in Qingyang, Gansu Province, as the engineering background, the design concept and implementation scheme of a foot steel pipe active bearing was put forward. The purpose was to solve the problem that it was difficult to control the surrounding rock settlement with the foot steel pipe. Numerical simulation and field experiments were used to verify the effect of the active bearing technology of the foot steel pipe. The main conclusions were as follows: (1) The effect of increasing the diameter of the foot steel pipe is better than that of increasing the number of foot steel pipes. (2) The active bearing mode of exerting its bearing capacity in advance by prepressing the foot steel pipe can effectively reduce the settlement of the vault. The settlement rate of the vault can be reduced by about 70% in 1–2 days and more than 50% in 1–3 days. (3) At the initial stage of surrounding rock deformation, this technology can provide a large bearing capacity, thereby reducing the overall deformation of the surrounding rock, slowing down the release of the surrounding rock pressure, and playing a positive role in the settlement control of the vault.

Research on the Evolution Mechanism of Large Deformation of Expansive Loess Tunnel under Rainfall

Large deformation, collapse, and destruction of supporting structure occurred many times due to rainfall in Xiaohegou tunnel, which is located at Taixing Railway in Shanxi Province. In this paper, the Xiaohegou tunnel is taken as an example, and the large deformation numerical simulation method is used to reproduce the collapse process of the slope at the entrance and the internal section of the tunnel. The reasons for the failure of the original initial support of the tunnel are analyzed, and the failure mechanism of the support structure of the expansive loess tunnel was clarified. The results show that the rainfall leads to the destruction of the tunnel support and the decrease of the bearing capacity of the support structure, which results in the collapse of the tunnel.

Local buckling characteristic of hollow π-type steel-concrete composite support in hilly-gully region of loess tunnel

Shallow tunnel can be directly affected by the terrain, and the traditional steel frame is prone to distorting and buckling failure when the tunnel is constructed in a loess region, as it is crisscrossed with hills and gullies. Therefore, the effect of hilly-gully terrain is crucial to the structural stability. The present paper aims to analyze the effect of longitudinal pressure bias on the stability of the π-type Steel-Concrete Composite Support (SCCS) arch. Through the in-situ monitoring of the Dayoushan Tunnel, the deformation and the pressure from surrounding soil affected by the “U” shaped terrain were analyzed. The buckling characteristic, the failure mode, the bearing capacity, and the in-plane and out-plane deformation of the π-type SCCS arch under different gradient distributed (GD) loads were investigated by establishing the nonlinear buckling simulation. The results show that the longitudinal pressure bias had a marked impact on the out-of-plane stability of the π-type SCCS arch. The superposition failure with in-plane local buckling and out-of-plane overall buckling was appeared, the level of gradient magnitude was linearly inversely proportional to the structural bearing capacity, and the out-of-plane deformation of π-type SCCS arch was significantly larger than its in-plane deformation. Also, the stability the π-type SCCS arch with different wall thickness was identified and the corresponding anti-buckling measures for the in-plane and the out-of-plane stability were proposed. This study can further provide reference for the design of loess tunnel support structure in longitudinal pressure bias terrain from the stability perspectives.

Construction and Quality Control of Subway Wet Loess in Concealed Tunnel Based on Particle Swarm Optimization Algorithm

To reduce urban pressure, urban rail transit has become an effective way to reduce traffic congestion, mitigate traffic accidents, reduce environmental pollution, and improve commuting efficiency. Subway as the main means of urban public transport travel, in recent years by people’s favor, although the construction industry of rail transit is developing rapidly and the industry scale is expanding, but because the construction of rail transit construction projects is very difficult, especially in the wet loess within the concealed excavation tunnel, but also frequent accidents, and the quality of the project is not easy to guarantee, so the underground railroad wet loess within the concealed excavation tunnel construction technology is poor. Therefore, it is especially important to study the construction technology and project quality management of underground railway concealed tunnel in wet loess. In this paper, based on the in-depth study of the basic principle of quantum particle swarm optimization calculation and the realization of key engineering technologies, the particle swarm optimization algorithm is programed using MATLAB software, and the coding scheme, operation specification, and operation parameters are designed. Then, combined with the particle swarm optimization algorithm and assisted MATLAB software, the main analysis of the construction and quality control of wet loess in concealed tunnels of subway projects was carried out, mainly by systematizing the relationship between the three major elements of subway project schedule, cost, and management and the construction and quality control of wet loess in concealed tunnels of a subway, and concluded that the construction and quality control of wet loess in concealed tunnels of the subway needed schedule. It is concluded that the construction and quality control of subway wet loess tunnel requires stable schedule, adequate cost budget, and management personnel.

Study on the Surrounding Rock Pressure Characteristics of Loess Tunnel Based on Statistical Analysis in China

A method of load calculation in a tunnel was adopted in consideration of the state of covering depth. Due to the macropore characteristics of loess soil, the existing demarcation regulations of deep and shallow tunnels are inappropriate for large-section and shallow loess in highway tunnels. To determine the load of large-section loess tunnel and provide a proper reference for structural design, the theoretical basis for distinguishing depth of deep and shallow tunnels, as well as the characteristics of common calculation methods for surrounding rock load on depth, were analyzed. Through statistical analysis and significance analysis, the significance of the variation of surrounding rock pressure with cover depth was studied. Correlation analysis was carried out on significant results to fit the relationship between cover depth and surrounding rock pressure. The results showed that the surrounding rock pressure varied linearly with the cover depth, and the surrounding rock pressure was about 0.3 times the initial pressure in deep tunnels. The surrounding rock pressure of shallow tunnels did not change significantly with the cover depth, and the surrounding rock pressure was loose pressure. A method based on statistics was proposed to distinguish the cover depth threshold (CDT) of deep and shallow tunnels.

Monitoring and Analysis of Deformation Refinement Characteristics of a Loess Tunnel Based on 3D Laser Scanning Technology

Loess tunnels often undergo large-scale deformation with complex spatial and temporal distribution. Mastering the characteristics of spatial and temporal deformation is conducive to precise policy implementation and the control of large deformation of the tunnel. In this study, relying on the Yulinzi Tunnel in Gansu Province, China, based on 3D laser scanning technology, the tunnel was monitored for a short period of 24 h and a long period of 36 days. The refined characteristics of the temporal and spatial deformation of the representative points of the interrupted surface, the tunnel face, and the excavation mileage during the excavation process of the three-bench and seven-step method of the tunnel were analyzed. The results show that the tunnel’s arch has large deformation, and there is twisting deformation. The distribution of the overall deformation of the tunnel is related to the excavation sequence, showing a stepped deformation law. With the construction of the following excavation process, the deformation rate of the tunnel always indicates the characteristics of significant in the early stage and small in the later stage, and the overall deformation changes with time in accordance with the distribution law of the exponential function. The research results provide a reference for predicting the deformation development trend of loess tunnels and providing reasonable deformation control methods.

Discussion on Treatment Measures of Highway Loess Tunnel Roof Fall

Discussion on Treatment Measures of Highway Loess Tunnel Roof Fall