Shallow Tunnelling Method Research Articles

Shallow Tunneling Method and Control Measure for Ground Surface Settlement

In order to ensure that the tunnel deformation and surface settlement are controlled within the allowable range during the construction process, the design unit has compiled technical measures and monitoring schemes for ground settlement control of this project. Based on the example of a shallow tunneling project on Subway line 8, this paper analyzes and discusses the shallow tunneling method in detail and puts forward corresponding technical measures for ground settlement control.

Seismic Behaviour of Shallow Tunnelling Method Tunnels Accounting for Primary Lining Effects

The shallow tunnelling method (STM) is usually used to construct shallow tunnels buried in soft ground. It consists of primary lining and secondary lining (tunnel). In the seismic design of STM tunnels, it is usually assumed that the secondary lining (tunnel) is resistant to all seismic effects. However, the soil–primary–secondary lining system may generate complex interaction phenomena during ground shaking. Compared with the case where the primary lining is not considered, the existence of the primary lining alters the seismic response of the secondary lining (tunnel). The paper attempts to investigate this complex interaction, focusing on the response of the secondary lining (tunnel). The full dynamic time history analysis is adopted to investigate the interaction in the transversal direction. A case history of the Hohhot (China) arched STM tunnel buried in a stratified soil deposit has been analyzed. Two tunnel configurations for a two-dimensional plane strain model of STM tunnels in Hohhot are studied and compared, including a model with primary lining and one without primary lining. A numerical parametric analysis was conducted to elucidate critical response characteristics of STM tunnels. Salient parameters that may affect the dynamic response of the tunnel were studied, including the characteristics of ground motion, the characteristics of contact interface, the characteristics of the soil, and the characteristics of the tunnel lining. The response characteristics of the tunnel are compared and discussed, including horizontal acceleration, deformation mode, lining internal force, and lining damage. The results show that the primary lining has a significant influence on the magnitude and distribution of the seismic response, especially considering the nonlinearity of the soil and the nonlinear characteristics of the tunnel lining. The effect of primary lining on the seismic response is about 5%–35 %.

Dynamic evolution in mechanical characteristics of complex supporting structures during large section tunnel construction

AbstractThe shallow tunnelling method (STM) often uses temporary supports to divide large section tunnels into several closed or semiclosed sections so as to share the upper load. The complex support system composed of primary and temporary supports can ensure safety during tunnel construction. Based on the large section tunnel of Beijing Subway Line 12, the mechanical characteristics of support system by the double‐side‐drift method (DSDM) during excavation and demolition were analyzed through numerical simulation and monitoring. The study showed that the middle cave excavation was the most critical stage of the DSDM, during which the load on the supporting structure increased significantly. The temporary vertical support bore most of the new load during middle cave excavation. During the demolition stage, the load was redistributed, which caused arch settlement and section convergence. The removal of the temporary vertical support exerted the greatest impact in this process. The lateral temporary inverted arch changed from axial compression to axial tension after the middle and lower caves were excavated. Based on the mechanical characteristics of the support system, some engineering suggestions were proposed for large section tunnel construction. These research results can provide reference for the design and construction of similar large section tunnels.

A comparative numerical analysis of design variation plans for a shallow tunnel in very soft ground after a sudden accident

A severe accident of surrounding soil failure occurred during the construction of Zizhi Tunnel, a shallow tunnel in very soft ground using the shallow tunneling method in Hangzhou, China. We provide an in-depth comparison with careful evaluation of three design variation plans through 3D numerical analyses on the stress distribution inside the advance core, ground deformation, and soil arching over the tunnel crown. Consequently, the third support design scheme, i.e., soil reinforcement using sub-horizontal Metro Jet System (MJS) piles, is recommended as it leads to the smallest disturbance to the surrounding ground, and is the most favorable for the soil arching formation. We also find that the advance core’s stability is crucial for preventing the face failure and reducing the surface settlement. Finally, the effectiveness of world’s first successful example of applying sub-horizontal MJS piles to reinforce tunnel excavation in very soft soil is verified by the field monitoring data.

Failure analysis and countermeasures of a tunnel constructed in loose granular stratum by shallow tunnelling method

To solve the failure problems encountered in the tunnels constructed by shallow tunnelling method in loose granular stratum, taking a tunnel construction project on the Sichuan-Tibet 318 Highway Line in China as an example. Failure phenomena, characteristics, and modes were systematically analyzed during tunnel construction. Then, the evolution at different failure modes was analyzed through DEM calculation and the prevention mechanism was discussed. Finally, corresponding construction countermeasures were put forward from the aspects of pre-support, excavation, bolt driving, and collapse prevention. The results show that: (1) The conventionally strong support system cannot play an active role in the tunnel. The core problems in construction are how to reduce the disturbance of excavation to surrounding rock and realize the restraint of the support system to the surrounding rock in the arch-foot area. (2) According to the loss of the surrounding rock, the failure modes can be divided into two types: Ⅰ-the rapid development failure from vault collapse and Ⅱ-the lag failure of ground cavity moving up. Construction optimization aims at avoiding the mode-Ⅰ and alleviating the mode-Ⅱ. (3) For the mode-Ⅰ, maintaining the stability of the initial ground-arch is the practical measure; For the mode-Ⅱ, when the cavity area is more significant than 5 m2, it must be filled in time. (4) The field deformation data of surrounding rock after treatment shows that the effect of the proposed countermeasures is remarkable, which can provide valuable references for the tunnel design, construction, and disaster control in a similar area.

Mechanical Behaviors of Existing Large-Diameter Tunnel Induced by Horseshoe-Shaped Undercrossing Twin Tunnels in Gravel

This article investigates and presents a case study on the Beijing Subway Line 12 excavation beneath the existing Qinghuayuan Tunnel. The composite pre-reinforcement technique was used in conjunction with the shallow tunneling method to control the distortion of the existing large-diameter tunnel. When building twin tunnels underneath, this strategy considerably decreased the impact on the existing large-diameter tunnel. To systematically study the mechanical response of the existing large-diameter tunnel, a variety of sensors was embedded in the prefabricated segments just above the new twin tunnels. During the undercrossing twin tunnels procedure, the earth pressure, tunnel crown settlement, opening width of the segment joint, and the circumferential strain of the large-diameter existing tunnel were all measured. The settlement development of the existing large-diameter tunnel was categorized under six stages: (1) sedimentation, (2) heave, (3) second sedimentation, (4) second heave, (5) third sedimentation, and (6) steady state. The joint opening of the existing large-diameter tunnel changed sharply during the new undercrossing twin tunnels. The earth pressure and concrete stress of the linings rapidly increased during the new undercrossing twin tunnels. The majority of the reinforcement and concrete stresses were compressive and far lower than the yield strength, indicating that the tunnel was in a safe working condition.

Comparison of shallow tunneling method with pile and rib method for construction of subway station in soft ground

Comparison of shallow tunneling method with pile and rib method for construction of subway station in soft ground

A new construction method for metro stations in dense urban areas in Shanghai soft ground: Open-cut shafts combined with quasi-rectangular jacking boxes

At present, the metro station constructions in dense urban areas face significant challenges during the rapid urban metro network expansion. The repeating disturbances to city traffic, commerce, and residents' daily life induced by conventional open-cut metro station constructions can no longer be accepted in dense urban areas. To solve these problems, some underground excavation methods, such as enlarging shield tunnels, shallow tunnelling method and pipe-roof mining method, have been attempted, but it is difficult for them to be adapted to the strict limitation of urban space in soft ground. In this paper, the planning, construction, challenges, and relevant key techniques of an innovative type of metro station – open-cut shafts combined with quasi-rectangular jacking boxes (SCJB) were introduced and investigated. The main conclusions are as follows: (1) For the advantages of efficient underground space utilization and little impact on traffic and environment, the SCJB method is recommended for the constructions of metro stations that need to traverse the heavy traffic roads and express-way viaducts. The flexible combinations of open-cut shafts and jacking boxes can form various layouts of underground space to adapt to various complex surrounding environment. (2) Steel-concrete composite lining was designed for jacking boxes considering the transportation convenience and bearing capacity enhancement, and the corresponding earth pressure balanced box jacking machines were developed. (3) By applying thick thixotropic slurry with small filtration loss, the jacking resistance of the large-section box culverts was effectively reduced to 4–6 kPa and the total jacking force was kept at 30,000 kN. (4) By surveying with laser guiding system and performing timely rectifications, the trajectory deviations induced by two large adjacent box jacking were controlled within ±50 mm. The horizontal convergence of east line box segments caused by the west line jacking was controlled within 1.5 mm, through strict trajectory control. The surface settlement induced by the crossing of three box jacking tunnels was effectively controlled within 15 mm. The experiences learnt from this project can provide valuable references for the planning and construction of metro stations in dense urban areas in soft ground.

Settlement characteristics of large-diameter shield excavation below existing subway in close vicinity

A case of Qinghuayuan tunnel excavation below the existing Beijing Subway Line 10 is presented. The new Qinghuayuan tunnel, part of the Beijing-Zhangjiakou High-speed Railway, was excavated by a shield machine with an outer diameter of 12.2 m. The existing subway was excavated by shallow tunnelling method. The project layout, geological conditions, reinforcement measures, operational parameters of shield machine and monitoring results of the project are introduced. During the Qinghuayuan tunnel excavation below the existing subway, total thrust, shield driving speed, cutterhead rotation speed and torque were manually controlled below the average values obtained from the previous monitoring of this project, which could effectively reduce the disturbance of the surrounding soil induced by shield excavation. The Gaussian fitting function can appropriately fit both the ground and the existing subway settlements. The trough width is influenced not only by the excavation overburden depth, but also by the forepoling reinforcement and tail void grouting measures.

Ground surface settlement response to subway station construction activities using pile–beam–arch method

A pile–beam–arch method is a shallow tunneling method suitable for constructing urban subway stations with dense surface traffic. The main construction process can be simplified into four stages: excavation of pilot tunnels, installation of piles and beams, installation of arches, and excavation of soil inside the subway station. This study uses numerical calculations to investigate the development law of ground surface settlement during the construction of subway stations. The results indicate that surface settlement gradually increases in the first three stages and rebounds after the fourth stage. Further, pilot tunnel excavation and arch installation are identified as key stages for controlling the surface settlement. Based on these facts, the effects of the sequence of pilot tunnel excavation and the spacing and cross-sectional area of the columns on the surface settlement are studied. The results show that the sequence of pilot tunnel excavation has a significant effect on the development of the surface settlement trough. Further, column spacing and surface settlement exhibit a linear relationship, wherein a shorter spacing leading to low surface settlement. The cross-sectional area of the column exhibits a nonlinear relationship with the surface settlement; the surface settlement is stable when the cross-sectional dimensions are 1.5. × 1.5 m. The Peck formula and stochastic medium theory are used to predict the surface settlement in the pilot tunnel excavation stage. The results are 30%–50% higher than that in the numerical calculation, which indicates their safety and conservatism. The results of this study provide a reference for the selection of construction parameters and the prediction of surface settlement in similar projects.

Numerical analysis on the features of road deformation in the construction process of heat supply pipeline works

Shallow Tunneling Method has been widely applied in the construction of pipe gallery. However, due to the complexity of strata and the particularity of environment, improper construction may cause uneven settlement of urban road, even pavement collapse. In the paper, in reliance on Beijing Xi’anmen Street Heat Supply Pipeline Works and by virtue of drilling and survey, the engineering geological conditions and pavement structure of the target area were ascertained, Finite Element Method was utilized for numerical calculation, so as to determine the surface settlement & deformation laws under different construction situations, including the change law of longitudinal and lateral pavement settlement following the progress of excavation, and also the final pavement settlement and settlement scope, etc. It was found by research that, under the joint action of variable section of pipe gallery and shaft pipeline, the section height of pipe gallery has marked effect on the change of pavement settlement, the settlement speed of pavement monitoring point appears to be “double-peak type”.

Displacement Characteristics of an Urban Tunnel in Silty Soil by the Shallow Tunnelling Method

The urban shallow tunnelling process in silty soil is easy to cause large displacement of surface and tunnel. Obviously, if the strata and the tunnel face are not treated by reasonable reinforcement method, instability and collapse phenomenon will be encountered during the tunnel excavation. There are a series of studies on construction methods of shallow tunnels, but these methods have limitations in silty soil. In this study, a comprehensive construction plan of the urban shallow tunnel in silty soil was proposed and applied to a case study in Fuzhou, Fujian Province in South China. The in situ monitoring tests and numerical simulation were employed to address displacement characteristics of surface and tunnel. Results indicated that the urban shallow tunnelling process could achieve good effect by dewatering of silty soil, reinforcing surface by vertical jet grouting piles, and advanced small pipes and circumferential grouting in the tunnel face; surface settlement during dewatering process accounted for about 30% of total surface settlement in silty soil; the excavation of the top heading, the middle, and lower benches had great effect on displacement of surface and tunnel for three‐bench seven‐step excavation method in silty soil; surface settlement troughs in silty soil were deeper and wider; lock‐feet bolts had good effect on restricting horizontal convergence; and ratio of total crown settlement and total horizontal convergence was in range of 1.43∼1.59 when b/h was 0.88 in silty soil. The construction plan proposed in this paper is helpful for further study of shallow tunnel tunnelling process in silty soil.

An Investigation into the Tunnel‐Soil‐Pipeline Interaction by In Situ Measured Settlements of the Pipelines

Response of buried pipeline to tunnelling is of great concern in the subway construction. It is of paramount importance but difficult to estimate the influence of tunnelling on adjacent pipelines because of the complex tunnel‐soil‐pipeline interaction. The technique of in situ measured settlements of buried pipelines remains the standard approach for understanding this interaction and addressing the issue. The Huangzhuang station of the Beijing Subway is located in a densely populated area, with many buried pipelines in the close proximity; the shallow tunnelling method of pile‐beam‐arch (PBA method) was used to excavate the station tunnels; the shallow tunnelling of the station tunnels inevitably causes settlements of the ground surface and the buried pipelines. Direct monitoring of the pipelines by digging holes from the ground surface was performed during the station tunnel construction. In situ measured settlements of the ground surface and the buried pipelines caused by the subway construction were obtained. It is observed from the obtained results that the pipeline settlement development can be divided into four stages that are associated with different construction periods of the PBA method. Sharp increases in the pipeline settlement occurred in the specific stages (stages 2 and 4). It is concluded from comparisons between the pipeline settlement and the ground surface settlement that separation between steel or cast iron pipelines and the soil beneath occurs due to the tunnel construction. And the pipeline settlement is smaller than that of the ground surface. This finding has the practical implication that the ground surface can be monitored instead of the buried pipeline. Using this indirect pipeline monitoring, the pipeline safety can be conservatively evaluated. This study is an example for evaluating the shallow tunnelling‐induced effects on adjacent buried pipelines and understanding the tunnel‐soil‐pipeline interaction under similar conditions.

Surface settlement of subway station construction using pile-beam-arch approach

This paper illustrates the surface settlement characteristics of subway station construction in the shallow-buried soft ground using the pile-beam-arch (PBA) approach of the shallow tunnelling method. The geological conditions, cross sections and measured surface settlements of 30 subway stations constructed by the PBA approach in Beijing are presented. The total surface settlements of the strip foundation station and the pile foundation station using the PBA approach mainly range from 40 mm to 80 mm and from 30 mm to 40 mm, respectively. The superposition of the surface settlements in Stage 1 and 3 of the PBA approach accounts for about 90% of the total surface settlement, and that in Stage 2 and 4 accounts for about 10%. The mean of the surface settlements produced in silty clay is smaller than that in silty sand, and is approximately 1.4 times as large as that in gravel. The surface settlement generally decreases with the increase of the overburden depth. The surface settlement could more than double due to the effect of groundwater. The surface settlement induced by the excavation of the latter level of drifts is commonly larger than that of the former level of drifts. In addition, the quality of workmanship has significant impacts on the surface settlement. This research is conducive to update the database of the world subway projects and can serve as a practical reference for future similar projects.

Performance Assessment of Arch-shaped Primary Lining during Construction in Weak Rock Shallow-buried Tunnel

Tunnels excavated through weak rock masses using the Shallow Tunneling Method (STM) call for rigid lining support to withstand and constrain the mobility of the unstable rock where its performance needs to be assessed with field measured data in case of potential failure. For fear of any further hazards after the occurrence of a rock fall at chainage km 86,685 m in the Longsheng Tunnels, the monitoring plan was specially modified to accurately obtain the relative vertical and horizontal displacements of the steel rib component in the arch-shaped composite rigid primary lining that was simply placed on the heading. In this paper, the weak rock masses’ interaction with the rigid lining is treated as unknown distributed loads divided into radial and tangential directions, where the influence line theory is applied to better understand the response of the lining so as to facilitate a displacement-based back analysis for unknown rock mass loads and the resultant stress outcome. By comparing the back analyzed stress outcome with the measured one, the proposed performance assessing method for the unclosed arch-shaped lining was proven to be time efficient with acceptable accuracy, which is of considerable application potential and can provide a reference for similar engineering.

Bridge Responses Induced by Adjacent Subway Station Construction Using Shallow Tunneling Method

This paper presents a case of subway station construction under an existing prestressed concrete bridge with a three‐span continuous beam located at the intersection of the 3rd Ring Road, Beijing. The Huayuan Subway Station of line 6, constructed crossing between #7 and the #8 piers of the bridge by the shallow tunneling method, is approximately perpendicular to the existing Huayuan Bridge. The minimum horizontal distance between the pile foundation and the subway station is only 0.08 m. The “Pile‐Beam‐Arc” construction sequence was used to ensure the safety of both the subway station and the bridge. Moreover, a series of reinforcement measures were adopted to safeguard the project, including deep grouting reinforcement surrounding the pile foundation from ground surface, temporary inverted arch in the middle of No. 5 drift, and the lateral steel support. Even though some cracks were observed on the bridge deck surface by the on‐site deformation monitoring, the results were still within the proposed control standard. To prevent the further development of the cracks, jacking protection measure and bonded steel constructed under the box girder were performed. The related measures proposed in this research can provide useful references for future similar projects.

Limit analysis of a shallow subway tunnel with staged construction

This paper presents a limit analysis of the series of construction stages of shallow tunneling method by investigating their respective safety factors and failure mechanisms. A case study for one particular cross-section of Beijing Subway Line 7 is undertaken, with a focus on the effects of multiple soil layers and construction sequencing of dual tunnels. Results show that using the step-excavation technique can render a higher safety factor for the excavation of a tunnel compared to the entire cross-section being excavated all at once. The failure mechanisms for each different construction stage are discussed and corresponding key locations are suggested to monitor the safety during tunneling. Simultaneous excavation of dual tunnels in the same cross-section should be expressly avoided considering their potential negative interactions. The normal and shear forces as well as bending moment of the primary lining and locking anchor pipe are found to reach their maximum value at Stage 6, before closure of the primary lining. Designing these struts should consider the effects of different construction stages of shallow tunneling method.

Surface settlements induced by tunneling in permeable strata: a case history of Shenzhen Metro

A case study of a significant surface settlement induced by tunneling in permeable strata with the shallow tunneling method is presented in this paper. The measurements of surface settlements along the excavation direction were first analyzed to highlight the impacts of groundwater seepage. Due to the groundwater inflow inside a double-arched tunnel, the surface settlement developed to a high level far beyond the measured crown settlement. The settlement-affected zone extended to 4 times the tunnel height ahead of the forefront heading and 1.5 times the tunnel height behind the hindmost heading. Consolidation resulting from high pore pressure change was considered to be the main mechanism for the large surface settlements. In addition, a 3D fluid-mechanical coupled numerical analysis was carried out to confirm the relationship between the significant surface settlement and pore pressure variation. This analysis reveals that lowering the permeability of the small pipe grouting zone, especially of the primary lining, could lessen the drop in pore pressure in the overlying strata, further reducing the total surface settlement. The numerical results also suggest that the transverse range of vertical displacement could be quite wide, and the settlement developed integrally from the tunnel crown towards the ground surface due to groundwater seepage. Moreover, the effect of advance drainage on surface settlement was investigated based on the same numerical model. Drainage with horizontal boreholes could considerably increase the safety of tunnel heading but had limited impact on surface settlement. Finally, the applications of pre-grouting and advance drainage measures were discussed for tunneling cases in permeable strata.

Ground surface settlements due to construction of closely-spaced twin tunnels with different geometric arrangements

This paper examines the ground surface settlement profiles due to the construction of closely-spaced twin tunnels using the shallow tunnelling method. In the concerned zone, where the twin-tunnelling was performed in stacked and offset arrangements, the ground surface settlements of in total 18 cross-sections were continually recorded during construction. To cater for different conditions of the twin tunnels in the concerned zone, partial face, full face and forepoling reinforcement schemes were adopted. The recorded surface settlements and settlement troughs of three typical sections are reported and illustrated. The surface settlement troughs induced by each of the twin tunnels are fitted by the Gaussian function. The parameters that characterize the surface settlement troughs induced by each of the twin tunnels, such as the maximum settlement, percentage of ground loss, trough width and empirical trough width parameter are presented and compared.

Effects of twin tunnels construction beneath existing shield-driven twin tunnels

This paper presents a case of closely spaced twin tunnels excavated beneath other closely spaced existing twin tunnels in Beijing, China. The existing twin tunnels were previously built by the shield method while the new twin tunnels were excavated by the shallow tunnelling method. The settlements of the existing tunnels and the ground surfaces associated with the new tunnels construction were systematically monitored. A superposition method is adopted to describe the settlement profiles of both the existing tunnels and the ground surfaces under the influence of the new twin tunnels construction below. A satisfactory match between the proposed fitting curves and the measured settlement data of both the existing tunnels and the ground surfaces is obtained. As shown in a particular monitoring cross-section, the settlement profile shapes for the existing tunnel and the ground surface are different. The settlement profile of the existing structure displays a “W” shape while the ground surface settlement profile displays a “U” shape. It is also found that due to the flexibility of the segmental lining, the ground losses obtained from the existing tunnel level and the ground surface level in the same monitoring cross-section are nearly the same.