Construction Safety Risk Assessment of Large-Sized Deep Drainage Tunnel Projects

Frequent extreme climate events and rapid global urbanization have amplified the occurrence of accidents such as waterlogging or the overflow of pollution in big cities. This has increased the application scenarios of large-sized deep drainage tunnel projects (LSDDTPs). The scientific and accurate evaluation of the construction safety risks of LSDDTP can effectively reduce the corresponding economic losses and casualties. In this paper, we employed the hierarchical holographic model to construct the safety risk list of LSDDTPs in terms of the risk source and construction unit. Based on social network analysis, we then screened key indicators and calculated the weights of all secondary indicators from the correlation between risk factors. We subsequently developed a construction safety risk assessment model of LSDDTPs based on the matter-element extension method. The Donghu Deep Tunnel Project in Wuhan, China, was selected as a case study for the proposed method. The results of empirical research demonstrated that eight indicators (e.g., failure to effectively detect the change of the surrounding environment of the tunnel project) were key factors affecting the construction safety risk of IV, which is within the acceptable risk level. Our proposed model outperformed other methods (the fuzzy comprehensive evaluation, analytic hierarchy process, entropy weight method, and comprehensive weight method) in terms of scientific validity and research advancements.

Flume tests on the hydraulic/mechanical behavior of the Tangjiashan landslide dam considering the influences of the different debris materials in heterogeneous strata

Landslide dams (LDs) usually form from natural debris materials and exhibit heterogeneous strata along both the depth and run-out directions. In addition, an LD usually has a weaker structure than that of undisturbed ground and is more vulnerable to seepage loading. Considering that the surface layer of naturally packed LD materials is generally in an unsaturated state, it is undoubtedly important to investigate the stability of the unsaturated debris materials in the heterogeneous strata of LDs. In this paper, a systematic flume test program was first conducted, in which the Tangjiashan LD was carefully referenced for model design. Three water level rising rates and two stratal arrangements were considered in the flume tests. Then, soil-water-air coupled finite element analyses were conducted to simulate the flume tests, and all the material parameters of the LD materials were carefully determined based on the results of the element tests. A comparison of the test and calculated results shows the possibility of using the proposed numerical method to estimate the occurrence of dam breaching and the risk of LD failure. Moreover, the hydraulic/mechanical behaviors of the LD materials and the heterogeneous strata of the LD were very important to the stability of the Tangjiashan LD. Finally, from an engineering viewpoint, the possibility of utilizing a naturally formed LD and thus not destroying it when it forms is also discussed, e.g., dam breaching risk can be reduced by excavation of a drainage tunnel, and the dam stability can be carefully estimated based on accurate geological data.

Errichtung eines Eisenbahntunnels in Deckelbauweise über dem Hauptsammler West der Stadt Stuttgart/Construction of a railway tunnel above the main drainage tunnel of Stuttgart using the cut-and-cover method

Im Zuge der Neuordnung des Bahnknotens Stuttgart erfolgte eine partielle Verlegung der S-Bahn-Zuführung nach Untertage. Dies erforderte im Bereich von Block 18 der Deckelbauweise die Überfahrung des bestehenden Hauptsammlers West der Stadt Stuttgart (HWS) bei einer verbleibenden Überlagerungshöhe von rund 1,0 m. Zur Begrenzung des Einflusses der aushubbedingten Entlastung auf das Bestandsbauwerk wurden unterschiedliche Lösungsansätze untersucht. Detaillierte numerische Berechnungen erlaubten eine optimierte Planung des ursprünglich vorgesehenen Amtsentwurfs, wodurch der ausführungstechnische Aufwand erheblich reduziert werden konnte.   Dieser Beitrag beschreibt zunächst die komplexen Randbedingungen des behandelten Projektabschnitts. Darauf aufbauend werden die entwickelten Lösungsvarianten diskutiert. Der Hauptteil des Beitrags behandelt die durchgeführten numerischen Untersuchungen. Dabei werden die Modellierung des HWS und die Änderung des Beanspruchungszustands während der einzelnen Bauphasen näher beleuchtet. Abschließend werden ausgewählte Aspekte der Ausführungsphase aufgezeigt.

Risks involved in vertical shafts construction for the eastern drainage tunnel in Mexico

This article describes the main stages involved in the construction of the vertical shafts (large-diameter vertical wells), which are necessary for the subsequent construction of the tunnel´s sections. The different risk situations existing during the construction of the Eastern Drainage Tunnel in the valley of Mexico City (in Spanish, "Tunel Emisor Oriente") are analyzed. In order for this 52 km-long and 7.5 m-wide tunnel to carry part of the city’s sewage, 25 shafts must first be built, ranging from 55 to 150 meters deep. The magnitude of such a project implies working in different geographical areas and varied geological strata involving the presence of groundwater, which increases the risks due to possible landslides or flooding during excavation. As digging will occur in different types of soil, varying procedures must be used depending on soil type. Likewise, due to the magnitude of this kind of project, detailed scheduling and planning are required as simultaneous works on different fronts are necessary to meet deadlines. The study mentions that, while projects like these involve high risks for workers, analysis of activities and situations are conducted precisely to demonstrate that such risks can be considerably reduced.

Slope stability analysis of the drainage tunnel portal in Tanju Dam, Dompu District, West Nusa Tenggara

This paper presents design results of the tunnel portal slopes at the Tanju Dam, Dompu, West Nusa Tenggara. The objective of this research was to analyse the stability of the tunnel portal slopes using circular failure chart (CFC) method, limit equilibrium method (LEM), and finite element method (FEM). Input parameters were obtained from drill core evaluations and laboratory tests. By considering the rock mass rating (RMR) values of rock masses, which are categorized as class II, at the two slopes, adjustments for the cohesion and inner friction angle values are made. The inlet slope (IL) have cohesion values of 350 kPa and 40º inner friction angle and the outlet slope (OL) have cohesion values of 400 kPa and 45º inner friction angle. The CFC method shows that the IL and OL have safety factor (FS) values of 3.5 and 3.44, respectively. The LEM shows that the IL and OL have the FS values of 3.69 and 3.65, respectively. Meanwhile, the FEM shows that the IL and OL have FS values of 4.78 and 4.79, respectively. The stability analysis results indicate that designed slopes are stable.

Slope and Groundwater Monitoring for 3D Numerical Modelling to Ensure the Structural Health of an Alpine Road Tunnel Crossing an Active Rock Slide

Railways and roads frequently cross natural corridors like alluvial plains and alpine valleys. Here, structures and infrastructures can be affected by natural hazards such as floods and landslides. In some cases, the design has disregarded the possible interactions between slope processes and linear infrastructures. This work summarizes a 20-year long research comprising monitoring and laboratory data, field investigations and numerical modelling about an active 25-million m3 rock block slide threatening the serviceability of a highway tunnel in the Eastern Italian Alps, along the Tagliamento River Valley. The effectiveness of 3D geotechnical and hydrogeological numerical modelling calibrated on long-term monitoring datasets in planning countermeasures for landslide risk mitigation is demonstrated. A correlation between rapid snowmelt and/or extreme rainfall events and landslide activity is found. Moreover, monitored stream and spring discharges, together with seepage along the tunnel, appear to be strictly related to the displacements measured by GNSS and in-place inclinometers. In particular, the landslide accelerates once the threshold of 20 l/s in the tunnel seepage discharge is overcome. The continuous monitoring of specific electrical conductivity in five points allows tunnel discharge to be characterized identifying two type of groundwater circulation, one deeper and one perched, developing during extreme event. These facts clarify the role played by rainfall infiltration and groundwater flow in the fractured rock mass in promoting slope movements and damage in the tunnel lining. Based on these observations, two different 3D codes are used for groundwater flow simulation (FEFLOW by DHI-WASY) and stress and strain analysis (FLAC3D by ITASCA). The actual conditions of the slope and the possible countermeasures have been simulated. In FEFLOW, the Equivalent Porous Medium (EPM) approach is adopted with a model domain of 8-km2 including the landslide and the infrastructures. In FLAC3D, the properties of the sliding surface are reduced to simulate the wetting caused by the rising of hydraulic head in the fractured rock mass during the snowmelt or rainfall events. The 300-m long extension of an already existing T-shape drainage tunnel is analyzed. The simulated countermeasure work induces a lowering of the hydraulic head in the rock mass; consequently the reduced geotechnical properties have to be applied to a smaller section of the slip surface, resulting in a decrease of displacements. Even though the stabilizing effect is not definitive, mainly because of the volume of the unstable slope, the extension of the drainage tunnel reduces both the intensity and the duration of the water seepage into the tunnel with direct benefits for the tunnel serviceability.