SEPARATING UNDERGROUND METRO LINES UNDER OPERATION IN BAKU

Transcription

1 SEPARATING UNDERGROUND METRO LINES UNDER OPERATION IN BAKU SYSTRA FR: Federico VALDEMARIN, Mott MacDonald CZ: Jan CENEK, Self-employed prof.: Elena CHIRIOTTI

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3 SEPARATING UNDERGROUND METRO LINES UNDER OPERATION IN BAKU SYSTRA FR: Federico VALDEMARIN, Mott MacDonald CZ: Jan CENEK, Self-employed prof.: Elena CHIRIOTTI SUMMARY The 28 May Station is the only transfer station between the existing Green and Red metro lines in Baku. As part of the huge development strategy of the metro network, the complete separation of the existing underground lines has been planned at this location, in order to simplify the operation of the future metro system. The extreme challenge of the project is represented by the need of achieve the separation of such lines over a breakdown of the operation of just five weeks. The lack of direct access from the surface, a complex geology with artesian water layers, a congested underground space and sensible buildings represent additional constraints. The chosen design solution consists in creating a complex steel pipe jacking frame around the operating tunnels and then excavating under such frame two large caverns where the separation of the existing lines and the construction of the new metro tunnels will be achieved. Soil treatments have been conceived to mitigate the geotechnical risks and to reduce possible impacts on the existing structures. The paper illustrate the challenges of this extraordinary project, one of the most complex underground structures under study, the design solutions and the risk management approach used to manage the complexity and to design the mitigations measures. 2

4 Figure 1. Existing Baku metro network and location of the 28 May Station INTRODUCTION The metro network in the capital of Azerbaijan is rapidly growing together with its metropolitan area. The beginnings of the existing metro system started in Baku in 1967 in Soviet times. To date, only two lines (Green and Red) are in operation with a total length of 34km, including 23 stations. The future network will be much wider: besides the construction of three completely new lines and the extension of two existing lines, the whole metro project includes the development and modernization of the existing metro sections. A consortium formed by SYSTRA (leader), Mott MacDonald (Praha) and Saman (Korea) awarded the contract for the preliminary and detailed design of infrastructures for the extension of Baku Metro Network. This extension consists in 83km of new lines and extension of existing lines. As part of this modernization project, the connection of the Green Line at the 28 May/Cefer Cabbarli Stations, focus of this paper, represent also it s Achilles heel given that the already very congested system have only one transfer station. The Project Prior to the extension of the southern part of the existing Red Line and Green Line, and in order to improve the current transport capacity, a key activity is the removal of the Y operation at the 28 May Station and the separation of the Red and Green Lines tunnels. The 28 May Station in fact, connects today the arrivals on the Red Line in the direction Icharishahar (Old Town) and the Green Line in the direction Darnagul. Passengers wishing to continue along the Green Line, must first go through a complex system of galleries to transfer at Cefer Cabbarli station where the trains are dispatched to the Khatai Station (Figure. 1). The new project aims to simplify the operation of the May 28 Station and to completely separate the tunnels of the two lines. Figure 2 presents the existing and the future layout of the tunnels and station s platforms at the 28 May and Cefer Cabbarli Stations. The Separation of the Red and Green lines at the 28 May Station consists of the construction of two tunnels of 190meters in total as shown in Figure 3. They are created by partially demolishing the existing garage and storage terminals of the Xatai shuttle and making available the connection to Nizami station trough the Green Line. 4

5 Figure 2. Existing (left) and future (right) metro network at 28 May station. Figure 3. 3D model 28 May station after separation THE INPUT DATA AND CONSTRAINTS To get accurate and reliable data for planning infrastructure and underground works in post- Soviet countries is quite complex because for secrecy, confidentiality, topographical information was deliberately distorted or shifted by tens of meters, mostly unavailable as built documents for existing structures. In addition, the local suppliers for exploratory work do not follow international standard methods and their output can be unclear. The Geological Investigations and their Uncertainties Baku urban area lays in the middle of a large syncline responsible for the amphitheatre morphology of the city and the differences between the formations. The folded geological formations present in Baku subsoil are sediments from the Quaternary, and Neogene. These sediments are mainly made of clays, sands, and some banks of limestone and sandstone. Based on the geological data made available from previous project in the area at the feasibility stage, the entire geological profile was considered a huge layer of relatively stiff clay. A first investigation campaign composed by 9 core recovery borehole and 8 SPT vertical logs was launched in 2012 in order to confirm these assumptions. The very first boreholes of the campaign detected loose soils and artesian water under the stiff clays starting 25-30m BGS. After performing laboratory analysis (more than 200 identification tests, 30 shear tests and 16 compressibility tests) the local contractor was not able to satisfactorily characterize the lower layer whose characteristics appeared contradictory and inconsistent with the description of the borehole logs. The designer requested additional geological surveys, which were supposed to satisfactorily identify and characterize the lower strata. For this purpose, 13 cone penetration tests 5

6 CPTu (the firsts ever conducted in the country) and 2 pumping tests were recommended and carried out. Based on the results, the bottom layer was characterized and classified as a sandy silt. During the implementation of the additional geological survey, the presence of artesian groundwater was confirmed, which increased the concerns about the underground works being executed in the layer of sandy silt. The pump tests also confirmed a continuous water horizon under a confined pressure of about 3 bars. The Geological Model Based on the results of the CPTu it was possible to clearly identify and distinguish the following Geotechnical Units: Unit 1 (0 to 4m depth): man-made ground unit Unit 2 (4 to 26-30m depth): stiff Silty Clay unit. The unit is composed mainly by silty clay, with subunits of sand and sandy silt. These sand lenses are almost always recognized trough the CPTUs. The connection between such lenses, although not surely established based on the actual data, was considered in the implementation of risk analysis. Unit 3 (26-30 to 60m depth): Silty Sand unit, consisting of silty fine sands laminated with thin to very thin interbedded silty clay and sandy silts. Based on the above characterization, the distribution of the geotechnical units has been studied and a geotechnical 3D model of the studied zone was produced. The study has been developed using the GDM software of BRGM that allows to build an interpretative 3D model of the geology, the existing and the future structures, and to represent such interpretation along oriented sections. The Geotechnical Characterization It must be mentioned that geotechnical campaigns (both in situ and laboratory tests), performed in Baku by local companies, did not comply with international standards in terms of methodology, equipment implementation and calibration, recovery, testing, and interpretation. Specifically, the sampling method (simple core barrel) led to significant sample disturbance and affected the reliability of mechanical test. For such reasons many parameters were derived more from CPTUs results or from designer s experience on Baku s soils than from Lab tests results. Also taking in to account the intrinsic variability of the Units, a set of worst credible values for each Unit has been defined further to the reference values (see Table 1), in order to develop the design by scenarios. Table 1. Geotechnical characterization for the geotechnical Units 6

7 Figure 4. Existing surrounding structures at project location The Existing Structures One of the biggest constraints of such project is represented by the surrounding existing structures. The presence and the sensitivity of such structures limits the jobsite access and space at the surface, potentially interferes with the designed temporary and permanent structures, and requires particular attention to the effects of induced settlement. Figure 4 present a 3D view of the main existing structures at the location of the future works. The main existing structures considered as design constants and for risk analysis are: the 28 May Station: a deep mined station consisting of a central tunnel, two side tunnels and transversal galleries for the connection with the Cefer Cabbarly station; the Cefer Cabbarli Station for the shuttle line to Xataï: a deep mined station consisting of two twin tunnels, two dead-end galleries and a network of adits for ventilation and drainage connected at the end; The existing running metro tunnels: circular shaped, lined by cast iron segments of 5.1m internal diameter with pressure gate chambers with a cast iron segmental lining of 6.2m diameter; an underground shopping passage consisting of a reinforced concrete box which extend foundations to about 5m below the surface; utilities such as a water pipeline (D 500mm), a medium pressure gas pipeline, 0,4 kv electric cables and multiple unspecified manholes, which need to be diverted before any works may take place; the Azerbaijan State Oil Academy: a five storey masonry structure, with 4,5m deep strip foundations considered a sensitive structure due to its public use. 7

8 Figure 5. The three options from the Feasibility Study DESIGN PROCESS AND SOLUTIONS As stated at the previous chapter, the seemingly simple task that involved the construction of two 100m long track tunnels is further complicated by the results of the investigations and by the time constraints imposed by the Client. Those constrains mainly consist of: Topography there is no direct surface access; access located immediately below university building precluded the use of a large excavated open pit, and in proximity to a complex system of existing underground structures; Geology - the risk of encountering saturated sand lenses, soft clays, artesian aquifers; Time - the client requested the work be completed in an extremely short period (during shutdown of the Green Line 5 weeks). During the Feasibility Phase, three different options were analysed, including the direct re-excavation of the existing tunnels (Fig. 5a), the excavation of a cavern preceded by simple side adits (Fig. 5b) or by adits combined with diaphragm walls (Fig. 5c). The client selected the third variant, despite to the longer period of construction and higher costs, because it resulted in the shortest shutdown period. The selected option was subsequently developed to the preliminary design level and then further modified to account for the topographical and geological surveys that allowed to better estimate the possible collisions with the network of existing tunnels, and the possible effects of the artesian waters. This was done in close collaboration with the contractor selected by the Client. The completed design was submitted for comments to the Azerbaijani state expert analysis and the Independent Checker selected by the Client. The Basic Design The challenge consisting of excavating two relatively short tunnels (about 100m long) which encountered a section of an existing tunnel made of cast iron segments and performing this work as quickly as possible to minimize interruptions to the metro operation. The work also has to be done in proximity to a network of existing tunnels without easy access from the surface. The whole design was developed in considering the following key-issues, with taking into consideration the project s risks and constraints: 1. To develop a design based on a systematic risk analysis process shared with the Employer and his representative: the design choices were driven by risk assessment and this process has been traceable and explicit. 2. To develop the design by scenarios: a reference scenario based on a deterministic set of parameters (most likely soil and groundwater characteristics and behaviour, volume loss, deconfinement ratio, works performance, geometry, etc.); a scenario based on worst credible parameters quantitatively considering the impact of a certain number of geotechnical 8

9 uncertainties and their variability to test the robustness of the proposed technical solution, and to incorporate the necessary corrective measures in the design. 3. To define mitigation and contingency measures to control the effects induced by the installation, excavation, and completion of the underground work, for both the reference (mitigation measures) and the worst (contingency measures) scenarios. 4. To define a set of driving parameters with their respective operational ranges (when applicable), and attention and alert thresholds to be used during construction to monitor the work and assist in decision making. 5. To predefine the sequence of actions when attention or alert thresholds are reached. 6. Given the multiple excavations and mutual interferences, the correct design method is a 3D approach. In particular, numerical, coupled analyses have been considered (combining mechanical and hydraulic challenges). 7. To select in a traceable way the most adequate construction methods and ground treatment, taking into consideration the soil and groundwater conditions and required space for jobsite logistics. 8. To cope with time and environmental constraints. The Basic Design s Solution The separation of the red and green lines is to be performed mainly the under protection of two underground caverns, both constructed below the Oil Academy Building. The depth of the new tunnel alignment is approximately 18 22m below the surface. The overburden of the upper lateral adits is around 12 15m. The caverns themselves are planned to have a maximum width of approximately 17m and a height of approx. 10m. The ground bearing structure of the caverns are to be built before the excavation of the caverns. The 3D model of Figure 6 gives an overview of the project at the 28 May station. The magenta show the new separation tunnels, while the objects in green, yellow and pink represent all the preliminary works needed to install the new separation tunnels in the cavern. Figure 6. 3D model of the basic design s solution The access shafts are excavated to the level of an intermediate slab. From there, the upper level adits are excavated. At the same time, the shafts are completed to the bottom level. If necessary, ground treatment (e.g. jet grouting, ground freezing) from the upper level adits is performed to support the excavation phase of the lower adits. Between the lateral adits, vertical and horizontal pipe jacks are installed. These pipes are then filled with concrete and reinforced at their ends (corner connections), within the adits, to ensure a stiff frame around the caverns excavated at a later time. Longitudinal adits will be kept close to the future and existing Green Line tunnel to form a square cavern shape which has 3 adits in 3 corners of the rectangle. The distribution of the longitudinal adits and pipe jacking frame can be seen in Figure 7. The access to the cavern excavation is opened from the shaft with an approximately 6.0x6.5m window to facilitate the excavation progress. From there, the first level of the cavern (5.5m under pipe jacking roof) is excavated as well as the head walls. The drainage system is also installed. After this phase, the operation of the green line needs to be interrupted. Then, the caverns excavation is completed and the existing running tunnel demolished. The invert and construction elements for the new running tunnel are installed in sequence. Once the connecting structures are built, the caverns are filled and traffic operation is resumed. 9

10 Figure 7. The section type for basic design s solution This solution allows reducing the traffic interruption on the green line for a period not to exceed 5 weeks. RISK MANAGEMENT The main geotechnical hazards identified for the construction phases are: The presence of the artesian groundwater during excavation of the lower adits and the connection between the pipejackings tubes may result in the instability of the excavation and unsafe conditions for the workers. Although the low permeability values registered during the tests, the presence of artesian groundwater in the Silty Sand Unit may result in water inflows and in stability problems at the base of the caverns. The presence of sand lenses interbedded in the Silty Clay Unit may contain water or, if thick enough and continuous, may be connected to the artesian aquifer, results in possible water inflows and instability during the excavations phases. The uncertainties due to poor recovery of the samples and questionable reliability of lab results oblige to carry out the design through parametric studies, analyzing the influence of key input parameters in the engineering solutions. The identified hazards have also to been considered in the context of a densely urbanized environment and heavily utilized underground space, which strongly limit the investigations distribution and that present a significant constraints for the geometry of the excavation and the application of soil support. The geotechnical hazards above mentioned have been thoroughly considered in the Risk Register Matrix. The process of minimizing of such risks has been an integrated part of the design phase and will be completed and implemented in the future stages of this project, together with: Non deterministic approach in design (most likely and worst credible conditions) Identification of mitigation measures, where and when needed Definition of predefined countermeasures (to be fully shared with the Contractor) Observational method during construction (role of monitoring) A preliminary Risk Register has been drafted in accordance with ISO: 3100 and other relevant standards [1-4] by the designer in the basic design phase of the 28 May Station Upgrade project. This has allowed to properly communicate and share the risks among the project s actors, establish shared geotechnical baselines to access the impact of geotechnical uncertainties, and to define the provisions for risk. The risk related to the uncertainty of certain geotechnical parameters and to the response of the soil to excavation and soil treatment, has pushed the designer and the Owner to propose that the access shafts be used as the location for the field trial tests for the soil treatment work and to confirm/ update as much as possible the geotechnical characterization by means of additional vertical and inclined boreholes and in situ tests. Consequently, the contractor has the responsibility of updating the design based on the results of such tests and on the surveys carried out through excavation of the shafts. This has led the project s actor to select a design -build form of contract in which the steps of the design update are formalized, together with the procedure in which such results are used to update the project. 10

11 CONCLUSIONS The project which consists in separating the running tunnels of the Red and Green Metro Lines in Baku is particularly challenging for many reasons: the Client s requirement of interrupting the metro traffic for no longer than 5 weeks; the complexity of the existing underground structures; the sensitivity of the buildings above the future work; the limited access space on the surface and the limitations in placing and dimensioning access shafts to support the excavation of the underground work; the existing condition of the subsoil and the pressurized aquifer; the uncertainties related to the ground characterization. This has led to develop a very challenging design involving innovative technical solutions requiring high accuracy and latest technologies for the first time used not only in Azerbaijan such as: two huge caverns supported by rigid closed water resistant pipe jacked frame completely jacked from small underground access galleries, the massive ground treatment measures (in alternatives, also installed from the underground) and taking in to account existing and operating tunnels and buildings in the close proximity, replacement of old by new metro tunnels including rail and MEP in extremely short time. Where the contractor s experience has been involved as early as possible in the study of the construction method. This was combined to a systematic risk management approach and to the definition of the most adequate form of contract considering the project constraints and characteristics. REFERENCES [1] The Code of Practice for Risk Management of Tunnel Works (2006). International Tunnelling Insurance Group (ITIG), presented at the ITA World Congress, Seoul, April [2] Guidelines for Tunnelling Risk Management: International Tunnelling Association, Working Group n 2 (2004). Tunnelling and Underground Space Technology, N.19, 2004, pp [3] The Joint Code of Practice for Risk Management of Tunnel Works in the UK (2003). Published by the British Tunnelling Society (BTS), prepared jointly by BTS and the Association of British Insurers. [4] A Guide to the Systematic Management of Risk from Construction (1996), CIRIA, Special Publication

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