SERVICE STATES OF AN OPERATIONAL TUNNEL: A VIEW FROM A STRUCTURAL ENGINEER

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1 SERVICE STATES OF AN OPERATIONAL TUNNEL: A VIEW FROM A STRUCTURAL ENGINEER Yong Yuan (1) (1) Key Laboratory on Geotechnical and Underground Engineering, Tongji University, China Abstract Understanding the service state of a tunnel is the fundamental aspect for its service life design. When tracing back the data collected from a site inspection of a tunnel structure it can be concluded that it is rather difficult to identify the service state of a tunnel structure given the current specifications of the serviceability limit state during the design stage. To illuminate the service state is a challenging task for given the complexity of durability of a tunnel structure which usually interacts with loads and environmental actions. This paper tempts to give a statement on concept of the serviceability limit state of a tunnel in full operation, seen from a structural engineer point of view. In order to achieve this, inspection data could be classified into levels of material, structural components, and structure. This implies that service could be specified according to the degree of decay in relation to their performance. In this paper, a classification of service states has been suggested based on this consideration and being a way to assess the actual state of development in the view of limit state design. 1. BACKGROUND A tunnel, being a major piece of infrastructure, usually serves for transportation, water transport, or as a pipeline. Conditioned by its situation in the operation stage it usually suffers attacks from mechanical, physical, and even chemical actions. Traditionally it is designed based on loads it would bear during its design service life. However, this paper is on a tunnel where degradation of its performance was reported during routine maintenance. It implied that the load-bearing capacity was not sufficient for such infrastructure. Furthermore, given the high costs for construction of a tunnel a long-term service life is desired which is leading to increasing requirements when considering its durability. For a service life design it is necessary to impose a desired state that tunnel structure should satisfy with. The approach of a limit state design is currently recommended by the national design guidelines [1] as well as other international design guidelines [2]. It specifies two types of limit states which are the ultimate limit state and the serviceability limit state. For these limit states, when designing a structural member, the first focuses mostly on load- 929

2 bearing capacity, while the later is associated with deflection of a member under loading. Obviously, it is impossible to account for the environmental actions that may lead to the degradation or deterioration of the structural member, and with this, its structural performance. Siemes and Polder [3] believed that more limit states should be defined to describe the structural performance. Vrouwenvelder and Schiessl [4] gave a probabilistic approach for the ultimate limit state that accounts for the chloride penetration in concrete members with the objective to disclose the limit state approach in the durability design and application [5]. Recently, as summarized by the structural design approache of Folić [6] it is concluded that the limit state is a sound way for durability design. These investigations showed that proper specifications of limit states are needed for achieving a service life design. This paper envisions a description of a service state for an urban tunnel in the light of site surveys. Defects of an operating tunnel were recorded not only on the material level but also on the structural level. In order to relate these defects to the causes that resulted in them could be a class of state. It could help to make a service design in light of a limit state approach. 2. STRUCTURAL DESIGN OF A SHIELD TUNNEL A tunnel is usually designed according to the loads that act on it within a specified working period, which associates with its service life. For a tunnel crossing a river, factors of structural safety should be checked for any typical cross-sections. When considering the design of a shield tunnel as provided in Figure 1 as an example, those typical sections are[7]: (1) The deepest section with the largest overburden; (2) The most shallow section with the lowest overburden; (3) The section with the highest groundwater level; (4) The section with the lowest groundwater level; (5) The section with largest surcharge; (6) The section with the most eccentric loads; (7) The section with uneven surface; and (8) The section with an tunnel adjacent at the present or planned in the future. 930

3 (a) A typical lining-ring (b) Loads acting on a typical cross-section Figure 1: Assumed design of a tunnel Based on a structural analysis of a given cross-section under specified loads, safety and stability of the tunnel lining can be verified in light of the limit state design. Ever though the magnitude and character of the loads acting on the tunnel would be stable, properties of materials that are used to construct the tunnel could vary during the time in service while under attack of its service conditions. That means to ensure a reliable tunnel structure the serviceability decay with age should be taken into consideration in the design stage. However, as indicated by Vrouwenvelder and Schiessl [4], requirements related to durability are usually specified in an implicit way. This hinders the implementation of a durability design for structures. 3. SERVICE STATES OF AN OPERATIONAL TUNNEL 3.1 Data from received from tunnel inspections The main restriction of site inspections is that usually only the inner surface of a tunnel is considered by visually observing of the actual state of its surface condition. The selected measurement instruments for observing the actual state of the restricted section profile were conducted arbitrarily. 931

4 (a) partial loss of a segment (b) seepage from joint (c) rusty bolt (d) stagger between segments Figure 2: Typical defects observed from a tunnel lining As can be observed from Figure 2, the inspections of a tunnel in operation show main defects given as: Surface appearance cracks on the surface of a tunnel lining; portion loss of lining segment; leaching of cement; scaring of cover that protects concrete or steel member, spalling of concrete; rust or pit formation on the surface of steel member. Waterproof leakage (including wet spots, seepage, drop, spring, or jet of water) from cracks or joints; Deformation movement of joint (including opening and stagger); 932

5 convergence of lining ring; differential settlement along tunnel. Usually a structural engineer will pay close attention to the deformation of a lining structure. However, leakage may result from either deficient durability or excessive deformation of a structural member. Carbonizing or leaching concrete members, rust formation or corrosion of steel reinforcing bars may trigger the loss of structural integrity that leads to a serious reduction of serviceability. No matter where these defects cause from, they may be grouped into three categories, viz. material, component, and structure, as listed in Table 1. Table 1: Defects observed on site. MATERIAL COMPONENT STRUCTURE CONCRETE leaching carbonation erosion ASR abrasion permeating spalling scaring STEEL rust, corrosion pit MEMBER cracking, bulking, deflection CONNECTION bulking, twisting, loosening JOINT opening staggering leakage leakage displacement fracturing 3.2 Limit states of a structural design During a design stage of a structure under action loads, a limit state is defined as the border that separates the desired state from the adverse state in described in structural codes. Usually the ultimate limit state and serviceability limit state are specified to be complied. The ultimate limit states defined in the codes could be categorized, according to [2], as: EQU Loss of equilibrium of the structure. STR Internal failure or excessive deformation of the structure or structural member. GEO Failure due to excessive deformation of the ground. FAT Fatigue failure of the structure or structural members In other words, the ultimate limit states refer to those states that are associated with the collapse of structure, the fracture of a structural component, the overturning of a member, the lifting or sliding on the soil, and/or other events where the safety of a structure is of importance. The serviceability limit state refers to comfort for the user, its functionality (fit for purpose), and/or to aesthetical or cosmetical reasons. Beyond that, the definition is wide and vague. The current code on serviceability limit state focuses mostly on the rigid aspects of a structural member in combination with load actions, before it can be quantitatively calculated by its indices that cause its deformation. 933

6 3.3 Service states in operation From this point of view, a tunnel structure designed in the light of a limit state is complying with several states during its operational service. Figure 3 displays the different functionalities required in a tunnel design. The evolution of the structural function from TIGHTNESS to CAPACITY indicates that the structural performance of an operational tunnel may vary during its service life. ACTIONS loads + environment TIGHTNESS APPEARANCE serviceability limit state ultimate limit state INEIGRITY RIGIDITY CAPACITY Figure 3: Relationship between service states Based on this concept we here try to level the service states of an operational tunnel in order to be able to make a reasonable assessment of its structural performance. (1) Perfect state: The structure and its components remain close to their design conditions. The tunnel structure could perform and function well. (2) Degradation state: The resistance of a structural member against penetration or permeation of environmental action decreases. Appearance or aesthetics of a structure may be affected by erosion or corrosion due to environmental actions; (3) Deterioration state: There is obvious evidence of loss of structural integrity, such as reduction of structural dimension, loose of structural connection, and so on. (4) Serviceability limited state: Deformation of structure or a structural member was observed and continuously developing with age but without proof that load conditions were changed. (5) Ultimate limited state: Excessive deformations or local collapse of structure was monitored during routine inspections and conservation work. 934

7 4. IMPROVE SERVICEABILITY THROUGH DESIGN PHILOSOPHY 4.1 Limit state design With the description of limit state approach, a limit state is reached if g X = (1) ( ) 0 where ( X ) g is the limit state function, vector X represents variables of actions (loads and environmental effects), material properties, geometrical properties, and uncertainties in the design model. If the limit state is exceeded it means that ( X ) < 0 g (2) hence, the specified functionality of structure or a structural member is insufficient. 4.2 Requirements of serviceability For a service life design of a structural member the long-term behaviour should be taken into account. Therefore the requirements of a limit function should include terms and/or variables that are time dependent. Service states Excellent Worse t d t s t u Time Figure 4 Design states during service life Suppose that the service states of structural members are gradually evolving with age, ref. Figure 4, even though the interaction between its deformation and resistance to environmental ingression exist. The degradation of its structural performance would reduce the service states from Excellent towards Worse under the combined actions of loads and environmental. While supposed that t d is the initial point of the durability limit state, t s is the onset of the serviceability limit state, and t u is the onset of the ultimate limit state. Therefore, a unified design formula that describes any adverse state could be given in the light of the limit state design as 935

8 D S( X, t) < 0 g( X t) = C S( X, t) < 0 R( X, t) S( X, t) where, ( X t), if t < t (3) < 0 if t < t if d s t < t g, is limit state function that accounts for the degradation of structural performance; D, C is prescribes the restrictions for durability or serviceability respectively; S ( X, t) is a function for the structural responses under combined actions (loads and R X, t is a function of the structural resistance. environmental ingress); ( ) 5. CONCLUSIONS This paper provides a proposal to evaluate the service states of structures in the light of the limit state design. Even though there may exist interactions between service states, the approach of the limit state design may be applied to assess the service state of an operational tunnel. The suggestion here is to divide service states of a structure according to its performance as follows: (1) Perfect state, tunnel structure could perform its function well; (2) Degradation state, there may be durable defects that affect appearance or aesthetics of a tunnel structure; (3) Deterioration state, there may be irreversible changes in material and/or structure related to the tunnel integrity; (4) Serviceability limit state, when the stiffness of a tunnel structure is affected; (5) Ultimate limit state, as if safety of a tunnel structure is affected. Assessing the service states of a tunnel structure could be base on the principle of a limit state approach if the limit values or criteria upon these states are defined. It would also be helpful to keep a tunnel in perfect state if it were designed in the light of the service states approach. ACKNOWLEDGEMENTS The author would like to acknowledge the financial support from NSFC under the number of grant and from STCSM. REFERENCES [1] Ministry of Construction, 'Unified standard for reliability design of engineering structures', 1992 [2] CEN, 'Eurocode 1, Basis of Design and Actions, Part I Basis of Design', [3] Siemes, T. and Polder, R., 'Design of concrete structures for durability', HERON. 43(4) (1998) [4] Vrouwenvelder, T and Schiessl, P., 'Durability aspects of probabilistic ultimate limit state design', HERON. 44(1) (1999) [5] Schiessl, P., 'New approach to service life design of concrete structure', Asian Jour. Civil Engng. 6(5) (2005) [6] Folić, R., 'Durability design of concrete structures - part 1: analysis fundamentals', Archit. & Civil Engng. 7(1) (2009) [7] ITA WG2, 'Guidelines for the design of shield tunnel lining', Tunl. & Undgd. Space tech. 5(3) (2000) 303~331. u 936