Durability Modelling of Behaviour of Different Concrete Types as Tunnel Lining Elements

Transcription

1 Durability Modelling of Behaviour of Different Concrete Types as Tunnel Lining Elements Marijan Skazlic 1 Nenad Gucunski 2 Dubravka Bjegovic 3 T 11 ABSTRACT Development of novel building materials based on a cement binder, and improvement of the properties of existing materials made considerable progress at the end of last century and at the beginning of this century. One of innovative types of concrete that occurred in this period is a high-performance fibrereinforced concrete (HPFRC). To the present day a small number of structures made of this kind of concrete were constructed. Tunnelling is one of rare fields of civil engineering in which application of HPFRC has not been closely investigated so far. In this paper is shown an experimental research work carried out on different concrete types: Conventional concrete and high-performance fibre-reinforced concrete. Experimentally obtained values of mechanical properties of the said concrete types were used in modelling the behaviour of secondary tunnel lining. In numerical analyses, the parameters that were varied were the following: the type and cross-section of concrete in secondary tunnel lining and the type of durability loading. The analyses carried out showed that HPFRC used for fabrication of secondary tunnel lining has higher durability and safety than other concrete types. KEYWORDS Durability behaviour, High-performance fibre-reinforced concrete, Tunnel lining University of Zagreb, Faculty of Civil Engineering, Kaciceva 26, Zagreb, Croatia, Phone , Fax , skazle@grad.hr Rutgers-The State University of New Yersey, 623 Bowser Road, Piscataway, New Yersey, USA, Phone , Fax , gucunski@rci.rutgers.edu University of Zagreb, Faculty of Civil Engineering, Kaciceva 26, Zagreb, Croatia, Phone , Fax , dubravka@grad.hr

2 1 INTRODUCTION The New Austrian Tunnelling Method (NATM) is a method for excavation of large cross-section underground structures without using temporary supports. This method has been developed as a result of relevant experience, a number of measurements both on structures and models, and theoretical analyses. According to NATM, a tunnel consists of primary and secondary supports. Primary support is used to permanently support the tunnel cross-section and, in interaction with rock mass, to carry the total load. Secondary support, i.e. tunnel lining is installed after settlement and deformation of the primary support system has been completed. [Kovacevic et al. 2004] All theories to date suggested that secondary lining has not been considered as a static portion of the system, but rather as structural and protective element and thus it is designed as such. Measurements of deformations in the secondary lining of constructed tunnels showed significant growth in stress and deformations during use. [Kovacevic 2003, University of Zagreb 2004] It was found that durability of the primary support elements is being reduced over time and the mechanical properties are being lost. Loss of mechanical properties of the primary support system results in additional load on secondary lining, causes changes in the state of stress and deformations, and consequently affects durability of the secondary tunnel lining. Such loss is caused, in most cases, by (1) creep, swelling or weathering of the rock mass, (2) corrosion of steel anchors and/or loss of shotcrete durability, and (3) creep of shotcrete. The factors influencing durability of primary and secondary support systems can be analyzed through numerical simulations. [Skazlic 2005] In this paper a description is given of the test results obtained experimentally for main mechanical properties of conventional concrete () and of two types of high-performance fibre-reinforced concrete ( and ). and differed according to the values obtained for compressive and flexural strengths, and modulus of elasticity. The said materials were selected because conventional concrete is the material that is most used for installing secondary tunnel lining, and HPFRC is a new type of concrete that has not been widely used for this purpose. Compared with other concrete types used for installing secondary tunnel lining, HPFRC has much higher strengths, stiffness, toughness and durability. Such higher tensile strength and toughness of HPFRC, relative to conventional and fibre-reinforced concretes, provide the mobilization of the rock mass and thus preservation of its strength, which is one of the main principles of NATM and modern tunnelling in general. By the use of materials having higher tensile strength and toughness, higher stability and durability of structures are ensured. [Skazlic 2005, Skazlic & Bjegovic 2005] The values of mechanical properties obtained in experimental investigation for the above three concrete types were used in modelling the behaviour of secondary tunnel lining. In numerical analyses, the parameters varied were: (1) the type and cross-section of the concrete in secondary tunnel lining; and (2) the kind of load. In addition, the durability of the secondary tunnel lining was analysed with respect to the parameters varied. The analyses carried out illustrated that loss of mechanical properties of some elements of the primary tunnel support can have significant effects on durability of secondary tunnel lining. Thus, it was established that secondary tunnel lining has higher bearing capacity and safety when HPFRC is used instead of conventional concrete. 2 EXPERIMENTAL WORK In experimental work, the values of mechanical properties required to numerically model the state of stress and deformations in secondary tunnel lining were determined. The mechanical properties tested were the following: (1) compressive strength (according to HRN.EN and HRN.EN196-1 standards); (2) flexural strength (according to HRN.EN and HRN.EN196-1 standards); and (3) static modulus of elasticity (according to HRN.U.M1.025 standard). All the values of the above mechanical properties were used in the case when specimens were cured in water until the date of testing. The results are shown in Table 1. The illustration of mechanical properties tests is given in Figure 1.

3 Table 1. Mechanical properties of various concrete types necessary for numerical modelling. Property Compressive strength (MPa) Flexural strength (MPa) Static modulus of elasticity (GPa) Conventional concrete () Concrete type Figure 1. Concrete mechanical properties tests required to numerically model compressive strength (left), flexural strength (middle), and modulus of elasticity (right). 3 NUMERICAL MODELLING OF THE BEHAVIOUR OF SECONDARY TUNNEL LINING The numerical simulation of the behaviour of the secondary tunnel lining was carried out using the final difference method by means of the Fast Lagrangian Analysis of Continua program package (FLAC). The tunnel was modelled on the basis of the parameters of the Konjsko Tunnel constructed on the Zagreb-Split Motorway. Mechanical properties of rock mass employed in calculations were the following: ρ = 2.3 Mg/m 3, c = 100 kpa, ϕ = 20, E = 200 MPa and ν = 0.3. The height of tunnel overburden was 30 metres. Primary support system was installed using shotcrete of 25 cmaverage thickness and steel IBO anchors of φ = 25 mm, L = 8 m, and steel RA 400/500. Non linear models of material behaviour were used in numerical modelling. The numerical simulation of excavation stabilization with primary support system was run in the following steps (Figure 2): 1. Determination of primary state of stress, 2. Excavation phase 1.and application of shotcrete and ground anchors, and 3. Excavation phase 2 and application of shotcrete and ground anchors. In the numerical analyses of the state of stress and deformations of the secondary tunnel lining the parameters varied were the following: (1) the type of concrete with which the tunnel lining elements were constructed (,, and ); (2) the thickness of secondary lining (10, 20, 30 and 40 cm); and the main load on secondary tunnel lining causing the loss of its durability (failure of ground anchors; failure of shotcrete; loss of rock strength by 50 %; failure of the primary tunnel support; and loss of rock strength by 25 %). For the chosen concrete types and cross-section of the

4 secondary tunnel lining, and for given combination of permanent load, the maximum stresses in the tunnel cross-section obtained by parameter analysis are shown in Figures 3 and 4. Figure 2. Numerical simulation of excavation phase 1 (left) and excavation phase 2 (right) COMPRESSIVE STRESS (MPa) TENSILE STRESS (MPa) SECONDARY TUNNEL LINING THICKNESS (cm ) GEOTECHNICAL ANCHORS LOSS SHOTCRETE LOSS ROCK STRENGTH 50% LOSS ROCK STRENGTH 25% AND PRIMARY SUPPORT LOSS COMPRESS. STRENGTH/SAFETY COEF. -30 SECONDARY TUNNEL LINING THICKNESS (cm ) GEOTECHNICAL ANCHORS LOSS SHOTCRETE LOSS ROCK STRENGTH 50% LOSS ROCK STRENGTH 25% AND PRIMARY SUPPORT LOSS TENSILE STRENGTH/SAFETY COEF. Figure 3. Results of numerical modelling of the behaviour of the secondary tunnel lining installed with concrete.

5 TENSILE STRESS (MPa) TENSILE STRESS (MPa) SECONDARY TUNNEL LINING THICKNESS (cm) -40 SECONDARY TUNNEL LINING THICKNESS (cm) GEOTECHNICAL ANCHORS LOSS SHOTCRETE LOSS ROCK STRENGTH 50% LOSS ROCK STRENGTH25% AND PRIMARY SUPPORT LOSS TENSILE STRENGTH/SAFETY COEF. GEOTECHNICAL ANCHORS LOSS SHOTCRETE LOSS ROCK STRENGTH 50% LOSS ROCK STRENGTH 25% AND PRIMARY SUPPORT LOSS TENSILE STRENGTH/SAFETY COEF. Figure 4. Results of numerical modelling of the behaviour of secondary tunnel lining with (left) and (right). 4 ANALYSIS OF RESULTS The analysis of the results obtained for the behaviour of the secondary tunnel lining was made taking into consideration the criterion of allowed stresses (See Figures 5, 6 and 7). Resistance, i.e. computational strength of secondary tunnel lining was obtained by dividing the values of strength shown in Table 1 by factor of safety. The factor of safety used was 1.5, based on French recommendations for HPFRC. [AFGC/SETRA Working Group, 2002] COMPUTATIONAL STRENGTH/MAX STRESS THICKNESS OF SECONDARY LINING (cm) COMPUTATIONAL STRENGTH/MAX STRESS THICKNESS OF SECONDARY LINING (cm ) Figure 5. Safety factors in tension obtained for different types of concrete in secondary tunnel lining in the case of shotcrete failure (left) and anchors failure (right).

6 In Figures 5, 6 and 7, safety factors in compressive and flexural failure of the secondary tunnel lining are shown versus the thickness of secondary lining and load. The safety factor is defined as a ratio between computational strength and maximum compressive strength and/or tensile stress. Only the cases are illustrated for which the safety factor is higher than or equal to 1, i.e. in which secondary tunnel lining has permanent bearing capacity for the load analyzed. The Table 2 presents the analysis of the results of numerical modelling of the behaviour of the secondary tunnel lining. For particular concrete types and the cross-section of the secondary lining, the cases are illustrated when bearing capacity in compression or in tension under main permanent loads is not allowed. X COMPUTATIONAL STRENGTH/MA STRESS THICKNESS OF SECONDARY LINING (cm ) Figure 6. Safety factors in compression obtained for different types of concrete in secondary tunnel lining in the case of rock strength loss of 25 % and primary support failure. COMPUTATIONAL STRENGTH/MAX STRESS THICKNESS OF SECONDARY TUNNEL LINING (cm) COMPUTATIONAL STRENGTH/MAX STRESS THICKNESS OF SECONDARY TUNNEL LINING (cm) Figure 7. Safety factors in tension obtained for different types of concrete in secondary tunnel lining in the case of loss of rock strength by 25 % and primary support failure (left) and loss of rock strength by 50 % (right). The analyses described above showed that the mixtures of new concrete types, i.e. and, exhibit significantly higher bearing capacity and safety than the concrete type used so far for installation of secondary tunnel lining. As for durability aspect, it can be said that secondary tunnel lining may, in time, be exposed to high load due to loss of mechanical properties in some primary tunnel lining elements. The calculations described did not take into account extraordinary actions on these structural elements such as possible explosion from terrorist attacks. When this kind of load is considered together with permanent loads, it is safe to conclude that HPFRC is the optimum concrete composition for secondary tunnel lining because it has significantly higher safety and bearing capacity than concrete types used for this purpose up to date. As in this paper two different types of HPFRC were analyzed, when deciding between these two HPFRC types, the site conditions and total costs should be taken into account.

7 Table 2. Results of numerical modelling in the cases when bearing capacity of the secondary tunnel lining, due to its reduced durability, cannot to carry main loads. Concrete type Type of failure tension Main load - primary support failure and loss of rock strength by 25 % Thickness of secondary lining (cm) 10, 20, 30, 40 tension - primary support failure and loss of rock strength by 25 % - loss of rock strength by 50 % - shotcrete failure -anchors failure 10, 20, 30, 40 10, 20, 30, 40 20, 30, 40 20, 30, 40 HPFRC1 tension - primary support failure and loss of rock strength by 25 % - loss of rock strength by 50 % 10, 20, 30, 40 20, 30, 40 HPFRC2 tension - primary support failure and loss of rock strength by 25 % 10, 20, 30 5 CONCLUSIONS In this paper analyses were made of the durability of secondary tunnel lining installed with three different materials (two different types of HPFRC and conventional concrete). The material that has been most commonly used for this purpose so far is conventional concrete, and HPFRC is a new concrete type exhibiting higher tensile strength and toughness the use of which for tunnel lining is yet to be more widely applied. In numerical modelling of the behaviour of the secondary tunnel lining the experimentally obtained values of mechanical properties of these concrete types were used. In numerical analyses the parameters varied were: (1) the type and cross-section of the concrete in secondary tunnel lining, and (2) the type of load (failure of ground anchors; failure of shotcrete; loss of rock strength by 50 %; failure of primary tunnel support; and loss of rock strength by 25 %). In addition, the durability of the secondary tunnel lining with regard to the above parameters was analyzed. The analyses performed illustrate that loss of mechanical properties of some elements of the primary tunnel lining may have significant effect on durability of the secondary tunnel lining. It was also found that secondary tunnel lining has higher bearing capacity and safety when HPFRC is used for its construction instead of conventional concrete. ACKNOWLEDGMENTS The authors express their acknowledgements to Ministry of Science, Education and Sports of the Republic of Croatia for their support. The experimental research described in this paper has been carried out as part of two research projects (Modern methods of engineering materials testing, , Project Leader Marijan Skazlic, PhD, Assistant Professor, Development of new materials and systems of concrete structures protection, , Project Leader Dubravka Bjegovic, PhD, Professor ) supported by the above Ministry.

8 REFERENCES Kovacevic, M.S., Skazlic, Z., Skazlic, M Durability of Tunnel Primary Support, Proceedings of the International Symposium «Durability and Maintenance of Concrete Structures», Dubrovnik, Croatia, October, 2004, pp Kovacevic, M.S The Observational Method and the use of geotechnical measurements, Proceedings Geotechnical problems with man-made and man influenced grounds, XIII European conference on soil mechanics and geotechnical engineering, Prague, Czech Republic, August, 2003, pp University of Zagreb, Faculty of Civil Engineering, Geotechnical Department 2004, Stabilization Measurements on Konjsko Tunnel, south and north tunnel tubes, the Zagreb-Split-Dubrovnik Motorway (in Croatian) Skazlic, M. 2005, High performance fiber reinforced precast segments for secondary tunnel lining, dissertation, University of Zagreb, Faculty of Civil Engineering Skazlic, M., Bjegovic, D High performance fibre reinforced concrete-composition, structure and technology, in Research and development of new materials, ed. Filetin, Croatian society of materilas and tribology, Zagreb, Croatia, pp (in Croatian) AFGC/SETRA working group 2002, Ultra-High Performance Fibre-Reinforced Concrete, Interim Recommendations