Application of risk analysis and assessment in tunnel design

Transcription

1 Volume 5, Number 1, March 2009, pp [INVITED PAPER] Application of risk analysis and assessment in tunnel design Young-Geun KIM* * General Manager, Ph.D, P.E., Civil Works Division, SAMSUNG C&T Corporation, Seoul, KOREA Received ; accepted ABSTRACT A new risk analysis system for estimating the risk factors in tunneling is suggested in consideration of the complex effect in tunnel construction. It is verified that various risk factors can be expressed by stability and environment index using numerical and statistical analysis. Stability index includes the factors of safety of ground condition, the amount of ground settlement, condition of inflow and earthquake as a variable, and environment index includes vibration and noise by blasting under construction and train operation. The risk analysis of geotechnical stability factors were performed by classifying the factor of safety calculated SSR method, the damage of neighboring structure due to settlement, the groundwater inflow rate into the tunnel and the potential damage by the certain earthquake. Also, the environmental factors can be grouped into two aspects such as the construction stage and the operation stage; vibration, noise and the drawdown of the groundwater level caused by tunnel construction. Each risk factor was evaluated as a classified term to be the fixed quantity based on various probabilistic and statistic technique, then it was analyzed the distribution characteristic of risk along tunnel line. Then, the impact was evaluated that how much each risk factor influences on the construction cost with a tunnel construction period by analyzing social charge, so it is possible to perform reasonable tunnel design which was capable of minimizing the risk in the construction stage as well as the design stage. Finally, the applicability of quantitative risk assessment method using stability and environment index are evaluated and utilization method in designing tunnels in Korea is reviewed. Keywords: Risk analysis, Risk assessment system, Geotechnical stability, Environmental impact, Tunnel design 1. INTRODUCTION Tunnel construction have many problems and risks such as safety, stability and environmental influence in excavation and operation, because that is generally constructed in the vicinity of the existing structures and excavated at weathered soil and rock in shallow depth. Therefore, it is important that support system and reinforcement method should be determined reasonably and quantitatively in consideration of stability of tunnel, the settlement of adjacent structures and environmental influences such as vibration and noise by blasting. However, tunnel design mainly depends on empirical method due to the absence of quantitative assessment criteria for the risk factors. It is necessary for the new assessment method to consider the risk factors including stability and environmental effect in designing the tunnel at urban region. In Korea, many subways are planned and constructed in Metro city, such as Seoul, Daegu, Daejeon and Kwangju, but the popular complaints are occurred, according to instability of buildings, noise and vibration by blasting and train operation. Thus, the various risk factors in tunneling must be studied at the planning stage for the reasonable construction of subway. The concept of safety factor was used mainly in the design of underground structure. Such safety factor could be expressed with the ratio of the resistance and the load. It is the most important disadvantage that the conventional design must assign the representative value about the design parameters although the ground itself has the variation and the uncertainty. Therefore, in the conventional design method, estimating the relative reliability of the underground structure is unreasonable. The risk analysis which is referring in this research is the method to use the various probabilistic technique and statistical data. To complement the conventional tunnel design performed by the analysis of the mean ground behavior, the risk analysis considers the uncertainty of ground investigation result and environment element of circumference rationally. Risk factors, which will be able to occur in the design and construction stage of the underground structure must be selected, and quantitative analysis about the risk factors must be performed. The risk about each risk factor can be represented as follows: RISK (1) = P f C f where P f = the probability of failure; C f = the cost of failure. JCRM All rights reserved.

2 12 Y.G. KIM / International Journal of the JCRM vol.5 (2009) pp In risk analysis, the assessment about the probability of failure which has the reliability is most important. We estimated the uncertainty of soil parameter and failure model using probabilistic techniques. But, risk factors such as environmental problems are very difficult to be evaluated by probabilistic techniques, so we evaluated the environmental risk using verified statistical data within the related laws and ordinances. In this study, new risk analysis system for considering the risk factors in tunneling is developed and suggested to assess quantitatively the risk in urban subway tunnel such as ground condition, ground water, adjacent buildings, noise and vibration. Firstly, considering the stability and environmental influence in urban subway tunnel, the risk factors are selected and can be expressed by stability and environmental index using numerical and multi statistical analysis. Stability index includes the factors of safety of ground condition, the amount of ground settlement, and condition of inflow, and environment index includes vibration and noise by blasting metro-train under construction and operation. Also, new risk assessment system is applied to verify the validity in tunnel design at subway and railway and planed the excavation, reinforcement and measurement. It is reviewed that the risk assessment system can be used as a quantitative index in tunnel design through the application case of tunnel in subway and railway. 2. RISK ANALYSIS SYSTEM IN TUNNEL 2.1 Selection of Risk Factors It is necessary for consideration for various factors in tunnel design because of safety and stability tunnel and buildings. Especially, importance of environmental factors such as noise and vibration under tunneling and operating are increasing. In this study, influence factors are selected considering stability and environment effect in tunneling. As shown in Table 1, many risk factors are selected and evaluated for risk analysis system. Table 1. Risk factors in risk analysis system. Stability and Safety Environmental influence and impact Risk Factors in Tunneling Ground Condition M F Ground Settlement M S Groundwater M W Earthquake M E Vibration and M V1 Noise by Blasting M N1 Vibration and Noise M V2 by Operation M N2 Drawdown of groundwater Md Index Stability Index Environment Index Risk Factors for Geotechnical Stability 1) Ground condition: Tunnel stability is analyzed by using shear strength reduction method (SSR), which is repeatedly calculated the safety factor of tunnel for quantitative evaluation in according to reduction of shear strength(c, φ) of soil and rock. 2) Ground settlement: Influence on adjacent buildings in tunnel excavation is evaluated by numerical analysis. In this study, maximum strain and settlement of buildings are analyzed for the stability of adjacent buildings. 3) Groundwater: Ground water flows into tunnel due to ground excavation and affected the stability of tunnel. In this study, seepage and flow analysis are conducted for the evaluation of groundwater inflow into tunnel. 4) Earthquake: It is important of influence of earth-quake in urban subway. Thus, tunnel stability for earthquake is evaluated by dynamic analysis using amplification of earthquake vibration Risk Factors for Environmental Impact 1) Vibration and noise by blasting under tunnel construction: Estimation the compensation amount for mental and property damage by tunnel blasting and noise. 2) Vibration and noise by train operation: Estimation the compensation amount for mental damage and property damage by train operation. 3) Drawdown of groundwater: Estimation the compensation amount for damaged wells by tunnel excavation. 2.2 Analysis for Risk Factors for Geotechnical Stability Analysis for Support Capacity of Ground The risk for support capacity of ground is analyzed by evaluating the probability of failure due to uncertainty of ground characteristics. Although the sufficient site investigation was performed, the characteristics of the ground behavior could not be grasped perfectly. Therefore, strength parameters of the ground include certain level of uncertainty. The coefficient of variation is the quotient of the empirical standard deviation and the expected value: C v = σ / m (2) The safety represent support capacity of ground itself is evaluated by using shear strength reduction method. Figure 1 shows the concept for evaluation safety factor by SSR method. Figure 1. Evaluation safety factor by SSR method.

3 Y.G. KIM / International Journal of the JCRM vol.5 (2009) pp In the case of normal distribution, the probability of the failure may be expressed by the central safety factor v c, as shown in Equation (3) Φ[ ( v 1) / v C C ] P f = 1 c c vr + vq (3) where C vr and C vq are the coefficient of variation of the resistance and the load respectively. The corresponding relationship for the lognormal distribution is as follows: Pf ln (1 2 ) /(1 2 vc + CvR + C vq ) = 1 Φ ln(1 + C 2 )(1 C 2 vr + vq ) The risk is evaluated by multiplying the unit cost of tunnel construction with the probability of failure obtained by Equation (4) Analysis for Ground Settlement The damage of the structure in proximity, which is happens to the ground settlement due to tunnel excavation is important factor in risk analysis. The equal settlement and the inclination of the structure are computed by the numerical analysis and empirical equation, and the damage degree of each structure is estimated. The risk of ground settlement can be evaluated from the damage degree and the unit cost of construction. Table 2. Assessment criteria for the inclination and damage. Deg. of Grade Management Inclination damage A normal maintenance < 1/750 0% B C D E when the continuous careful observation is needed, normal maintenance after simple repairing. partial repairing, reinforcement ; the continuous observation judgment of the utility limit ; overall large scale reinforcement the utility prohibition, emergency reinforcement; the dismantle and reconstruction > 1/750 < 1/500 (4) 5~10% >1/500 <1/300 20~40% >1/300 <1/200 40~60% >1/200 60~100% method in consideration of the corresponding RMR value of the phase which is good. As the rating is continuously charged by the stage, the continuous RMR value against the parameter value is expressed as shown Figure 3. The continuous RMR graph of groundwater parameter represent the rating to zero, but in the case which the inflow quantity is large, the rating has a negative value. The continuous RMR rating for tunnel inflow can be expressed by numerical equation as like Equation (5). Reduction rating = -8 (Inflow quantity-0.125) (5) The probability density of an each interval in Figure 2 can be evaluated as shown in Table 4 and the probability density of RMR reduction rating can be expressed in normal distribution as shown in Figure 3. The probability of failure for each RMR reduction rating is represented in Table 5. The risk for groundwater is evaluated by multiplying the unit cost of tunnel construction with the probability of failure obtained. Table 3. Groundwater parameter in RMR rock classification criteria. Parameter Range of values Inflow per 10m tunnel length(l /min) 0 <10 10~25 25~125 >125 Joint water Ratio pressure Major 0 < ~ ~0.5 >0.5 principal stress Rating Figure 2. The continuous graph for RMR rating Analysis for Inflow of Groundwater As the failure probability according to the reduction rating from a groundwater inflow at the tunnel face is computed, uncertainty of rock classification is considered in risk analysis. RMR rock classification is computed to the sum total of the assignment rating to be composed of five parameters. Because the rating to be assigned is represented by a unity rating about the case of the schedule range of the parameter, the assignment of continuous rating is impossible and the assessment rating is overestimated or underestimated about a specific parameter. Charging the rating against the center-value vicinity for the groundwater parameter is performed using interpolation Table 4. The probability density for RMR rating. Inflow 0~10 10~25 25~125 Rating 10~15 7~10 0~4 Probability density graph

4 14 Y.G. KIM / International Journal of the JCRM vol.5 (2009) pp where D = damage, C st = estimated cost of structure, V c = vibration contribution, V o = operation vibration level, S st = status estimation of structure, V pr = presumption value of vibration, V cr = vibration velocity criterion Analysis for Noise by Blasting The risk of mental damage by blasting noise is evaluated with deductive equation (Equation (10)) through regression analysis based on the cases of actual damage compensation. M = 12.5 D (10) Figure 3. The probability density for RMR reduction rating. Table 5. The probability of failure for RMR reduction rating. Rating Probability of failure Rating Probability of failure % % % % % % % % % % % % Analysis for Earthquake At the sections where the computed earthquake acceleration exceed the acceleration criteria based on the coefficient of the earthquake area, it is assumed structures in that dangerous area are collapsed when the earthquake occurs. There, the risk for earthquake is evaluated by considering the damage repair cost for the structure which the collapse is predicted. 2.3 Analysis of Risk Factors for Environmental Impact Analysis for Vibration by Blasting The estimation criteria of compensation follow the related laws and ordinances and the risk (social cost) by the vibration is divided by mental and property damage. The compensation amount for mental damage can be represented as follows: D = E vt P (6) ( ) E vt = V e - V t T (7) where D = damage, P = unit compensation cost, E vt = total over exposure of vibration(db day), V e = evaluated vibration level, V t = threshold vibration level, T = exposure time, day. The compensation amount for damage of property can be represented as follows: D = C V (8) st c ( 15 ) 1.5 pr Vo V Vc =, Cv = 4 S + V V st o cr (9) where M = damage amount per a person, D = value in damage conversion table. The noise by excavation blasting can be estimated as follows: 1/3 ( ) ( ) db A = log d/ W (11) where d = distance of the object point, m; W = charging amount of the explosive, kg/delay. The noise by blasting can be predicted by Equation (11) based on the charging amount of general blasting in tunnel. And, the compensation amount by general blasting method can be estimated by multiplying the unit compensation cost from Equation (10) with the number of residents Analysis for Vibration and Noise by Operating Train The noise by operating train can be calculated by Lange Equation (11) assuming the design life of the tunnel in 30 years. ( ) L = log d [ db A ] (12) a where d=distance of the building in tunnel. The vibration by operating train can be calculated by Tokita Equation (13) through the correction of the vibration level, the transfer characteristic of the vibration wave and the correction about the interaction with structures, etc. L = L - [ A log ( r / r ) + A ] (13) v o 1 o 2r It is assumed that the noise increases as shown in Equation (14) than the initial value as the period to be happened the noise increases. Δ L = 5 / log 3 log T [ db] (14) Because the damage by the operating train has many continuance periods relatively, the risk by the operating train has very big value than the risk by the blasting Analysis for Drawdown of Groundwater The risk against wells of the neighborhood area which is caused by the tunnel excavation is evaluated. Groundwater levels at each bore hole are measured by groundwater flow analysis before and after the excavation, the influence range which the drawdown of the groundwater is happened is computed. And, the damage compensation cost is estimated by grasping the number of wells in the range of the damage influence.

5 Y.G. KIM / International Journal of the JCRM vol.5 (2009) pp APPLICATION OF RISK ANALYSIS SYSTEM IN SUBWAY TUNNEL DESIGN 3.1 The Outline of Subway Tunnel This project is one of the extensions of Line 3 at Seoul subway, from Garak-dong to Ogum-dong. Total length is 1577m including two stations. The tunnels consist of 2-Arch tunnel at Station 302 and 303, main tunnels with double and three tracks. The geological longitudinal profile of this subway is shown in Figure 4, the deep layer of alluvial soil at beginning zone and the faults at terminal region are investigated. (a) Risk for the ground condition - M F. (b) Risk for the ground settlement - M S. 3.2 Risk Analysis in Subway Tunnel The risk factors of the risk assessment index are selected for influence of stability and environment for tunnel. Also, the value for the factors were calculated from numerical analysis at 20m intervals and showed the distributions of value of risk value for all tunnel sections in Figure 5 and Figure 6. As shown in Figure 5, M F is showed the low values at the section with weathered soil and rock and faulted zone. M S, that is, stability for adjacent buildings is the low grade at main tunnel sections because of shallow depth to buildings. Also, inflow of ground water at 302 tunnel sections is greatly increased and M F is showed the low values. The influence of earthquake for tunnel, M E is showed the low values at faulted zone in terminal section. The results of vibration and noise by blasting and metro train under construction and operation are shown in Figure 6. The effects of noises by blasting and metro train are greatly increased at cut and cover section in shaft and station. Also, the effects of vibration by blasting are greatly increased at Station 302 section that many buildings are located at near distance. Finally, the distribution of risk index, stability index and environment index are obtained from these analyses for the tunnel stability and environmental effect, as shown in Figure 7. The classes of stability index are high ranked at Station 302 section and faulted zone in terminal because of ground condition and large cross section. The class of environment index is high ranked at main tunnel section with double track cross section due to near excavation. (c) Risk for the groundwater - M w. (d) Risk for the earthquake - M E. Figure 5. Distribution of the geotechnical risk grades at the subway tunnel. (a) Risk for the vibration by blasting - M V1. (b) Risk for the noise by blasting - M N1. (c) Risk for the vibration of operating train - M V2. Figure 4. Geological longitudinal profile of extension of Line 3 at Seoul subway. (d) Risk for the noise of operating train - M N2. Figure 6. Distributions of the environmental risk grades at the subway tunnel.

6 16 Y.G. KIM / International Journal of the JCRM vol.5 (2009) pp Application of Risk Assessment System in Subway Tunnel Risk assessment system is applied to the optimum design of subway tunnel, such as the selection of excavation, reinforcement, grouting and measurement. For total section of tunnel, the value and class of stability index and environment index are evaluated and compared to the class of rock mass, such as RMR and Q-system. The case of application for risk assessment system for subway tunnel is shown in Figure 8. In this case, mechanical excavation is designed at the section of the class of stability index S5 and the class of environment index E5. Blasting excavations are applied at the others. Ground grouting for preventing inflow of groundwater is applied at the section of the class of stability index S5. Also, measurements of vibration and noise are planned at the tunnel section of the class of environment index E4-E5 for minimizing the environmental influence in tunneling at urban region. 4. APPLICATION OF RISK ANALYSIS SYSTEM IN RAILWAY TUNNEL DESIGN Figure 9. The overview of the railway tunnel. (a) Risk for ground condition. 4.1 The Outline of Railway Tunnel This project is one of railway line from Seongnam to Yeojoo. The length of tunnel is 3,515m. The tunnels consist of inclined shafts for excavation under construction and exit in emergency. The overall view of this tunnel is shown in Figure 9, a road tunnel under this tunnel at center region is constructed and rock mass at tunnel site is evaluated as good condition. (b) Risk for ground settlement. (c) Risk for groundwater. (a) Stability index. (d) Risk for earthquake. Figure 10. Distributions of risks for geotechnical stability. 4.2 Risk Analysis in Railway Tunnel (b) Environment index. Figure 7. Distributions of risk index at the subway tunnel. As it is explained from the preceding paragraph, from the risk assessment for risk factors, the relative distribution of the risk in tunnel can be obtained as shown in Figure 10 and Figure 11. Tunnel engineers can select relative dangerous zones and consideration items in the design stage. And, the risk to be inverted to the cost of the failure or the compensation can be expressed as shown in Figure 8 and Figure 9. The risk amount is very high at the tunnel portals and at the vicinity of tunnel section with shallow soil-depth. Therefore, various design solutions to lower the risk amount were examined in such danger zones. Figure 8. Applications of risk assessment system at the subway tunnel design.

7 Y.G. KIM / International Journal of the JCRM vol.5 (2009) pp CONCLUSION (a) Risk for the vibration by blasting. (b) Risk for the noise by blasting. (c) Risk for the vibration of operating train. (d) Risk for the drawdown of groundwater. Figure 11. Distributions of risks for environmental impact. Figure 12. The result of the construction cost analysis for geotechnical risks Figure 13. The result of the construction cost analysis for environmental risks. As the tunnel construction increases, there have been risen many environmental issues as well as the stability of the tunnel itself caused by the tunnel excavation. Because tunneling works have many risk factors in both design and construction stage due to the uncertainty characteristics of the ground, the risk assessment and its control is very important for the tunnel construction. There are many risk factors in tunneling in urban region. In this study, a new risk analysis system is suggested to assess quantitatively the risk factors in subway and railway tunnel, such as ground condition, groundwater, adjacent buildings, noise and vibration. The factors are selected considering the stability and environment and graded through numerical analyses and empirical equation, thus two groups are classified using statistical analysis, that is, the index for the tunnel stability and the index for environmental influence. The index is divided by the 5 classes and evaluated the stable and risky grades. Also, each risk factor was evaluated as a classified term to be the fixed quantity based on various probabilistic and statistic technique, then the distribution characteristic of the risk along tunnel line was analyzed. The impact was evaluated that how much each risk factor influences on the construction cost with a tunnel construction period by analyzing social cost, so it is possible to perform reasonable tunnel design which is capable of minimizing the risks in the construction stage as well as the design stage. Risk assessment system is applied to verify the validity in tunnel design at subway and railway in Korea and planed reasonably the excavation method, reinforcement type and measurement. It is showed that the risk assessment system can be used as a quantitative index in tunnel design through the case of subway and railway tunnel. But the criteria and the evaluation method for the risk factors must be reviewed through the verification and application in tunnel design and construction. With applying the assessment technique for the risk to the conventional tunnel design which is dependent on the class of rock mass preponderantly, we attempted the rational design for tunnel which considers both the geotechnical stability and the social environment. Then, it is needed that the difference between the predicted risks in design stage and the actual risks during tunnel construction will be studied in detail including the risk management for optimizing the risks. REFERENCES D.H. Kim, Y.G. Kim, C.S. So, S.J. Oh, K.B. Kim and J.K. Park Risk analysis and evaluation considering geotechnical stability and environmental effect in tunnel design; The Proceedings of Korean Society for Rock Mechanics and Engineering: C.R. Choi, J.K. Park, D.U.You, Y.G. Kim and D.H Lee Development and Application MI System for Tunnel Design; The Proceedings of Korean Tunneling Association: Y.G. Wye, J.K. Park, S.K. Jwun and Y.G. Kim Development and Application Multiple Index for Reasonable Tunnel Design; The Proceedings of Korean Society for Rock Mechanics and Engineering:

8 18 Y.G. KIM / International Journal of the JCRM vol.5 (2009) pp Y.G. Kim, 2005, Tunnel Design for Seongnam-Yeojoo Double Track Railway; The Report for Construction Lot-5. Korea Rail Network Authority. László Rétháti Probabilistic solutions in geotechnics, Developments in Geotechnical Engineering 46: Elsevier. Milton E. Harr Reliability-based design in civil engineering: Dover Publications, INC. E.M. Dawson et al Slope Stability Analysis by Strength Reduction; Geotechnique Vol. 49, No. 6: