A re-appraisal of scoring items in state assessment of NATM tunnel considering influencing factors causing longitudinal cracks

종방향균열 영향인자 분석을 통한 NATM터널 정밀안전진단 상태평가 항목의 재검토

Abstract

State assessment of an operational tunnel is usually done by performing visual inspection and durability tests by following the detailed guideline for safety inspection (SI) and/ or precision inspection for safety and diagnosis (PISD). In this study, 12 NATM tunnels, which have been operational for more than 10 years, were inspected to figure out the cause of longitudinal cracks for the purpose of modifying the scoring items in the state assessment NATM tunnel related to the longitudinal crack and the thickness of concrete lining. All investigated tunnels were classified into four groups depending on the shape and usage of each tunnel. The causes of longitudinal crack occurrence were analyzed by investigating the correlations between the longitudinal crack and the following four factors: the patterns of ground excavation; construction state of primary support system; characteristics of material properties of the concrete lining; and thickness of lining which was obtained by Ground Penetration Radar (GPR) tests. It was found that influencing factors causing longitudinal cracks in the lining were closely related with the construction condition of the primary support system, i.e. shotcrete, rockbolt, and steel-rib; crack occurrences were not much affected by the excavation patterns. As for the properties of concrete lining materials, occurrence of the longitudinal crack was mostly affected by the following three items: w/c ratio; contents of cement; and strength of lining. When estimating the lining thickness of the concrete lining by GPR tests and taking thickness effect into account in the statement assessment, it was concluded that increase of the index score by an average of 0.03 (ranging from 0.01 up to 0.071) is needed; a more realistic way of state assessment should be proposed in which the increased index score caused by lack of lining thickness should be taken into account.

공용중인 터널의 조사 및 평가는 시설물안전법의 안전점검 및 정밀안전진단 세부지침(터널)편에 따라 외관조사와 내구성조사를 근거하여 상태평가를 수행하고 있다. 본 연구에서는 종방향균열의 발생요인 파악을 위해 준공 후 10년이 경과되어 최초 정밀안전진단이 실시된 NATM터널 12개에 대해 균열과 라이닝의 두께를 검토하여, 이를 반영한 상태평가 수정사항을 모색하였다. 종방향균열에 대한 굴착지반의 영향을 검토하기 위해 지보패턴, 이를 구성하는 지보재 시공 조건, 라이닝의 재료적 특성, GPR탐사를 통한 라이닝 두께 등을 터널 형상 및 용도에 따라 4개의 그룹으로 나누어 자료를 분석하였다. 균열발생밀도는 숏크리트, 록볼트, 강지보재의 지보재 지지능력에 따라 변화되나 굴착지보패턴에 따른 균열발생과의 연관성은 낮은 것으로 나타났다. 라이닝 재료적인 특징인 물-시멘트비, 시멘트함유량, 강도 등이 균열발생에 영향을 주는 것으로 나타났다. 또한, GPR탐사의 라이닝 두께를 반영한 상태평가에서는 평균 0.03 정도(분포, 0.001~0.071)의 점수증가를 야기하는 것으로 확인되어 향후 라이닝 두께 부족을 고려하는 현실적인 상태평가 방안에 대한 검토가 필요하다.

Fig. 1. General procedure for detecting defects in NATM tunnel lining by optical scanning

Fig. 2. General process of GPR survey performed on NATM tunnel lining

Fig. 3. Results of GPR survey performed on NATM tunnel lining

Fig. 4. Excavation patterns depending on size of tunnels and on ground conditions for the four groups of NATM tunnels

Fig. 5. Crack ratio dependent upon support patterns and tunnel groups

Fig. 6. Correlation between primary support systems and longitudinal cracks

Fig. 7. Effect of material properties and construction circumstances on the crack occurrence frequency in the lining

Fig. 8. Measured vs. designed lining thickness

Fig. 9. Correlations between crack occurrence and the lack of lining thickness

Fig. 10. Samples of typical and problem causing longitudinal cracks (BR tunnel)

Table 1. Summary of longitudinal cracks occurred in NATM tunnel lining

Table 1. Summary of longitudinal cracks occurred in NATM tunnel lining (continue)

Table 2. Scores for state assessment of unreinforced NATM lining

Table 3. Scoring depending on width of cracks in state assessment of unreinforced NATM lining

Table 4. Scoring depending on damaged thickness of unreinforced NATM lining

Table 5. Summary of tunnels studied in this paper

Table 5. Summary of tunnels studied in this paper (continue)

Table 6. Summary of crack ratios according to excavation patterns

Table 7. Summary of longitudinal cracks occurred in the lining dependent upon lining properties and construction circumstances

Table 8. GPR survey to assess the relationship between lining thickness and the occurrence of crack