Abstract:
Assessing the possibility of rock collapse is important when tunneling in unfavorable geological conditions because damage can be minimized. A new methodology based on the Rough Set (RS) theory and the Rock Engineering Systems (RES) is proposed to quantitatively identify the tunnel surrounding rock conditions, and to provide the corresponding hazard level. Rock collapse hazards from recent tunnel construction projects in Fujian province, China, were investigated. On the basis of 164 compiled datasets, the ground index (GI) was defined. RS theory was implemented to identify reduct sets associated with the attributes while rock engineering systems (RES) approach was employed to quantify the interactions between the factors affecting the ground conditions. 24 independent cases were selected to validate the proposed GI by comparing predictions given by the GI with the field observations. The final results showed excellent agreement between predictions and observations which suggested that the GI could be used for a quantitative rock collapse hazard assessment of the tunnel surrounding rock. Both methods indicated that the rock quality design, intactness coefficient, groundwater and discontinuity condition, are among the most important parameters controlling the conditions of the surrounding rocks.
Introduction
The quantitative identification of the tunnel working face and the surrounding rock conditions in tunneling is very important since it can help in evaluating geo-engineering hazards associated with the tunnel construction and minimizing the related risks by adopting appropriate design and construction methods. In areas with complicated geological setting and prone to adverse geological hazards, unfavorable ground conditions are often encountered. This can considerably affect underground construction works. Hazardous situations such as tunnel boring machine jamming, rock squeezing phenomena, water or mud inrush, rockburst, rock spalling, tunnel collapse, rockfall threaten the achievement of cost and schedule milestones.
Recently, methods that could be used to quantify hazard occurrences in tunnel have been investigated, including: using a vulnerability index to assess geotechnical hazards (Benardos and Kaliampakos, 2004), laboratory simulations for rock burst and slabbing failure occurrence (Gong et al., 2012), the interaction matrix of the rock engineering system to quantify rock behavior (Kim et al., 2008;Shin et al., 2009), geological drillings to assess the stability of ancient landslide crossed by tunnels (Jiao et al., 2013), as well as a qualitative approach for rock mass classification to determine ground behavior and rock mass composition in underground excavations (Stille and Palmström, 2008). Irrespective of the adopted methodology, it is noted that the rock mass classification plays a key role in quantifying a hazard occurrence. Meanwhile, a prime requirement to achieve this depends on the establishment of an objective rating system in which the weight of the system parameters are determined in a quantitative manner. As matter of fact, one of the problems with classification systems in rock engineering or in other related fields is the subjectivities they involve which lead to difficulties in determining unequivocally the rating system of a particular rock mass since the qualitative attributes that are often used, propagate a variety of uncertainties into the system.
