Mandal, S.K and Singh, M.M. (2009) Evaluating extent and causes of overbreak in tunnels. Tunnelling and Underground Space Technology, 24 (1). pp. 22-36. ISSN 0886-7798

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Damage or overbreak not only endangers safety of structure but also increases cost of construction and time of completion. Drilling and blasting being cost-effective for excavation of any underground structure should strictly adhere into specialized controlled blasting pattern to minimize the unacceptable impact on peripheral in situ rock mass. The paper reveals that in addition to geo-technical properties of rock mass, in situ stress condition plays an important role in enhancing the magnitude of overbreak. Implementation of same blast pattern throughout the length of tunnel results into different magnitudes of overbreak and the magnitude increases in highly stressed zone. Furthermore, implementation of same controlled perimeter blast pattern along the tunnel cross-section may result into different magnitudes of overbreak. Different magnitude of overbreak along the tunnel cross-section i.e., in left and right wall and crown has been observed even with implementation of same controlled blast pattern throughout the tunnel cross-section. Feasibility and compatibility of drilling equipment with respect to tunnel cross-section also adds to the quality and magnitude of overbreak. Undersize drilling equipment leads to angular drilling on either walls or crown and enhances the magnitude of overbreak. The paper with the help of statistical and graphical analysis revealed that blast pattern for peripheral rock mass should consider geo-technical properties and in situ stress condition of rock mass to minimize the magnitude of overbreak. The authors also emphasized that to contain magnitude of overbreak within allowable limit; the implemented blast pattern should be different for different sections viz., right wall, left wall and crown of tunnel. Furthermore, in poor rock mass condition or in highly stressed zone, drivage of tunnel should be carried out in small sections and in different phases until the excavation reaches the required excavation profile in that area. Excavation in small sections and in different phases would lead to proper excavated profile and minimize overbreak and damage of peripheral rock mass.

Item Type: Article
Uncontrolled Keywords: Overbreak; Drilling and blasting; Detonating pressure; Foliation plane
Subjects: Blasting
Depositing User: Dr. Satyendra Kumar Singh
Date Deposited: 28 Dec 2011 12:35
Last Modified: 28 Dec 2011 12:35

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