Abstract
Columnar jointed basalt (CJB), characterized by unique geometric and geological properties, poses a great challenge for geotechnical designs due to its unloading relaxation and damage during excavation. In this paper, the geometrical structure and field failures of the CJB exposed on Baihetan left dam foundation were briefly presented. A systematic field test was conducted to study the excavation damage zone of the CJB on the dam foundation using ultrasonic P wave measurements. The excavation damage zone of the CJB not only showed time-dependent damage in depth but also showed spatially inhomogeneous distribution in depth. Corresponding three-dimensional numerical analysis was also carried out to analyze the unloading relaxation mechanism of CJB. The unloading relaxation of CJB on dam foundation was mainly caused by the joint opening under tension stress during excavation. The difference of excavation damage zone of the CJB located in different test zones was also discussed in this article.
Highlights
Bedding, joints, and fissures widely exist in engineering rock mass after complex diagenetic and mineralization processes and supergene evolution, which exhibits extremely complicated mechanical behavior [1,2,3]
As a typical example of an embedded joint rock mass, the columnar jointed rock mass contains a large number of intercolumn joint surfaces and hidden joint surfaces, which are more prone to unloading relaxation [10, 11]. e socalled excavation damage zone (EDZ) is often used to describe unloading relaxation of rock masses [12, 13]. e relaxation characteristics of jointed rock masses are mainly evaluated by the relaxation depth and degree of relaxation obtained by the in situ single-hole acoustic test [14]. e in situ testing technology provides a key tool to understand the real damage behavior of jointed rock masses, because no laboratory experiments can reproduce the actual geostress conditions and the complex engineering situation
Unloading Relaxation Failure of Columnar Jointed Basalt on Left Dam Foundation. e columnar jointed basalt (CJB) was widely exposed on the left dam foundation during excavation, located at below 670 m in altitude. e CJB is characterized by small columnar blocks with irregular polygons in cross-section, and the axis trends towards the valley with a dip angle of approximately 70°∼85°. e intercolumnar joint surface is straight and rough at the decimeter scale, and the microfissures are developed in columns at the centimeter scale
Summary
Joints, and fissures widely exist in engineering rock mass after complex diagenetic and mineralization processes and supergene evolution, which exhibits extremely complicated mechanical behavior [1,2,3]. Fan et al [15, 16] studied the deformation and relaxation characteristics of columnar jointed basalt (CJB) under excavation and Advances in Materials Science and Engineering unloading by means of field investigation and in situ testing. Jiang et al [17,18,19] conducted long-term continuous observations on the unloading relaxation characteristics of the CJB on the Baihetan left dam foundation and the columnar joint of the diversion tunnel, revealing its transversely isotropic mechanical properties and time effect of the excavation damage zone. E difference of damage depth of CJB located in different zones on the Baihetan dam foundation is discussed with numerical analysis and the aid of additional field observations
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