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Columnar jointed basalt (CJB), characterized by a hexagonal joint network, poses a great challenge for geotechnical designs due to its time-dependent propensity to deformation and damage. In this paper, the geometrical structure and failure modes of CJB exposed on a dam foundation is presented. The study was conducted during the unloading excavation of the Baihetan dam, China, aiming to provide a more in-depth understanding of the CJB damage mechanism. A systematic field test for the columnar jointed rock mass was also carried out on the dam foundation using ultrasonic P wave measurements. It was found that the excavation damage zone of the CJB not only showed time-dependent damage both in depth and degree but also showed spatially inhomogeneous distribution of the damage degree. Sequential regression analysis further demonstrated the time-dependent behavior of the CJB. Successive observations using digital borehole cameras in the field showed that the unloading damage mechanism of the Baihetan CJB was mainly related to the time-dependent opening of the initially closed joints. Several reinforcing methods for improving the stability of the damaged CJB, including the injection of liquid cement and the installation of the prestressed rock bolts, are discussed. [ABSTRACT FROM AUTHOR]

The columnar jointed basalt (CJB) in the left-bank dam foundation of the Baihetan hydropower station, China, is a special jointed rock mass with a columnar cylinder structure. A field investigation revealed three types of joints developing in the CJB: (1) columnar joints, (2) internal steeply dipping joints (ISDJ), and (3) internal gently dipping joints (IGDJ). A uniform distribution survey lines method (UDSLM) is proposed to describe quantitatively the spatial structural features of CJB exposed in limited outcrops. The visible spacing of internal joints in CJB is found to evolve; joint spacing decreases with increased degrees of weathering, disturbance, and time after unloading excavation. Combined with the actual structure, the formation mechanism of CJB is discussed from the structure of a single column to the whole rock mass, based on the contraction hypothesis. A columnar joint tensor, which is closely related to the CJB deformation characteristics, is established for describing the complex columnar joint network in the three-dimensional space based on the measurement data with the UDSLM. In addition, the structural characteristics of intact columns were exposed through dismantling cuboid specimens of 0.5 m edge lengths and 1.0 m height, verifying the rationality of the UDSLM and the evaluation of structural characteristics of CJB in the Baihetan dam foundation. This in situ study presents an in-depth understanding of CJB structure and provides meaningful guidance for excavation and supporting reinforcement for the CJB exposed on the Chinese Baihetan dam foundation. [ABSTRACT FROM AUTHOR]