Insights into statistical structural characteristics and deformation properties of columnar jointed basalts: field investigation in the Baihetan Dam base, China.

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]