Geometrical Characteristic Investigation of the Baihetan Irregular Columnar Jointed Basalt and Corresponding Numerical Reconstruction Method

Columnar jointed rock mass with unique geometric and geological properties is one spectacular example of geometrical order in nature. Columnar joints are generally accepted to be formed by spatially uniform volume contraction during cooling. In this paper, substantial field work was performed to study the geological characteristics of irregular columnar jointed basalt on the left bank dam foundation in the Baihetan Hydropower Station, where the columnar jointed rock mass is extensively exposed due to excavation. The quantitative measurements of the sizing of polygonal crack pattern of columnar joints and assessment of their degree of irregularity were summarized. Considering the irregularity of polygonal crack pattern, a modified Voronoi polygon (MVP) method was developed to model the special polygonal crack pattern of columnar joints. The new polygonal pattern obtained by the MVP method consists of a large number of irregular polygons, of which the degree of irregularity is consistent with the field measurement results. This method can reproduce the rapid evolution from an initial ideal regular hexagonal pattern to a final actual irregular polygonal pattern as the degree of irregularity increases. The compression tests of columnar jointed rock mass with different irregularity show that the geometric irregularity has a great influence on its mechanical properties. This numerical construction method provides a reliable way to reconstruct columnar joint structure with specific polygonal crack pattern, which is consistent with onsite columnar jointed basalt.