Abstract
A columnar jointed rock mass is a type of rock mass with strong geometric anisotropy and high interface permeability. Its seepage characteristics pose new challenges to the construction and maintenance of the Baihetan Hydropower Station on the Jinsha River. The research object in this study is the columnar jointed rock mass (basalt) in the dam area of Baihetan Hydropower Station. Similar-material model samples of the columnar jointed rock mass with different column dip angles ( α = 0 ° ~90°) were prepared following a similar principle. A true triaxial seepage–stress coupling test was conducted to evaluate the seepage characteristics of similar-material samples with different dip angles under intermediate principal stress and minimum principal stress. The experimental results showed that the columnar jointed rock mass exhibited apparent seepage anisotropy. The relationship curve between the volume flow rate Q and the pressure gradient − d P / d L of the samples with different dip angles showed evident nonlinear seepage under intermediate principal stress, which could be well expressed using the Forchheimer equation. It shows the characteristics of a typical linear Darcy flow under minimum principal stress. The law of variations in the permeability of the samples with different dip angles under intermediate principal stress can be well expressed using the one-dimensional quadratic function equation k = a + b σ 2 + c σ 2 2 , and the law of variations in the permeability of the samples with different dip angles under minimum principal stress can be well expressed using the logarithmic function k = a + b ln σ 3 . The permeabilities of the columnar jointed rock mass with dip angles of 0°, 15°, 30°, and 60° were most sensitive to changes in stress, and the seepage characteristics increased in complexity after changes in stress.
Highlights
A columnar joint structure is a primary tensile fracture structure with a regular or an irregular columnar shape formed by volume shrinkage during rapid cooling of hightemperature lava
The evaluation coefficient R2 under all testing conditions exceeds 0.89, and the fitting accuracy is high. These results indicate that the law of variation in intrinsic permeability of similar-material samples with the columnar jointed rock mass under intermediate principal stress can be expressed using a quadratic polynomial of one variable
The final permeabilities of the samples with 0° and 60° dip angles were similar to the initial permeability, whereas those of the samples with other dip angles were largely reduced. These results indicate that except for these two angles, intermediate principal stress significantly decreases the permeability of similarmaterial samples with the columnar jointed rock mass
Summary
A columnar joint structure is a primary tensile fracture structure with a regular or an irregular columnar shape formed by volume shrinkage during rapid cooling of hightemperature lava. Columnar jointed rock masses are present in many large-scale water conservancies and hydropower engineering geological structures worldwide [1]. Its bedrock in the dam site and high slope of intake mostly consist of massive basalt [2,3,4,5]. The rock mass exhibits strong geometric anisotropy and high interface permeability, in addition to discontinuity, heterogeneity, and nonlinearity, presenting new challenges to the safety and stability of engineering construction and subsequent operations [6,7,8,9]. Numerous studies have been conducted worldwide on the mechanical properties and anisotropic deformation and failure laws of columnar jointed rock masses [10,11,12,13]
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