Rock Mass Classification for Columnar Jointed Basalt: A Case Study of Baihetan Hydropower Station

Shanghai Research Institute of Materials, Shanghai 200437, China Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing 210098, China Research Institute of Geotechnical Engineering, Hohai University, Nanjing 210098, China Huadong Engineering Corporation Limited, Hangzhou 311122, China Nanjing Yangtze River Management Office, Nanjing 210011, China Key Laboratory of Coastal Disaster and Defense, Ministry of Education, Hohai University, Nanjing 210098, China


Introduction
Columnar jointing is a typical fracture pattern (shown in Figure 1), best known from such locations as the Giant's Causeway or Fingal's Cave, in which cracks self-organize into a nearly hexagonal arrangement, forming an ordered colonnade [1,2]. As a miraculous natural phenomenon, there is a long research history since the Giant's Causeway was first reported in the 17th century [3]. Nowadays, the formation of columnar joints is reasonably understood as a result of cracks propagating into cooling lava flows [4][5][6][7]. As a kind of adverse geologic condition, a reasonable and suitable rock mass classification is extremely important for engineering projects.
Developing the hydropower energy to the west is a trend in China [8][9][10]. With the booming development of the water conservancy and hydropower projects, more and more com-plicated engineering geological conditions, such as columnar jointed rock, are encountered. At the foundation of Baihetan Hydropower Station, columnar jointed rock mass is widely distributed. A large number of laboratory and in situ tests give a clear illustration of the characteristics of columnar joints and similar jointed rock mass [11][12][13][14][15][16]. Unlike other traditional types of rock mass, the geologic structure of columnar jointed basalt is very complex [17,18]. Due to the cuts of joints, the value of rock quality designation (RQD) is quite low, and it will be identified as fragmentized rock mass using existing rock classification methods [19,20]. However, the seismic velocities V p are high and the rock strengths are high in certain stress states. It has typical characteristics of discontinuity in geometry and continuity in mechanical properties. The rock quality will be substantially underestimated, and traditional rock classification system is not suitable for such type of rock mass.
Considering the exact measurement of mechanical property is very difficult [21][22][23][24][25][26][27], rock mass classification methods are commonly used at the preliminary design stage of a construction project. Considering the inapplicability of existing rock classification systems, a rock classification system, integrating a set of key factors controlling the rock quality such as weathering, degree of stress release, degree of integrity of rock mass, and permeability, is proposed. A case study on columnar jointed basalt is conducted; the results may be extended to other similar cases or used as reference for other engineering projects.

Existing Rock Mass Classification Systems
A proper rock mass classification can give engineers a quick and reliable estimation of the rock mass without complicated calculations of various parameters. It forms the basis for design and estimation of the required amount and the type of rock support in groundwater control measures. A wellknown early attempt is Terzaghi's classification system for support of tunnels. Based on the most important inherent and structural parameters, several other classification systems, such as RMR, GSI, and Q, have been proposed and widely used in engineering. A detailed list of rock mass classification systems is presented in Table 1.
From these rock classification systems, the factors involved in the existing empirical classification systems are related mainly to the general information of rock mass, geometric characteristics of discontinuities, and construction method. Over the past several decades, the existing rock mass classifications have been applied successfully in tunneling, underground mining, and slopes. However, it is noted that considerable caution must be exercised in applying a rock mass classification to other rock engineering problems, although the classification scheme is appropriate for its original applications.
Columnar jointed basalt rock, as a kind of special rock mass, is not suitable to be classified with these factors, and sometimes, the results are evidently inconsistent under different classification schemes. Furthermore, many effective factors in rock mass descriptions, such as underground water and seismic velocity, are absent in existing classification schemes. The objective of this paper is to propose a classification scheme for columnar jointed basalt based on a comprehensive understanding of its properties.

Geological Conditions
Baihetan Hydropower Station is located on the downstream reaches of the Jinsha River, at the border of Ningnan County of Sichuan Province and Qiaojia County of Yunnan Province   The dam site is about 260 km from Kunming and about 400 km from Chongqing, Chengdu, or Guiyang, respectively. The straight distance between the dam site and Shanghai of the East China Region is around 1850 km. The site is located to the east of Zemuhe and Sikai-Jiaojihe fracture zones and to the north of Xiaojiang fracture zone. Although active facture zones exist in this region and the regional seismic activities are intense, no intense earthquake has been recorded in a 40 km range around the dam site.
3.1. Geomorphology and Topography. The river valley at the dam site is asymmetrical and is of a V shape, with massive mountains on both sides. On the left bank, the landform above El. 850 m is wide and flat at No. 1 Exploration Line, while the landform below alternates with gentle and steep slopes or cliffs. On the right bank, the landform above El.
1170 m consists of gentle slopes, whereas cliffs and steep slopes cover the land below El. 1170 m. The outcrops at the dam site are mainly Emei basalt formed in the late Permian Period, with underlying Maokou limestone formed in the early Permian Period. The river alluvial, about 5 to 25 m in thickness, is composed of Holocene boulders with sand. The Q4 unconsolidated deposits distribute mainly on riverbed, terraces, and sloping mesas. An illustration of the distribution of columnar joints is presented in Figure 3.
It can be seen that the middle dam site is mainly composed of P 2 β 2~P2 β 6 basalts, basically including microliticaphanitic basalt, amygdaloidal basalt, and varied basaltic brecciated lava (Figure 4). The rocks are hard but developed with some columnar joints, especially in the middle portion of P 2 β 3 stratum where the diameters of columns range from 5 cm to 10 cm; the columnar joints in other strata range from 20 cm to 30 cm. Weak basalt or brecciated tuff interlayers in 5 Geofluids composed of mainly faults with thickness less than 1.0 m, fracture zones, and joints, mostly striking NW with a few striking NE, and most of them are dipping at high angles. Outcrop rocks in both banks are weakly weathered, and the underlying rocks are slightly weathered and relatively intact. The load release effect influences rocks on both banks to various depths and deeper in the left bank. The underground water level and relatively impermeable layer are comparatively deep.
3.2. Hydrometeorology. The main source of runoff in the Jinsha River is from precipitation and replenished by the melt snow from the upstream area. The annual runoff is concentrated mainly in June to November, accounting to around 80 percent of the total. An analysis on runoff data of 61 years from June 1939 to December 2000 indicates that the average annual discharge at the Baihetan dam site is 4110 m 3 /s. Floods in the Jinsha River originate mainly from rainstorms. The maximum peak floods of the Jinsha River occur mainly in July to September (over 95 percent of the total numbers), and the remaining 5 percent of floods occur in the last ten days of June or the first twenty days of October.
The annual average temperature at the Baihetan dam site is 21.7°C, with the highest and lowest recorded temperatures being 42.2°C and 2.1°C, respectively. The annual average water temperature is 17.4°C. The mean annual rainfall at the dam site is 715.9 mm, and there are about 100 precipitation days in a year. The annual average wind velocity is 2.1 m/s at the dam site, with the maximum being 13 m/s in south direction.

Columnar Jointed Basalt in Baihetan
The development of joint fissures in columnar jointed rock mass is heterogeneous. The columnar jointed basalt can be classified into three categories based on the columnar size and length.
Type I rock masses have columnar mosaic structures with high fractural densities, and the joint planes are undulating and rough ( Figure 5). The columns have a length of 2 to 3 m and a diameter of 13 to 25 cm, and microfractures develop to cut the columns into small blocks with 5 different diameters. Type I columnar basalt distributes in mainly two sublayers: P 2 β 2 and P 2 β 3 . Type II columnar jointed masses, distributed in mainly P 2 β 3 2 , P 2 β 6 1 , P 2 β 7 1 , and P 2 β 8 2 , have a height of 0.5 to 20 m and a diameter of 25 to 50 cm. They are irregular, and the columns are not cut off completely. There are microcracks cutting the columns into 10 cm diameter blocks. The growth of Type III columnar joints is poorly developed, and the rock can be treated as intact basalt.
The rock block shown in Figure 6 is compact cryptocrystal basalt. The rigid bearing plate method is used to estimate the strength, and a mean deformation modulus about 51.6 GPa is obtained. It indicates that rock blocks have good stiffness and strength properties and the weak planes are the key factor affecting the mechanical properties of columnar basalt. The weak planes in columnar jointed basalt are mainly columnar joints, microfissures, and low-angle structural planes (including shearing band and fractures). Columnar joints and microfissures have typical distribution features like Voronoi diagram. They are closed without disturbance and open after perturbation. Low-angle shearing band is gently cutting the columns transversely.  For the distribution of weak planes without the consideration of the compact degree of structural planes, the linear density of rock joints is as high as 15 strips per meter. Therefore, the RQD of columnar basalt is low and would be categorized as weak rock mass. However, the mechanical properties, such as strength and seismic velocity ðV p Þ, are quite high so that columnar jointed basalt can be classified as good rock mass in some rock mass classification systems. Due to the special features of columnar jointed basalt and the inconsistency between geometric discontinuity and mechanical properties, conventional classification schemes may be not suitable; special analysis is required in the classification of columnar jointed rock mass.
In the development of a classification scheme of columnar jointed basalt, the following factors are taken into account. Based on the application environment, the main factors controlling the structure and quality of columnar rock mass are estimated first. In most rock classification schemes, the factor of water is absent. However, columnar jointed rock mass in Baihetan distributes at the dam foundation and will suffer from super high pore water pressure with a height over 200 m. Therefore, water permeability is inevitable in classifying rock mass with underground water. Furthermore, the degree of compact of joints, geometric integrity, structural type, and weathering are also taken into consideration. Adopting the strength parameter from RMR and employing the description and analysis of geometric and geological characteristics, a rock classification scheme for columnar jointed rock is developed in the following.  The percentage of surface area for the column with a diameter over 10 cm.

The Main Factors and Indexes in Classification
The mean distance between two adjacent transverse hidden joints.
The mean distance between two adjacent intraformational faulted zones.   7 Geofluids uniaxial compressive strength (UCS) of intact rock is employed as the representative strength parameter. Considering RMR has been widely applied in a large number of engineering projects, the classification of Baihetan columnar joints uses the same parameter as RMR classification standard (Table 2).

Rock Mass Integrity.
Rock integrity is a key factor affecting the quality of rock mass. For hard rock blocks, the integrity is determined by the development of joints. In the analysis of the degree of integrity of columnar basalt, column sizes and hidden joints are also considered. For a comprehensive analysis of columnar jointed basalt, five quantitative parameters are employed (Table 3).
Columnar jointed rock masses have complex structures, and a single factor cannot describe the integrity of rock masses accurately. In this rock classification system, a quantitative categorization of rock mass integrity is listed in Table 4.

Weak Plane Tightness.
Existing rock classification systems usually neglect the tightness of weak planes. However, the tightness of columnar joints is extremely important in the rock characteristics. For columnar jointed basalt, the tightness of rock joint is determined based on open distance, filling material, and degree of weathering. A detail description of weak plane tightness is presented in Table 5.

Rock Mass Permeability.
Underground water condition is one of the five important parameters in RMR rock classification system. Considering the columnar jointed rock mass is in the foundation of the dam with a height of 289 m, the underground water problem is serious and the columnar jointed rock mass is saturated [53]. Permeability obtained from site packer permeability test is used for the classification of columnar jointed rock mass (Table 6).

Columnar Jointed Rock Classification System and Application
5.3.1. Classification Scheme. The proposed columnar jointed rock mass classification system is based on three main factors. This scheme categorizes rock mass into five levels and seven sublevels. Related quantitative indices are listed in 8 Geofluids Table 7. Rock mass in levels I to III can be considered in the construction of dam foundation.

Application.
After the introduction of the proposed classification system, a simple example for rock mass in P 2 β 3 3 is used to demonstrate the application of this new classification scheme. Take the information in PD37 (shown in Figure 3) as example; this kind of rock mass is distributed at a distance about 20 to 40 m to the entrance. The slightly weathered columnar basalt is shown in Figure 7 and it indicates that this kind of rock has relatively obvious columnar outline.
The columns are irregular and twisty with a length of 2 to 3 m and a diameter of 13 to 25 cm. The dipping angle is about 70 to 85°, and the shape of section is mainly pentagon or tetragon. Besides the columnar joints, there have developed a certain number of microfissures in the columns, and the per-centage of microfissures with diameters over 10 cm is about 27%. Microfissures are mainly parallel to the columnar joints with trace lengths from 0.3 to 2 m, and the density is shown in Figure 8(a). The faces of microfissures are tortuous and closed. The mean J v value is about 8.8 per m 3 . From Figure 8(b), the average seismic wave velocity is 3800 which leads to an integrity factor which is about 0.48 meaning a poor contact condition. Besides, the mean distance between two adjacent transverse hidden joints is less than 8.5 cm. The mean distance between two adjacent intraformational faulted zones is about 5.5 m and can be categorized as "slightly developed." The opening widths of columnar joints are about 0.5 to 1.0 mm and those of microfissures are less than 0.5 mm. The weak planes are "relatively tight" based on the categorization scheme. Water pressure test shows that the permeability is about 2.6 to 6.2 Lu. Referring to the classification criteria, the rock integrity is "poor intact," and the weak    10 Geofluids planes are "relatively tight." Hence, the level of columnar jointed basalt in this part of the foundation is III 2 . Rock mass cannot be employed as dam foundation directly, but can be used under effective reinforcement treatment. Using the classification for columnar jointed basalt in Baihetan, the excavation scheme is designed for rock mass with different levels. The whole excavation process is very successful, and the final image of the dam foundation is shown in Figure 9. Now, the construction of dam is nearly finished. The result indicates that the rock classification is feasible for the related engineering with columnar jointed rock.

Conclusion
Baihetan hydropower station is the first arch dam built on the foundation with columnar jointed rock mass. Columnar jointed rock mass is special in that it is discrete in geometry but is still of high quality like intact blocks. For columnar jointed basalt, the strength and deformability features are mainly governed by rock-to-rock contacts. The tightness of weak planes plays an important role in the rock classification. Since the rock mass is located at the dam foundation, the effect of permeability is important. Existing rock classification schemes cannot give a comprehensive estimation due to the absence of these critical factors.
A rock mass classification scheme for columnar jointed basalt is proposed based on rock integrity, weak plane tightness, and permeability. With the help of a number of measures, such as seismic wave velocity, and geology descriptions, such as microfissures, this work tries to present a more suitable scheme. As an attempt for columnar jointed basalt, this work is also valuable as reference for similar rock masses.

Data Availability
Data are available on request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.