In Brazil, natural underground caves are protected by federal environmental legislation and several technical-scientific studies are required in the environmental licensing processes that regulate the preservation of Brazilian speleological heritage. Caves abound in iron formation terrains so in recent years researchers have intensified a balance between iron mining production and issues of cave protection.
Most of the iron caves in Brazil are found in two large ferruginous geosystems: the Iron Quadrangle region of Minas Gerais State, southeastern Brazil, and the Carajás ridge, of Pará State, in northern Brazil (Valentim & Olivito 2011). Currently, the National Speleological Database of the federal environmental agency, Chico Mendes Institute of Biodiversity (ICMBio), has about 4,300 iron caves registered, most of which are located in the Carajás ridge of Pará State. This is due to a greater effort of cave prospecting in areas where iron ore extraction is higher, demonstrating the strong link between the evolution of speleological knowledge and the iron ore mining industry.
Federal Decree 6.640/08 required that caves must be classified according to 11 physical, biological, ecological, and/or historical-cultural attributes, to establish their level of significance (importance for the speleological ecosystem). About these attributes, scientific studies are carried out to advance the knowledge on each of the themes allowing the correlation and the precise classification of the significance. In the archaeological context, extremely important in the recovery of the ancient history of past populations, all sites are duly rescued for Brazilian museums.
The mentioned Decree also regulates the different ways to license cave impacts, depending on their level of significance, or requiring studies that guarantee that any anthropic actions will not compromise the physical integrity of the caves (Brasil 2008).
During these often long-term studies within the licensing issues, another regulation, CONAMA 347, still requires that a buffer zone of 250 m should be protected around each cave (MMA 2004). In the context of mineral industry, this regulation directly interferes with mining operations, blocking large areas with the substantial amount of mineral reserves, which may lead to the stopping of mines and/or making the mine design more complex increasing production costs.
Therefore, it is important and urgent for the mining companies to understand the geomechanical mechanisms for the structural stability of caves in mining regions, which will allow greater security for the operations and leading confidence for the environmental agencies that may speed up the licensing of the areas.
This paper introduces the Cave Geomechanical Index (CGI), a novel system for classifying the susceptibility of iron cave spans to structural instability. This index was based, conceptually, on the Bieniawski (1989) rock mass geomechanical quality rating (RMR) system, but is focused exclusively on natural iron caves, considering other essential variables for stability analysis.
A simplified development flow of the main activities for the development of CGI is shown in Fig. 1.
Figure 1. Simplified development flow of the main activities of the Cave Geomechanical Index (CGI).
The CGI was applied to 63 spans of 27 strategically selected caves, being the most representative in speleological terms and the closest to the mining areas. They occurred near the N4 and N5 mines in Carajás ridge which were used for development, adjustments and calibrations of the index between 2015 and 2018.
Iron caves are poorly studied when compared with caves in carbonate rocks or other types of terrain. Generally, iron caves are shallow with thin ceilings (20 m average), with their development coincident with the slope of the overlying land surface. The spans of iron caves are small (20 m length × 10 m width average), and irregular in shape, with structural control and discontinuities observed on their walls and ceilings (Piló & Auler 2009; Piló et al. 2015). White & White (1969) and White (2012) reported that due to the natural evolution of caves and the breakdown along discontinuities, it is common to observe fragments and blocks of rocky material on the cave floors.
The genesis of iron caves is attributed to four main processes of generation of empty spaces in rock masses: erosion, leaching, dissolution, and biogenesis (Simmons 1963; Vann 1963; Moss 1965; Morris 1985; Pinheiro & Maurity 1988; Buchmann et al. 2009; Calux 2013; Dutra 2013). Studies of these processes are keys to understanding the geotechnical issues and stability analysis of this type of cave.
As an example, Fig. 2 shows the topographic map and photos of cave N4E_0026, one of the studied caves.
Figure 2. Photos of cave N4E_0026 (one of the studied caves).
Geotechnics Applied to Speleology
The vast majority of cave stability studies reported in the literature have been undertaken in carbonate terrains (Waltham 2002; Parise & Trisciuzzi 2007; Hatzor et al. 2010; Szunyogh 2010; Gutiérrez et al. 2014; Parise et al. 2015; Jordá-Bordehore et al. 2016, 2017; Andriani & Parise 2017; Fiore et al. 2018). Carter and Miller (1995) observed that in carbonate rocks there is some relationship between ceiling width and cave stability. Criteria based on RMR (Bieniawski 1989) have also been used in the geotechnical mapping of these areas (Siegel & McCrackin 2001).
Some ideas about structural instability from carbonate caves however can be transferred to the iron caves environment, leading to recent studies aiming to predict irreversible negative impacts on speleological heritage on mining sites (Sánchez 2007; Brandi 2015a, b; Araújo et al. 2016; Renó 2016; Valentim 2016; Dutra 2017; Lacerda 2017; Santos Junior 2017; Novas et al. 2017, Brandi 2019a).
Also regarding the ferriferous cave environment, Renó (2016) developed a cave zoning system to support geotechnical studies. Using this approach, the geotechnical analysis of the spans of some iron caves was undertaken based on traditional geomechanical classification systems adapted to caves. In line with this research, the works of Araújo et al.(2016) and Valentim & Olivito (2016) corroborated the evolution of the geotechnical theme applied to speleology in iron caves from, respectively, the survey of geomechanical parameters with a 3D laser scanner, and the proposed classification of a cave through the method of geomechanical classification of rock masses adapted to caves. Dutra (2017) evaluated the geotechnical susceptibility of two iron caves in the Serra do Gandarela region by mapping parameters as geomechanical classification of the rock mass, structural discontinuities, ceiling thickness, water input points (drips, speleothems, basins), sediment cones, root incidence and characterizing the rainfall and water infiltration into the caves.
The study area is located at the N4 and N5 iron mines in Carajás ridge southeastern of Pará State. The selection of the caves took into account their physical representativeness of the Mineral Province of Carajás as horizontal projection (from 5 to 200 m) and volume (from 5 to 1000 m³), and their proximity to active mining areas (Mines N4EN, N4WS and N5S). Twenty seven caves were selected which have been monitored over 4 years between 2015 and 2018 (Fig. 3).