Our planet as we see it today has been shaped over the millennia by the interaction of earth, water and wind. This interaction is ongoing, without pause. Some of it, which happens on the surface, is visible and easy to understand and keep track of. Just as important, however, is what is happening below the surface. What seems to us like solid ground might not be so solid after all; the underlying layers of the ground we stand on might have different kinds of strengths, density, and structural peculiarities.
For example, layers of rock underground will vary in density, crystallinity, structure and porousness. These characteristics determine how it behaves when acted upon by extraneous factors like added stress/pressure, or release of stress/pressure, modification by large excavation, drilling or blasting, or even by running or seeping water.
Water seepage and flow is of special concern as it is a never-ending phenomenon. Given the presence of groundwater at varying depths all over the globe, it is vital to understand the dynamics of the interaction between water and the rock substrate. Understanding these dynamics is also important for understanding the flow of other fluids such as oil and liquid gas below the earth’s surface. This information, therefore, is highly relevant to the mining and drilling industry. Moreover, understanding the nature of the rock mass is vital for constructing either above or below ground.
Researchers around the world have employed different methods, such as empirical, numerical and a combination of the two, to understand fluid flow through rock substrata. In general, investigations have taken into consideration factors such as the nature of the substrate, the geometry of fracture (length, aperture, orientation, roughness etc.), and state of stress.
It has been observed that the behavior of fluid flow through the substrate depends upon the properties of the substrate (strength, modulus, and Poisson’s ratio), properties of fractures (orientation, length, density, aperture, roughness, in-filling materials, and inter-connectivity) and properties of the fluid (density and viscosity) as well. It has also been noticed that the pore dimensions viz., the fracture aperture, or width of the fracture, change with stress, and hence, permeability is quite stress-dependent.
Research has also been conducted under in-situ conditions to evaluate the behavior of fluid flow through fractured rock mass on large scales. Most of this research has focused on the fluid flow behavior of intact rock or rock mass having interconnected voids/fracture network.
However, fluid flow through rock mass is governed by the flow properties of the fractures or the most prominent ‘single fracture’. Fluid flow through a fracture network, which is a conglomeration of several individual fractures, is quite intricate. Simulating this is difficult and remains a challenge to the scientific community, due to heterogeneities involved with the rock and rock mass. In general, most engineering activities, such as civil or hydro-geological engineering projects, extraction of ores/minerals, geothermal energy, exploring for and extraction of petroleum and natural gas from the deep earth crust and disposal of nuclear/radioactive waste and greenhouse gases, have to deal with hard and crystalline rock mass, which occurs at great depths inside the crust. The matrix permeability of such rock is considerably low. Fluid flow in such conditions mainly occurs through fractures rather than the intact rock. In general, fluid flow through fractures is higher than flow through intact rock. This leads to seepage/fluid-in-rush related problems during the execution of any engineering projects.
Moreover, the activities mentioned above themselves bring about a concentration of stresses. As a result, local joints or tension fractures in the rockmass come into existence. Consequently, the strength of the rock mass decreases. In addition, fluid flow through such rock mass results in the development of excess internal water pressure, which substantially reduces the effective normal stress and hence, further reduces the shear strength of the rock mass. This may lead to failure of underground constructions (tunnel/mine roof, long-wall mines, rock caverns etc.) and instability of deep and steep open cut slopes.
Therefore, understanding the behavior of fluid flow (mechanics and transport of fluid) through a rockmass, especially occurring at great depth (>500 m) and with an elevated groundwater pressure (>5 mPa), becomes essential for all kinds of surface and underground engineering and mining activities involving water, oil and gas fluid. Given the time and expense involved when employing conventional methods such as drilling to great depths to acquire rock samples, it makes sense to device alternate methods that are quicker and cost-effective.
Kunal Kumar Singh, a research scholar at the IITB-Monash Research Academy in Mumbai, has developed such a technique. Kunal used paraffin wax as an analogue material to simulate the flow of water through a fractured rockmass to measure its permeability under varied confining stresses and ground water pressures. This technique would be quite helpful in providing insights into the seepage induced instability of rockmasses and fluid-rock interactions. Moreover, it is easy to adopt, cost and time effective.
The fluid flow properties of the fractured rock mass has been determined under high confining pressures (?40 MPa), which covers stress occurring below the earth’s crust at depth of upto 1000 m and at elevated fluid pressures (?25 MPa). These are the parameters encountered during the execution of civil and geological engineering projects such as: (a) Extraction of ores/minerals, (b) Geothermal energy, (c) Petroleum and natural gases exploration and extraction, (d) Construction of large-scale structures, such as dams, tunnels, underground caverns etc. on the earth’s crust, (e) Construction of geological repositories for disposal of high level radio-active nuclear waste, (f) Greenhouse gases-CO2 sequestration in the deep earth crust.
The results obtained from the experiments conducted using analogue material have been validated by comparing them directly with those using natural material (hard and crystalline rock). The use of analogue material facilitates easy and fast simulation of fluid flow properties of the rock mass. This method is extremely cost-effective as it eliminates the need to collect the undisturbed rock samples from the deep locations and creation of the fracture(s) in it.
It has been observed that fluid flow through a fractured rock mass decreases with an increase in effective stresses. Variation in flow and water flow pressure shows linear relationship. Deviation occurs due to high water flow pressure. Reynolds number and pressure gradient relationship is quite sensitive to the flow pressure and fracture roughness. It has also been observed that flow decreases with increase in the sample size, and increases with an increase in fracture aperture.
Kunal was guided by Prof. Devendra Narain Singh from IIT Bombay and Prof. Ranjith Pathegama Gamage from Monash University.
IITB-Monash Research Academy is a Joint Venture between IIT Bombay and Monash University. Research scholars study for a dually-badged PhD from both institutions, and enrich their research and build collaborative relationships by spending time in Australia and India over the course of their degree. Established in 2008, IITB-Monash Research Academy aims to enhance scientific collaborations between Australia and India.
Research scholar: Kunal Kumar Singh, IITB-Monash Research Academy
Project title: Investigations on Permeability of Fractured, Steep and Deep Rock Slopes with High Groundwater Pressures
Supervisors: Prof. Devendra Narain Singh; Prof. Ranjith Pathegama Gamage
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