There is a growing global awareness of the need to protect future generations against potential risks from today’s industrial activities. Deep under the bedrock of northern Sweden, Golder has helped demonstrate safe disposal of residual waste from a smelter, in a repository designed to last for 3,000 years.
This disposal site for the smelter’s waste is dug into crystalline rock 350 meters (1,150 feet) below sea level. A spiral access tunnel leads from the surface down to the repository level. It is there that nine caverns, 18 meters wide and 15 meters high (60 by 50 feet), with a total length of 1,500 meters (4,920 feet), are planned to be constructed to accommodate the waste.
Understanding rock fractures key to safe disposal
An important step in achieving safe disposal is to understand the impact of groundwater flow through the natural fractures in the bedrock and how that can potentially spread hazardous materials to the biosphere.
Accordingly, environmental regulations require the owner to investigate and design a repository with a specified maximum flow of groundwater through the repository.
To help meet those requirements, the facility’s owner called on Golder’s expertise in hydrogeology and fracture analysis. Golder’s role was to determine the capability of the fractures in the rock to carry groundwater based on such factors as the pattern of the fractures and how well they are connected to each other.
Golder’s first step was to leverage the company’s proprietary FracMan® software, utilizing core mapping data and pump test information from existing boreholes to create a calibrated Discrete Fracture Network (DFN) hydrogeological model to forecast the groundwater flow through the repository rocks.
Only limited fracture trace mapping data were available for estimating the size and distribution of fractures in the rock. This, in turn, limited the team’s ability to estimate the connectivity of the fractures, resulting in a need for more accurate and consistent data.
Golder looked for a way to incorporate direct observations enabling our client to “ground-truth” the findings of the FracMan®-generated model. This could only be achieved by analyzing the actual rock from inside the tunnels. One critical success factor in gathering this information was doing it in a way that did not interfere with the ongoing construction work inside the tunnels.
This meant that the data would have to be rapidly collected during a brief window of time — between the time a section of tunnel was excavated and the excess rock removed, to the moment construction crews started installing steel supports to improve stability, and covering the walls with shotcrete.
Photogrammetry provides needed data, minimizes disruption
Accordingly, Golder brought in its expertise in photogrammetry, to create spatially correct three-dimensional (3D) models of the tunnels on which fractures could be measured directly.
To do this, Golder designed site-specific methodology and made available appropriate equipment to the tunnel contractor to quickly capture the necessary image data. About 10,000 high-quality images were taken of the 300-meter-long access tunnel to the repository area.
Photogrammetric 3D models were created continuously as the excavation proceeded, resulting in high-resolution 3D data serving as a permanent digital record for future use, even after the tunnel walls are covered behind structural supports, shotcrete and cabling.
Likewise, the fracture trace mapping was executed continuously as 3D models became available and will be used to update the FracMan®-generated DFN model, to support the estimation of groundwater flow through the repository.
Given the size of the project and the data management requirements, Golder worked with a commercial software developer to evolve that company’s technology to meet the client’s requirements.
The result of the project is that Golder has established a functional system for executing large-scale photogrammetry projects in tunneling, including processes for continuously mapping geology and discontinuities remotely, that delivers a fast, comprehensive and detailed picture of the fracturing of the rock mass. The methodology met the client’s requirements for documentation of the excavated caverns, supporting their need to predict the long-term and safe disposal of residual waste.