Abstract
In this study, we show that development of a detectable permeation grouting system is feasible, based on the addition of magnetic materials to the grout, specifically, magnetite. A magnetic-based detection system is selected for development because unlike other previously trialled detection methods, magnetic fields are detectable over large distances within the subsurface, and importantly, attenuation of the magnetic field is not strongly dependent on the material properties of the surrounding rock. To test the conceptual feasibility of such a system, a finite element based numerical model is developed to simulate the magnetic field anomaly that can be achieved by the addition of magnetic materials to a cement grout. The model is verified against an analytical solution and then used to predict the magnetic field generated by a grouted cylinder of rock, assuming a fixed percentage of uniformly distributed magnetic minerals, and a central injection borehole. Two field trials are conducted to verify the detectable grouting concept, the first using a walkover survey that allowed mapping of the magnetic signal in 2D. The second is designed to mimic magnetic field measurements from a borehole monitoring array, with a single central magnetic grout block (representing the grout close to the injection point). Results of the two field trials show that the magnetic cement is detectable, even when the background magnetic noise within the surrounding soils/rocks is significant. A good agreement is obtained between the measured and the modelled magnetic anomaly. This research opens the door to the development of a ‘detectable’ magnetic grouting system, that can increase confidence in the integrity of grouted rock volumes and reduce the inefficiencies currently present in the grouting industry, enabling in-situ real-time optimisation of grouting campaigns.
Highlights
Cement grouting of rocks is widely used for reducing the hydraulic conductivity of underground structures (Li et al, 2016), of founding layers below dams and their interface with the dam core (e.g. Rastegarnia et al, 2017), for hydraulic containment of underground storage and disposal sites (e.g. Tsuda et al, 2012) and for the filling of underground shafts and cavities prior to construction of major infrastructure (Meier and Hoffmann, 1999)
This difference in maximum anomalies may be due to errors in the magnetometer location: for the stacked bricks the magnetic anomaly will be smaller in diameter and, it would be easier to miss the peak value during the walkover survey with 0.5 m gaps between the survey lines
The magnitude of the peaks and troughs in each vertical profile will be governed by the distance between each individual monitoring borehole and the magnetic grout front at that depth, whereas the direction of the magnetic field will be related to the shape of the grouted rock volume
Summary
Cement grouting of rocks is widely used for reducing the hydraulic conductivity of underground structures (Li et al, 2016), of founding layers below dams and their interface with the dam core (e.g. Rastegarnia et al, 2017), for hydraulic containment of underground storage and disposal sites (e.g. Tsuda et al, 2012) and for the filling of underground shafts and cavities prior to construction of major infrastructure (Meier and Hoffmann, 1999). Cement grouting of rocks is widely used for reducing the hydraulic conductivity of underground structures (Li et al, 2016), of founding layers below dams and their interface with the dam core The GIN method combines observations of pressure and injected volume to calculate a grouting intensity number. Rafi and Stille (2015) demonstrated via a case study that the GIN value should be defined based on knowledge of grout spread, for fractures at shallow depths or fractures requiring a high level of sealing. They presented an analytical solution for estimating grout penetration that includes the deformation of fractures during grouting
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