This paper shows a result of the part of the study on characterisation of penetration of the colloidal silica grout (CSG) under saline groundwater. The purpose of this study is to understand the impact of salinity on grout penetration by verifying the improved penetration theory with laboratory tests. This study was performed as a part of the public offered grouting research project established in 2016 in Japan, and the overview of its research is shown in Tsuji et.al [2019].

When CSG is applied against saline groundwater conditions, such as in deep underground conditions or in coastal regions, there is a concern of insufficient grouting due to fast and inhomogeneous gelling as CSG gets mixed with saline groundwater. Based on the analysis of the previous experimental study in Finland by Lehtonen [2011], it was proposed to add a time factor φ (fitting factor or constant number) to the existing penetration theory of Funehag [2007]. This improved theory is aimed to reflect the penetration length, especially under the saline groundwater conditions. A series of grout injection tests under no-flow (14 tests) and flow-through (3 tests) conditions were performed. The CSG of one European and two types of Japanese (conventional and pH adjusted) were injected into an artificial fracture test system, filled with the five different groundwater simulants (from freshwater to saline water), using a constant head of 2 kPa under static or flowing conditions (10 ml/min). The test system was built to dimensions of 635 mm (length) × 535 mm (width) × 0.08 mm (aperture). The injected volumes and filling patterns with sizes were measured and recorded.

All no-flow tests were successful, resulting in the formation of relatively homogeneous gelled zones. Overall, the Japanese pH adjusted CSG was far less sensitive in terms of penetration length to groundwater composition than the other grouts resulting in the narrowest range of distribution of factor φ. Therefore, as an alternative method, designing a longer gel time by multiplying the inverse value of φ can be proposed to attain the required penetration. Moreover, the penetrations of the pH adjusted CSG into both the seawater and the de-ionised water were precisely the same, which suggests that constant φ = 1 can be applied for the penetration into the saltwater. However, the appropriate value for φ needs to be further determined. In the test with the flow, grout mass loss (due to erosion) was observed. Interaction between the grout type and groundwater composition, and the hydraulic gradient at the grout front seem to affect erosion resistance, and the further test with longer penetrations is found to be necessary to understand the hydraulic gradient at the grout front to withstand the erosion.

Although the results were fruitful, further investigation is necessary to develop this penetration theory. Laboratory tests with a larger and/or improved equipment and grouting tests at the site could be the future steps.

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