September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.
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In situ soil permeability reduction through Al and OM
precipitation as a geoengineering tool for dike stabilization
J. Zhou
Delft University of Technology, The Netherlands
S. Laumann
1,2& T.J. Heimovaara
1 1Delft University of Technology2Tauw bv, The Netherlands
Keywords: SoSEAL, in situ permeability reduction, building with nature, podzolization
The utilization of natural processes for engineering purposes has been widely discussed in recent years since they might enable the development of cost-effective, robust and sustainable engineering technologies. We hereby propose a novel geoengineering tool for in situ permeability reduction, namely Soil Sealing by Enhanced Aluminum and organic matter Leaching (SoSEAL). SoSEAL is inspired by podzolization, which is a soil formation process where the mobilization and subsequent leaching of aluminum (Al), iron (Fe) and organic matter (OM) in the topsoil is followed by their precipitation at greater depth (Sauer et al., 2007). The accumulation of Al/Fe-OM precipitates results in the formation of an almost impermeable soil layer. In such way induced in situ permeability reduction is interesting for several engineering questions, including the prevention of internal erosion.
Within the last year laboratory experiments, model development and the first pilot demonstration at a dike stretch in the Netherlands have been carried out to test the hypothesis of the SoSEAL concept. All experiments have been performed using humic acid (HUMIN P775, Humintech, Germany) as an OM source and aluminum chloride as the metal component.
The interaction of Al and OM, which results in precipitation/flocculation of the two components, is a well-known process in drinking water treatment for OM removal (Matilainen et al., 2010). Whether or not flocculation occurs depends on the applied Al concentration and therefore the metal/carbon (M/C) ratio. Batch experiments demonstrate that flocculation of the humic acid starts to occur at a molar M/C ratio of 0.01. Almost all OM is removed from solution at M/C ratios larger than 0.04. Al-OM floc sizes, measured by laser diffraction, range between 20 and 1000 µm, depending on the pH of the solution and the applied stirring rate (figure 1). This size range enables the flocs to cover micro- and mesopores in a porous media and therefore reduce the permeability.
Figure 1. Changes in Al-OM floc size distribution depending on solution pH (test performed at 300 rpm).
In order to determine the permeability reduction that can be achieved by Al-OM flocs, saturated column experiments using sand with three different grain size distributions were performed. The results show that the hydraulic conductivity reduction ranged between 60 and 90% with the highest reduction achieved in the finest sand.
J. Zhou
Delft University of Technology, The Netherlands
S. Laumann
1,2& T.J. Heimovaara
11Delft University of Technology, 2Tauw bv, The Netherlands
In situ soil permeability reduction through Al and OM
September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.
43
At the pilot location along a dike stretch in the Netherlands a highly permeable sand layer is located at a depth between 7 to 13 m below ground surface (bgs). Reducing its permeability will increase the stability of the dike. The aim of the pilot was to create a circular low-permeability-zone in the sand layer via separate injections of Al and OM solutions. The setup is shown in figure 2.
Figure 2. Injection and monitoring system of the first SoSEAL pilot implementation.
A COMSOL based 3D reactive transport model is used to simulate the solute transport, the geochemical reactions and their effect on the permeability at the site, and to predict the possible outcome of the pilot implementation.
Model results show that the applied injection strategy results in a locally reduced hydraulic conductivity, mainly at the interface between the two injection circles. The distribution of Al-OM flocs within the circle is however not homogeneous. This is related to the local groundwater flow at the site and the favorable mixing of the two injection solutions along the flow direction. The highest amount of Al-OM flocs is therefore expected in the regions around I4-I2 and I7-I9. The circular barrier formed induces hydraulic head changes in the system. Under natural flow conditions these changes are however in the range of mm to a few cm. Taking into account the tidal effect at the site, local heterogeneity and the measurement accuracy, these difference are not measurable. According to the model simulations the differences in hydraulic head before and after the treatment are the most prominent (up to 40 cm) under pumping conditions (figure 3). Using the existing monitoring and injections wells as measurement points it is in addition possible to localize the barrier approximately. Field experiments are carried out at this moment to verify these model results.
Figure 3. Hydraulic head differences under background flow and pumping conditions before and after the treatment.
Sauer, D., Sponagel, H., Sommer, M., Giani, L., Jahn, R. and Stahr, K. (2007). Review Article Podzol: Soil of the Year 2007 A review on its genesis, occurrence, and functions. Journal of Plant Nutrition and Soil Science 170: 581–597.
Matilainen, A., Vespäläinen, M. and Sillanpää M. (2010). Natural organic matter removal by coagulation during drinking water treatment: A review. Advances in Colloid and Interface Science 159: 189-197.
