Principle
There are two basic techniques to apply an in-situ under-seal, which are both based on the principle of pore injection - normal pore injection and injection after processing (jet-grouting). In injection after processing, the ground’s fine grain structure is disturbed and replaced. A mix is injected which ensures that the soil's permeability is greatly reduced. Injection fluids can consist of bentonite-cement mixes or Water glass (=Na2Si2O5) 6H2O: Soluble sodium salt from silicone dioxide or organic resin (polymers) mixed with bentonite. Injection fluids are injected in vertical filters that are placed at distances of 0.8 to 1.5 metres; approximately the radius of influence at the depth of the injection. In injection after soil processing, fluid is injected with water at an extra high pressure; thereafter, loosened soil mixes with the injection fluid and hardens.
Figure 3‑1: Under-sealing diagram
In addition, it is also possible to excavate the soil and then to replace it once a (HDPE) foil, clay mat or concrete layer has been applied.
Implementation area and implementation conditions
The areas of interest, relating to the implementation area, include the soil type, type and concentration of polluted substances and the geo-hydrological circumstances, as already stated in the former paragraph.
Clay or loam are only suitable for implementation if they contain a lutite fraction (particles <2 µm) of more than 35%. The lutite fraction must be made up by more than 15% swelling minerals. The clay layer must be at least 0.4 m thick. The clay must be processed at a similar moisture content - higher than the optimum established in the laboratory - and is thus greatly weather dependent. Sand-bentonite mixes must be implemented with a layer thickness of at least 0.25 m. Clay and bentonite mats only have a thickness of a few cm. After implementation, bentonite needs to be pressured from above to realise suitable sealing. A drainage layer, for the drainage of rainwater, and a protective layer are applied to the mineral sealing layer to prevent damage being caused to the sealing layer as a result of root growth.
For a definitive seal, a maximum depth of 20 metres is retained. This technique cannot be implemented in difficult to permeate soil. The lower limit has a permeability of circa 10-6 m/s.
Once again, an inventory can be made of implementation conditions applied by the government. The requirements below are related to the role fulfilled by the under-sealing. A qualitative entry has not been provided.
- The inhibition/reduction of the transport of polluted substances from the soil to the surroundings (vertical);
- The inhibition/reduction of the in-flow of non-polluted ground-water to the soil pollution via seepage (evaluation of a seepage situation can have various aspects): It would be preferable if there was no downward dispersion, though a large volume of to-be-treated water can be unfavourable;
- The inhibition/reduction of dispersion of (volatile) polluted substances via soil air transport.
Costs
For the method of under-sealing with a mineral seal, amounts should be set aside for research, preparation, implementation, exploitation and after-care. The table below contains an estimate of the amounts.
Table: Costs for implementing under-sealing (Van der Gun et al., 2000).
|
Implementation type |
Specifications material/implementation format |
Costs |
|
Application of clay sealing layer |
€ 17 -22 per m2. |
|
|
Application of sand/bentonite sealing layer. |
€ 17 -27 per m2. |
|
|
Application of sand/bentonite sealing. |
€ 17 -27 per m2. |
|
|
Application of clay/bentonite mats |
€ 10 -15 per m2. |
|
Environmental burden and measures to be implemented
In the application of under-sealing, the dispersion of polluted substances into the air can be disregarded. Because toxic components in the yet to (via reaction) harden mix are avoided, process water can possibly be burdened. Polluted works water can potentially be released. Possible spread to ground-water should be a point of concern. When pre-drilling holes for the installation of injection lances, polluted soil may be loosened. In addition, soil and cement particles may come to the surface along with the injection lances.