Principle
Figure: Excavation diagram
Excavation of soil is implemented in the removal of polluted ground above the ground-water level (potentially supported by ground-water extraction). In some soil types (clay, loam) light excavation activities without drainage may also take place below the ground-water level. The disadvantage is the high water content in the excavated soil. In Achilles (OVAM 2002c) it has been stated that excavation should take place when dry, unless for the removal of floating layers where ground-water extraction only takes place once the floating layer has been removed.
Excavations are carried out with the help of hydraulic equipment; in practice, a shovel is used for shallow large-scale excavations and a hydraulic digging machine for deeper and more accurate excavations. After excavation, the soil is stored in a (temporary) depot or is immediately transported to a final processing location.
During ‘dry’ excavation, in the presence of volatile pollutants, one should consider potential volatilisation of the pollutant or odour emissions. If there are unacceptably high risks in the environment, it may be necessary to remove volatile pollutants from the soil - via soil air extraction - prior to excavation, to excavate in phases or excavate under a tent.
Implementation area and implementation conditions
Excavation of soil is possible if the (lowered) ground-water level is approximately 0.5 metres (depending on soil type) below the soil in the excavated section of the ground. If the ground-water level is higher, the stability and bearing capacity of the surface level can be influenced.
Excavation is also possible in or under buildings by using special smaller cranes (a mini-digger for example). However, this means that the capacity of the excavation is relatively low (< 100 m³/day). The excavation depth is unlimited, so long as sufficient operating surface has been created for the crane. In difficult circumstances, (a lot of underground infrastructure), manual excavation (partial) can be considered.
A safe and stable slope line is maintained for excavation, depending on the soil composition. In case of close buildings with a shallow foundation, a distance of at least 1 meter from the building wall is retained. By taking civil-technical measures (for example, trench shuttering, protective casing or a dam wall) the slope line can be made steeper, after a stability study has been carried out.
Excavation is often implemented in combination with ground-water extraction, with the purpose of clean rinsing the soil. Excavation is and remains a fast, effective and economically viable clean up method in situations where the terrain is well accessible and the pollutant is shallowly located. Further, there are specific situations where excavation will be implemented:
- Within the framework of target-specific clean up, the non-mobile pollutant is removed and replaced by an active soil layer.
- For the removal of hot-spots or for the removal of a source with pure product.
- In cases where, as a result of special developments or acute risks, there is limited time or when long-turn in-situ approaches are selected.
- Polluted substances that are very difficult or impossible to break down, for which there is no suitable in-situ clean up technique; for example, a few (mobile) heavy metals such as zinc, nickels and arsenic.
Costs
The costs for excavating polluted ground are determined by the capacity of cranes, the depth to be excavated, the quantity and the location. An average price for excavation amounts to 2.5 to 10 euro per m³.
Environmental burden and measures to be implemented
Volatile organic components or dust may be released during excavation, depending on the type of pollutant present, the concentration and the dispersion of the components, the porosity and the ground’s moisture content. The most important parameters are the duration of the excavation and the size of the implemented equipment. The longer and more intensive the excavation activities, the greater the chance that organic components will volatilise.
Excavation and removal of polluted ground is a reliable method except in cases where the release of emissions into the atmosphere form a great risk to on-site work activities or the surroundings; for example, ground polluted with benzene or other volatile carcinogenic compounds.
Emissions that are released during excavation vary from case to case and are determined by the speed at which excavation takes place, the ground’s height of fall, the quantity of excavated ground, wind speed, temperature, excavation surface and the time period for which the excavated ground remains at the site.
The main emission sources during excavation are:
- the exposed excavated well
- the dumping of the excavated ground
- the excavated ground itself (in the storage area)
- emissions from digging machines and lorries during transport.
The largest quantities of VOS appear to volatilise during excavation. In the EPA report [EPA, 1997], it was stated that approximately 70% of the mass of volatile organic substances, such as xylene, can be emitted during the excavation of soil. For moist grounds one should operate on the basis that, depending on the outside temperature, only 5 to 10% of VOS is emitted during excavation.
A number of measures can be taken to limit emissions of VOS and dust. In general, measures that are taken to reduce dust will also have an influence on emissions of VOS and vice-versa. In comparison to point sources, VOS emission-reduction in excavation is more difficult to implement and is only moderately efficient.
The most commonly used method for limiting emissions of VOS is the placement of covering layers over the ground to form a physical barrier. The simplest method is to use clean soil as a covering layer; so long as the addition of clean soil does not lead to a dilution of the pollutant (mixing with clean soil is not permitted). These soil layers thus increase the transport distance that the VOS has to travel and also reduce, temporarily, the emission speed. The soil covering layers are commonly used to restrict the emission of odour components and air-bound pollution. The efficiency of these soil layers is determined by the thickness and the percentage of polluted soil that can be covered in this manner. Emission speeds can be reduced considerably (e.g. > 95%) by adding compressed soil, though this causes additional lateral migration of VOS and is only a temporary measure. Soil layers are less effective over longer periods and are is an approach that adds to the quantity of soil that needs to be processed.
Synthetic covering layers are used to limit VOS emissions in short-term storage areas. They are also implemented for soil transport. Covering layers can be thin (0.4-0.6 mm plastic) or relatively thick (3-4 mm plastic or geo-textiles). These covering layers cannot be implemented for an unlimited duration where the polymer will begin to degrade, thus the life-span is limited - for thin layers, this is only a few weeks. The efficiency of these layers is determined by their permeability for present vapours and the percentage of soil that is covered.
Modified foam is often used for the limitation of VOS emissions in the treatment of dangerous waste which contains volatile toxic components. Generally, there are two types of foam: Temporary and long-term Temporary foams provide coverage for up to one hour. Long-term foams contain a stabilisation additive which extends the foam’s life-span to a few days and even weeks. Short-term emission-reduction of 75% to 95% is measured for total paraffin and total aromatics. Emission-reductions for total VOS of 99% to 100% are measured with the use of stabilised foam.
The two most important benefits of foams are that they can be very efficient and can be very quickly implemented on polluted soil. However, this technique also has a few disadvantages. The thick foam layers can only be implemented on horizontal surfaces, and thus not on the side walls of the excavated well The addition of foam, which has a water content of >90%, increases the weight of the soil and makes the soil less manageable and less suitable for thermal processing.
Water sprinklers are often implemented to limit the emission of dust. The addition of chemical additives such as polymers or acrylics will increase efficiency. By adding water, the temperature of the soil will also be lowered, which will reduce the transport of VOS through the soil. Water sprinkling is often less effective than water foams and has the same limitations and disadvantages as described above. Chemical additives may cause future pollution. Under normal circumstances, a waste water stream is not created in areas where only a limited quantity of water has been sprayed.
Operational measures can also be taken to reduce VOS emissions. These measures would involve checking the speed of excavation, the quantity of polluted ground that is exposed and the time period for which the soil is left unprotected. Excavations are better implemented on days when the outdoor temperature and wind speed are low.
If possible, one could fully cover the excavated site with a tent. This cover enables emissions to be contained and treated via an emission-reducing technique for point sources. However, there are a few disadvantages of doing so, such as the cost of coverage.
The final alternative is to ensure that the wind speed above the polluted ground is restricted by implementing wind screens.
Cost prices for VOS-reducing measures in excavation are not extensively available in the literature. The table below provides an overview of the available cost price data.
Table: Overview of cost prices for VOS-reducing measures in excavation [EPA 1997]
|
Technique |
Material cost (euro/m²) |
Comment |
|
Clay |
4,15 |
Covering layers, mat and membrane |
|
Soil |
1,33 |
Assumed to be 150 mm deep, not including transport of soil |
|
Synthetic covering layer |
5 - 9 |
Determined by thickness |
|
Temporary foam |
0,04 |
Assumed to be 65 mm thick, 0.7 euro/m³ of foam |
|
Long-term foam |
0,13 |
Assumed to be 40 mm thick, 3.3 euro/m³ of foam |
|
Wind screen |
40/m |
Per running metre |
|
Water sprinklers |
0.001 (water cost) |
Varies with cost price of water; it is assumed that the hire price of the water container is 500 euro per week and €1/1000L of water. |
During the excavation of polluted ground, hydraulic cranes are used which are driven by diesel motors. It is estimated that circa 0.1 litres of diesel is used for per m³ of excavated soil.
Under the influence of rain or in the presence of free product, polluted soil can, if stocked at the site, further leach into soil at the storage place. This can cause secondary pollution. Placing foil on ground where storage is taking place, enveloping and draining into a reservoir with suitable waste water extraction/cleaning, are possible approaches to reducing these risks. Naturally, immediately loading on to a lorry for transport is a better solution.
Excavation activities may create noise and odour problems in the immediate surroundings.