Principle
The most important function of horizontale top-sealing is the prevention of contact with the pollution and infiltration of rain or evaporation of the pollution.
The following categories of sealing layers can be found:
- natural sealing (geo-textile and clean soil)
- synthetic membrane
- concrete/asphalt
- clay
Technique 1: Asphalt/concrete layer
Figure: Diagram for top-sealing of construction materials
Asphalt concrete and the (cement) concrete layer are two examples of civil-technical sealing.
The asphalt concrete layer consists of bitumen, (5 to 10%), sand, aggregates and filling substances and is applied in a thickness of 6 to 8 cm.
The cement concrete layer consists of a mix of sand, cement, water and possibly gravel. The final permeability is determined by the composition of the sand/cement mix. A concrete layer can be applied by laying a thick concrete floor, a few centimeters thick (5 to 20+ cm, depending on application), or by placing prefab boards.
Prefab boards are less sensitive to tensions. The joints between boards need to be flexibly sealed in order to deal with minor displacement.
Technique 2: synthetic foil
Top-sealing with synthetic foil can be implemented using a number of synthetic materials. The most commonly used are thermoplasts - synthetic materials that remain flexible after small increases or decreases in temperature, which are not easily damaged and which have rows of foil that can be welded together. In general, the foil is delivered in rolls that are 5 to 10 meters wide.
The most commonly used type is HDPE (high density poly-ethylene) which has a density of 900 to 1000 kg/m³. Another type is the plasticized PVC-P. One downside of this is that plasticizing agents can, over time, fade from the PVC whereby it becomes less flexible, and thus more likely to break or tear.
Technique 3: clean soil layer
Figure: Diagram of covering soil layer
The covering soil layer generally consists of a layer of soil which is placed on top of the polluted under-soil. The covering soil layer is placed after the top layer of the polluted soil has been excavated. This type of covering soil layer is implemented if the pollutant is present in close proximity to buildings.
In addition to this, the covering soil layer can be implemented as a ridge above polluted soil. Thus, the ground level is raised.
A typical covering soil layer has a depth of ca. 1 meter. This layer prevents, in normal human use (cables and pipes) and in contact with animals (inc. light plant growth), direct contact with the pollutant.
A covering soil layer is often implemented in combination with top-sealing (foil or natural materials), in order to limit leaching.
Implementation area and implementation conditions
When assessing the compatibility of insulation materials, in addition to the technical realisation possibilities, the implementation conditions also play a role.
The following conditions are important to the technical realisation possibilities:
- If the asphalt concrete layer is applied with a thickness of 6 to 8 cm, a permeability of less than 10-9 m/s can be realised. Asphalt concrete and concrete are poor at dealing with irregular pressures and percolate water from disposal sites, for example. Prefab concrete boards are able to deal with irregular loads, if the joints between the boards are well filled with a flexible sealant.. Volatile organic solvents are able to migrate through concrete.
- A minimum thickness of 2.5 m is required for foils. The under-soil must be levelled and rid of vegetation and sharp objects which may damage the foil. Welding of durable foil-sealing must be carried out in a standardised and controlled manner. The foil must be coated with a draining layer to prevent mechanical damage and to allow excess precipitation to run off.
- A covering soil layer can be implemented as a means of insulation against non-volatile compounds and poor and unleachable compounds, with which direct contact should be avoided. Due to the limited thickness of the covering soil layer, no deep excavation activities may take place through the covering soil layer.
The following implementation conditions are deemed applicable to vertical sealing:
- Water permeability: For this, an assessment needs to take place to determine the extent to which the technique meets the set requirements for permeability; expressed in a value for the maximum permeable infiltration volume/year..
- Life-span: This refers to the time period within which the function can be performed or within which a prior set objective can be attained.
- Subsidence/subsidence differences: If top-sealing or top-screening is implemented, subsidence may be caused as a result of the extra surface load. The occurrence of subsidence can influence the effectiveness of the top-seal.
- Leaching: In the interest of soil quality, leaching requirements can be set for materials used in top-sealing and top-screening;
Costs
The table below contains an indication of the sealing layers in a top-sealing construction. The following comments are applicable:
- Prices are greatly determined by the scale and location of the work;
- Prices are greatly determined by market conditions;
Table: Costs for implementing top-sealing
|
Sealing layer methods of implementation |
Costs |
|
Laying an asphalt or concrete seal |
€ 40 – 75 per m2. |
|
Placing of foil (depending on thickness) |
€ 5 -9 per m2. |
|
Implementation of a layer of clean soil |
€ 12 per m³ clean soil (sand) |
Environmental burden and measures to be implemented
This paragraph contains a qualitative assessment with regards to environmental burdens. The table below contains an overview of the assessment:
Table: Overview of environmental burden from top-sealing
|
Aspect |
Civil-technical hardening materials |
Synthetic foil |
Construction of covering soil layer |
|---|---|---|---|
|
Use of raw materials |
Low, use of sand, cement, gravel |
Low, only usage layer, poss. support layer |
Soil: Dependent upon applied thicknesses and to-be-implemented materials (poss. re-usable materials) |
|
Used soil-burdening substances and aids |
Ground-water is altered (dehydration or saturation of the soil with water above foil). |
Ground-water is altered (dehydration or saturation of the soil with water above foil). |
Not applicable |
|
Emission into air, soil or water |
Relevant where leaching is possible, via leakage through hardening materials. |
Relevant when, over time, leaks occur through the foil |
Dependent upon assessment of pollutants |
|
Odour-problems |
Not applicable |
Not applicable |
Not applicable |
|
Noise problems |
Transport, sealing equipment |
Transport |
Transport |
|
Energy use |
Sealing, production of asphalt, concrete |
Welding, foil production |
Soil transport |
Environment-friendly materials can be selected for the remediation of the covering soil layer. The only problems from this type of remediation are related to the quantity of required clean soil. Instead of this clean soil, one can, within certain boundaries, use re-usable materials.