Principle
A vertical retaining seal serves as a temporary measure, supported by the excavation.
The most common vertical seals at the excavation are as follows:
- Installation of shuttering
- Installation of a Berliner wall
Technique 1: Shuttering
Shuttering is a temporary measure made from wood and/or steel, used to contain soil, which is used in limited-depth excavation. Because mine props are used, the excavation width is limited and the excavation is in strip-form. Shuttering can be implemented in a variety of ways:
- Horizontal, soil-containing planks or struts placed behind or between vertically placed and propped stiles
- two horizontal wales, propped against each other, behind which dam wall planks are placed.
- inter-stage, whereby the shuttering consists of boards propped against each other.
There are two types of shuttering systems: In the first, soil-containing boards are used and in the second, use is made of dam wall planks with a light profile.
Shuttering diagram
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 technical realisation possibilities should be considered:
- The implementation of shuttering as a containing construction in excavation is useful if noise and pile-driving vibration need be avoided, if work space is limited or if the excavation width must remain limited due to the risk of ground cracks. A heavy-duty hydraulic crane is often required for placing the shuttering. Shuttering cannot be implemented if there is a horizontal flow of ground-water. The excavation depth is limited to 6 metres. The width is a minimum of 1 meter. Obstacles are a hindrance during excavation and during placement of shuttering. Where pipe networks are present, boards or planks can be partly omitted.
For vertical sealing, the following implementation conditions are deemed applicable:
- 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 hydraulic resistance for vertical sealing.
- 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. This is only applicable to the life-span of the technique;
- Distortion of sealing soil layers (not permitted): during the implementation of a vertical seal, sealing soil layers are often cut and distorted, which can have an impact on the spread of pollutants.
- Vibrations (civil-technical requirements): In the implementation of vertical sealing techniques, vibration may take place which has a negative impact on the surroundings;
- Leaching: In the interest of soil quality, leaching requirements can be set for materials used in vertical sealing;
- Emissions from the construction: In the interest of soil quality, requirements can be set for emissions which occur from the vertical sealing construction.
Costs
The table below contains the costs for materials and the implementation of the technique:
Costs for shuttering (OVB, 2004)
|
Specifications material/implementation format |
Costs |
Costs mob/demob |
|---|---|---|
|
Shuttering |
€ 115-275 per m² per week, depending on the excavated depth. |
|
Environmental burden and measures to be implemented
There are no problems associated with the placement of shuttering. In comparison to dam walls, noise problems may occur.
Technique 2: Berliner wall
The Berliner wall is a temporary soil-containing construction which consists of steel I-shaped profiles, between which wooden or steel props are placed. The wall can be fitted with anchorage or mine props. I-shaped steel profiles are placed at regular distances of 1 to 3 m around the area that is to be excavated, whereby the ‘flanges’ of profiles run equidistant to the length of the planned excavation. During excavation, wooden beams or plates are inserted from above into the I-shaped profiles, which drop down with the well. The ground pressure is transferred to the I-profiles via these beams or plates; which means that they must be placed deep enough below the excavation level. I-shaped profiles can be pile-driven, vibrated or they can be placed in an earlier drilled whole that is filled with concrete. In fact, the wooden beams only drop down once the excavation has reached a certain depth. As a result of this, the ground behind the wall is de-stressed due to an active shearing zone.
Berliner wall diagram
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 technical realisation possibilities should be considered:
- The Berliner wall is mainly implemented in loose-grain sandy soils. Underground obstacles form a hindrance for the walls The elastic arching of the profiles and the de-stressing of the soil will result in sinking of the ground behind the wall. This can result in damage to foundations situated behind it, namely when they are not on support poles. The elastic re-shaping of the wall can be reduced by implementing mine props or anchorage. For an excavation next to a foundation on steel, the use of a Berliner wall is strongly discouraged due to too much soil de-stressing from the excavation and the low rigidity of the wall. The Berliner wall is a temporary construction. When the wall is removed, there will be a relatively wide space that is difficult to close.
This results in a soil formation, which once again has a negative effect on the present foundations. The excavation capacity is low, due to the labour-intensive excavation approach. The Berlin wall does not dam water. This means that in the presence of ground-water, the water level outside the well must be lowered to 0.50 meters below the well bottom. This is why this technique is primarily implemented in cases of deep-lying ground-water.
For vertical sealing, the following implementation conditions are deemed applicable:
- 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 hydraulic resistance for vertical sealing.
- 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. This is only applicable to the life-span of the technique;
- Distortion of sealing soil layers (not permitted): during the implementation of a vertical seal, sealing soil layers are often cut and distorted, which can have an impact on the spread of pollutants.
- Vibrations (civil-technical requirements): In the implementation of vertical sealing techniques, vibration may take place which has a negative impact on the surroundings;
- Leaching: In the interest of soil quality, leaching requirements can be set for materials used in vertical sealing;
- Emissions from the construction: In the interest of soil quality, requirements can be set for emissions which occur from the vertical sealing construction.
Costs
The table below contains the costs for materials and the implementation of the technique:
Costs for Berliner wall (OVB, 2004)
|
Specifications material/implementation format |
Costs |
Costs mob/demob |
|---|---|---|
|
Berliner wall |
€ 125-250 per m² |
€ 4000-6000 |
Environmental burden and measures to be implemented
There are no or very few problems associated with the placement of a Berliner wall. In comparison to dam walls, noise problems may occur.