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For gravity pipelines, the reduction of the carrying capacity is usually compensated by the large safety margins included in the calculating method. For pressure pipes it may be necessary to use the reduced wall thickness. Example: Groove depth: 10 % of the wall thickness Increase of stress from normal forces (e.g. internal pressure): 1-(1-10 %)1 = 10 % Increase of stress from bending moment (e.g. traffic): 1-(1-10 %)2 = 19 % Increase of bending deformation (… |
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During the construction phase, the pipeline is loaded in the direction of its axis by tensile forces delivered by the bursting unit. As a result of planned and/or unplanned steering movements, uneven tensile stresses arise around the circumference and must be limited to the permissible tensile stress of the pipe material. In addition, the minimum wall thicknesses listed in the DWA worksheet - [DWA-A 161] must be adhered to. (Table: Minimum wall thickness … |
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In order to protect the new pipeline the bursting process can be modified to reduce the influence that the, into the soil displaced, old pipeline fragments have on the new pipeline. Measures for the protection of the new pipeline include:
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(Image: Reducing the size of the broken pipe pieces) (Image: Bursting unit with cutter knives [FI-Cerma]) The angle of the cone-shaped bursting head has a significant effect on the load introduction and thus on the destruction of the old pipeline [Falk95b]. The cone angle should be selected with the goal of creating the smallest possible pipe fragments which in turn lead to a more even distribution of the loads on the new pipeline, and thus to smaller … |
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(Image: Reducing the size of the broken pipe pieces) Increasing the thickness of the pipe walls (sacrifice layer) by 1 or 2 mm may serve to protect the new pipeline against damages. The increase in wall thickness is largely dependent on the material type of the old pipeline, and thus the fracture pattern of the fragments (such as geometry and sharpness), as well as the material type of the new pipeline. |
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(Image: Application of pipes with an outer protective coat) A further possibility is the application of an outer layer (protective coating) onto the new pipeline. (Image: HD-PE long pipes DA 500 with an outer protective coat) (Image: PE-pipe with an outer protective coat against groove and notch formation [FI-Egepla]) |
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(Image: Peeling off the protective coat of an Egeplast Saftey-Line coated pipe (SLM) [FI-Egepla]) (Image: Preparing the welding seam (left: removing the welding bead, right: bandaging the welded seam with a GRP bandage) [FI-Egepla]) Because of the additional outer coat, special cover sleeves are necessary for butt welding. In the region of the welding seam and the connections, it is necessary to peel off a sufficient width of the protective coat. (Image: … |
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(Image: Application of double-walled pipe systems) (Image: Double-walled pipe system [Jürge05]) Damage to the product pipeline and loading by point or line loads is preventable through the implementation of a 2-phase installation. This process involves using a bursting unit to first install a casing pipeline into which the product pipeline is placed as part of the second working step. |
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(Image: Lubrication of the new pipeline and fixing of the pipe fragments of the replaced pipe with a clay cement suspension) During the pipe bursting process the broken pieces of the old pipeline are fixed into place by grouting the annular space, formed by the bursting head, between the new pipeline and the cavity (also called the overcut) with a clay cement suspension [FI-Tracta] [Miege90] [Falk95b]. This procedure is also referred to as annular … |
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(Image: Plus/Minus Icon) Advantages of annular space grouting:
(Image: Bursting process in combination with an annular space grouting [FI-Tracta] - Pipe piece removed showing clay cement and adhering fragments … |
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Dec 20, 2013 Question: Upon which factors is the required bursting force dependant? 1st Factor (Image: Fragezeichen) 2nd Factor (Image: Fragezeichen) 3rd Factor (Image: Fragezeichen) |
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Dec 20, 2013 The required bursting force is dependant on: 1. Breaking resistance of the old pipeline 2. Displacement capability of the soil in the embedment 3. Amount of expansion (Image: Breaking resistance) (Image: Displacement capability) (Image: Amount of expansion) |
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(Image: Breaking resistance) The breaking resistance of the old pipeline is established by the:
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(Image: Displacement capability) The displacement capability of the soil is established mostly by its density. Main influencing factors include the:
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(Image: Amount of expansion) The amount of expansion corresponds to the radius of the expansion minus the internal radius of the old pipeline. The overcut (i.e. radius of expansion minus external radius of the new pipeline) should be as small as possible in order to minimize the movement of the old pipeline fragments. (Image: Amount of expansion and overcut) |
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In determining the amount of expansion, the following questions must be answered while taking into account the existing soil conditions:
In order to prevent heaving or settling at the surface, a minimum pipeline depth of cover of 10 x the amount of expansion has proven to be … |
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According to DWA-M 143-15 [DWAM143-15:2005], a buffer between parallel pipelines of at least three times the amount of expansion (minimum 40 cm) has proven to be effective for cohesive soils. (Image: Fragezeichen) Example calculation for the amount of expansion in cohesive soils: Diameter of the old pipeline: DN 250 Calculation according to DWA-M 143-15: 3 x (340 mm -– 250 … |
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For non-cohesive and/or stony soils special considerations apply which concern the diameter and the material of the neighbouring pipelines. For pipe materials < DN 200 that are brittle and easy to break, the minimum spacing should be at least 5 times the amount of expansion. For nominal sizes > DN 200, the spacing should not be less than 1.0 m. (Image: Fragezeichen) Example calculation for the amount of expansion in non-cohesive soils: Neighbouring … |
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For non-cohesive and/or stony soils special considerations apply which concern the diameter and the material of the neighbouring pipelines. For pipe materials < DN 200 that are brittle and easy to break, the minimum spacing should be at least 5 times the amount of expansion. For nominal sizes > DN 200, the spacing should not be less than 1.0 m. (Image: Fragezeichen). Example calculation:
The calculated distance … |
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(Image: Measured soil deformation as a result of an expansion [Zimme88]) Supporting findings come from the studies by ZIMMERMANN and the LGA Nuremberg. These are the resulting relationships: Eb = (A – DN) x (4 to 6) Eb = Area of influence A = Expansion (De + 15 %) Example: |
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Dec 20, 2013 |
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Dec 20, 2013 The minimum distances to neighbouring utilities and structures depend on:
In case that minimum distances cannot be adhered to, special protective measures, such as open cut excavation at the crossing points, must be taken in order to prevent the transfer of loads. (Image: Underground utilities … |
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