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The pipe bursting process includes additional load-increasing factors caused by the ruptured old pipe, compared with the structural calculation in [DWA-A 161] for pipe jacking. These additional load-increasing factors are compensated for by the load-reducing forces of the process itself. See table below. Conclusion: A structural analysis according to [DWA-A 161] produces results that have a greater safety factor. (Image: Point load test with internal … |
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For gravity pipes, 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, loads are applied on the pipe 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 … |
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To protect the new pipe, the bursting process can be modified to reduce the influence of the old pipe fragments on the new pipeline in the displaced soil. Measures for protecting the new pipe 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 applying loads 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. This results in a more even distribution of the loads on the new pipe and smaller notches and grooves … |
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(Image: Reducing the size of the broken pipe pieces) Increasing the thickness of the pipe walls (sacrifice layer) by 0.04 or 0.08 in (1 or 2 mm) may serve to protect the new pipe against damages. The increase in wall thickness is largely dependent on the material type of the old pipe, and thus on the fracture pattern of the fragments (such as geometry and sharpness), as well as on the material type of the new pipe. |
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(Image: Application of pipes with an outer protective coat) Another possibility for protecting the new pipeline is the application of an outer layer (protective coating). (Image: HDPE long pipes DA 20 in 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]) A two-phase installation method can prevent damages to the product pipe due to point loads or line loads. This process involves using a bursting unit to first install a casing pipe into which the product pipe 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 pipe are fixed into place by grouting the annular space, formed by the bursting head, between the new pipe and the cavity (also called the overcut) with a clay cement suspension [FI-Tracta] [Miege90] [Falk95b]. This procedure is also referred to as annular space grouting |
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(Image: Plus/Minus Icon) Advantages of annular space grouting:
(Image: Bursting process in combination with annular space grouting - Pipe piece removed showing clay cement and adhering fragments of the old … |
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1st Factor (Image: Fragezeichen) 2nd Factor (Image: Fragezeichen) 3rd Factor (Image: Fragezeichen) You will find the correct answer on the following page. |
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The required bursting force depends 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 resistance to breaking of the old pipe is established by the:
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(Image: Displacement capability) The soil displacement capability is mostly established by its density. The main influencing factors include the:
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(Image: Amount of expansion) The amount of soil displacement (expansion) corresponds to the difference between the radius of the expansion and the internal radius of the old pipe. The overcut (i.e. radius of expansion minus external radius of the new pipe) should be as small as possible in order to minimize the movement of the old pipe fragments. (Image: Amount of expansion and overcut) |
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In determining the amount of soil displacement, the following questions must be answered while taking into account the existing soil conditions:
In order to prevent surface heaving or settling, a minimum depth of cover of 10 times the amount of expansion has proven to be effective. … |
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A buffer between parallel pipes of at least three times the amount of expansion (minimum 40 cm) has proven to be effective for cohesive soils [DWAM143-15:2005] (Image: Fragezeichen) Here is an example for the calculation of expansion amounts in cohesive soils: Diameter of the old pipeline: DN 10 in (250 mm) Calculation according to DWA-M 143-15: 3 x ( … |
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For non-cohesive and/or rocky soils, special considerations apply regarding the diameter and the material of neighbouring pipes. For brittle pipe materials and diameters smaller than DN 8 in (200 mm), the minimum spacing should be at least 5 times the amount of expansion. For nominal sizes larger than DN 8 in (200 mm), the spacing should not be less than 3.3 ft (1.0 m). (Image: Fragezeichen) Here is an example for the calculation of expansion amounts … |
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For non-cohesive and/or rocky soils, special considerations apply regarding the diameter and the material of the neighbouring pipes. For brittle pipe materials and diameters less than DN 8 in (200 mm), the minimum spacing should be at least 5 times the amount of expansion. For nominal sizes larger than DN 8 in (200 mm), the spacing should not be less than 3.3 ft (1.0 m). (Image: Fragezeichen) Example calculation: 5 x (13 in - 10 in) = 1.5 ft < min. … |
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(Image: Measured soil deformation as a result of an expansion [Zimme88]) Supporting findings come from the studies by ZIMMERMANN and LGA Nuremberg. These are the resulting relationships: Eb = (A – DN) x (4 to 6) with: Eb = Area of influence Example:
This results in an influence area of: Eb = (575-400) x 4 = 27.5 in (0,7 m) Eb = (575-… |
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May 13, 2020 |
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