The first phase of the so-called Secondary condition I involves the bursting of the old sewer and the radial displacement of the pipe fragments and the surrounding soil.

Subsequently, the soil settles into the overcut ( …

The destruction of the old pipe wall from the inside and the displacement of its fragments into the surrounding soil can lead to damages to the new product pipeline.

The service life of product pipelines installed using the pipe bursting method is greatly reduced by :

• Grooves and/or notches,
• Stress peaks due to point or line loads.

Grooves and notches can occur in all pipe materials.

According to DVGW, grooves of up to 10 % of the wall thickness are permissible for pressure pipelines.

(Table: Characteristics of the E-modulus (short term and long term) for various materials)

The displaced old pipeline fragments lead to the creation of point loads on the new pipeline which can cause peak stresses and thus cracks in the pipe wall.

Therefore, plastic pipes should be highly resistant to stress cracking.

Information on the slow crack growth of a pipe material is provided through the Notch or FNCT test. The point load test in combination with a creep rupture test is another method.

Peak stresses due to point or line loads occur especially with large fragments.

First theoretical investigations in this respect showed stress differences in the pipe of up to 400 % for differently shaped fragments with the same assumed soil stresses [Naneg88].

The Notch-Test according to ISO 13479 or DIN 33479 is a modified creep rupture internal pressure test in which the failure points are defined as four evenly distributed notches about the circumference (apex angle 60°, the notch depth equals to 20 % of the wall thickness).

The test is carried out with a constant hydrostatic internal pressure in an 80 °C water bath until fracture occurs.

(Image: Standstill periods for the Notch-test with reference to […

With the FNCT test, sharp-edged circumferential notches are cut around small test rods which are loaded to failure under a constant tensile stress of 4 N/mm² in a 80°C water bath.

(Image: Full-Notch-Creep-Test)

The point load test for pipes installed on sand bedding using the trenchless method involves applying an external point load to the pipe. The 10 mm-diameter indentor pin is pressed into the pipe wall up to a defined depth (depression = 0.06 · D_e) at room temperature. The test piece is then subjected to a creep rupture test.

For pipelines installed using the bursting method, the following two influences must be considered in the static calculations:

1. Loads lateral to the pipeline axis resulting particularly form soil pressure as well as water and traffic pressure
2. Loads in the direction of the pipeline axis resulting from the pulling-in operation.

The static calculations of pipes for loads lateral to the pipeline axis are to be carried out according to [DWA-A 161]. Although this calculation method is geared towards pipe jacking, it does offer results that also apply to the pipe bursting procedures.

In a more exact static calculation according to the finite element method (FEM), substantially …

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 (…

Measures for the protection of the new pipeline include:

• Reducing the size of the pipe fragments
• Application of pipes with an increased wall thickness or pipes with an outer protective coat
• Application of double-walled pipe systems (casing pipes)
• Lubrication of the …

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 …

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.

(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])

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.