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 new product pipes 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.

Grooves of up to 10 % of the wall thickness are permissible for pressure pipelines (according to DVGW).

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

¹ _{The Deutscher Verein des Gas- und Wasserfaches (DVGW) is the German association for gas and water with headquarters in …}

The displacement of the old pipe fragments leads to the creation of point loads on the new pipeline, which can cause peak stresses and consequently can result in 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 Full Notch Creep Test (FNCT). The point load test in combination with a creep rupture test is another method.

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

The Notch-Test 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) [ISO 13479]

The test is carried out with a constant hydrostatic internal pressure in an 175 °F (80 °C) water bath until fracture occurs.

The point load test for pipes installed on sand bedding using the trenchless method involves applying an external point load to the pipe. The 0.4 in-diameter (10 mm) indenter 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 pipes installed using the bursting method, the following two factors must be considered in the static calculations:

1. Lateral loads to the pipeline axis resulting from soil, water, and traffic pressure

2. Axial loads resulting from the pulling-in operation.

The static calculations of lateral loads to the pipe axis are to be carried out according to the manufacturer's specifications.

In a more precise static calculation according to the finite element method (FEM), substantially lower results are usually obtained as the bursting head causes a circular compaction and thus an "improvement" of the soil. This reduces the loads on the pipe. …

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 (…

Measures for protecting the new pipe include:

• Reducing the size of the pipe fragments

• Using pipes with an increased wall thickness or pipes with an outer protective coat

• Using double-walled pipe systems (casing pipes)

• Lubrication of the pipe and fixing of the pipe fragments with a clay cement …

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 …

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.

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.