Underground Infrastructure - March 2023 - 25

Directional Drilling
addition to external pressures, due to the head of the relatively
dense drilling fluid/slurry or subsequent (post-installation)
soil loads, the pipe must withstand the axial tensions induced
during the pullback process.
While the document provides useful information for evaluating
the potential for collapse under lateral pressures, either
during of following installation, the most widely used portion
of ASTM F1962 are the formulae for estimating the pulling
tensions corresponding to the leading end of the pipe reaching
point A, B, C and D.
These formulae account for the frictional drag acting on the
pipe along the surface of the borehole, primarily due to the
high buoyant weight of the HDPE pipe, within the relatively
dense drilling fluid, especially for vacant pipe. However, they
are sufficiently general to consider the possible implementation
of anti-buoyancy measures to reduce these otherwise
high frictional forces.
The theoretical basis of the formulae for estimating the required
pulling force is provided below.
Thus, a commonly used procedure for difficult or very long
installations is to fill the pipe with water to reduce the buoyancy.
In the absence of ballast, the maximum total calculated
tension will typically occur towards the end of the installation;
e.g., at point C or D.
The net resulting peak tensile stress is required to be less
than the safe pull tensile stress of the HDPE pipe. Physical
properties of the PE material(s) are provided in the appendices,
which allow determination of an appropriate wall thickness
for the pipe, depending on its diameter and estimated
peak pulling force (or tendency to collapse). Software tools
are based on the ASTM F1962 document and model.
Implementation (Section 9)
Due to the magnitude and complexity of maxi-HDD
equipment and control systems, a well-trained, experienced
crew is essential to plan and execute the operation. The initial
decision involves selecting the size and capacity of the
machine. At a minimum, it should be able to provide the necessary
pulling force, based on the estimated required pulling
force for the pipe itself, with possible additional capacity for
accomplishing reaming.
It is important to properly use the drilling fluid for the
initial pilot bore and reaming operations, and to accurately
locate and track the bore path. The pipe must be securely
gripped, including a swivel and possible breakaway link. Asbuilt
drawings must be provided, preferably supplemented
with details of the soil characteristics and drilling operation.
Inspection and site cleanup (Section 10)
The HDPE pipe should not be cut prematurely. It should
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Underground Infrastructure - March 2023

Table of Contents for the Digital Edition of Underground Infrastructure - March 2023
