Page 155 - Buried Pipe Design

Design of Gravity Flow Pipes 129

Safety factors
Safety factors for live load analysis may be close to unity. In the above
analyses, the arching action of the soil cover was neglected and thus,
the analyses are very conservative.

Parallel trench
In 1968, parallel trench research was conducted at the Buried
Structure Laboratory at Utah State University. This research was con-
ducted under the direction of Reynold K. Watkins with the primary
objective to answer questions that had arisen concerning embedment
stability when a trench is excavated parallel to an existing buried flex-
ible pipe. Questions such as these were posed:

1. What is the stability of the trench?
2. At what minimum separation between the pipe and the parallel
trench will the pipe collapse?
3. Compacted soil at the sides supports and stiffens the top arch. What
happens to a buried flexible pipe when some or all the side support
is removed in a parallel excavation?
4. What are the variables that influence collapse?

Answers to these questions as determined by Dr. Watkins are summa-
rized here.
In order to reduce the number of variables, ring stiffness was
assumed to be zero. Results were conservative because no pipe has
zero ring stiffness. For the most flexible plain steel pipes, D/t is less
than 300. For the test pipes, D/t was 600 in an attempt to approach
zero stiffness. It was necessary to hold the pipes in shape on mandrels
during placement of the backfill.

Vertical trench walls
Figure 3.27 is the cross-section of a buried flexible pipe with an open
cut vertical trench wall parallel to it. If trench wall AB is cut back
closer and closer to the buried pipe, side cover X decreases to the point
where the sidefill soil is no longer able to provide the lateral support
required to retain the flexible ring. The ring deflects, thrusting out a
soil wedge, as indicated in Fig. 3.28. As the ring deflects, a soil prism
breaks loose directly over the ring. The soil prism collapses the flexi-
ble ring. In order to write pi terms to investigate this phenomenon, the
pertinent fundamental variables must be identified. Ring stiffness is
ignored because the ring is flexible. In fact, at zero ring deflection, the