Page 160 - Trenchless Technology Piping Installation and Inspection

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displaced by it. Therefore, if trenchless technology methods were
used, where the pipe is installed with minimal disturbance to the
surrounding soil, the only variable in design would be the strength
of the pipe. So, for trench installations, restoring soil to its undis-
turbed state is desirable although it may be practically impossible
(Petroff, 1995).

4.3.3 Pipe-Soil Interaction
Whether a pipe is rigid or flexible has profound effect on the way
in which it interacts with the surrounding soil. The interaction
between pipe and soil influences the magnitude of loads exerted on
the pipe and the manner in which the pipe transfers these loads to
the surrounding soils. Calculation of loads exerted on underground
pipelines can be traced back to the studies conducted by Anson
Marston during the early part of the twentieth century. The results
were later expanded by M. G. Spangler and R. K. Watkins and are
still in use today (Moser and Folkman, 2008). Figure 4.4 provides
an illustration of the soil load distribution on rigid and flexible
pipes. In the case of rigid pipes, the theory proposes that the soil in
the side prism tends to settle relative to the central prism. This
causes the pipe to assume full load of the central prism and a por-
tion of the load from the side prisms. In contrast, a flexible pipe
tends to deflect, which result in a lowering of the pressure from the
central prism.

Side prism Central prism Side prism Side prism Central prism Side prism

Rigid pipe Flexible pipe
FIGURE 4.4 Trench load comparisons for rigid and fl exible pipe. (Najafi , 2005.)