Page 161 - Trenchless Technology Piping Installation and Inspection

Pipe and Pipe Installation Considerations 127

Soils Arching Effect
Marston’s theory (Spangler, 1982) proposes that the load due to the
weight of the soil prism above an underground pipeline is modified
by arching action. According to this theory, part of the soil weight is
transferred between side prisms and the central prism, resulting in
either an increase or decrease in the effective weight of soil above the
pipe. Arching effects could be classified as either positive or nega-
tive. Positive arching effects decrease the vertical pressure exerted
on the pipeline whereas negative arching effects increase the vertical
pressure.
For pipelines installed in a trench, insufficiently compacted back-
fill and embedment material are more compressible than the adjacent
native soil that has become well-compacted through natural consoli-
dation. The more compressible backfill and embedment material has
a tendency to consolidate and settle more than the native soils. As a
result, some of the vertical soil load is transferred through shearing
stresses between the side prisms and the central prism and creates
positive arching. The degree of transfer depends upon the type of
backfill material and how well it is compacted. This arching action
can be used advantageously for both rigid and flexible pipes by mak-
ing the bedding immediately underneath the pipe more compressible
than the adjacent bedding. However, care must be taken in order to
avoid differential settlement of the pipe, especially for rigid pipes.
Assuming well-compacted soils in bedding and backfill, positive
arching effects occur in the case of flexible pipes that deflect owing
to their lower stiffness. This phenomenon is expected to naturally
occur in trenchless installation of pipelines. The relative downward
movement of the central prism within the trench mobilizes upward
shearing stresses along the sides and creates an arching action that
partially supports the soil column weight above the structure (see
Fig. 4.5). In addition to this action, passive resistance of the soils adja-
cent to the pipe is mobilized and aids in transfer of loads. In contrast
and again assuming well-compacted soils in bedding and backfill,
negative arching occurs in the case of rigid pipes that do not deflect
owing to their high stiffness. Owing to the relatively low stiffness of
soils on the sides, shearing forces are transferred from the side prisms
to the central prism increasing the effective vertical soil load on the
pipe (see Fig. 4.5).
Installation conditions can have a significant effect on the soil
loads acting on underground pipelines. In a long-term behavior study
in Norway (Vaslestad et al., 1994), a 5.25 ft (1.6 m) diameter circular
concrete culvert was constructed with imperfect trench condition (the
compressible material was expanded polystyrene foam) with 46 ft
(14 m) embankment height. Vertical earth pressure directly above the
culvert was only 25 percent of the soil prism weight. Horizontal earth
pressure at the mid elevation of the culvert, however, was 73 percent
of the soil column weight above that elevation. In the same research,