Page 162 - Trenchless Technology Piping Installation and Inspection

128 Cha pte r F o u r

Soil arch Pillar
Prism Prism

Flexible pipe Rigid pipe
FIGURE 4.5 Arching effects for fl exible and rigid pipes. (Gabriel, 2006.)

the vertical pressure above a box culvert with 32.8 ft (10 m) backfill
height but with 1.7 ft (0.5 m) of expanded polystyrene foam placed
immediately above the culvert was 50 percent of the pressure due to
the weight of the soil prism above. However, the vertical pressure
above an identical box culvert under the same embankment height
without the foam, and with normally compacted backfill, was about
120 percent of the soil prism weight. These examples indicate that
installation methods and culvert shape can strongly influence the
magnitude and distribution of earth pressures on rigid culverts.
The magnitudes of the loads exerted on culverts depend on arch-
ing effects, which are the result of relative deformation of the backfill
in a certain zone above the culverts. This deformation is related to
both the soil and the structural stiffness. In the case of flexible culverts,
culvert deformation results in arching effects, which reduce the verti-
cal loading regardless of the installation method. Typically, the verti-
cal earth pressure on flexible culverts is less than that due to the weight
of the prism of soil above the culvert. For rigid culvert installation,
arching effects can also be achieved by introducing compressible
material into the backfill. In the case of positive projecting embank-
ments, arching results in vertical earth pressures that are greater than
that due to the weight of the prism of soil above the culvert. Thus,
according to the Marston’s theory, the vertical earth pressures on cul-
verts are a function of the installation method, the soil and structural
stiffness, the geometry of the structure, and the boundary condition
with the natural ground. It is worth noting that even for flexible cul-
verts, a load reduction is achieved as differential settlements transfer
loads to the surrounding soil adjacent to the structure.