Page 160 - Buried Pipe Design

134 Chapter Three

Excavation
Depth of the excavation must include overexcavation required to
remove unstable subbase material. It should be replaced by approved
bedding material. Some tank manufacturers consider soil to be unsta-
2
ble if the cohesion is less than C 750 lb/ft based on unconfined com-
2
pression test or if the bearing capacity is less than 3500 lb/ft . In the
field, bearing capacity is adequate if an employee can walk on the
excavation floor without leaving footprints. A muddy excavation floor
can be choked with gravel until it is stable. These are conservative cri-
teria for soil stability.
Of greater concern are OSHA safety requirements for retaining or
sloping the walls of the trench. Excavations for tanks are usually short
enough that OSHA trench requirements leave a significant margin of
safety. Longitudinal, horizontal soil arching action is significant.
These criteria for bearing capacity and cohesion are equivalent to a
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vertical trench wall over 20 ft deep. Bearing capacity of 3500 lb/ft can
support more than 29 ft of vertical trench depth at soil unit weight of
120 lb/ft . Cohesion of 750 lb/ft can support a vertical open cut trench
2
3
wall that is more than 20 ft deep.
Critical depth of vertical trench wall. Granular soil with no cohesion can-
not stand in vertical cut. Much of the native soil in which pipes and
tanks are buried has cohesion. Therefore, the wall of the excavation
can stand in vertical open cut to some critical depth Z. See Fig. 3.31
(left). Greater depth will result in a “cave-in” starting at the bottom
corner O, where the slope of the failure plane is 45°
/2. For a two-
dimensional trench analysis, the infinitesimal soil cube O is subjected
to vertical stress Z, where

soil unit weight
Z critical depth of vertical trench wall

soil friction angle
C soil cohesion
Mohr’s circle is shown in Fig. 3.31. The orientation diagram (x-z) of
planes on which stresses act is superimposed, showing the location of
the origin O. The strength envelope slopes at soil friction angle
from the
cohesive strength C. At soil slip, Mohr’s stress circle is tangent to the
strength envelope. From trigonometry,

2C
tan 45° 2 Z
This is the critical depth, Eq. (3.31).