Page 101 - Buried Pipe Design

Design of Gravity Flow Pipes 77

migrate from trench wall into the embedment, the trench wall may
settle, but the pipe is unaffected. If embedment particles migrate into
the trench wall, the shift in sidefill support may allow slight ring
deflection. This could occur only if the trench wall soil is loose enough,
or plastic enough, that the embedment particles can migrate into it.
Soil particle migration is unusual, but must be considered. Remedies
include (1) embedment with enough fines to filter out migrating parti-
cles in groundwater flow and (2) trench liners. Geotextile liners may
be required under some circumstances.

Voids in the embedment
Soil should be in contact with the pipe in order to avoid piping (chan-
nels of groundwater flow) under the haunches. Voids are avoided if the
embedment is flowable fill—a good idea when trench widths are too
narrow for placement of soil under the haunches. Trench boxes and
sheet piling should be designed so tips of the piles or shield are above
the spring lines of the pipe. If they are designed and used properly,
voids left by the withdrawal of sheet piling or trench shield will not
affect the performance of the pipe.

Heavy equipment
If the buried structure has a low inherent strength or is so flexible that
heavy compactors can deform it, then only light compactors can be
used close to it. Heavy compactors must remain outside of planes tan-
gent to the structure and inclined at an angle less than 45°
/2 from
horizontal. Soil cover H greater than minimum is required above the
structure. The heavy equipment zone is often specified and is usually
2 ft or D/2, whichever is greater. Operators should be reminded that a
large structure gives a false illusion of strength. It achieves its
strength and stability only after the embedment has been placed about
it. Because the structure cannot resist high sidefill pressures during
soil placement, operators should think, “If it were not there, how far
back from the edge of the sidefill would I keep this equipment in order
not to cause a soil slope failure?” The answer is found from experience
and from the tangent plane concept. A margin of safety is usually
applied to the 45°
/2 plane by specifying a 45° tangent plane. The
minimum cover H min for various types and weights of equipment can
be determined by the methods suggested in Chap. 2. As a rule of
thumb, the minimum soil cover should not be less than 3 ft for H-20
truck loads, D8 tractors, etc. For scrapers and super-compactors, 5 ft
of soil may be a more comfortable minimum.