Page 98 - Buried Pipe Design

74 Chapter Three

windrow of soil on the trench bank until a soil slide develops. This soil
slide can be directed by the jet into place against the pipe with enough
energy to fill in the voids. Windrows are added on both sides simulta-
neously in order to keep the soil in balance. Of course, the water must
drain out rapidly for best compaction.

Slurry and flowable fill. Under some circumstances, the best way to
assure support under the haunches is by flowable fill (soil cement or
slurry). The pipe is aligned on mounds. Flowable fill is poured into the
haunch area on one side of the pipe. If flowable fill is required to a depth
greater than flotation depth, it can be poured in lifts. Full contact is
ensured when the flowable fill rises on the other side of the pipe.
Flowable fill should not shrink excessively. Some agencies specify com-
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pressive strength of 200 lb/in . Less strength (40 lb/in ) may be desirable
to reduce stress concentration and to facilitate subsequent excavations.
Recommended slump is about 10 in.
High-velocity impact. Soil compaction can be achieved by dropping
from a sufficient height, blowing, slinging, etc. Better control is
achieved if the embedment is “shot-creted” into place or if dry soil is
blown or slung into place.

Trench Width
Trenches are kept narrow for rigid pipes. The Marston load on a pipe
is the weight of backfill in the trench reduced by frictional resistance
of the trench walls. The narrower the trench, the lighter is the load on
the pipe. The pipe has to be strong enough to support the load. The
trench only needs to be wide enough to align the pipe and to place
embedment between pipe and trench wall. If ring deflection is exces-
sive, or if the pipe has less than minimum soil cover when surface
loads pass over, the soil at the sides can slip. Ring inversion is incipi-
ent. If there is any possibility of soil liquefaction, the embedment
should be denser than critical density. Ninety percent standard den-
sity (AASHTO T-99 or ASTM D 698) is often specified. In loose satu-
rated soil, liquefaction can be caused by earth tremors. Soil
compaction may or may not be required depending upon the quality
of the embedment. For example, gravel falls into place at densities
greater than 90 percent. Loss of embedment by washout of soil parti-
cles by groundwater flow (piping) should be prevented. Spangler
observed that a flexible ring depends upon support from sidefill soil.
His observation led to the inference that if the trench is excavated in
poor soil, the trench walls cannot provide adequate horizontal support.
The remedy appeared to be wider trenches, especially in poor native