Page 100 - Buried Pipe Design

76 Chapter Three

Some concerns for embedments

1. Wheel loads over a pipe with less than minimum soil cover
2. Water table above the pipe and/or vacuum in the pipe
3. Migration of soil particles out of the embedment
4. Voids left by a trench shield or sheet piling
5. Heavy equipment near pipe

Wheel loads
(See the section Minimum Soil Cover in Chap. 2.) Sidefill soil
strength must support the pipe under a live load. However, minimum
cover of compacted granular soil is about H 1 ft for HS-20 dual
wheel, and H 3 ft for the single wheel of a scraper. Manufacturers of
large steel pipes with mortar linings recommend that a margin of
safety of 1.5 ft be added to the minimum cover. Recommended mini-
mum cover is 2.5 ft for HS-20 loads and 4.5 ft for scraper wheel loads.
With soil cover greater than minimum, wheel load pressure is less
2
than P l W/2H . Trench width could be critical if the sidefill embed-
ment were so poor that it could not support wheel loads anyway.

Water table
(See the section Flotation in Chap. 2.) When the water table is above
the pipe, sidefill soil strength is the effective (buoyant) strength
x K
y .
The effective vertical soil stress is (
y ) eff
y u, where u is the pore
water pressure; that is, u w h, where w is the unit weight of water
and h is the height of the water table (head) above the spring line of
the pipe. If the pipe tends to float, for analysis, P is the hydrostatic
buoyant pressure on the bottom of the pipe, P w (h r), rather than
soil pressure on top.

Soil particle migration
ASTM D 2321 has some rules to follow concerning soil particle size and
possible soil particle migration. In general, open-grained coarser mate-
rial should not be used for foundation and bedding if finer materials are
used in haunching and initial backfill. In such a case, the finer mater-
ial can migrate down in the coarser material, and pipe support can be
lost. Also, groundwater flow may wash trench wall fines into the voids
in coarser embedments.
Wheel loads and earth tremors may shove or shake coarser particles
from the embedment into the finer soil of the trench wall. If fines