51
I-800
Backfilling
During periods of hot weather, backfilling should only be done early in the
morning when the line is fully contracted, and there is no chance of insuf-
ficiently dried joints being subjected to contraction stresses.
The pipe should be uniformly and continuously supported over its entire
length on a firm, stable material. Blocking should not be used to change
pipe grade or to intermittently support pipe across excavated sections.
Pipe is installed in a wide range of sub-soils. These soils must be stable
and applied in such a manner to physically shield the pipe from damage.
Attention should be given to local pipe laying experience, which may
indicate particular pipe bedding problems.
Backfill materials that are free of rocks with a particle size of
1
/
2
-inch or
less should be used to surround the pipe with 6 to 8 inches of cover. It
should be placed in layers. Each soil layer should be sufficiently com-
pacted to uniformly develop lateral passive soil forces during the backfill
operation. It may be advisable to have the pipe under pressure, 15 to 25
psi, during the backfilling.
Vibratory methods are preferred when compacting sand or gravels. Best
results are obtained when the soils are in a nearly saturated condition.
Where waterflooding is used, the initial backfill should be sufficient to
ensure complete coverage of the pipe. Additional material should not be
added until the water flooded backfill is firm enough to walk on. Care
should be taken to avoid floating the pipe.
Sand and gravel containing a significant proportion of fine-grained mate-
rial, such as silt and clay, should be compacted by hand or, preferably,
by mechanical tamper.
The remainder of the backfill should be spread in uniform layers to fill the
trench completely so that there will be no unfilled spaces around rocks or
lumps of earth in the backfill. Large or sharp rocks, frozen clods, and
other debris, greater than 3 inches in diameter, should be removed. Roll-
ing equipment or heavy tampers should be used only to consolidate the
final backfill.
Maintenance shall be in accordance with the Standard for Inspection,
Testing and Maintenance of Water Based Extinguishing Systems, as
defined by NFPA 25.
CAUTION
• Underground pipe must be thoroughly inspected and tested for leaks prior to
backfilling (refer to section on hydrostatic pressure testing).
Failure to follow this instruction could result in system failure.
I-800 2755 Rev_I.fm Page 51 Tuesday, September 13, 2005 8:46 AM