TM 5-818-4/AFM 88-5, Chap. 5
susceptible to frost penetration. Drainage zones are of-
larly adapted to areas containing a maze of pipes or
ten extremely important to the satisfactory construc-
conduits. Adequate downward drainage, generally es-
tion and subsequent performance of the structure. To
sential to the success of this method, can be provided
maintain the proper functioning of these zones, care
by sump pumps or, if necessary, by pumping from well
must be taken to ensure that the material placed has
points. Sluicing should be accompanied by vibrating to
the correct gradation and is compacted according to
ensure adequate soil density. Concrete vibrators have
specifications.
been used successfully for this purpose. This method
should be restricted to areas where conduits or pipes
c. Special problems. In open zones, compaction of
have been placed by trenching or in an excavation that
backfill will not generally present any particular prob-
provides confining sides. Also, this method should not
lems if proper compaction procedures normally associ-
be used below the groundwater table in seismic zones,
ated with the compaction of soils are exercised and the
since achieving densities high enough to assure stabili-
materials available for use, such as backfill, are not un-
ty in a seismic zone is difficult.
usually difficult to compact. The majority of the prob-
(c) Another method is to place clean, granular
lems associated with backfill will occur in confined
bedding material with pneumatic concrete equipment
zones where only small compaction equipment produc-
under the haunches of pipes, tunnels, and tanks. The
ing a low compaction effort can be used or where be-
material is placed wet and should have an in-place wa-
cause of the confined nature of the backfill zone even
ter content of approximately 15 to 18 percent. A noz-
small compaction equipment cannot be operated effec-
zle pressure of 40 pounds per square inch is required to
tively.
obtain proper density. Considerable rebound of materi-
(1) Considerable latitude exists in the various
al (as much as 25 percent by volume when placed with
types of small compaction equipment available. Unfor-
the hose nozzle pointed vertically downward and 50
tunately, very little reliable information is available on
percent with the nozzle pointed horizontally) occurs at
the capabilities of the various pieces of equipment. De-
this pressure. Rebound is the material that bounces off
pending upon the soil type and working room, it may
the surface and falls back in a loose state. However,
be necessary to establish lift thickness and compaction
the method is very satisfactory if all rebound material
effort based essentially on trial and error in the field.
is removed. The material can be effectively removed
For this reason, close control must be maintained
from the backfill by dragging the surface in the area
particularly during the initial stages of the backfill un-
where material is being placed with a flat-end shovel.
til adequate compaction procedures are established.
Two or three men will be needed for each gunite hose
(2) Circular, elliptical and arched walled struc-
operated.
tures are particularly difficult to adequately compact
(d) For structures and pipes that can tolerate
backfill beneath the under side of haunches because of
little or no settlement, lean grouts containing granular
limited working space. Generally, the smaller the
material and various cementing agents, such as port-
structure the more difficult it is to achieve required
land cement or fly ash, can be used. This grout may be
densities. Rock, where encountered, must be removed
placed by either method discussed in (b) and (c) above.
to a depth of at least 6 inches below the bottom of the
However, grouts may develop hard spots (particularly
structure and the overdepth backfilled with suitable
where the sluice method is used that could cause segre-
material before foundation bedding for the structure is
gation of the granular material and the cementing
placed. Some alternate bedding and backfill placement
agent), which could generate stress concentrations in
methods are discussed below.
rigid structures such as concrete pipes. Stress concen-
(a) One method is to bring the backfill to the
trations may be severe enough to cause structural
planned elevation of the spring line using conventional
distress. If lean grouts are used as backfill around a
heavy compaction equipment and methods. A tem-
rigid structure, the structure must be designed to
plate in the shape of the structure to be bedded is then
withstand any additional stress generated by possible
used to reexcavate to conform to the bottom contours
hard spots.
of the structure. If the structure is made of corrugated
metal, allowance should be made in the grade for pene-
tration of the corrugation crests into the backfill upon
5-2. Installation of instruments. Installa-
application of load. Success of this method of bedding
tion of instrumentation devices should be supervised,
is highly dependent on rigid control of grade during re-
if not actually done, by experienced personnel from
excavation using the template. This procedure is
within the Corps of Engineers or by firms that special-
probably the most applicable where it is necessary to
ize in instrumentation installation. The resident engi-
use a cohesive backfill.
neer staff must be familiar with the planned locations
(b) Another method of bedding placement is to
of all instruments and necessary apparatus or struc-
sluice a clean granular backfill material into the bed
tures (such as trenches and terminal houses) so that
after the structure is in place. This method is particu-
necessary arrangements and a schedule for installa-
5-4
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