UFC 3-220-01N
15 AUGUST 2005
Another method is to place clean, granular bedding material with
pneumatic concrete equipment under the haunches of pipes, tunnels, and
tanks. The material is placed wet and should have an in-place water
content of approximately 15 to 18 percent. A nozzle pressure of 40
pounds per square inch is required to obtain proper density. Considerable
rebound of material (as much as 25 percent by volume when placed with
the hose nozzle pointed vertically downward and 50 percent with the
nozzle pointed horizontally) occurs at this pressure. Rebound is the
material that bounces off the surface and falls back in a loose state.
However, the method is very satisfactory if all rebound material is
removed. The material can be effectively removed from the backfill by
dragging the surface in the area where material is being placed with a
flat-end shovel. Two or three men will be needed for each gunite hose
operated.
For structures and pipes that can tolerate little or no settlement, lean
grouts containing granular material and various cementing agents, such
as portland cement or fly ash, can be used. This grout may be placed by
either method discussed above. However, grouts may develop hard spots
(particularly where the sluice method is used that could cause segregation
of the granular material and the cementing agent), which could generate
stress concentrations in rigid structures such as concrete pipes. Stress
concentrations may be severe enough to cause structural distress. If lean
grouts are used as backfill around a rigid structure, the structure must be
designed to withstand any additional stress generated by possible hard
spots.
8-5.4
Installation of Instruments. Installation of instrumentation devices
should be supervised, if not actually done, by experienced personnel from Contracting
Officer's Organization or by firms that specialize in instrumentation installation. The
resident engineer staff must be familiar with the planned locations of all instruments and
necessary apparatus or structures (such as trenches and terminal houses) so that
necessary arrangements and a schedule for installation can be made with the
contractor and with the office or firm that will install the devices. Inspectors should
inspect any instrumentation furnished and installed by the contractor. Records must be
made of the exact locations and procedures used for installation and initial
observations. Inspectors should ensure that necessary extensions are added for the
apparatus (such as lead lines and piezometer tubes) installed within the backfill as the
backfill is constructed to higher elevations. Care must be used in placing and
compacting backfill around instruments that are installed within or through backfill.
Where necessary to prevent damage to instruments, backfill must be placed manually
and compacted with small compaction equipment such as rammers or vibratory plates.
8-5.5
Post-construction Distress. Good backfill construction practices and
control will minimize the potential for postconstruction distress. Nevertheless, the
possibility of distress occurring is real, and measures must be taken to correct any
problems before they become so critical as to cause functional problems with the
facility. Therefore, early detection of distress is essential. Some early signs of possible
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