[Adopted 6-3-1968]
The success of any street resurfacing is, to a large extent, dependent
upon proper backfilling. To obtain maximum load-bearing capacities and to
prevent settlement, it is necessary that the backfill be made of good material,
properly placed and completely compacted. In backfilling around and over a
structure, the loads of heavy compacting equipment may exceed those for which
the structure was designed. Therefore, it is always advisable that the structure
be covered adequately before direct loads are applied.
Selected, drainable backfill material is preferred, but most local fill
material can be used, provided that it is carefully placed and compacted.
It should be free from large rocks and hard lumps or clods larger than three
inches in diameter. Do not use frozen fill, sod, cinders or earth containing
a high percentage of organic material. Granular material containing a small
amount of silt or clay is ideal, since it makes a dense, stable fill. In instances
where the excavated material is unsuitable for backfill and the opening is
soon to be repaved, use sand for the backfill material.
Fill material under haunches and around the structure should be placed
alternately in six-inch layers on both sides of the pipe to permit thorough
tamping. The fill is placed alternately to keep it at the same elevation on
both sides of the structure at all times. The fine material should be tamped
in these uniform layers with a small tamper under, around and over the structure
to a depth of two feet above the top. It should proceed evenly on each side
of the structure so that injurious side pressures cannot occur. Until two
feet of fill has been placed, earth must be dropped very carefully into the
opening.
A.
The backfilled material shall be consolidated by tamping
or with water. Compaction of fills by puddling or jetting of water is not
recommended, except for cohesionless, sandy or sandy-gravel material. The
reason that water will not work satisfactorily in clayey soil is that it tends
to produce a permanently unstable condition.
B.
The fill should be placed in twelve-inch layers across
the opening and then tamped. Tamping can be done with hand or mechanical equipment,
depending on field conditions. The tamping should proceed carefully and thoroughly
to ensure a completely tamped backfill.
C.
If the water method is used, the first flooding should
be applied only after the first two feet of fill above the structure has been
placed and compacted by tamping, and the second flooding during or after the
subsequent filling of the opening. The method is to attach a piece of pipe
to the water hose and push it through the backfill until it touches the structure
or previously flooded layers. The water is then allowed to run until it reaches
the surface. An excess of water should be avoided to prevent disturbance of
the earth under and around the structure and excess pressure upon it.
A.
Hand equipment. For tamping under the haunches of a structure,
a pole or two-by-four is generally needed to work in the small areas. Hand
tampers for compacting horizontal layers should weigh not less than 20 pounds
and have a tamping face not larger than six inches by six inches. Ordinary
sidewalk tampers are generally too light.
B.
Mechanical tampers. Most types of power tampers are satisfactory
and can be used in all except the most confined areas. However, they must
be used carefully and completely over the entire area of each layer to obtain
the desired compaction. Avoid striking the structure with power tamping tools.
C.
Vibrating compactors. Vibrating equipment can be used
to compact granular backfills, but generally is unsatisfactory for clay or
other plastic soils.
A.
After the opening has been properly backfilled, repave
the area to match the adjoining pavement surface. If convenient, provide temporary
paving over the opening with crushed stone bonded with limestone screenings.
After six months or within one year, place the permanent pavement.
B.
If it is necessary or desirable to repave the area with
a permanent surface immediately after backfilling, use a one-to-fifteen cement
mix in the top three to four feet of the excavation, being careful to maintain
one foot of fine material above the pipe.
