CHAPTER 9
EXCAVATION AND BACKFILL
EXCAVATION AND BACKFILL
Standard Specification 2565.3, 2451, and 1805
A basic understanding of specifications is needed to
properly replace the disturbed soil and restore the
condition of the excavated area. The size of the
disturbed area should be kept to the minimum
required to do the job. All areas disturbed during the
project must be restored to the satisfaction to of the
Engineer.
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Trenching must be located at a distance from the edge of the pavement, back of curbing, or
edge of surfaced shoulders as indicated in the Contract Documents, or as directed by the
Engineer. The distance must be such that no damage, or undermining, is done to the pavement
or curb. The trench must be of uniform alignment for accurate referencing of the underground
installation.
If trenching or excavation operations require the removal of concrete pavement or sidewalk, it
must be first cut with a concrete saw and then removed. Removal must be to the nearest
existing joint or as directed by the Engineer.
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The Contractor must have the proper operating compaction equipment on the site before any of
the excavation has begun. A mechanical tamper is usually used in this type of work. The foot
of the tamper must fit within the trench or excavation.
Excavations around foundations, handholes or
conduits must be backfilled with the same material
as the adjacent soils and compacted to
approximately the same density. Spec. 2451.3D
requires that backfill material be uniformly
distributed in approximately 8 inch (200 millimeter)
thickness, or less, and thoroughly compacted as
required.
Once the excavation for a structure or conduit is finished, the structure must be installed as
soon as practical. When placing conduit or direct buried cable, backfilling and proper
compaction must be done as soon as possible to prevent water contamination of the roadbed,
shoulder, or slopes.
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Precast concrete foundations for roadway lighting must be installed as detailed in special
provisions which are shown below. The foundations must be installed in drilled shafts and fine
filtered aggregate or lean mix backfill must be used to back fill the void between the foundation
and the surrounding soil.
B.
Precast Concrete Light Foundation Design E & H Installation Requirements
Install Precast Light Foundations Design E and Design H in accordance with Standard Plates 8127 and 8128, 2545.3 F, and
as follows:
1.
Excavate a cylindrical 30 in to 36 in diameter drilled shaft into the ground with a 30 in or 36 in diameter earth auger bit.
(1.1)
Use auger drilling equipment with adequate capacity, including power, torque, and down thrust to auger a 30
in to 36 in diameter cylindrical drilled shaft to the depth indicated in contract documents.
2.
Level and firmly compact the bottom of the shaft so it is flat and horizontal while maintaining the required depth of the
shaft.
3.
Remove the forming tube from the precast foundation before placing the precast foundation in the shaft.
4.
Center, set, and level the precast foundation in the shaft and maintain a 3 in minimum to a 6 in maximum continuous
symmetrical annular void between the precast foundation and surrounding soil.
5.
Before backfilling place in the annular void the ground rod electrode with the ground rod electrode conductor in
accordance with contract documents.
6.
Completely backfill the annular void between precast foundation and the surrounding soil of the shaft with firmly
compacted fine filter aggregate in accordance with 3149.2J.2.
(6.1)
Use a mechanical pole tamper to firmly compact each 6 in layer of fine filter aggregate to uniform density.
(6.2)
Substitution of lean mix backfill per specification 2520 instead of fine filter aggregate is permitted.
(6.2.1)
Protect the conduit stub out with end bell opening from lean mix backfill to ensure the end bell opening is
accessible and the conduit is free of debris for the installation of direct buried lighting cable.
(6.2.2)
After lean mix backfill has been poured and cured excavate the open trench for the direct buried lighting
cable and install the direct buried lighting cable in the foundation.
7.
Test and verify the compaction of the filtered aggregate backfill material around the precast foundations in accordance
with 2105.3F.3 at a rate of one test per every 10 (ten) foundations with a minimum of two tests per project.
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Due to the ongoing issues with improper compaction around precast light foundations as shown
below the installation requirement changes described above were necessary to ensure long
term stability of the light foundation.
The following are examples of pre-cast light foundations that were not adequately compacted
when initially installed.
Excavated material must be placed where it will not cause damage or obstruction to vehicles,
pedestrians or interfere with surface drainage. Holes resulting from excavation must be covered
and protected as soon as practical.
The Contractor and Inspector must not allow unprotected hazards to exist for motorists or
pedestrians during the construction of the project.
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