Rev. 9-1-95
DRAINAGE STRUCTURE
BACKFILL REQUIREMENTS
715-R-178
All soft surface sediments
encountered in the excavation for the drainage structure shall be
removed as directed along the entire length of the proposed
structure and replaced with B borrow for structure backfill
compacted to 95 percent of the maximum dry density. The outer
3050 mm (10 ft) of such B borrow under the ends of the structure
shall be enveloped at the top, bottom and outside ends with a
continuous length of permeable geotextile. The geotextile shall
extend the entire width of the excavation. As an alternate to the
geotextile, the outer portion of 3050 mm (10 ft) under the
structure ends may be backfilled with No. 2 aggregate. All B
borrow for structure backfill shall be in accordance with 211.
For structures over 920 mm (36 in.)
wide, the following shall also apply:
All soils surrounding the drainage
structure shall be compacted to at least 95 percent of the
maximum dry density in accordance with 203.24. The soil in the
bottom of the excavation, all bedding material, and the B borrow
for structure backfill shall be tested to ensure compliance with
these density criteria.
If 95 percent of the maximum dry
density cannot be obtained at the bottom of the excavation or in
other areas, the Division of Materials and Tests shall be
contacted for additional recommendations. If during construction,
soft soils are encountered at depths that make removal
impractical, the Division of Materials and Tests shall be
contacted for additional recommendations.
The drainage structure shall be
protected during construction. Heavy construction equipment shall
not cross the drainage structure prematurely or be moved too
close to the walls of the drainage structure.
The drainage structure shall be
encased with 610 mm (2 ft) of B borrow for structure backfill.
The B borrow for structure backfill shall be placed around the
structure in uniform layers not exceeding 150 mm (6 in.) loose
lift thickness, and shall be mechanically compacted thoroughly to
produce the required dry density. The backfill material shall be
placed and compacted in layers of approximately equal thickness,
simultaneously on each side of the structure. Compaction
equipment shall run parallel to the axis of the structure
starting at the extremity and progressing toward the structure.
When the level of fill reaches the
top of the structure, two lifts of B borrow for structure
backfill shall be carefully spread and hand compacted over the
structure without traversing the structure with heavy equipment.
Such B borrow shall be compacted to at least 95 percent of the
maximum dry density in accordance with 203.24. Compaction with
heavy equipment shall not begin until a minimum of two lifts have
been placed, hand compacted, and tested. The compaction equipment
shall traverse the drainage structure perpendicular to its axis.
Riprap and geotextile, both in
accordance with 616, shall be used at the ends of the structure
to protect the exposed B borrow for structure backfill above the
existing ground.