Unless otherwise shown in the
contract drawings, the Contractor is to provide a minimum of 24 inches of
porous backfill behind the full-length of abutments, wing walls, and retaining
walls. The placement width is normally
erratic due to construction means and methods.
Make sure the minimum width is maintained. If the underdrain is at the footing
elevation, the 24-inch dimension is measured from the edge of the footing, not
the back face of the wall. If there are weep holes in the concrete, the Contractor
must place 2 cubic feet of bagged No. 3 aggregate at each weep hole to retain
the porous backfill.
Porous backfill is No. 57
size gradation; it must be compacted.
Even rounded No. 57 gravel is not self-compacting.
It is imperative that the
filter fabric used to encapsulate the porous backfill be continuous and
properly overlapped. The fabric gets
flipped back and forth as the Contractor alternates from porous backfill to
Type B granular. Working the fabric in
this manner can cause misplacement or tearing.
A non-continuous or misplaced layer of filter fabric allows the fine
material to “pipe” into the porous backfill, which can lead to settlement and
lack of drainage.
All drains should be free
flowing. They need to have a positive
fall. Special care needs to be taken
when the drainage hangers, anchors, or pipes are attached to a structure prior
to final dead load deflection. As the
bridge deflects, the grade of the pipes may be affected and cause stagnant or
pooling sections of pipe.
1. Porous Backfill.
a. Type of stone used.
b. Pay in appropriate unit.
2. Pipe.
a. Metal pipe per various sections in 707.
b. Plastic pipe per 707.33,
707.45.
c. Laid to grade, outletted per
plan.
d. Measure each type per linear foot.
3. Scuppers, structural steel, cast steel.
a. Verify prequalification of fabricator to level SF.
b. Shop drawings per 501.04.
c. Test reports per 501.06.