518 Drainage of Structures

Porous Backfill (518.05)

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.


Documentation Requirements - 518 Drainage of Structures

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.