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.