Statement_of_Work.pdf

USDA SEPRL - College Station Road Facility – Sinkhole Repair

STATEMENT OF WORK FOR REPAIR OF SINKHOLES

Dropout at Drain 2

• An excavation should first be made along the east side of the manhole structure to remove any undermined surface soil or loose materials associated with the current dropout. The small hole at the base of the northwestern part of the current dropout (see image attached below) should be “chased” until the hole is no longer evident;

• After removing obvious loose soils and “chasing out” the small hole, the base of the “repair excavation” should be taken to 10 feet below the existing ground surface on the east side of the manhole structure and to 8 feet below the surface on the north and south sides of the structure for the minimum dimensions shown on attached Sketch A. The base of the excavation should be enlarged if necessary to remove loose materials or to completely “chase out” the small eroded hole.

Sidewalls of the excavation can be left in a vertical or near-vertical configuration if workers will not be entering the hole (precautions must be taken to protect workers working near the top of the excavation);

• Low-strength concrete or flowable fill (design 28-day compressive strength of about 1,000 psi) should be placed in the excavation up to 3 feet below the top of the manhole structure;

• After the concrete has hardened, the remainder of the excavation can be backfilled with the previously excavated on-site soil compacted to at least 92% of the standard Proctor maximum dry density.

• Sod should be placed at the surface to prevent erosion.

• Haul excess soil material to on-site location as directed by the USDA.

Small Hole at Drain 1

• We recommend backhoe excavation of the backfill soil around all sides of Drain 1 three feet in depth to a distance of 5 feet outside the perimeter of the manhole structure.

Assuming no voids are observed at the base of or in the sidewalls of the excavation, a filter fabric such as Mirafi 140NL should be placed: along the exposed face of the manhole structure; across the base of the excavation; and up the sidewalls of the excavation. The excavation should then be backfilled with the previously excavated on-site soil placed and compacted to at least 92% of the standard Proctor maximum dry density, with sod applied at the surface. Excess soil material removed from the Drain 2 location or on-site stockpile soil can be used if additional fill material is required to reach the finish grade around the Drain 1 structure. We believe this approach has a high chance of success in preventing a future dropout at this location.

View of washout on east side of Drain 2

View of hole at northwest corner of washout at Drain 2

View of small hole at Drain-1

Basic view of work area – Drain 2 is in foreground – Drain 1 is beyond

Sketch A – Recommended Repair At Drain 2

Sketch B – Recommended Repair

At Drain 1

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