212Structure Excavation and Backfill

Structure Excavation and Backfill

212.1DESCRIPTION

1 This section describes the requirements for excavation and backfill for the construction of bridge foundations, retaining walls, bin walls, and other structures and for the disposal of excess materials.

2 Structure excavation will be classified as follows:

212.2MATERIALS

1 Provide materials in accordance with the following:

Materials Subsection Pervious Backfill Material 803

2 Provide backfill material consisting of clean gravel from sources other than surface water sources and free from lar ge or frozen lumps, wood, rocks, or other potentially harmful matter.

212.3EQUIPMENT—V acant

212.4CONSTRUCTION

212.4.1 General

1 Where practical, construct structures in open excavation. Shore or brace, and protect with co fferdams, where necessar y. Forms may be omitted, with the approval of the engineer, when the site is dry and footings can be placed without cofferdams. Fill the excavation with concrete to the top of the footing, in accordance with Section 513, Structural Concrete. Provide and place additional concrete required because of eliminating the forms, at no additional cost to the department. If additional excavation depth is required by the engineer, the department will pay for additional Class B concrete.

2 Restore channels to as near original condition as possible.

Structure Excavation and Backfill SECTION 212

212.4.2 Depth of Footing

1 Excavate for footings to the specified elevations. If the material encountered at the specified elevation is unsuitable for foundation, the engineer will require additional excavation. Fill with class B concrete to the specified elevation for the bottom of the footing.

212.4.3 Preparation of Foundations for Footings

1 When excavations for footings are in rock or other hard foundation material, cut the final surface level and remove loose debris. Clean seams and fill with concrete, mortar, or grout.

2 When p lacing concrete on an excavated surface other than rock, avoid disturbing the bottom of the excavation. Place concrete immediately after removal of foundation material to grade.

212.4.4 Cofferdams

1 Construct cofferdams to protect fresh concrete against damage from a sudden rising of the stream and to prevent damage to the foundation by erosion. When required, provide cofferdams, as water tight as possible, designed to safely satisfy the requirements of the site and construction. Construct earthen cofferdams using clean material from sources other than surface water sources. To provide clearance, straighten, reset, or enlar ge cofferdams that tilt or move sideways during sinking. Provide interior clearance of at least 24 in [600 mm] between forms and co fferdam to allow for construction, inspection, and pumping.

2 When dewatering is not feasible before placing concrete, it may be necessary to construct a concrete foundation seal in accordance with Subsection 513.4.11.6, Cofferdam Seals.

3 After completion of the substructure, remove cofferdams, including all sheeting and bracing, without disturbing or damaging the finished concrete.

212.4.5 Evacuation of Water from Structural Excavation

1 Pump water from inside foundation enclosures during and at least 24 hours after concrete placement. Use a suitable sump pump located away from the concrete work. Do not start pumping to dewater sealed cofferdams until the seal is sufficiently set to withstand the hydrostatic pressure.

212.4.6 Inspection

1 Notify the engineer after excavation is complete. Do not place concrete until the depth of the excavation and the character of the foundation material is approved.

Structure Excavation and Backfill SECTION 212

212.4.7 Backfill

1 If necessary, wash material to remove fines that would make stream water turbid.

2 Backfill excavated areas to the surface of the surrounding ground or stream bed, and compact the areas to the same density as adjacent material . Backfill material placed below water level does not require compaction. Neatly grade the top of the backfill material to blend with the adjacent area.

3 To prevent forward movement, first place the backfill material in front of such structures, or bring it up evenly with the material behind. Deposit backfill material behind abutments, wingwalls, retaining walls, and similar structures in horizontal layers, approximately 8 in [200 mm] loose in thickness, and compact it to the same density as adjacent material. Step or adequately roughen excavation slopes next to concrete components to prevent wedging action during backfilling. Do not jet backfill material behind abutments, wingwalls, or retaining walls.

4 When placing backfill around piers, deposit material equally on both sides to the final elevation.

5 Place backfill to drain water away from structural elements.

6 Do not place backfill against abutments, retaining walls, wingwalls or other structures until the concrete has cured in accordance with Table 513.4.14- 1, Form and Support Removal and Loading of Concrete.

212.4.8 Pervious Backfill Material

1 Provide pervious backfill material of the same grading at any one location. Where otherwise at risk of erosion, cover pervious backfill with at least 12 in [300 mm] of approved material.

212.5MEASUREMENT and PAYMENT

1 The engineer will measure:

cubic yard [cubic meter] and by computing the theoretical volume within the vertical planes 18 in [450 mm] outside of the footings and the horizontal planes at the bottom of the footing and the existing ground line. The existing ground line is the natural ground line, roadbed excavation line, or roadbed embankment line, whichever is in place at the time of excavation. The measurement will include the volume for additional excavation required by the engineer or required as a result of slips, slides, cave-ins, or silting beyond the contractor’s control.

Structure Excavation and Backfill SECTION 212

volume of material used.

2 The department will pay as follows:

Pay Item

Pay Unit Measure to the Nearest Pay to the Nearest Dry Excavation CY [m3] 0.1 ft [0.05 m] 0.1 CY [0.1 m3] Pervious Backfill Material CY, TON [m3, t] 0.1 ft, 0.05 ton [0.05 m, 0.05 t]

0.1CY, 0.05 TON

[0.1 m3, 0.05 t] Wet Excavation CY [m3] 0.1 ft [0.05 m] 0.1 CY [0.1 m3]