Section 208 — Structure Excavation and Backfill for Selected Major Structures

FOR SELECTED MAJOR STRUCTURES

Description

208.01This work consists of excavating material for the construction of selected structures. This work also includes preserving channels, shoring and bracing, constructing cofferdams, sealing foundations, dewatering, preparing foundations, backfilling, and removal of safety features and cofferdams.

Material

208.02Conform to the following Subsections: Neat hydraulic cement grout Soil and soil- aggregate materials 701.04(a) (2) 704.01

Construction Requirements

208.03General . Slope the sides of excavations, use shoring and bracing, and use other safety features according to OSHA safety standards 29 CFR, Part 1926, Subpart P, Excavations. If sides of excavations are sloped for safety considerations, submit one copy of the design that demonstrates compliance with OSHA standards. If support systems, shield systems, or other protective systems are used, design the shoring according to Section 562. Submit drawings and construction details according to Subsection 104.06. Clear the area of vegetation and obstructions according to Sections 201 and 203. Saw cut existing pavements or concrete structures adjacent to the area to be excavated that are designated to remain. Where necessary, blast rock according to Section 205. Conserve suitable material for structural backfill from excavated material. Do not deposit excavated material in or near a waterway. From the edge of the excavation, do not stockpile excavated material or allow equipment closer than specified in OSHA safety standards 29 CFR, Part 1926, Subpart P, Excavations. Use suitable material in embankment construction if approved. Dispose of unsuitable or excess material according to Subsection 204.14. Remove safety features when no longer necessary. Remove shoring and bracing to at least 48 inches below the surface of the finished ground.

208.04Channel Preservation. Perform work in or next to waters as follows:

(a)Excavate and conserve material inside cofferdams, sheeting, or other separations (such as dikes or sandbags). (b) Do not disturb the natural bed of the waters adjacent to the work.

(c)Backfill the excavation to original ground- line with conserved material.

208.05Cofferdams. Use cofferdams when excavating under water or when the excavation is affected by groundwater. Submit drawings showing proposed methods and construction details of cofferdams according to Subsection 104.06 and Section 562. Design and construct cofferdams that conform to the following:

(a)Extend below the bottom of the footing;

(b)Brace to withstand pressure without buckling and secured in place to prevent tipping or movement;

(c)Construct as watertight as practical;

(d)Provide sufficient clearance for the placement of forms and the inspection of their exteriors;

(e)Provide for dewatering;

(f)Protect fresh concrete from damage; and

(g)Prevent damage to the foundation by erosion. Remove cofferdam material down to the natural bed of the waterway or to the top of seal, whichever is lower. Remove cofferdam material outside the waterway to at least 48 inches below the surface of the finished ground.

208.06Foundation Seal. Seal the foundation area from water by placing Class S (Seal) con crete if the area cannot be pumped reasonably free of water. Provide and place Class S (Seal) concrete according to Section 552. Maintain the water level inside the cofferdam at the same level as the water outside the coffe rdam while placing a foundation seal. Vent or port the cofferdam at low water level when a foundation seal is placed in tidal water. Do not dewater a concrete -sealed cofferdam until the Class S (Seal) concrete strength is sufficient to withstand the hydros tatic pressure.

208.07Dewaterin g. Remove and dispose of water according to Federal, s tate, and local rules and regulations. 208.08 Foundation Preparation and Approval. Provide a firm foundation of uniform density throughout its length and width. Obtain CO approval of the foundation when the excavation is complete. Prepare foundations as follows:

(a)Footings placed on bedrock. Excavate to the specified elevation. Clean the foundation surface of deleterious material. Clean and grout seams and crevices greater than ½ inch wide. Where needed to backfill over -excavations or over -break areas, place flowable backfill or structural concrete (Class A) to provide a level bearing pad for footing as directed. Do not place soil or aggregate backfill under the footing unless approved for the design bearing resistance.

(b)Footings placed on an excavated surface other than bedrock. Excavate material to foundation grade and compact the foundation before footing is placed.

(c)Footings keyed into undisturbed material. Excavate the foundation to the limits of the footing and compact the foundation. Where material does not stand vertically, fill the space between the limits of the bottom of footing and the undisturbed material with structural concrete (Class A). Fill only to the top of the excavation if the excavation is below the top of the footing. Concrete placed against steel sheet piles in cofferdams is considered as being against undisturbed material.

(d)Unstable material below footing elevation. Excavate unstable mater ial below foundation grade as directed and backfill with foundation fill. Place foundation fill in horizontal lifts that do not exceed 6 inches in compacted thickness. Compact each lift according to Subsection 208.10.

(e)Foundations using piles. If foundation seals are required, drive the piles before placing Class S (Seal) concrete unless otherwise specified. Remove loose and displaced material and reshape the bottom of the excavation to the foundation elevation. Grade and compact the bed to receive the footing.

208.09Backfill . Place structural backfill in horizontal lifts that do not exceed 6 inches in compacted thickness. Compact each lift according to Subsection 208.10. Place structural backfill lifts evenly on all sides of the structure as appropriate. Extend each lift to the limits of the excavation or to natural ground. Do not place structural backfill against concrete until 80 percent of the design strength is attained. Backfill in or ne xt to a running waterway according to Subsection 208.04.

208.10Compacting. Determine optimum moisture content and maximum dry density according to AASHTO T 99, Method C. Adjust the moisture content of the backfill to a moisture content suitable for compaction. Compact material placed in each lift to at least 95 percent of the maximum density. Determine the in -place density and moisture content according to AASHTO T 310 or other approved test procedures. Do not apply densit y requirements as measured by AASHTO T 310 to material that cannot be tested or compacted to maximum values determined by AASHTO T 99. For this material, fill the voids around the rocky material in each lift with structural backfill or foundation fill as a ppropriate. Compact each lift until there is no visible evidence of further consolidation.

208.11Acceptance. See Table 208-1 for sampling, testing, and acceptance requirements. Material for grout will be evaluated under Subsections 106.02 and 106.03. Material for structural backfill and foundation fill will be evaluated under Subsections 106.02 and 106.04. Grout will be evaluated under Subsections 106.02 and 106.04. Excavation and backfill work will be evaluated under Subsections 106.02 and 106.04. Clearing will be evaluated under Section 201. Concrete will be evaluated under Section 552. Flowable backfill w ill be evaluated under Section 614. Removal of obstructions will be evaluated under Section 203. Rock blasting will be evaluated under Section 205. Temporary works will be evaluated under Section 562.

Measurement

208.12Measure the Section 208 pay items listed in the bid schedule according to Subsection 109.02 and the following as applicable: When measuring structural excavation by the cubic yard in its original position, do not include the following volumes:

(a)Material excavated outside the vertical planes located 18 inches outside and parallel to the limits of the footings or foundations;

(b)Material included within the staked limits of the excavation (such as contiguous channel changes and ditches) for which measur ement is covered under other Sections;

(c)Water or other liquid material;

(d)Material excavated before measurement of the original ground;

(e)Material re- handled, except when the contract specifically requires excavation after embankment placement; or (f) Excavation for cofferdam seals. When measuring foundation fill and structural backfill by the cubic yard in place, measure the volume placed inside the vertical planes located 18 inches outside and parallel to the limits of the footings or foundations.

Payment

208.13The accepted quantities will be paid at the contract price per unit of measurement for the Section 208 pay items listed in the bid schedule. Payment will be full compensation for the work prescribed in this Section. See Subsection 109.05 . Payment for structure excavation, shoring and bracing, and cofferdams will be full compensation for excavation to a depth of 6 feet below the lowest elevation shown in the plans for each foundation structure. When the excavation exceeds 6 feet, either the Contractor or the CO may request an equitable price adjustment for the depth in excess of 6 feet.

(a)Progress payments for dewatering by the lump sum will be paid as follows:

(1)25 percent of the pay item amount, no more than 0.5 percent of the original contract amount, will be paid at the start of dewatering operations.

(2)The remaining portion of the pay item amount will be prorated based on the structural excavation work completed.

(b)Progress payments for shoring and bracing by the lump sum will be paid as follows:

(1)25 percent of the pay item amount will be paid after the design submittal is approved.

(2)An additional 25 percent of the pay item will be paid after 50 percent of the shoring and bracing work is completed. (3) The remaining portion of the pay item amount will b e paid after structural excavation work is completed and shoring and bracing material is removed.

(c)Progress payments for cofferdam by the lump sum will be paid as follows:

(1)25 percent of the pay item amount will be paid after the design submittal is approved .

(2)An additional 25 percent of the pay item will be paid after 50 percent of the cofferdam work is completed. (3) The remaining portion of the pay item amount will be paid after structural excavation work is completed and cofferdam material is removed. Table 208-1 Sampling, Testing, and Acceptance Requirements

Material or

Product

(Subsection) Type of

Acceptance

(Subsection) Characteristic Category Test Methods

Specifications Sampling

Frequency Point of

Sampling Split

Sample Reporting

Time Remarks

Source

Foundation

fill(1) (704.01) Measured & tested for conformance (106.04 & 105) Classification − AASHTO M 145 1 per soil type Source of material Yes Before using in work − Gradation − AASHTO T 27 & T 11 " " " " −

Structural

backfill(1) (704.01) " Plasticity index − AASHTO R 58, T 89, & T 90 " " " " − Gradation − AASHTO T 27 & T 11 " " " " −

Production

Foundation

fill (704.01 ) Measured & tested for conformance (106.04 ) Moisture - density − AASHTO T 99, Method C(2) 1 per soil type Source of material Yes Before using in work − Density − AASHTO T 310 or other approved procedures 1 per 300 yd3 In place No Before placing next lift −

Structural

backfill (704.01 ) " Moisture - density − AASHTO T 99, Method C(2) 1 per soil type Source of material Yes Before using in work − Density − AASHTO T 310 or other approved procedures 2 per lift In place No Before placing next lift −

(1)Not required when using Government -furnished source.

(2)At least 5 points per proctor.