2-36 Section 206. Excavation and Backfill for Structures
206.01 Description
This work consists of clearing, removing old structures or parts of structures, removing materials required for constructing structures, disposing of surplus or unsuitable material in accordance with subsection 205.03.P, and backfilling completed structures.
206.02 Materials
Provide materials in accordance with the following sections : Sound Earth .......................................................................................... 205 Granular Material Class II .................................................................... 902 Aggregate, 6A , 46G ............................................................................. 902 Geosynthetics ...................................................................................... 910
A.Bridges, Pump Stations, Retaining Walls , and Culverts ( Other Than Pipe) . Provide granular material Class II.
B.Miscellaneous Structures. Unless otherwise required, provide sound earth. Miscellaneous structures are structures other than bridges, pump stations, retaining walls, and culverts other than pipe.
206.03 Construction
A.Foundation Excavation and Rock Foundation Excavation . Excavate to allow for foundation uni t construction. If shown on the plans or approved by the Engineer, trim the footing excavation to the exact size of the footing and omit the footing forms. For concrete placed on or against an excavated surface other than rock, do not disturb the bottom and side surfaces of the excavation before placing concrete. Excavate to the required grade immediately before concrete placement. Before placing concrete, check the excavation depth and secure the Engineer’s approval of the foundation support material. Place concrete in the absence of free-standing water. Change the elevations for the bottom of footings as directed by the Engineer to ensure a stable foundation. If directed by the Engineer, remove and replace unsound material under proposed structures and replace with Department -approved material. Remove loose fragments and clean and cut rock surface or other hard material before placing concrete on required surface. Level, step, or serrate the surface as directed by the Engineer.
1.Foundation Excavation . Excavate materials, including portions of the existing structures, within the foundation excavation limits, except rock foundation excavation. MDOT Standard Specifications for Construction Section 206 2-37 2. Rock Foundation Excavation . Excavate in accordance with subsection 205.03.B.
B.Backfill Placement and C ompaction. Place and compact backfill around completed structures.
1.Placing Backfill . Provide material appropriate for the type of structure requiring backfill in accordance with subsection 206.02. If soil that is excavated from the site meets material requ irements , the Contractor may use it to backfill around completed structures. Place backfill against the concrete structure after completion of the required curing, surface finishing, and waterproofing. Cover the inlet of each weep hole with geotextile blanket prior to placing backfill. Place backfill evenly around the structure to equalize horizontal loadings.
2.Compacting Backfill
a.Bridges, Pump Stations, Retaining Walls , and Culverts ( Other Than Pipe) . Place backfill material in 6-inch layers. Compact each layer to 100% of the maximum unit weight in the load-bearing area. The load- bearing area is the area within the 1:1 slope, down and away from the outer limits of the bottom of the footing to the bottom of the excavation. Place backfill behind and arou nd substructure units, between the outer limits of the bottom of the footing and the surface elevation, in layers no greater than 12 inches deep. Compact backfill to at least 95 % of the maximum unit weight. Place backfill between the bottom of footing elev ation and the bottom of slope paving subbase in layers no greater than 12 inches deep. Compact to at least 95% of the maximum unit weight.
b.Miscellaneous Structures. For miscellaneous structures, place backfill in 12- inch layers and compact to at least 95% of the maximum unit weight. The Engineer may approve an increase in the thickness of layers if the Contractor obtains the required compaction results.
206.04 Measurement and Payment
Pay Item Pay Unit Excavation, Fdn .............................................................................. Cubic Yard Excavation, Rock Fdn ..................................................................... Cubic Yard Backfill, Structure, CIP .................................................................... Cubic Yard Backfill, Structure, LM ..................................................................... Cubic Yard MDOT Standard Specifications for Construction Section 206 2-38 Aggregate ....................................................................................... Cubic Yard
A.Excavation. The Department does not consider excavation to include removal of ice, water, or liquids. Unless otherwise required, the cost of sheeting, shoring, and dewatering is included in the unit prices for related structure excavation pay items.
1.Excavation, Foundation . The Engineer will base payment for Excavation, Fdn on plan quantity in accordance with subsection
109.01 A. Unless otherwise shown on the plans, the Engineer will
determine the plan quantities using the space bounded by the existing ground surface or exposed portions of the existing substructure, the elevation of the bottom of the foundation, and the 1:1 slopes extending outward and upward from points 18 inches outside the bottom of the footing. The Department will pay for , as extra work , the removal of piling below the bottom of footing elevations that i s not shown on the plans and encountered during structure excavation.
2.Excavation, Rock Foundation . The Engineer will measure Excavation, Rock Fdn in its original position for the amount of rock excavated within vertical planes through the footing neat lines. The Engineer will make allowance for overbreak if the Engineer determines that it is impractical to excavate to the neat lines of the footing. The Engineer will measure the amount of overbreak by actual cross sections of the footing excavation. Overbreak allowance is limited to vertical planes 6 inches outside and parallel to the neat lines of the footing and to a depth of 3 inches below the elevation of the bottom of the footing, as shown on the plans. Excavation, Rock Fdn does not include removal of portions of existing structures.
B.Backfill
1.Backfill, Structure, CIP . The Engineer will base payment for Backfill, Structure, CIP on plan quantity in accordance with subsection
109.01 A regardless of the foundation excavation slope. The Engineer
will not measu re material placed outside the maximum pay limits shown on the plans. The Department will pay for granular material Class II used to backfill bridges, pump stations, retaining walls, and culverts other than pipe as Backfill, Structure, CIP. MDOT Standard Specifications for Construction Section 206 2-39 2. Sound Earth . The cost of sound earth used as backfill material for miscellaneous structures is included in the unit prices for related pay items.
3.Aggregate . The Engineer will measure Aggregate by volume, loose measure. The unit price for Aggregate includes the cost of providing, hauling, and placing 6A or 46G ma terial at locations as directed by the Engineer.
Source: Michigan Standard Specifications for Construction, 2020 Edition. Pages 162–165 of 1,146.