206 - Excavation and Backfill for Structures
Suitability of Foundation Materials
Final excavation for the foundation to grade will not be made until just before the concrete is to be placed. Concrete will not be placed until the excavation depth has been checked and the suitability of foundation support material has been approved. The concrete will be placed in a dry hole. The elevations for the bottom of footings will be subject to change, as directed, to ensure a satisfactory foundation. When directed (usually based on the recommendation of region soils personnel), all unsound material underlying proposed structures will be removed and replaced with approved material. Some of the approved materials may include Granular Material Class II (for use in dry holes), bank run gravel (for use in wet holes), coarse aggregate (for use in wet holes) and Grade X Concrete (for use in soft clay areas where compactive effort would only drive the backfill into the remaining material, or where it is not desirable to impound water in impervious soils that would tend to keep them saturated, resulting in loss of stability).
Preparing Abutment Area
The importance of the proper construction of bridge abutment embankments cannot be overemphasized. The abutments are rigid structures and must have stable support. The following procedures must be followed in preparing foundations for abutment fills regardless of height:
Remove topsoil from the entire footing area and extend for a distance outside the footing equal to the height of the embankment in all directions.
Remove all frost from the abutment embankment area.
Compact the ground to not less than 95 percent of maximum unit weight to a depth of 9 inches (250 mm). Compacting original ground is not a pay item in this instance. Density tests are required for documentation.
Structure embankment fill material will be Granular Material Class III when used under structural footings for which piling is not specified. Density of structure embankment material will not be less than 100 percent when piling is not required.
For use under structure footings supported by piling, structure embankment material will be Granular Material Class III, except that sound earth is allowed as structure embankment material if placed between April 1 and November 15. If piling is required, density of embankment material will not be less than 95 percent.
Testing of the granular material will be in strict accordance with the Materials Quality Assurance Manual.
MEASUREMENT AND PAYMENT
Backfill for Structures
Backfill for structures that are bridges, grade separations, pump houses and retaining walls shall be Granular Material Class II, unless otherwise specified, and will be paid for as Backfill, Structure. Structures other than bridges, grade separations, pump houses, and retaining walls are defined as miscellaneous structures. Backfill for miscellaneous structures shall be sound earth, unless otherwise shown on the plans. Final payment for Backfill, Structure, CIP will be made at plan quantity, regardless if material is placed outside of the maximum pay limits shown on the plans. Structure Backfill in all conditions where sound earth is allowed will not be a pay item (backfill in this instance is included in the contract unit prices for other items).
Backfill for structures that are bridges, grade separations, pump stations and retaining walls (Granular Material Class II) will be placed in 6 inch (150 mm) layers. Each layer will be compacted to 100 percent of the maximum unit weight in the load bearing area. The load bearing area is the area within the 1 on 1 slope down from the outer limits of the bottom of footing to the bottom of the excavation. All backfill behind and around the substructure units bounded by the outer limits of the bottom of the footing and the surface elevation will be placed in layers not more than 6 inches (150 mm) in depth, and compacted to not less than 95 percent of the maximum unit weight, unless otherwise specified. All backfill between the bottom of footing elevation and the bottom of slope paving subbase will be placed in layers not more than 6 inches (150 mm) in depth, and compacted to not less than 95 percent of the maximum unit weight. Granular material will be at a point short of saturation as determined by the One Point Michigan Cone Test. If the material contains an excess of moisture, it will be dried to the required moisture content before being compacted.
Backfill for miscellaneous structures (sound earth) will be placed in 6 inch (150 mm) layers and compacted to not less than 95 percent of the maximum unit weight. Increased layer thickness may be approved, provided the required compaction is obtained.
Backfill behind abutments requires proper inspection and the best possible quality control. To prevent the dips or bumps that often occur at bridge approaches, special equipment should be used to compact backfill in confined areas. Backfill for structures requires a gradation test, as well as density tests, to document the conditions where granular material is used according to plan.
Excavation for Structures
Foundation excavation is set up for the excavation of all materials, except rock, encountered within the foundation excavation limits as shown on the plans. Foundation excavation includes portions of the existing structure that are within the foundation excavation limits and designated to be removed. Rock foundation excavation is set up for rock removal encountered within the foundation excavation limits and does not include those portions of the existing structures that are to be removed. Final quantity for foundation excavation will be based on plan quantity. Plan quantity will be determined by the space bounded by the existing ground surface or exposed portions of the existing substructure, the elevation of the bottom of the foundation, and 1 on 1 slopes extending outward from points 18 inches (450 mm) outside the bottom of the footing, unless otherwise indicated on the plans. Final quantity for rock excavation will be based on measurement by volume in its original position for the actual amount of rock excavated within vertical planes through the neat lines of the footing.