Cofferdams, Cribs, and Sheeting (503.03)
Documentation Requirements – 503 Excavation for Structures
The Contractor may elect to use whatever materials or methods he considers necessary to accomplish this item unless specific details are required by the plans. Many times when sheeting is installed into streambeds, the streambeds consist of sand or gravel. Sand and gravel are pervious materials and will allow water to flow through them. If this condition exists, water can flow under the sheets and come up through the bottom of the cofferdam. This can loosen the soil in the bottom of the cofferdam and cause it to be very soft and unstable. It can also result in water coming up through any freshly-placed concrete. If this situation exists, the Contractor should take measures to prevent the flow of water up through the bottom of the cofferdam. These measures can consist of driving the sheet piling deep enough to cut off the flow of water, or placing a concrete seal in the bottom of the cofferdam prior to pumping out the water. If additional measures are required, they are considered to be part of the cofferdams, cribs, and sheeting item and no additional compensation should be allowed for these items.
In order to qualify as Cofferdams, Cribs, and Sheeting for a particular substructure unit, the Contractor must perform work to protect and maintain the excavation at that particular substructure unit. This work can include pumping out water, installing cribs or sheeting, or building an earthen cofferdam.
This item may include bedrock and requires the removal of all materials necessary for construction of structures according to plan. It also includes subsequent backfill and disposal of excavated material.
Sides of excavation should be protected from caving. If side failure occurs, the disturbed soil should be removed and replaced with properly compacted soil. The sides must not be laid back to the extent where the slope will endanger the stability of adjacent foundations. The stability of the slopes needs to be determined by a “competent person” on the Contractor’s staff.
When footings are not on piling, any material undercut or disturbed below plan or authorized elevation must be replaced with concrete at the Contractor’s expense. If the excavation is allowed to remain exposed for a considerable period of time and the material becomes unsuitable, it must be removed and replaced with concrete at the Contractor’s expense. The additional concrete may be placed with the footing concrete; however, the footing reinforcing steel must be located at the elevation indicated on the plans.
Many Contractors will place gravel in the bottom of the excavation to assist in dewatering and to provide a better work surface for the workers. This is unacceptable as any over excavation of spread footing subgrades must be replaced with concrete, not gravel.
When footings are supported on piling, any material undercut or disturbed must be replaced with properly compacted material. If the bottom of the excavation becomes muddy, the Contractor may remove the muddy soil and replace it with suitable granular material.
When the Cofferdam, Cribs, and Sheeting item is not provided, drainage outside the forms and pumping necessary to keep the surface suitable for placement of concrete are included in the excavation item.
This item includes removal and disposal of material that, in the opinion of the Engineer, is rock or hard shale. Shale that is removed by the same methods and comparative effort as soil should be classified as soft shale.
Rock or hard shale may be removed by whatever methods the Contractor elects. These usually are blasting, jack hammering, or ripping. Note the option to excavate by blasting may be excluded due to the close proximity of existing facilities. It is desirable to have rock excavation below the tops of footings as near to the sides of the footings as practical.
To qualify for payment as rock excavation, the Engineer must determine that the excavated material is indeed rock or hard shale. In addition, all of the rock excavation below the footing top must be filled with concrete. Rock excavation performed above the top of footing may be to any width; however, payment above as well as below the top of footing is to the plan dimensions of the footing only.
In the event bedrock is encountered over 1 foot (0.3 m) higher than indicated by the borings, or bedrock is not encountered at plan elevation, report the findings to the District Construction Engineer for consideration of a change in elevation of the footing. A plan note will usually be provided indicating when raising the footing can begin. When bedrock is not encountered at footing elevation, an investigation of the soil should be made as deep as practical. Hand augers or probes are recommended for initial investigation.
Generally, when bedrock is found less than 1 foot (0.3 m) lower than plan elevation, the additional height of pier or abutment can be provided by additional footing concrete; however, reinforcement should be placed at plan elevation.
When bedrock is found 1 foot (0.3 m) or more below plan elevation, consideration should be given to lengthening the pier or abutment above the footing.
Relative costs should be investigated in either case and, if the cost difference is significant, should be reported to the District Construction Engineer for review.
When the foundations for a bridge are spread footings, they are designed to be supported on soil or bedrock as indicated by the soil borings and drive rods. The Engineer must examine the soil or bedrock encountered at plan elevation for agreement with soil boring data and to assure that it will provide the intended bearing capacity. This bearing capacity will be listed in tons per square foot (tonnes per sq. meter) in the plan notes.
Listed below are bearing capacities that various materials generally will provide, and may be used as a guide in evaluating materials encountered.
|
Normal Bearing Capacity |
|
|
Material |
Tons per Sq. Ft. (Tonnes per Sq. Meter) |
|
Clay, Silt, or Clay and Silt |
˝ to 4 (5 to 39) |
|
Sand or Gravel |
1 to 4 (0 to 39) |
|
Cemented Sand and Gravel |
5 to 10 (0 to 98) |
|
Soft Shale |
3 to 5 (9 to 49) |
|
Hard Shale |
5 to 12 (9 to 117) |
|
Solid Rock |
5 to 30 (9 to 293) |
The District Construction Engineer should be consulted whenever there is doubt that the material encountered at plan elevation will provide the necessary bearing capacity. Whenever the material encountered is different and of lesser quality than indicated by the borings, an investigation similar to that described in the section titled “Elevation Changes” should be made, and the findings reported to the District Construction Engineer for review.
Footings placed on pile foundations that were exposed to temperatures below freezing sometimes settle during the setting of the concrete and result in unsatisfactory footings. It is therefore imperative that the soil in such cases be free from frost and, if disturbed by freezing, compacted to proper density.
When excavation for footings is performed and freezing temperatures are expected during the time it is exposed, insulation such as an adequate thickness of straw is recommended for protection from frost.
When the excavated area has become frozen and the area is heated in an enclosure, the effect of the supplied heat on the frozen soil is slight, and a thorough examination for complete removal of frost is required. Satisfactory temperatures found in spot checks of soil over the entire area to as deep as frost may have penetrated is an indication of frost removal.
When frozen soil is thawed out, it requires re-compaction since frost heaving has lessened the density. If reinforcing steel has been placed in a footing area at the time the soil was frozen, it will be necessary for the Contractor to first remove the reinforcing prior to re-compacting the soil.
The backfill material behind the abutments and beneath the approach slabs shall conform to Item 203 Granular Material Type B. The use of this material should facilitate compaction and help alleviate the settlement of the approach slab.
When the plans do not require the original ground line to be altered by removal of the embankment, and when structural excavation is performed prior to building an embankment, elevations or measurements that establish the elevation of the original ground must be made. Measurements made and recorded from the Contractor’s footing grade stakes can be used to establish the elevation of the original ground.
When the original ground will be altered by removal or construction of an embankment prior to excavation, use the plan line of the excavation or embankment items for top boundary of excavation.
The bottom elevation of the footing is to be as shown in the plans. This elevation is to be verified by subtracting the total verified height of the substructure unit below the beam seat, from the beam seat elevation.
1. Verify existing ground elevations
2. Verify that contractor excavated to plan dimensions
3. Make sure that the volume of the existing structure is deducted from the pay quantity for unclassified excavation
Dispose of excavated material not needed or suitable according to 105.16 and 105.17. Note: In recent years this item has been bid as a lump sum. Numbers 1 & 2 above still apply.