EXCAVATION AND BACKFILL

Transcription

1 Standard Specification , 2451, and 1805 A basic understanding of specifications is needed to properly replace the disturbed soil and restore the condition of the excavated area. The size of the disturbed area should be kept to the minimum required to do the job. All areas disturbed during the project must be restored to the satisfaction to of the Engineer. 9-1

2 Trenching must be located at a distance from the edge of the pavement, back of curbing, or edge of surfaced shoulders as indicated in the Contract Documents, or as directed by the Engineer. The distance must be such that no damage, or undermining, is done to the pavement or curb. The trench must be of uniform alignment for accurate referencing of the underground installation. If trenching or excavation operations require the removal of concrete pavement or sidewalk, it must be first cut with a concrete saw and then removed. Removal must be to the nearest existing joint or as directed by the Engineer. 9-2

3 The Contractor must have the proper operating compaction equipment on the site before any of the excavation has begun. A mechanical tamper is usually used in this type of work. The foot of the tamper must fit within the trench or excavation. Excavations around foundations, handholes or conduits must be backfilled with the same material as the adjacent soils and compacted to approximately the same density. Spec D requires that backfill material be uniformly distributed in approximately 8 inch (200 millimeter) thickness, or less, and thoroughly compacted as required. Once the excavation for a structure or conduit is finished, the structure must be installed as soon as practical. When placing conduit or direct buried cable, backfilling and proper compaction must be done as soon as possible to prevent water contamination of the roadbed, shoulder, or slopes. 9-3

4 Precast concrete foundations for roadway lighting must be installed as detailed in special provisions which are shown below. The foundations must be installed in drilled shafts and fine filtered aggregate or lean mix backfill must be used to back fill the void between the foundation and the surrounding soil. B. Precast Concrete Light Foundation Design E & H Installation Requirements Install Precast Light Foundations Design E and Design H in accordance with Standard Plates 8127 and 8128, F, and as follows: 1. Excavate a cylindrical 30 in to 36 in diameter drilled shaft into the ground with a 30 in or 36 in diameter earth auger bit. (1.1) Use auger drilling equipment with adequate capacity, including power, torque, and down thrust to auger a 30 in to 36 in diameter cylindrical drilled shaft to the depth indicated in contract documents. 2. Level and firmly compact the bottom of the shaft so it is flat and horizontal while maintaining the required depth of the shaft. 3. Remove the forming tube from the precast foundation before placing the precast foundation in the shaft. 4. Center, set, and level the precast foundation in the shaft and maintain a 3 in minimum to a 6 in maximum continuous symmetrical annular void between the precast foundation and surrounding soil. 5. Before backfilling place in the annular void the ground rod electrode with the ground rod electrode conductor in accordance with contract documents. 6. Completely backfill the annular void between precast foundation and the surrounding soil of the shaft with firmly compacted fine filter aggregate in accordance with J.2. (6.1) Use a mechanical pole tamper to firmly compact each 6 in layer of fine filter aggregate to uniform density. (6.2) Substitution of lean mix backfill per specification 2520 instead of fine filter aggregate is permitted. (6.2.1) Protect the conduit stub out with end bell opening from lean mix backfill to ensure the end bell opening is accessible and the conduit is free of debris for the installation of direct buried lighting cable. (6.2.2) After lean mix backfill has been poured and cured excavate the open trench for the direct buried lighting cable and install the direct buried lighting cable in the foundation. 7. Test and verify the compaction of the filtered aggregate backfill material around the precast foundations in accordance with F.3 at a rate of one test per every 10 (ten) foundations with a minimum of two tests per project. 9-4

5 Due to the ongoing issues with improper compaction around precast light foundations as shown below the installation requirement changes described above were necessary to ensure long term stability of the light foundation. The following are examples of pre-cast light foundations that were not adequately compacted when initially installed. Excavated material must be placed where it will not cause damage or obstruction to vehicles, pedestrians or interfere with surface drainage. Holes resulting from excavation must be covered and protected as soon as practical. The Contractor and Inspector must not allow unprotected hazards to exist for motorists or pedestrians during the construction of the project. 9-5

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