Backfilling is the process of putting the soil back into a trench or foundation once excavation and other project-related has been completed. The backfill process requires skills and heavy equipment as well as knowledge of the specifications, contract requirements, and soil conditions. Backfiling specialists and contractors must take care to prevent impact loading of any pipeline, shaft, structure, cabling, or other buried elements when placing and compacting backfill, as well as accounting for the unique characteristics of different areas of soil, all of which require different construction techniques to ensure optimum performance.
There are a few common methods of backfilling and compacting backfill.
In Compacting, after soil is backfilled into a trench, the loose material is compacted using some mechanical means, such as a compactor, an excavator, or a "jumping jack"-type compactor. As a general guide, soils should be compacted to at least the minimum percentages of maximum dry density as determined by ASTM D698 Method A (Standard Proctor). Using this method, soil is typically backfilled in layers or lifts. The soil lift will depend upon the nature of the backfill and the compaction equipment that is used. Water may be added during the compaction process, to assist with compaction. The general process follows three steps that are repeated until the backfill is at grade level:
- Backfill in layers of 4 to 6 inches, using non-organic fill material that is free of debris
- Compact with a 1,000-pound compactor, or as appropriate
- Water thoroughly
Water jetting is a backfilling technique that does not involve mechanical compaction. Instead, the backfill is compacted by pressurized water applied to the bottom of the fill with a probe. Water jetting is recommended for sand or sandy soils or with highly fissured bedrock. It is not suitable for plastic soils or heavy clay soils. With the jetting technique, you pump the water under pressure and use the force of the jetted water to move the bedding or backfill material around. As with any backfilling, the material should be placed slowly and in lifts. After the water is applied, it is allowed to drain from the soil to improve compaction. Due to the mixing of water and soil, crews must take preventive measures to contain sediment-filled water and prevent it from entering drains and watercourses.
The third and final method of backfilling can be done using flowable fill, a cementitious material with a low water-cement ratio that is delivered to the job site by a ready-mix truck. Typically, the utility pipe or other equipment in the trench is first covered with an aggregate material, then the flowable fill is placed in the trench directly from the truck, just like regular concrete. The aggregate surrounding the pipe provides for easier access to the pipe for future repairs. One of the challenges when using flowable fill is the liquidity of the fill. Contractors must contain or block the fill to prevent it from flowing into other trench areas.
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