Backfilling Trenches and Other Excavations
Backfill is the key to a solid and stable grading
Backfilling is the process of putting soil back into a trench or foundation once excavation and the related work has been completed. The backfill process requires skills and heavy equipment as well as knowledge of the specifications, contract requirements, and soil conditions. Every area soil has unique characteristics, requiring different construction techniques to ensure optimum performance.
In addition, backfill crews must take care to prevent impact loading of any pipeline, shaft, structure, cabling, or other buried elements when placing and compacting backfill. There are a few common methods of backfilling and compacting backfill. Filling and compacting trenches for utility lines involves special considerations.
Compacting in Trenches
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).
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 are 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, all in accordance with EPA Guidelines.
Backfill can also 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.
Backfilling Utility Trenches
Backfilling trenches containing utility lines involves special techniques and considerations. Standard recommendations include:
- Backfill trenches and excavations immediately after the pipe is laid, unless other protection is directed or indicated.
- Select and deposit backfill materials with special reference to the future safety of the pipes.
- Fill the lower portion of the trench by depositing approved backfill and bedding material in layers of 6 inches maximum thickness, and compact with suitable tampers to the density of the adjacent soil until there is a cover of not less than 12 inches, using special care not to damage pipe and pipe coatings.
- Backfill the remainder of the trench with material free from stones larger than 6 inches or 1/2 the layered thickness, whichever is smaller (in any dimension). This may not apply to special materials for pavements,
- Mechanically tamp in 6-inch layers under roads and other paved areas, using heavy-duty pneumatic tampers (or equivalent). Tamp each layer to a density of not less than 100 percent of an ASTM D698 Proctor Curve. Provide additional compaction by leaving the backfilled trenches open to traffic while maintaining the surface with crushed stone.