There are 2 primary types of pipe trench backfill conditions. The first type is for areas not underneath or near pavement or a building structure (usually to within +/- 5 feet of). This type of trench backfill can typically be filled with the excavated spoils or other soil types which are accepted by the project owner or the engineer. The second type of trench backfill is done utilizing ‘premium’ backfill which can consist of coarse sand, coarse-graded aggregate, fine graded aggregate, or a mixture of the two. No matter the backfill material being used, the correct way to backfill a trench is by doing it in lifts of material, typically between 4-12” thick, and watering/compacting each lift prior to placement of the next. This process helps keep the trench above the pipe from settling over time due to loading, erosion, or other means.
There are several different ways to compact the trench backfill material. The most common piece of equipment used is a hydraulic plate tamp attachment (or ‘Ho-Pac’) to the end of a backhoe or excavator boom. Some of these attachments currently have a ‘quick-connect’ option now where the Ho-Pac can be attached to the boom without the need for the operator to even leave the machine.
For other hard to reach and smaller areas, gas powered walk behind plate compactors or manual ‘jumping jack’ compactors may be used.
Once enough material has been compacted above the pipe to safely protect it, vibratory or static rollers may even be used depending on the engineer. County and state agencies have become more particular about trench backfilling operations as intermittent ‘settling’ issues can become contentious and costly operations to fix, especially when they occur long after the project was originally completed. No matter the equipment utilized, the engineer or inspector will typically want to be assured that the trench backfill lifts have been compacted to maximum dry density results obtained from a previously obtained Proctor or Modified Proctor lab test. Water is typically added to increase the efficiency of the compaction and cohesion of the backfill material, enough to reach the proper moisture percentage as determined from the materials proctor curve.
The ideal material for pipe trench backfill under pavements is crushed limestone with rock ranging from fines all the way up to 2-1/2″. Properly graded crushed limestone has just the right balance of aggregate sizes and square shaped aggregate to ensure interlocking of the particles which will ultimately maximize density, strength, and resist future settlement.
Bank run or crushed gravel can be used for pipe trench backfilling as well but many engineers may argue that the reduced particle interlock isn’t conducive to resist future settling.
A smaller gradation of crushed limestone or gravel (max particles in the range of 3/8-3/4″ or ‘screenings’) can also be used for pipe trench backfill but this material may take excessive amounts of water because the fines absorb alot of moisture. Also diligent compaction techniques become more critical to assure peak density. Also ‘screening’ type aggregate may ‘bridge’ on the top portion of large lifts during compaction leading the crew to believe peak compaction has been achieved, when later on the material will settle at non-dense pockets of material. For these reasons many experienced contractors will avoid the use of gravels, sands, and ‘screening’ for pipe trench backfill.
Flowable or ‘Controlled Density Fill’ is often permitted and used to backfill certain trenches, and may even be required in some areas when the engineer is particularly concerned with settlement. FLowable fill is a cementitious material with a higher water/cement ratio than concrete and is typically ‘self-leveling’ which means it essentially places itself.
Another consideration when backfilling and compacting pipe trenches is the strength of the pipe installed below. For the less rigid commonly used pipes such as SDR35 PVC pipe and Corrugated HDPE pipes, etc, the backfill and compacting operations must be done with more caution as damaging the pipe below can happen quickly and can go unnoticed. For the more rigid commonly used pipes such as Ductile Iron and Concrete Pipe etc, caution should still be taken during installation, but the pipe can typically withstand more pressure above. Also caution must be used when backfilling and compacting pipe trenches to assure the trench walls do not cave in. It is common for a pipe trench crew to only lay as much pipe as they can backfill that same day, to avoid leaving open trenches overnight or for longer periods of time. When backfilling trenches around new and existing structures it’s common to hand tamp or tamp with a compact plate compactor to avoid damage. Because of this compaction results may be slightly less than that of heavier more efficient equipment.
The prevailing engineering concept of trench backfill is the proper compaction of the pipe ‘envelope’ (bedding and immediate cover) particularly the ‘haunch’ without damaging or upsetting the pipe. The common bedding and immediate pipe cover material has historically been uniformly graded porous crushed stone, but a recent belief has emerged with many public inspection authorities that well graded crushed gravel with few voids is just as effective if not more so. This assures the pipe is secure and is most resistant to movement.