Backfilling & Excavation

This technique involves some mechanical means such as a compactor on an excavator or a “jumping jack”-type compactor. Compact soils to not less than the percentages of maximum dry density as determined in accordance with ASTM D698, Method A (Standard Proctor). The soil lift will depend upon the nature of the backfill, and the compaction equipment to be used. Water may be added during the compaction process, to assist with compaction. The following steps are recommended:

Backfill in layers of 4″-6″ (non-organic fill and clean of debris)

Compact with a 1,000 lb. compactor

Water thoroughly

Repeat first three steps

Water jetting is a technique that relies on specific material but no compaction. Backfilling with jets allows the contractor to leave the trench and come back later forcing pressurized water into the trench using a long metal device. Water jetting should be avoided on plastic soils or heavy clay soils, however, it is recommended in sand or in highly fissured bedrock. In jetting, 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. Flooding or ‘jetting’ backfill generally produces poor to very poor compaction. It is necessary to take preventive measures to contain water containing sediment, and in particular, prevent it from entering drains and watercourses, all in accordance with EPA Guidelines. If the water cannot drain from the backfill soils, the material will be set up for future collapse.

Backfill can also be done using flowable fill, a cementitious material with a low water/cement ratio, delivered to the job site by a ready-mix truck. The material is then placed as backfill directly from the truck, just like regular concrete. Backfilling with flowable fill should be done carefully, making sure that the utility pipe is covered first with an aggregate material. Allowing flowable fill to engulf the pipe can create problems for those needing to hand dig around the pipe in the future.

The other challenge when using flowable fill is that the material does flow so the contractor must block or prevent the backfill material to flow continuously to other trench areas.