Trench Compaction: Backfilling Methods

When is comes to backfilling, there are several methods in the construction industry to achieve the same results. Depending on the requirements for a specific project, trench backfilling allows for different options for filling areas. The most common techniques in the industry include jetting, filling, dumping, and compaction.

Whether the project involves filling over a pipe for gas, water, or power under the pavement, some contractors will prefer one method over the other. In the end, these useful techniques are helpful for your future trenchwork projects. We will cover four main methods of trench backfilling:

1. Jetting: Also known as, “water jetting”, this practice uses pressurized water sprayed into the trench through the use of a long metal device.

Note: Water dissipation decreases the trench volume, thus in the long term this method is best recommended for granular soil types. For example, clay, a cohesive soil type, is not the best type of soil for conducting jetting. As a result, pavement around the area of the trench can dip or fall creating a depression in the roadway. If you are not sure how to identify the soil type of you are working with, learn more in our "Optimum Compaction" blog.

2. Filling: refers to “flowable fill”, a type of fragile concrete mix used in construction, particularly for road bases or backfilling. By using a “cementous” material with a low water/cement ratio, filling is delivered straight from the concrete mixer truck onto the trench.

3. Dumping: This method involves filling the trench with granular aggregate without compaction.

Note: One of the common issues found with this method is the granular aggregate naturally contains voids, meaning that fine material can enter. In most cases, contractors prefer this method because it does not require additional time with trench compaction. One of the reasons for contractors to use this method is that the stone compacts itself.

4. Compaction: Is a process used to densify, or reduce the volume of a mass of material. This method requires knowing your soil well. Before making an informed decision, understand the soil mechanics of the area to be compacted. In cohesive materials, such as clay, tamping rammers work more effectively due to their lower frequencies. Otherwise, for granular materials like sand, the best choice is vibratory plates for their higher frequencies. 

Note: Mechanical compaction demands many passes that depend on the lift thickness and soil moisture content to achieve desired results. This method certainly brings successful outcomes if it is done correctly. Compaction requires higher costs associated with the appropriate equipment and proper labor, but will definitely prove its worth when the project is completed.

Backfill Material:

It is imperative to gauge how much material you use when using backfill material, but occasionally lifts can overflow. As a result, you will not be able to achieve proper compaction. Therefore, we recommend placing backfill material one foot approximately (1’) above the top of the pipe. It must be deposited the full width of the trench. Additionally, the fill material should not cause movement of the pipe or damage the pipe joint. Make sure the backfill material is gently rolled into the trench before compaction begins.

Trench Compaction Equipment:

Choosing the right equipment for trench work is very important. You cannot compact with backfill material without understanding the limits of each machine. Trench compaction will always be achieved more effectively if the right equipment is used.

Tamping Rammers:

Tamping rammers, also known as tampers or “Jumping Jacks,” use vibratory movement to compact material, using two large springs driven by the rammer’s engine itself. The size and percussive force of rammers make them a beneficial tool for compacting cohesive materials, such as clay.

Rammers deliver around 650 to 700 blow per minute, are recommended for tight spaces, such as trenches, and can also be used in a variety of soil types. Construction specialists mention the most common application of rammers are included but not limited to compacting backfill, base preparation for asphalt patching, and line trench work.

Vibratory Plates:

Also known as plate compactors, vibratory plates are especially favorable for working in granular soils, such as sand. These tools are classified as “single direction” or “reversible” meaning they have the capacity to change direction increasing maneuverability in cramped spaces. The machine’s vibration compacts soil particles, with its engine-driven exciter. 

Furthermore, vibratory plate compactors operate best through an “echo” principle. When vibratory waves travel down, waves will reflect back once the plates waves hits a hard surface. We recommend using an appropriate trench fill height that ranges between 4” to 6” maximum. As mentioned before, knowing the soil's moisture content will help to determine the proper amount of compaction needed to complete the job. 

We hope this guide served you well. We would be very interested in hearing your favorite backfill methods.