Clean Fill Terminology

Working with fill material for your construction, landscape, or farming projects can seem complicated. Inconsistent terminology can make it even more confusing.

First, you have to be aware of your local laws to ensure that you are compliant with any safety or environmental regulations.

Next, you have to understand the terminology so you can select the right fill type for your project or properly dispose of excess fill.

After 20 years in the construction industry, I still find the terminology a challenge. Part of the problem stems from the fact that each province or municipality might use different language to describe the same material.

In this post, I will outline some of the most common types of construction and fill material used in the province of British Columbia.

This blog post will be a living document that my team and I will regularly update to include new terminology as we encounter them.

Fill Categories

From a construction point of view, the first thing you need to be aware of is the fill category.

In general, there are three types of fill categories:

  • Structural material can include recycled concrete, recycled asphalt, sand, gravel, river sand, clay, and similar material. Clean structural fill is suitable for construction project sites that can support construction development.
  • Non-Structural material can include top-soil, organics, silt, peat, and similar material. Clean non-structural fill is suitable for landscaping and farming projects as they can support water retention and air circulation, making them an ideal choice for plant life.
  • Boulder material can include riprap, boulder, crushed rock, and similar material. Clean boulder fill, including riprap, are commonly used near water banks.

When you make a post listing using Fill Connect, you’ll be asked to select the fill category first. Then you can choose a fill type based on the category you selected.

Now that you know the main categories, let’s dive deeper into the types of fill you might encounter on a typical construction project.

Road Construction

Base: This is the layer right below the asphalt. The depth of this layer changes based on the design loading, but it’s usually 150-200mm. The gradation of this gravel is specified in contract documents, and it is generally 25mm minus.

Subbase: This is the layer below the subbase, and the gravel size of this layer is more significant than the base layer. It is usually about 300mm, but again this could change depending on the design. What is confusing is that sometimes, this is called base! But I usually refer to typical sections on the drawing to clarify this.

Subgrade: This is the layer below subbase, and in most cases, it’s just the native material.

Depending on the project location and type of soil, sometimes, this layer also needs to be compacted and should be made of compactable material. But usually, there are no specific requirements for the gradation.

If the native material is not compatible, then the un-compactable soil should be removed to the depth that is acceptable to the engineer on record and replaced by compactable material. Gradation requirements are not that strict, and the material could be sourced from nearby excavation sites.

Bridge Construction

Embankment Fill: The two end of a bridge or an overpass sits on a structure called abutment. Behind abutment, the ground is brought to bridge surface elevation with embankment fill.

This fill material usually needs to be compatible and is made of free-draining soil. Often, the material from cut areas within the same project or nearby projects could be used for embankment fill or if not available purchased from a gravel pit.

Foundation Backfill

Utility Trench Construction:

Bedding: When installing pipes in a trench, the pipe is placed on bedding gravel, and again depending on the design, it could be 100mm to 150mm thick. The pipe is then placed on this layer. The bedding gravel is placed in layers and compacted until it’s about 300mm over the top of the pipe.

Pipe zone: Sometimes, the type of bedding gravel and pipe zone is different as per the contract documents. In this case, bedding is placed under the pipe up to the spring line of the pipe. Then different types of gravel are placed from there to 300mm over the pipe.

Import Backfill: Once the bedding is placed, the trench is brought to the subgrade level by backfill. If the trench is under a new or existing roadway, import backfill is used, and it should be compacted to the required density. If the trench is in greenfield or somewhere that is not exposed to any loading; native backfill could be used.

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