Structural Fill & Embankment
What are Structural Fill and Embankments?
Structural fill is a screened earthen material used to create a strong and stable base.For example, the native soil at a site may be too weak to support a structure, so the soil is replaced by compacted structural fill to provide the needed structural support. In roadway applications, structural fills are often used as fill for abutments or slabs, backfill for retaining structures, or filling of trenches and other excavations that will support roadways or other structures when completed. An embankment is an earthen structure that is used to raise the elevation of a roadway or railway above the elevation of the surrounding area. One common example of an embankment is the approach to a bridge, where the embankment raises the road to the level of the bridge deck. Embankments also tend to be large structures, potentially miles long, while structural fills tend to be smaller, discrete structures. The key point is that they are both earthen structures.
Structural fills and embankments are typically constructed by compacting earthen materials in place, so the compaction properties of the material (optimum water content and maximum dry density) are very important to performance. The compressibility and shear strength are also important measures of the compacted material. In addition, drainage is an important consideration to prevent the loss of shear strength due to saturation.
How is Foundry Sand used in Structural Fill and Embankments?
Structural fill and embankments are major applications for foundry sands, especially for "green" sands that contain bentonite. Traditionally, structural fills and embankments have been built using soil and natural aggregates including small stones and sand. Foundry sand has the particle size distribution and mechanical properties necessary to be a good material for structural fill and embankments. In structural fill, foundry sand is typically placed in lifts and compacted to form the base for a building, roadway or other structure. In embankment use, foundry sand is generally placed and compacted to form the fill in the embankment, whose slopes are then covered with soil.
How do Structural Fill and Embankments perform using Foundry Sand?
Foundry sands in structural fill and embankments behave like natural sands with fines.
Foundry sands are essentially high quality sands, and so their performance in fill applications has been good. The friction angle for sands without clay is typically 30-36 degrees, which is comparable to natural sands. Sands with clay can have a lower friction angle (around 28 degrees.) The numerous successful field projects are an indication that foundry green sands also perform well in fill. Clay bonded sands can be sensitive to moisture, but that is managed by controlling the water content. In addition, foundry sands do not require any special material handling equipment or procedures. Contractors have reported that working with working with foundry sand is very similar to working with natural materials.
What are the technical issues associated with Foundry Sand in Structural Fill and Embankments?
Foundry sands generally perform very well as fill materials, and have been used for decades in projects ranging from a few thousand cubic yards to more than 100,000 yards. When used as fill materials, foundry sands should be subjected to the same types of physical testing as other fill materials. Plasticity index, shear strength and moisture density need to be understood so that the geotechnical engineer can properly design the fill. As with base and subbase applications, foundry sands containing clays should be compacted dry of optimum water content. Resin sands generally will drain satisfactorily, but high bentonite green sands may need additional drainage features in large fills.
Traditional embankment material consists of a granular mix with particles both larger and smaller than most foundry sands. Therefore the design engineer needs to be familiar with the physical characteristics of the specific sand proposed for embankment use. The shear strength of foundry sands is the key design factor for embankments designed with foundry sand because shearing resistance keeps the slopes stable. To prevent erosion caused by flowing water, foundry sand embankments should have a vegetated soil cover, which is standard construction practice in most states.
In addition to large engineered fills on transportation projects, foundry sands are used to level building sites for commercial properties, as pipe bedding and backfill, and in a host of other common earthwork projects where they replace other granular materials. The AASHTO Subcommittee on Materials is developing a “Standard Practice for Foundry Sand for Structural Fill and Embankments.” ASTM also plans to adopt a version of the AASHTO standard when it is complete.
Are there any specific QA/QC issues that suppliers and/or end users need to be aware of?
Foundry sands typically do not require any special handling equipment or procedures, and are transported, placed and compacted with conventional construction equipment. Green sands may need moisture added during transportation and placement in order to prevent dusting. Foundry sand can sometimes have chunks of sand and binder from the molding process that may need to be screened out or crushed prior to use, depending on the project requirements. For most fill projects, care should be taken to maintain consistent moisture content in order to achieve the proper compaction values on the job site.
Are there any specific environmental issues associated with the use of Foundry Sand in Structural Fill or Embankments?
After a comprehensive review of foundry sand data, U.S. EPA has declared that sands from foundries casting iron, steel and aluminum products are safe for use in soils-based applications. However, some states have not yet updated their environmental regulations to be consistent with the newer research findings. It is important to know, therefore, what is permitted in the state in which the fill or embankment will be built. Although structural fill and embankments are the largest volume uses for foundry sands nationwide, some states still have limitations on the placement of foundry sand in unbound applications. If your state regulatory agency does not currently permit the use of foundry sand in structural fill and/or embankment, data from recent research may help convince the state agency to proceed.