In this article, we briefly cover some background information about ground improvement techniques. Then, we do a deep dive into aggregate piers, one of the most economical ground improvement methods available today. In a previous article, we covered the basics of ground improvement.
An overview of ground improvement techniques
Before ground improvement methods like aggregate piers became prevalent, shallow foundations could only be used if marginal soils were removed and replaced with engineered fill. If the structural loads were still too high to warrant the use of shallow footings, the only option for the building site was to use deep foundations — a much more costly method. Today, aggregate piers provide a way to construct shallow foundations in marginal soils, which is often more cost-effective than removing and replacing with fill or using deep foundations.
Aggregate piers may also be referred to as vibro stone columns, Vibropiers® or Geopiers®. Rammed Aggregate Piers® are a proprietary system developed by the Geopier® company. Aggregate piers are made up of 20-to-36-inch diameter columns of highly compacted stone, constructed in groups within a footing or loaded area. The stone replaces or displaces the existing soft soil, resulting in a footing area with both stone columns and existing soils. That combination can produce bearing pressures up to three or four times the bearing pressures of the in situ (previously present on-site) soils. The high modulus columns also help significantly reduce the anticipated settlement of the loaded area.
Types of aggregate piers
There are two primary types of aggregate piers: Rammed Aggregate Piers® and vibrated aggregate piers (or vibro stone columns). The primary difference between the two is the way they’re constructed. Rammed Aggregate Piers® are constructed by pre-drilling a hole, putting aggregate into lifts, and then tamping or ramming the lifts down into the hole. The process is repeated until the pre-drilled hole is filled with highly compacted aggregate.
The basics of vibrated aggregate piers
Vibrated aggregate piers use large vibratory probes that vibrate at a high frequency to compact granular material. The probes can be inserted in a predrilled hole, or using some unique rigs, crowded into the ground to laterally displace the soft soil and create an open column. The stone can then be poured into the hole and compacted in lifts with the vibratory probes. At Subsurface Constructors, we use several such rigs to install stone columns without pre-drilling. As a result, we are able to minimize or even eliminate the generation of spoils. Because this technique eliminates a substantial part of the ground improvement process, vibrated piers that are installed without pre-drilling are often the most cost-effective approach. Of course, in some soil types, it’s necessary to pre-drill. This is usually the case with stiff soils and some fill soils.
Stone compaction with aggregate piers
When the stone for aggregate piers is dumped from the ground surface, the process is called “top-feed installation,” especially when referring to vibro installation methods. The top-feed technique is used when the soils are relatively stable, and a pre-drilled or probed hole will stay open during the stone placement and compaction process. But when the existing soils are unstable, such as in silty or granular soils, a bottom-feed installation technique can be used. In this case, the stone is tremied to the bottom of the hole through the compaction device, and the tool is not removed from the hole until the aggregate pier is complete.
Factors that may impact your choice of aggregate piers
Rammed Aggregate Piers® typically require a graded stone to reach maximum compaction, while vibrated piers use clean stone — stone that is all roughly the same size. Both rammed and vibrated piers can provide bearing pressures in the range of 4,000 pounds per square foot (psf) to 6,000 psf or higher, depending on the soil makeup. The main concerns owners have regarding different types of aggregate piers are one, that they provide the structure with the necessary support, and two, choosing the least expensive option.
Learn more about ground improvement techniques and aggregate piers
This article is an excerpt taken from a white paper: The Desktop Guide to Aggregate Pier Ground Improvement. In the white paper, we cover ground improvement methods and aggregate pier ground improvement in further detail. Fill out the form below to download it.