When the soil conditions and loading requirements of a structure necessitate ground improvement, the geotechnical team will consider multiple techniques. After eliminating remove-and-replace as an option but before considering costly deep foundations, contractors can consider rigid inclusions and aggregate piers as a means to support shallow foundations.
Specific soil characteristics dictate whether rigid inclusions or aggregate piers are a best-fit for cost-effective ground improvement that will meet bearing requirements and settlement criteria. Here we’ll discuss the fundamental characteristics of rigid inclusions and aggregate piers, their installation methods and case studies demonstrating these ground improvement methods in action.
What are rigid inclusions?
Rigid inclusions are unreinforced, grouted or concrete columns installed in very soft soils to meet settlement criteria and improve bearing capacity for support of shallow foundations of a structure. They are considered ground improvement because they are not structurally connected to the building they support. In addition to the grouted columns, an essential element of rigid inclusion ground improvement is the load transfer platform, or LTP. This is a layer of granular, structural fill that bridges the load between the rigid inclusions and prevents too much point stress on the footing.
This method of ground improvement is used for very weak soils, such as organic silt, peat, very soft clay or any soil where there is little to no lateral confinement from the surrounding soil. This is also indicated by weight-of-hammer (WOH) soils or soil with very low blow counts. Geotechnical contractors use rigid inclusions for ground improvement on projects such as multi-story buildings, large tanks, earthen embankments and bridge approaches with mechanically stabilized earth walls.
The term “rigid inclusion” is a generic term for ground improvement with unreinforced, grouted columns, and there are different installation methods and names for rigid inclusions across geotechnical contractors. Rigid inclusions installed with vibrating probes to displace the soil are called vibro concrete columns (VCCs). Others are installed using a reverse flight rotating auger to displace the soil. Those installed with the later method may be called Controlled Modulus Columns or DeWaal piles. (These are proprietary phrases trademarked by specific companies.) But the industry is generally moving toward the umbrella term “rigid inclusion,” making it the most straightforward way to describe this ground improvement technique.
Installation of rigid inclusions
A geotechnical contractor will install rigid inclusions by laterally displacing the soil by either advancing a vibrating probe or by using a reverse flight rotating auger. Typical depths of rigid inclusions using vibratory probes are on the order of 25 to 40 feet, while drilled auger methods can reach depths of 60 to 70 feet or even deeper. They can be installed at a rate of 30 to 40 columns per day, depending on their depth, and range in diameter from 18 to 24 inches. The columns will set within a day and cure to allow further construction in a week to 10 days, depending on the grout mix.
The LTP is installed on top of the rigid inclusions and is composed of a layer of granular, structural fill with varying thickness (from 4 inches to 3 feet), depending on how widely loaded the area is. Thicker LTPs may also be reinforced with one or more layers of geogrid or high-strength geotextile.
Finite element modeling (FEM) will sometimes play a role in rigid inclusion design — especially when an LTP is used, as the interaction between the concrete column and the LTP is quite complex. The geotechnical team may use FEM to design the columns to provide the desired bearing pressure and meet settlement criteria, or to optimize the number of columns installed on a site and provide maximum cost-efficiency.
Finally, scenarios exist in which rigid inclusions are installed in tandem with aggregate piers. This hybrid technique can be used where grouted columns are used in the zone or layer with the soft organic soils and then aggregate piers are used elsewhere. Another scenario for hybrid ground improvement would be to address different loading conditions across the site. For example, the columns of a multi-story building on soft soil may be supported by rigid inclusions to carry the higher load, while the floor slab (which isn’t as highly loaded) can be supported by aggregate piers. This can provide a more cost-effective solution than using rigid inclusions across the entire site.
Rigid inclusion case study
To further illustrate how rigid inclusions can be used on a project to provide adequate bearing pressure and cost-effective ground improvement, here is a case study of our work at the Fairfield Regional Fire School.
For a regional fire training facility in Fairfield, Connecticut, the new school was replacing a post-World War II-era facility. It included a 9,500-square-foot classroom building, 40,000-gallon drafting pit, a 5,600-square-foot support facility and a three-story 3,200-square-foot “burn” building.
To complete ground improvement, our team needed to provide a cost-effective solution that would meet the soil needs of the site. The site is located in a flood plain about 600 feet from the ocean and has a typical soil profile consisting of 4 to 12 feet of urban fill over 1 to 4 feet of organic silts and peats — and below that is medium-dense mixtures of sand, silt and gravel. Through the organic zone, we installed a combination of a grout column with highly compacted ¾-inch aggregate above the grout below the footings. Below the floor slab, we installed approximately 575 aggregate pier columns.
What are aggregate piers?
Aggregate piers are another method of constructing shallow foundations in marginal soils. This ground improvement technique consists of columns of compacted stone installed in groups to reinforce soft, cohesive soil and poor fill. While rigid inclusions are installed in very soft soils, aggregate piers are employed in soils that are likely still soft, but have enough strength to provide lateral confinement. Examples include soft clays, silts or man-made fills. Aggregate piers are used to increase bearing pressure and mitigate settlement under structural footings, and can support a variety of structures — ranging from multi-story buildings, agricultural and industrial facilities, liquid storage tanks and MSE walls in transportation applications.
Similarly to rigid inclusions, various terms for “aggregate piers” exist in the industry. Those installed using a high-frequency vibratory probe to compact granular material are called vibro stone columns (VSCs). This technique allows the stone columns to be installed without pre-drilling, minimizing or even eliminating the generation of spoils. Rammed Aggregate Piers® (a proprietary term and method) 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. Even more so than rigid inclusions, it’s important to use the umbrella phrase “aggregate piers” in the geotechnical report and construction specifications, as this opens the bidding process to the widest variety of contractors. This ensures the best-fit ground improvement method is chosen for your project.
Installation of aggregate piers
Installing aggregate piers is a straightforward process, starting with excavation. Using hydraulic crowd pressure, a hole is excavated by advancing the probe into the ground and displacing the soil laterally. If the probe is used (in soft soil) to advance the hole, minimal to no spoils are generated. In some cases where stiff layers of soil or man-made fill may slow or prevent probe advancement, the hole can be pre-drilled.
To feed the stone into the hole to form the aggregate pier, there are three methods: Dry Top Feed, Dry Bottom Feed and Wet Top Feed. (Click here for a full explanation of each method.) The Dry Top Feed method is the fastest and most often used. Finally, lifts are compacted using vibration and down pressure from the vibroflot until a high modulus column is complete. Aggregate piers can be installed at a rate of 40 to 60 piers per shift, and typical bearing pressures are in the range of 4,000 pounds per square foot (psf) to 6,000 psf.
Aggregate piers case study
To further illustrate how aggregate piers can be used on a project to provide adequate bearing pressure and cost-effective ground improvement, here is a case study of our work at the Union University Library.
The Union University Library in Jackson, Tennessee was designed to be an iconic structure on campus with a ground entrance from the campus lawn. To be constructed as designed, the structure needed aggregate pier ground improvement to achieve the high bearing pressure required — a significant 7,000 per square foot (psf).
Soils across the site generally consisted of medium stiff to stiff lean clay changing to clayey sand with depth, becoming very stiff at roughly 20 feet. Although the on-site soil was relatively competent, it would not support the required bearing pressure of 7,000 psf, so our geotechnical engineering team designed and installed an aggregate pier system to achieve this requirement.
We supported the shallow foundations using approximately 425 aggregate pier elements, and several of the foundations required uplift resistance. Our team used grouted soil anchors to handle the uplift requirements on the project.
Comparing rigid inclusions and aggregate piers
It’s important to note that the best ground improvement technique for a project ultimately depends on the soil data gathered on-site. All things held equal, aggregate piers are typically the cheaper of these ground improvement techniques, as stone is cheaper than concrete and aggregate piers can usually be installed much faster than rigid inclusions.
But at the end of the day, a geotechnical contractor will recommend a solution that will ensure adequate bearing pressure and minimal settlement for the structure based on the analysis of the soil properties as described in the geotechnical soil report as well as the structure’s loading conditions. For sites with very soft soil, this often means rigid inclusions, while sites with soils that have bit more strength and ability to provide lateral confinement will be able to support the installation of aggregate piers.
An experienced geotechnical contractor will ensure your ground improvement is engineered in a safe and cost-effective manner. Read up on more of our case studies here, and learn more about our ability to engineer and install rigid inclusions and aggregate piers. Or, to begin a discussion with a geotechnical engineer about ground improvement options for your next project, reach out to us today.