In the wake of the tragic Surfside condominium collapse, Florida lawmakers passed legislation focused on enhancing building safety and preventing similar disasters. The “Milestone Inspection Bill,”
The Economical Alternative to Deep Foundation Systems-Vibro Stone Columns
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Increasingly, Florida general contractors faced with the prospect of having to construct expensive, time-consuming deep foundations beneath multi-story residential or commercial buildings are turning to a specialized type of aggregate pier known as vibro-piers or vibro stone columns.
Compared to deep foundation structures, vibro stone columns install far more quickly, reduce the handling of spoils and avoid soil settlement delays. A qualified geotechnical company can advise you how to use this advanced ground improvement method for your next project. They can design cost-effective treatments and installation procedures to match your needs.
Vibro Stone Columns Versus Aggregate Piers
Vibro stone columns are an advanced version of aggregate piers used to improve ground bearing capacity beneath foundations or slabs without the disadvantages of conventional excavation techniques.
Modern vibro stone columns displace soil with high-frequency vibrating probes known as vibroflots. Vibro stone columns are installed in a pattern whose design depends on the type of structure to be supported and the quality of the soil beneath, which is determined by geotechnical analysis.
Specific pier aggregate addition techniques depend on soil conditions, but the aggregate is added in measured stages, each of which is thoroughly compacted using the vibroflot. These series of aggregate lifts continue upward to the surface of the building site.
The result is a set of extremely dense, stable columns of rock with significantly higher load-bearing capacity than the surrounding ground. Shallow spread footings or concrete slabs are well supported without the expense of deep foundation systems.
As we discuss below, this technique has several additional advantages over deep foundation systems in terms of function, cost, and time-savings.
Determining Required Bearing Capacity
Your geotechnical contractor’s engineering team will work closely with the project’s structural engineers to determine the allowable bearing capacity and settlement requirements of multi-story buildings or other industrial construction projects you take on.
Depending on the quality of the soil, typical bearing pressure using vibro stone columns of 3,000 up to 8,000 pounds per square foot can be achieved. The columns are spaced apart depending on the engineer’s specifications.
The geotechnical engineer also considers the permissible structure settling, including total settlement and differential settlement, which is usually one and one-half inch respectively. At times, the precise determination of these factors may require boring test holes.
Vibro Stone Column Design Based on Soil Conditions
As appropriate to surface and subsurface soil conditions, vibro stone columns are installed as deep as 50 feet. Delivery and compaction of the aggregate will differ depending on the specific type of soil beneath the structure.
- Cohesive Soils
In cohesive granular soils, the borehole will not fall into itself, so the vibroflot can be removed after the correct depth is reached. Vertical lifts of aggregate are added directly in a technique known as dry top feed. After each dump of aggregate, the vibroflot is reinserted and compacts the lift with high-frequency vibration. This process is repeated until the pier reaches the surface.
- Soft Clay and Silt
In less stable soils, a dry bottom feed method is used to deliver the pier material. In this case, once the vibroflot has completed the hole, it is kept in place so the hole cannot collapse. A tremie is attached to the vibroflot, which places rock poured into a top-side hopper into the bottom of the hole. The vibroflot is then retracted a small amount to compact the added aggregate at each lift.
- Sandy, Silty Soils
For loose soils that are below the water table, the wet bottom-feed technique is employed. As in dry bottom feed, the vibroflot remains in the hole to prevent collapse and aggregate is added from above. As aggregate is added, water or compressed air jets from all sides keep the hole open. The vibroflot is extracted a small distance at each lift to compact the fill.
Vibro Stone Column Advantages
Vibro stone columns have become one of the most popular ground reinforcement techniques for modern building projects. They are especially economical for improving the bearing capacity of mixed fills and granular soils. Savvy builders utilize vibro stone columns rather than bearing the high costs and additional time of constructing deep foundations.
- Fast Installation
Florida contractors are often under strict completion deadlines. Vibro stone columns give you a schedule boost in the earliest construction stages with an installation that takes a fraction of the time of deep foundations. The installation of upwards of 40 vibro stone columns per shift is typical.
- Reduced Spoils Management
Boreholes created with a vibroflot avoid creating the spoils of traditional excavation. Your costs of moving and hauling away spoils are nearly nil. On sites with soil contamination, the reduction in spoils is a double benefit.
- Rapid Soil Settlement
Especially where your project needs high bearing capacities, compared to deep foundation systems the settlement period of vibro stone columns is far less due to compaction. Thus, subsequent construction occurs sooner. Vibro stone column compaction also makes it an ideal solution to building on fills.
- Frees Up Resources
The vibration soil displacement process hastens soil drainage, mitigates soil liquefaction and eliminates dewatering. The building of temporary shoring structures is unneeded. All these features mean your building crew can be assigned to more urgent tasks.
- Detailed Results
Using state-of-the-art data collection, analysis and logging, the process of column installation can be continuously monitored by your structural engineers in real-time, which increased schedule confidence.
Consult a Vibro Stone Column Geotechnical Expert Today
When choosing a geotechnical firm to apply vibro stone column technology on your next Florida project, select one with many years of experience in improving ground bearing capacity for a variety of projects from single-story to multi-story buildings.
Helicon provides you with the most economical and practical solution that fits your specific building goals. Contact Helicon today to speak to a Project Manager about your next geotechnical project.
