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SESI Learning Series: Cost­ Cutting Utilizing Deep Dynamic Compaction
October 22, 2013

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When preparing a building site, developers have many choices with varied costs. SESI Consulting Engineers recommends and designs buildings on spread footings, piles, caissons, drilled shafts, and on soils improved by excavate and backfill, preloading, and dynamic compaction. When applicable, SESI often recommends dynamic compaction in lieu of other foundation preparation techniques to make a project economically feasible.

On Solid Ground

History

The process of dynamic compaction is one of the oldest forms of ground improvement. It has been used on thousands of projects throughout Europe and in the United States. The use of modern equipment started in the early 1970s. Realizing the potential cost savings, William St. Pierre, founder of SESI, has been recommending the dynamic compaction process for projects in the United States since the early 1980s.

Dynamic compaction (also known as deep dynamic compaction) is the process by which energy is applied at the ground surface to increase its density by reducing the void spaces in the soil. The application of the energy results in a decrease in soil volume when the air and/or water is expelled from the void spaces of the soil, creating a more uniform, dense soil mass.

The dynamic compaction process has been performed on sites containing silts, clays, loose sands, organic soils, granular soils, cinders, fly­ash, refuse, coal fragments, and miscellaneous debris. Among the most common sites are landfills, brownfields, demolition rubble dumps, and other loose, uncontrolled fills. Materials up to 30 feet deep can be successfully densified using dynamic compaction. The process is an excellent alternative when piles or excavate & backfill are cost and time prohibitive. SESI has been involved in the design and inspection of over 200 dynamic compaction projects including sanitary landfills, brownfields, and miscellaneous fills over soft soils.

The Process

Dynamic compaction is a procedure used to densify weak soil deposits in­place by dropping a large weight (typically 8 to 16 tons) from heights of up to 75+ feet with standard equipment. It is typically done on a grid pattern varying from 10 to 15+ feet on centers with 4 to 6 drops per location per pass. Most often the procedure is done in two or more complete coverages over the area requiring improvement.

The Goal

The primary goal of dynamic compaction is to change an uncontrolled fill into a controlled fill. This is done by providing sufficient energy at the ground surface to cause densification of the underlying fill deposits, thereby reducing the compressibility of these deposits and providing suitable bearing capacity for building foundations. Dynamic compaction is an exploration tool as well as a ground improvement method. If weak deposits are present below the ground surface, they will be revealed during the impact process by a greater than normal lowering of the ground surface.

FAQ

How long does the process take?

The length of time to complete the process depends on the size of the project and the soil conditions. Generally, a 100,000 sf building can be completed in less than a month.

How much ground lowering will occur?

The amount of ground lowering depends on the site conditions, but generally, an average ground lowering throughout the area being compacted is 6 inches to one foot.

How much does it cost?

The cost of dynamic compaction will depend on the size of the building. Larger buildings cost roughly $1 to $2 per sf while smaller buildings can cost $3 per sf or more.

For more information or to find out if deep dynamic compaction can work for your site contact Michael St. Pierre, PE at (973) 808-­9050

*Information was posted in 2013