Compacts Loose Soil
Dynamic compaction is one of the more versatile and least expensive of the available ground improvement techniques. It involves the repeated dropping of large steel tampers by means of crawler cranes. Tampers typically range from 6 to 20 tons, and are dropped from heights of about 40 to 80 feet. The repeated application of the high energy impacts causes deep compaction in a soil mass. Densification occurs by rearrangement of the soil particles (loose sands, silts, etc.) or collapse of voids within the soil mass (old landfills, sinkholes, etc.).
Depending upon the particular site characteristics and structural requirements, an area treatment may be performed, or the individual footing locations may only be compacted. Footing treatment by dynamic compaction is very cost-competitive when compared to alternate foundation systems. Foundation costs can often be reduced by a factor of three to four in comparison with deep foundations. With an area treatment, dynamic compaction tends to serve as large-scale deep proof-testing, with additional tamping being performed in softer areas, thus creating more uniform subsoil conditions.
The successful application of a dynamic compaction program requires a carefully-controlled application of impact stresses to the ground, using a predetermined grid pattern. The craters that are created by the tampers may be up to 7 feet in depth and are backfilled by either dumping fill into the craters or by pushing the craters in with a bulldozer. Multiple phases of energy application may be required, depending upon the level of improvement required, the depth of the water table, the soil types, etc. Following completion of the "high-energy" tamping, a low-energy or "ironing" phase is generally performed to densify the crater backfill and the disturbed soil between the craters. The ironing phase consists of dropping the tamper from a height of 10 to 20 feet on nearly overlapping centers.
Applicable Soil Types
Most soil types can be improved, including some silts and clays. The best improvement occurs in loose sandy soils; however old fills, including rubble fills and some sanitary landfills, can be successfully treated. Dynamic compaction is routinely performed on soils below the water table, however, it may be necessary to stage the energy application to allow dissipation of excess pore pressures created by the impact energy.
Depth and Degree of Improvement
The depth of soil improvement depends on the energy per drop and can be approximated by
The degree of soil improvement depends primarily on the total energy applied to the soils, i.e., the more energy input into the soil, the greater the degree of improvement. The degree of improvement is generally expressed in terms of relative density or density as determined by SPT, CPT, PMT, DMT or some other testing technique.
Vibration levels produced by the tamper impact generally dampen out quickly with distance. Typically, vibrations can be maintained below a peak particle velocity of 0.3 inches per second at a distance of 100 feet from the impact point. Please see our article entitled Ground Vibrations from Dynamic Compaction in the Technical Articles section of this web site.
TerraSystems personnel have helped pioneer the engineering theory and methodology behind dynamic compaction and have performed nearly 800 dynamic compaction projects in the United States and abroad during the past 30 years. Our projects have ranged in size from less than 10,000 square feet to over 7,000,000 square feet and in building height from one to 37 stories. Please visit our Experience section on this web site to see a representative cross-section of our projects.
TerraSystems owns eight specially-modified dynamic compaction cranes and over twenty tampers of various tonnages and shapes. Our equipment is continuously maintained in order to minimize costly on-site downtime.