A deep foundation system consisting of a mono-pole or drilled/driven piles is often recommended by the geotechnical engineer when the geotechnical exploration at a planned wind turbine tower location reveals that the subsurface conditions consist of loose, clean, granular soils (sands and silty sands). However, often the loose soils may be improved economically to enable the tower to be constructed on a shallow mat foundation. The improvement process permits the use of a higher soil bearing capacity and reduces both overall and differential settlement of the proposed foundation. Seismic liquefaction potential is also reduced as a result of the loose soils being densified beyond the threshold relative density for liquefaction. One ground improvement technique consists of performing a regular pattern of compaction probes within the loose soils, which is known as vibro compaction.
Vibro compaction is performed using a specially designed down-hole vibrator. The vibrator is typically between 10-15 feet long. Extension tubes are added to the vibrator to enable the vibrator to penetrate to the required depth at each particular site. Treatment depths in excess of 100 feet have been performed. The vibrator—also known as a vibro probe, or vibro flot—consists of a ±18 inch diameter closed end steel casing with an internal electric or hydraulic motor spinning an eccentric weight. The rotating eccentric weight causes the vibrator to oscillate horizontally, thereby imparting vibrations at the treatment depth. The vibratory energy allows the soil particles to move into a denser configuration by reducing the inter-granular forces between the soil particles. The improvement results in higher bearing capacity, lower settlement, and lower liquefaction potential. The vibrator can be suspended from a crane, excavator, or attached to a fixed-mast rig.
At each compaction point location the vibrator penetrates the ground by means of its weight or rig down-thrust, and it is assisted by the vibratory energy and (occasionally) water jets integral to the vibrator. After reaching the bottom of the treatment zone the soils are densified in situ for a specific time, and the vibrator is then raised several feet and the process repeated until reaching the ground surface. A crater will form at the compaction point location, indicating that deep densification is taking place. Clean granular backfill is added with a front-end loader to fill the crater. Oftentimes on-site material is utilized as backfill.
The improved soil characteristics depend on the soil type, the soil gradation, the spacing of the compaction points, and the time spent compacting the soils. The spacing for compaction points is generally between 6 and 14 feet, with centers arranged on a triangular or square pattern. The treatment is carried out to a depth sufficient to meet the design. Although the process is typically used to densify sands and silty sands it can also be used on mine spoils, provided they are granular in nature.
A quality control program is essential to assure the successful performance of a vibro compaction program. Field-scale testing should be performed at the beginning of the program to verify construction quality and design parameters. Consistent compaction point quality is ensured by monitoring the construction procedure and backfill quantity. The effectiveness of vibro compaction in granular soils can be readily verified using common test methods such as Standard Penetration Testing (SPT), Cone Penetrometer Testing (CPT), or Dilatometer Testing (DMT). The testing is performed between compaction points to verify that the design densification has been achieved.
Vibro compaction is available as a design-build service by specialty contractors. When properly designed and constructed, a vibro compaction program is an efficient and economical alternative to traditional deep foundation systems for many wind turbine sites with loose granular soil conditions.