OVERVIEW:
Dole’s salad processing plant in Soledad, CA, recently began a $10 million project to install two wind turbines. The turbines will provide power to the facility to aid Dole in achieving its goal of net zero carbon emissions from Dole-managed operations by 2030. The two 500′ tall wind turbines are capable of producing 2.7-megawatts each for a combined total of 5.4 megawatts of renewable energy.
CHALLENGE:
The most significant obstacle encountered was the soil conditions at the project locations. Due to the nature of the structure of a wind turbine, the foundation must be able to resist high tension and compression loads. Achieving the loads was challenging as the soil mainly consisted of silty sands with sandy clays in a few areas. Geotechnical borings taken at the install locations found the subsurface conditions to consist primarily of loose, granular soils. Within the 50-foot boring, a suitable bearing layer was not encountered.
Grouted helicals were considered for this project in the design phase, but they would have failed in tension. The lack of a suitable bearing layer also introduced many uncertainties when designing the foundations with helical piles. Mobilizing a dedicated piling rig for only two foundations would have been cost prohibitive. Therefore, traditional micropiles, auger cast piles, and other piling methods that rely on a dedicated rig were too costly.
SOLUTION:
The engineer specified STELCOR DDMS to support the wind turbines as alternate pile types were either cost prohibitive or incapable of resisting the loads in the conditions present. Drilled-in Displacement Micropiles have been proven to achieve lower cost per kip than alternate piling methods and perform well in sandy soils, making STELCOR a perfect fit. The IDEAL design team configured a STELCOR DDM with a 16″ grout column and a 7″ steel core to meet the loading requirements. The designed STELCOR pile was tested to 163 tons in compression and only moved 3/8 of an inch. The same pile was also used for the tension piles and is capable of resisting the designed tension load of 123 tons.
Avalon Structural, a certified STELCOR installer, installed 42 total STELCOR DDMS. Each foundation consisted of 21 piles, with an inner ring of 7 compression piles and an outer ring of 14 tension piles. STELCOR was installed using only an excavator with a hydraulic drive motor attachment and a colloidal grout mixer. Mobilization and moving between install locations while on-site is very efficient as there is minimal setup. Even when set up, the installation equipment is highly mobile. Due to the efficiencies of STELCOR, installation rates were increased, saving time, money, and a lot of headaches.
INSTALLER:
Avalon Structural, INC.
DESIGN LOADS:
Compression load: 123 kips
Tension load: 123 kips
TEST LOADS:
Compression load: 163 kips
Tension load: 163 kips
PILE DETAIL:
STELCOR 1600
18” Tip or Drive Plate
16” Corrugated Grout Column
13” Solid Grout Column
11” Reverse Grout Auger
7.0” O.D. X 0.408″ W.T. – 80 ksi central shaft
SOILS + EMBEDMENT DEPTH:
Subsurface explorations at this site mainly encountered loose to medium dense sands with layers that contained silt and clay. The borings were terminated at 51 feet, and the final 10 feet consisted of sandy silt and stiff lean clays with varying amounts of sand and gravel.
NUMBER OF PILES:
42
EMBEDMENT DEPTH:
50′
VIEW/DOWNLOAD CASE STUDY AND SOILS REPORT (PDF)
