Case Studies
4. Determination of Foundation Conditions for wind turbine sites
NORCAL has conducted geophysical surveys in many of the western states for numerous wind turbine projects. These projects involve the placement and construction of 50 to over 200 wind turbines. Normally about 10 to 20% of the turbine locations are actually investigated with geophysics, depending upon the site geology. The turbine projects are generally remote with limited access for drill rigs and other heavy equipment. However certain preliminary subsurface information is required to design the foundations for these structures that may be 300 feet tall and must withstand significant lateral and vibratory forces.
A geophysical method referred to as multi-channel analysis of surface waves (MASW) is being used to obtain compressional (P) and shear (S) wave velocities to depths over 50 feet. This information is needed by geotechnical engineers to evaluate the dynamic properties of the soil and rock for foundation design. MASW data can be obtained in 1-D mode to determine P and S velocities beneath a particular point or in 2-D mode to determine the respective velocities versus depth and distance along a profile. For sites that have difficult access this method is preferred over other investigative methods since boreholes are not required (making it more cost effective) and equipment is man portable.
The following 1-D P and S velocity profile was taken at a wind turbine site in eastern Washington.
The profile below was obtained at a wind turbine site in New Mexico. This profile shows lower S wave velocity material underlying a harder surficial caliche zone.
