|
By Dr. William Gates, Senior Principal Professional, Kleinfelder
As the wind industry affirms its commitment to providing 20 percent of the nation's electricity by 2030, wind energy developers ramp up efforts to build the necessary infrastructure. Amidst this drive to meet the growing demand for clean power, it is critical that engineers and designers look carefully at the geological conditions below ground before they erect these powerful machines.
A careful study of the geologic and geotechnical issues beneath a proposed wind farm are as important as the proper blade angles and sizes. The subsurface geology forms the foundation of these large structures. These turbines are supported on large monopole towers upwards of 350 feet high with blade diameters of 250 feet. The structures are subjected to large wind loads that stress the foundation with combinations of torque, tension and compression forces. Added to this, many wind projects are located on geologically challenging and unstable terrain.
Not long ago, Kleinfelder geotechnical engineers were contracted to evaluate the geologic and geotechnical characteristics and provide design considerations for the Pine Tree Wind Project in California and the Elkhorn Wind Project in Oregon.
The design-build Pine Tree Wind Project was initiated in 2007 by the Los Angles Department of Water and Power (LADWP). Pine Tree Wind Project is located on a large cattle ranch near the boundary of the Mojave Desert and the foothills of the Sierra Nevada Mountains. Here the warm winds rise from the desert floor and race over the ridges of the mountain front at well over 50 miles an hour. The ridges and wind gaps provide ideal locations for the turbines to capture the wind. When the project is complete it will feature 80 wind turbines providing some 120 MW of power.
Kleinfelder geologists began mapping the engineering geology of the rock slopes along the access roads to the proposed turbine sites in 2006 in preparation for this project. Presently, engineering geologists are installing exploration borings and test pits at the turbine sites to verify the subsurface geology and gather geomechanical information to aid in design of the foundations. The site geology (bedrock) is composed primarily of granitic rock (granodiorite) typical of the Sierra Nevada. The rock crops out in various states of weathering and strength. Access to the turbine pad locations requires about 36 miles of pioneered roads for the proposed eighty turbines.
Similarly, in 2006, Kleinfelder contracted with DEA and Horizon Wind Energy to provide a geotechnical design report for the Elkhorn Wind Project. The Elkhorn Wind Project is located near North Powder in the Wallowa Mountains of northeastern Oregon. The turbine locations occupy foothill ridge spurs and saddles where the winds are channeled up from the valley around North Powder. At completion, the project will include 61 wind turbines providing over 90 MW of electricity. Engineering geologists mapped the surficial geology, geologic hazards and installed exploration borings and test pits at the foundation pads to verify the subsurface geology and gather geomechanical information for design. The geology (bedrock) is composed of volcanic rock consisting of flows of strong Columbia basalt. Interspersed between the basalt flows are weak ash tuffs and other sedimentary rock.
Both wind projects exhibit similar geotechnical challenges, most obviously that the turbine pads are located in remote rugged terrain on soil and rock. Rock material characteristics ranged from very weathered weak rock to fresh unweathered, very strong rock. Moreover, the roads to the pads are controlled by rights-of-way property issues as well as the geologic conditions.
Beneath Our Feet
To get a handle on the geology and geotechnical issues of the project site, an old sage geologist once told me, "It is important to see the geology through the souls of your feet." In other words, get out there and put your hand on the rock and taste the soil. Spend a little time and observe; "what is going on?"
To gain information on the geomechanical characteristics of the rock and soil in the project areas, Kleinfelder engineering geologists conducted field mapping of the outcrops, excavation of test pits and drilling of exploration boreholes. Much of the information collected during the outcrop mapping activities dealt with the condition of fractures (discontinuities) within the exposed rock masses. Key mapping windows were established for horizontal and vertical mapping traverses.
Rock mass classification is a means to characterize the geomechanical characteristics of the rock mass by testing the rock strength and evaluating the characteristics of the fractures within the rock mass. The classification techniques are accomplished using the field data and therefore are more of a design tool for both slope stability and foundations than actual field data collection.
Will the Slopes Stand When Excavated?
Most access roads, lay down areas, and pads require stable cut slopes in either soil or rock. In rock, the geomechanical characteristics of the rock mass, including the strength and weathering, control the height and geometry of cut slopes. Therefore, it is imperative that the engineer have a working knowledge of cut slope stability and how it relates to the geomechanical characteristics of the geologic media including strength and weathering.
Typically in soil cuts, depending on the mechanical characteristics, the slopes are designed to lay back at a 2:1 or 1.5:1 (horizontal to vertical) geometry to promote stability. However, with rock slopes there is the occasional misconception that excavations present no real engineering problems because the rockmass is typically composed of hard strong rock that will stand steeply (like Half Dome in Yosemite National Park) when excavated.
Unfortunately, this assumption is commonly made with no evaluation of the actual stability of the potential rock slope. Some of the most catastrophic failures in the world have occurred because of rock slope failures. Structural geology (joints in the rock) rules and large blocks may slide out of excavated faces even at shallow angles.
1 | 2 | Next Page | Full Article
| 
Articles 
| View all PennWell sites | View all PennWell events
Add RSS Feed
