GEOLOGIC CONSIDERATIONS FOR FOUNDATIONS OF LARGE DAMS IN THE SOUTHEASTERN US

The southeastern United States has experienced rapid population growth and periods of extended drought conditions over the past few decades. Concerns over reliable water supplies have led to the planning and/or construction of several new large water supply dams.

Because of the size and complexities of large dams, geologists typically lead the subsurface site characterization effort prior to design of the dam and provide input to the design engineers during design. Once construction begins, geologists evaluate the foundation material after excavation and cleaning, and develop geologic maps of the foundation.

Geological aspects that affect the design and construction of new dams are changes in lithology and the presence of discontinuities. Lithologic changes affect broad areas in terms of depth of weathering, permeability, compressibility, and strength. Discontinuities, such as joints, faults, and shear zones affect the same properties, but in more limited and typically linear zones. The geologist's role is to define the lithologic and discontinuity characteristics in a manner and in terms that can be used directly in seepage and stability models, estimation of the top of rock surface, and to estimate the difficulty of excavation of earth materials. These parameters constitute the fundamental input required by engineers in their design work. One of the key responsibilities of the engineering geologist is to communicate the geological characteristics of the site to the design engineering staff in a manner that is useful and understandable to those with training in another field.

Specific examples of geological characteristics that have affected the design and construction of recent dams include:

A lithologic change along the dam centerline at Deep Creek Dam, central North Carolina, led to a deeper weathering profile on one abutment. A composite roller compacted concrete and earthfill dam was designed to address the variable depth to rock in the foundation.

Complexities in the weathering profile at Hickory Log Creek Dam, northern Georgia, was caused by differential weathering along joints and foliation parallel joints. Additional excavation was needed in these areas to remove the more weathered material near the joints.