GEOTECHNICAL AND GEOPHYSICAL INVESTIGATION - LAKE TOWNSEND DAM REPLACEMENT PROJECT, GREENSBORO, NC

In 1966, the City of Greensboro (City) had the Lake Townsend Dam constructed to augment the City's available water supply. Lake Townsend Dam consists of a gated concrete spillway and integral intake/pump station flanked by earth embankments. Prior to 1980, major concrete elements of the dam, particularly the concrete spillway piers, began exhibiting cracking. The City attempted repairs of these areas to prevent further cracking and concrete deterioration without success. An investigation completed in 2006 revealed that the cracking distress was caused by alkali-silica reactivity (ASR) in the hardened concrete of the spillway and that dam rehabilitation and/or replacement was needed. It was determined that a replacement dam had to be constructed downstream of the existing dam, using the existing dam as a coffer dam during construction.

An extensive geological and geotechnical characterization of the site was conducted. Field explorations were performed to evaluate the performance of the existing dam as a hydraulic barrier, develop an understanding of the complex foundation conditions, and understand how site development, grading, and operations could impact dam replacement. Field exploration techniques used for the dam included:

Test Borings in soil and rock

Six-inch diameter concrete cores through the concrete spillway

Vibrating wire piezometers to measure water pressure upstream and downstream of seepage cut-offs

Cone Penetration Testing

Seismic Refraction

Ground Penetrating Radar (GPR), and

Test pits

The field explorations affected the alternative selected and required detailed analysis of the complex foundation conditions of the existing and new dam locations.