By: Alexander D. Krumenacher, E.I.T
The Texas Hill Country covers about 25 counties in Central and South Texas, including the western part of Austin, and is known for its picturesque terrain and popular geologic landmarks including Enchanted Rock, the Colorado River, and Mount Bonnell. The scenic landscape has also made West Austin a desirable area for development. Rapid growth in recent years has created the need to increase capacity and reliability of the electricity transmission infrastructure – which, within this geologically complex and environmentally sensitive region, is easier said than done.
POWER Engineers, Inc. called upon Terracon to provide geotechnical engineering services for the 14.5-mile reconstruction of the Lower Colorado River Authority’s (LCRA) T-160 McNeil to Marshall Ford substation electrical transmission line. The majority of the alignment traverses the rugged Hill Country region northwest of Austin. This particular area is dissected into many closely spaced crevices and gorges by erosion from the Colorado River and contains slopes ranging from 5 to over 15 percent with many steeper bluffs. Consequently, Terracon faced drilling in areas with challenging access and environmental conditions and near existing transmission lines and support structures.
Managing geologic, environmental priorities
The Hill Country’s limestone formations, and primarily the Edwards Formation, commonly contain karst features, which are formed by the dissolution of soluble rocks and include solution channels, vugular zones (small cavities in the rock), voids, caves, and aquifers – all of which present engineering and construction challenges in the form of questionable subgrade for foundation support. These same features are also the habitat of several wildlife species unique to the Hill Country, including the Barton Springs, Austin Blind, and Jollyville Plateau salamander species and the Golden Cheek Warbler bird species, the last three of which are currently listed as threatened or endangered under the U.S. Endangered Species Act. Conserving these important habitats added yet another level of complexity for Terracon’s drilling operation.
Terracon worked closely with POWER, LCRA, the environmental firm ZARA, and the entities managing the various preserve areas along the alignment to develop a drilling program that would yield quality engineering data with limited disturbance to the habitats of any of the threatened species. The team decided that Terracon would drill a “probe” hole at each structure along the alignment to explore for the presence of karst features exhibiting the potential to serve as a habitat for the endangered salamander species. Drilling was immediately paused upon encountering asoft or highly vugular zone, a potential void within the limestone, or loss of air/water return to the surface. The probe hole was then visually inspected using a camera borehole-logging system operated by ZARA to determine if a void existed and, if so, what the vertical and lateral extents were. If a karst feature with potential to serve as a habitat was encountered, ZARA team members determined whether or not to proceed with drilling. At some locations, drilling was halted while ZARA performed further environmental research by placing traps in the borehole and leaving them for several days to determine if wildlife was present and, if so, what types. To minimize disruption to the Golden Cheek Warbler, drilling was suspended during its nesting season, between late February and September.
Sensitive solutions, necessary data
No engineering data other than basic material classification could be collected from the probe drilling, so the team drilled geotechnical borings at selected structures to gather the geotechnical data necessary for foundation design. Standard geotechnical drilling methods in limestone require the use of water or weighted slurry to remove cuttings from the borehole, allowing for deeper borings. For those drilled in potential habitat areas, Terracon used only untreated water pumped from the nearby Colorado River in an effort to minimize alterations to the aquifer water quality, which could potentially affect the salamander species.
In addition, air rotary drilling methods were used whenever possible. Air rotary drilling techniques use a drill bit to grind up the soil/rock to fragments small enough such that, when compressed air is forced into the bottom of the hole, the air will circulate back to the surface and carry the cuttings with it. One shortcoming to using this method in rock formations containing vuggy zones and voids, such as the Edwards Formation, is that the air can’t circulate back to the surface with the same pressure it went in with because the karst features allow the air to flow laterally through the rock formation preventing cuttings from being carried back to the surface. When cuttings can’t be adequately removed from the borehole, a core barrel or drill pipe can become lodged in the hole and snap.
Successful outcomes, minimal impacts
In all, the Terracon team drilled a total of 22 geotechnical borings along the alignment up to depths of 40 feet, and 63 probe holes were drilled up to depths of 20 feet, for a total of 2,032 feet of drilling – all within budget and with minimal impacts to the unique Hill Country landscape and the threatened and endangered species living in the work zone. Despite the numerous challenges presented by the T-160 project, the team delivered accurate and valuable information to our client through intelligent use of various drilling techniques, application of experience and knowledge gained on previous similar projects, and close coordination with all members of the design team.