GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 101-5
Presentation Time: 9:00 AM-6:30 PM


LEDBETTER FERRILL, Nathaniel, Department of Geosciences, Trinity University, San Antonio, TX 78240

This project investigates influences of mechanical layering and fractures on rockfall in Canyon Lake spillway, central Texas. In 2002, 86 cm of rain fell in 8-days across the Guadalupe River basin causing Canyon Lake to fill and overtop the emergency spillway. The flood incised up to 12 m downward into the Glen Rose Formation, and weathering and rockfall continue to shape the gorge. Recent rockfalls of a 7 m high cliff were documented by field observations and historical aerial imagery. Fracture orientations and spacings were measured using a compass, clinometer, and tape measure, and mechanical stratigraphy was characterized using an N-type Schmidt hammer to develop a mechanical rebound profile. Results show that the dominant capping limestone layer exhibits two systematic orthogonal near-vertical fracture sets, with the following mean strikes and standard deviations: Set 1 = 22.8° ± 5.3° and Set 2 = 110.8° ± 7.9°. Set 1 fracture spacings range from 48 cm to 296 cm, averaging 185.3 cm. Set 2 fracture spacings are 32 cm to 233 cm, averaging 130.1 cm. Rebound measurements show interlayered competent (limestone) and incompetent (shale, marl, argillaceous limestone) typical of the Upper Glen Rose Formation. Shale, marl, and argillaceous limestone are less competent due to elevated clay content – percent clay has a negative correlation with rebound. Two rockfalls occurred along the 7 m cliff from 2012 through 2016. Rockfall 1 occurred between September 5, 2012 and February 16, 2013, and produced 15 m maximum runout, with 10 m runout of the massive limestone bed coming to rest overturned. Rockfall 2 occurred between May 5, 2016 and October 20, 2016, with 17 m maximum runout, including 14 m runout of the massive limestone bed which came to rest overturned. Both rockfalls released along intersecting pre-existing fracture surfaces in limestone above more easily eroded incompetent beds, and involved both wedge and toppling failure. Fracture spacing controlled block size, with mechanically weak layers destabilizing the cliff section. Planning road cuts through mechanically layered rock and assessing rockfall hazard can directly benefit from this research. The National Highway Institute and state rockfall hazard rating systems do not fully consider fracture spacing, which is one of the biggest factors for toppling and wedge failure.