Variable Piedmont rock conditions in a Washington, DC Water Shaft - PowerPoint PPT Presentation

Variable Piedmont rock conditions in a Washington, DC Water Shaft Geohazards 2018 Steve Mogilnicki – WRA Acknowledgements Corman, DC Water, Davis, Bradshaw

Outline 1. purpose 2. geologic setting 3. methods 4. observations 5. rock reinforcement 6. conclusion

1. purpose C.S.O. Drop Shaft 21 Kennedy Center DC Water Clean Rivers Project

Shaft in rock drill & blast 28 feet diameter 96 feet deep

2. geologic setting Coastal Plain Modified from USGS

Schist

Melange of Rock Creek Shear Zone turbidite sediments metamorphosed into schist USGS Wash. W. quad (1994) fill Fleming, Drake, McCartan

Image from http://www.scotese.com PALEOMAP

3. methods strength & weathering foliation, joints, mineralization spacing/roughness/infill - discontinuities weak zones & faults strike & dip

8 pre-blast visits, Aug. to Oct. 2017

Quadrants Approach Channel A D B C

blast hole drilling

4. observations Quadrant A Foliation dips out of cut Steep joints (2)

largest joint - planar

wedge

Discontinuities Foliation - strike NE, moderate dip SE, spaced 1 – 3 feet, rough Joints - strike often NW, steep dip, spaced ½ - 2 feet, smooth

Quadrant C Foliation dips into cut Joints

Clay on smooth joint

Weak rock geohazard intensely fractured highly weathered

Slickenside on joint fault strike N 15 W dip 70 NE

foliation data strike NE dip SE regional shear

joint data 2 sets NW

5. rock reinforcement – bolts & mesh

filler concrete - rockfall

Shotcrete

6. In conclusion Geologic work aided multidisciplinary drop shaft construction effort