Investigation and Remediation for Existing 300 foot Tall Communication Towers. Presented at Annual Kansas City Geotechnical Conference April 25, 2013

Transcription

1 Investigation and Remediation for Existing 300 foot Tall Communication Towers Presented at Annual Kansas City Geotechnical Conference April 25, 2013

2 Presentation Topics Objective of Investigation Methodology Results

3 Objective Determine length of drilled shaft foundations supporting existing communication towers This information was to be used to determine viability for installing more equipment (co-locate)

4 Project Site

5 Project Site Cont.

6 Tower Overview 275 foot tall, 3 leg tower Each leg is supported by drilled shaft foundations, 4 foot diameter Total length of each drilled shaft was unknown FTC was contacted to non-destructive determine shaft length under each tower leg

7 Data Collection Program

8 Foundation Length Determination Options Core one or more of the shafts Install borehole casing adjacent to one or more shafts for performing geophysical surveys Perform sonic echo/impulse response testing Last option preferred due to site access constraints and was most cost effective

9 Sonic Echo/Impulse Response Testing Requires access to top of shaft Mounting of accelerometers Striking top of shaft with instrumented hammer

10 SE/IR Site

11 SE/IR Site

12 SE Data 110 th and I-70 Shaft 1 - Apparent toe reflection at 18 feet (SE data, accelerometer).

13 IR Data 110 th and I-70 Shaft 1 - Apparent toe indication at 18 feet (IR data, accelerometer)

14 SE Data 110 th and I-70 Shaft 2 - Apparent toe reflection at 18 feet (SE data, accelerometer).

15 IR Data 110 th and I-70 Shaft 2 - Apparent toe indication at 18 feet (IR data, accelerometer)

16 SE Data 110 th and I-70 Shaft 3 - Apparent toe reflection at 18 feet (SE data, accelerometer).

17 SE Data 11 th and Steele Shaft 1 - Apparent toe reflection at 12.5 feet with bulb at 8 feet (SE data, accelerometer).

18 Summary of Results Drilled shaft foundations were unexpectedly short Next step was to perform design review to determine adequacy of foundations for existing and additional loading

19 BPU Performed Additional Archive Search Obtained original geotechnical reports for both sites Reports included subsurface information from borings and lab tests and recommended design basis for shafts Calculations were found for original tower designs from 1989

20 Original Shaft Design 334 kips compression, 300 kips uplift, 39 kips shear Design basis was for 4 foot diameter, 8 foot long shaft over 6.5 foot long bell (11 feet diameter at base Geotechnical uplift resistance calculated as weight of foundation, weight of soil wedge and cohesion along presumed cylindrical surface

21 Revised Loading 377 kips compression, 300 kips uplift, 45kips shear (increase in compression and shear) Geotechnical uplift resistance calculated as weight of foundation, weight of soil wedge and cohesion along presumed cylindrical surface

22 Soil Profile Stiff Clays to 30 feet where sandstone bedrock was encountered (110 th and I-70 site). Stiff Clays to 13 feet where limestone bedrock was encountered (11 th and Steele site).

23 Design Review Results Drilled shaft foundations were not adequate to support current or anticipated design loading Needed to retrofit drilled shaft foundations to provide additional capacity (100 kips in compression and 182 kips in uplift per shaft) Given proximity to underlying bedrock, we recommended use of rock anchors tied to each drilled shaft

24 Conclusions Excellent SE/IR data results were obtained Original shaft design was unconservative in our opinion relative to reliability issues (construction of bells, reinforcement of bells, potential for groundwater rise and desiccation cracking

25 Acknowledgments Kleinfelder initial referral and laboratory testing Palmerton and Parrish for Investigative Drilling Lanny Uden Kansas City Board of Public Utilities Jeff Hill Hayward Baker

26 Questions?

27 Contact Information Casey Jones, P.E, P.G