Vibratory Caisson: A Case Study in the Southwestern Deserttsdos.org/media/presentation2019/Farahani,...

SEPTEMBER 4 - 6, 2019

Vibratory Caisson: A Case Studyin the Southwestern Desert

By Majid Farahani, P.E.Leidos Engineering, LLC


INTRODUCTION• Tucson Electric Power (TEP) is a municipal utility located in Tucson, AZ

• TEP operates 46, 138, 345 and 500 kV overhead lines

• Maintenance had identified foundation of structure#30 on TEP’s 138kV Transmission Line-114 to be out of alignment

• Closer inspection showed the piles below the reinforced concrete cap to be twisted and bent

• The decision was made to replace this structure as soon as possible.

• The structure was placed on order and one of the options considered was use of a vibratory caisson. No one had tried in the Southwest at that point.

• The intent of this presentation is to share that experience with you.


INTRODUCTION

What is a Vibratory Caisson?A vibratory caisson is a hollow end tubular steel shaft with typically zero taper. The cross section may be round or polygonal. A hydraulic hammer mounted on top of it allows it to be accelerate/vibrate the soil in contact with the caisson. The soil loses its axial capacity and effectively becomes liquefied. This allows the caisson to drop through the soil. Once the hammer is turned off, the soil returns to its post vibration natural state and foundation is ready for use.


INTRODUCTIONThere are 3 types of common connections available between the pole and caisson:

• Base-Plated/Flanged Connection

• Drop-in Type

• Slip-Jointed Connection


DESIGN CONSIDERATIONSGeotechnical Investigation• 1 boring to the depth of 86.5 ft

• No water encountered at the time of boring

• Wash protected by soil-cemented banks which rose to 10 ft above the bottom of wash at the time

• Soil was sand & gravel to gravelly sand with varying layers of cemented clay in between

• At 30 ft N=94, at 45 ft N=59 and at 50 ftN=81


DESIGN CONSIDERATIONSFoundation Design• No specific guide or standard for design of vibratory caissons in transmission

lines• As of early 2018, CEATI was attempting to form a study group on Vibratory

Caisson Design• Approach: no different than other deep foundations• Impact of vibration on coefficient of friction• Soil confined on the inside of caissonGeotechnical Recommendation:

22 ft below the scour depth with 0.5 degree rotation15 ft below the scour depth with 1.5 degree rotation(both per Broms method*)

* Broms, Bengt B. (1964). “Lateral Resistance of Piles in Cohesion-less Soils.” Journal of The Soil Mechanics and Foundation Division, Proceedings of The American Society of Civil Engineers. Vol. 90, Issue 3, pp. 123–156.


DESIGN CONSIDERATIONS

Structural Aspects of Design• Matching resulting

moment capacity above and below the scour depth

• Connection plates


DESIGN CONSIDERATIONSOther Considerations –

Scour• 100-year flow

• Flood control use permit

• Scour depth changed from 29 ft to 22 ft with newer velocity data

• F.S. = 1.3



Rotational Movement of Caisson• Oversizing the holes in base plate

• Rotate the pole above its slip joint

• Drilling new holes for line post insulators

• Slotting the holes in the base plate



Impact by Floating Objects and Corrosion• Top 30 ft of caisson out of 7/8-in plate

• The balance was 1/2-in plate

• The caisson was hot dipped galvanized

• The caisson was coated with CorroCote II Classic, both inside and outside to a thickness of 35 mils, minimum average


PUBLIC OUTREACH

• Vibration Impact: Residents adjacent to the area were placed on notice, however, the vibration could not be felt 40-50 feet away from the location

• Noise Impact: Most of the noise generated was muffled by the heavy traffic in adjacent River Avenue


CONSTRUCTION SEQUENCE

• Environmental restrictions: drive over riverbed• Hammer and its supervisor rented by

contractor• Excavation of 5 ft deep hole for alignment• Boulder encountered• It took 18 minutes to drive the caisson 45 ft

deep



Cost Comparison• Vibratory Caisson $132,400 Total 1-Day Installation

• R.C. foundation $ 142,500 Total 4-Days installation and 28 days to cure

• 250 ton-ft hammer at $4,000

• 600 ton-ft hammer at $600 more!



Intended Versus Actual

• The objective was to drive the caisson 50 feet deep

• The caisson was vibrated in 45 feet before we hit a layer that we could not break through

• Design required 44 feet of burial depth


LESSONS LEARNED/BENEFITS/DISADVANTAGES

Lessons Learned• Below grade coating application – special attention

• Additional soil borings to develop a better profile

• Load case combinations – don’t need worst case river flow + worst case overhead simultaneously

• Hammer size selection

• Take noise level readings before and during the installation



Benefits• Quick installation: don’t need to wait for concrete to

cure; usable the moment you turn off the hammer

• Minimal disturbance: no excavation; no spills to remove; reduced environmental impact

• Helicopter lift, if access is an issue

• Cost competitive depending on proximity to hammers



Disadvantages• It falls in a unique construction category, so not every

contractor has or has dealt with vibratory hammers

• Need more design guidelines – hopefully CEATIprogram will provide this

• Not sufficient guidelines on selection of hammer

• No data on any that have been installed and removed after 30 years


Questions and Open Discussion

Please contact:Majid Farahani, P.E.Sr. Transmission Line Engineering Manager(312) [email protected]

mailto:[email protected]

Vibratory Caisson: A Case Study in the Southwestern Deserttsdos.org/media/presentation2019/Farahani,...

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