7 minute read
Future Foundations #6
Fairview Ave. N. Bridge Replacement Project
Large Diameter Helical Piles and Innovative Equipment Collabortion Win Contract
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A joint effort between material supplier and equipment supplier breaks barriersand records and drives innovation in helical pile installation.
When there is no available technology to meet the project needs within environmental limits or criteria... When there is no design guide for the proposed solution ... When there is no precedent project or model specification to put in front of the governing body for approval of the solution...
Marine Project. Noise Limitations. Vibration Limitations.
A replacement for the Fairview Ave North Bridge was designed with 8’ Drilled Shafts into the bed of Lake Union in Seattle. A working platform would be necessary for the crane to install the shafts as designed. A temporary trestle structure with piles to provide the necessary service load at 500 kips per pile was designed. There could be no impact hammer used due to the environmental restrictions in place to protect Lake Union’s water mammals and fish. Additionally, vibration limits imposed by adjacent structures also played into the limitations of installation options.
Pileworks, the Magnolia, TX steel pile manufacturer knew that large diameter helical piles could supply the bearing capacity and the vibration of installation would be minimal, and noise relative to other installation would be greatly reduced.
Working backwards, the potential solution, large diameter helical piles would need to be designedto support an ultimate axial load of 750 kips. The piles would be installed to a depth where the alluvial sands were present and would achieve these loads. With driven piles of course, the blow counts would determine the capacity of each pile. Determining the load capacity of helical piles is somewhat different. With helical piles the torsional resistance of the pile during advancement is measured by the torque exerted on the member via the drill head, torque data at pile toe creates a base line for an applied Kt value to be assigned, which is used to determine the axial load the pile has achieved.
The helical pile industry has limited empirical information on large shafts and a fair bit of conservatism must be used when determining the theoretical performance. Applying a conservative Kt value, relying heavily on subsurface data, confirming the ability to place the pile toe into the denser strata are precursory steps used to determine that the performance of the pile could achieve the required design load.” Fenwick continued, “As for as a top drive rotary head capable of delivering over 350,000+ ft-lbs while resting on top of an 80’ section of pile, I don’t think a configuration of mast and rotary like this is readily available and combining existing equipment to do so will be necessary”.
Then Fenwick recommended a drill motor manufactured by Eskridge in Kansas City capable of achieving the required 375,000 ft-lb of torque necessary to install these large diameter helicals....
Enter APE...
At the time, APE had the HD 200 as its largest HD Driver and one of the largest, most powerful helical drills on the market. The internal mechanics of the drill are designed to support the rotation of the 200K ft-lb drive motor and the 200 as built could not support a 375K ft-lb drill motor. APE had to design a new drill system on demand. The requirement of nearly twice the torque of the largest drill in the APE HD line required a leader system that would handle the higher forces. The housing was designed to accommodate the larger motor and the internal systems, including hydraulics, filtration systems and X were all beefed up in order to drive that size motor. The setup featured APE’s unique pile centering guide and lofting clamp, which enabled the operator to get hold of the pile at ground level, loft the pile over 80 feet high, then lower the drive extension on to the pile engaging the drive. Getting on to a long pile this way is considerably faster than traditional methods, thereby dramatically increase the production rate of the project.
The APE C15 hydraulic power unit was setup to provide power to all of the auxiliary functions in addition to the main driver which featured infinitely variable speed control from zero to full, in both forward and reverse.
The pile installation was monitored and recorded to provide the drive torque and depth data for each pile.
As APE innovated in the background, Pileworks along with Atlas Engineering proceeded to approach the project owner, the City of Seattle, and the project GM, Orion, with this solution.
Typically, innovation is met with resistance. With no precedent, no model specification for helical piles of this size, no equipment, no design standard, this was going to be an uphill climb. “It’s never been done before “ is not something that owners and GC’s generally like to hear.
However opportunity often is disguised as a challenge. When all bids failed in Orion’s initial bid round due to the vibration requirements, the door opened for helical pile solution.
With 2nd round bidding in full motion Helical pile started to show great promise to mitigate some of the project limitations that rendered driven pile impractical for the Fairview site, all contractors bidding in this round started to garner as much information as possible to develop a design and cost construct that would equal an award. Orion Marine emerged successful upon award date and the daunting task of verifying what was proposed in the second round will meet the job requirements. A key to pulling this off was attributed to having an aligned design team in place, that being the Contractor, Engineering Firm, Material Supplier and Equipment Manufacturer all working to facilitate an alternate foundation system, great things are possible. As Matt Fenwick stated, “It truly is the holy trinity of design when you can get this many different groups huddled up to accomplish a common goal and it generally equals very quick results.” Riding on the back of an environmentally reduced design concept and installation approach, the proposed helical design chosen was approved by the city in under two months allowing for manufacturing of the equipment and materials to take place in the interim.
Helicals had been presented to the city in advance of the re-bid by multiple contractors, this was in part by Pileworks and other helical manufacturers seeking to get their products on the job and proposing a wide range of potential shaft sizes that would replace the current design with a 4:1 ratio, not much of a brilliant part to incorporate?
Reference material for a 22’ helical pile is not in existence as of the time this solution is being tendered. The 2019 DFI (Deep foundations Institute) Helical Foundations Design Guide presented allowable stress design and conventional correlation between torque and capacity (based on H. Perko 2009). Although the 2019 torque correlation data consider no pile shafts beyond a diameter of 10 inches. Would it be scalable?
2 dynamic load tests and 1 static load test were conducted. The static load test at 12 days following install needed to be stopped prior to failure due to the bending capacity of the cap beam was exceeded when the test reached 800 kips.
The Dynamic tests were performed at 3 days and 13 days after setup with a 13.3 kip drop hammer. Conventional pile gauges were used to measure forces and velocities owing to the piles similarity to a standard pipe pile. At the maximum blow recorded on the 13 day dynamic load test, the maximum case method capacity for the blow was 927 kips.
The noise test found that the installation of the pile measured less than the noise generated from the testing inspectors outboard motor, and equal to the measurements recorded from the General Trestle work above.
