Last winter I had a small project making sure some piles were embedded deeply enough for a dock and landing at a small inlet off of Hood Canal on the east side of the Olympic Peninsula. (Hood Canal is named a canal but is a natural body of water formed during the last glacial period). I was not involved in the initial design or scoping of this project, but a clever engineer placed a note on the design drawings that the pile depth should be approved at the time of pile driving - passing responsibility to whoever took on the oversite.
One of the permit conditions was that the piles were to be driven using a vibratory pile driver.
Driving a steel pile with a vibratory driver using a crane on a barge
This permit condition is the result of studies on water noise impacts to fish and sea mammals. The more typical hammer pile driving can generate intense noise pulses in the water that are intense enough to tear fish bladders and destroy the hearing of fish (Laughlin WSDOT, 2005 and 2010). Lots of piles are driven into water for docks and bridges. Hence, this issue is posing a challenge for in water works - think ferry docks, bridges, and marinas. Washington State Department of Transportation has studies using bubble curtains around piles to dampen noise with some success, but as always "more study" is needed. Lots of physics, biology and acoustics; reading the studies is a good brain work out and certainly a regulatory challenge.
Studies on hammer driving which is basically dropping a big weight on top of the pile to drive the pile versus vibratory driving indicate that the sound intensity and pulses are substantially lower for vibratory pile driving. Hence, the condition my client had on pile driving. The vibratory pile driving basically shakes the pile through the sediment. I was surprised how well it worked, but we were unable to drive through the old log road that had been constructed along the shoreline without moving the pile locations slightly.
Steel piles prior to dock and landing reconstruction. The two sheds are 100+ year old structures from old fishing days. Grassy area at the base of the slope was an old road bed as tide lands were the first "highways".
Hi Dan, I think we are the only company in Whatcom county with a vibratory pile driver.
ReplyDeleteIf you ever want to see the work done locally let me know.
Ours is mounted on an excavator that travels around by barge on Lake Whatcom or is trucked to sites like Blaine where we just finished driving sheet pile on a steep slope.
I agree with you about issues with drive conditions at different sites. I last year I shot many piles into an alluvial fan on South Bay in Lake Whatcom. Thousands of years of silt created a need for deeper than forcasted pile lengths. The real issue were the entombed objects in the alluvial fan. Large cedar and doug fir logs ( greater than 36" diameters )were encountered and set the pile placement off by up to 8 inches on some piles. We overcame the majority by using the excavator arm to whrench the piles to the side and under the logs in many cases. Our goal was to liquify the soil next to the artifacts in the alluvial fan and to nudge them through the soil a few inches by using the pile as a lever.
It was very time consuming but in the end we prevailed.
My crew recently drove the Pattle Point piles and encountered hard driving conditions. We hit refusal using our pnuematic pile hammer.
I carry three different hammers with me for different types of conditions but I do love the vibro as it is much easier to be around and easier to pull piles with. The down side is it is much more expensive to maintain.
I thought I would let you know that are in the area if you ever have any question on these subjects as we drive piles and build based out of Lake Whatcom using vibros and impact hammers.