How are large wooden piers, like the Santa Monica pier, maintained? I imagine that the wooden piles are treated to last for many decades, but surely they must eventually rot or erode, or might get damaged by storms, wildlife, or vandals. How do they replace a pile that’s still holding up the pier, or fit a new one into a gap that’s been left by a broken one? Is this something that can be done from underneath, or do they have to (partially) disassemble the upper structure first in order to drive a new pile down into the sea bed?
Piles in marine structures decay mainly in the inter-tidal zone that would be 1-2 metres of their total length. The constant wetting and drying allows for far more organic growth and decay to take place than in the more constant conditions further down. A pile that is almost rotted through at water level can - if the right timber was chosen, or is properly treated - be completely sound in the remainder of its length above and below the decay.
The simplest fix is to remove the damaged section and scarf in new timber or you can contain the damaged section in a sleeve and pump in various chemical gunks and epoxies to fill it. Both of these can be achieved without having to dismantle the deck. Less aesthetic is to attach more timber to span the joint and connect the sound upper and lower parts of the piles. These transfer load and can hold until a major refurbishment allows better access.
Wharves and piers are engineered structures . The spiderweb of cross-bracing below the deck transfers loads so an individual pile can fail without compromising overall strength and cohesion. A 100 year+ design life is normal, and only the failed sections really need to be maintained at frequent intervals.
Thanks for the explanation! I do have one question concerning this part:
I’m not familiar with the word “scarf” in this context. Could you elaborate on what this means?
I’m not an engineer, but I am a surfer. I while away a lot of hours looking at the understructure of piers and to these untrained eyes, it looks like there is a ton of redundancy built into the design. Removing one (or even a dozen) pilings for maintenance won’t make a difference in the stability unless it coincides with abnormally large surf and winds.
I’m not @Banksiaman but look at this. Its a standard woodworking technique:
In a pier the goal isn’t aesthetic but rather structural. With a properly cut scarf, the interface surface area to be glued or bolted is much larger, leading to a lower load per unit area and a joint stronger than the surrounding good material.
To me, “Scarf” as a verb means to eat something rapidly.
Treated wood, certain species of wood last longer.
Also to repair they can use floating thingys to hold stuff up.
At the lake near me, they had drained it to fix a bridge(it’s a smallish manmade lake).
I was alarmed to see some of the pier pilings at docks were very thin in places. Then saw where they had to have chainsaws to cut thru the thin spots to scarf pieces in or replace the whole piling.
So yeah the wood they use is pretty tough. It’s not the stuff you buy from the lumber yard.
My dock(small) on our pond has old creasoted telephone poles as piers. They aren’t thin as far down as we see when the pond is low. Almost 20 years old.
Ours is 30 years old. We recently had the top structure rebuilt, but the piers were solid.
What about the lifetime of boardwalks? Usually not affected by water (unlike piers) , but ocean-side salt spray could be a factor. Are entire sections replaced periodically, or is it just board by board as they start to rot or break apart? Maintenance must be continuous on extremely long ones such as in Atlantic City or Coney Island.
Fresh water piers are treated wood that may never get repaired. Boaters should be aware of the danger of old partially submerged piers near shore in bodies of fresh water. Pulling out old piers doesn’t work well since they are pounded into the river or lake bottom and can rot anywhere so attempting to pulling them out can break them below the water line. There’s an assumption they’ll rot away over time unlike in saltwater which helps preserve them.
They’re in a completely different pier group and they don’t suffer from piles to the same extent.
The lifespan of boardwalks [or any timber structure] depends entirely on the characteristics of the species chosen. Pine flooring you might use on your back porch will fare poorly in open maritime conditions. A good maritime resistant hardwood, like turpentine (Syncarpia), which is the go-to species in Australia, will last decades or more. In that circumstance you will only need to replace the odd board where a knot may have allowed internal decay to start, or where warping becomes a trip hazard.
It may be possible to periodically replace a section of deck every year over a period as an incremental upgrade. Fully closing a structure to take off the entire deck can be a major production, and best justified when there are other things like replacement of under-deck supports that can be achieved at the same time.
Nicely done!
Much appreciated 