What material for piling marina? electrolytic corrosion? pile strength

[markdj]

Hi, just found this forum and think it's great. Tons of interesting posts.

I am researching what is the best material to use for a pile in a merina? The marina pontoons will be supported by piles and at the moment the existing piles are a combination of rotting wooden poles, I-beams and mild steel poles sunk into drilled holes in the rock and back filled with concrete. One wooden pole is only a stump in concrete and needs replaced. The entrance to the marina is small so a pile driving barge is too large.

Regarding the strength of the pole. We require to drill out or otherwise remove rotting stump, insert new pole and refill with concrete. This needs to be done underwater. Or we could drill a number of holes into rock, make shuttering and fill with concrete with reinforced steel into holes and upstand pile in setting concrete. Any other ideas?

The material options for pole are mild steel coated, galvanised steel pole or stainless steel 316 pole. Surprisingly the SS pole is the cheapest and is one we are considering. Would this corrode quicker than a mild steel pole? Coating a mild steel pole is about 3 times the price of SS pole.

The pole needs to be 7 metres long and there is a rise and fall of tide of around 3.5 metres.

Also, side loading on pole (most at high tide) will be 2 boats windage. IS there a recommended formula for calculating windage in high winds?

We are also looking at the AC earthing on the pontoons. What is the recomended resistance to earth. We are hoping to get down to 2-5 ohms with around 10 copper earth spikes on shore as this has been a problem with the marina. IS using a SS pole going to provide an easy way to earth compared with a galvanised or coated steel pole?

 

[MikeHalloran]

Here in SoFla, given universal presence of termites above the waterline and various nasty fauna below, the universal choice for marine pilings seems to be precast concrete.



Mike Halloran
Pembroke Pines, FL, USA

 

[markdj]

What way is the concrete fixed to the seabed? We can get down to the bedrock as this marina once was a quarry.

 

[4wilmar]

The mild steel pole would rust quickly if the coating is damaged during installation or subsequent use and galvanizing doesn't last very long around salt water. If you do consider precast concrete piles, rebar corrosion will be a problem (particularly in the tidal zone), so I think your best option is the SS 316 pole or pile.

If you can drill large enough holes into the rocky seabed to set the poles/piles in place, you could then pump concrete or grout down the inside of the pile to force the water out and thus bond the pile into the rock.

 

[unclesyd]

Here are several alternatives I've seen being used around this area. There is also another local company that was using a SS fiber reinforced concrete piling, but they don't answer their phone.

http://www.cmilc.com/index.htm

http://www.pearsonpilings.com/m/_general/default.asp

http://www.durablewoods.com

 

[stevenal]

Don't you have mud above the rock? Sounds like a location likely to be anaerobic. I understand SS depends on the presence of O2 to maintain its corrosion resistance, and rots quickly without it. Around here, treated wood and mild steel are used.

Marina problems with AC are usually caused by improperly wired boats. Reducing ground resistance to remote earth is unlikely to help anything.

 

[stevenal]

I had not intended to be the last word here, just asking a question that I thought might be relevant to the experts here and generate further discussion. (corrosion is outside my area of expertise, but inside my area of interest).

AC earthing is something I'm qualified to answer. If further discussion in this area is required, I'd suggest the electric power engineering forum.

 

[cliff011]

The best pile for this application is pre-cast concrete. A reputable piling company could provide you with the pile and would have the ability to auger the rock. A standard size pile is 400mm or 16 odd inches in diameter.

If metal is your choice i would not use S/S for any foundation or marine pile, the reasons are long and many but trust me S/S is not the pile.

 

[markdj]

Thanx for the info. This is an ongoing project which is yet to be carried out so all info is gratefully received. If going for steel poles we would fit anodes to the poles either by steel wire or welded to reduce corrosion. I'm interested in the corrosion of SS as I know that it rusts with pitting if no oxygen which would occur where the pole would be set in concrete in the rock. The mud build up on the seabed is also an issue but these will be cleared first. The problem with concrete is the weight. This is a confined space with only access via one side (other side is the sea). We were also considering a plastic gas pipe filled with concrete when in place but this would require reinforcing I think. Side loading would be up to 2 tons at 3 meters from the top of the pole.

 

[Metalguy]

markdj,
You haven't indicated that cost is a big factor, so why not use a Ti alloy? Great strength/weight ratio and no corrosion concerns.

 

[markdj]

Oops! Yes cost is an issue but if a Ti Alloy is not x10 the pricew it may be considered. What is the cost compared with mild steel?

 

[Metalguy]

Unfortunately I'm not familiar with current prices of either, so you'll have to do the legwork. But keep in mind, as I'm sure the Ti vendors will tell you, that you'll be able to use a considerably smaller and lighter pile if it's a good Ti alloy. They'll also recommend the best alloy for your purpose.