Cover requirements for steel piles socketed into rock?

[unners]

Can anyone help point me towards a good resource regarding grout cover requirements for steel piles that are grouted into rock? The original design consists of outer casing piles driven to refusal into rock, and then an inner 'anchor' pile grouted into a socket that has been drilled (or down-the-hole hammered) another few meters into the rock. I am comfortable with assessing the exposure classification for the casing pile but can't find a good reference on what cover is required for the socketed section. The grout is cementitious, not epoxy.
I would think that it would be similar to grouted permanent rock bolts but I can't find much on that either.
Thanks very much in advance. Cheers

 

[saikee119]

There is a free program called Life365 available with which you can calculate the life of your pile. Just search

http://www.life-365.org

and download the software and manual.

The durability of a pile is hinged on how corrosive the environment generally represented by the chloride concentration. Thereafter by how the rebar protected (naked, epoxy coated, stainless steel), cover to the rebar and concrete mix (mostly on how much void or w/c ratio). Cement replacement is used is considered and so any corrosion inhibitor applied.

The life365 has been for some years and there is a ACI committee on it too.

It is a definitive design procedure on the life expectancy on reinforced concrete. I used to program the diffusion theory myself and found good agreement with this program so I have been using for years ever since. Done this with piles too and all sorts of RC infrastructures.

I have seen RC structures that disintegrated after 12 to 15 years and Life365 can model it. So I can recommend that this is not olny mathemically rigorous but has been proven accurate in the field.

Specific to your pile section socketed into rock you have to decide on its chloride concentration and then test different concrete cover to see how long the pile can last. There is a lot of examples available to help you to decide on the chloride concentration. Being socketed into rock your grouted pile section should last longer than the normal pile I would have thought as the section should be sealed when properly constrcuted. This means you are justified to use a chloride concentration lower than expected. Your soil report, ground water analysis and the site proximity to the sea should help you to establish source and concentration of the chloride. The Life365 is the structural calculation to back you up on your decision. In general concrete degraes due to caboration and chloride able diffuse through concrete to reach a concentration level to cause the onset of corrosion. The time taken is the life of your pile.

 

[STrctPono]

I would have to check our office publication of AASHTO to see if there is a section on steel piles socketed into rock and if there is a minimum cover required. 3" is the minimum for cover concrete of drilled shafts. Our DOT requires 4" however. This is grout and not concrete so it has a lower durability. Even with that being said, I don't think that grout is treated differently and 3"-4" would be sufficient

 

[HTURKAK]

The original design consists of outer casing piles driven to refusal into rock, and then an inner 'anchor' pile grouted into a socket that has been drilled (or down-the-hole hammered) another few meters into the rock. I am comfortable with assessing the exposure classification for the casing pile but can't find a good reference on what cover is required for the socketed section. The grout is cementitious, not epoxy.

I want to tell our past experience ; the outer casing pile was structural steel pipe and inner anchor pile was steel H profile and, shear keys provided at the exterior of inner anchor pile and interior of outer pile . The annulus between rock, pile and anchor pile filled with cementitious grout. The exterior pipe pile was PE coated.

I will suggest you to look API RP 2A RECOMMENDED PRACTICE FOR PLANNING, DESIGNING AND CONSTRUCTING FIXED OFFSHORE PLATFORMS.

The same code of practice states ( Unless specified otherwise by the designer, the systems for corrosion protection should be designed in accordance with NACE RP-01-76.)

If you provide the type of corrosion protection for the outer pile , depth of rock , water depth etc.. you may get better responses.
If you put this thread to Off-shore str. eng forum , you may get better responses.

 

[unners]

Thanks for the replies. I've downloaded the life-365 program. It looks like a reasonable approach for typical applications although like any 'black box' type program it would only be as good as the input information. In my case, I have no way of knowing the chloride concentration as I'm in solid dolerite rock.

To provide some more background, these piles are for a marine structure. The outer pile is protected from corrosion using a HDPE sleeve. There is only 1-2m of soft sediment above the hard rock layer, and the outer piles have only achieved around 0.5m embedment into the rock before refusing. I'll look into the API guidelines, thanks Hturkak.

The Contractor has proposed an alternative with a lower cover than was originally specified, and I'd just like some good reference material to justify our initial design!

Cheers

 

[Settingsun]

Is the design case that the sediment is lost to scour so the rock-embedded part of the piles isn't buried? How are you protecting/assessing the outer casing which sounds more exposed that the inner? I assume the HDPE stops at top of rock.

As far as references go, books/standards/Main Roads specifications for bored-cast insitu piles might be relevant. There is the practical aspect of reliably getting the grout in there in somewhat less than ideal circumstances.

I suspect there are many example of piles in similar corrosion exposure conditions with no grout around them and minimal corrosion allowance, without any problems. Bear that in mind when responding as the contractor may bring it up.

 

[unners]

Thanks Steve

We have allowed for scour to rock level although it is unlikely given the water depths and hydrodynamics. The HDPE stops at rock. I'm a bit more comfortable with the design for the outer casing. While the HDPE sleeve remains in place, and sealed at the toe and top, we expect corrosion to be very limited. The steel section is designed with a corrosion allowance based on a compromised sleeve however.

The Main Roads specs are probably the most useful, although they often require cover in the order of 50-75mm. This is what we generally allow (min 65mm) in the splash zone and intuitively something embedded in rock below the surface should see significantly lower corrosion rates than this. We completely agree that QA of the construction process is also a key component.

I'm sure I'll hear all about the many examples too