Concrete seawall/I-wall

[DaveVikingPE]

I posted this on the Coastal Engineering Forum, but I'd like to run it by the straight structural gang...

I have an instance where a vertical concrete wall will extend at least six feet above grade (landward side; the mudline on the seward side is about 10 feet below landward grade, thus the wall is about 16 feet high) and will be subject to direct wave impacts. The concrete wall is to be no greater than two feet thick. The return period is a 100-year storm.

The wave force, computed by the Minikin method, has a resultant of 69 kips/ft of wall. The location of the force's resultant is 14 feet above the mudline.

Constraints: the seawall is to be a vertical bulkhead-type wall with minimal excavation allowed; that is, it's not allowed to be a T-wall. NB: scour protection will be incorporated into the design.

Idea: Make it an I-wall.

Questions: OK, so it's an I-wall. What's the required section modulus of my steel that extends into the ground forming the "I"? Etc.

Other info: I am of the mind that the configuration desired is impossible. What do you folks think?

 

[austim]

DaveViking,

I don't consider your suggested configuration to be impossible. In fact, structurally your completed wall would be identical to thousands of run-of-the mill sheetpiled walls.

Just follow the standard texts for sheet piling design, and they will give you the design moments involved (including the required depth of penetration below the mudline).

There is a basic question of structural concept that you should sort out for yourself - will your wall be a simple free cantilever or will you provide anchor ties at the top? Depending on the soil profile, you could well find that a the depth of penetration required to fix a free cantilever will be impractical, and that you cannot avoid upper level anchors.

I wouldn't be too quick to assume that a simple cantilevered wall would involve less excavation than a T-wall. The BSP pocket book in its section on cofferdams shows typical geometries with embedment up to twice the height above mudline. So with very soft soils you might possibly need to excavate a trench up to 30 feet below mudline.

What consideration have you given to the question of suitable construction methods? If you have very soft soils below mudline, then construction of a free cantilevered concrete wall might be hugely impractical (with a deep trench requiring its own support). Even an anchored wall could be difficult.

I would be seriously questioning the mandated choice of material in the first place. My preferred choice would be a classic steel sheet-piled wall, which would really involve minimum excavation (ie none at all). It should be simple to construct (unless you have boulders or other obstructions to pile driving).

As in most seawall/wharf projects, it all depends on the soils, doesn't it?

 

[DaveVikingPE]

Thanks, austim!

More info:

The big problem is that a substantial portion of the wall is above grade, hence cantilevered and is subject to direct wave impacts (no buttresses on the landward side are allowed, either). Also, the scour protection is to be at a minimum so that the major "aesthetic" feature of the structure is the sheet pile rather than rip-rap. Thus, the wall itself is supposed to take all the forces.

The moment at the mudline due to the wave impact force is about 12,000 k-in/ft of wall. This is a pretty big moment, though it can be handled by a variety of larger steel sections. The original concept involved simply pouring a 2-ft thick wall behind a sheet pile wall; the wall wouldn't extend below the mudline. The moment at the toe of the wall/mudline, however, was larger than could be handled by the sheet pile originally proposed. (I didn't come up with the concept, I was tasked with running the numbers.)

The next concept (again, I didn't come up with this) was an "I-wall/soldier pile wall" comprising the poured wall as above but this time somehow connected to pile piles or caissons embedded into the soil and spaced economically. The problem with this one was that the pipe piles are fine if they're spaced at 1-ft on-center, that is, next to each other; a 10-ft spacing was desired...

My gut instinct is that a vertical sheet pile wall is not the appropriate type of structure given the environment (long fetch, big wave forces, etc.). I'm of the mind that a large, curved concrete seawall is the best, most economical structure for the area. That, or go with the sheet pile and tons and tons of rip-rap.

 

[dlew]

DaveViking,

Many years ago I had to survey 5-miles of seawalls on the Florida East coast after a strong hurricane passed through the area. The seawalls were of steel, prestressed concrete or reinforced concrete sheet piling. All the steel sheet piling walls held in pretty good condition. Most of the concrete walls (prestressed concrete sheet piling or reinforced concrete slab with king piles) were badly damaged. The age of the concrete walls was an important factor in the amount of damage received. Fifteen year seemed to have been the useful life of the concrete walls. Most of the failures included the lost of sand from behind the wall. Once the soil was gone, the concrete could not resist the pounding of the waves. The lowering of the "sandline" by erosion was also an important factor.

The problem I see with the original concept is that steel sheet piling is supposed to deform to reduced the soil pressures behind the wall. If the steel sheet piling deforms, the rigid concrete wall attached to its front will trend to crack, and would not last too long.

I agree with you that the I-wall is not the solution for the dimensions of your wall.

A massive curved concrete wall that could dissipate the wave energy would be a good structural solution, althought expensive.

If that were my seawall and money, I would use a steel sheet piling wall, and hire an architect with good artistic sense to make it aesthetic.

Just my two-cents worth

Regards

 

[DaveVikingPE]

Austim and dlew,

Thanks again for your considerations!

This project has an engineering design team of two people: myself as the structural engineer, and a coastal engineer. The effort is overseen (not "supervised&quot by a senior coastal engineer with about 35 years of experience.

Yesterday the coastal engineer asked me how things were going and I explained that I had spent half the day proving on paper that the concept, from a strctual enginering perspective, wouldn't work. We discussed the wave forces she had calculated and I asked: "so, what do you think of the Minikin method [see the USACE Shore Protection Manual] for computing wave forces?" This was the method used and is wholly appropriate, albeit very conservative as it results in very high wave loads. The coastal engineer, who actually grew up in the area explained that she, in fact grew up in the area, and is in aggrement with the locals that, well, they simply don't recall (even elderly folks there) waves being a problem.

Now, to digress a bit... If engineering judgement reveals that the wave forces are too conservative (and if I get it in writing from the Senior Coastal Engineer - a repsected P.E. with more experience than I'll ever have at computing wave forces and designing successful - i.e., they've been built and have performed beyond their design capcaity, etc. - coastal projects) the structural requiremenst will change, the moment will be greatly reduced and this wraps up the issue and I can get on with designing something for construction rather than "negatively" designing something.

Thoughts? Thanks again!