Marco Island Seawalls -a discussion
Click here to see a recently added diagram of Seawalls
on Marco.
Last updated
Friday, July 25, 2008
A friend of mine asked my opinion
on our seawalls. I'm not an expert, but I have an engineering background
and I've studied the subject extensively. I think I have a good handle on
many aspects of the situation. The text below is extracted from an email
exchange. [Italics were added for clarity]
9/20/2003 (updated 2007)
Jose,
I went over and took a look at your seawall. There's plenty of theories on
seawalls. Here's my take about what is going on.
Our walls were put in place in the mid 70's, one of the last areas to be
developed by the Mackle Brothers. [Goldenrod/Capistrano area].
When I bought my lot in 1987 I reasoned that it was good to have the newest
walls with hopefully the latest technology. It turns out that Deltona was
in bankruptcy at the time and may have scrimped on the wall length and perhaps
the concrete and rebar quality. I've been told that our walls are 10'
long, while they are 12 and 14 feet long on other parts of the island.
Typically we have about 3 feet in the sand and 7 feet up to the cap.
Deltona advertised that the walls were designed for a 30 to 35 year lifespan.
Our walls are now 30+ years old. It is not surprising that many are
failing. When walls are replaced the code requires that the new wall
panels be 16 feet long, 5 feet wide and 6 inches thick, as I understand it.
There's 3 possible modes of failure in our seawalls.
1. Tow out at the bottom.
This is mostly a myth, except in areas with extreme current, like on the river. I've talked to workers on the job
repairing seawalls and they say that they almost never see a towed out wall.
Even with only 3 feet in the sand, moving the wall out at the bottom would take
tremendous force. No such force exists.
2. Falling over at the top.
Another myth. For this to happen the tie back rods
need to fail. They are large steel rods that are cast into the cap and
extend back toward the street to a concrete mass (called a deadman) buried in the sand.
Again, this failure is rare. The rods are encased in plastic and rinsed with rain
water so they are not exposed to excessive salt. In addition, for an
entire section of wall to fail at once, multiple tie rods would have to fail at
the same time, an unlikely event, eh?
3. Breaking in the middle. This is the cause of most failures.
Workers on site agree. I've seen it time and time again as I watch walls
being repaired. Here's what happens. A horizontal crack develops
about mid-way up the wall near the barnacle line because of the pressure from
the land behind it. This is the weakest part
of the wall, since it is supported at the top and the bottom, but not in the
middle. Seawater gets in
the crack and attacks the rebar, eventually rusting it through.
At the left is
a picture of a section of rebar
that I removed from a failed wall on Bald Eagle in 2002. The end that is intact was embedded in
the concrete. I chipped off the concrete and sawed off the sample. The rusted end was exposed at the crack. Note that it
is rusted all the way through. Once that happens the wall has considerably reduced
ability to handle pressure from the sand behind the wall (see
force diagram). With the wall
secure at the top and secure at the bottom, it breaks in the middle and the top
portion slides over the bottom part, and into the canal.
The bottom piece tends to stay upright, since it is buried in the sand.
But the upper piece is only held at the top and tends to slide out over the
bottom piece. From the top, it looks like the wall has "towed out", when
in fact the bottom piece is still intact and in place, and the top piece has
slid out over it. Hence the myth in #1 above.
Click the small
picture to see some pictures of a wall that failed. Use your browsers
return button to come back to this page.
Now, with that
said, what about water runoff and your "holes" behind the wall? [Jose was
losing sand through seams in the wall, creating settling and "holes" behind the
wall]
As the man who gave you the quote said, this is a pretty common problem. I
frequently see repair being made as he suggests with Hydraulic Cement (HC), and
no doubt that will at least reduce if not solve the problem of sand leaking
through the wall. However, one school of thought says that it creates
another problem. Water needs to be able to get from the land to the canal.
That includes water from rain and tides. We have about a 3.5 foot tide.
When the tide is up the ground behind the wall is saturated with seawater up to
the high tide mark. As the tide goes out this water needs to get back to
the canal. Deltona understood this. They installed screens on the
land side of the wall at each crack to let the water out but hold the sand back. The Hydraulic Cement tends to restrict this,
holding both water and sand back, creating added
hydrostatic pressure behind the wall. (one cubic
foot of water weighs 62.43 pounds!) Some suggest this increases the likelihood
of an eventual wall failure. Rain has a similar issue. When a house
is built on a lot it makes the problems worse by focusing the water flow at the
sides of the house and near the drains from the lanai. What was once an
even flow across the length of the seawall now becomes a river in certain spots.
With the HC treatment this water will have increased trouble getting to the
canal and will increase the pressure on the wall at certain spots. As we
ride around the canals in the boat I notice that many wall failures seem to
start at the area between two houses.
When walls are replaced they put a vertical screen over the entire length
of the seam between the wall panels on the land side. This serves to let water
out to the canal, but tends to hold the sand back. Deltona did the same
thing, but the screens have deteriorated over time, and in some cases the seams
have opened up as the walls settled. Then they fill the area behind the wall
from top to bottom with small stone to create a "French drain". This
allows water to move freely behind the wall, equalizing the pressure over the
entire length of the wall and allowing the water to drain through the seams and
out of the "weep holes".
The original
French drains tend to get filled in with sand when houses are built on the lots.
The contractor typically puts a thin layer of stone over the sand to give the
appearance of a real French drain. Take a shove to yours and I'll bet
you'll find that the stone isn't very deep. Mine was less than a foot.
One company on Marco offers a very questionable service, in my opinion.
They seal the seams with HC, and put "filters" into the drain holes. This
"repair" targets leakage of sand from behind the wall into the canal. Of
course the filters plug up quickly, so, for a fee, the company will come out
frequently and change them. A nice steady business for them!
Meanwhile, lots of water is trapped behind the wall. Not a good thing for
our old walls.
Another company
offers "sea wall inspections" for a price. Typically they get in the canal
and inspect your seawall, sometimes even using an underwater camera. They
usually say that you have some cracks but that your wall is still standing.
It's unlikely that this is new information to you!? The inspectors cannot
give you any idea how long your wall will last. Wall failures tend to
occur dramatically. You may wake up one morning, especially after a big
rain storm, to find your wall broken and sliding into the canal.
Another popular approach, and I think the best, is to recreate the French drain.
They dig out all the sand behind the wall creating a trench about 2 feet wide
and down below the weep holes. Then they put the screens on the seams and
put in a barrier at the edge of the grass (usually a 2X8 pressure treated
board). They attach a screen to the board. The screen extends down
into the trench, reaching the bottom. Then they refill the trench with
clean small stones. Island Rock on Marco (now out of business,
unfortunately) used to do this. It will be very expensive if there are
houses on both sides, since it is difficult to get machinery in and everything
will have to done by hand.
Another thing you can do in conjunction with the French drain above is to better
control rain water run off with gutters. Ideally, the gutters need to be
tied into 4 inch underground drain pipes that carry the water right out to the
canal. [Best to check on town rules...there may be an issue with this,
even though it's a common practice.] Or have them installed so that the
water goes toward the street.
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Last is the "rip-rap" rock like I have. Several companies will dump rock
into the canal alongside your seawall, piling the rock up nearly to the cap.
The rock supports the wall, hopefully preventing failure mode #3 above.
They usually figure on about a ton of rock per foot of seawall. The
service cost me $5000 in 2001 for 80 tons of rock along 80 feet of seawall.
In addition the rock seems to effectively reduce the loss of sand through the
seams. I've done all of the above to may wall. In the last year and
a half there has been no noticeable settlement or loss of sand (i.e.
holes).
Well, these are some long winded thoughts on seawalls. Unfortunately there
is no silver bullet and none of the solutions are cheap. I've done
business with Island Rock (now out of business) and Crocodile Rock (also out of
business). You might want to come over and take a look at what I did to my
wall before you make a decision.
-ED CRANE
357 Capistrano Court
