slurry wall viewed
from the top of the levee
The picture above is of a slurry wall in a levee in the Pocket Area of Sacramento. According to a geotechnical engineer with the USACE in Sacramento, this slurry wall has SOLVED the "serious problems" (seepage and sand boils) this levee had previously experienced during high water in 1986. This slurry wall was built after the high water of 1986. The slurry wall is about 30 feet deep in most area's and approx. 15 to 16 inches wide. (The original design spec's called for a 12 inch wide wall)(see below)
The USACE estimates the cost of installing a slurry wall in a levee to be approximatly $150.00 per lineal foot. Or $800,000.00 per mile. The cost per foot is affected by several factors (see below) The walls can be installed useing current construction methods at approx. 100-200 feet per day (note that multiple SMW machines can be used on a single levee! rcm).
(The method used for installing this slurry wall involved the use of a backhoe. I suspect much greater speeds and economy could be acheived with a custom automated slurry wall building machine. Regarding the cost of doing large scale slurry wall work in our levee system. We are in a unique time in history. We have recently spent over a trillion dollars stopping the flood of Communism in the world. Since the Berlin Wall fell we have been in a "sword to plowshares" mode. I thinks it's time we declare a war on floods, to protect our own people. We can fix our levee's for the cost of a single B1 bomber. rcm)
cross-section of
a levee with slurry wall
This picture is a rare photo of an exposed slurry wall. A construction project in the Sacramento area required temporary removal of a section of the levee (the levee section was removed to allow access for a project unrelated to the levee). This levee happened to have a slurry wall core. The USACE was impressed with the structural integrity with this slurry wall. In fact he has saved a piece of the slurry wall removed from this levee. The USACE is an excellent resource for information about our levee system in Northern California.
Notice, in this close-up, the layering of the sand strata in the levee compared to the solid construction of the slurry wall.
An e-mail clarification from The USACE 2/17/97... "A little clarification on slurry walls and their cost. The depth of the slurry wall in the Pocket area was 30 feet (mostly) and thus the cost of about $150 per foot is correct as you indicate. However, the 30-foot wall was appropriate for that levee and not necessarily for all levees. The design depth is really a function of levee height, depth of water against the levee, levee geometry, and soil type in the levee and it's foundation. In order to determine the depth of cutoff, seepage analyses are performed on a section of levee that models the site conditions. I don't want people to think that a 30-foot deep slurry wall is the magic cure for all levees. The reason it worked well in the Pocket Area is that for most of the area where the wall was constructed, there is a relatively shallow soil type that is not susceptible to seepage (clay layer). Deeper sand deposits below this clay layer still result in seepage during high water in the River. But most of this seepage is quite some distance from the levee, so we feel it is more of nuisance seepage than anything else. Also, there are some areas where the slurry wall did not get down to that clay layer and some seepage still does exist during high river stages, but again, it is relatively confined to two or three small areas and again we feel it is more nuisance seepage and not a threat to levee stability."
Date: Fri, 21 Feb 97 13:50:19 PST To: <safelevee@yahoo.com> Subject: levees RC I'll try to answer your many questions as best I can. 1. I don't have any pictures of the 1955 flood. However, you might check with the Corps Public Affairs office. They might be able to help you. Jim Taylor 557-5101. 2. I don't know the cause of the 1955 break. I would assume it was seepage and piping related. 3. Failures: Sutter Bypass (Meridian). I don't know that anyone knows the cause of the failure. Possibly the local reclamation district people might be able to provide you some information. But I doubt it. I haven't heard that anyone was at the site when it was going. Feather River. What I've gathered from various sources is that the failure occurred from a sand boil near the toe of the levee. 4. The slurry wall would not be susceptable to damage due to any amount of river velocity because the wall would not subjected to the dynamic force of the river. However, what is susceptible to damage due to high river velocity are the banks, which if the erosion is severe enough can erode toward the levee and consequently threaten the levee for obvious reasons. If in an extreme case, the erosion was really bad, it could eroded up to and undermine the levee. In this case, the slurry wall would be subjected to forces it was not designed to handle and it would probably fail. The slurry wall has nowhere near the strength of a concrete wall. 5. Have slurry walls ever failed? I can only say that those constructed along the Sacramento River have so far performed very well and have not failed. But, I don't know of any failures anywhere else. 6. As far as I know, there is no one best solution for levees. Different solutions, ie. slurry cutoff walls, landside stability berms, and erosion protection may be needed to varying degrees depending on the particular site conditions. So, there is no single answer to your question about what's the safest solution. 7. Why slurry walls in Sac? Economics. Where homes back up against the levee, the slurry wall is generally cheaper. Where the land is undeveloped, it's normally more economical to purchase land and build stability berms. This is why in North Natomas a landside berm was constructed in 1992 from Verona to Powerline Road to control seepage (stability) and a slurry wall was constructed (to cut off seepage) from Powerline Road to the East Main USACE |
(I hope to post soon to this site an animated GIF explaining the destructive process of "piping" which occurs at high water in a sand levee. This animations will be based on sketches drawn by The USACE
r.c.m. 2/23/97)