How It Works: Protecting New Orleans With The World’s Largest Water Pump

Posted on 08/20/2009 by Jeff Lineberger, P. E.
New Orleans sits smack dab between the Gulf of Mexico and Lake Pontchartrain, and when a hurricane comes rolling in, those bodies of water tend to spill into the streets. This
summer, the U.S. Army Corps of Engineers started construction on a barrier that can block a 16-foot swell blown in from the Gulf and a massive pumping station that will blast
floodwaters back to sea.
The $500-million station—the newest installment of a $14-billion federal project to fortify the Big Easy against the type of fierce storm
the city sees once in 100 years—will protect the 240,000 residents living in New Orleans, a high-risk flood area because of its
nearby shipping canals. The Gulf Intracoastal Waterway is one of the city’s most trafficked industrial waterways, but it provides a
perfect path from the Gulf for a 16-foot storm surge to flood homes and businesses. When a major storm threatens, the waterway’s
new West Closure Complex will mount a two-point defense. First, operators will shut the 32-foot-tall, 225-foot-wide metal gates to
block the surge. Then they’ll fire up the world’s largest pumping station, which pulls 150,000 gallons of floodwater per second. And
unlike the city’s notorious levees, the WCC won’t break when residents need it most. “This station is designed to withstand almost
everything,” including 140mph winds and runaway barges, says Tim Connell, the U.S. Army Corps of Engineers’s project manager
for the complex.
Army engineers will bring the gates and pumps online as they go, with the goal of wrapping up construction by 2011—provided there
are no interruptions, says Dennis Kamber, a senior technical adviser on the project: “We have our fingers crossed that Mother
Nature will be kind to us this year.”

By Susannah F. Locke

Posted 08.18.2009 at 11:54 am

New Orleans sits smack dab between the Gulf of Mexico and Lake Pontchartrain, and when a hurricane comes rolling in, those bodies of water tend to spill into the streets. This

summer, the U.S. Army Corps of Engineers started construction on a barrier that can block a 16-foot swell blown in from the Gulf and a massive pumping station that will blast

floodwaters back to sea.

The $500-million station—the newest installment of a $14-billion federal project to fortify the Big Easy against the type of fierce storm

the city sees once in 100 years—will protect the 240,000 residents living in New Orleans, a high-risk flood area because of its

nearby shipping canals. The Gulf Intracoastal Waterway is one of the city’s most trafficked industrial waterways, but it provides a

perfect path from the Gulf for a 16-foot storm surge to flood homes and businesses. When a major storm threatens, the waterway’s

new West Closure Complex will mount a two-point defense. First, operators will shut the 32-foot-tall, 225-foot-wide metal gates to

block the surge. Then they’ll fire up the world’s largest pumping station, which pulls 150,000 gallons of floodwater per second. And

unlike the city’s notorious levees, the WCC won’t break when residents need it most. “This station is designed to withstand almost

everything,” including 140mph winds and runaway barges, says Tim Connell, the U.S. Army Corps of Engineers’s project manager

for the complex.

Army engineers will bring the gates and pumps online as they go, with the goal of wrapping up construction by 2011—provided there

are no interruptions, says Dennis Kamber, a senior technical adviser on the project: “We have our fingers crossed that Mother

Nature will be kind to us this year.”

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Foundation Repair? Understanding Residential Slab Underpinning

Posted on 08/18/2009 by Jeff Lineberger, P. E.

Slab on Grade Foundation Systems gained popularity on a mass scale after World War II.  Many returning veterans needed affordable and reliable housing in primarily suburban areas of the United States.  To build homes in volume, developers and builders chose shallow bearing concrete foundations systems because they were relatively inexpensive, and could be built in about half the time it took to build basement type foundation systems.  Structurally, these primarily residential foundations resemble the geometry of a  “waffle”, with “ribs” on the bottom, and a flat smooth surface on top.  The ribs stiffen the slab to prevent excessive movement, and the smooth top surface accommodates living area floors. Over the past 50 years, slab on grade design & construction techniques have not changed much.  The design principles are essentially the same, and it is arguably one of the most reliable and popular foundation system types on the market today.

But what to do when these foundation systems structurally fail? What is the proper repair method?  How does a proper repair method compare to less reliable repair systems.

One of the crucial underlying design principles of shallow bearing slab on grade foundation systems is its “single unit” behavior.  To do their job, slab-on-grade foundation systems act as single structural elements with uniform stiffness.  These foundation systems are designed to respond uniformly to resist upward or downward movement caused by soil pressure from below, and heavy structural loads from above. In doing so, these slab-on-grade foundation systems are stiff enough to buffer potentially damaging soil and subgrade material movement, yet flexible enough to protect the supported structural elements from unsafe or excessive planar tilting.

Properly repairing a failed or structurally compromised slab on grade foundation system requires a design method that mirrors its original design principle. Since shallow bearing foundation systems are designed as single units, they likewise must be repaired as a single unit. At a minimum, the remedial structural design must uniformly stabilize the entire foundation so it can resist the soil, dead and live loads imposed upon it.

Case #1:  Repairing Part of the Slab-on-Grade Foundation System-Partial Piering.  Stabilizing only part of the system restrains and limits its movement in a localized area, allowing the remaining unrestrained portion of the foundation to continue to freely move. Like moving a lever up and down, the unrestrained part of the slab transfers and focuses its stress to the area between the restrained and movable portion slab. This transferred stress can and often does cause additional damage to the brittle concrete foundation structure, thus repeating the cycle of foundation system structural failure.

Case #2:  Repairing the Entire Slab-on-Grade System-Full Piering.  Whether repaired or not, the foundation system is designed to behave as a single structural element.  When an engineer designs this type of foundation system, shear, deflection, and strength is evaluated for the entire foundation, not just part of it.  So why should an appropriate repair address only part of the slab?

The single unit slab-on-grade design principle allows the entire foundation system to react to soil movement uniformly.  In doing so, the impact of excessive soil movement to the brittle house materials (brick, sheetrock, etc.) is minimized, thus protecting the slab and house from cosmetic or structural damage.

A reliable, durable, and time tested method for uniformly stabilizing (full Piering) slab-on-grade foundation systems includes underpinning, where a slab on grade foundation (interior and exterior grade beams) bears upon and span underpins (drilled and steel reinforced concrete piers, or DRCP’s). For residential foundation systems, these underpins are strategically placed beneath the expanse of the foundation system to depths exceeding eight feet to provide maximum support. Unlike the typical (and inexpensive) multiple segmented concrete pile components stacked atop one another like unstable kinder blocks, DRCP’s are poured in place, reinforced with vertical steel, and placed to bear on deep, yet stable soils, free from seasonal moisture variations. More importantly, and unlike segmented piles, DRCP’s provide superlative resistance to soil friction that tends to push the underpin up (float) or from side to side (lateral). As such, DRCP’s provide a stable bearing surface for the damaged foundation system by limiting its ability to move vertically or horizontally. For a residence of like kind, quality age and size, 30-40 reinforced concrete underpins drilled to a 10-15 foot depth and placed at the interior and perimeter of the foundation system may be expected.

So, if your unbiased engineer recommends foundation repair, DRCP’s, while expensive, must be strongly considered by the owner as the repair method of choice for slab on grade foundation systems. Other methods, while inexpensive, such as mudjacking, soil stabilization, using pilings, or partial piering may satisfy limited budgets in the near term, they will probably cost more in the longer term.

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How Residential Foundations Work: Slabs 101

Posted on 08/18/2009 by Jeff Lineberger, P. E.

One of the most frequent questions I get from my consumer clients is:  Do I need to get my house slab repaired, and if so, what is the proper repair method?

As you can imagine, there are a myriad of variables that need to be considered before foundation repair is warranted.  The fact is, knowing how slab-on-grade foundation systems work will help optimize the process of determining whether or not your foundation needs to be repaired.  In any case, if you think you may need your foundation repaired, you will be in a better position to assist the structural engineer in determining the health of your foundation, and which repair method is best for you.  So, here are some items worth investigating before taking the significant economic step of getting your foundation repaired:

Get your house plumbing system statically tested.  Assure your drain system (and domestic water supply, irrigation system, etc.) is leak free. Should plumbing leaks be present, get a flow test done to determine the leak discharge water volume.

Check your foundation drainage.  Assure that the surface water drains away from the foundation along its perimeter, and no low areas allow water to pond for longer than a day or so after a heavy rain.

Understand how your foundation system works. The basic purpose of your house foundation system is to safely separate habitable areas from the exterior environment and limit damage or distress to interior/exterior brittle building materials.

One of the crucial underlying design principles of shallow bearing slab on grade foundation systems is its “single unit” behavior.  To do their job, slab-on-grade foundation systems act as single structural elements with uniform stiffness.  These foundation systems are designed to respond uniformly to resist upward or downward movement caused by soil pressure from below, and heavy structural loads from above. In doing so, these slab-on-grade foundation systems are stiff enough to buffer potentially damaging soil and subgrade material movement, yet flexible enough to protect the supported structural elements from unsafe or excessive planar tilting.

Know the limitations of your foundation system.  The weight of your house walls, roof, appliances, furniture, or perhaps fireplace stone or brick  are compressive loads that act downward on top of your foundation floor.  These compressive loads average perhaps 300-500 pounds per square foot.  Conversely, soil pressure acts upward from beneath your foundation system.  Soil pressures average perhaps 3000-5000 pounds per square foot. And the winner is…?  You get the idea.

Understand your local soil conditions.  Sand is non- plastic and quite stable.  Clay is plastic and subject to volumetric changes with the addition or removal of moisture.  When water is added to clay soil it expands, and when water is removed from the same soil, it shrinks.
As previously mentioned, expanding clay can produce tons per square foot of pressure on the underside of the foundation system, while the weight of a typical house produces less than 500 pounds per square foot average along its perimeter. With that said, excessive water input to clay soil poses the greatest threat to lightly loaded, shallow clay bearing foundations systems.

Get an independent engineer to evaluate your foundation. A competent structural engineer can provide an unbiased opinion and technical guidance based upon what the structural status of your foundation system. If your foundation system is structurally sound, then it is likely that underpinning will not be needed. If the engineer finds that your foundation is not structurally sound, then the proper repair will likely require underpinning.

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