I have already raised the question of whether the Corps of Engineers did soil borings (cores sampling) to determing the elastic parameters of the soils below the levees. The independent study by the Engineering committee headed by Dr. Seed criticizes the analysis and decision making based upon those samples. In the previous blog entry I have illustrated the hydrostatic effects of the storm surge on the strenght of the soils. Now I will raise the issue of anisotropy. Soils and rocks are not homogeneous bodies. Their heterogeneity is not usually random but rather consists of thin quasi homogeneous layers of differing properties. These layers can be characterized by the terms, Coarse, Medium, and Fine Sand, Silt, Clay, Calcareous (shell), and Organic (peat). Each of these layers has a different set of elastic parameters (strength). The layers are often very thin (less than a mm). The effect of this layering is to introduce anisotropy into the elastic parameters. Anisotropy causes the strength of the soils or rocks to vary with direction of measurement. The usual directions of interest are parallel and perpendicular to the bedding planes. Differences on the order of 20% between the parallel and perpendicular velocities have been reported in the Geophysical Literature. The orientation of the elastic parameters is such that the weakest direction is parallel to the bedding and thus in the horizontal direction. I propose that the Corps of Engineers familiarize themselve with the effects of anisotropy as well as overpressure and make sure that their lab measurements on soil borings measure all of the effects and further, that their levee models include these effects. I believe that the Katrina failure of some of the canals was predictable if correct and complete modeling based on detailed soil boring analysis was done during design.
EXAMPLE FROM THE LITERATURE The Corps of Engineers Soil Strength Analysis taken from the Independent Analysis headed by Dr. Seed is below. The analysis and decisions made based on the analysis have been criticized in the report. Errors of under estimation of the weakest soil strength of the order of 100% are visible on the graph. The decisions made from this analysis undoubtably contributed to the disaster. All Engineers know that a chain is only as strong as its weakest link. Their graph is below.
I have contributed some addition considerations. My analysis of the hydrostatic effects of the surge imply a 20-30% weakening of these measurements at the depth of failure . Anisotropy considerations add a 5-25% weakening of the shear parameter in the horizontal direction parallel to the bedding planes of the soil. Most core measurements are made perpendicular to the bedding planes unless additional core preparation is done to get a sample that can be measured parallel to the bedding planes. My conclusion is that the U.S. Corps of Engineers modeled the levy response to a storm surge using soil strengths a least 150% higher than reality and that this was a major factor in the levee failure.
The modeling of stress and strain and the breaking or "failure" of solid bodies is a science that has at least 200 years of development behind it. It is an integral part of the training of Civil Engineers, Structural Geologists and Solid Earth Geophysicists. The forces involved are usually due to Gravity. The parameters of interest are the weight distribution of the object, and the strength of the materials in the object. The strength of the materials is broken into two independent parameters, the compressional strength and the shearing strength. Levees are primarily made of soil which is a porous and permeable solid which is water wet. The physical properties of water wet rocks both consolidated and unconsolidated are studied by Geophysicists for the purpose of determining their effects upon Seismic (sound) waves propagating through them. I am not an expert on near surface stress and strain modeling but I propose that the parameters that are used in such modeling are closely related to the elastic parameters of the water wet soils. This is the subject I wish to address in this blog entry. Both the shear and compressional elastic moduli are known to be a well behaved function of the "effective pressure". Effective pressure is the weight of the overburden (soil and rock above our depth of interest) minus the pressure of the fluid (water) in the pore spaces. Effective pressure is essentially the weight born by the solid frame of the rock or soil by grain to grain contact and is thus a primary factor in determining the elasticity of the entire rock. Laboratory measurements of the velocity of Seismic waves through a core specimen measured at different effective pressures are available in the Geophysical literature. Examples Below:
The velocity and elastic moduli increase with effective pressure which represents the normal variation seen with depth within the Earth. Note that the variation is the most rapid at the shallowest depth. In the case of the New Orleans levee failures, the depth at which failure occurred seems to be at about 20 feet. The effective pressure would then be about 20 - 9 = 11 psi. Now consider what happens when Katrina's storm surge raises sea level by 10 feet. The pressure at 20 feet of depth is now 20 - 15.5 = 4.5 psi. This is a very large relative change in effective pressure and may have had a profound effect on the soil's elastic parameters and contributed to their failure. My simple analysis shown in the figure above shows a reduction in the shear strength of 35% at a depth of 17 feet. I do not recall reading that the Corps or SWB did any soil borings and strength measurements. Dr. Seed in the independent Engineering report did mention noting a layer of organic peat material and measured the strength of it and found it very low. I believe that soil boring and core measurements of elasticity should be a part of all levee construction and further postulate that the effects of changes in the hydrostatic head (water level) need to be included in the modeling. Because the elastic parameters vary quite rapidly with depth in the near surface, I believe that this variation should be an integral part of the Levee Failure Model. In the measurement of the soil's elastic parameters, care needs to be taken to measure any anisotropy in those coefficients. We Geophysicists have become quite familiar with the phenomenon of elastic anisotropy in layered (banded, bedded) rocks. The direction parallel to the bedding planes usually being the minimum values (weakest strength).
Again, near surface stress and strain are not my area of expertise and others undoubtedly have more authority than I do in this area but I felt it necessary to raise the issue in consideration of the Katrina disaster and other recent levee failures.
We depend upon the Levees and Dams of the US to protect us from floods. The Corps of Engineers is responsible construction, inspection and maintenance. The following article was copied out of the Times Picayune and addresses the quality of the Corps inspection efforts.
By Bob Marshall
Times Picayune Staff writer
Before Hurricane Katrina, levee inspections in New Orleans were so superficial that one engineer who used to work for the Army Corps of Engineers said he conducted more diligent inspections on Florida levees that protected cattle. Engineers familiar with proper levee inspection routines across the country said the annual tours of New Orleans’ vital hurricane protection levees—described by critics as cursory drive-bys more about fellowship and lunch than looking for problems—sounded nothing like the serious geotechnical investigations conducted in other places. Further, the hasty approach taken on the 100-mile, five-hour tours by the Corps of Engineers and its local partners, the Orleans Levee Board and the state Department of Transportation and Development, did not come close to meeting federal regulations. “Annual inspections take many forms, but any way you slice it, you couldn’t inspect 100 miles in five hours—not properly, anyway,” said Thomas Wolff, assistant dean of engineering at MichiganStateUniversity and a member of the National Science Foundation team that investigated the levee failures in New Orleans. “You certainly couldn’t meet the requirements by looking down those levees with binoculars from a roadway,” Wolff said. Section 208.10 of Title 33 in the Code of Federal Regulations details how the corps and its local partners must maintain levees and floodwalls. The mandate includes regular inspections to look for encroachments on the levee or its right of way such as fences, patios and pools; the growth of shrubs and trees whose root systems could damage the system’s integrity; signs of seepage or sand boils; subsidence; animal burrows; and the accumulation of trash or debris. A long list of possible problems is spelled out, and inspectors are given a grading system to rank the seriousness of any problems found. The regulation calls for the thorough inspections to be made “immediately prior to the beginning of the flood season, immediately following each major water period and otherwise at intervals not exceeding 90 days.” Further, in the corps’ Flood Control Operations and Maintenance Policies, Regulation 1130-2-530 states, “Projects that protect urban areas or ones where failure would be catastrophic and result in loss of life should be inspected annually.” It also tells corps personnel to report nonfederal sponsors who are not complying with the regulations.
Lunch, not levees
Corps and Levee Board officials last month admitted their annual inspections typically were little more than quick driving tours. Inspection of the 17th Street Canal levee usually took place from a stop on the Old Hammond Highway, rather than a thorough walk or drive of the flood control structure that is 15,000 feet long. Records show the inspections were scheduled to end early enough for a taxpayer-paid lunch, costing as much as $900 for 57 people.The agencies said they conducted more frequent informal inspections, typically when personnel were going to the levees for other purposes. For instance, the Levee Board said it relied on grass cutters to report any problems they might see. But Wolff, who worked for the corps from 1970 to 1985, said inspections of levees in the districts where he worked, from Missouri to California, were much more formal events. They involved crews of as many as eight engineers who drove and walked the levees, and seldom covered more than 10 miles in a day. He said major inspections of the Mississippi River levees were conducted once every five years, and resulted in written reports, which were used as references before the next such inspection. Less rigorous, but still thorough, inspections were done annually. “You pretty much physically inspected the entire levee over two days,” he said. “Normally that might be as long a stretch as 20 miles. We seldom did more than 10 miles in a day. You couldn’t if you were conducting a thorough inspection.” Bob Bea, a University of California-Berkeley engineering professor who was also on the NSF team, said his levee inspections in Florida during a corps career in the 1950s were even more rigorous. “In some cases we actually came back and did soil borings to inspect suspicious sites,” he said. “And those levees were protecting cattle.”
Warning signs missed
Bea said careful inspection of the levees was critical because the earthen embankments often hide defects until failure occurs. “That’s one of the problems that plagues engineers in this field,” he said. “The levee can look absolutely solid, tall and green and safe. But just below the surface you could have all kind of trouble brewing. “That’s why you have to stay sharp, you can’t be complacent. My basic conclusion, based on my experience, is that you can’t see something that is impugning the integrity of a levee by looking at it through binoculars from a road.” The engineers agreed that the mistake thought to be at the heart of the New Orleans catastrophes—sheet piling too short to prevent seepage—would not have been visible through a walking inspection. And while the use of new, sophisticated ground sonars might have detected weakened soil layers before Katrina, justifying the expense of such a search would have been difficult without evidence, especially in view of recent budget cutbacks. But that’s why these inspections have to be done conscientiously,” Bea said. “You’re talking about a system that is supposed to keep half a million people safe.” In fact, investigators now believe there were warning signs that trouble was brewing below the grassy surface of the levees. Residents living along the 17th Street Canal levee had reported flooded lawns to the Sewerage & Water Board, a problem the Levee Board and Corps of Engineers said they never heard about. Further, NSF investigators said several residents reported other leaks, sand boils and depressions in the levees near the floodwalls—warning signs that the federal regulations tell the corps to look for in the mandated inspections.
I myself have witnessed large amounts of water flowing under the river levees at high water stage and wondered how safe we are from River flooding. The citizens of NOLA have great reason to suspect that all of the levees in th US are similarly neglected.
The history of the drainage ("outfall") canal system in New Orleans goes back more than a century. The 17th street outfall canal dates to before WWI (at least 1913). The political control was the purvey of the Orleans Levee Board (OLB) who had authority over the land primarily, the Sewer and Water Board who had control over the use of the canal for drainage, and the Corps of Engineers who were charged to protect the city from storm surges. In the 1960's enter the Lake Pontchartrain's wetlands environmental groups who wanted to preserve wildlife, fishing, swimming and boating interests. The Corps was always under the the fiscal control of Congress, but this is mainly a problem of sales and consensus building for the Corps that they are used to and expert at. After hurricane Betsy, in the 1960's the Corps developed a "barrier plan" similar to the Dutch ZuiderZee. It would have controlled the storm surge in Lake Pontchartrain by means of a lock and dam installed in the Rigolettes ). This proposal was delayed and stymied by environmental and economic concerns and was abandoned in the late 1980's for the "high level plan" which involved raising the height of the levees and flood wall along the outfall canals. This decision put NOLA at great risk and transferred the responsibility of the canals to the OLD and SWB. Then the SWB in attempting to improve its ability to drain rainfall from the City requested permission from the Corps to dredge the 17th street canal to improve it drainage efficiency. In spite of warnings, the Corps granted the permit and dredging was completed in the late 1990's. Warnings were ignored that pointed to the exact spot where the levee later failed. All of this can be read in the Legal Brief on the class action suit against the Corps and the U.S.
The Judge dismissed the complaints because citizens can't sue the government but his words are damning to say the least:
This story–fifty years in the making–is heart-wrenching. Millions of dollars were squandered in building a levee system with respect to these outfall canals which was known to be inadequate by the Corps’ own calculations. The byzantine funding and appropriation
methods for this undertaking were in large part a cause of this failure. In addition, the failure of Congress to oversee the building of the LPV and the failure to recognize that it was flawed from practically the outset–using the wrong calculations for storm surge, failing to take into account subsidence, failing to take into account issues of the strength of canal walls at the 17th Street Canal while allowing the scouring out of the canal–rest with those who are charged with oversight. The cruel irony here is that the Corps cast a blind eye, either as a result of executive directives or bureaucratic parsimony, to flooding caused by drainage needs and until therwise directed by Congress, solely focused on flooding caused by storm surge. Nonetheless, damage caused by either type of flooding is ultimately borne by the same public . Such egregious myopia is a caricature of bureaucratic inefficiency. It is not within this Court’s power to address the wrongs committed. It is hopefully within the citizens of the United States’ power to address the failures of our laws and agencies. If not, it is certain that another tragedy such as this will occur again.
STANWOOD R. DUVAL, JR.
UNITED STATES DISTRICT COURT JUDGE
