Stability of I-Walls in New Orleans during Hurricane Katrina

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Stability of I-Walls in New Orleans during Hurricane KatrinaJ. Michael Duncan, M.ASCE1; Thomas L. Brandon, M.ASCE2; Stephen G. Wright, M.ASCE3; and

Noah Vroman, M.ASCE4

Abstract: Failures of I-walls during Hurricane Katrina were responsible for many breaches in the flood protection system in NewOrleans. Six breaches were examined in detail by Task Group 7 of the Interagency Performance Evaluation Taskforce. Four of thesefailures and breaches, which occurred before the water levels reached the top of the wall, were not caused by overtopping erosion. Thefailure of the I-wall at the 17th Street Canal resulted from shear through the weak foundation clay. The south failure of the London AvenueI-wall was caused by subsurface erosion, which carried massive amounts of sand inland, and removed support for the wall, leading tocatastrophic instability. At the north breach on London Avenue, the failure was caused by high pore pressures, combined with a lowerfriction angle in the loose sand, which resulted in gross instability of the I-wall under the water pressure load from the storm surge.Looking back, with the benefit of 20-20 hindsight, these stability and erosion failures can be explained in terms of modern soil mechanics,exploration techniques, laboratory test procedures, and analysis methods. An important factor in all of the cases investigated wasdevelopment of a gap behind the wall as the water rose against the wall and caused it to deflect. Formation of the gap increased the loadon the wall, because the water pressures in the gap were higher than the earth pressures that had acted on the wall before the gap formed.Where the foundation soil was clay, formation of a gap eliminated the shearing resistance of the soil on the flood side of the wall, becausethe slip surface stopped at the gap. Where the foundation soil was sand, formation of the gap opened a direct hydraulic connectionbetween the water in the canal and the sand beneath the levee. This hydraulic short circuit made seepage conditions worse, and erosiondue to underseepage more likely. It also increased the uplift pressures on the base of the levee and marsh layer landward of the levee,reducing stability. Because gap formation has such important effects on I-wall stability, and because gaps behind I-walls were found inmany locations after the storm surge receded, the presence of the gap should always be assumed in I-wall design studies.

DOI: 10.1061/�ASCE�1090-0241�2008�134:5�681�

CE Database subject headings: Walls; Louisiana; Hurricanes; Failures; Levees; Floods.

Introduction

I-walls are used to raise the level of flood protection withoutwidening the footprint of a levee. As shown in Fig. 1, I-walls areconstructed by driving steel sheet piles through the levee, oftenpenetrating into the foundation soils. In some cases the portion ofthe I-wall that projects above the levee crest is encased in rein-forced concrete.

During Hurricane Katrina, I-wall failures resulted in breachesat many locations in New Orleans. Six of these beaches are listed

1University Distinguished Professor, Emeritus, Dept. of Civil and En-vironmental Engineering, 200 Patton Hall, Virginia Tech, Blacksburg, VA24061. E-mail: [email protected]

2Associate Professor, Dept. of Civil and Environmental Engineering,200 Patton Hall, Virginia Tech, Blacksburg, VA 24061 �correspondingauthor�. E-mail: [email protected]

3Brunswick-Abernathy Regents Professor, Civil Engineering Dept.,The Univ. of Texas, 1 University Station C1792, Austin, TX 78712-0280.E-mail: [email protected]

4Research Engineer, USACE ERDC, 3909 Halls Ferry Rd., Vicks-burg, MS 39180-6199. E-mail: [email protected]

Note. Discussion open until October 1, 2008. Separate discussionsmust be submitted for individual papers. To extend the closing date byone month, a written request must be filed with the ASCE ManagingEditor. The manuscript for this paper was submitted for review and pos-sible publication on May 14, 2007; approved on January 25, 2008. Thispaper is part of the Journal of Geotechnical and GeoenvironmentalEngineering, Vol. 134, No. 5, May 1, 2008. ©ASCE, ISSN 1090-0241/
2008/5-681–691/$25.00.
JOURNAL OF GEOTECHNICAL AN

J. Geotech. Geoenviron. Eng

in Table 1. The mechanisms of failure of these walls involvedinstability due to shear failure within the foundation clay at the17th Street Canal and the Inner Harbor Navigation Canal �IHNC�north breach on the east side, instability due to underseepageerosion and high uplift pressures in the sand foundation soils atLondon Avenue, and overtopping erosion that removed supportfor the walls at the IHNC southeast and northwest breaches.These breaches resulted in devastating flooding in the areas thewalls were designed to protect. Most disturbing were the failuresthat occurred before the canal water level reached the tops of thewalls.

Following Hurricane Katrina, the U.S. Army Corps of Engi-neers formed the Interagency Performance Evaluation Taskforce�IPET� to conduct a comprehensive investigation of the storm andits consequences. The writers worked on the team that investi-gated floodwall and levee stability. The findings of the investiga-tion are detailed in Volume V of IPET �2007� and the relatedAppendices. This paper summarizes the results of the IPET inves-tigation that are related to limit equilibrium analyses, underseep-age, and erosion of the 17th Street Canal and London AvenueI-walls.

A key finding of the IPET studies was the fact that gapsformed at many locations on the flood side of the wall as thewater level rose and the wall deflected, reducing stability of theI-walls.

Fig. 2�a� shows an I-wall with clay beneath the levee. In thiscase, formation of a gap eliminates the shearing resistance of the
soil on the flood side of the wall, because the slip surface stops at
D GEOENVIRONMENTAL ENGINEERING © ASCE / MAY 2008 / 681

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the gap. In addition, the water pressure in the gap is higher thanthe earth pressures on the wall before the gap formed. Both ofthese results of gap formation lead to reduced stability of the wall.

Fig. 2�b� shows an I-wall with sand beneath the levee. In thiscase, formation of the gap opens a direct hydraulic connectionbetween the water in the canal and the sand beneath the levee.This hydraulic short circuit makes seepage conditions worse, anderosion due to underseepage more likely. It also increases theuplift pressures on the base of the levee and any impermeablelayers landward of the levee, reducing stability.

For these reasons, formation of gaps behind the I-walls re-duces I-wall stability. Gap formation was found to be an impor-tant factor in all of the failures and breaches �except for those dueto overtopping� that occurred in New Orleans, and it was con-cluded that design studies for I-walls should always assume that agap will form behind the wall.

The following sections describe studies of the failures andbreaches that occurred at the 17th Street Canal, where the foun-dation soil was clay, and at the London Avenue Canal, where thefoundation soil was sand.

17th Street Canal I-Wall

A photograph of the breach in the 17th Street Canal I-wall isshown in Fig. 3. The breach is about 450 ft long. The remaining26,000 ft of the I-wall along the canal remained stable.

A cross section at Station 10+00, the center of the breach, isshown in Fig. 4. The canal side of the levee was made lower thanthe protected side to improve stability toward the canal when thecanal water level was low.

Table 1. Soil Conditions and Failure Mechanisms at Investigated I-Wall

Location Soil conditions

17th Street Canal Clay levee fill/marsh/foundation c

London Avenue south breach Clay levee fill/marsh/dense sand

London Avenue north breach Clay levee fill/marsh/loose sand

IHNC east bank south breach Clay levee fill/marsh/clay/sand

IHNC east bank north breach Clay levee fill/marsh/clay/sand

IHNC west bank north breach Clay levee fill/marsh/clay/sand

Fig. 1. I-w

682 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINE


A considerable number of borings had been made in thebreach area and in neighboring areas before the failure. Addi-tional borings have been drilled, cone penetration tests have beenperformed, and test pits have been excavated after the failure. Thetopography, before and after the hurricane, was established usingLIDAR surveys. A compilation of all of these data is included inAppendix 1 of the IPET report �IPET 2007�. The cross sectionshown in Fig. 4 is based on the information derived from theseexplorations, and from laboratory tests performed on samples re-trieved from the area. Several hundred unconfined compressiontests and unconsolidated-undrained �UU� tests have been con-ducted on the soils at the 17th Street Canal. Undrained shearstrengths measured on samples from borings within and adjacentto the breach area are plotted against elevation in Fig. 5. Thestrength values shown in Fig. 5 were obtained from boringsdrilled at the centerline of the levee and at the toe of the levee.The data shown are from a number of different types of undrainedstrength tests:1. One point Q is an unconsolidated-undrained �UU� triaxial

compression test, using one value of confining pressure. Thepoint represents the strength measured in a single test;

2. Q is a set �three or four test specimens� of UU triaxial testsperformed using a range of confining pressures;

3. UCT is an unconfined compression test. The point representsthe strength measured in a single test;

4. Crest strength interpretation is the strength profile beneaththe levee crest that was used in the IPET stability analyses;and

5. Toe strength interpretation is the strength profile beneath thelevee toe and beyond the toe that was used in the IPETstability analyses.

h Locations

Failure mechanism

d Stability failure through foundation clay

Underseepage erosion of foundation sand leading to removalof support for I-wall

Underseepage erosion and/or foundation instability due tohigh uplift pressure

Overtopping erosion of levee fill leading to removal ofsupport for I-wall

Stability failure through foundation clay

Overtopping erosion of levee fill leading to removal ofsupport for I-wall

ss section

Breac

lay/san

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ERING © ASCE / MAY 2008

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Soil Properties

The levee fill is compacted CL and CH material, with an averageliquid limit of about 45 and a total unit weight, �t, of 109 pcf.Beneath the fill is a layer of peat-like material called “marsh” thatis 5–10 ft thick. The marsh is composed of organic material fromthe cypress swamp that occupied the area, together with silt andclay deposited in the swamp. The average moist unit weight of themarsh layer is about 80 pcf. Beneath the marsh is a lacustrine CHclay layer, with an average liquid limit of about 92, a PI of 65,and �t=100 pcf. A compilation of all of the laboratory data col-lected for the 17th Street Canal investigation can be found inAppendix 1 of the IPET report �IPET 2007�.

The measured shear strengths of the levee fill scattered verywidely, from about 120 to more than 5,000 psf. Placing the great-est emphasis on data from UU tests on 5-in.-diameter samples,which appear to be the best-quality data available, su=900 psf is areasonable value to represent the levee fill.

Although the scatter in measured values is great, close analysisof the available data shows that the marsh deposit is strongerbeneath the levee crest where it was consolidated under theweight of the levee, and weaker at the toe of the levee and be-yond, where it was less compressed. The measured shearstrengths of the marsh scatter very widely, from about 50 to about920 psf. Values of su=400 psf beneath the levee crest and su

=300 psf beneath the levee toe appear to be representative of themeasured values. Considerable judgment was needed to interpretthe strength test results because of the scatter. Fortunately, thefactor of safety is not influenced greatly by the strength of thelevee fill and the marsh materials.

Field explorations after the failure showed that the rupturesurface passed through the clay, beneath the levee fill and themarsh material. A photograph of the side of an exploration trenchthat was excavated at the toe of the slide mass is shown in Fig. 6.The dark marsh material can be seen both above the lighter-colored clay and below the clay, although the clay is found onlybeneath the marsh in its undisplaced position. The lower part ofFig. 6 shows that the marsh-clay-marsh sequence was createdwhere the rupture surface within the clay continued upwardthrough the marsh. Lateral displacement of the sliding mass over

Fig. 2. Potential I-wall failure mechanisms showing; �a� foundati

the underlying undisplaced material results in the marsh-clay-



marsh alignment after the failure. Age dating showed that themarsh above and below the clay was the same material �IPET2007�.

As can be seen in Fig. 6, the rupture surface passed throughthe clay and the marsh, but not through the levee fill. The clay isnormally consolidated beneath the levee crest, and perhapsslightly overconsolidated beneath the toe. Although the results oflaboratory strength tests performed on the clay were very scat-tered, much more consistent strength values were derived fromthe results of cone penetration tests with pore pressure measure-ment �CPTU tests�. Undrained shear strengths from four CPTUtests performed through the levee, all within 250 ft of the breach,are shown in Fig. 7. It can be seen that the four tests are in closeagreement, and there is little scatter in the results. The laboratoryshear strength results shown in Fig. 7 were collected from testspecimens that were obtained from centerline borings.

The undrained strength values shown in Fig. 7 were calculatedfrom the CPTU test data using a method developed by Mayne�2003, 2005�. The writers have found that, where pore pressuresmeasured in CPTU tests are of high quality, this method provides

tability through clay; �b� underseepage and erosion through sand

Fig. 3. Photograph of breach in 17th Street Canal I-wall

on ins


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values of undrained strength that are consistent with laboratorytests on the best-quality test specimens, with less scatter in re-sults. Where pore pressures measured in CPTU tests are of lesserquality, the method is not so effective.

Experience with a large number of cone penetration tests per-formed to evaluate undrained clay strengths in the New Orleansarea since completion of the IPET investigation have shown thatit is often difficult to measure reliable pore pressures in CPTUtests due to problems with maintaining saturation of filters andother practical difficulties. As a result, production-level testing inthe New Orleans area is now often being performed without re-quiring pore pressure measurements. Undrained strengths of clayare being computed by dividing the total cone tip resistance �qc�by a “cone factor” Nc. It has been found that consistency withhigh-quality CPTU tests can be achieved using values of Nc in therange of 19–25 for the clays in the New Orleans area. Using theNc method simplifies testing and produces a greater amount ofuseful data for expenditure of less effort, as compared with themore exacting CPTU tests.

The undrained strength line shown in Fig. 7, which was cal-culated from the CPTU test data using Mayne’s �2003� method, isconsistent with the use of a value of Nc equal to 23. The und-rained shear strength increases with depth at a rate of 11 psf / ft ofdepth. Although there is a large amount of scatter in the results ofthe laboratory tests on the clay, there is very little scatter in theresults of the CPTU tests, and these values thus provide a solidbasis for establishing undrained strength profiles in the clay.

In the IPET report, a total unit weight of 109 pcf was mistak-enly used for the lacustrine clay. This value of total unit weight,combined with the 11 psf / ft rate of increase of strength withdepth, corresponds to a value of su / p�=0.24. We have since foundthat a more appropriate average value of the total unit weightwould be about 100 pcf, which would result in su / p�=0.29.Owing to the fact that the clay is at the bottom of the slip circlesanalyzed, a change in clay unit weight has essentially no effect onthe overturning moment and the computed factors of safety.

Strength Model

The IPET strength model, developed using the data discussed in

Fig. 4. Cross section of 17th S
treet Canal I-wall at Station 10+00
the previous paragraphs, was as follows:



Fig. 5. Laboratory undrained shear strength test results from crestand toe borings and strength interpretation for 17th Street CanalI-wall at Station 10+00


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1. The undrained strength of the levee fill was su=900 psf, �u

=0;2. The undrained strength of the marsh material beneath the

levee crest was su=400 psf, �u=0, decreasing to su

=300 psf, �u=0 at the toe. Beyond the toe, the strength wasconstant, su=300 psf, �u=0; and

3. The undrained strength of the clay was taken as 0.24 timesthe effective overburden pressure at the top of the clay, andincreased at a rate of 11 psf / ft at all locations. Thus thestrength was highest beneath the crest, decreased from thecrest to the toe. The 0.24 strength ratio was determined fromthe rate of increase of strength with depth �11 psf / ft� and atotal unit weight of 109 pcf.

Fig. 6. Photograph of exploration trench at failure area o

Two strength profiles are shown in Fig. 5: one for strengths



beneath the levee crest where consolidation stresses are higher,and a second for strengths beneath the toe and beyond, wherestresses are lower.

This IPET strength model involves two simplifying approxi-mations regarding clay strength: �1� By using the same rate ofincrease of su with depth, 11 psf / ft, throughout the clay, it isimplicitly assumed that the clay is normally consolidated through-out. While the clay beneath the levee crest is most certainly nor-mally consolidated, it is perhaps slightly overconsolidatedbeneath the toe. �2� By using effective vertical stress equal tosimple overburden pressure at all locations, redistribution ofstress within the foundation from the center toward the toe of thelevee is ignored. These approximations tend to overestimate clay

Street Canal I-wall, and schematic of failure mechanism
f 17th
strength beneath the crest, and underestimate strength beneath the


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toe. However, a detailed study for two locations showed that theseapproximations have a very small effect �about 2%� on calculatedfactors of safety. Because this difference is smaller than the rea-sonably expected accuracy of the strength evaluations and stabil-ity analyses, it was concluded that further refining the IPETstrength model was not justified.

Fig. 7. Undrained shear strength increase with depth in lacustrineclay layer interpreted from laboratory tests on crest boring specimensand cone penetration tests

Fig. 8. Critical circle determined from slo



I-Wall Stability

Stability analyses were performed for a range of canal water lev-els, bracketing the measured height of the storm surge at the timethat eye-witness accounts indicated that the failure occurred. Theresults of analyses with and without a gap between the wall andthe canal-side levee fill are shown in Fig. 8. The slip circle shownin the figure is the one for the higher water level with a gapbetween the wall and the levee fill.

The results shown in Fig. 8 were calculated using Spencer’smethod �Spencer 1967�. The analyses were performed with thecomputer program SLIDE �Rocoscience 2005�, and were checkedusing the computer program UTEXAS4 �Wright 1999�. Addi-tional analyses were performed using noncircular slip surfaceswith UTEXAS4. The critical noncircular surface was very similarin shape and position to the critical circle, and the factor of safety�FS� for the noncircular surface was 6% lower. This 6% lowerfactor of safety corresponds to a water level for FS=1.00 that is0.8 ft lower than the FS=1.00 water level found using circularslip surfaces.

It can be seen that the computed factors of safety are lower forthe higher canal water level, as would be expected, and are about25% lower for the condition with a gap behind the wall than forno gap. Based on these results, and on the fact that gaps wereobserved at locations where instability did not occur and condi-

ility analysis for 17th Street Canal I-wall
pe stab

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tions could be examined after the hurricane, it was concluded thata gap did form behind the wall, and that gap formation was a keyfactor in the failure of the 17th Street I-wall. One of the mostimportant conclusions of the IPET investigation is that gaps canform behind I-walls, and that these gaps significantly reduce wallstability. It is therefore prudent to always assume that a gap willform, and this condition should be considered in all I-wall stabil-ity analyses.

London Avenue I-Wall South Breach

A photograph of the south breach in the London Avenue eastI-wall is shown in Fig. 9�a�, and a photograph of sand that waswashed through this breach into the neighborhood is shown inFig. 9�b�. The breach is about 60 ft wide.

A cross section through the center of the breach area, beforethe failure, is shown in Fig. 10. The levee is founded on a layer ofmarsh that overlies a dense sand layer �SP and SP-SM with anaverage D10=0.12 mm�.

Seepage Analyses and Uplift Pressures

Owing to the high permeability of the sand in the foundation,underseepage effects are important at this location. Finite-element

Fig. 9. �a� Photograph of south breach area of London Avenue Canal;�b� photograph of deposited sand from south breach of London Av-enue Canal

analyses of seepage beneath the I-wall were performed using the



computer program SLIDE. All of the cases analyzed represented acondition in which deflection of the wall would open a gap behindthe wall, from the levee crest down to the top of the sand. Thiscondition is consistent with the observation that gaps formed innearby locations where failure did not occur.

The permeability of the sand, based on field pumping tests,was 1.5�10−2 cm /s. The permeability of the marsh layer wasestimated as 1�10−5 cm /s based on consolidation test results,and the permeability of the levee fill and the Bay Sound clay wasestimated as 1�10−6 cm /s. Transient and steady finite-elementseepage analyses show that: �1� steady seepage through the sandwas established quickly; and �2� the pore pressures within thesand and the uplift pressures on the base of the marsh layer arenot affected by the permeability values assigned to the marshlayer and the levee fill, provided that those materials are at leasttwo orders of magnitude less permeable than the sand.

The hydraulic boundary conditions used in the seepage analy-ses are shown in Fig. 10. Two canal water elevations were ana-lyzed �7.1 and 8.2 ft NAVD88�, covering the range of estimatedcanal water levels at the time of failure. A constant-head boundarycondition was imposed at the location of the drain beneath War-rington Drive. Two head values at this location were analyzed:−8.4 ft NAVD88 �the normal ground water level with pumps op-erating�, or −5.1 ft NAVD88 �a higher level equal to the groundsurface elevation, which might have been realized with pumps notoperating�. Reports indicate that the pumps stopped operatingwhen the wall failed, severing the power line. A no-flow boundarycondition was used at the canal center line.

Computed pore pressures, or uplift pressures, at the base of themarsh layer are shown in Fig. 11 for the four cases analyzed,together with the total overburden pressure at the base of themarsh layer. It can be seen that in all four cases the computedpore pressures exceed the total overburden pressure at the base ofthe marsh layer beyond about 15 ft from the wall. This resultindicates that the marsh layer would be heaved off the underlyingsand by the high uplift water pressures.

How events would proceed beyond this stage cannot be de-fined precisely. A likely result of upward heave of the marshwould be rupture of the marsh layer at one or more weak points,and upward flow of water and sand through the rupture. This flowwould relieve the high water pressure locally, and create a newhydraulic boundary condition with high hydraulic gradientswithin the sand at the point of rupture. Although these hydraulicgradients cannot be evaluated precisely, they would certainly behigh, and would undoubtedly be capable of eroding the sand up-ward into the breach in the marsh. This erosion would progressrapidly back toward the levee, resulting in rapid removal of ma-terial from the landward side of the levee, quickly leading tocatastrophic instability, breach of the wall and levee, and inwardrush of water through the breach. Though it is not possible todocument the details of this failure sequence, because there wereno eyewitnesses to its development and progression, it is consis-tent with the known facts, and with the great volume of erodedsand shown in Fig. 9�b�.

Slope Instability

At the south breach the sand was dense ��t=120 pcf�, with stan-dard penetration test blow counts greater than 50, which wouldcorrespond to friction angles in the range of 40–46°. Cone pen-etration tests performed after the breach showed high tip resis-tance in the sand adjacent to the breach, which correspond to
similar values of ��. A value of ��=40° was used in analyses of

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the stability of the levee and I-wall, with the uplift pressuresshown in Fig. 11. The calculated factors of safety are shown inFig. 12.

The marsh was treated as undrained, with su=300 psf and �u

=0, based on the available test results. A value of su=300 psf isconsidered appropriate for the areas beneath the canal-side leveeslope and beyond the levee toe, where the slip circles passthrough the marsh. The average unit weight of the marsh is about80 pcf at both the north and the south breaches.

The levee fill was also treated as undrained, with su=900 psfand �u=0. The slip circles do not intersect the levee fill, however,and the levee strength therefore has no influence on the calculatedvalues of factor of safety. The average unit weight of the levee fillis about 109 pcf at both the north and the south breaches.

Analyses were performed with canal water levels at 7.1 and8.2 ft NAVD88, using pore pressures in the sand from finite-element seepage analyses without a rupture through the marshlayer. At the bases of the slices where the calculated pore pres-sures exceeded the overburden pressures near the top of the sandon the inboard side, zero shear strength was assigned for the sand.

As discussed earlier, it was assumed that deflection of the walltoward the land side resulted in formation of a gap through thelevee fill and the marsh in back of the wall, down to the top of thesand. It was assumed that the gap would not extend into the sand,because the sand is cohesionless and would slump and fill thegap.

Factors of safety against instability were calculated for therange of canal water levels estimated at the time of the breach�7.1 and 8.2 ft NAVD88�, and the two inland water levels �−5.1and −8.4 ft NAVD88�. The calculated factors of safety rangedfrom FS=1.19 to 1.56. Thus, based on the available data, amechanism of failure involving erosion and piping is clearly in-dicated at the south breach, but a slope stability failure mecha-nism is not.

An analysis was performed, with the landside water level at−8.4 ft, to determine the canal water level corresponding to acalculated factor of safety equal to 1.00. It gave a level of 9.7 ft,

Fig. 10. Cross section of south

which is 1.5 ft higher than the highest estimated water level at the



time the breach occurred. Thus, instability without removal ofmaterial by erosion and piping is unlikely at the south breach.

London Avenue I-Wall North Breach

Analyses of failure due to erosion and piping, and due to insta-bility, were also examined for the London Avenue north breach onthe west side of the canal, which failed about 1 h after the southbreach �IPET 2007�. The differences between the London southbreach analyses and the London north breach analyses were asfollows:1. The seepage boundary conditions were different. The canal

water level at the time of the north breach was 1.1–1.3 fthigher than at the south breach because the north breach

area of London Avenue Canal

Fig. 11. Computed pore pressures at base of marsh layer at southbreach of London Avenue Canal

breach


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Fig. 12. Critical circle determined from slope stability analysis for south breach of London Avenue Canal

Fig. 13. Cross section of north breach area of London Avenue Canal

JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / MAY 2008 / 689

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occurred later. The inland seepage boundary conditionranged from −8.4 to −3.9 ft because Pratt Drive on the northbreach is at a slightly higher elevation than Warrington Driveon the south breach.

2. The cross sections are somewhat different. On the inland sideof the wall at the north breach, there is a thin layer of lacus-trine clay between the marsh layer and the sand, as shown inFig. 13.

3. The sand is less dense in the north breach area ��t=115 pcfin the north area, �t=120 pcf in the south area�. Standardpenetration test blow counts �NSPT� in this area range from 2to 14, with an average of about 10 blows / ft. This range ofvalues of NSPT corresponds to values of �� in the range of30–34°. Cone penetration tests performed after the breach, inthe area adjacent to the breach, showed tip resistances thatcorrespond to about the same values of ��. A value of ��=32° was used in the stability analyses for the north breacharea.

Finite-element seepage analyses were performed with a gapbehind the I-wall, with canal water levels equal to 8.2 and 9.5 ft,and with inland water levels equal to −3.9 and −8.4 ft. As for thesouth breach, it was found that the calculated uplift pressures atthe base of the marsh layer were larger than the overburden pres-sures, although the calculated uplift pressures did not exceed the

Fig. 14. Critical circle determined from slope stab

total overburden pressures by as high a margin as at the south.



The maximum uplift pressure at the south section was about350 psf, compared to a maximum value in the north section ofabout 400 psf. Thus, while the same progressive failure mecha-nism of heave and rupture of the marsh, followed by erosion ofthe sand through the rupture is possible at the north breach, itwould be expected that these events would not have progressed asrapidly or as vigorously as at the south breach.

Slope stability analyses were also performed for the conditionsat the north breach. The calculated factors of safety for theseanalyses are shown in Fig. 14. The values of FS for all fourconditions analyzed are less than 1.0, indicating a high likelihoodof instability at his location, even without erosion of materialfrom the landward side.

Conclusions

Failures of the I-wall during Hurricane Katrina were responsiblefor numerous breaches in the flood protection system in NewOrleans. Task Group 7 of IPET examined six of these breaches indetail. Four of these failures and breaches occurred before thewater levels reached the top of the wall, and were therefore notcaused by overtopping erosion. The analyses described here indi-cate that the failure of the I-wall at the 17th Street Canal resultedfrom shear through the weak foundation clay. It seems probable

nalysis for north breach of London Avenue Canal
ility a
that the south failure of the London Avenue I-wall was caused by


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subsurface erosion, which carried massive amounts of sand in-land, and removed support for the wall, leading to catastrophicinstability. At the north breach on London Avenue, it appears thatthe failure was caused by high pore pressures, combined with alower friction angle in the loose sand, which resulted in grossinstability of the I-wall.

Looking back, with the benefit of 20-20 hindsight, these sta-bility and erosion failures can be explained in terms of modernsoil mechanics, exploration techniques, laboratory test proce-dures, and analysis methods.

In all of the cases investigated, an important factor was devel-opment of a gap behind the wall. Formation of the gap increasedthe load on the wall, because the water pressures in the gap werehigher than the earth pressures that had acted on the wall beforethe gap formed. Where the foundation soil was clay, formation ofa gap eliminated the shearing resistance of the soil on the floodside of the wall, because the slip surface stopped at the gap.Where the foundation soil was sand, formation of the gap openeda direct hydraulic connection between the water in the canal andthe sand beneath the levee. This hydraulic short circuit madeseepage conditions worse, and erosion due to underseepage morelikely. It also increased the uplift pressures on the base of thelevee and marsh layer landward of the levee, reducing stability.

Because gaps behind I-walls were found in many locationsafter the storm surge receded, and because gap formation has suchimportant effects on I-wall stability, it should always be assumedin I-wall design studies that a gap will form behind the wall.

In a companion paper in this volume, Brandon et al. �2008�examine the effects of gaps on I-wall stability, and explain howgaps can be modeled in stability analyses.



Acknowledgments

The results of the investigation and analysis presented in thispaper represent the efforts of many individuals involved in theIPET study initiated after Hurricane Katrina. Joe Dunbar, ReedMosher, George Sills, and Ron Wahl, all of ERDC, provided valu-able contributions to the work presented in this paper.

References

Brandon, T. L., Wright, S. G., and Duncan, J. M. �2008�. “Analysis of thestability of I-walls with gaps between the I-wall and levee fill.” J.Geotech. Engrg., 134�5�, 692–700.

Ineragency Performance Evaluation Task Force �IPET�. �2007�. “Perfor-mance evaluation of the New Orleans and southeast Louisiana hurri-cane protection system.” Final Rep. of the Interagency PerformanceEvaluation Task Force, U.S. Army Corps of Engineers, �https://ipet.wes.army.mil�.

Mayne, P. W. �2003�. “Class ‘A’ footing response prediction from seismiccone tests.” Proc., 3rd Int. Symp. on the Deformation Characteristicsof Geomaterials, Vol. 1, Swets & Zeitlinger, Lisse, 883–888.

Mayne, P. W. �2005�. “Integrated ground behavior: In-situ and lab tests.”Deformation characteristics of geomaterials, Vol. 2, Taylor & Francis,London, 155–177.

Rocscience, Inc. �2005�. Slide v5.0—2D limit equilibrium slope stabilityanalysis, Toronto.

Spencer, E. �1967�. “A method of analysis of the stability of embank-ments assuming parallel inter-slice forces.” Geotechnique, 17�1�, 11–26.

Wright, S. G. �1999�, UTEXAS4—A computer program for slope stability
calculations, Shinoak Software, Austin, Tex.

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Stability of I-Walls in New Orleans during Hurricane Katrina

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