Fil Failure Analysis of fthe Breached Levee System at the 17th Street Canal, New Orleans

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1 Fil Failure Analysis of fthe Breached hd Levee System at the 17th Street Canal, New Orleans Jay Wang, Ph.D., P.E. Department of Civil Engineering and Construction Engineering Technology Louisiana Tech University, Ruston, LA Mark Castay, E.I.T. C.H. Fenstermaker & Associates, Inc., New Orleans, LA Louisiana Transportation Conference 2/18/2009 1

2 Presentation Outline n Objectives n Brief review of the Investigation by the IPET n Independent investigation performance n Results from the effective stress-based analysis n Results from the total stress-base analysis n Conclusions 2

3 Objectives Conduct tindependent d research on the failure mechanism of the breached levee system at the 17 th street t canal. Compare the results from the independent research with the outcomes released by the IPET. 3

4 Background Information n IPET---the Interagency Performance Evaluation Task Force. n A distinguished group of government, academic, and private sector scientists and engineers. n IPET was created by the Chief of Engineers, U.S. Army Corps of Engineers. n Leaders of Floodwall and Levee Performance Analysis. Dr. J. Michael Duncan, a University Distinguished Professor and Director of the Center for Geotechnical Practice at Virginia Polytechnic Institute and State University. Dr. Reed L. Mosher, a Senior Scientific Technical Manager at the U.S. Army Engineer Research and Development Center. n Latest report issued at June n Independent Levee Investigation Team led by Dr. Dr. Raymond B. Seed, Professor of UC Berkeley, reports at July 31,

5 17 th Street Canal Breach and other breaches (IPET 2007) 5

6 Prefailure Cross-Section at Sta (Most scour occurred near this station, IPET 2007 ) 6

7 Centerline of the cross-section of the breach (the 17th Street Canal) 7

8 Exposed failure plane at the 17 th street canal (IPET 2007) Clay that was initially below the marsh layer was displaced above some of the marsh material during the failure. The marsh material above and below the clay is the same layer, confirmed by age dating. (IPET 2007) 8

9 Sketch for the displaced clay layer (IPET 2007) 9

10 Key findings of the IPET studies n Gaps formed at tthe I- wall on the flood side. n Huge hydrostatic water pressure applied onto the I-wall. n Shear failure occurred through the weak lacustrine clay layer. (Duncan et al. 2008) 10

11 A photograph of a gap behind the Michoud Canal I-wall (Brandon et al. 2008) 11

12 Cross section at Sta used in the investigation 12

13 Key points of the independent n Total stress-based research n Fully coupling of pore water-soil (X) n Effective stress-based (X) n Staged gravity-loading n Interaction between soil and flood wall 13

14 1 2 3 The sequential loading steps of the levee system Staged loading type Load phase Total-stress based analysis Effective-stress t based analysis Apply gravity load All of the levee Different soil Bay sound clay (1) for initial stress. embankment and layers activated in Elasto-plastic foundation soil four loading steps Beach Sand (2) analyses are layers are activated sequentially, with Lacustrine clay (3) performed. at the same time. prescribed zeropore pressure on Marsh (4) each nodal point. Levee fill + flood wall (4) Apply canal water pressure to the level of El+1. Raise canal water to El + 5. No pore pressure is involved. The added water pressure is applied in 10 steps. Form steady --- Keep canal water at EL +1 by imposing water seepage through pressure and hydrostatic pore pressure on the the levee fill and relevant boundaries. other soil l ayers ---Keep hydrostatic pore pressure on the boundary by performing on the right hand side. pore-elasto-plastic --- Maintain the ground water table consistent with coupling analysis. the ground surface at the protected side. Transient coupled seepage analysis runs for 1616 days to reach a steady flow. The increased water pressure and pore pressure are applied in multiple steps within 24 hours. Raise canal water The added water The increased water pressure and pore pressure are applied in multiple 4 to El pressure is applied in steps within 5.5 hours, 10 steps. 14

15 The effective stress-based analysis n Staged gravity load application (staged construction) n Slowly raise water table up to El+1.0 to establish steady water flow through soil strata n Raised water table to El+5.0 within 24 hours n Raised water table to El+11.0 within 5.5 hours 15

16 Hydrograph proposed by the IPET for the 17 th Street drainage canal 16

17 Soil properties employed in the total stress based analysis Levee Embankment Marsh Beach Sand Lacustrine Clay (Fat Clay) Bay Sound Clay Young s Modulus E Psf/Pa) Poission s Ratio Unit Weight (Pcf /N/m 3 ) Frictional Angle ( o ) Cohesion C (Psf/N/m 2 ) D-P coefficient D-P coefficient 234,000/ 14,400/ 1000,000/ 180,000/ 234,000/ 11,203, ,457 4,787, ,457 11,203, / 80/ 110/ 95/ 102/ 18,071 12,571 17,286 14,928 16, /43,091 1/48 1/48 600/28, /37, e e e-4 49, ,192 43,094 17

18 Soil properties employed in the effective stress based analysis Soil Properties Levee Embankment Marsh Beach Sand Lacustrine Clay (Fat Clay) Bay sound Clay Young s Modulus E (Psf/Pa) / 10,083, / 62, ,000/ 4,787, / 775, / 10,083,310 Poission s Ratio Unit Weight (Pcf/N/m 3 ) 115/ 18,071 80/ / / / Frictional Angle ( o ) Cohesion C (Psf/N/m 2 ) 41.80/2,000 1/ / / /2,000 D-P coefficient D-P coefficient Permeability (m/day) k x e e e e -5 k y e e e e -6 y 18

19 Material properties of the I-wall and sheet pile wall Flood Wall properties I-Wall Sheet Pile Wall Young s Modulus E (Psf/Pa) 59.0E+10/ 2.824E+13 30E+10/ 1.436E+13 Poission s Ratio Unit Weight (Pcf/N/m 3 ) 150/ /

20 Friction angles of levee embankment and lacustrine clay n No experimental results available n Correlation from PI n Levee embankment at breach area: LL = 43, PL = 20 n Lacustrine clay: average LL = 46, PL = 17 20

Descritization of levee system Finite element meshes of the 17 th street Canal levee system Element groups and number of elements for different levee domains Element

21 Descritization of levee system Finite element meshes of the 17 th street Canal levee system Element groups and number of elements for different levee domains Element group Number of elements Levee clay (1) 600 Marsh (2) 560 Lacustrine clay (3) 1406 Beach sand (4) 434 Concrete I-wall (5) 40 Sheet pile wall (6) 112 Bay sound clay (7)

22 Overburden pressure contours immediately after the staged construction 22

23 Pore pressure contours for the steady flow with canal water level at El+1 23

24 Pore pressure distribution across the multi soil layers around 12 meters away from the floodwall 24

25 Effective overburden pressure contours subjected to a steady flow at El+1 25

26 Effective overburden pressure distributions across multi soil layers around 12 meters away from the floodwall ll 26

27 Yield zone on the protected side of the sheet pile at El+11 27

28 Configurations of the contact surfaces at canal side 28

29 Configurations of the contact surfaces at protected side 29

30 Total overburden pressure at El+1 (total stress-based undrained analysis) 30

31 Total overburden pressure at El+11(total stress-based undrained d analysis) 31

32 Configurations of the contact surfaces at canal side (Total stress-based undrained analysis) 32

33 Schematic of the gap between floodwall and levee fill at canal side (Total stress-based undrained analysis) 33

34 Configurations of the contact surfaces at protected side (Total stress-based undrained analysis) 34

35 Conclusions n n n n n n The IPET-mentioned gap formed between flood wall and levee and foundation soils at canal side after running the total stress-based analysis. No gap was found at the same place if the effective stress-based analysis is performed. Large pore pressure was developed in the beach sand layer, which is overlain by the lacustrine clay layer. The pore pressure greatly reduced the overburden effective stress in the lacustrine clay, in which a localized shear zone was formed. Shear failure might have occurred at first in the clay layer, which h made a portion of clay displaced into the marsh layer. A gap was possibly developed after shear failure occurred in the lacustrine clay layer. 35

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