Effective Stress Design For Floodwalls
on Deep Foundations
Glen Bellew, PE
Geotechnical Engineer
USACE-Kansas City
23 April 2015
Contributors
James Mehnert, PE
USACE-Kansas City
Paul Axtell, PE, D.GE
Dan Brown and Associates
US Army Corps of Engineers
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Outline
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Project Background
Load Cases Considered
Seepage Analysis
Foundation Analysis
Observed Performance 1993 Flood
Existing Wall Stability
Alternatives Considered and Selected
Design Verification Load Test
Major Findings/Lessons Learned
Construction Photographs
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Project Location – Fairfax Jersey Creek Levee
Missouri River
BPU Floodwall
Fairfax-Jersey
Creek Levee
Unit
Kansas River
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Project and Leveed Area Details
BPU Floodwall
1400 ft
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Levee/Flood Wall
constructed 1940’s
by USACE
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Highly Developed
Area (~$3.3 billion)
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Critical
Infrastructure
(Power Plant, water
treatment)
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Major
Manufacturing (GM
Plant)
Kansas River
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Existing Floodwall and Subsurface Conditions
~16 ft
~20 ft
CL/ML
g=119 pcf
~80 ft Sand
g=116 pcf
~20 ft
Sheet Pile
Fluted, Tapered
Steel Pipe Piles
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Non Critical Load Case – Short Term Flood
Typical
infrastructure
analysis, buildings,
bridges, etc.
No time for
blanket seepage
Pre-flood s’ and
stress history
control Su
Sand, f’, g’
~Horizontal Seepage
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Critical Load Case – Long Term Flood
Analysis specific to
water retention
structures
~Vertical
Seepage,
reduces s’
Sand, f’, g’
Effective Stress
Controls behavior,
f’, gflood
~Horizontal Seepage
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Effective Stress Design Process
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Establish seepage conditions (effective stress)
Determine Ultimate Axial pile capacity
Lateral response of pile group (often controls design)
Calibrate analysis to observed performance
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Seepage Analysis Criteria
Dh
i=Dh/z
z
Historically criteria has focused on preventing rupture/heave of
topstratum by limiting vertical gradients to less than critical gradient
(ic = g’/gw).
Original design (1940’s) design ensured H < z.
Current requirements are FS >1.6
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Seepage Analysis Methodology – calculating h
 Blanket Theory (EM 1110-2-1913)
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Simple geometric inputs (great for simple stratigraphy)
Decades of performance to verify adequacy of method
Spreadsheet solutions – quick to perform
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Seepage Analysis Methodology – calculating h
 Finite Element Modeling (next EM 1110-2-1913)
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Unlimited complexity in geometry and boundary conditions
Modeling quirks can lead to unrealistic results for a novice user
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In situ permeabilities, boundary conditions, model extent
User interface improving, but can be time consuming to set up
Use when complexity warrants
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Pile Design Methodology – Axial Capacity
 Overall
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Drained Strength Parameters
Effective State of stress reasonably assumed for flood conditions
EM 1110-2-2906
Criteria - FSmin = 1.7
 Side Resistance
► b method
• Nordlund for driven, tapered piles
 Tip resistance
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Bearing Capacity Factors
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Pile Design Methodology – Lateral Response
 Typical to use Ensoft’s Lpile and/or Group Software
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p-y curves by soil type (drained sand, undrained clay)
Unit weight
Friction angle
p-y modulus (kp-y)
Group effects – auto p-mult.
Criteria – Max D = 1.5”
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Effective Stress Lateral Response - Ensoft
 Design Case – Long Term Flood
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P-y curves not available for drained conditions in cohesive soil
• Use Sand Curves with appropriate f’
►Cannot input U>hydrostatic directly
• Reduce g of “blanket” by gflood = g’-igw
• also accounts for artesian sand
►p-y modulus (kp-y) estimated based on soil type/strength
• Loose-Medium Sand or Soft-Medium Clay
►Group requires an estimate of the axial load response (auto or input)
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Performance Observations- 1993
~3 ft
~45 Day duration
Seepage – some reports of concentrated
seeps with possible pin boils, no major boil
activity
Structural Performance – no performance
observations noted
Documentation limited…
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Calibrate with Back Analysis of 1993 Flood?
iavg = 0.7
RESULTS
Seepage: FS~1.3
Pile Capacity: FS = 1.5
Pile Structural >failure
Deflections – 1.5” max
gflood = 13 pcf
f’ = 29 deg
kp-y = 50 pci
P-y curve – API Sand
g' = 53.6 pcf
f’ = 36 deg
kp-y = 60 pci
P-y curve – API Sand
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No failure predicted, none observed…
Probability of Failure – “Brittle” Response
45
Maximum
Possible
Load
40
Probability of Failure (%)
35
30
Maximum
Historical
Load
25
20
15
10
5
0
0
1
2
3
4
Example fragility curve, not BPU floodwall
5
Loading
6
7
8
9
10
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Existing Floodwall – Analysis w/ water @ TOW
iavg = 0.83
FSi = 1.1
gflood = 9 pcf
f’ = 29 deg
kp-y = 50 pci
P-y curve – API Sand
g' = 53.6 pcf
f’ = 36 deg
kp-y = 60 pci
P-y curve – API Sand
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Existing Floodwall – Results w/ water @ TOW
Axial FS <1
Deflections >>1.5”
Floodwall
modification needed
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Design/Site Constraints
Landside
Riverside
The Good:
 Well Defined Site (<100’ spaced borings)
 Laboratory Data (consol, R-bar, class.)
 Foundation Load Test during construction
The Bad:
 Constrained ROW
 Maintain similar pile
spacing
 No driven/vibrated
elements – Drilled Shafts
 Difficult Design Case (low
effective stresses)
 Lateral Deflections a major
design constraint (limit to
1.5” under extreme load)
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Modification Alternatives – 1. Cut off and Found.
New Foundation
$
gflood = 53.6 pcf
f’ = 29 deg
kp-y = 50 pci
P-y curve – API Sand
Full Depth Cut-Off (~100 feet)
$$$
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Modification Alternatives – 2. RW and Found.
gflood = 25.4 pcf
f’ = 29 deg
kp-y = 50 pci
P-y curve – API Sand
New Foundation
$$
Relief
Wells
$
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Selected Modification Alternative RW and Found.
Relief
Wells –
2nd
contract
Structural Modification – 1st Contract
24” Steel Casing,
HP 12x74
New Foundation
Cap
Extension
and
Buttresses
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Load Test Planning and Considerations
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ASTM D 1143 loading procedures B “Maintained Load Test”
and C “Loading in Excess of Maintained Test” (2 hr holds)
Estimate drained response (need extended static holds – 2
24-hr holds lateral and 1 24-hr hold axial)
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“Production Style” shafts for combined/lateral
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~40 kip lateral and ~35 kip axial design loads
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Groundwater conditions and stress states from load test
to design condition are very different (link with s’)
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Load Test Goals
 Variables in Axial Analysis
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f’
g
Interface friction, d
Reasonably Known for Design Case
Nice to Validate with Load Test
 Variables in Lateral/Group Analysis
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f’
Reasonably Known for Design Case
g
Sand p-y curve
Need to Validate with Load Test
Kp-y
Axial response curves
Nice to have from Axial Load Test
 Combined Load Test – structural performance of hybrid shaft
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Load Test Overview – Axial
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Figures and photos courtesy Dan Brown and Associates.
Load Test Overview – Lateral/Combined
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Figures and photos courtesy Dan Brown and Associates.
Axial Load Test Results
2 hr
130 kip 24 hr hold
Axial Results
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Data courtesy Dan Brown and Associates.
Lateral Load Test Results
120 kip 24 hr hold
head
Lateral Results
60 kip 24 hr hold
2 hr
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Data courtesy Dan Brown and Associates.
Load Test Results – Applicability to Design Case
 Drained conditions “reasonably” approximated during load
test
 Back analyze load test responses to calibrate lateral model
 Need state of stress during lateral load test (including suction)
 effective stress model applicable to both design and load test
conditions (Lpile is frictional - f, g )
 Kp-y will be over-estimated in back analysis of load test if suction is
ignored.
Design Water Surface
Normal Ground Water
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Load Test Effective Stress - Soil Suction
 Soil Water Characteristic Curve (SWCC) ASTM D 6836
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Relates in situ volumetric water content to soil suction
Suction profile with depth = effective stress profile
“gunsat” > g
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Shear Strength with Soil Suction
 Estimating shear strength with soil suction
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Khalili and Khabazz (1998)
ts = c’ + svtanf’ + Cytanf’
Where,
ts = unsaturated shear strength
c’ = drained cohesion (zero)
sv = gravity stress
y = matrix suction
f’ = drained friction angle
C = fitting parameter
 Can’t input ts directly into a frictional L-Pile model…
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Considering Soil Suction in LPile
 Calculate a Modified Friction Angle to account for soil suction
svtanf’ + Cytanf’ = svtanfm’
where fm’ = modified friction angle
 Solve for fm’ for blanket to get an applicable friction angle that
is f(suction).
 Assumes fm’ that results in appropriate ts is reasonable to account for
suction in a frictional model.
Material
f’
fm’
Blanket
29
39
Sand
36
36
 Necessary because Ensoft doesn’t have ability to directly account for U.
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Axial Load Test Interpretation
 Soil/Casing interface friction angle
 Assumed f=d, measured 1.1f=d (conservatism or
incomplete drainage?)
 Axial Response curves
 Develop normalized (to ultimate capacity) side resistance
and tip resistance response curves for use in Group
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Lateral Load Test – Back Analysis w/ normal
GWT and suction
 Calibrate Kp-y for verification of design
 Assumes Kp-y same for all states of stress for effective stress
analysis
Solve for this
ggravity = 115 pcf
fm’ = 39 deg
kp-y = Variable
P-y curve – API Sand
g’gravity = 53.6 pcf
f’ = 36 deg
kp-y = Variable
P-y curve – API Sand
Verify this is
appropriate
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Lateral Load Test Back Analysis Results
Load Test
Calibrated Analysis
Working
Load
30 kip
60 kip
Original
Calibrated
Material
Kp-y
Kp-y
Blanket
50
55
Sand
60
130
Conservative original estimate?
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Major Findings and Lessons Learned
 Load Test –
 “Drained” conditions approximated during 2 hr load steps
 A complete test with 24 hr minimum holds next time?
 “Sand” p-y curves approximate drained behavior of fine grained soil
 Modified friction angle can account for soil suction in Lpile
 Load and temperature variations can be problematic during extended
static holds
 Consider direct U dissipation measurement adjacent to shaft
 Design –
 Can reduce FSmin if load test performed during design
 Kp-y was reasonably estimated prior to load test
 Ensoft programs account for effective stress design
 Accounting for U directly would be an improvement
 FLAC or finite element could improve understanding
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Construction – Shaft Installation
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Construction – Cap Extension
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Construction – Completed Wall Modification
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Questions?
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