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US Army Corps of Engineers
BUILDING STRONG®
Effective Stress Design For Floodwalls
on Deep Foundations
Glen Bellew, PE
Geotechnical Engineer
USACE-Kansas City
23 April 2015
Paul Axtell, PE, D.GE
Dan Brown and Associates
James Mehnert, PE
USACE-Kansas City
Contributors
BUILDING STRONG®
Outline
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
BUILDING STRONG®
Project Location – Fairfax Jersey Creek Levee
Kansas River
Missouri River
Fairfax-Jersey
Creek Levee
Unit
BPU Floodwall
BUILDING STRONG®
Project and Leveed Area Details
Levee/Flood Wall
constructed 1940’s
by USACE
Highly Developed
Area (~$3.3 billion)
Critical
Infrastructure
(Power Plant, water
treatment)
Major
Manufacturing (GM
Plant)
Kansas River
BPU Floodwall
1400 ft
BUILDING STRONG®
Existing Floodwall and Subsurface Conditions
~80 ft Sand
~20 ft
~16 ft
CL/ML
~20 ft
Fluted, Tapered
Steel Pipe Piles
Sheet Pile g=119 pcf
g=116 pcf
BUILDING STRONG®
Non Critical Load Case – Short Term Flood
~Horizontal Seepage
No time for
blanket seepage
Pre-flood s’ and
stress history
control Su
Typical
infrastructure
analysis, buildings,
bridges, etc.
Sand, f’, g’
BUILDING STRONG®
Critical Load Case – Long Term Flood
~Horizontal Seepage
~Vertical
Seepage,
reduces s’
Effective Stress
Controls behavior,
f’, gflood
Sand, f’, g’
Analysis specific to
water retention
structures
BUILDING STRONG®
Effective Stress Design Process
Establish seepage conditions (effective stress)
Determine Ultimate Axial pile capacity
Lateral response of pile group (often controls design)
Calibrate analysis to observed performance
BUILDING STRONG®
Seepage Analysis Criteria
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
i=Dh/z z
Dh
BUILDING STRONG®
Seepage Analysis Methodology – calculating h
Blanket Theory (EM 1110-2-1913)
► Simple geometric inputs (great for simple stratigraphy)
► Decades of performance to verify adequacy of method
► Spreadsheet solutions – quick to perform
BUILDING STRONG®
Seepage Analysis Methodology – calculating h
Finite Element Modeling (next EM 1110-2-1913)
► Unlimited complexity in geometry and boundary conditions
► Modeling quirks can lead to unrealistic results for a novice user
► In situ permeabilities, boundary conditions, model extent
► User interface improving, but can be time consuming to set up
► Use when complexity warrants
BUILDING STRONG®
Pile Design Methodology – Axial Capacity
Overall
► 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
► Bearing Capacity Factors
BUILDING STRONG®
Pile Design Methodology – Lateral Response
Typical to use Ensoft’s Lpile and/or Group Software
► 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”
BUILDING STRONG®
Effective Stress Lateral Response - Ensoft
Design Case – Long Term Flood
► 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)
BUILDING STRONG®
~3 ft
Performance Observations- 1993
Seepage – some reports of concentrated
seeps with possible pin boils, no major boil
activity
Structural Performance – no performance
observations noted
Documentation limited…
~45 Day duration
BUILDING STRONG®
Calibrate with Back Analysis of 1993 Flood?
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
iavg = 0.7
RESULTS
Seepage: FS~1.3
Pile Capacity: FS = 1.5
Pile Structural >failure
Deflections – 1.5” max
BUILDING STRONG®
No failure predicted, none observed…
0
5
10
15
20
25
30
35
40
45
0 1 2 3 4 5 6 7 8 9 10
Pro
bab
ilit
y o
f F
ailu
re (
%)
Loading
Probability of Failure – “Brittle” Response
Maximum
Historical
Load
Maximum
Possible
Load
Example fragility curve, not BPU floodwall
BUILDING STRONG®
Existing Floodwall – Analysis w/ water @ TOW
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
iavg = 0.83
FSi = 1.1
BUILDING STRONG®
Existing Floodwall – Results w/ water @ TOW
Axial FS <1
Deflections >>1.5”
Floodwall
modification needed
BUILDING STRONG®
Design/Site Constraints
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)
Riverside
Landside
BUILDING STRONG®
Modification Alternatives – 1. Cut off and Found.
Full Depth Cut-Off (~100 feet)
$$$
New
Foundation
$
gflood = 53.6 pcf
f’ = 29 deg
kp-y = 50 pci
P-y curve – API Sand
BUILDING STRONG®
Modification Alternatives – 2. RW and Found.
Relief
Wells
$
New
Foundation
$$
gflood = 25.4 pcf
f’ = 29 deg
kp-y = 50 pci
P-y curve – API Sand
BUILDING STRONG®
Selected Modification Alternative RW and Found.
Relief
Wells –
2nd
contract
Cap
Extension
and
Buttresses
New
Foundation
Structural Modification – 1st Contract
24” Steel Casing,
HP 12x74
BUILDING STRONG®
Load Test Planning and Considerations
► 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)
► “Production Style” shafts for combined/lateral
► ~40 kip lateral and ~35 kip axial design loads
► Groundwater conditions and stress states from load test
to design condition are very different (link with s’)
BUILDING STRONG®
Load Test Goals
Variables in Axial Analysis
► f’
► g
► Interface friction, d
Variables in Lateral/Group Analysis
► f’
► g
► Sand p-y curve
► Kp-y
► Axial response curves
Reasonably Known for Design Case
Reasonably Known for Design Case
Need to Validate with Load Test
Nice to have from Axial Load Test
Nice to Validate with Load Test
Combined Load Test – structural performance of hybrid shaft
BUILDING STRONG®
Load Test Overview – Axial
Figures and photos courtesy Dan Brown and Associates.
BUILDING STRONG®
Load Test Overview – Lateral/Combined
Figures and photos courtesy Dan Brown and Associates.
BUILDING STRONG®
Axial Load Test Results
130 kip 24 hr hold
Axial Results
Data courtesy Dan Brown and Associates.
2 hr
BUILDING STRONG®
Lateral Load Test Results
60 kip 24 hr hold
120 kip 24 hr hold
Lateral Results h
ea
d
Data courtesy Dan Brown and Associates.
2 hr
BUILDING STRONG®
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
BUILDING STRONG®
Load Test Effective Stress - Soil Suction
Soil Water Characteristic Curve (SWCC) ASTM D 6836
► Relates in situ volumetric water content to soil suction
► Suction profile with depth = effective stress profile
► “gunsat” > g
BUILDING STRONG®
Shear Strength with Soil Suction
Estimating shear strength with soil suction
► 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…
BUILDING STRONG®
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.
Necessary because Ensoft doesn’t have ability to directly account for U.
Material f’ fm’
Blanket 29 39
Sand 36 36
BUILDING STRONG®
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
BUILDING STRONG®
Lateral Load Test – Back Analysis w/ normal
GWT and suction
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
Solve for this
Verify this is
appropriate
Calibrate Kp-y for verification of design
Assumes Kp-y same for all states of stress for effective stress
analysis
BUILDING STRONG®
Load Test
Calibrated Analysis
Lateral Load Test Back Analysis Results
30 kip
60 kip
Original Calibrated
Material Kp-y Kp-y
Blanket 50 55
Sand 60 130 Conservative original estimate?
Working
Load
BUILDING STRONG®
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
BUILDING STRONG®
Construction – Shaft Installation
BUILDING STRONG®
Construction – Cap Extension
BUILDING STRONG®
Construction – Completed Wall Modification
BUILDING STRONG®
Questions?