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?