field performance data and support of excavation …...field performance data and support of...

Field performance data and support of excavation design

Richard J. Finno, PE, PhD, DGE

Ralph B. Peck Medal Lecture presented at 50th Annual GEC, University of Kansas / Nov 8, 2018

AcknowledgementsHayward BakerSchnabel Foundation CompanyThatcher Engineering Corp.Case FoundationWJE & AssociatesGeoEngineers, Inc.Getec, Inc.

Turner ConstructionWalsh ConstructionBoard of Underground – City of Chicago

Funding provided by National Science Foundation and Infrastructure Technology Institute at Northwestern University

Scott Perkins, Steve Nerby, Sebastian Bryson, Michele Calvello, Paul Sabatini, Dan Priest, Jill Roboski, Kristi Kawamura, Tanner Blackburn, Terry Holman, Wanjei Cho, Young-Hoon Jung, Greg Andrianis, Miltos Langousis, Cecilia Rechea, Taesik Kim, Fernando Sarabia, Luis Arboleda, Charlotte Insull, Zhenhao Shi, Nathan Van Winkle, Sangrae Kim – former Northwestern University graduate students


Professor Ralph B. Peck

TeacherEngineerResearcher

Assistant subway engineer for the city of Chicago, 1939-1942


• Stability based design (pre-1980)• Serviceability based design

– Design based on stiffness of support system– Adaptive management (getting the most out of

field measurements): Chicago-State and SQBRC excavations

• Concluding remarks

OutlineRalph B. Peck Medal Lecture presented at 50th Annual GEC, University of Kansas / Nov 8, 2018

Presenter

Presentation Notes

Tonight I will discuss how field measurements have impacted the design of excavation support from the beginnings of stability based design to now when at least in urban areas, more often than not, the main design consideration is serviceability

After Terzaghi (1936) Fig.4

Experience did not match Coulomb or Rankine earth pressure distributions for retained sands

Higher apparent stresses at top and lower at bottom of cut

𝑝𝑝𝑎𝑎 = 𝛾𝛾𝑧𝑧 𝑡𝑡𝑎𝑎𝑡𝑡2 �45 −𝜑𝜑2� = 𝐾𝐾𝑎𝑎 𝛾𝛾𝑧𝑧


Apparent Earth Pressure Envelopes• Measured loads in cross-lot braces• For a given soil condition

– At each excavation• Loads in each brace divided by tributary area• Selected maximum apparent pressure at each

level– For all excavations, defined envelope of

maxima • Developed loading diagrams for sands,

stiff clays and soft clays

Details found in PhD thesis by Flaate (1966)


Data base: 9 Chicago 9 Oslo 4 Japan 1 England

T&P Apparent Earth Pressure EnvelopeSoft clay


Factors affecting strut loads• Earth and water pressures• Workmanship• Preloading• Temperature


Peck (1969) diagram for settlements measured during excavation projects

Zone I: Sand and clay with average workmanship

Zone II: Very soft to soft clay with limited depth below b/cut

Zone III: Very soft clay to large depth below cut

Settl

emen

t_ (%

)

Dep

th o

f exc

avat

ion

Distance from excavationDepth of excavation

Problems with caisson installation (soil squeeze or

pumped fines)


failure of wale

Effects of redundant (very conservative) designRalph B. Peck Medal Lecture presented at 50th Annual GEC, University of Kansas / Nov 8, 2018

Summary of stability-based design

• Stability-based design arose from field measurements of loads in bracing

• Conservative design based on apparent earth pressure envelopes – generally leads to a redundant system

• Does not consider explicitly consider ground movements associated with excavation


Serviceability-based design• Select support components to maintain

ground movements within acceptable levels• Became possible because of application of

finite element analyses to excavation support problems

• Mana and Clough (1981) and Clough et al (1989) - semi-empirical approach based on system stiffness and FS basal heave

• Today - not uncommon to use FE analysis to size support elements


Movements in soft clays

Clough and O’Rourke (1990)

Sands and hard clays0.3


Soil structure interactionRalph B. Peck Medal Lecture presented at 50th Annual GEC, University of Kansas / Nov 8, 2018

Clough et al. 1989

Typical stratigraphy Undrained strength (kPa) Water content (%)


Movement predictions

• Depend on soil conditions, constitutive response of soil, retention system stiffness and construction procedures

• Two step process – Precedent – Site specific (numerical method)


=

Simulation GoalConstructionFE proceduresConstitutive modeling

Instrumentation - verification


Chicago-State ExcavationRalph B. Peck Medal Lecture presented at 50th Annual GEC, University of Kansas / Nov 8, 2018

Presenter

Presentation Notes

I am going to discuss several excavations to illustrate how field performance data can and should be integrated into the design of excavation support. The first is Chicago-state project where the excavation allowed for renovation of a subway station. – one that Prof. Peck worked on in 1940. In this case, allowable movements were limited to 1 ¼ inch of building settlement.

Section view of excavation support

Original design: sheet pile wall and internal bracing

Original bracing levels

3 story reinforced concrete frame

structure


Approach to design• Computed maximum horizontal wall deformation

as a function of system stiffness• Computed the settlement distribution behind the

wall per Hsieh and Ou (1998)• Settlement profile input to a FE structural model of

building to evaluate damage potential in terms of computed tensile stresses

• Established following performance specifications:


Instrumentation plan

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-5

0

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10-50 -25 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350

Elev

atio

n (m

CC

D)

1 - Install SP Wall (Day 2 to Day 11)2 - Install Strut 4 (Day 60)3 - Install Strut 5 (Day 74)4 - Stress 1st Level Tieback (Day 87)5 - Stress 2nd Level Tieback (Day 105)

7 - Pour Base Slab (Day 172 to Day 177)8 - Pour E. Ext. Wall (Day 197 to Day 207)9 - Remove Struts 4, 5, and 6 (Day 258)6 - Chip Face to Flange EL +3.35 to EL -7.62 (Day 110 to Day 140)

1 1 2 3

4 5

6 6 7

7

8 8 9

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Construction Day Number

Mov

emen

t (m

m)

EL -3.7 (m CCD)

EL -7.9 (m CCD)

EL -11 (m CCD)

School Settlement (W10)

Wall installationExcavation and support Station renovation and backfill


2 4 6 8 10 12 14 16 18 20Distance from Centerline of Wall (m)

30

25

20

15

10

5

0

Settl

emen

t (m

m)

SettlementDay 15Day 49Day 86Day 103Day 117

30 25 20 15 10 5 0

LateralDeformation (mm)

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Elevation(m CCD)

Lateral DeformationsDay 15 - 1st Lat. and vert. measurement after wall installedDay 59 - Excavate below strut level (EL +2.7 m CCD)Day 81 - Excavate below 1st tieback level (EL -1.4 m CCD)Day 87 - 1st tieback level stressedDay 102 - Excavate below 2nd tieback level (EL -5.2 m CCD)Day 116 - Excavate to final grade (EL -7.9 m CCD)

Secant Pile Wall

Frances Xavier Warde School

CL

San

d an

d Fi

llS

tiff

Cla

yS

oft

Cla

yM

edC

lay

Med

ium

Cla

yS

tiff

Cla

yH

ard

Cla

y

(Day 105)

(Day 116)

(Day 60 to Day 74)(Day 59)

(Day 87)

(Day 81)

(Day 102)

W11 W10 C1 C2

Lateral movements and settlement

Wall installationExcavation


Summary of Chicago-State project

• Stiffness based design of support system and proximity to adjacent structures necessitated detail performance monitoring

• Requirement for small allowable movements dictated identifying all sources of movements – Secant pile installation, cycles of excavation

and support and long-term movements• Detailed performance data allowed

development of “adaptive management” approach


• Quantitatively evaluate performance of a system based on observations

• Use observations to update performance predictions

• Plan project so alternative procedures can be applied depending on observed performance – a design approach

What is adaptive management?


Adaptive management – automated observational approach

Input

Calculation

Curvesor Output

Prediction

Initial Design

Minimize differencebetween Observed andCalculated Responses

Data Collection

Data Analysis

Curvesor Output

Monitoring

New Parameters

Improved prediction/adjusted design

Optimization


NO YES

Parameter optimization algorithm

Observations

Computedresults

Optimizedinput parameters

Is the modeloptimized?

FE run

Compute Objective Function

ComputeSensitivity X

Perform NonlinearRegression

Updatedinput parameters

Initialinput parameters

FE run Updatedresults

Perturb bk byuser-defined amount

FE run

Computed results

Calculate Xik=∂yi/∂bk(for i = 1 to ND)

Repeat fork=1 to NP

ND = Number of observationsNP = Number of parameters to optimize

MATLAB

( ) ( ) ( )' 'T

S b y y b y y bω = − −


• Identify control parameters (inclinometer data supplemented by settlement data)

• Prediction method ~ finite element method• Observations must be reliable, accurate and

obtained in a timely fashion• Constitutive model must have capability to

replicate behavior that is observed• Assume differences in observed and

computed responses are due to soil responses• Contingency options must be planned in

design stage

Key elements of adaptive managementRalph B. Peck Medal Lecture presented at 50th Annual GEC, University of Kansas / Nov 8, 2018

Presenter

Presentation Notes

Interaction between items 3 and 4 Included in bid package

Benefits of adaptive managements

• Optimize soil parameters at an early stage of excavation so updated predictions of later stages can be made

• Use optimized parameters from one site to from the basis of predictions of performance of another project in similar conditions


Optimize parameters at early stage of

excavation


Stage 4 Stage 5 Stage 1 Stage 2 Stage 3

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displacement (mm)

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displacement (mm)

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displacement (mm)

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displacement (mm)

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diplacement (mm)

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Field data

Computed displacements (initial)

Computed displacements (best-fit)

Soil parameters based on laboratory tests

Computed

Observed

Computed

Observed

East

West


Stage 4 Stage 5 Stage 1 Stage 2 Stage 3

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Computed displacements

Observations used forregression analysis

1

2

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45

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45

EASTSIDE

WESTSIDE

Optimize with this data set


Use of parameters optimized at other projects in same geology

• Lurie Center excavation• Ford Center excavation• Block 37 excavation• SQBRC excavation


Louis A. Simpson & Kimberly K. Querry Biomedical Research Center (SQBRC) excavation


Support system at north wall

PZ 3707 and PZ 2607

36 in. x 5/8 in. pipe struts

Urban Fill

Beach sand

Medium clay

Stiff clay

Basal till “hardpan”

Utility tunnel

Shape accelerations arraysRobotic total stationWeb cameraProject website


Lateral wall movements

Values taken at elevation where maximum value occurred


SAA1 - North Side

Wall movements as excavation is lowered below strut

Large incremental movements

Stage 3

Stage 5

Stage 7


Use of parameters optimized at other projects in same geology

• Three soil models – Hardening soil (Schanz et al 1999): Chicago-

State, Ford and Lurie Centers excavations– Hardening soil small (Benz (2007): Block 37

excavation– Hypoplasticity clay (Masin 2014): Laboratory

data from block samples taken from Ford and Block 37 excavation

• Applied to SQBRC excavation with computed results compared to observed wall deformations


Computed and observed lateral wall movements

Stage 3

Stage 5

Stage 7

± 5 mm± 5 mm


HS model: no small strain stiffness

Optimized values based on movements at end of excavation


Stress-strain characterization

Bender elements Internal instrumentation


Block 37 dataG0 based on bender element results at end of consolidation

Obtainable in conventional TX device

Gconstant ?

Stiffness variation with strainRalph B. Peck Medal Lecture presented at 50th Annual GEC, University of Kansas / Nov 8, 2018

Lateral displacements near wall dominated by εH max

Settlement distribution depends on all strain levels

Variable moduli (e.g. elasto-plastic model) can be used to compute lateral movements near wall

Small strain non-linearity and dilatancy must be included for settlement distributions

γ (%)

Shear strains at end of excavation


Shear strain

Maximum shear strain behind wall - SQBRC

Need for use of small strain stiffness Ralph B. Peck Medal Lecture presented at 50th Annual GEC, University of Kansas / Nov 8, 2018

Ignored when small strain stiffness not considered

Effect of constitutive model on computed settlements

2 DE

MC – underpredicts max. settlement and distortion but overpredicts extent of movements: true for any model with constant elastic modulus

Less distortion


Summary – adaptive management

• Current capabilities– Plane strain simulations– Soil models should include small strain

stiffness – Updates possible in “real” time

• Removes uncertainties with construction procedures

• Design approach if contingencies included

• Cannot simulate all activities


Movements from causes other than excavation and bracing cycles

• Site preparation• Removal of existing foundations• Wall installation

– Densification of sands from vibrations– Displacements arising during wall installation

• Slurry or secant pile wall• Sheet-pile wall

• Deep foundation installation• Concrete shrinkage during top-down

construction


Concluding remarks

• Field observations form basis of design of excavation support

• The process of predicting, monitoring and updating (adaptive management) is a useful design tool

• Relatively small movements that arise from sources other that stress relief from excavation - and not normally considered in design - become more important when allowable movements are small


Presenter

Presentation Notes

Peck’s legacy lives on (first bullet)

Concluding remarksTrend is to use FE simulations to size support components; Approach minimizes amount of support and computes deformations associated with excavation

• But loads are not based on “envelopes of maxima” strut loads

• temperature effects neglected and idealized construction process, less redundant design

• Movement predictions generally only include those from the excavation process

• Warrants detailed monitoring program


Presenter

Presentation Notes

“It is what your learn after you know it all that

counts”John Wooden


field performance data and support of excavation …...field performance data and support of...

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