MICROPILES RESEARCH AT WASHINGTON STATE

UNIVERSITY

Dr. Adrián Rodríguez-Marek,

Dr. Balasingam Muhunthan, and Dr. Rafik Itani

Civil and Environmental Engineering Department. Pullman, WA 99164-2910

International Workshop on MicropilesVenice, Italy

June 1, 2002

Objectives Comprehensive literature review

Update to FHWA State of the Practice State of the Art in analytical methods Experimental data

Develop and validate analytical tools for Micropile networks Static loading Dynamic loading

Design Guidelines Design guidelines for battered micropiles Take systematic advantage of network effects (static

and dynamic)

Research Approach

MICROPILE PERFORMANCE

DATA

NUMERICAL MODELS

Finite Element Ousta and Sharour WSU FE implementation

(e.g. Modak 2000)Finite Difference Pseudo-Static (e.g. LPILE, GROUP) Dynamic (e.g. FLAC)

Empirical p-y curves

Calibration and Validation

SIMPLIFIED ANALYTICAL

APPROACH

- Center of rotation/Elastic Center

- Transfer Matrix

Calibration Calibration

DESIGN GUIDELINES

Outline Of Presentation

Focus on: Experimental needs (Rodríguez-Marek) Considerations for static design of Micropiles

(Muhunthan)

Available Data on Micropile Performance

Vertical, static loading Extensive availability of data

Static lateral loading Field test: Bruce, Weinstein, and Juran

Dynamic lateral loading Centrifuge tests with seismic loading (Juran et al.

1998) Shaking table tests (Kishishita 2001)

NEEDS Full scale lateral load tests with dynamic loads Field instrumentation

National Earthquake Simulation Network

NSF funded network of test facilities for advancing the understanding of earthquake engineering

Objective: Develop test facilities that will become available to the earthquake engineering community in general (to be ready by 2004)

OPPORTUNITY: Greater access to test facilities (e.g. centrifuge testing) and field testing equipment

Eccentric shaker, MK-15

• Uni-directional eccentric mass vibrator

• Operating frequency range: .25 – 25 Hz

• Force capability: 440 kN (100,000 lbs)

• Weight: 27 kN (6000 lbs)

• Dimension: 1.8 m x 3 m

Eccentric Mass ShakerUCLA NEES equipment site (PI: Dr. John Wallace)

Dynamic Lateral-Load Field Tests: Objectives

Quantify the effects of inclination, configuration, and spacing on load transfer mechanism and foundation response of micropile groups (and networks)

To obtain ultimate lateral capacities for single micropiles and micropile groups

Obtain field p-y curves Effect of cyclic loading at varying strain levels “Scale” effects Comparison with commonly used p-y curves Validation of pseudo-static analyses (e.g. GROUP)

Characterize dynamic impedance functions for micropile foundations

Tentative Test Site

Site: Caltrans’ property Low marginal cost for

Micropile tests Fully-characterized site

Field tests: SCPT, SPT, PMT, and down-hole suspension logging

Laboratory tests: Atterberg Limits, Consolidation & UU Triaxial Tests

Extensive field tests of Drilled Shafts performed at this site

F I L L , c o n c r e t e

a n d a s p h a l t d e b r i s

C L A Y , s i l t y s a n d y

S I L T , f i n e s a n d s

C L A Y , s i l t y s a n d y

S A N D , s i l t y m e d i u m

t o f i n e - g r a i n e d

C L A Y , s l i g h t l y s i l t y

S A N D , f i n e - g r a i n e d

0

5

1 0

1 5

2 0

2 5

3 0

3 5

4 0

4 5

5 0

De

pth

(ft

)0 1 0 2 0 3 0 4 0 5 0

( N 1 ) 6 0

0 1 2 3 4 5 6

S u ( k s f )

U - U

P M T

0 3 6 9 1 2

S o i l S t r e s s ( k s f )

w t

0 2 5 5 0 7 5

W n ( % )

W nL LP L

v

' p

0 1 0 0 2 0 0 3 0 0

N o r m a l i z e d C o n e R e s i s t a n c e , Q

M e a n Q

M e a n Q + / - 1 S t d

Summary

Full scale dynamic lateral load tests of micropiles are important Assess “Scale Effects” associated with:

• Model tests• Design formulas based on large-diameter piles

Field tests will be performed side by side to full-scale tests of drilled shafts

One tentative test site has been identified (other sites will be explored)

• Sand Site: Group efficiency factors as a function of construction methods

• Soft-Clay sites: Evaluation of ultimate capacities

Summary

Other issues Include pile non-linearity in evaluation of field

p-y field curves Incorporation of measurement errors into

back-calculation of p-y curves Quantification of lateral soil pressures during

testing

Static Design Of MICROPILES

PROBLEM:  PILE CAPACITY & SETTLEMENT

SINGLE GROUP

• VERTICAL• RETICULATED NETWORK

CURRENT STATE (Capacity)

Most design based on relative density (Dr

or ID )

Influence of stress level on strength of soil (rarely taken into account)

No account of compressibility (intended for quartzitic sands; other weak minerals?)

Contradictory results (Literature)

P r e d i c t e d q c ( M P a ) I D ( % ) z / ( 0 . 5 B ) D ( m ) o N q M e a s u r e d

q c ( M P a ) E q . 1 ( M P a ) E q . 2

5 8 2 1 . 4 3 8 . 7 1 3 3 . 6 1 . 1 9 2 . 8 4 1 . 6 5

5 8 4 2 . 8 3 7 . 2 1 8 6 . 3 3 . 5 1 7 . 9 3 4 . 7 3

5 8 6 4 . 2 3 6 . 2 2 5 0 . 1 6 . 8 8 1 6 . 0 9 . 7 0

8 0 2 1 . 4 4 2 . 3 2 2 7 . 7 1 . 2 5 . 1 0 2 . 8 1

8 0 4 2 . 8 4 0 . 0 2 7 4 . 6 3 . 9 9 1 2 . 3 7 . 0 3

8 0 6 4 . 2 3 8 . 7 3 4 3 . 3 8 . 2 5 2 3 . 0 7 1 3 . 4 5

8 0 8 5 . 6 3 7 . 8 4 1 7 . 2 1 3 . 3 1 3 7 . 3 8 2 2 . 1 0

8 9 2 1 . 4 4 3 . 6 2 7 7 . 4 1 . 4 6 . 3 3 3 . 4 2

8 9 4 2 . 8 4 0 . 9 3 1 1 . 2 5 . 2 1 4 . 2 0 8 . 0 0

8 9 6 4 . 2 3 9 . 4 3 7 1 . 5 1 0 . 9 1 2 5 . 4 3 1 4 . 6 7

8 9 8 5 . 6 3 8 . 4 4 6 6 . 7 1 7 . 3 6 4 2 . 6 0 2 5 . 0 0

C o m p a r i s o n o f q c b e t w e e n c e n t r i f u g e a n d c o n s t a n t – a n a l y s i s

( G u i a n d M u h u n t h a n , 2 0 0 2 )

qvcuNcNq E q . ( 1 )

qv

0

cuN

3

K21cNq

E q . ( 2 )

Predicted qc (MPa) ID(%) z/(0.5B) D(m) o Nq Measured

qc (MPa) Eq.1 Eq.2

58 2 1.4 38.7 106.3 1.19 2.26 1.32

58 4 2.8 37.2 172.7 3.51 7.35 4.38

58 6 4.2 36.2 248.0 6.88 15.86 9.62

80 2 1.4 42.3 146.6 1.2 3.28 1.80

80 4 2.8 40.0 225.3 3.99 10.09 5.77

80 6 4.2 38.7 303.0 8.25 20.36 11.87

80 8 5.6 37.8 415.6 13.31 37.24 22.00

89 2 1.4 43.6 164.0 1.4 3.74 2.02

89 4 2.8 40.9 247.1 5.2 11.28 6.36

89 6 4.2 39.4 333.1 10.91 22.80 13.15

89 8 5.6 38.4 421.6 17.36 38.48 22.00

Comparison of qc between centrifuge and variable – analysis (accounting for stress level) (Gui &

Muhunthan, 2002)

ID(%) z/(0.5B) D(m) o Nq Predicted qc (MPa)

G50 Fqc Measured qc

(MPa)

Predicted qc(Ir)

Fqc*Eq.2 2 1.4 38.7 106.3 1.32 2971 0.85 1.19 1.13

4 2.8 37.2 172.7 4.38 3999 0.84 3.51 3.67

6 4.2 36.2 248.0 9.62 4817 0.83 6.88 8.00

2 1.4 42.3 146.6 1.80 4938 0.77 1.2 1.38

4 2.8 40.0 225.3 5.77 6655 0.83 3.99 4.77

6 4.2 38.7 303.0 11.87 8020 0.85 8.25 10.07

8 5.6 37.8 415.6 22.00 9189 0.86 13.31 18.87

2 1.4 43.6 164.0 2.02 6193 0.75 1.4 1.51

4 2.8 40.9 247.1 6.36 8353 0.85 5.2 5.39

6 4.2 39.4 333.1 13.15 10070 0.89 10.91 11.66

8 5.6 38.4 421.6 22.00 11540 0.91 17.36 19.92

Comparison of qc between centrifuge and variable – analysis (accounting for compressibility) (Gui &

Muhunthan, 2002)

SOIL BEHAVIOR (Critical State Soil Mechanics)

Zones of stable plastic yielding

Capacity of piles in sands is a function of the “in situ state” of soil as defined by the “state parameter, Rs” as compared with the relative

density, Dr, used in the conventional practice.

 Normalized pile capacity tends to decrease with increasing Rs or increasing depth.

Normalized pile capacity tends to converge or remain constant when the in situ soil state nears critical state or Rs converges to unity.

Constant Rs, would yield constant pile capacity,

stiffness, and compressibility Even Constant Cyclic strength of sand

Parallel contours of normalized cyclic strength of Ottawa sand (Pillai and Muhunthan 2001)

State Parameter

c

as p

pR

 

Rs < 1 - dilative behavior

 

Rs >1 - contractive behavior

Summary

NEED TO INTERPRET Single, Group, Network effects based on

STATE BASED SOIL MECHANICS Soil parameters (Capacity, stiffness) as

functions of Rs