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