MICROPILES RESEARCH AT WASHINGTON STATE UNIVERSITY

1. 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 Micropiles Venice, Italy June 1, 2002

2. 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)

3. 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 Calibration Calibration • SIMPLIFIED ANALYTICAL APPROACH • Center of rotation/Elastic Center • Transfer Matrix DESIGN GUIDELINES

4. Outline Of Presentation • Focus on: • Experimental needs (Rodríguez-Marek) • Considerations for static design of Micropiles (Muhunthan)

5. 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

6. 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

7. Eccentric Mass Shaker UCLA NEES equipment site (PI: Dr. John Wallace) • 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

8. 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

9. 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

10. 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

11. 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

13. Static Design Of MICROPILES PROBLEM: •  PILE CAPACITY & SETTLEMENT • SINGLE • GROUP • VERTICAL • RETICULATED NETWORK

14. 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)

18. SOIL BEHAVIOR (Critical State Soil Mechanics) Zones of stable plastic yielding

19. 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. • Normalizedpile 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 ConstantCyclic strength of sand

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

21. State Parameter Rs < 1 - dilative behavior Rs >1 - contractive behavior

22. Summary • NEED TO INTERPRET • Single, Group, Network effects based on STATE BASED SOIL MECHANICS • Soil parameters (Capacity, stiffness) as functions of Rs