MEDAT-2: Some Geotechnical Opportunities. Site Characterization -- Opportunities. Down-hole CPT & vane (Fugro)

Transcription

1 MEDAT-2: Some Geotechnical Opportunities Ross W. Boulanger Department of Civil & Environmental Engineering University of California Davis, California Presentation for Workshop on Mitigation of Earthquake Disaster by Advanced Technologies (MEDAT-2). Sponsored by the MCEER Las Vegas, Nevada November 3 & December 1, 2 MEDAT-2: Some Geotechnical Opportunities Advanced technologies can be used to enhance: research to improve hazard assessment & understanding of physical systems field techniques for site characterization, damage assessment, quality control, and construction efficiency. Ground motions & structural response High-density networks of instruments around structures & treated sites Site characterization Modernize in situ tests & automate imaging techniques Liquefaction phenomena that remain poorly understood Void redistribution effects on residual shear strength Damage assessment and case history documentation Remote sensing for ground deformations, lateral spreading, or landslides Inspection tubes or remote sensors installed in deep foundations Remedial measures for liquefaction Treatment methods & important mechanisms Construction efficiency Quality assurance Site Characterization -- Opportunities Need to modernize SPT & Becker Penetration Test (BPT): advanced instrumentation & electronics automated recording of the delivered energy energy & force measurements at the sampler seek repeatability similar to CPT Large Penetrometer Test (LPT) needs development in USA. Down-hole CPT & vane (Fugro) Uses a wire-line retrieval system, with remote data acquisition and control. Can advance a hole through deposits with intermittent obstructions (e.g., large particles, hard strata) that a CPT could not otherwise advance through. Some advances in CPT tests: Vision Resistivity & electrical properties Down-hole remote system Tomographic imaging methods Advances in other in situ tests Remember the CPT s history of development.

2 Remote vane CPT Opportunities to Understand Basic Phenomena: Residual Shear Strength (S r ) of Liquefied Sand What is the residual shear strength of liquefied soil for locally undrained loading? Definition of strength? Stress path (including cyclic loading history) Fabric What is the strength mobilized in the field under the influence of pore pressure or void redistribution (i.e., local drainage) and other factors? To what extent are back-calculated strengths from case histories affected by void redistribution and other factors? Under what field conditions are void redistribution and other factors important or unimportant? Effect of Void Redistribution on Residual Shear Strength (S r ) Test 5: D r = 2%, Before shaking & after 2 shaking events 6.4 m Kulasingam, Malvick, Kutter, & Boulanger 4.4 m Mechanism B by NRC (1985) - Example of potential void redistribution within a globally undrained sand layer. Seed & Harder (199) Seed (1986) argued that S r values backcalculated from case histories of flow failures implicitly accounted for any effects that void redistribution and/or other factors may or may not have had.

3 Test 8: D r = 35%, Before shaking & after a short-duration event that triggered high r u values Test 8: D r = 35%, Before & after a 2 nd shaking event, which was the 1 st motion plus long-duration aftershocks 6.4 m 6.4 m Implications Void redistribution is more pronounced at the field scale than in laboratory test devices. The in situ S r of liquefied soil depends on: in situ boundary and loading conditions stratigraphy permeabilities earthquake characteristics stress path pre-earthquake soil properties and state. Improved Centrifuge Modeling of Liquefaction & Remediation using Advanced Technology Improving representation of prototype processes (in situ testing, construction) Instrumentation that is smaller, remote, more detailed -- better define the physics. What is the composite behavior of a heterogeneous treated mass (effects of reinforcement, drainage during and after shaking, time)? Designing for system performance: How much to treat, to what degree, and how does it affect the larger system? NEES funding will produce major advances in capabilities for the community. Understanding this phenomena will improve our estimates of S r and provide guidance on remediation strategies. Need advanced instrumentation to better define the physics of the behavior.

4 MEMS: 2 dual-axis accelerometers per cable σ v Network Controller and PC Interface Ac accelerometer on 2-sided PC Board µcontroller, SRAM, tranceiver Sensitive Axes Accelerometers σ v Exploring the ability to operate wireless. UC Davis - NEES Geotechnical Centrifuge Facility, Kutter et al. (2) Subsurface tomography for NEES project (Santamarino et al. 2) Centrifuge Robot hinged plate container, robot tools, tomography, and mesh of MEMS TOOL INSERTING PENETROMETER SECOND TOOL & CAMERA AT RACK probing with CPT penetrometers stereo robot eyes cross hole tomography ground improvement by deep vibration mesh of MEMS SHAKER TOOL RACK pile installation surface inspection with stereo video robotic vibroflot NORTH UC Davis - NEES Geotechnical Centrifuge Facility, Kutter et al. (2)

5 In-flight S-Wave & P-Wave Hammers Vacuum In-flight V S measurements in Nevada sand.8 A1 Forward hit Reverse hit A A2 Nevada Sand (D r 8 %) A2 3.2 m Air Hammer A3 A4 3.2 m 3.2 m Acceleration (g) A A4 Arulnathan, Boulanger, Kutter, & Sluis (2) Arulnathan, Boulanger, Kutter & Sluis (2) Time (s) Comparison to piezo-ceramic element tests on triaxial specimens Receiving Comparison of V S data from the centrifuge & triaxial specimens Receiving V s (m/s) 1 1 (a) Test 1 Test 2 Centrifuge Models V s = 65.8(σ m/ ).25 R 2 = / σ m (kpa) l b Specimen d l b Specimen d 1 Test 1 Test 2 Bender Elements in TX Transmitting Transmitting V s (m/s) 1 V = 67.9(σ s m / ).25 R 2 =.99 (b) / σ m (kpa)

6 Deep Soil Mixing & In-ground Walls Oriental Hotel in Kobe, 1995 In-ground walls improve liquefiable deposits by: Reducing earthquake-induced shear strains in the treatment zone, thereby limiting pore pressure development. Containing the enclosed soil should it liquefy, and thus contributing to the composite strength. Acting as a barrier to the migration of excess pore pressures from untreated to treated zones. Concerns include quality assurance & poor understanding of seismic behavior (e.g., what if they crack?) Crosshole Sonic Logging & Tomography for DSM or Jet Grouted Walls? Oriental Hotel: Extensive liquefaction around the perimeter & deformation of quay walls. No damage to foundation or evidence of liquefaction within the DSM walls (building footprint). (shown here for quality assurance of CIDH Piles -- Olson 1999) Other limited experiences in Kobe suggest that in-ground walls can be effective in mitigating liquefaction hazards (Hamada & Wakamatsu 1996) Physical model studies could be used to understand the earthquake behavior of in-ground wall systems. Automation by strings of receivers & transmitters lowered into preset casings in DSM or jet grout walls. Effect of irregular shapes and lower impedance contrasts between soil-cement and surrounding soil? Can we improve upon the limited success of past trials? Plus automated recording of construction sequence (depth, duration, pressures, etc.), injection volumes, spoil volumes, and other quantities. How to visualize and interpret such large volumes of data?

7 Visualization of Complex Systems: Physical model or FEM results Rendering tables can improve our comprehension of complex time-varying behavior. Boris Jeremic Thank you.