Domain

Explanation

Speaker CV

Hon-Yim Ko is Glenn L. Murphy Chair of Engineering at the University of Colorado at Boulder, USA. A civil engineering graduate of the University of Hong Kong, he obtained his Ph.D. from the California Institute of Technology. Prior to joining the faculty at Colorado in 1967, he was a member of the lunar surface mechanical properties team on NASA's unmanned spacecraft Surveyor project managed by Caltech's Jet Propulsion Laboratory. At Colorado, his research has focused on multiaxial testing of geo-materials such as soil, rock and concrete, constitutive modeling of their properties, and centrifuge modeling of geotechnical structures. He has been engaged in numerous professional activities, including the chairmanship of ASCE's Soil Properties Committee, and of ISSMFE's Technical Committee on Centrifuge Testing.

Abstract

The performance of geotechnical structures is heavily influenced by the effects of gravity which provides the predominant loading while contributing to the overburden pressures that control the soil's stiffness and strength properties. Simulation of such effects during scaled model testing is essential and can be conveniently obtained through testing in a centrifuge. The principles of centrifuge modeling are briefly reviewed in this lecture, followed by illustrations of the application of this method in a number of diverse areas such as foundation systems for offshore structures, reinforced earth construction, and earthquake liquefaction studies.

Contents

• Need for centrifuge
• Examples
• Scaling relations
• Principle & philosophy
• Modeling seismic problems
• Centrifuge modeling
• Project VELACS
• Applications

Need for centrifuge

• To geotechnical physical modeling
• Gravity effects: self-weight effects & self-weight induced stresses governing material properties (strength & stiffness)
• Examples:
• Slope stability: stability number & scale factor,
• Where m: model & p: prototype
• Footing load: stress bulb propagation & stress contours
• Need to test scale models in centrifuge
• Centrifuge increases speed of consolidation
• Easier to do 3-D modeling in centrifuge due to friction effects in 2-D problems
• Must do something to centrifuge to deal with seismic dynamics

Scale relations

• Prototype & model often smaller than actual
• Hence, scaling relations need to be derived based on continuum mechanics
• By dimensional analysis of similarity requirement
• Differential equations of physical phenomena: analysis of consolidation equation shows that time factor T is inversely proportional to square of length:
• 
• 
• [model size] + [acc, g] = constant for appropriate scaling

Some scaling

Q

• l
• Acceleration
• Time (dynamics)
• Frequency

Prototype

• N
• 1
• N
• 1

Model

• 1
• N
• 1
• N

Principle of centrifuge

• To obtain a stress field that simulates the prototype condition to allow observations to be made that can otherwise be made only by full-scale prototypes
• Philosophy:
• Gravity-dependent problems: liquefaction, explosive cratering & impacts
• Design verification
• Parametric study for database to compare results
• Validation of analysis

Centrifuge modeling

• Stages:

1. Lab testing: soil properties & parameter initiation
2. Constitutive modeling: model development, initiation, data reduction
3. Numerical modeling: FEM of prototype & centrifuge model
4. Centrifuge model: scale model; problem of field; modeling of models
5. Field problem/prototype prediction

Modeling seismic problems

• Need: occurrence of unpredictable; increased numerical pitfalls
• Difficulties:
• B.C.: laminated or rigid container, e.g. retaining wall
• Conflict in time scaling for diffusion & dynamic phenomena
• Need substitute materials

Project

• VELACS
• Objective: database by centrifuge testing for verification of liquefaction analysis methods

Applications

• Stability of gravity offshore platform:
• Platform subjected to impacts by ice-bergs
• Modeling: 200-scale model
• Test 1: saturated sand without clay layer
• Test 2: saturated sand with clay layer
• Cone penetration test: with load cells, pore pressure transducers
• Experiment vs. numerical: stresses, pore pressures, modes of failures
• Parametric study
• 500-year earthquake:
• Max 0.2g: far-field EQ
• Centrifuge container with accelerometers & pore pressure transducers in sand layers
• Effects of sinkholes on earthdams:
• Sinkholes: holes sinking in the middle of dams due to erosion of materials due to presence of weak seams in soil layers
• Model equivalently reduced to level sand layer in container with a central piston that can rise or sink to form sinkhole
• Numerical validation using PLAXIS
• Suction anchors:
• For anchoring cables to seafloor: ease of installation & apply suction to anchor more firmly
• Downward forces:
• Self-weight
• Suction force: by pumping out water from the top of inside of piston
• Upward forces:
• Tip soil resistance
• Side soil resistance
• Objective: find optimal haul-line to attach to the suction anchor (after full immersion in bedsoil) to generate max. horizontal tension in cable

Q&A

• Max grain size: ensure assumed continuum satisfied
• Pore pressure transducer:
• With contact due to soil intrusion à affects observations
• Measures pore pressures
• Avoid more difficult effective stresses & strains

Lessons

• Calm & slow pace
• Ensure all AV working & ready to start
• Cartoon overview
• Model layout
• Simplification & reduction