Shear Wave Velocity-based Characterization Of Ageing Effect On Soil Liquefaction

Recent years, strong earthquakes happened frequently all over the world, resulting in wide range of disaster, among which liquefaction problem is particularly prominent. In terms of liquefaction evaluation, the present procedure widely used throughout the world is based on the correlation between liquefaction resistance and different in-situ soil indices such as the blow counts of standard penetration test (SPT), tip resistance of cone penetration test (CPT), and small strain shear wave velocity (Vs). During the past decades, many research indicate that engineering properties including stiffness and strength of soil will vary with time, a phenomenon known as ageing. This will greatly influence the validity of simplified procure when assessing liquefaction resistance of soil with different age.This paper mainly concerns ageing effect on liquefaction resistance of sandy soils. With shear wave velocity used as the characterization parameter, laboratory tests including cyclic triaxial test and dynamic geotechnical centrifuge test were conducted to reveal possible mechanisms that cause ageing and develop a shear wave velocity based liquefaction evaluation method considering time effect. In detail, this study includes topics as follows:(1) By analysis of strong motion station records of Christchurch, it reveals that site characterization will drop a lot during earthquake and gradually restore afterwards. Influenced by such change following an earthquake, the simplified method widely used now faced great risk.(2) Amounts of triaxial tests were conducted to study ageing effect factors during engineering time. Results show that ageing effect of sand with fines is far more apparent than clean sand. And stress level will not influence ageing effect with a fixed trend, the test results indicate that time effect will reach the best at proper stress level. When under low stress level, loose sand exhibit more time effect than dense sand.(3) Based on theoretical consideration and cyclic triaxial tests, relationship between liquefaction resistance and over-burden stress corrected shear wave velocity is developed. The correlation of CRR-Vs1can characterize the unsynchronized ageing effect on stiffness and strength, and evaluate liquefaction potential of sandy soils with different age reliably.(4) Through geotechnical centrifuge shaking table test, structure, density and stress state which are main factors influencing small strain shear modulus and penetration resistance are divided. It is found that the evolution of soil structure is synchronized with consolidation. Frequent liquefaction is helpful to the improvement of soil structure, and thus increase its liquefaction resistance.(5) Since soil deposition time often doesn’t correspond to the change of engineering properties, this article proposed an ageing index (AI) characterizing soil ageing based on small strain shear modulus (Gmax) and cone penetration resistance of CPT test (qc), which will provide possible methods for liquefaction evaluation from in-situ indices.