Determination of Small Strain Modulus and Degradation for in-Situ Weathered Rock and Old Alluvium Deposits

The small strain shear modulus (G0) and the degradation of shear modulus (G/G0) with increasing magnitude of shear strain are important soil properties required for the evaluation of site response due to earthquake effects. While these properties are well established for geologically recent alluvial sand and clay materials, published data on the properties of materials derived from in-situ rock weathering and ancient alluvial deposits are limited. This paper presents the results of laboratory testing on completely decomposed granite and tuff in Hong Kong, and weathered Jurong Siltstone and Old Alluvium in Singapore. The small strain shear modulus (G0) of the materials was determined from bender element tests, while the shear modulus degradation (G/G0) was assessed from cyclic triaxial test with local strain measurement. The results are compared with the published data of similar materials. Apart from the laboratory bender element tests, G0 can also be determined by the in-situ shear wave velocity test. It has been found worldwide that there is generally a reasonable relationship between shear wave velocity and the SPT N value. In this paper, various in-situ shear wave velocity (Vs) testing results obtained from Singapore and Hong Kong have been reviewed and the observed correlations between VS and SPT N values for various soils are presented

The Importance of Creating Value in Seismic Design

Major earthquakes have resulted in devastating consequences in terms of human and economic loss. In almost all the earthquakes we observe the failure of structures, sometimes due to poor construction but also due to designers not identifying the specific geo-hazards (iIntensity of ground motion, faults, liquefaction, slopes etc) which affect these structures. In many cases these damages could have been avoided if the original design had correctly identified the geohazards at the site and incorporated the philosophy of performance based design. In this paper several examples will be presented where the different stages of risk assessment will be identified and possible solutions incorporated in the final design. The paper provides examples where existing studies and codes in certain countries may be storing up problems for the future.This paper also highlights some gaps in existing knowledge where more research is needed. Design examples will also cover the advantages of performing detailed design accounting for soil structure interaction effects. In many cases these will offer potential saving to the clients and thus provide value in seismic design. Examples are shown where structures which have accounted for the geohazards will be shown to perform satisfactorily during past earthquakes

Seismic Microzonation Assessment in Hong Kong

This paper describes a comprehensive investigation on the potential seismic site response effects for the ground conditions in Hong Kong. The ground conditions in the North-west New Territories region of Hong Kong have been selected for this investigation and the results are formulated into seismic microzonation maps. Published geological maps and detailed ground investigation information from over 3000 boreholes have been used for the microzonation assessment. The site classification for microzonation is based on the site classification system developed in the United States and China, with modifications to suit Hong Kong's ground conditions. One-dimensional site response analyses have been carried out to establish how various types of site profiles, representative of Hong Kong subsoil conditions in the study area, will potentially respond to earthquake ground motion. The soil properties are determined based on field and laboratory dynamic tests. The response of a suite of soil profiles was investigated using three input earthquake ground-motion levels corresponding to rock motion having a 63%, a 10% and a 2% chance of being exceeded in the next 50 years. The results are presented in terms of defining site classes and deriving surface response spectra for each of these site classes