Application of suspension P-S velocity logging method in Hong Kong

Abstract

Ground Investigation must be done in order to interpret the complex underground profile. In traditional way, lots of drilling works and lab tests would be carried out for the ground investigation. One of the purpose of doing lab tests is to look for the engineering parameters of different soil / rock types and estimate the ground condition for further geotechnical works, however laboratory works takes time. If the designer wants to know the physical properties of soil / rock on site, some field tests could be done to find them out in advance. Elastic properties such as Poisson’s ratio, shear modulus, bulk modulus and young’s modulus can be computed by measuring P-wave velocity (Vp) and S-wave velocity (Vs) on site. There are intrusive and non-intrusive ways for seismic measurement. Method like seismic refraction survey and seismic reflection survey, they are non-intrusive methods, these require lots of space and not applicable to many places with plenty underground utilities which produce lots of noise in the background. Also, the measuring depth is relatively shallow. On the other hand, methods like uphole, downhole, cross hole and suspension P-S velocity logging – a relatively new technology which can obtain both Vp and Vs data, these are intrusive methods, they are repeatable, velocity data can be used for comparing with the logging data and verifying geological strata in the vicinity of the borehole and relatively deeper data can be collected. Therefore, to maximize the use to data by proposing correlation between geophysical parameters, such as P-wave, S-wave velocities etc. and other engineering parameters, which can be a useful tool to encounter the geology-related problems during the construction.

This dissertation aims to interpret the direct velocity measurements of suspension P-S logging from different geological setting underlying the part of Tung Chung offshore area and Tseung Kwan O in Hong Kong with a correlation between the shear wave velocity and SPT-N value measured on site and compare it with other published correlation with same parameters in other countries to study the behavior of seismic wave in different geological strata. Also, to evaluate and discuss the capabilities and applicability of the Suspension P-S Velocity Logging Method in Hong Kong. The seismic velocity measurement obtained in Hong Kong were carried out as part of ground investigation works for Tuen Mun to Chek Lap Kok Link and Cross Bay Link, Tseung Kwan O respectively. Whereas, the SPT-N values are obtained from the same boreholes for the same ground investigation works from the Geotechnical Engineering Office (GEO) internet database, while the source of overseas data are from literature review including the published well-known formulas from other professionals.

The correlation was established by means of comparison of S-wave velocities and SPT-N value for different types of soil stratum. The measurement show that Fill layer with SPT-N values of 11 – 42, has S-wave velocities ranging from 180m/s to 300m/s. Alluvium layer with SPT-N value of 11 – 61, has S-wave velocities ranging from 240m/s to 360m/s in Tseung Kwan O site while the Alluvium layer in Lantau with SPT-N values of 9 – 63 has, S-wave velocities ranging from 175m/s to 550m/s. Marine Deposit (MD) layer with SPT-N values of 9 – 19, has S-wave velocities ranging from 196m/s to 243m/s in Tseung Kwan O site while the MD layer in Lantau with SPT-N value of 0, which the Standard Penetration Tests were proceeded, but the SPT probe penetrated through 450mm under its dead weight, has S-wave velocities ranging from 190m/s to 400m/s. for the in-situ soils, completely decomposed tuff (CDT) with SPT-N values of 40 – 107, has S-wave velocities ranging from 270m/s to 470m/s. Completely decomposed granite (CDG) with SPT-N values of 32 – 99, has S-wave velocities ranging from 230m/s to 460m/s. These results are used to establish a correlation of SPT-N value and S-wave velocities in Tseung Kwan O and Lantau, to study any difference in the seismic wave behavior between volcanic rock materials and granitic rock materials. Since the correlations proposed in this study is site specific, it is essential to the point that places outside Hong Kong with different geological strata, may not fit into the correlations. Apart from the soil / rock type issue, there also a limitation relating to the sampling size in this study. Only two areas have been chosen for the study and only those seismic data with corresponding SPT-N value data should be counted. It is emphasized that the formula of correlation should be update from time to time by filling more data sets into the correlation to achieve a prediction closer to the real measurement under various geological and site conditions.