site characterisation study - phases 1 and 2 - HKU Libraries - The ...

- 5 -ABSTRACTIn response to recommendation (d) contained in Volume 1 of the report on the KwunLung Lau Landslide of 23 July 1994 (Hong Kong Government, 1994), the Planning Division ofthe GEO initiated a Site Characterisation Study on the use of non-invasive geophysicaltechniques to investigate masonry walls and man-made slopes in Hong Kong. Seven noninvasivegeophysical methods have been assessed through two phases of field trials to determinetheir applicability to the identification of features which affect slope and retaining wall stabilityin Hong Kong. Selection of the seven methods was based on a literature review of internationaland local land-based geophysical practice. The selected methods were ground penetrating radar(GPR), shallow seismic reflection, spectral analysis of surface waves (SASW), resistivityimaging (RI), self potential, electromagnetic methods (EM) and thermal imaging. Two othermethods, seismic refraction and sonic method, were tested at individual sites by two contractorsbut were not stipulated in the contract requirements.The field trials were carried out by six contractors for Phase 1 and five out of the originalsix for Phase 2. The contractors were selected based on their technical expertise and track recordin geophysical investigations for civil engineering works. The participating contractors for thePhase 1 field trials were Guandong South China EGD Co. (GSC), Bachy Soletanche Group(BS), Meinhardt Works (MW), Golder Associates (HK) Ltd (GA), Fugro Geotechnical Services(HK) Ltd. (FGS) and the Institute of Geophysical and GeochemicaJ Exploration (IGGE). ThePhase 2 contractors were the same as for Phase 1 apart from Meinhardt Works who decided notto participate.The Phase 1 field trials, which tested all seven methods were made at four sitesconsisting of two pre-war masonry walls, one cut slope and one fill slope. All four sites hadsome existing ground investigation data which were provided to each of the contractors prior tothe trials for purpose of calibrating the geophysical results. Based on observations of thefieldwork and the geophysical results, the techniques were divided into three groups, as follows:Group 1. Techniques which produced some promising results andmay be applied widely to sites in Hong Kong. These areGround Penetrating Radar and Resistivity Imaging.Group 2. Techniques which are affected by environmental noiseand limited by characteristics inherent in the technique,but might be usefully applied at specific sites. These areElectromagnetic Conductivity (frequency domain) andSpectral Analysis of Surface Waves.Group 3. Techniques which did not provide any usefulinformation and do not warrant further consideration.These are High Resolution Seismic Reflection, SelfPotential, Electromagnetic Conductivity (time domain)and Thermal Imaging.