Report No. : GEO Report No. 71

Report Title : Site Characterisation Study - Phases 1 and 2 (1999), 207 p.

Author : N.P. Koor

Abstract

In response to recommendation (d) contained in Volume 1 of the report on the Kwun Lung Lau Landslide of 23 July 1994 (Hong Kong Government, 1994), the Planning Division of the GEO initiated a Site Characterisation Study on the use of non-invasive geophysical techniques to investigate masonry walls and man-made slopes in Hong Kong. Seven non-invasive geophysical methods have been assessed through two phases of field trials to determine their applicability to the identification of features which affect slope and retaining wall stability in Hong Kong. Selection of the seven methods was based on a literature review of international and local land-based geophysical practice. The selected methods were ground penetrating radar (GPR), shallow seismic reflection, spectral analysis of surface waves (SASW), resistivity imaging (RI), self potential, electromagnetic methods (EM) and thermal imaging. Two other methods, seismic refraction and sonic method, were tested at individual sites by two contractors but were not stipulated in the contract requirements.

The field trials were carried out by six contractors for Phase 1 and five out of the original six for Phase 2.

The Phase 1 field trials, which tested all seven methods were made at four sites consisting of two pre-war masonry walls, one cut slope and one fill slope. All four sites had some existing ground investigation data which were provided to each of the contractors prior to the trials for purpose of calibrating the geophysical results. Based on observations of the fieldwork and the geophysical results, the techniques were divided into three groups, as follows:

• Group 1. Techniques which produced some promising results and may be applied widely to sites in Hong Kong. These are Ground Penetrating Radar and Resistivity Imaging.
• Group 2. Techniques which are affected by environmental noise and limited by characteristics inherent in the technique, but might be usefully applied at specific sites. These are Electromagnetic Conductivity (frequency domain) and Spectral Analysis of Surface Waves.
• Group 3. Techniques which did not provide any useful information and do not warrant further consideration. These are High Resolution Seismic Reflection, Self Potential, Electromagnetic Conductivity (time domain) and Thermal Imaging.

In Phase 2 the Group 1 and 2 techniques were further tested at four new sites again consisting of, two pre-war masonry walls, one cut slope and one fill slope.

The Phase 2 trials concluded that the overall quality of the raw data and interpretations varied significantly with the expertise of the individual geophysics teams. Certain contractors were able to demonstrate that a combined geophysical investigation utilising GPR and RI can locate the back of masonry walls if they are less than 3 m thick and define zones of elevated moisture content and voids with reasonable accuracy. Only limited success was achieved in determining the location of corestones and the thickness of loose fill at the two slope sites.

In the light of the conclusions from the field trials, it is recommended that due to the inconsistent raw data and interpretations it would not be advisable at present to let a term contract specifically for non-invasive geophysical techniques for site characterisation in Hong Kong, or to require the use of such techniques in LPM studies. Also, further trials of the four or other techniques are not warranted. However, since some of the results were encouraging it is recommended that further research work on developing GPR and RI for masonry wall investigations is carried out through a local University with support from GEO. The research should focus on ways to enhance the resolution of the two techniques and also to develop a site and data processing methodology which would ensure more consistent results from contractors.

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