This paper presents a case study on characterization of cavities in marble in karst terrain. A recently developed insitu technique, namely Drilling Process Monitoring (DPM) is applied in the cavity characterization. This technique is able to characterize the mechanical strength properties and their spatial distribution along the drillhole via automatically and continuously monitoring the full process of hydraulic rotary drilling in real time during site characterization. According to the monitored data of the drilling parameters in time, this technique can efficiently and sensitively characterize either tiny discontinuities or massive cavities present in marbles. The DPM results are compared against the results from core logging in the case study. The results of this case study show the DPM data can provide more factual data on cavities.
1. INTORDUCTION
Karst is a distinctive topography in which the landscape is largely shaped by the dissolving action of water on carbonate bedrock (usually limestone, dolomite, or marble). Cavity is one of the common karst features and its formation is very complex. The buried karst cavities can cause serious problems on foundation design and building constructions [1,2]. It is, therefore, of importance to establish an accurate geological model to describe the spatial distribution of karstic cavities during ground investigation. The geophysical surveys were commonly applied for the detection of different types of subsoil anomalies and they have often given adequate response to the problem of the cavities [3]. However, geophysical methods, such as ground penetrating radar, micro gravimetry, resistivity imaging and natural potential survey still have some limitations in practical applications [4]. Besides the geophysical surveys, the drilling method is extensively used in ground characterization in practice. Over the last three decades, a number of researchers and engineers have attempted to use drilling parameter recording techniques for ground investigation [5,6,7,8]. The idea was basically to collect the drilling information t a pre-selected depth interval during the drilling into ground. This approach has the problem in analysis and interpretation of the electronic data. High quality data collected in a systematic manner are important [9]. Over the last ten years, the research team at The University of Hong Kong has carried out in-depth studies to develop innovative approach in the in-situ devices and the associated data analysis methods for using drilling parameters in ground characterization. The innovative approach involves the automatical monitoring of the full drilling process in real time at a given time sampling frequency. Consequently, a new insitu technique, namely, the Drilling Process Monitor (DPM) has been developed by the HKU research team for ground soil and rock characterization [10-18]. The DPM device can be easily mounted onto an existing hydraulic rotary drilling machine and continuously record the associated drilling parameters in real time with a given time sampling rate. It can monitor the full drilling process and operations that are experienced by a drilling machine while drilling. The analysis software of DPM data can provide additional and valuable factual results to characterize the structural geometries and relative strength of subsurface profiles in depth along the hole.
