Application of Electrical Resistivity Tomography (ERT) and Arial Photographs Techniques in Geo Hazard Assessment of Karst Features in Constructing Sites in Perak, Peninsular Malaysia

Abstract

This paper illustrates the application of geophysical survey, aerial photographs and satellite images as identification techniques for geohazard assessment of karst features in housing complexes construction sites north of Ipoh city, Perak state, Peninsular Malaysia. The engineers in Perak  face numerous problems while designing structures that would be situated over marbleized limestone (carbonate) karst terrains, due to its categorization to various bedrock solution features such as sinkholes, caves, cavities, depressions, conduits enlarged joints and fractures, and internal drainages, which directly exerts a negative impact on the use of this land for construction projects and structures. Furthermore, it can also indirectly have the potential to cause catastrophic damages in the near future perhaps many years after the project has ended.  The subsidence damages which result in construction works will cause massive losses which maximize the project overall cost leading to enormous financial costs, to the developers and the lands. In this study, Two-dimensional (2D) electrical resistivity tomography (ERT) survey was performed across three housing complex construction sites north of Ipoh in order to image the subsurface and locate evidence for near surface karstic features such as voids or cavities, including sinkholes and to estimate the depth of the bedrock. It is also done in order to estimate whether geophysical techniques are capable of identifying such features.  Six resistivity traverses or profiles were conducted along the survey area at each of the three construction sites. The orientation, extension and the degree of inclination of those profiles are shown in Google’s satellite map. The interpretation of the geophysical data indicated that both low resistivity and high conductivity anomalies extends along the proposed area in all of the construction sites. The ambiguous anomalies observed in construction site # 1 indicated that the area has been exaggerated by a sinkhole, thus it contains non-stiff clay and is saturated with water, rendering it less resistant to electrical currents (high conductivity). The ambiguous anomalies observed in construction site #2 indicated that the area has been affected by several sinkholes and tubular anomalies containing both stiff and sandy clay. The anomaly observed in construction site #3 indicated that the area has been affected by several soil cover collapse sinkholes and tubular anomalies containing stiff, non-stiff and sandy clays. This study also demonstrated that high-resolution Electrical Resistivity Tomography (ERT) can be effectively applied to reflect and differentiate surficial soil, clay, weathered rocks, compact or intact rocks, and air-filled karstic voids or cavities. The appearance of many sinkholes in the area is mostly attributed to karstic activity. In accordance to the classification of the characteristics of morphological features of karstic ground conditions by (A. C. Waltham and P. G. Fookes, 2005), the karst in construction site site#1 found between profile 1 and profile 6 is an older or complex karst type KIV, while the karst in construction site#2 found between profile 1 and profile 3 is a youthful karst type KII. Afterwards, the karst type changed over profile#5 to profile #6 to mature into karst type KIII. The karst in construction site#3, found between profile 1 and profile 3, is a youthful karst type KII. Then, the karsts change in profile 4 to an older, mature karst type KIII. The karsts found between profile #5 and profile#6 is of the older or complex karst type KIV. Early planning is needed to mitigate or minimize the risk of structures in these construction sites over karstified carbonate bedrock. Initial consolidation of geo grids, driven piles to rock head pinnacles, and control drainage works must be put into operation in these respective sites. Keywords: Application, ERT and arial photographs Techniques, Geo hazard Assessment, Constructing sites, Perak- Peninsular Malaysia