Abstract

The position and condition of carbonate bedrocks such as limestone or dolomite and its appearance are important for engineering construction sites such as buildings, housing complexes and road projects. In spite of the subsidence damage resulting from carbonate dissolution causing massive losses all over the world, the causes are well addressed in a few areas only. The application of a geophysical field survey and aerial photograph will be represented through identification techniques.

In this case study, a two-dimensional (2D) electrical resistivity tomography (ERT) survey was performed across two housing complex construction sites, north of Ipoh city, in the Perak state, Peninsular Malaysia, to image the subsurface and locate evidence for the near surface karstic features such as voids or cavities including sinkholes. Furthermore this survey was carried out to estimate the depth of the bedrock and to assess the reliability of the (ERT) electrical resistivity method whether it can identify such features or not.

Six resistivity traverses or profiles were conducted along the survey area at each of the two construction sites. The orientation, extension and the degree of inclination of those profiles are shown in the location map. The correct resistivity data was interpreted using res2dinv software. The interpretation of E R Tomography/ image sections revealed that many anomalies with very low resistivity and high conductivity extend along the project areas in these construction sites. Massive sinkhole has affected many sections of the project area in construction site #1. Thus it contains non-stiff clay and is saturated with water, rendering it less resistant to electrical currents (high conductivity). Enormous longitudinal channel pipe containing both stiff and sandy clay has affected many sections of project area in construction site #2. An assessment of the situation was surmised from the subsurface images. Subsequently, an estimation of the possibility of a collapse occurring in the near future due to the sinkhole was prepared.

This study also demonstrated that the 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 Waltham and Fookes (2003), the karst level in construction site#1 found between profile 1 and profile 6 is an older or complex karst type KIV, while the karst level in construction site#2 found between profile 1 and profile 3 is a mature karst type KIII. Afterwards, the karst type changed over profile#4 to profile #6 to an older karst or complex karst type KIV.

The hazards of the sinkholes and other karst features such as, cavities and dissolution channel pipe can cause problems to the construction projects in the near future resulting to mismanagement during the initial phase of the projects, as the developers did not carry out prior geophysical technique and geological studies. Moreover, the borings within these karsts regions is incapable of providing sufficient subsurface data for analysis, and then might misrepresent the subsurface geological model, which might in turn lead to an additional cost for corrective design or an ad-hoc analysis.

Consequently, early engineering subsurface remediation techniques are needed to minimize the potential of geohazard of sinkholes and other karst features in these construction sites over karstic carbonate bedrock. Initially, it was considered to utilize the reverse graded filter technique to fill the huge sinkhole in construction site#1. Skin friction piles driven into the layers contained non-stiff materials (soil, clay, silt, sand). New chemical grouting techniques such as deep injection chemical grouting technique are utilized in construction site#2. Controlling of the surface and ground water drainages will be in force when work begins at these construction sites.

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