A microgravity survey over deep limestone bedrock


Author : Samsudin Bin Hj TaibPublication : Bulletin of the Geological Society of MalaysiaVolume : 46Page : 201-208Year : 2003


Description

Bulletin of the Geological Society of Malaysia, Volume 46, May 2003, pp. 201 - 208

 

A microgravity survey over deep limestone bedrock

SAMSUDIN BIN HJ TAIB

Department of Geology, University of Malaya, 50603 Kuala Lumpur

 

Abstract: A microgravity survey had been carried out within a residential area. A total of 141 gravity values was obtained over an area of about 500 by 300 m2. These data were obtained along the roads within the study site. The boreholes within the area indicate that the area is underlain mainly by fine grained material namely silty clay, silty sand, clayey silt and sandy clay. Near the bedrock coarser material such as pebble and gravel may be present. The density of this clayey material is 1.4 to 1.8 g/cm3. The bedrock is limestone and has density of 2.64 to 2.75 g/cm3. From the borehole data the limestone within the study area is at about 27 to 33 m deep.

A major problem within the study area is the presence of subsurface mass movement. This is indicated by the formation of large scale undulation on the ground both on the road and within the house areas and the cracks on the building structure including drains, walls and pillars. Both lateral and vertical movement occur on these structures indicating lateral and vertical mass movement. It has been suggested that collapsing of cavity roof within the limestone can be the cause. The microgravity survey was aimed at trying to determine the presence of the cavity. With the borehole data, the gravity modelling is very well constrained. The modelling indicates that the subsurface topography of the limestone is made up of a series of ridges and valleys. Possible occurrence of the cavity within the ridges has also been modelled.

 

Abstrak: Satu survey mikrograviti telah dijalankan di kawasan perumahan. Sejumlah 141 nilai gravity telah dikutip di kawasan seluas lebih kurang 500 x 300 meter2. Data gravity ini dikutip di sepanjang jalan yang terdapat di kawasan perumahan tersebut. Lubang gerudi di kawasan ini menunjukan kawasan ini dilapisi oleh bahan berbutir halus seperti lempong berlodak, pasir berlodak, lodak berlempong dan pasir berlempong. Berdekatan dengan batudasar bahan berbutir kasar seperti pebel dan gravel hadir. Ketumpatan bahan berlempung ini berjulat 1.4 hingga 1.8 g/cm3. Batudasannya adalah batukapur dan mempunyi ketumpatan berjulat 2.64 hingga 2.75 g/cm3. Dari lubang gerudi dalam kawasan ini kedalaman batukapur ini adalah di sekitar 27 hingga 33 meter.

 Satu masalah besar dalam kawasan kajian adalah terdapatnya pergerakan jisim subpermukaan. Ini dikenalpasti dari pembentukan mukabumi yang beralun di jalan dan di kawasan rumah,dan retakan pada struktur bangunan termasuk parit, dinding dan tiang. Pergerakan tegak dan mendatar berlaku pada struktur-struktur ini menandakan pergerakan jisim mendatar dan tegak telah berlaku. Ada cadangan mengatakan bahawa runtuhan bumbong lohong dalam batukapur adalah penyebabnya. Survei gravity adalah ditujukan untuk cuba menentukan kedapatan lohong dalam batukapur. Dengan adanya data lubang gerudi pemodalan gravity dapat di kawalan dengan baik. Ia membolehkan penentuan topografi batukapur yang terbentuk dari satu siri rabong dan lembah ditentukan. Kemungkinan adanya lohong dalam rabong batukapur tersebut juga telah di modelkan.