Bulletin of the Geological Society of Malaysia, Volume 49, April 2004, pp. 5 – 11
Geology Department, University of Malaya, 50603 Kuala Lumpur, Malaysia
Abstract: The Sungai Kelalong Dam site is founded in the interbedded mudstone-sandstone of the Miocene age Nyalau Formation. The formation has been mildly deformed and resulted in very broad, open syncline-anticlinal folds plunging in the South to Southwest direction. The rock masses are dissected by at least 4 sets of discontinuities, mainly the bedding planes, joints and faults. The occurrence of massive landslide on the spillway slope and part of the neighbouring core trench wall was largely attributed to the intersection of low angle, listric normal fault and the subvertical, E-W striking fault zone. The former is interpreted as a sliding plane of an ancient slide, which probably took place during the Pliocene uplifting in a condition of wet soft sedimentary deformation, to allow for the formation of well-developed normal listric faults system. Reactivation of the ancient slide is caused by the extensive earthworks activities for the dam construction which resulted in massive landslide to the spillway slope. This case study gives clear examples of the importance of geological inspection during the construction stage to check and to detect any structural defects that were not identified in the previous site investigation works.
https://doi.org/10.7186/bgsm49200402
Abstrak: Tapak pendasaran Empangan Sungai Kelalong, Bintulu, Sarawak terdiri daripada selang lapis batu lumpur dan batu pasir Formasi Nyalau yang berusia Miosen. Batuan Formasi Nyalau ini tercangga lemah dan terlipat dalam bentuk sinklin-antiklin yang sangat landai dan paksinya menjunam dalam arah Selatan – Barat Daya. Ketakselanjaran pada jasad batuan di kawasan ini dibentuk oleh satah-satah perlapisan, kekar, sesar dan zon sesar. Kejadian tanah runtuh di bahagian cerun alur limpah dan sebahagian daripada parit teras empangan dipercayai berpunca daripada persilangan di antara sesar normal listrik bersudut-landai dengan zon sesar curam yang berjurus hampir T-B. Satah sesar listrik itu dipercayai merupakan satah gelinciran kepada sebuah gelinciran kuno yang dipercayai terbentuk semasa Pliosen dalam keadaan canggaan ketika sedimen masih basah dan lembut, yang mana membolehkan terbentuknya suatu sistem sesar normal listrik yang baik. Pengaktifan semula gelinciran kuno ini disebabkan oleh aktiviti-aktiviti kerjatanah semasa pembinaan pendasaran alur limpah dan empangan utama. Kajian kes yang dipersembahkan ini jelas menggambarkan betapa pentingnya kerja-kerja penyiasatan geologi semasa pembinaan untuk menentusahkan kehadiran sebarang unsur-unsur kecelaan geologi yang tidak dapat dikenalpasti semasa penyiasatan tapak sebelum pembinaan.
