Summary A mass rapid transit (MRT) railway system is soon to be constructed in Singapore. Attention is being paid to details of the engineering geology of the island, which are generally not well documented or understood. The main solid formations, both igneous and sedimentary, are deeply weathered, often with an abrupt boundary between completely weathered and slightly to moderately weathered rocks. They have a high degree of fracturing and the sedimentary Jurong Formation has a high but variable dip. They are overlain by Quaternary deposits, many of which have poor geotechnical properties. A soft marine clay and overlying peaty deposits present particular problems in excavations. Each has a low strength and high compressibility and water tables are close to the ground surface. Variations in the geotechnical characteristics of the marine clay are now being recognized. The soft deposits are mainly found in buried channels cut during lower stands of sea level in the recent geological past. The channels have steep sides, and at many sites geotechnical conditions change very rapidly.
In both sand quarry faces and boreholes the Old Alluvium of Singapore is a matrix supported pebbly sand with minor beds of better sorted sands and clays. The beds can be grouped into four textural classes (pebbles, coarse sand with fine pebbles, medium to coarse sand, clay, and silt), each with characteristic sedimentary structures. The deposit appears to be the proximal facies of an ancient braided river alluvium of possible Pleistocene age.This widespread but scattered alluvium is found both on land and offshore on the Sunda Shelf in Southeast Asia. It is believed to have been deposited during low sea levels. In Singapore, the mineralogy of the grains forming the Old Alluvium suggests a mixed provenance of granitic and low-grade metamorphic origin. The volume and freshness of the deposited material indicate an environment of considerable relief, seasonal rainfall, and extremely active erosional processes, conditions considerably different from the current ones. The Old Alluvium was probably deposited by seasonal rivers which experienced periodic large floods. Such regional conditions could have prevailed over much of Southeast Asia at the time of deposition of the Old Alluvium.
A chronology of landsliding is presented, including suggestions as to a date for initiation. Periods of activity known from historical sources are correlated with known periods of climatic deterioration. The current morphology of the landslide slopes is closely related to the geological succession and structure. These permit the landslides and their development to be differentiated on the basis of whole slope and toe morphology, and much of the whole-slope activity can be related to conditions at the toe. Examples of geomorphological maps, slope categories maps, and cross sections are presented along with a n example of the 'evolutionary' maps which may be derived from Ordnance Survey plans and aerial photographs. These generally indicate that weakening of materials by weathering, seepage erosion at the toe, and marine erosion result in frequent mass movement events of a low magnitude. These events ultimately influence the stability of larger slipped blocks behind, which fail less frequently. It is suggested that whole-slope failures in this region have a maximum frequency of once in 120 to 150 years, and that problems of interpretation of historical accounts may mean that it is very much less frequent than this.
Summary Morphological mapping has proved extremely useful in a renowned area of coastal landslipping. The undercliffs form a coastal strip nearly six miles long, and up to 500 m in width. The antiquity (possibly late Pleistocene in part), and the large scale of much of the landslipping have assisted in the development of a mature, luxuriant vegetation cover, which masks much of the ground detail. However, the main back and side scars have often remained clear from the air, enabling photogrammetry to be used to produce base maps at a scale of 1:2500. Ground details where added to the base maps from fieldwork clearly reveal the patterns of mass movement. Examples of the final geomorphological maps are presented. Some 19th century interpretations of the mechanisms of landslipping indicate the advantages of observing the features in a fresh, lightly vegetated state. The present programme of morphological mapping has, hopefully, revealed a sufficient quantity of the detail to enable a reassessment of the area in the light of modern theory. Detailed morphological mapping is also presented as a site investigation technique of considerable merit in areas of difficult access and terrain, or in protected areas.
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