The Geology of Singapore as Seen by a Civil Engineer

Few weeks ago, I moved to Singapore to join a consulting firm as a Tunnel Design Engineer. Although I had been to Singapore few times earlier, on small "assignments", it is still like starting all over. New job, new country, now challenges and hey, more importantly, new Geology!! 

The value of Geology to any Geotech/ Tunnel engineering problem is very apparent. Balance in geological system is a delicate condition which takes longs periods to establish. This balance can easily be disturbed/ altered by certain civil engineering activities and hence it is of great importance to understand how these systems are likely to react to such disturbances. Particularly for tunnelling projects, geological factors affects the overall success of TBM, right from TBM selection, project planning, and tunnel excavation to tunnel operation. So I started to learn about Singapore Geology from a Civil Engineering perspective. In this effort, I happened to stumble over this excellent reference in Singapore National Library [4] based on which I named this post. 

The geology of Singapore has been well presented by the Singapore Public Works Department [1] in their 1976 report and by DSTA [2] in their 2009 report. The report covers the stratigraphy of the nine recognized formations, structure and geologic history of different formations. Below figures show the simplified geological maps of Singapore. 

Simplified Geological Map, Scrivenor (1924)

Simplified Geological Map, Alexander (1950)

This post summarizes some of the critical aspects which needs to be borne in mind while dealing with certain geological units in Singapore. 

Sedimentary Rocks (Zone 1) - Extremely quick variation in properties, varying from good to bad within few centimeters. Boulder Clay (SE Zone) - Consists of big, hard and tough rounded sandstone boulders. In between boulders - silty hard clay which tends to soften and slide on exposure. Moisture content - 10 to 27%, Cohesive strength ~ 5MPa and no internal friction. 

Igneous Rock (Zone 2) - In Central Singapore, Changi and Pulau Ubin. They are excellent for foundations. The soil from weathering of granite in-situ can be very firm and can stand high excavations without support (in dry conditions). Can be difficult to drive plies (if required, to carry tension). 

Older Alluvium (Zone 3) - It covers most of eastern Singapore. Consists of semi-consolidated sands, gravels, pebble beds, and silty clay beds. It is generally a good soil but has to be watched carefully. It is difficult to predict penetration of piles. The compressibility of the soil must be studied carefully for heavier buildings. 

Recent Alluvium (Zone 4) - Characterized by high water content, low strength and high settlements. They are fairly homogeneous and behave quite theoretical but need to watch for sudden appearance and disappearance of thin sand layers. 

Four Zones Discussed  Above

References: 

[1] PWD, Singapore. "Geology of the Republic of Singapore." Published by Public Works Department (PWD), Singapore (1976). 

[2] Lee, K. W., and Y. Zhou. "Geology of Singapore" Defense Science and Technology Agency (DSTA) (2009). 

[3] Pitts, John. "A review of geology and engineering geology in Singapore." Quarterly Journal of Engineering Geology and Hydrogeology 17.2 (1984): 93-101. 

[4] Sehested, K. G. The geology of Singapore as seen by a civil engineer. Public Works Department, 1960.

[5] Sharma, J. S., Chu, J., & Zhao, J. (1999). Geological and geotechnical features of Singapore: an overview. Tunnelling and Underground Space Technology, 14(4), 419–431. doi:10.1016/S0886-7798(00)00005-5