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

Week 2 Tunnelling & TBM Course: Geological and Geotechnical Aspects of Tunnelling

Week 2 (27th Jan '14 to 31st Jan '14) of 2nd Level specializing course in Tunnelling and TBM was completely dedicated to Geological and Geotechnical aspects of Tunnelling. We had expert lectures from Geodata: Mr. Luca Soldo (blog post here) and from National Research council of Italy: Mr. Corrado Fidelibus. Mr. Fidelibus gave us an overview of Soil Mechanics and presentations related to Natural and Induced stress states because of Tunnelling. The lectures also covered the investigation methods,  lab tests, instrumentation and interpretation of Geophysical tests.

Geological studies were explained with a case study on Gotthard Base Tunnel by explaining the geological and geotechnical explorations carried out to detect Piora fault.

Mr. Fidelibus discussed the procedure and the precaution to be taken in calculating the Natural stress states while using in a numerical model. An example of prediction of induced stress state using a FEM program was demonstrated. Key concepts of stress measurements methods were discussed with emphasis on stress relief methods, Hydrofracturing method and flat jack method along with other available methods. Carrara marble project case study was used to demonstrate the use of stress measurement methods.

In addition to the above lectures, following literatures are studied for case studies and to understand the prevailing guidelines:

• "Some new Q-value correlations to assist in site characterisation and tunnel design", N. Barton, International Journal of Rock Mechanics & Mining Sciences 39 (2002) 185–216
• "ISRM Suggested Methods for land geophysics in rock engineering", T. Takahashi, International Journal of Rock Mechanics & Mining Sciences 41 (2004) 885–914

More about the above papers later.

-Senthil Nath G T

Geological Aspects of Tunnel

Today we had a guest speaker from Geodata spa. Mr. Luca Soldo is a Sr. Engineering Geologist and principal contributor of the book: Mechanized Tunnelling in Urban Areas, Taylor and Francis Publication. Mr. Soldo gave us a Geological point of view for drilling and geophysical testing to be performed for Tunnel alignment and typical examples of Tunnel projects.

Mr. Luca Soldo's Lecture on Geological Aspects of Tunnel on 27th January, 2014