Myanmar Geologists Society, Singapore (Technical Seminar No. 1) The Occurrences of Old Alluvium, Singapore 6th June, 2010 Naing Maw Than (
[email protected]) B.Sc.(Hons.), M.Sc.(Geology) (Mandalay) D.C.Sc. (Yangon), M.Sc.(Petroleum Geoscience) (Brunei) KOH Bros Building & Civil Engg. Contractors Pte. Ltd. S’pore Content 1) A Brief Singapore Geology 2) Old Alluvium 2.1.Distributions and Composition 2.2 Basement and Thickness 3) Depositional Variations 3.1 Bartley Road Outcrop 3.2 Correlation of Soil Profile from Bartley road to Marina South 3.3 Depositional Variation of the Old Alluvium 4) Engineering Properties of OA 5) The Concepts of Channel Development
Geological Map of Singapore (College of Engineering, TU)
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1) A Brief Singapore Geology Kallang Formation (Pebbles,Sands, Clays, peat) – Late Pleistocene Holocene Older Alluvium (Quartzose feldspathic, Sand, clay, silt) - Pliocene to Mid Pleistocene Jurong Formation (Interbedded Sandstone, siltstone, mudstone, Schist) - Upper Triassic to Lr-Mid Jurassic Bukit Timah Granite(Acid igneous rocks, Granite, Granodiorite) – Lower to Middle Triassic Gombak orite (Norite & Gabbro) - Upper Paleozoic Sajahat Formation (Hard, quartzite, Sandstone, mudstone) - Lower Paleozoic 2) The Characteristics of Old Alluvium Alluvium (from the Latin, alluvius, "to wash against") is loose, unconsolidated (not cemented together into a solid rock), soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of silt , clay, sand and gravel. When this loose alluvial material is deposited or cemented into a lithological unit, or lithified, it would be called an alluvial deposit. The term "alluvium" is included in another geologic process: lake sediments (lacustrine), river sediments (fluvial), or glacially-derived sediments (glacial till). 2.1) Compositions and Distributions The Old Alluvium in Singapore is mainly composed of thick pile of Fluvio-Deltaic sediments continued through Upper Tertiary to mid-Pleistocene (inter glacial period) with occasional incursions of marine sediments. PUB test holes may indicate marine deposits. Some discussions described that consists of an alluvial fan of colluvial deposits or piedmont-plain type deposits (Verstappen, 1975) (See in Geology of Singapore, 2009). It consists of poorly lithified, quartzose feldspathic Silty to Clayey SAND, bluish to greenish grey, medium to very dense, coarse, angular, alternated with some layers of gravels and Silt in dark grey colour with cross bedding, scour marks. The upper section is completely weathered OA Clay, yellowish, reddish to dark brown.
Silt
Clayey SAND
OA Clay Silty SAND (All samples collected from Marina South area)
Silty SAND
Gravelly SAND
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The OA covers about 12 Km2 as extension of southern Johore to east of S’pore Offshore. The OA is found lying to the north and northeast of the Kallang River basin between the central granite and the granite at east, sediments were rapidly deposited in initially subsiding faulted trough plunging down to Bedok that was probably derived from granitic terrain. Types of Chemical Weathering Reactions - Hydrolysis - H+ or OH- replaces an ion in the mineral. Example:
2.2) Basement and Thickness The underlying units of the OA are Granite, the Jurong Fm. and Sajahat Fm. in descending order and in turn capped by by the Kallang formation. The granite basement is commonly found in the central and eastern part of the Singapore ranged the depth between mBGL. (-10.4 to -200m). Another basement is metamorphosed quartz sandstone of the Sajahat Fm. that extends down to at least –149m at Bedok that give aggregate thickness in 195m(PUB Test bore hole No.1). In Punggol area, OA laid down against the Jurong Fm. as part of the contact is faulted. OA largely varies in thickness from (2 to 195m) in the area. A Sketch for Basement & Thickness of OA (data from Geology of Singapore, 2009)
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3) Depositional Variations From Bartley Road to Marina South 3.1) Bartley Road Outcrop The upper section is completely weathered Clay, yellowish to reddish brown and Lower section is Silty to Clayey SAD, bluish to greenish grey wedged by estuarine Clays. The cutting section occurs about 30m with cross-bedded Sands showing flow direction from north to south and Cut-and-Fill structure in reverse directional.
OA Clay
Silty to Clayey SAND
3.2. Correlation of Soil Profile from Bartley Road to Marina South .
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3.3) Key points of the OA development The OA was probably dominant by fluvial condition with subordinate marine sediments (embayment deposits) during Pliocene to Mid-Pleistocene (correlated with Lower Pleistocene Low eustatic sea level). The Kallang river was main channel and characterised by aggradation, multistacked, low– sinuous braided nature initially and high-sinuous in later phase. Deposited in faulted trough with repeated vertical movement and small block faulting. A bay line (place where ceased fluvial dominance) is probably passed at the junction of Upper Payaleber road and Mac Pherson road.
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4) Engineering Properties of OA The engineering properties of OA is based on BS 5930 for SI works and BS 1377 for laboratory works. Common practice based on SPT ‘N’ value has been adopted by K.S.Wong et al., (2001) and currently used the weathering grades in terms of A, B, C, D, E by Code of Practice for Foundations CP4: 2003, Table 2.5.
Residual Soil
Classification Clayey to Silty SAND,
Consistency Loose for Sand & Gravel,
Zone (OA I)
Silty SAND, Silty CLAY,
Medium Stiff for SILT and CLAY
(OAw2, OAw1) Weathered Zone (OA II)OAsw2 Weathered Zone (OA II)OAsw1
and locally
Cemented Zone (OA III)- Oacz
SPT Range < 12 to 25
Medium dense for SAND, Clayey SAND, Silty SAND Stiff for clayey soil
26 to 49
Dense for SAND, Clayey SAND, Silty SAND V Stiff for clayey soil
50 to 99
V Dense for SAND, Clayey SAND, Silty SAND Hard for clayey soil
> 100
Geotechnical Properties of the Old Alluvium (J.S. Sharma et. el., 1999)
Zone Water Content (w), %
OA 1
OA 2
OA 3
22
18.2
16.3
Bulk Unit Weight (g), kn/m2
20.3
20.7
20.3
Dry Unit Weight (gd), kn/m2
16.6
17.6
17.8
Specific Gravity
2.65
2.64
2.64
Liquid Limit
55
49
38
Plastic Limit
23
20
19
Plasticity Index
32
28
19
Avg. Undrained Shear Strength , Kpa
100
195
362
Cohesion in Kpa
1.9
8.4
30.3
Angle of Internal friction
36.1
35.9
35
Horizontal Permeability
18.8
6.4
3.4
compression Index Recompression Index
0.2
0.1
0.07
0.025
0.02
0.015
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Overview on Engineering Geology of OA (Tan. S.B. et. el. 1980) It has favourable physical and geotechnical parameters, This indicates that the deposits is by and large strata to carry to structure of any magnitude. The poorly graded dense sands which characterize the deposits are practically impervious when compacted, posses excellent shearing strength, low compressibility and good workability as construction materials. 5) The Concepts of Channel Development (From Lecture Notes of J.J. Lambiase, Petroleum Geoscience Dept., UBD, 2004) Channel Types There are four basic types of channel, Straight, Anastomosing, Meandering and Braided. Generally, meandering and braided are the most common and meandering channels are the most important in SE Asia. Gupta (1980) noted OA formed by Braided system (See in Sharma et.al. 1999)
anatomising meanderin
straig
braide
(After Miall, 1977) Channel flows, Outer bank, Inner bank, Point bar, Thalwag Spiral flow drives sediment towards the inner bank and creates an asymmetrical channel cross-section, with the deepest part, called the Thalweg, near the outer meander bank. The outer bank also is the area of highest shear stress and therefore erosional, while deposition occurs on the inner bank, or Point Bar. As a result, the channel migrates laterally towards the erosional outer meander bank, a sedimentary succession is deposited on the inner bank that reflects the grain size and bedform distribution of the bedload in the channel as fining upward sequence in general.
Deposition Point Bar
Erosion
Inner Bank
Outer bank Thalweg
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Flood Plain, Levees, Crevasse splay (Silt & Clay sediments) The excess water spreads over a large area known as the Floodplain
Braided Channels Braided channels have multiple smaller channels that separate bars within banks that define the channel belt. Because braided channels form at higher slopes than meandering and deposits are mostly stacked braid bar sands and gravels. Braided bars migrate downstream and laterally, most often during flood events. Aggradation Stacked Channels One example is the progression that results from a relative sea level rise. High gradient, low sinuosity streams occupy incised valleys at a lowstand Stream gradient decreases and sinuosity increases during transgression, forming amalgamated channel deposits. The highstand succession consists of channel deposits from low gradient, high sinuosity streams isolated within floodplain sediments.
floodplain
high sinuosity
tidal sediments
channel deposits
high gradient, low sinuosity
(After Shanley and McCabe 1993) References: 1. Geology of Singapore , 2009 (2nd Ed, DSTT) 2. Geotechnical Site Investigation and Interpretative BASELINE Report (LTA) 3. J.J.Lambiase,2004, Reservoir Sedimentology Lecture notes (Petroleum Geoscience Dept.) Universiti Brunei. 4. J.S.Sharma et al.,1999, Geological and geotechnical features of Singapore : An Overview 5. K.S.Wong et al.2001, Old Alluvium Engineering Properties and Braced Excavation Performance, Proceedings of underground Singapore 2001 6. Tan. S.B. et el. 1980 (Engineering Geology of the Old Alluvium in Singapore)
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