Characterisation and engineering properties of Singapore residual soils

E.C. Leong, H. RahardjoNanyang Technological University, Singapore

S.K. TangCPG Consultants Pte Ltd, Civil & Structural Engineering Division, Singapore

ABSTRACT: Residual soils are formed by the in situ weathering of rocks and can be found in manyparts of the world. In Singapore, residual soils of granitic and sedimentary rocks occupy about two-thirdsof the land area. As Singapore has a small land area of about 647.5 km2 and a population of 4 million,many developments are in these residual soil deposits. The formation process of residual soils is complexand their characteristics are very different from those of transported soils. The thickness of the residualsoils varies from a few metres to several tens of metres. The residual soils are highly heterogeneous andtheir properties appear to be highly variable. Generally, residual soils are unsaturated and their behaviouris influenced by their degree of saturation. However, in engineering practise, the degree of saturation ofthe residual soils is seldom accounted for when evaluating their engineering properties. This paper summarises the current understanding of the characteristics and engineering properties of Singaporeresidual soils.

1 INTRODUCTION

Singapore, located between latitudes 1°09�N and 1°29�N and longitudes 103°36�E and 104°25�E andapproximately 137 km north of the Equator, is a typical tropical island with uniform temperature andabundant rainfall throughout the year. The average daily maximum temperature is 30.9°C and the dailyminimum temperature is 23.9°C with the average daily temperature being 26.8°C. The annual rainfall isabout 2350 mm and the average daily relative humidity is about 84.3%. Singapore consists of one mainisland and 60 small ones. The main island of Singapore is about 42 km from east to west and 23 km fromnorth to south. The total land area is 647.5 km2.

Singapore lies close to the southern extremity of the Eurasian tectonic plate, north and northeast of theSumatra-Java oceanic trench. Singapore and the surrounding region on the east form a stable crustalblock known as Sundaland. The geology of Singapore consists essentially of three formations: (i) igneousrocks of granitic or similar composition (Bukit Timah Granite) in the centre and northwest, (ii) sedimen-tary rocks (Jurong Formation) in the west, and (iii) a semi-hardened alluvium (Old Alluvium) which masksolder rocks beneath in the east. Figure 1 shows a simplified geology map outlining the distribution of thethree major geological formations of Singapore.

The oldest rocks in Singapore probably come from the Palaeozoic era, which ended about 225 millionyears ago. Granite occurs in two separate masses. The larger one is found in the central and northernareas, the smaller one in parts of northeastern Singapore. Granite or igneous rocks underlie the BukitTimah Nature Reserve and the Central Catchment Area in the centre of the island. The granite in Singapore,according to radioactive age determination, is more than 200 million years old. The sedimentary rocksof the Jurong Formation skirt the central granite and form extensive areas in southern, southwestern andwestern Singapore. These variations of conglomerate, sandstone and shale are also found on the islandsto the south and west. The semi-hardened Old Alluvium was deposited by an ancient river system, prob-ably in the Pleistocene epoch, during a low stand of the sea.

The present day configuration and much of the morphology of the low-lying areas of Singapore is a resultof erosion and deposition during the period of fluctuating sea levels in the late Tertiary and Quaternary.As the sea level rose after the end of the last cold stage about 11,000 years ago, it formed Singapore as

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a group of islands, separated from the Malay Peninsular by the Straits of Johor. Extensive marine clay,beach deposits and associated terrestrial sediments were deposited around the rocky flanks of the island.

The relief of Singapore is relatively gentle. Only 10% of Singapore is over 30 m high and more than60% of Singapore is less than 15 m high (Pitts, 1984a). The area of granite and other igneous rocks in thecentre of the island forms a landscape of rounded hills and gentle spurs and valleys. In this area, steeperpeaks of Bukit Timah (161 m), Bukit Gombak (139 m), Bukit Panjang (132 m), Bukit Batok (106 m) andBukit Mandai II (88 m) can be found. To the west and southwest of the island, the sedimentary rocks giverise to a series of narrow ridges, generally trending northwest/southeast, which can be quite steep locally.Examples are the ridges of Pasir Laba, Pasir Panjang and Mount Faber. The coast is flat, but in a fewplaces cliffs can be found. Considerable stretches of the coastline have been markedly modified by recla-mation work, building of embankments and swamp clearance.

The intent of this paper is to present the characterisation and engineering properties of the Singaporeresidual soils. Blight (1997) defines residual soils as soil-like material derived from the in situ weathering,and decomposition of rock which has not been transported laterally from its original location. Residualsoils can be classified into three categories according to their parent rock types, namely, igneous, sedi-mentary and metamorphic. The Singapore residual soils consist of the Bukit Timah Granite residual soilsand the Jurong Formation sedimentary residual soils. Each of these residual soil formations occupiesapproximately one-third of the total land area.

2 ENGINEERING GEOLOGY

The soil pattern of Singapore has been largely determined by the tropical weathering of a variety ofrocks over a period of time. The granitic residual soils, derived from granite or other igneous rocks(Bukit Timah Granite), are found in the central and northern parts of Singapore and on Pulau Ubin. Onthe western side of Singapore, the residual soils are derived from the Jurong Formation sedimentaryrocks. Weathering and leaching have resulted in a uniformly low level of chemical nutrients in thesesoils, but the physical properties of the soils still show a relationship to the nature of the parent rock.

The Old Alluvium is a quartenary alluvial deposit consisting mainly of medium to very dense, partially cemented, clayey quartzo-feldspathic sands and fine gravels with some coaser gravel and lenses ofsilt and clay (Pitts, 1984b). Weathering has also been observed in parts of the Old Alluvium and sometimesthe weathered Old Alluvium has been treated as residual soils (Zhao & Lo, 1994). However, this is not cor-rect following the definition of residual soils adopted in this paper and hence will not be included here.Table 1 summarises the ages and materials of each of the residual soil formations. There are other minorrock formations with residual soils that are not covered in this paper. These include Sajahat Formation,Palaeozoic Volcanics and Gombak Norite. These formations are minor geological formations found at

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MuraiIgneous Rocks

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Figure 1. Simplified geological map of Singapore.

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localised areas in Singapore. Briefly, the Sajahat Formation generally consists of well-lithified quartzite,quartz sandstone and argillite, while the Palaeozoic Volcanics generally consist of andesitic ashy tuff andagglomeratric tuff. Both the Sajahat Formation and the Palaeozoic Volcanics are found in the eastern part ofSingapore. Both the formations are partially metamorphosed. The Gombak Norite, which includes noriticand gabbroic rocks, is found on the western side of the Bukit Timah Granite.

2.1 Bukit Timah Granite

The Bukit Timah Granite, one of the oldest formations in Singapore, is widely distributed in the centraland northern parts of Singapore Island. The intrusion of the Bukit Timah Granite is believed to havetaken place during the lower to middle Triassic period (200 to 250 million years ago). The rock in theformation varies from granite to granodiorite. Several hybrid rocks and dykes are also included in theBukit Timah Granite.

The granite is generally light grey and medium grained, with grain sizes measuring from 3 mm to5 mm. Quartz, which often accounts for 30% of the minerals present, has a glassy grey appearance and arough surface. It occurs interstitially to the feldspar crystals and has interlocking boundaries with them.Feldspar is the most abundant mineral in the granite and often constitutes 60% to 65% of the rock. Thecolour is commonly cream in appearance with the more weathered ones being pale to brownish yellow.The pink variety of orthoclase is present. Biotite and hornblende, which make up the remaining con-stituents, are easily recognised by their dark brown colour and by their cleavage.

Top portions of the Bukit Timah Granite have generally been heavily weathered and decomposed intoresidual soils consisting mainly of reddish to yellow brown clayey soils. A typical section profile throughthe Bukit Timah Granite, A-A in Figure 1, is shown in Figure 2a. As water seeps through fissures in the

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Table 1. Summary of age and materials of geological formations.

Formation Age Material

Bukit Timah Granite Lower to The granite varies from granite through middleTriassic adamellite to granodiorite and several hybrid (� 200 to 250 million rocks are included within the formation. Both years ago) hornblende-rich granite and biotite-rich granite

occur. Zones of norite-granite mixed rocks are also present.

Jurong Queenstown Facies Red to purple mudstone and sandstone with Formation minor conglomerate. Minor tuffs can also be

found within the member. The red colourationis thought to come from tropical weathering inTriassic time, but volcanic material present mayalso have added to the colour.

Rimau Facies Transitional to marine quartz conglomerate andquartz sandstone. The clasts are usually angularto sub-angular and loosely packed but the rockis well cemented.

Ayer Chawan Upper Triassic to Well-bedded marine muddy sandstone andFacies early Jurassic mudstone, often black in colour. Red

(�100 to 200 million roundstone conglomerate is common and all years ago) beds are tuffaceous. Lithic tuff and spilite are

mapped separately. A number of fossilcollections have been made from this member.

Tengah Facies Muddy marine sandstone with occasional gritbeds and conglomerate. The member is usuallydeeply weathered but appears not to have beenstrongly lithified at any stage.

St John Facies Flysch-like marine muddy fine sandstone withminor laminae of carbonaceous matter. Usuallymoderately well lithified.

Jong Facies Well-cemented roundstone conglomerate andsandstone with occasional mudstone beds andspilite pillows.

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granite mass, chemical processes take place between the groundwater and the rock particles, which aredirectly in contact with the water. As a result, the subsurface of the rock blocks are gradually “eroded”leaving numerous boulders in place. These boulders can be as large as several metres in diameter. Thedegree of weathering of the granite decreases with depth. The top portion of the granite has been com-pletely weathered into residual soils, while the fresh granite bedrock remains at great depths. The graniterocks, directly underneath the residual soils, are usually highly weathered and fractured. The residualsoils derived from the heavily weathered granite have undergone further weathering resulting in changesin colour. Thus the colour of the soils varies from yellowish grey to reddish brown.

2.2 Jurong Formation

The Jurong Formation was formed in the period of late Triassic to early Jurassic (about 100 to 200 millionyears ago). The formation is composed of a series of sedimentary rocks such as sandstone, mudstone,shale, tuff, conglomerate and limestone. The formation has been severely folded and faulted in the past asa result of tectonic movements. The general strike of the formation is northwest/southeast. Dips of the for-mation may vary over short distance from a few degrees to vertical or overturned. There are a number ofmajor and minor faults in the Jurong formation with displacement ranging from unknown distances to afew decimetres. The major faults are normally infilled with clay gouge, which is extremely soft when wet.

Degrees of weathering of this formation depend on types of rocks. Sandstone is usually more resis-tive to weathering than mudstone and tuff. Upper portions of the Jurong Formation have been heavilyweathered and have changed into residual soils of silty clays. A typical section profile through theJurong Formation, B-B in Figure 1, is shown in Figure 2b. Swelling of most of the mudstone and tuff inthe formation takes place upon exposure to the atmosphere especially in the presence of water resultingin a drastic decrease in strength.

The Jurong Formation consists of six facies (PWD, 1976): Queenstown Facies, Rimau Facies, AyerChawan Facies, Tengah Facies, St John Facies and Jong Facies. The relationship of the six facies isshown in Figure 3. Detailed description of the facies can be found elsewhere (PWD, 1976), a briefdescription of the facies is given below:

(a) Queenstown Facies (Upper Triassic to Lower Jurrasic)The Queenstown Facies consists predominantly of thinly bedded red and purple mudstone with some redto purple shale. The mudstone often has a white coating on the joint surfaces. Vugs, possibly initially con-taining pyrite, are common, and these are now seen as holes up to 5 mm in diameter. Greenish stains areoften found on the fracture planes. Massive red to purple mudstone is also common, and this is closelyjointed or sheared. In the more sandy portions sharply angular quartz dominates and the quartz is coated

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Figure 3. Diagrammatic representation of facies relations in the Jurong Formation (from PWD 1976).

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with haematite and goethite to give the red-purple colour. A thin section of this rock contains quartz, sub-ordinate clouded feldspar, and a few flakes of muscovite and pale brown biotite. Tuffaceous materialcould be identified in hand specimen in some of the coarser grained rocks in this member, particularlytowards the northwest, and this volcanic material may have also contributed to the red coloration.

(b) Rimau Facies (Upper Triassic)The Rimau Facies is typified by quartzite and conglomerate. The conglomerate contains subangular torounded fragments, usually less than 5 cm in diameter, but sometimes of cobble grade, quartz, tuff, quartzsandstone, chert, rhyolite, basic igneous pebbles and pebbles of red sandstone presumed to be derivedfrom the Queenstown Facies and schist presumably derived from the Murai Schist. Quartz, probably veinquartz, is by far the dominant lithology of the clasts. The matrix is usually coarse sub-angular sand, butfeldspar has been recognized, usually weathered to clay.

Beds of Rimau Facies are usually 0.5 m to 1.5 m thick and coarse cross beddings and scour featuresare common. Beds between 1 cm and 10 cm thick of silt and fine sand grade may be seen between thethicker beds, and these may show a greater variety of sedimentary current features. The general coarsenature of the facies and the presence of cross-bedding and current-bedding features suggest a shallowwater, near-shore, probably deltaic environment, close to a rising land mass that has been deeply leached.The clay residual has been removed, possibly incorporated in the Queenstown and Tengah Facies prior tothe deposition of Rimau Facies.

(c) Ayer Chawan Facies (Upper Triassic to Lower Jurassic)The Ayer Chawan Facies is generally a well-bedded tuffaceous muddy sandstone facies. Bed thicknessvaries between 1 mm and 1 m. Graded beds are common but few other sedimentary structures can beobserved, except minor scour channels in the top of mud beds and filled with sandy beds. Quartz grit beds,silt and clay beds, tuff and tuffaceous conglomerate, often red in colour, are also common within themember. It is possible that there was reworking of the sediments by the biota at the time of deposition.The sandstone usually comprises fine to medium grained sand made up of quartz, with a significantamount of polycrystalline quartz grains, tuffaceous clasts and secondary chalcedony and chert. Biotite micais also present with zircon, tourmaline and opaque ore, sometimes rimmed with haematite. Up to 20% claycan be found in the rock. The sediments of the Ayer Chawan Facies are dominantly fine grained withlamella bedding and only minor current features. The finer sediments are more characteristically black orgrey and contain a few angular grains of sand size quartz in clay silt matrix of silica minerals, sericite,opaque ore and heavy minerals. It is possible that the characteristic black coloration is due to the presenceof finely divided pyrite. These features, together with the occurrence of finely disseminated carbonaceousmatter suggest a low energy environment. It is thought that these anaerobic conditions occurred afterphases of volcanic activity and were responsible for the elimination of the presence of biota.

(d) Tengah Facies (Upper Triassic to Lower Jurassic)The Tengah Facies consists of a poor assortment of sandstone and conglomerate. The rapid change ingrain size from mudstone to grit suggests flysch-like deposition. The sediment is a muddy quartz-richfine to medium sandstone, usually poorly indurated. As a result of weathering there are few natural out-crops. The member is usually well bedded with beds being 2 cm to 30 cm thick, but moderately welllithified beds up to 1 m thick can be seen. The well-lithified beds are generally quartz-rich and appearto have been cemented by silica.

(e) St. John Facies (Upper Triassic to Lower Jurrasic)The St. John facies is pale grey muddy sandstone with well defined ripple marked beds, current bed-ding, graded bedding and minor intraformational breccia. Lenses of coal, less than 2 mm thick, are alsocharacteristic of this facies and were not recorded in any other facies.

(f) Jong Facies (Upper Triassic)The Jong Facies consists of beds of conglomerate 50 cm to 6 m thick and occasionally much thicker. Theconglomerate with subrounded to rounded clasts is usually about 6 cm to 10 cm in diameter, but fre-quently conglomerates with diameter up to 30 cm occurs in the beds. These beds grade up into a muddyfine to coarse sandstone to make up the bulk of the facies. Beds of hard muddy sand grit, ranging from20 cm to 2 m thick, form most of the remainder. Mudstone beds, often dark grey to black, and seldommore than 1m thick, are less frequent. The sand in the unit is quartz rich, with lithic-volcanic, tuffaceous,and pumiceous fragments are also present. The clasts in the conglomerate are dominantly siliceous asfine sandstone or siltstone, or as quartz porphyry. Dark grey mudstone clasts are also common. Chertfragments are common within the facies.

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3 COMPOSITION

The formation process of a residual soil profile is complex, difficult to understand and difficult to gen-eralise (Blight, 1997). The three main agencies of weathering are physical, chemical and biologicalprocesses. Physical processes break down the rock into smaller fragments, expose fresh surfaces tochemical attack and increase the permeability of the material. Chemical processes involve hydrolysis,cation exchange and oxidation which alter the parent rock to form more stable clay minerals (Mason,1949; Mitchell, 1976). Biological weathering includes physical action such as splitting by root wedgingand chemical action such as bacteriological oxidation, chelation and reduction of iron and sulphur compounds (Pings, 1968; Jackson & Keller, 1970). The variability of the residual soils can be observedfrom its large band of grain size distributions as evident in Figure 4. Some of the variability in the grain size distribution is attributed to the difficulty in preparing the soil sample. There is a tendency for the soil not to be fully broken down to the individual particles (Brand & Phillipson, 1985;Netterberg, 1994).

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Figure 4. Grain size distribution envelopes of residual soils.

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In general, the degree of weathering is largely governed by factors such as rock type, fracturing andtopography. Weathering of the Bukit Timah Granite is predominantly due to chemical action breakingdown of its primary minerals. The initial process of weathering, known as primary weathering, involvesthe chemical decomposition of less resistant minerals. In secondary weathering, laterization and drift for-mation takes place. The Bukit Timah Granite has undergone the primary weathering stage where the soilsare mainly sandy clayey silt. During primary weathering, the primary minerals in the granite undergochemical changes under the action of water. The main chemical reaction is hydrolysis where the hydrogenand hydroxyl ions ensure the breakdown of the primary minerals and destroy the original rock structure.When part of the feldspar decomposes, the rock breaks down to platy fragments. With most of the feldspardecomposing to kaolinite, the rock changes to a silty sand. Quartz being a more chemically resistant min-eral, remains almost unchanged and exists as sand grains. At this stage, the silt-sized particles containessentially kaolinite. With further weathering, the silt-size kaolinite decomposes to clay size particles. Thedepths of weathering are variable. In most areas, the depth of weathering of the Bukit Timah Granite is between 20 and 50 m (Zhao, 1993). The granitic residual soil is mainly sandy clayey silt and sometimesa layer of a few metres of reddish brown sandy silty clay can be found above the clayey silt indicating secondary weathering.

The Jurong Formation consists of a variety of sedimentary rocks ranging from weak mudstone andshale to strong sandstone, conglomerate and limestone. Due to the extensive weathering, a thick over-burden of residual soil often overlays the Jurong sedimentary bedrocks. The primary mineralogy of thesediments in the Jurong Formation rendered them susceptible to chemical decomposition under the hotand humid tropical conditions of Singapore. The only rock types of the Jurong Formation that have notexperienced extensive weathering are the highly quartzose sandstones and quartz conglomerates of theRimau Facies. The siliceous-cemented quartz grains are chemically inert and more resistant to tropicalweathering. The depth of weathering ranges from a few metres to 50 m. Generally, the degree of weath-ering of the Jurong Formation decreases with depth. Most of the Jurong Formation residual soils has aclay content between 10 and 50% and may be described as clayey silt or silty clay. The remainder hashigher sand contents, and ranges from clayey sand to silty sand.

The variation in grain size distribution of the residual soils over depth can be observed from Figure 5.It can be observed that as degree of weathering increases from the deeper depths to the shallower depths,the clay size fraction of the soil increases. The variation in clay mineral compositions with depth is

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summarised in Table 2. The Bukit Timah Granite shows a higher proportion of quartz compared to theJurong Formation residual soils. Both kaolinite and illite are the dominant clay minerals in the JurongFormation residual soils.

4 CLASSIFICATION AND INDEX PROPERTIES

At present, there is no suitable classification system for residual soils (Leong & Rahardjo, 1998). Thedifficulties in classifying residual soils may be summarised as: (i) different parent rocks result in vari-able mineral composition, (ii) formation is a function of climate, (iii) properties change by remoulding,drying and wetting, and (iv) residual soils are of interest to several disciplines. Many efforts have beendirected into the development of classification system for residual soils. However, these developmentshave taken different approaches (geological, pedological and engineering) and the classifcation systemscannot be directly adopted across disciplines. In some classification systems, confusion arises from thedifferent use of the same terminology. Leong & Rahardjo (1998) had suggested extension of the UnifiedSoil Classification System for residual soils to include also a weathering grade classification. InSingapore, a modified weathering grade classification system (Table 3) suggested by Dames & Moore(1983) in the preliminary study for the Mass Rapid Transit System Phase I has been adopted in localengineering practice. Recently, however, there has been an attempt to abolish the use of the modifiedweathering grade classification by Dames & Moore (1983) and to use the weathering classification inthe British Standard for Site Investigation, BS 5930, revised in 1999 (Anon, 2001).

The plot of the Atterberg limits of the residual soils on the plasticity chart is indicative of their claymineral compositions. These plots are shown in Figure 6. Most of the data of the Bukit Timah residualsoils plot below the A-line, indicating that they consist of mainly silts. The Bukit Timah Granite resid-ual soil is generally described visually as silty clay or clayey silt. The particle size distribution curvestend to show a predominance of silt, Atterberg limits that plot above and below the A-line on the plas-ticity chart indicate that the soil can be a clay or silt of intermediate to high plasticity. Frequently, thedescription of silt is retained despite the Atterberg limits being plotted in the CL or CH regions as it isconsidered that the sample preparation pretreatment and test procedure could destroy the silt particleaggregation (Frost, 1976). Dames & Moore (1983) and Poh et al. (1985) had divided the Bukit Timah

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Table 2. Variation of clay mineral compositions of residual soils with depth.

Residual Depth Quartz Kaolinite Illite Muscovite K-felspar Smectite soils (m) (%) (%) (%) (%) (%) (%)

Bukit Timah 1.5–2.5 93 5 – – – –Granite 3.0–4.0 92 5 �1 – – –

7.5–8.5 – 62 – 30 – –12.0–13.0 56 30 �1 11 – –16.5–17.5 – 39 1 7 48 –21.0–22.0 83 3 �1 – 11 �1

Jurong 3.15–3.3 – 56 43 – 11 �1Formation 4.65–4.8 – 54 45 – – –

12.3–12.5 – 52 15 – – 32*

Table 3. Modified weathering grade classification for granitic and sedimentary residual soils (Dames & Moore,1983).

Equivalent Symbol Rock type weathering grade General description

G 1/S 1 I and II Fresh to slightly weathered rockG 2/S 2 III and IV Moderately to highly weathered rockG 3/S 3 Bouldery soil: Boulders of rock of variable

Granite (G)/ weathering within completely weathered rocksedimentaries(S) or residual soil

G 4/S 4 V and VI Completely weathered rock or residual soil

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residual soils into two groups: Group I – clayey silts, and Group II – silty clays. Clearly from Figure 6,most of the Bukit Timah residual soils are of Group I. The dominant clay mineral of the Bukit Timahresidual soils is kaolinite where some support is provided by the clay mineral composition shown inTable 2. The Atterberg limits of the Jurong Formation residual soils plot mostly above the A-line, indi-cating that both kaolinite and illite are the dominant clay minerals present. This is clearly supported bythe clay mineral compositions indicated in Table 2.

The effect of drying on the properties of soils is well-known (Mitchell & Sitar, 1982; Fookes, 1994;Fourie, 1997). Because of their formation process, residual soils are more prone to changes in proper-ties caused by drying and exposure to air. Drying causes partial or complete dehydration of the clayminerals and can change their properties irreversibly. As the drying temperature increases, the apparentwater content increases (Fourie, 1997). Stricter control is needed on sample pretreatment for index testson residual soils. Modifications to the index tests or new index test methods (Vaughan et al. 1988;Fookes, 1994) may be needed for more consistency.

The variation of the natural water content of the residual soils with depth is shown in Figure 7 togetherwith their plastic and liquid limits. Generally, the natural water content of the residual soils is close to

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Figure 6. Plasticity charts of residual soils.

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Figure 7. Variation of natural water content of residual soils with depth.

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or is lower than the plastic limit. The natural water contents also exhibit a higher variation at shallowdepths compared to those at deeper depths. The deeper residual soils tend to have more uniform watercontents. Except for low-lying areas where the groundwater table can be found a short distance from theground surface, most of the residual soils are in an unsaturated state. The greater variability of the nat-ural water content near the ground surface is an indication of the flux boundary condition between thesoil and the atmosphere as affected by the climate.

The density of the Bukit Timah residual soils ranges from 1.6 to 2.4 Mg/m3, with an average of1.8 Mg/m3, whereas the density of the Jurong Formation residual soils ranges from 1.6 to 2.2 Mg/m3,with an average density of 2.0 Mg/m3. The specific gravity of the residual soils can range from as lowas 2.4 to as high as 2.75.

5 ENGINEERING PROPERTIES

Generally residual soils are in unsaturated conditions. Therefore, theories for shear strength and permeability of unsaturated soils will be presented before describing the engineering properties ofresidual soils in Singapore. The unsaturated shear strength and permeability equations revert smoothlyto the shear strength and permeability equations for saturated soils that are a special case of the unsaturated soils.

5.1 Shear strength

The shear strength of an unsaturated soil can be represented by the “extended” Mohr-Coulomb criterion(Fredlund et al. 1978):

tff � c� � (� � ua) tan �� � (ua � uw) tan �b (1)

where tff � shear stress on the failure plane at failure, c� � effective cohesion, � � normal stress,ua � pore-air pressure, (� � ua) � net normal stress, �� � effective angle of shearing resistance,uw � pore-water pressure, (ua � uw) � matric suction, and �b � angle indicating the rate of increase inshear strength relative to matric suction.

As the soil approaches saturation, the pore-water pressure, uw, approaches the pore-air pressure, ua,and Equation 1 can be rewritten as:

tff � c� � (� � uw) tan �� (2)

which is the Mohr-Coulomb strength criterion for saturated soils. In applying Equation 2 to unsaturatedsoils, the shear strength component due to matric suction, i.e. (ua � uw) tan �b, is masked as the cohesionintercept, c [� c� � (ua � uw) tan �b]. Therefore the cohesion intercept, c, in residual soils appears tovary widely. Zhu (2000) conducted Iowa borehole shear tests in Jurong Formation residual soils. The testresults indicate that cohesion decreases with increased soaking time, whereas the effect on �� is muchless. Similar findings were reported by Brand et al. (1983) for in situ direct shear tests on the Hong Kongdecomposed granitic residual soils.

Typical plots of the effective shear strength parameters of saturated residual soil specimens (effectivecohesion, c�, and effective angle of shearing resistance, �� obtained from consolidated undrained triaxialtests with pore-water pressure measurement) with depth are shown in Figure 8. The effective angle ofshearing resistance, ��, is shown to vary over a very narrow range whereas the effective cohesion, c�, rangeis not so narrow. A summary of the effective shear strength parameters reported in the literature is shownin Table 4. A glimpse of the average effective angle of shearing resistance, ��, showed that the Bukit TimahGranite residual soils has a narrow range of average ��, from 29° to 30° whereas the range of average ��of the Jurong Formation residual soils is larger, from 27° to 35°. The larger range of average �� of theJurong Formation residual soils is attributed to its more variable parent rock types.Very limited tests havebeen performed to determine the angle �b. Generally �b is expected to range from 0.5�� to ��. The �b val-ues reported for the Jurong Formation residual soils are in the range of 23° to 35° (Lim, 1995; Gasmo, 1997;Hritzuk, 1997). More recently, �b is shown to vary with matric suction, (ua � uw) (Rahardjo, 2000). At lowmatric suctions, there is experimental evidence that �b can exceed ��. Variation of �b with (ua � uw) forthe Singapore residual soils is shown in Figure 9. The air-entry values of the Bukit Timah granite andJurong Formation residual soil specimens shown in Figure 9 are 25 kPa and 50 kPa, respectively. It can be

1290

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1291

00 4020 2060 80 100

10

20

30

40

50

60

70

c' (kPa)D

epth

, z (

m)

0

10

20

30

40

50

60

70

25 30 35 40φ ' (

o)

Dep

th, z

(m

)

Bukit Timah GraniteJurong Formation

Bukit Timah GraniteJurong Formation

Figure 8. Variation of effective shear strength parameters with depth.

Table 4. Effective shear strength parameters of Singapore residual soils.

Range of Average Range of Average Residual soils References c� (kPa) c� (kPa) �� (°) �� (°)

Bukit Timah Dames & Moore (1983) 0–125 0 13–36 30Granite Poh et al. (1985) 0–42 – 20–35 –

Yang & Tang (1997) 5–10 – 35–40 –Tan et al. (1988) 0–40 15 30–35 32Rahardjo (2000) 12–50 26 29–33 30

0–14 9 27–31 29KarWinn et al. (2001) – – 20–40 –Zhou (2001) – 7 – 32

Jurong Formation Dames & Moore (1983) 5–100 – 17–46 2810–65 17–36

Yong et al. (1985) – 12 13–40 3517 28

Lo et al. (1988) – 6 – 32Lim (1995) 19–50 31 24–40 27Gasmo (1997) 15–22 20 – 27Hritzuk (1997) – 95 – 35Rahardjo (2000) 5–9 7 29–32 30.5Zhu (2000) 10–30 – 24–40 –Seah et al. (2001) 0–40 – 24–40 –Orihara et al. (2001) – – – 33

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observed in Figure 9 that at low matric suction, in the vicinity of the air-entry value, �b is large and canexceed ��. This is attributed to tensile strength of the residual soil specimen due to the high bond strengthbetween soil particles. At higher matric suctions, above the air-entry value, the value of �b decreases as thecontribution of matric suction to shear strength of the residual soil reached an asymptotic value due to thehigh stiffness of the soil structure.

It is more appropriate to analyse the shear strength of a residual soil using the stress state of the soil,i.e., (� � ua), (ua � uw) or (� � uw) as given in Equation 1 or 2. In practice, however, undrained shearstrength of residual soils is commonly determined and used in design. In this case it is important thatthe undrained shear strength is associated with the appropriate stress state of the soils. It is more diffi-cult to determine the stress state of a residual soil in an unsaturated condition since it involves two stressstate variables which are net normal stress, (� � ua), and matric suction, (ua � uw), as opposed to theone stress state variable of effective stress, (� � uw), when the soil is in a saturated condition. For exam-ple, the failure envelopes obtained from the Unconsolidated Undrained (UU) tests are commonlycurved at low effective stresses because the soil has not become fully saturated. The failure envelopewill become horizontal (i.e., � � 0 concept) only when the soil has become fully saturated (Figure 10).In addition, the matric suction variable is highly affected by the environmental conditions such as pre-cipitation and evapotranspiration while the effective stress remains generally constant when the water

1292

0

10

20

30

40

50

60

70

Matric suction,

Matric suction,

(ua-uw) (kPa)

Coh

esio

n in

terc

ept,

c (k

Pa)

φb = 27.5o

(a) Bukit Timah Granite residual soil

00

10

50 100 150 200 250

0 50 100 150 200 250

20

30

40

50

60

(ua-uw) (kPa)

Coh

esio

n in

terc

ept,

c (k

Pa)

φb = 50o

(b) Jurong Formation residual soil

Figure 9. Variation of �b angle with matric suction (data from Rahardjo 2000).

09031-14.qxd 20/Oct/02 1:04 AM Page 1292

table does not change its position. Therefore, the problems associated with strength in residual soils arerather complex.

In situ tests are usually the preferred means of strength characterisation of a site. One of the most usedin situ tests in Singapore is the standard penetration test (SPT). Though the SPT is useful for soil strat-ification, its use in the residual soils for strength estimation requires considerable engineering judgementin its interpretation. Typical SPT results for Bukit Timah Granite and Jurong Formation residual soilsites within a single borehole are shown in Figure 11. Generally, there is a trend showing SPT N valueincreasing with depth through the weathering profile. However when the SPT results from several

1293

Figure 10. Shear stress versus normal stress relationship at failure for undrained test (modified from Fredlund &Rahardjo 1993).

0

10

20

30

40

50

60

70

100500 150SPT N

Dep

th, z

(m

)

Bukit Timah Granite

Jurong Formation

Figure 11. Typical SPT results with depth within a single borehole.

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1294

0

10

20

30

40

50

60

70

0 50 100 150

SPT N

Dep

th, z

(m

)

AuthorsDames and Moore (1983)Zhao (1993)

N = 0.5z

N = 4z

0

10

20

30

40

50

60

70

0 50 100 150SPT N

Dep

th, z

(m

)

Authors

N = 0.5z

N = 3z

(a) Bukit Timah Granite residual soils (b) Jurong Formation residual soils

0

10

20

30

40

50

60

0 50 100 150SPT N

Dep

th f

rom

top

of B

ukit

Tim

ah G

rani

te (

m)

N = z

0

10

20

30

40

50

60

0 50 100 150SPT N

Dep

th f

rom

top

of J

uron

g Fo

rmat

ion

(m)

N = 1.5z

(a) Bukit Timah Granite residual soils (b) Jurong Formation residual soils

Figure 12. Typical SPT results with depth.

Figure 13. Typical SPT results with depth from top of formation.

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boreholes are plotted with depth (Figure 12), the scatter is considerable. More valuable information canbe obtained when the SPT N value is plotted against depth from the top of the respective formation asshown in Figure 13. The degree of weathering decreases with depth from the top of the formation.Although there is still a scatter in the plot, a lower bound of the SPT N value with depth can be obtained(Figure 13). Attempts have been made to correlate SPT N value with undrained shear strength, su, fromunconsolidated undrained triaxial test. The correlations are shown in Figure 14. Figure 14 shows thatthe correlation has a wide range from su � 2 N to 12.5 N (kPa) for the Bukit Timah Granite residual soilsand su � N to 8 N for the Jurong Formation residual soils. Stroud’s (1974) correlation of su � 5 N pro-vides a reasonable average for the Singapore residual soils. The wide scatter in the correlations could beattributed to the difference between the degree of saturation of the soil specimens used in the undrainedtriaxial tests and the degree of saturation of the soils in the field. As a result, the matric suctions of thesoils in these two tests are different and this causes the difference in strength.

A more successful correlation is achieved when cone penetration resistance, qc, is correlated withSPT N (Figure 15). The correlation works out to be qc � N/5 (MPa) for both Bukit Timah Granite andJurong Formation residual soils. The better agreement between the two in situ tests is attributed to thesimilar degree of saturation of the soils where both in situ tests have been performed. As an illustrationon the effect of degree of saturation on shear strength, unconfined compression tests were performed on

1295

0

100

200

300

400

500

0 20 40 60 80 100

SPT N

su (

kPa)

AuthorsKarWinn et al. (2001)

su = 12.5N

su = 2N

0

100

200

300

400

500

0 20 40 60 80 100

SPT N

su (

kPa)

AuthorsKarWinn et al. (2001)

su = 8N

su = N

(a) Bukit Timah Granite residual soils (b) Jurong Formation residual soils

Figure 14. Correlation of SPT N with su from UU tests.

0

2

4

6

8

10

0 10 20 30 40 50 60SPT N

qc (

MP

a)

Poh et al. (1985)Chang (1988)KarWinn et al. (2001)

qc = N/3

qc = N/7

qc = N/5

0

2

4

6

8

10

0 10 20 30 40 50 60SPT N

qc (

MPa

)

KarWinn et al. (2001)

qc = N/3

qc = N/7

qc = N/5

(a) Bukit Timah Granite residual soils (b) Jurong Formation residual soils

Figure 15. Correlation of cone resistance, qc, with SPT N value.

09031-14.qxd 20/Oct/02 1:04 AM Page 1295

a number of Jurong Formation residual soil samples from various depths at the in situ water contentsand after saturation. The changes in undrained shear strength of the soil samples are shown in Figure 16.

The effect of matric suction on undrained shear strength of the soil samples at various depths isshown in Figure 17. The matric suctions of the soil samples were determined using the contact filterpaper method (ASTM 1997, D5298-94). Figure 17 shows that at low matric suction values, there is arapid increase in undrained shear strength and the undrained shear strength of the samples at shallowdepths is less than that at deeper depths. For the soil samples at deeper depths, the shear strengths mayvary considerably at high matric suction due to the more variable degree of weathering.

An extended analysis on the data shown in Figure 17 can be performed using Equation 1. Theundrained shear strength of the unsaturated soil samples can be estimated from Equation 1 using typi-cal values of density and effective friction angle for the Jurong Formation residual soils, i.e. 2.05 Mg/m3

and 32°, respectively. Using an assumed value of �� for �b for matric suctions less than the air-entryvalue of the soil (assumed to be 100 kPa) and 0.5�� for �b for matric suctions greater than the air-entryvalue of the soil, the estimated undrained shear strength versus the measured undrained shear strengthis shown in Figure 18. Only data with measured matric suction values less than 1000 kPa are shown as

1296

00 10 20 30 40 50

200

400

600

800

1000

1200

Water Content, w (%)

Und

rain

ed S

hear

Str

engt

h, s

u (k

Pa) Saturated

UnsaturatedSaturated - Best-fitUnsaturated - Best-fit

1

10

100

1000

10000

0 1000 2000 3000Matric Suction,(ua-uw) (kPa)

Und

rain

ed S

hear

Str

engt

h, s

u (

kPa)

2m to 3m 4m to 5m 6m to 7m 8m to 9m

Samples depth

Figure 16. Differences in undrained shear strengthfor saturated and unsaturated Jurong Formation resid-ual soil samples.

Figure 17. Relationship between undrained shearstrength and matric suction of Jurong Formation resid-ual soil samples at various depths.

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600 700Measured su (kPa)

Est

imat

ed s

u (k

Pa)

Figure 18. Comparison of measured and estimated undrained shear strength using Equation 1 for residual soilsfrom Jurong Formation.

09031-14.qxd 20/Oct/02 1:04 AM Page 1296

above 1000 kPa, the filter paper gives the total suction values (Leong et al. 2002). Figure 18 shows goodagreement between estimated undrained shear strength and measured undrained shear strength at lowmatric suctions but Equation 1 underestimates the undrained shear strength at high matric suctions. Theunderestimation is attributed to the lower accuracy of the filter paper method in measuring matric suc-tion near 1000 kPa.

6 STIFFNESS AND COMPRESSIBILITY

6.1 Undrained Young’s modulus

The change in undrained Young’s modulus, Eu, obtained from pressuremeter tests with depth is shownin Figure 19. Generally, Eu increases with depth. The range of Eu for both the Bukit Timah Granite andJurong Formation residual soils is from 1 to 100 MPa. Plots of Eu normalised with su determined fromconsolidated undrained triaxial tests are shown in Figure 20. The ratio of Eu/su for the Bukit TimahGranite residual soils ranges between 30 and 300, while that of the Jurong Formation residual soilranges between 40 and 400. The ratio of Eu/su can therefore vary by one order in magnitude. The largerrange of Eu/su for the Jurong Formation residual soil is attributed to its more varied parent rock types.As a preliminary estimate, a Eu/su value of 200 for the Singapore residual soils is reasonable.

6.2 Compressibility parameters

Oedometer tests are not often carried out for residual soils due to the predominance of coarse particles.However, when the oedometer tests are carried out, they can provide an indication of the compressibilityof saturated residual soils. All residual soils showed overconsolidation behaviour (Barksdale & Blight,1997). The compressibility is relatively low at low stress levels but the compressibility increases oncethe threshold yield stress or equivalent preconsolidation pressure has been exceeded. The compression

1297

0

10

20

30

40

50

60

0.1 1 10 100 1000

Eu (MPa)

Dep

th, z

(m

)

Dep

th, z

(m

)

AuthorsPoh et al. (1985)

0

10

20

30

40

50

60

0.1 1 10 100 1000

Eu (MPa)

AuthorsDames and Moore (1983)

(a) Bukit Timah Granite residual soils (b) Jurong Formation residual soils

Figure 19. Variation of undrained Young’s modulus with depth.

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1298

0

10

20

30

40

50

60

1 10 100 1000 10000

Eu/su

Dep

th, z

(m

)

AuthorsPoh et al. (1985)

Eu/su = 30 Eu/su = 300

0

10

20

30

40

50

60

1 10 100 1000 10000

Eu/su

Dep

th, z

(m

)

AuthorsDames and Moore (1983)

Eu/su = 40 Eu/su = 400

(a) Bukit Timah Granite residual soils (b) Jurong Formation residual soils

Figure 20. Variation of normalised undrained Young’s modulus with depth.

curves of the residual soils are generally gentle and it is difficult to determine the yield stress or equiv-alent preconsolidation pressure. The preconsolidation pressure in residual soils can be more correctlyviewed as the destructuring stress of the residual soil (Vaughan et al. 1988; Barksdale & Blight, 1997).It is therefore reasonable to expect preconsolidation pressure to increase with depth. Barksdale & Blight(1997) showed that the preconsolidation pressure does increase with depth for three profiles of andesitelava. Although Dames & Moore (1983) indicated that the preconsolidation pressures determined for the Bukit Timah Granite residual soils are variable and do not show any clear relationship with depth,investigation by Rahardjo (2000) showed preconsolidation pressure increases with depth. Test resultsindicated that most of the residual soils exhibit an overconsolidation ratio, OCR, greater than 1. Dames &Moore (1983) suggested that the OCR for Bukit Timah Granite residual soils is in the range 1.5 to 3,and for Jurong Formation residual soils is in the range 1.5 to 5.

A summary of the compression index, Cc, and recompression index, Cr, reported in the literature isgiven in Table 5. Compression index, Cc, has been correlated with a number of index properties for sedi-mentary soils. These relationships may not be directly applicable to residual soils. However, it appears that

Table 5. Compression and recompression indices.

Residual soils References Cc Cr

Bukit Timah Granite Dames & Moore (1983) 0.007–0.4 0.02–0.075Poh et al. (1985) 0.16–0.4 –Rahardjo (2000) 0.18–0.64 –

Jurong Formation Dames & Moore (1983) 0.003–0.108 –Yong et al. (1985) 0.10–0.60 0.025–0.11Rahardjo (2000) 0.05 –Seah et al. (2001) 0.1–0.6 –

09031-14.qxd 20/Oct/02 1:04 AM Page 1298

Cc of the Singapore residual soils shows a good correlation with the initial void ratio, e0. The relationshipcan be expressed in the following form:

Cc � 0.4eo � b (3)

where b is a constant. Plots of Cc with e0 for the Bukit Timah Granite and the Jurong Formation resid-ual soils are shown in Figure 21 together with the bounding lines given by Equation 3. A survey of theCc and the Cr values in Table 5 shows that the ratio Cr/Cc is about 0.25 to 1.

Coefficient of consolidation, cv, for the Bukit Timah Granite residual soils spans a wide range. Mostof the cv values fall in the range 35 to 55 m2/year (Dames & Moore, 1983). Very limited cv data is avail-able for the Jurong Formation residual soils. Dames & Moore (1983) suggested a design cv value of40 m2/year for Jurong Formation residual soils.

7 HYDRAULIC PROPERTIES

7.1 Saturated coefficient of permeability

The permeability of residual soils is expected to exhibit variability both in the vertical and lateral extent.The saturated coefficients of permeability for the Singapore residual soils reported in the literature aresummarised in Table 6. Agus et al. (2001a) had shown that the permeability test methods used and themeasured permeability should be interpreted based on the flow mechanism of the test methods. To com-pare field and laboratory measured permeability, the direction of water flow in the test should be con-sidered. In Table 6, the range of saturated coefficients of permeability for Jurong Formation residualsoils obtained using laboratory falling head test reported by Rahardjo (2000) is probably too high. Thediscrepancy is attributed to side-wall leakage as a rigid-wall permeameter was used in the test. Theranges of saturated coefficients of permeability for the Bukit Timah Granite and Jurong Formationresidual soils are of the order of 10�10 and 10�5m/s and 10�11 and 10�6m/s, respectively.

7.2 Soil-water characteristic curve

As most of the residual soils are in an unsaturated state, the rate of water flow through these soils willbe a function of its water content as well. The amount of water retained in a soil at different soil suctionscan be expressed using the soil-water characteristic curve. The soil-water characteristic curve can beused as the basis to derive a number of engineering properties of unsaturated soils (Fredlund, 1995).Using the soil-water characteristic curve and the saturated coefficient of permeability, the permeabilityfunction for an unsaturated soil can be derived (Fredlund et al. 1994; Leong & Rahardjo, 1997b). Shear

1299

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0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.5 1 1.5 2Initial void ratio, e0 Initial void ratio, e0

Com

pres

sion

inde

x, C

c

Authors

Poh et al. (1985)

KarWinn et al. (2001)

Cc = 0.4e0 - 0.2

Cc = 0.4e0 + 0.05

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.5 1 1.5 2

Com

pres

sion

inde

x, C

c

Authors

Yong et al. (1985)

KarWinn et al. (2001)

Cc = 0.4e0 - 0.2

Cc = 0.4e0 + 0.05

(a) Bukit Timah Granite residual soils (b) Jurong Formation residual soils

Figure 21. Relationship of compression index with initial void ratio.

09031-14.qxd 20/Oct/02 1:04 AM Page 1299

strength relationship with matric suction of unsaturated soils can also be obtained from the soil-watercharacteristic curve (Vanapalli et al. 1996; Öberg & Sällfors, 1997).

The soil-water characteristic curve is usually plotted with water content as its ordinate and soil suc-tion as its abscissa. It is useful to describe the soil-water characteristic curve using an equation. Leong& Rahardjo (1997a) evaluated a number of popular expressions for the soil-water characteristic curveand found that the Fredlund & Xing (1994) equation given by Equation 4 is the most suitable.

(4)

where ) � normalised volumetric water content, i.e. *w/*s, *w � volumetric water content � volumeof water/volume of soil, *s � saturated volumetric water content, e � base of natural logarithmn, and a,n and m � constants. If the volume of soil remains constant at all matric suction levels, ) is equivalentto the degree of saturation, Sr.

Leong (2000) obtained a number of soil-water characteristic curves for Singapore residual soils. Aguset al. (2001b) established the upper bound, average and lower bound soil-water characteristic curves forBukit Timah Granite and Jurong Formation residual soils. These curves are shown in Figure 22.

The permeability function for unsaturated soil may be obtained using the following expression(Leong & Rahardjo, 1997b):

kw � ks)p (5)

where kw � unsaturated coefficient of permeability, ks � saturated coefficient of permeability andp � constant depending on soil types. When the soil becomes fully saturated, i.e. ) is 1, kw becomes ks(Equation 5). Fredlund et al. (2001) determined the value of p for several hundred experimental datasets and found that p varies from 2.4 to 5.6 for different soil types. The overall average value of p for allsoil types is 3.29.

) �

��

1

ln eu u

aa w

nm

1300

Table 6. Saturated coefficients of permeability.

Field – Laboratory – Laboratory – Laboratory – Residual falling head falling head triaxial consolidation soils References test test permeameter test

Bukit Timah Poh et al. (1985) 4 � 10�9 to 3 � 10�10 to – 1.5 � 10�10 toGranite 4 � 10�7m/s 2 � 10�7m/s 2.5 � 10�8m/s

Yang & Tang (1997) 1 � 10�8m/s – – –Knight-Hassell (2000) 1 � 10�6 to – – –

1 � 10�8m/s*Agus et al. (2001a) 1.2 � 10�8 to 9.5 � 10�7 to – –

7.3 � 10�5m/s 1.2 � 10�6m/sAung (2001) – 4 � 10�9m/s – –

Jurong Dames & Moore 3.6 � 10�11 to 1 � 10�9 to – 5.5 � 10�10 to Formation (1983) 6 � 10�8m/s 4.5 � 10�7m/s 3.5 � 10�9m/s

Gasmo (1997) – – 1 � 10�6m/s –Hritzuk (1997) – – 3 � 10�9m/s –Agus et al. (1999) 1 � 10�8 to 3 � 10�9 to – –

1 � 10�6m/s 5 � 10�8m/s – –Rahardjo (2000) – 1.7 � 10�5 to 8.9 � 10�10 to –

5.2 � 10�4m/s** 1 � 10�9m/sOrihara et al. (2001) 1 � 10�7m/s* – – –Seah et al. (2001) 1 � 10�6 to – – –

1 � 10�9m/s*Authors 1.4 � 10�8 to – – –

1 � 10�10m/s

09031-14.qxd 20/Oct/02 1:04 AM Page 1300

8 ENGINEERING PROBLEMS

The variable nature of the residual soils makes quantification of their engineering properties difficult.This is further compounded by the fact that the index properties of the residual soils are affected by thedrying of the soil samples. At present, there is no suitable classification system for the residual soils.Sampling for residual soils for laboratory tests invariably introduces soil disturbances (Lee et al. 1985;Zhao & Lo, 1994) and therefore introduces a larger scatter in the test results. As such, site investigationin residual soil sites should involve a larger number of soil samples compared with transported soil sites.Better quality residual soil samples can be obtained using Mazier sampler or triple tube core barrels(Brand & Phillipson, 1985). In the Singapore residual soils, Zhao & Lo (1994) had indicated that 50 mmdiameter jacked-in thin-walled samplers or 100 mm diameter open samplers are suitable in firm to stiffdeposits and 63 mm diameter triple tube is suitable in stiff to hard deposits. Modified procedures in sam-ple preparation for index tests as suggested by Fourie (1997) are needed to ensure greater consistency inthe test results. The weathering grade classification should be used in conjunction with the Unified SoilClassification System to obtain a better description of the residual soils. The degree of saturation of thesoil samples should be ascertained for unconsolidated undrained triaxial tests and unconfined compres-sion tests. The degree of saturation affects the magnitude of matric suction, one of the stress state vari-ables for unsaturated soils.

Problems related to design and construction for excavation and tunnelling in the Singapore residualsoils have been described by Shirlaw et al. (2000). Shirlaw et al. (2000) highlighted that construction inresidual soils present difficulties in terms of a mixed ground condition where the construction processencounters several weathering grades within the same rock formation. In design, there is an uncertaintyin choosing the appropriate design approach as the properties of the residual soils lie between those of arock and a transported soil. Although residual soil is technically a soil, material features such as residualcementation and bonding, and a pore structure developed from the original rock structure is present inthe residual soil. Typical design parameters relevant to transported soils must be applied with caution andengineering judgement to residual soils.

Generally, the stiffness and strength of the residual soils is high when unsaturated but may decreaserapidly with saturation. Lee et al. (2001) reported the effects of groundwater ingress in a soldier-piledexcavation in a granitic residual soil site. The groundwater causes the soil to soften and leads to a largeincrease in wall deflection. However, the failure occurs only in localised soil zones.

Broms et al. (1988) evaluated methods for estimating bearing capacity and settlement of bored pilesin residual soils and weathered rocks in Singapore. At working load, the bored piles carry the appliedload mainly by skin friction. Uncertainties are associated with the determination of the undrained shearstrength of the residual soils and weathered rocks as gradual softening takes place in the material aroundthe borehole after the drilling and during the casting of the concrete. Pile load tests on the bored pilesare difficult and costly due to the high required applied load.

1301

0

0.2

0.4

0.6

0.8

1

1 10 100 1000 10000 100000Matric suction, Matric suction, (ua - uw) (kPa)

Nor

mal

ised

vol

umet

ric

wat

er c

onte

nt, Θ Θ

w

Upper Bounda = 159 kPan = 0.792m = 0.704

Lower Bounda = 32 kPan = 0.525

Averagea = 36 kPan = 0.565m = 1.147

0

0.2

0.4

0.6

0.8

1

1 10 100 1000 10000 100000 (ua - uw) (kPa)

Nor

mal

ised

vol

umet

ric w

ater

con

tent

, w Upper Bound

a = 432 kPan = 0.93m = 1.004

Lower Bounda = 1185 kPan = 0.448m = 5.387

Averagea = 299 kPan = 0.554m = 1.869

(a) Bukit Timah Granite residual soils (b) Jurong Formation residual soils

Figure 22. Soil-water characteristic curves.

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One common engineering problem encountered in Singapore residual soils is the failure of slopesdue to rainfall (Tan et al. 1987; Yang & Tang, 1997; Toll et al. 1999). The factors affecting the stabilityof a residual soil slope due to rainfall are complex and are usually not independent of each other (Leong& Rahardjo, 1997c). The quantification of the soil suction contribution to shear strength, the rate of suc-tion loss due to rainwater infiltration and the rate of suction recovery after rainfall, are still topics ofintense research (Rahardjo et al. 1998).

9 CONCLUSION

A summary of the characteristics and engineering properties of Singapore residual soils is presented inthis paper. As residual soils exhibit high heterogeneity spatially, it is expected that the generalisationsgiven in this paper may not be all encompassing. However, it is believed that the observed trends areapplicable to the residual soils. Refinement of descriptions and correlations is possible with more site-specific data and should always be attempted whenever possible. There is still much research needed inthe understanding of residual soils particularly with respect to their engineering properties as governedby the degree of saturation.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the assistance of Mr. Marcus Tong Swei Yeh and Ms. Leong PohLing in producing Figure 2 and the collation of some of the data reported in the paper.

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