Bukit Timah Research Articles

Updated Inventory of Amphibians, Reptiles and Mammals of the Bukit Timah Nature Reserve, Singapore

Updated Inventory of Amphibians, Reptiles and Mammals of the Bukit Timah Nature Reserve, Singapore

Synthesis of results from the Comprehensive Biodiversity Survey of Bukit Timah Nature Reserve, Singapore, with recommendations for management

Synthesis of results from the Comprehensive Biodiversity Survey of Bukit Timah Nature Reserve, Singapore, with recommendations for management

Diversity and Distribution of Dragonflies in Bukit Timah Nature Reserve, Singapore

Diversity and Distribution of Dragonflies in Bukit Timah Nature Reserve, Singapore

Estimating saproxylic beetle (Coleoptera: Cerambycidae and Buprestidae) diversity in Bukit Timah Nature Reserve, Singapore, with a methodological and biological review

Estimating saproxylic beetle (Coleoptera: Cerambycidae and Buprestidae) diversity in Bukit Timah Nature Reserve, Singapore, with a methodological and biological review

Butterflies of the Bukit Timah Nature Reserve, Singapore, and its vicinity

Butterflies of the Bukit Timah Nature Reserve, Singapore, and its vicinity

The springtail catchers of the genus Neurigona (Insecta, Diptera, Dolichopodidae) in the primary forest of Bukit Timah Nature Reserve, Singapore

The springtail catchers of the genus Neurigona (Insecta, Diptera, Dolichopodidae) in the primary forest of Bukit Timah Nature Reserve, Singapore

Historical review of Bukit Timah Nature Reserve, Singapore

Historical review of Bukit Timah Nature Reserve, Singapore

Hydrogeomorphic characteristics of streams in Bukit Timah Nature Reserve, Singapore

Hydrogeomorphic characteristics of streams in Bukit Timah Nature Reserve, Singapore

The other 99%: exploring the arthropod species diversity of Bukit Timah Nature Reserve, Singapore

The other 99%: exploring the arthropod species diversity of Bukit Timah Nature Reserve, Singapore

Ascher1

Ascher1

The plant diversity in Bukit Timah Nature Reserve, Singapore

The plant diversity in Bukit Timah Nature Reserve, Singapore

Effect of hysteresis on the stability of residual soil slope

Soil-water characteristic curve (SWCC) is an essential parameter in unsaturated soil mechanics. Matric suction in unsaturated residual soils changes with varying climatic conditions associated with cyclic drying-wetting conditions that result in hysteresis in the SWCC. The soil mechanical behaviour under wetting is crucial since numerous rainfall-induced slope failures occur during the wetting process. However, many slope stability analyses were carried out using drying SWCC. Consequently, the factor of safety (FoS) calculation may not represent the actual field condition. This paper presents the effect of hysteresis in SWCC on the stability of unsaturated residual soil slopes from Bukit Timah Granite in Singapore. The study focused on the analyses considering the differences in pore-water pressure and water content variations under drying and wetting conditions by performing numerical seepage and stability analyses. Each analysis was carried out on a slope subjected to dry and rainy periods under three different conditions: i) using only drying SWCC; ii) using only wetting SWCC; iii) using combined drying and wetting SWCCs. The results indicated that the FoS variations obtained from the numerical analyses incorporating combined SWCC matched more closely those obtained by incorporating only wetting SWCC than those obtained by incorporating only drying SWCC, regardless of the wetting or drying processes that the soil experienced. Moreover, the numerical analyses under high rainfall intensity by incorporating only wetting SWCC gave a more conservative FoS as compared to those obtained by incorporating combined SWCC. Numerical analyses incorporating only drying SWCC gave the most conservative FoS regardless of rainfall intensity.

A First Order Quantification of Effects of Uncertainties in Hydro-fracturing Parameters on Tunnel Ovalization Estimates

In-situ stresses are always present in rock-masses due to gravitational and tectonic forces. Excavation of a tunnel in such a pre-stressed media causes deformation of tunnel cross-section. Ovalization of tunnels due to in-situ stresses in rock-mass is an important design parameter. For estimation of tunnel ovalization, state of in-situ stresses needs to be determined first. In-situ stresses determined through hydro-fracturing technique (HFT) are dependent upon the three HFT parameters: (a) shut-in pressure, (b) re-opening pressure, and (c) fracture orientation. A critical review of previous studies indicates that HFT parameters are subjected to uncertainties due to (1) limitations of testing procedures and equipment, (2) assumptions and subjective engineering judgment associated with interpretation of test results, and (3) inherent variability of geological formations. Therefore, tunnel deformation estimates based on in-situ stresses determined through HFT would also be affected by these uncertainties. In this paper, a framework based on the first-order second moment method is developed to evaluate the effects of uncertainties in hydro-fracturing test data on tunnel deformation. The analysis indicates that uncertainty in tunnel deformation depends upon the uncertainty levels as well as magnitude of the three HFT parameters along with Poisson’s ratio, height of overburden, and the angular location of the point on the tunnel periphery where deformation is being estimated. It is also found that among the three parameters, the shut-in pressure has the maximum relative contribution in the resulting uncertainties in tunnel ovalization with an average of 68% (± 8% standard deviation). The proposed methodology is explained through an example case-study from Bukit Timah Granite rock-mass of Singapore. It is found that for a range of coefficient of variation of shut-in and re-opening pressure from 0 to 50%, the maximum coefficient of variation of tunnel deformation varies between 63 and 332%. In view of such high uncertainties, it is recommended that uncertainties of HFT parameters must be taken into account in the design procedure to avoid unsound engineering judgments.

Influence of Initial Conditions on Unsaturated Groundwater Flow Models

Slope failure in unsaturated soils is common in tropical countries due to the seasonal pattern of abundant rainfall preceding a period of prolonged drying. Much research has been undertaken to understand the behaviour of unsaturated slopes and reduce the number of catastrophic failures. The initial conditions are important factors in numerical modelling of the groundwater flow yet rarely considered in detail in the literature. This paper presents a parametric study of the initial conditions at a slope in Bukit Timah, Singapore. The intensity and duration of rainfall are varied to assess the effect on the pore-water pressure in the slope. The generated pore-water pressure profile is compared with field measurements and previous numerical studies. It is discovered that a low rainfall, with an intensity of 1x10-7 m/s over a period of 62 days, results in initial pore-water pressure which is consistent with data recorded at the field. Unlike the duration, changes in the rainfall intensity are shown to have a significant effect on the pore-water pressure in the slope. This study, therefore, demonstrates the importance of determining appropriate initial conditions in unsaturated groundwater flow analysis.  Â

Unloading-induced failure of brittle rock and implications for excavation-induced strain burst

Understanding the unloading-induced failure characteristics of brittle rock is essential for predicting excavation-induced strain burst. We conduct both the laboratory experiments and discrete element simulations to investigate the deformation, failure pattern, and strain energy evolution of the Bukit Timah granite. The laboratory tests with radial unloading show that the radial strain mainly contributes to the change of volumetric strain in the unloading process. The numerical tests with the flat-jointed contact model reproduce the Hoke-Brown failure envelope of the rock, and simulate the unloading-induced rock failure under different combinations of initial confining pressure and unloading rate. The evolutions of strain energy release and failure pattern show that higher unloading rate likely induces violent failure, in terms of greater lateral expansion and more ejected fragments. The micromechanical analyses on the particle velocity and tensile contact force reveal that the non-uniform rock deformation concentrates at lateral surfaces and results in strain burst with fragment ejection. The Hoek-Brown failure criterion well predicts the confining pressure at failure when the rock is laterally unloaded at lower unloading rate, but likely overestimates the stress level at higher unloading rate.

Spatial variations of air-entry value for residual soils in Singapore

Rainfall-induced slope failures are commonly observed in residual soil. The slopes in Singapore are associated with deep groundwater table. As a result, residual soils in Singapore are commonly observed within unsaturated condition above groundwater table. The unsaturated soil properties (i.e. soil-water characteristic curve) are rarely used as parameters in the development of slope susceptibility map. To incorporate important unsaturated soil properties, such as air-entry value (AEV), in the slope stability analysis, the sampled soil data must be transformed into a continuous field output. Geostatistical methods of ordinary kriging (OK) and simple kriging (SK) were used to estimate the spatial variability of air-entry value for soils in Singapore. The effect of soils from different geological formations in Singapore was considered in the analyses. The first case of analysis assumed Singapore as one region whereas the second case of analysis assumed Singapore as two regions – (i) Jurong Formation (JF); and (ii) Bukit Timah Granite and Old Alluvium (BTG & OA). Leave-one-out cross validation and one additional sampling point were used to assess the performance of the kriging methods and determine a suitable method for digital soil mapping. OK and SK had similar root mean square error (RMSE), of 66.6 and 66.4 respectively, for analyses with the assumption of Singapore as one region. For analyses with the assumption of Singapore as two regions, the RMSE using OK was estimated at 32.6 and 85.6 for BTG & OA and JF, respectively. SK estimated the additional sampling point with greater accuracy and was concluded as the better kriging method to estimate the AEV distribution of soils in Singapore as compared to OK method.

Engineering properties of the Bukit Timah Granitic residual soil in Singapore

Extensive site investigations were conducted prior to the construction of a Mass Rapid Transit project in Singapore that was predominantly in the Bukit Timah Granite (BTG) formation residual soil. This paper evaluates the engineering properties of the BTG formation residual soil based on data from 208 site investigation boreholes from four different sites. Based on the results from 2481 conventional laboratory tests and 1192 in-situ tests, this paper summarizes the engineering properties of the highly variable BTG residual soil, including conventional composition analysis, index and hydraulic properties, and strength and deformation parameters required for geotechnical analysis and design. Based on these results, the BTG formation is found to be quite heterogeneous. As the degree of weathering decreases with depth from the top of the formation, the BTG residual soil becomes sandier, with reduced silt and clay fractions. The coefficient of permeability and the compression index of the BTG residual soil vary significantly. In addition, the empirical equations relating the shear strength (index) to the standard penetration test (SPT)-N, as well as the equations and charts for determining stiffness, are proposed. These findings, together with the proposed equations or charts, can be used for design guidance of similar projects related to granitic residual soils in Singapore.

Influence of groundwater drawdown on excavation responses – A case history in Bukit Timah granitic residual soils

Performances of a braced cut-and-cover excavation system for mass rapid transit (MRT) stations of the Downtown Line Stage 2 in Singapore are presented. The excavation was carried out in the Bukit Timah granitic (BTG) residual soils and characterized by significant groundwater drawdown, due to dewatering work in complex site conditions, insufficient effective waterproof measures and more permeable soils. A two-dimensional numerical model was developed for back analysis of retaining wall movement and ground surface settlement. Comparisons of these measured excavation responses with the calculated performances were carried out, upon which the numerical simulation procedures were calibrated. In addition, the influences of groundwater drawdown on the wall deflection and ground surface settlement were numerically investigated and summarized. The performances were also compared with some commonly used empirical charts, and the results indicated that these charts are less applicable for cases with significant groundwater drawdowns. It is expected that these general behaviors will provide useful references and insights for future projects involving excavation in BTG residual soils under significant groundwater drawdowns.

PENERAPAN MODEL PEMBELAJARAN KOOPERATIF TIPE NHT UNTUK MENINGKATKAN HASIL BELAJAR IPS SISWA KELAS VI SD NEGERI 016 BUKIT TIMAH KECAMATAN DUMAI SELATAN

This study is based on the learning outcomes of social studies students of grades VI SD Negeri 016 Bukit TimahKecamatan Dumai Selatan Kota Dumai is still very low. This study aims to improve social studies learningoutcomes. From the data analysis, there is an increase of both teacher activity, student activity, and studentlearning result, that is teacher activity at meeting 1 cycle I percentage is 58,3% (enough) and at the secondmeeting increased to 70,8% (good). In cycle II the third meeting of teacher activity value from 70,8% (good) to83,4% (very good) and at the fourth meeting increased to 95,8% (very good). Judging from the activity ofstudents also experienced an increase from 54,2% (enough) to 62,5% (good). In the second cycle of the thirdmeeting to be 79,2% (good) and at the fourth meeting increased to 91,7% (very good). Judging from the resultsof the students also experienced increase from the average score of students on the basic score of 55.25 after thefirst cycle the average student score increased to 73,75 with an increase of 18,5 points from the baseline score.In the second cycle student learning outcomes also experienced an increase of 12 points from cycle I withaverage student scores 85,75. From the data analysis there is an increase both from teacher activity, studentactivity, and student learning outcomes. It can be concluded that the advancement of model Numbered HeadsTogether (NHT), can improve the learning outcomes of social studies learning outcomes of class VI SD Negeri016 Bukit Timah Kecamatan Dumai Selatan Kota Dumai.

An annotated list of new records for Singapore: results from large-scale tree surveys at the Bukit Timah Nature Reserve.

We report new records for Singapore of 16 tree species discovered between 1993 and 2008 during large-scale surveys of Bukit Timah Nature Reserve. These species are: Aglaia crassinervia Kurz ex Hiern (