Modeling of suction distributions in an unsaturated heterogeneous residual soil slope

Engineering Geology 131–132 (2012) 70–82

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Modeling of suction distributions in an unsaturated heterogeneous residual soil slope Azman Kassim a, Nurly Gofar a,⁎, Lee Min Lee b, Harianto Rahardjo c a b c

Faculty of Civil Engineering, Universiti Teknologi Malaysia, Skudai 81300, Malaysia Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Kuala Lumpur 53300, Malaysia School of Civil and Environmental Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 2263, Singapore

a r t i c l e

i n f o

Article history: Received 26 June 2011 Received in revised form 31 January 2012 Accepted 3 February 2012 Available online 14 February 2012 Keywords: Suction distributions Soil heterogeneity Relict discontinuity Residual soil Slope stability

a b s t r a c t The formation of residual soil of Grade V and Grade VI due to tropical weathering process introduces small hydraulic heterogeneities in the soil mantle which greatly alter the suction distribution during rainfall infiltration, and hence the stability of the residual soil slopes. This paper presents field evidences of suction distributions in a heterogeneous residual soil slope. Several modeling approaches were attempted to simulate the observation by considering the presence of thin layer of Grade VI, the variation in the hydraulic conductivity of Grade V layer as well as the effect of evaporation. The soil hydraulic heterogeneity in Grade V layer was modeled by adopting continuum method, in which the residual soil was subdivided into three zones of average hydraulic conductivities. The analysis results show that the presence of thin layer of Grade VI residual soil and the relict discontinuities in Grade V soil must be considered in the analysis as these features introduced permeability disparity and thus a natural capillary barrier effect that limited the downward movement of infiltrated rainwater even during the exceptionally wet condition. The results also show that the inclusion of evaporation effect provided a better prediction to the suction distributions during wet condition than dry condition. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Rainfall-induced slope failure is a common geohazard in many parts of the world, particularly in tropical regions which are covered extensively with residual soils (Brand, 1984; Shaw-Shong, 2004; Huat et al., 2005; Rahardjo et al., 2005; Rahimi et al., 2010). These slope failures are commonly shallow with typical depth of slip surface of 1 to 3 m oriented parallel along the slope surface (Muller and Martel, 2000; Kim et al., 2004; Matsushi et al., 2006). The failures are mainly attributed to the total or partial loss of matric suction during rainfall infiltration, and hence cause a reduction in shear strength of soil, with not much evidence on the rise of groundwater table (Fredlund and Rahardjo, 1993; Fourie et al., 1999; Chen et al., 2004; Lu and Godt, 2008; Travis et al., 2010). The analysis of suction distribution and geohydrological flow in the vadoze zone is thus vital to the study of slope failure in tropical residual soil. The tropical weathering process involved in the formation of residual soil introduces small hydraulic heterogeneities in the soil mantle that are strongly controlled by relict discontinuities (Gerscovich et al., 2006). As a result, the hydraulic heterogeneities greatly modify the suction distribution in the residual soil slopes during rainfall infiltration. This condition can lead to discrepancies in the suction distributions developed in vadoze zone which can eventually cause slope ⁎ Corresponding author. Tel.: + 60 6 07 5531596; fax: + 60 6 07 5566157. E-mail addresses: [email protected] (A. Kassim), [email protected] (N. Gofar), [email protected] (L.M. Lee), [email protected] (H. Rahardjo). 0013-7952/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2012.02.005

instabilities. Nonetheless, the influences of the relict discontinuities on the stability of tropical residual soil slopes are often overlooked by geotechnical engineers because of the highly erratic nature of the hydrological influences and the inherent heterogeneity of local soils (Au, 1998). The mechanism of rainfall infiltration induced landslides has been studied through field, laboratory experiments, and numerical analyses (i.e. Pradel and Raad, 1993; Rahardjo et al., 2001; Tsaparas et al., 2002; Cai and Ugai, 2004; Babu and Murthy, 2005). However, the studies that account for the effect of small hydraulic heterogeneities of the residual soil materials were very limited. Agus et al. (2005) who worked on Bukit Timah granitic residual soil in Singapore found that the variation of saturated coefficient of hydraulic conductivity in the residual soils mantle is within two orders of magnitude. Hicks and Spencer (2010) studied the influence of soil heterogeneity on the performance of a long clayey cut slope. However, their investigation focused mainly on the heterogeneity in shear strength properties but not the hydraulic properties of soil. Eaton (2006) highlighted the importance of geological heterogeneity in subsurface flow simulation. The concepts of continuum and discrete approaches of heterogeneity modeling were discussed in detail. Cesano et al. (2003) introduced a heterogeneity index for quantifying the fractured rock hydraulic heterogeneity which was subsequently used to predict the inflow in underground excavation. From the foregoing, it can be concluded that the importance of geological heterogeneity on the subsurface flow has been highlighted by a number of recent studies. The correlation between the geological

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heterogeneity and the mechanism of rainfall-induced slope failure, however, is still exposed to several uncertainties. The effect of soil hydraulic discrepancies due to relict discontinuities on the suction distribution under certain rainfall condition has been identified neither in the field nor in the numerical simulation. The collection of field evidences is required to validate the responses of the tropical residual soil to rainfall infiltration. As heterogeneities are often responsible for the occurrence of localized abnormalities, realistic models incorporating effects of these features can help to predict how and where abnormal seepage flow and pore-water pressure pattern may develop. This paper presents field evidences of suction distributions in a heterogeneous residual soil slope. The modeling approaches that incorporate the effect of soil heterogeneity due to relict discontinuities were reviewed. Several approaches were attempted and the simulated suction distributions were compared with those of field measurements to identify the most adequate approach that best fit the field data set. 2. Site descriptions The selected study site was a slope in Universiti Teknologi Malaysia, Skudai Campus, namely Bukit Cerapan. The variations in temporal suction and rainfall intensity were monitored for a period of one year, i.e. from 12th September 2006 to 11th September 2007. The data serves as verification of suction distributions predicted from several modeling approaches adopted in this study. Topography survey revealed that the study site poises a sloping angle of approximately 21°with an average length of 47 m. The geologic profile of the site based on two boreholes performed at the crest and the toe of the slope. The borelogs of BH1 and BH2 indicate that the subsurface is made up of three main layers as shown in Fig. 1. Four plots were constructed on the upper part of the slope to account for the effect of different surface covers on the infiltration characteristics of the slope. This study, however, focuses on the well turf slope only which is of typical natural slope. Each of these plots was bordered by metal zinc sheets with dimension of 2 m in width × 10 m in length. The borders guided the surface runoff flowed into a self-fabricated runoff collector that was placed at the lowest end of the sloping plot. The plot was instrumented with nine tensiometers (Soil Moisture Corp. model 2725), installed in three rows, to measure the matric suction at depths of 0.5 m, 1.0 m and 1.5 m below the ground surface. Significant matric suction changes occur mainly within a depth of 1 to 1.5 m from the

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ground surface. Therefore, tensiometer measurements were limited to 1.5 m depth in this study. Previous researches have demonstrated the viability of taking suction measurement in deep soil layers by installing the tensiometers laterally from inside a pit hole (Stannard, 1986), however such technique was not employed in the present study in view of the large number of suction measurement points required for the study plot. The suction measurements were captured manually three times per day, i.e. 7 a.m., 12 noon, and 7 p.m. to represent the daily cyclic suction variation. A rain gauge (HOBO RG3-M) was installed at the middle of the study site to quantify the real-time rainfall. Fig. 2 shows the topographical contour and cross-sectional view of the study plots. In general, two distinct types of residual soil were generalized from the site. The upper portion was completely weathered and transformed by the laterization effect into Grade VI residual soils. The thickness of this layer varies from 0.6 m at the crest (BH1) to 4.5 m at the toe (BH2). This variable thickness of totally weathered material could probably be the result of the accumulation of colluvial deposit at the toe due to gravitational force. The second layer consists of reddish-brown completely weathered rock (Grade V). The layer is almost uniform in thickness, i.e. about 5.5.m. The most apparent manifestation of residual soil at this layer or saprolitic zone was the rough texture of soil particles. The Grade V soil has more coarse-grained fraction than the fine-grained fraction and was classified as silty gravel. The fine content decreased as the degree of weathering decreased. To a great extent, the sandy silt (Grade VI) and silty gravel (Grade V) interestingly exhibited similarity in the pattern of particle size distribution that may be associated to an identical origin soil during weathering process. Highly to moderately weathered rock, Grade III to IV is observed as the third layer. Apart from the presence of boulder, the soil is classified as silty sand. The properties of the third soil layer are not elaborately discussed further since the primary focus of this study is persistently on residual soils of Grade V and Grade VI. Although relict joints were indistinctly observed due to the poor recoveries of soil sample during ground investigation, the formations of corestones of gabbroic rock were markedly identified in the Grade V soil at depth of 0.6 to 3.7 m from BH1. Aydin et al. (2000) pointed out that the existence of corestones is a common form of heterogeneities in the soil matrix of typical saprolites developed over jointed igneous rocks. These heterogeneities can significantly affect water movement in soils by creating a non-uniform velocity flow. In the same way, the relict joints and corestone boundaries in weathering profile of Bukit Cerapan residual soil may develop preferential flow paths, resulting in a high inflow rate. This was supported by the observation of a significant

Fig. 1. General subsurface profile of Balai Cerapan's slope.

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Fig. 3. SWCC of the Grade VI and Grade V soils.

Fig. 2. Topographical contour and cross-sectional view of the study site.

loss of flushing water during borehole drilling within the Grade V soil at depth of 0.6 to 3.7 m. A series of physical property tests were conducted on the residual soils of Grade V and Grade VI. Table 1 tabulates the values of physical, hydraulic, and mechanical indices of the soils while the soil water characteristic curves (SWCC) along with some of their key variables, i.e. volumetric water content at saturation (θs), residual volumetric water content (θr), and air-entry value (AEV) are presented in Fig. 3.

Table 1 Physical, hydraulic, and mechanical properties of the residual soils.

The SWCC was obtained experimentally by conducting pressure plate tests using the axis translation technique whereby the air pressure (ua) inside the apparatus was increased while the pressure of the water phase (uw) was maintained at the atmospheric level (Simms and Yanful, 2004). X-Ray Diffraction (XRD) analysis and scanning electron microscope (SEM) imagery were performed on the selected residual soil specimens. The tests are of particular interest on the presence of type and amount of clay mineral. The mineral constituents determined from the XRD test, and the mineral compositions obtained from the mineralogy test are tabulated in Tables 2 and 3, respectively. From the mineral constituents, it was evidenced that the fine-grained Grade VI soil was predominantly consisting of kaolinite while the less weathered Grade V soil consisted of gibbsite. Moreover, the presence of feldspar mineral confirmed the less weathering effect on Grade V. For the mineralogy test, the mineral compositions of Grade VI and Grade V were found almost identical to each other which may be associated to the same origin of gabbroic rock. The SEM images illustrated in Fig. 4 show certain similarity in terms of the soil microstructures of Grade VI and Grade V. Moreover, the images revealed that the surfaces of Grade VI particles were weathered, with voids on the surface of the soil particles and exhibited a porous structure (refer to point A of Fig. 4B), while the surfaces of less weathered Grade V particles were smooth with no sign of intra-elementary (refer to point A of Fig. 4A). According to microfabric characterization introduced by Collins and McGown (1974), at elementary level the microfabric of Grade VI is dominated by clay-size clusters, whereby for Grade V the fine particles have close contact in granular form. At the assemblage level, the soil microfabric of Grade VI shows that some granular particles are covered with kaolinite clay matrix (refer to point B of Fig. 4B). Soil particles from Grade V are mostly granular, with few examples of bonding at the interparticle level (refer to point B of Fig. 4A).

Grade V (silty gravel)

Grade VI (sandy silt)

3. Modeling heterogeneity

Composition Gravel (%) Sand (%) Silt (%) Clay (%) LL (%) PL (%) PI Soil classification BSCS Gs Laboratory Ksat (m/s)

48 15 20 17 53.2 35.5 17.7 GMH 2.65 3.68 × 10− 6

0 33 34 33 59.3 31.9 27.4 MHS 2.63 5.00 × 10− 7

Soil geological heterogeneity often controls the mechanism and failure locations of saprolitic slopes (Aydin, 2006). Heterogeneity

Shear strength c′ (kPa) ϕ′(°)

3.3 39.5

7.6 32.1

Table 2 Mineral constituents obtained from XRD test. Grade V (silty gravel)

Grade VI (sandy silt)

Major constituents

Gibbsite

Minor constituents

Quartz, Kaolinite, Iron Oxide, Geothite, Smectite Feldspar

Kaolinite, Smectite, Iron Oxide Geothite, Quartz

Trace amounts

–

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Table 3 Mineral compositions obtained from the mineralogy tests. Grade V (silty gravel)

Grade VI (sandy silt)

Minerals

(%)

(%)

Clay minerals Quartz Feldspar Iron-oxides Rock fragments Magnetite Hematite Ilmenite Hidroilmenite Zircon Tourmaline Monazite Total

65.0 8.5 19.0 7.5 Tr Tr Tr Tr Tr Tr Tr Tr 100

72.0 15.5 11.0 1.5 Tr Tr Tr Tr Tr – – – 100

Note: Tr — Concentration less than 0.5%. Fig. 5. Infiltration cell apparatus for the effect of relict discontinuities on Grade V soil.

includes variations in grain-size, porosity, mineralogy, lithologic texture, rock mechanical properties, structure and diagenetic processes (Eaton, 2006). While most of the previous studies focused on the stress path and weakness fractures forming the slip plane in heterogeneous soil, the spatial heterogeneity in hydraulic properties of soil is another important factor that should be considered for the characterization of subsurface flow and slope instability. In general, the modeling concept of hydraulic heterogeneity can mainly be divided into two, namely equivalent porous medium/continuum method, and discrete fracture method. The former method assumes that the flow is on a volumetrically averaged basis at a macroscopic scale in what is assumed to be equivalent to an ideal porous medium (Eaton,

Fig. 4. SEM images of (A) Grade V, and (B) Grade VI residual soils.

2006). In other words, the hydraulic properties of the heterogeneous soils can be subdivided into multiple zones in which the properties of each zone are represented by an average constant value. These constant hydraulic properties can be determined from the laboratory or field tests with a condition that the representative elementary volume (REV) must be smaller than the major variations in hydraulic properties. The REV is defined as the minimal volume over which the governing equations of flow apply. The discrete fracture method, on the other hand, takes into account the flow through each fracture characterized by a location, orientation, size and transmissivity (Eaton, 2006). Under such circumstances, the subsurface flow domain has continuously variable properties, in which the spatial distribution of these properties is uncertain. The major difficulty associated to this approach is that it is extremely computationally intensive (Long et al., 1985). Recent developments are increasingly blurring the lines between the two as mixed formulations of zonal and geostatistical approaches to parameterization of model domains become more widespread. For instances, Cesano et al. (2003) developed a new approach to quantify the degree of heterogeneity of a fracture network called heterogeneity index (Ih). The index combines the variance of various fracture characteristics that regulate both flow and the geomechanical behavior of a rock mass. Their findings proved the existence of a correspondence between the Ih and the direction and magnitude of flow. However, the method require vast amount of field fracture data for the statistical analysis to obtain the Ih. From the foregoing reviews, it can be concluded that discrete method offers a better prediction to the characterization of subsurface flow, but involves intensive computational and statistical analyses. On the

Fig. 6. Results of in-situ hydraulic conductivity tests in Grade V residual soil.

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Fig. 7. Variation of ksat versus relict joints spacing ratio in Grade V residual soils.

other hand, continuum method appears to be a simpler method and capable to give acceptable results for a macroscopic scale problem. Considering the ease of analysis and the practicality in solving a realistic slope problem, it is thus justified to select the continuum method for modeling hydraulic heterogeneity in this study. 4. Hydraulic conductivity heterogeneity Variation in hydraulic conductivity is one of the most significant implications from the soil geological heterogeneities. Mitchell (1993) demonstrated that the values of saturated coefficient of hydraulic conductivity (ksat) varies several orders of magnitude as a result of changes in fabric, void ratio and water content besides its dependency on clay content, sedimentation procedure, compaction energy level and compression rate. Agus et al. (2005) worked on Bukit Timah granitic residual soil in Singapore. They found that the effect of depth of weathering on ksat showed no discernible trend, which highlighted the difficulty in generalizing the properties of residual soils. They, however, concluded that the variation of ksat in the residual soil mantle was within two orders of magnitude. In this study, laboratory tests were conducted to assess the hydraulic conductivity of soil. Firstly, 1 liter-mould undisturbed samples were collected from each layer of residual soils predetermined from the ground investigation. The ksat of the undisturbed samples were tested through standard laboratory falling head permeability test. Secondly, modified laboratory permeability test were performed to fundamentally demonstrate the effect of relict discontinuities on the hydraulic conductivity of Grade V residual soils. Disturbed samples were compacted in a self-fabricated cell made of steel framed with acrylic sheet sidewalls, 750 mm in length, 200 mm in height, and 150 mm in width (Fig. 5). The procedures to perform constant head test as outlined by Head (1986) were adopted with modification made on the sampling in order to accommodate for the presence of structural discontinuities within soil sample. Artificial relict joints were simulated by placing 1 mm thick aluminum sheet at various

Table 4 Fitting parameters for predicting hydraulic conductivity functions of soils. Soil

α

n

m

Ksat

Layer 1 Layer 2, Zone 1 Layer 2, Zone 2 Layer 2, Zone 3 Layer 3

0.178 0.150 0.150 0.150 0.150

1.966 1.792 1.792 1.792 1.792

0.491 0.442 0.442 0.442 0.442

5.00 × 10− 7 1.23 × 10− 5 8.28 × 10− 6 6.12 × 10− 6 3.68 × 10− 6

spacing in the permeability cell. The tests were performed at nine different relict joint spacing (RJS) of 50 mm, 100 mm, 150 mm, 200 mm, 250 mm, 300 mm, 350 mm, 400 mm, 450 mm and also a homogeneous sample (Gofar et al., 2011) Lastly, in-situ hydraulic conductivity test was performed using a Guelph constant head permeameter at arbitrarily spots and depths in the Grade V residual soils. Despite the maximum measuring depth suggested by the instruction manual is only 0.75 m, the measurements at deeper depths of soil were still made possible by clearing the soils at upper layers. Fig. 6 shows the results of in-situ hydraulic conductivity tests at various depths. The data sets revealed that the saturated hydraulic conductivity varies approximately one order of magnitude despite the residual soils specimens were obtained from the same generalized layer. The overall trend of the scattered data demonstrated that the degree of weathering and saturated hydraulic conductivity of the soils decreases over depth. Consequently, the residual soils between 0.6 m and 3.7 m were subdivided into three zones (i.e. zone 1, 2, and 3) in which each zone was represented by an average ksat, as shown in Fig. 6. The ideal number of layer was determined through careful calibration of the numerical simulation. Fig. 7 shows the correlation between the hydraulic conductivity and the relict joints spacing based on the results obtained from selffabricated permeability cell. The hydraulic conductivities that ranged broadly between 1.41 × 10 − 5 m/s and 2.81 × 10 − 6 m/s indicated the enormous influence of structural discontinuities on the soil hydraulic conductivity. These results were in close agreement with the ksat values of Grade V residual soils obtained from the in-situ and standard laboratory hydraulic conductivity tests. It implies that relict discontinuities exist intensively within the in-situ soil mass of Grade V residual soil. The spacing of relict discontinuities was predicted by incorporating the in-situ hydraulic conductivity data into Fig. 6. It can be traced that the predicted spacing of residual soils in Zones 1, 2, and 3 are 110 mm, 160 mm, and 230 mm, respectively. The saturated hydraulic conductivities obtained from the in-situ tests and SWCC presented in Fig. 3 were used to predict the hydraulic conductivity functions of unsaturated soils via Van Genuchten's (1980) method. According to the method, the hydraulic conductivity of a soil can be expressed as a function of matric suction (Kw) as follows:

K w ¼ K sat

n
 o2 1−ðαΨÞn−1 1 þ ðαΨÞn −m ½1 þ ðαΨÞn m=2

:

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Fig. 8. Hydraulic conductivity functions used for numerical analysis.

5. Numerical modeling

Where, is saturated hydraulic conductivity of soil Ksat α, n, and m are curve fitting parameters Ψ is matric suction The curve fitting parameters α, n, and m for different layers of soils, as identified from their respective SWCC are tabulated in Table 4. The detailed procedures to obtain these parameters are explained in Van Genuchten (1980). The hydraulic conductivity functions predicted for the Grade VI and the three zones of Grade V layers are presented in Fig. 8.

A commercial finite element unsaturated and saturated seepage software, Seep/W (GeoSlope International Ltd., 2007), was used to simulate the transient suction distributions. The finite element analysis adopts the shape function proposed by Bathe (1982). Fig. 9 shows the simplified model of the slope inclined at an angle of 21°. The modeled sloping length was 30 m. It was shorter than the actual length of the slope (i.e. 47 m) because the suction measurements at the site were only available for 10 m long. Additional 10 m long elements were extended at both left and right edges to minimize the boundary effect. The seepage model comprised 1333 nodes and 1260 quadrilateral mesh elements with 4 nodes and 4 integration orders. First order very

Fig. 9. Simulated slope model.

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Fig. 10. Field suction envelopes and rainfall distributions.

fine quadrilateral elements (0.1 × 1 m) were designed for the ground surface to a depth of 0.6 m. Fine quadrilateral elements (0.25 × 1 m) were used from 0.6 m to 3.7 m. Large quadrilateral elements (0.5 × 1 m) were used below 3.7 m depth. The heterogeneous soil layers were created by applying different mesh regions to the model. Water table was located at 10 m below the ground surface, as indicated from the borelog of BH1 and BH2 (Fig. 1). The left and right edges above the water table were specified as a no flow boundaries (Q = 0), while the edges below the water table were assigned as head boundaries

with pressure head equal to the vertical distance from the water table. These boundary conditions were necessary for enabling lateral flow to take place within the saturated zone. The bedrock located at 15 m from the ground surface was assumed as an impermeable layer. A hydrostatic initial condition was established at the beginning of the transient seepage analysis. The initial pore-water pressure condition identical to that of actual site measurements was simulated by applying an arbitrary rainfall for a sufficient long duration. Upon achieving the desired initial condition, infiltration due to rainfall

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Fig. 11. Comparison between field suction envelopes and simulated suctions obtained from modeling scheme A.

was simulated on the exposed sloping surface by applying unit flux (q) of actual intensities with no ponding option to the slope. The time interval of the analysis was set at 1 h, which eventually resulted in a total of 8760 time steps to cater for the analysis period of 1 year. Table 5 tabulates the details of the modeling schemes applied in this study with different approaches adopted to account for the heterogeneity effect. The modeling scheme A assumed that the slope consisted of homogeneous soil by neglecting the thin Grade VI layer and the relict discontinuities in Grade V soil. The modeling scheme B accounted for the thin Grade VI layer but neglected the relict discontinuities. The modeling scheme C accounted for both thin layer of Grade VI soil and relict discontinuities in Grade V soil, while the modeling scheme D adopted the same approach as the scheme C with the inclusion of evaporation effect. The relict discontinuities in Grade V soil from 0.6 to 3.7 m (i.e. layer 2) were simulated by subdividing the layer into 3 isolated zones (i.e. zones 1, 2, and 3) with hydraulic conductivity functions presented in Fig. 8. The effect of evaporation was included in the modeling scheme D by applying a negative unit flux of 5.787 × 10− 8 m/s, which was equivalent to the average daily evaporation rate of approximately 5 mm/day in Malaysia (Ali et al., 2000). 6. Analysis and results 6.1. Field suction measurements Fig. 10 illustrates the daily rainfalls over a period of one year (12th September 2006 to 12th September 2007). The temporal distribution of the recorded rainfall was the typical of tropical region, with rainfall

consistently found throughout the year, but at higher intensity during the months of monsoon seasons from May to September (Southwest Monsoon) and from November to March (Northeast Monsoon). During the course of monitoring, a series of exceptionally intense rainfall events were recorded from 17 to 20 December 2006. The amount of rainfall within these 4 days reached 450.4 mm, which was much higher than the average monthly rainfall of 250.2 mm (Meteorology Department Malaysia). Fig. 10 also shows field suction envelopes measured at 0.5 m, 1 m and 1.5 m depths with dashed lines indicate the upper and lower bounds of suction readings. These suction envelopes were formed by the highest and lowest daily suction measurements. From the overall trend of suction distributions, it was obvious that the suctions at 0.5 m depth were generally lower and more sensitive to climatic change compared to those of 1.0 m and 1.5 m. The suctions at 0.5 m depth decreased to 0 kPa after a number of short and intense rainfall events. However, the suctions at 1.0 m and 1.5 m have never totally disappeared. The results implied that a soil interface exists between 0.5 m and 1 m, which has limited the propagation of wetting front due to the capillary barrier effect induced by the presence of thin layer of Grade VI soil. Another consistent observation emerged from the field data was that the daily suction variation was considerably large. For the rainy days, the suction could drop dramatically from 20 kPa to 0 kPa. As for the days of no rainfall, the variations in suction were approximately 5 kPa, which can be attributed to the daily cyclic evaporation process. A possible explanation for these daily cyclic suction variations is that the suction measurements could be influenced by the pressure

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Fig. 12. Comparison between field suction envelopes and simulated suctions obtained from modeling scheme B.

variation of air accumulated in the tensiometers due to the daily temperature variation (Hillel, 1998; Durner and Or, 2005). Field observations also showed that the highest daily suction was recorded during the noon time while the lowest suction was obtained at the night time and early in the morning. The results implied that the field suction variation was not only governed by the rainfall, but also the cyclic evaporation process. 6.2. Comparison between various modeling approaches Figs. 11–14 compare the field suction envelope and the simulated suction obtained from the modeling schemes A, B, C, and D, respectively. The modeling scheme A (Fig. 11) with an assumption of homogeneous soil for the entire slope showed that the simulated suctions at 0.5 m were very close to the upper bound of the field suction envelope. However, the suctions tend to be lower than the field measurements when deeper depths were considered. Apparently, the predicted suctions at 1.5 m depth were completely out of the lower bound of the field suction envelope. For the modeling scheme B (Fig. 12), the penetrations of infiltrated rainwater into deeper depths were marginally reduced with the introduction of the thin 0.6 m depth of Grade VI layer which was of lower hydraulic conductivity (Fig. 8). The suctions at 0.5 m depth were generally lower than those obtained in modeling Scheme A under most of the conditions, except for a few heavy rainfall events. During these heavy rainfall events, the advancement of wetting front in Scheme A was deeper and faster due to higher hydraulic conductivity of Grade V material. The overall lower suction at 0.5 m depth for modeling scheme B indicates more rainwater was actually retained in

the upper layer. The phenomenon can be explained by the theory of capillary barrier effect. The rainwater that infiltrates through the upper Grade VI layer will enter the underneath layer of Grade V soil only when the matric suction at the surface decreases to the value near the water entry point of the Grade V layer (Stormont and Anderson, 1999). As shown in Fig. 3, the water entry value of Grade V material is 3.5 kPa which is lower than that of Grade VI soil (9 kPa). This natural capillary barrier effect creates a hydraulic impedance that limits downward water movement (Khire et al., 2000). Nevertheless, the suctions at 1.5 m depth were still lower than those of field measurements. Fig. 13 shows that modeling scheme C gives appreciably better simulated suctions as compared to schemes A and B. With the inclusion of the thin Grade VI layer and the relict discontinuities in Grade V layer, the simulated suctions have fitted perfectly into the field suction envelope. The presence of relict discontinuities in Grade V layer has induced larger macro pores in the soil, hence resulted in a higher saturated permeability. Based on the theory of capillary barrier, the hydraulic impedance at the soil interface will be more significant if the contrast in hydraulic conductivity between the two soil layers forming the interface is greater. Consequently, less rainwater has infiltrated into the Grade V layer. The modeling scheme D (Fig. 14) adopted the similar soil layers as the modeling scheme C, but with the inclusion of a constant negative flux representing the evaporation rate. Apparently, the scheme further improved the prediction of suction. The simulated suctions at 0.5 m depth were almost identical to that of mean field suction. During heavy rainfall event, the simulated suctions at all depth are lower than those simulated in Fig. 13. However, the suctions tend to be higher than the field measurements when deeper depths were considered

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Fig. 13. Comparison between field suction envelopes and simulated suctions obtained from modeling scheme C.

especially during dry condition. The results imply that the actual field evaporation rate could be higher or lower than the average evaporation rate used in the present study. Apparently, the predicted suctions are at the upper bound of the field suction envelope during dry condition.

the numerical model providing a better prediction for the suction profile during the wet condition than the dry condition.

6.3. Suction profiles in heterogeneous soils

In this study, the field monitoring of a typical heterogeneous residual soil slope was carried out for a period of one year. From the field observations, the suctions at 0.5 m depth have dropped to 0 kPa in a number of occasions, whereby the suctions at deeper depths, i.e. 1 m and 1.5 m have never been totally dismissed even during the exceptionally intense rainfall events that occurred from 17 to 20 December 2006. The observations initiated the hypothesis of the presence of a hydraulic impedance interface that controls the downward movement of rainwater in the soil. The results from the numerical simulations confirmed the hypothesis. The incorporation of the thin Grade VI residual layer coupled with the effect of relict discontinuities in the seepage analysis has generated the suction distribution that fit perfectly into the field suction envelope. The thin Grade VI residual soil layer has functioned effectively as a soil cover that controlled the downward movement of rainwater through the capillary barrier effect. On the other hand, the relict discontinuities in the Grade V layer has caused an increment in the soil hydraulic conductivity, which eventually enhanced the effect of capillary barrier because of the greater contrast in hydraulic conductivity at the soil interface between Grade VI and V layers. Despite of the fact that the Grade VI layer is very thin (i.e. 0.6 m only) and the differential hydraulic conductivity in Grade V layer are small (varies within one order of magnitude only), these factors cannot be overlooked in the analysis as neglecting these factors

Two distinctive climatic conditions were isolated from the course of monitoring, i.e. a series of exceptionally intense rainfall events from 17 to 20 December 2006, and an 18 days period of no rainfall from 29 January to 15 February 2007. Through the numerical modeling, the displays of the complete vertical suction profiles for both conditions were made possible. Fig. 15(a) and (b) compares the simulated and measured suction values on 20 December 2006 (wet condition) and 15 February 2007 (dry condition), respectively. For the wet condition, the total breakthrough of rainwater into the Grade V layer had not occurred. As a result, the soil in the bottom Grade V layer still had certain amount of suction. As for the dry condition, it was unexpectedly to find that the simulated suctions at 0.5 m depth were still lower than those of deeper depths. The field measurement demonstrated a more reasonable result, whereby the suction at 0.5 m had increased to 26 kPa. It was thought that the evaporation process played a more important role during the dry period. The evaporation process constitutes a very complex problem which involves a number of parameters including solar radiation, relative humidity, ambient temperature, wind speed etc. In this study, the effect of evaporation was incorporated by just reasonably assuming a constant evaporation rate of 5 mm/day. This explains the reason for

7. Discussions

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Fig. 14. Comparison between field suction envelopes and simulated suctions obtained from modeling scheme D.

would result in a deeper propagation of wetting front, as proven by the results of modeling schemes A and B. Fig. 16 illustrates a typical conceptual profile of residual soil. A very thin Grade VI layer is found at the ground surface as the result of sedimentation process. The soil normally consists of very fine-grained materials coating on sand particles. In Grade V soils, continued weathering results in a collapse of the soil skeleton with a loss of microfabric and continued discoloration of the material to a characteristic dark reddish brown. It is likely that the finer textured material and the discoloration observed in the mass structure is the residual soil picking out relict discontinuities (Howat, 1985). The relict discontinuities are found more intensively at the upper layer than the soil at deeper depth. The continuum approach of zoning the soil into multiple layers of decreasing saturated hydraulic conductivity with depth is thus justified.

The highest suction value recorded in this study was 26 kPa. In another study carried out by Gofar et al. (2008) on a clayey soil slope, the highest suction could reach 70 kPa. Previous studies (i.e. Gofar et al., 2008; and Lee et al., 2009) have suggested that the suction at dry condition can be approximated to the suction corresponding to residual water content in SWCC. It is thus important to use an appropriate SWCC for numerical modeling of suction distribution as the SWCC input is a very sensitive parameter to the quality of the simulated suction. 8. Conclusions A detailed numerical modeling was carried out to simulate the suction distributions for an actual heterogeneous residual soil slope in Malaysia. In general, the numerical results show good agreement with

Table 5 Modeling schemes adopted in the numerical analysis. Modeling scheme

Layer 1

A

Grade V (3.68 × 10− 6) Grade VI (5.00 × 10− 7) Grade VI (5.00 × 10− 7) Grade VI (5.00 × 10− 7)

B C D

Layer 2 Zone 1

Zone 2

Zone 3

Grade V (3.68 × 10− 6) Grade V (3.68 × 10− 6) Grade V (1.23 × 10− 5) Grade V (1.23 × 10− 5)

Grade V (3.68 × 10− 6) Grade V (3.68 × 10− 6) Grade V (8.28 × 10− 6) Grade V (8.28 × 10− 6)

Grade V (3.68 × 10− 6) Grade V (3.68 × 10− 6) Grade V (6.12 × 10− 6) Grade V (6.12 × 10− 6)

Note: Numbers in blanket are saturated hydraulic conductivity (ksat) values in m/s unit.

Layer 3

Layer 4

Grade V (3.68 × 10− 6) Grade V (3.68 × 10− 6) Grade V (3.68 × 10− 6) Grade V (3.68 × 10− 6)

Grade IV (3.44 × 10− 7) Grade IV (3.44 × 10− 7) Grade IV (3.44 × 10− 7) Grade IV (3.44 × 10− 7)

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(iii) The presence of relict discontinuities in Grade V soil can be considered by adopting the continuum method in which the residual soil is subdivided into multiple zones of average hydraulic conductivities. Higher hydraulic conductivity is employed at the upper part of Grade V soil due to more intense occurrence of relict discontinuities. (iv) Evaporation is an important factor that causes the daily fluctuation of soil suction, particularly during dry period. The numerical model provides a better prediction to the suction distribution for the wet condition than for the dry condition. The suction distribution during the dry period is affected by the evaporation, which in turn is governed by a number of environmental factors. Acknowledgments This project is funded by the Ministry of Science, Technology & Innovation, Malaysia. Besides, the authors acknowledge the useful comments from the reviewers. References

Fig. 15. Vertical suction profiles on (a) 20th December 2006 (wet condition), and (b) 15th February 2007 (dry condition).

the field measurements. The following conclusions can be drawn from the study: (i) The thin layer of Grade VI residual soil and relict discontinuities in Grade V soil must be considered in the analysis of suction distributions in residual soil slope in order to produce the simulated suctions that best fit to the field measurements. (ii) The presence of the thin layer of Grade VI residual soil introduces a small hydraulic heterogeneity between sandy silt (Grade VI) and silty gravel (Grade V); hence this natural capillary barrier effect restrains the downward movement of infiltrated rainwater. As a result, the wetting front could not advance to the deeper layers of soil even during the exceptionally wet condition.

Fig. 16. Conceptual profile of typical residual soil.

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