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Performances of two instrumented laboratory models for the study of rainfall_图文

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Engineering Geology 117 (2011) 78–89

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Engineering Geology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e n g g e o

Performances of two instrumented laboratory models for the study of rainfall in?ltration into unsaturated soils
Lee Min Lee a,?, Azman Kassim b, Nurly Gofar b
a b

Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Kuala Lumpur 53300, Malaysia Faculty of Civil Engineering, Universiti Teknologi Malaysia, Skudai 81300, Malaysia

a r t i c l e

i n f o

a b s t r a c t
Rainfall in?ltration and suction variation in unsaturated soils must be taken into consideration in the analysis of most slope stability problems, particularly in the tropical regions where the annual precipitation is high. The process of rainfall in?ltration into unsaturated soils is an extremely complex problem attributed to the non-linearity of the hydraulic property functions of the unsaturated soils. This paper describes in detail two instrumented laboratory models, i.e. one-dimensional soil column, and two-dimensional slope model used to provide experimental evidences for the transient suction variations in the unsaturated soils under certain rainfall conditions. The performances of the laboratory models were tested on four typical types of residual soils, i.e. sand-gravel, silty gravel, sandy silt, and silt (kaolin), and a two-layered soil system, i.e. sandy silt underlain by silty gravel. The results showed that the suction distributions for the single-layered homogeneous soils obtained from the simpler one-dimensional soil column were almost identical to that of two-dimensional slope model. However, the two-dimensional slope model should be employed for the two-layered soil system because of the dominant effect of the lateral ?ow mechanism. The capillary barrier effect was observed when a less permeable soil layer was underlain by a more permeable soil layer. The minimum suction value in soil is governed by the rainfall intensity, rainfall duration, and the saturated permeability of soil. The in?ltration rate of the ?ne-grained soils that subjected to shrink and crack was independent of the soil permeability, but was signi?cantly governed by the preferential ?ows developed in the soils. ? 2010 Elsevier B.V. All rights reserved.

Article history: Received 16 March 2010 Received in revised form 20 September 2010 Accepted 9 October 2010 Available online 16 October 2010 Keywords: Rainfall in?ltration Unsaturated soil Laboratory model Slope stability Suction distribution

1. Introduction Slope failures can broadly be attributed to the convergences of three factors, i.e. rainfall, steepness of slope, and soil geological pro?le. Of these, rainfall appears to be the most signi?cant triggering factor for the slope failure occurrences in the tropical regions, owing to the high annual precipitation in the regions. Experience has shown that many slope failures occurred during or shortly after rainfall (Gavin and Xue, 2008). The rainfall in?ltration produces a downward ?ux that changes the water content and pore-water pressure gradients with depth, hence reduces the soil shear strength and subsequently triggers the slope failure (Gitirana et al., 2005). The process of rainfall in?ltration into a soil slope is an extremely complex problem, involving numerous parameters such as soil permeability, soil initial moisture condition, soil water retention

? Corresponding author. Tel.: + 60 3 41079802; fax: + 60 3 41079803. E-mail addresses: mllee@utar.edu.my, amuellee@yahoo.com (L.M. Lee), azmankassim@utm.my (A. Kassim), nurly@utm.my (N. Gofar). 0013-7952/$ – see front matter ? 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2010.10.007

ability, soil porosity, and evaporation rate etc. The problem becomes more complicated when dealing with the unsaturated soils because the hydraulic properties of the soils are strongly non-linear functions (Zou et al., 2001). Capillary barrier effect is another phenomenon that has added enormous complexity to the seepage ?ow in heterogeneous unsaturated soil. Natural weathering process leads to the variation in hydraulic properties of tropical residual soil in which the soils closer to the ground surface usually have lower permeability as compared to those of deeper layer. Consequently, capillary barrier effect developed at the interface between the soil layers. Taking advantage of this natural phenomenon, recent studies have emerged to develop a sloping cover system with capillary barrier effect. The system typically consists of two layers of granular materials designed so that the contrast in hydrologic properties between the layers creates the hydraulic impedance that limits downward water movement (Khire et al., 2000). Rainwater that in?ltrates through the upper ?ne grained layer will enter the bottom coarser soil only when the matric suction at the surface of the coarse layer decreases to the value near the water entry point (Stormont and Anderson, 1999). The fundamental characteristics of the capillary barrier system, including the hydraulic properties of the layers, inclination angle, thickness of the layers,

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diversion length and in?ltration rate can be traced from the previous studies by Ross (1990), Stormont and Anderson (1999), Tami et al. (2004), and Parent and Cabral (2006) etc. There are numerous approaches that can be adopted when studying the in?ltration characteristics of a sloping soil. Analytical analysis and numerical simulation of the in?ltration characteristics as well as the pore-water pressure changes in the saturated and unsaturated soils can contribute to an understanding of the trigger mechanisms leading to failure. Simple physical models such as Green and Ampt (1911) and Horton (1933) equations are commonly adopted to solve the problem analytically, while ?nite element analysis which employs Richards's (1931) equation is used to model the problem numerically. Field monitoring of the suction and water content could also provide an insight into the mechanisms leading to the rainfall-induced slope failure (Gasmo et al., 2000; Tsaparas et al., 2002; Li et al., 2005; Gofar et al., 2008; Tu et al., 2009). In general, the in?ltration models that rely on the analytical solutions are highly simpli?ed, thus its applicability to the real soils is always questionable. Moreover, the cracking of low permeability soils may cause preferential ?ow paths to develop, in which the ?ow of water is independent of the mass permeability of the soil matrix (Gavin and Xue, 2008). Such effects are usually overlooked in the analytical and numerical in?ltration models. On the other hand, ?eld monitoring which involves the measurements of pore-water pressure and rainfall is not only time-consuming, but involves expensive instrumentations as well. Controlling the parameters such as rainfall

intensity, rainfall duration and soil properties are normally not viable under the ?eld condition. Laboratory experiments on the instrumented slope models represent an effective way to provide data sets useful for understanding the rainfall in?ltration mechanisms under controlled environment. The in?ltration models could be in the types of one– dimensional soil column, two-dimensional slope model, or threedimensional large-scale model. Yang et al. (2006) investigated the effect of rainfall intensity and duration on the in?ltration mechanism through a soil column apparatus, and provided experimental evidences for the soil water redistribution and hysteresis. Similar laboratory model was employed by Stormont and Anderson (1999) to study the in?ltration behaviors of layered soils. Experiments through large scale slope models have been conducted by several researchers including Tohari et al. (2007), Damiano and Olivares (2010), Jia et al. (2009), Tami et al. (2004). Despite of the fact that extensive studies have been conducted through the laboratory experiments, very limited literatures can be traced on the comparison of onedimensional and two-dimensional models, as well as the in?ltration characteristics in different types of homogeneous and layered soils. Selecting an appropriate laboratory model to accommodate the experiment objectives and requirements is equally important as assigning the correct boundary conditions for a numerical model. The objective of this paper is to describe in detail two instrumented in?ltration models, i.e. one-dimensional soil column and twodimensional slope model used for the study of rainfall in?ltration into

Fig. 1. Diagram of the soil column setup.

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Fig. 2. Overview of the soil column after construction.

unsaturated soils. The performances of the laboratory models were tested on four types of soils, i.e. sand-gravel, silty gravel, sandy silt, and silt (kaolin), and a two-layered soil system, i.e. sandy silt underlain by silty gravel. The runoff rate, suction variation, and percolation ?ow rate were quanti?ed continuously under two extreme rainfall conditions. Some preliminary results obtained from the laboratory experiments are reported in this paper. 2. Laboratory models Two laboratory models, i.e. one-dimensional soil column, and twodimensional slope model were employed in this study. The details of the models are described in the following sections. 2.1. One-dimensional soil column model The one-dimensional soil column model designed for this study consisted of two main parts, namely acrylic soil column, and water ?ow system. A three-dimensional diagram of the soil column model is illustrated in Fig. 1, while the overview of the soil column after construction is shown in Fig. 2. The soil column was made of acrylic transparent tube with a 5 mm-thick wall and 190-mm internal diameter. The soil column consisted of two separated tubes (900 mm high each) that connected securely by rubber O-ring and clamp system. This arrangement was necessary for the ease of compaction and removal of soil specimens. Two types of threaded holes were fabricated on the soil column model wall. One type was used for the installation of tensiometer probes (ceramic cups), while the other one for gypsum moisture block. Both threaded holes were spaced at 200 mm along the length of the soil column. The holes that were not in use during an experiment were sealed with the threaded plugs. Screw clamp system was employed to prevent water leakage at the joint between two separated cylinders, and the joint between the cylinder and base plate (Fig. 3). An O-ring was placed in groove, and fastened with bolts and nuts. The silicon grease was applied on the surface plates of the joints to improve the resistance to water leakage.

Fig. 3. Components of the soil column model.

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Fig. 4. Diagram of the slope model setup.

The water ?ow system of the in?ltration column comprises three parts, i.e. in?ow/rainfall control, over?ow/runoff discharge, and percolation discharge (Fig. 1). The in?ow/rainfall control consisted of a water storage tank, a constant head tank, a ?ow regulator (ball valve), and a rainfall distributor. The water storage tank with a storage capacity of 216 L was placed at a height of 2.8 m from the ground level. The function of the water storage tank is to provide continuous water ?ow into the constant head tank. The constant head tank, which was placed immediately below the water storage tank, had a storage capacity of 216 L and a constant head of 0.3 m. Water in the storage tank ?owed into the constant head tank through a control valve. An over?ow outlet was placed at the same level with the inlet ?ow of the constant head tank to create a constant head condition during the test. Beneath the constant head tank was a ?ow regulator, by which simulated rainfall rate was precisely controlled. Note that this system could only produce a ?ow rate which is greater than 5 mL/min or equivalent to a rainfall intensity, q = 2.94 × 10?6 m/s. A perforated aluminum plate was placed on top of the soil column to avoid excessive raindrop energy that may cause erosion on the

surface of soils. When a rainfall was applied, the water ?owed through the holes of the plate and dripped onto a piece of ?lter paper that was placed in contact with the surface of the soil column. Through these arrangements, the simulated rainwater can be delivered onto the soil surface in a relatively uniform pattern. The second component of the water ?ow system is the over?ow/ runoff discharge. The over?ow discharge system was used to create a no-ponding upper boundary condition for the soil column. The over?ow was discharged as runoff through the outlet located at the soil surface. The runoff was then directed to a load cell that has a capacity of 2 kg to quantify the runoff rate. Alternatively, the ponding condition can be created by sealing the runoff outlet with a threaded plug. The last component of the water ?ow system is the outlet for the discharge of percolated ?ow. A constant head tank was placed on the ?oor to maintain the water table at the bottom of the soil column. This was intended to create a free-?ow lower boundary condition. The constant head tank with a large open area helped to produce a constant water table with a minimum ?uctuation and to allow

Fig. 5. Overview of the slope model after construction.

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Fig. 6. (a) Three-dimensional diagram, and (b) cross-sectional view of the tensiometer connector.

percolated water in the soil column to drain out freely. The constant head tank was connected to the soil column through a ?exible tube. Gravels with the average size of 5 mm and a ?lter paper were placed at the bottom of the soil column to avoid turbulent discharge ?ow. Upon percolation through the soil column, the water ?ow into the constant head tank and drain out through an over?ow outlet located on the wall of the tank. The over?ow was directed to another load cell to quantify the percolated ?ow rate. 2.2. Two-dimensional slope model The two-dimensional slope model is 2000 mm in length, 1100 mm in height and 100 mm in width, as illustrated in Fig. 4. The model is deemed to be a two-dimensional model because of the low width to height and length ratios. Thus, the water ?ows along the width direction of the model (y-axis) were neglected in the study. The watertight slope model was designed and constructed with de?ection limit considerations in order to avoid leakages. The frame of the model was made of steel, and the sidewalls were made of acrylic sheets of

5 mm thickness. The overview of the slope model after construction is shown in Fig. 5. A total number of 27 threaded holes with the similar speci?cations as the one-dimensional soil column were drilled at various spacing along one sidewall for the installations of the instrumentations. Some perforated holes of 3 mm were drilled at the base of the model to facilitate percolation discharge. In addition, two additional holes were drilled along the sidewall at top end of the model for run-off collection purposes. In order to accommodate for future researches that may look into the effect of slope steepness on the rainfall in?ltration and stability of slope, the model was designed for slope angles of 0°, 18° and 27° by setting the left side of the model at different hooks located at an adjacent steel column. Exception to the rainfall simulators, the two-dimensional slope model has a similar water ?ow system to that of one-dimensional soil column. The rainfall simulators for the two-dimensional slope model consisted of 12 sprayer units with numerous punching needle holes distributed at 150 mm spacing. The rainfall intensity was controlled using the ?ow regulators.

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3. Instrumentations and Data Acquisition System Two types of soil suction measurement instruments were used in the study, i.e. tensiometer and gypsum block. The tensiometer (Soil Moisture Corp. Model 2100F) is equipped with pressure transducer (Soil Moisture Corp. Model 5301-B1). During calibrations, attempts to measure soil suction higher than 70 kPa were found unsuccessful. Therefore, the gypsum block (Soil Moisture Corp. model 5201F1L06 G-Block) with a measuring capacity of 10 kPa to 1500 kPa was introduced. In this study, the tensiometer was used to measure the soil suctions within the range of 0 kPa to 70 kPa (valid for most of the suctions measured in this study), whereas the gypsum block with a higher measuring capacity was used to ensure that any suction higher than 70 kPa could be traced during the process of setting up the initial conditions. Upon the soil compaction, the instruments were installed into the soil column through the predrilled holes. The method offers the advantage of protecting the instruments from damage during soil compaction, but care should be taken to ensure that the sensors of the instruments were closely contacted with the soil particles. To mount the ceramic cup and the tube assembly of the tensiometer on the wall of the acrylic column, holes with threaded housing were fabricated on

the column wall. A specially designed connector that ?t well into the threaded housing, O-ring, and sealing tape were used to form a good seal at the connection. The details of the connector are shown in Fig. 6. The connector for the gypsum block consisted of two parts (Fig. 7). The ?rst part was ?tted into the housing, while the second part, equipped with an “O” Ring, was used to seal the wire ?tting to the connector. Since the gypsum block sensor was connected to the data logger via two independent wires, it was essential to use a wire ?tting to produce a cylindrical shape for the ease of sealing. The silicon grout sealer was injected into the space in between the wire ?tting and wires to provide a good seal. The data acquisition system used in the study comprises two units of data logger, a solid state relay, an external power supply, and a personal computer, as shown in Fig. 8. The tensiometers and gypsum blocks were connected to the Campbell Scienti?c Data Logger, model CR10x (Campbell Scienti?c Inc.), while the load cells were connected to the GDS 8 Channel Serial Data Acquisition Pad. The CR10x data logger consisted of two units of 32 single-ended channels multiplexer (model MUX AM416). A program was written to set up communication and data collection between the data logger and instruments. Besides, a controlling software named PC208W version 2.3 was used to execute the data logger.

Fig. 7. (a) Three-dimensional diagram, and (b) cross-sectional view of the gypsum block.

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Fig. 8. Data acquisition system.

The CR10x data logger was powered up by an internal 12 V battery, but the optimum power requirement for the tensiometer transducer system was 24 V. Therefore, the tensiometer transducer system was connected to an external 24 V power supply via a solid state relay. The functions of the solid state relay are to protect the data logger circuit and to power up the tensiometers only when the triggering signal was received from the data logger. These functions are essential to protect

the tensiometer transducer system from over-heated due to long operating durations. The GDS 8 Channel Serial Data Acquisition Pad is a data logger with eight channels of 16-bit data acquisition. The con?guration of the data logger was originally designed to log the data for shearing machine. Some modi?cations were made to allow the logging of four load cells concurrently. A controlling software named GDSLAB v2 was used to communicate with the GDS data logger.

Fig. 9. Soil water characteristic curve (SWCC) of the soils used in the study.

Fig. 10. Permeability function of the soils used in the study.

L.M. Lee et al. / Engineering Geology 117 (2011) 78–89 Table 1 Physical properties of soils used in the study. Sand-Gravel Composition Gravel (%) Sand (%) Silt (%) Clay (%) LL (%) PL (%) PI BSCS Gs ρb (kg/m3) ρd (kg/m3) MDD (kg/m3) OMC (%) Max ρ (kg/m3) Ksat (m/s) 50 50 0 0 – – – G/SP 2.65 – – – – 1856 3.4 × 10?4 Silty Gravel 48 15 20 17 53.2 35.5 17.7 GMH 2.65 1805 1366 – – – 3.7 × 10?6 Sandy Silt 0 33 34 33 59.3 31.9 27.4 MHS 2.63 – – 1415 31.0 – 5.0 × 10?7

85

Silt (Kaolin) 0 11 81 8 44.8 30.6 14.2 MI 2.52 – – 1587 19.3 – 6.8 × 10?8

The data from the data logger units were transferred to the personal computer periodically through the serial ports. The data stored in the personal computer were normally set in a format of pressure versus real time at a desired interval. An interval of 15-min was used in this study in conjunction with the rate of changes in transient pore-water pressure observed from ?eld monitoring conducted by Gofar et al. (2008) under rainfall condition. 4. Soil properties Four types of soil were employed in the study, i.e. sand-gravel, silty gravel, sandy silt, and silt (kaolin). These soil types were chosen to simulate a maximum, a minimum, and the intermediate conditions with respect to the hydraulic properties and particle sizes of soils. A series of laboratory tests were conducted to determine the soil properties. Figs. 9 and 10 show the soil water characteristic curve (SWCC) and permeability functions of the soils, respectively. The physical properties of the soils are tabulated in Table 1. 5. Experimental programs The experimental program focused mainly on the performances of the two in?ltration models that comprised four different types of soil subjected to two combinations of extreme rainfall intensities and durations. Prior to the tests, an initial condition was created for each type of soils in accordance to their actual ?eld suction measurements during dry condition. The ?eld measurements showed that the initial suctions for sand-gravel, silty gravel, sandy silt, and silt (kaolin) were 10 kPa, 23 kPa, 30 kPa, and 70 kPa, respectively. Previous studies (i.e. Gofar et al., 2008; Lee et al., 2009) suggested that the suction at dry condition for most types of soil can be approximated to the suction corresponding to residual water content (ψi) in SWCC, as shown in Fig. 9. These initial conditions were simulated in the laboratory experiments by air-drying the compacted soils in the models. In most of the cases, the initial conditions can be achieved after 3 to 30 days of drying duration depending on the permeability of the soil types. The details of the test programs are tabulated in Table 2. The intensities of the 1-hour and 24-hour rainfalls applied in the experiments were equivalent to 70% of the extreme rainfalls obtained from the Intensity-Duration-Frequency (IDF) curve of wet zone in Malaysia, developed by Gofar and Lee (2008). It is a conservative approach to assume that the remaining 30% of the rainfalls contribute to the surface runoff (Ng et al., 2003; Rahardjo et al., 2004).

All the experiments involving the two-dimensional slope model (i.e. S1 to S10) were constantly tilted at an angle of 18o (typical steepness for the tropical residual soil slopes) because the slope

Table 2 Experimental programs designed for the study. Experiment no. C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 Soil type Laboratory model 1D-soil column 1D-soil column 1D-soil column 1D-soil column 1D-soil column 1D-soil column 1D-soil column 1D-soil column 1D-soil column 1D-soil column 2D-slope model 2D-slope model 2D-slope model 2D-slope model 2D-slope model 2D-slope model 2D-slope model 2D-slope model 2D-slope model 2D-slope model Rainfall Rainfall intensity (m/s) duration (hours) 1.84 × 10?5 3.35 × 10 1.84 × 10
?6

Sand-gravel Sand-gravel Silty gravel Silty gravel Sandy silt Sandy silt Silt (Kaolin) Silt (Kaolin) Sandy silt underlain by silty gravel Sandy silt underlain by silty gravel Sand-gravel Sand-gravel Silty gravel Silty gravel Sandy silt Sandy silt Silt (Kaolin) Silt (Kaolin) Sandy silt underlain by silty gravel Sandy silt underlain by silty gravel

1 24 1 24 1 24 1 24 1 24 1 24 1 24 1 24 1 24 1 24

?5

3.35 × 10?6 1.84 × 10?5 3.35 × 10
?6

1.84 × 10?5 3.35 × 10?6 1.84 × 10 3.35 × 10
?5

?6

1.84 × 10?5 3.35 × 10?6 1.84 × 10
?5

3.35 × 10?6 1.84 × 10?5 3.35 × 10 1.84 × 10
?6

?5

3.35 × 10?6 1.84 × 10?5 3.35 × 10
?6

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Fig. 11. Suction distributions as the results of 24-hour extreme rainfalls for the homogeneous soils, i.e. (a) sand-gravel, (b) silty gravel, (c) sandy silt, and (d) silt (kaolin).

steepness is not the main variable in this experiment. The thickness for both sandy silt and silty gravel layers were set at 300 mm. 6. Experimental results and discussions 6.1. Comparisons between one-dimensional soil column and two-dimensional slope model Fig. 11 shows the suction distributions for the experiments C2, C4, C6, C8, S2, S4, S6, and S8. For the two-dimensional slope models (i.e. experiments S2, S4, S6, and S8), only the suction measurements at the middle slope were presented because the suction measurements observed at the crest, middle slope, and toe were almost identical to each other. Also noted that the thicknesses of the homogeneous soils in the two-dimensional slope model were only 600 mm. Obviously, there were no signi?cant differences in suction measurements between the experiments C2 and S2, C4 and S4, C6 and S6, and C8 and S8. These experiments employed single-layered homogeneous soils as the testing mediums. Under the same rainfall and soil

conditions, the suctions measured from both models were almost identical to each other. The results implied that the vertical ?ow plays a more dominant role than lateral ?ow in homogeneous soil slope. This ?nding was supported by Ritsema et al. (1996) who found that the average lateral water movement only varied between 1.6 and 4.7% of the total water displacement through the hillslopes. It can thus be concluded that when there is no signi?cant lateral ?ow observed in the two-dimensional slope model, the simpler one-dimensional soil column can be adopted for the investigation of the suction distributions in the single-layered homogeneous soil. The suction measurements of the two-layered soil system that obtained from the experiments C10 and S10 are shown in Fig. 12. With the soil layers interface at 0.3 m level, it is apparent to ?nd that the bottom soil layer (silty gravel) of the two-dimensional slope model has a lower suction than that of the one-dimensional soil column. The results implied that more water was actually in?ltrating into the bottom layer of the sloping model. The soil suctions obtained from the toe of the two-dimensional slope model were generally lower than those of the middle slope, crest and the one-dimensional

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values for 1-hour extreme rainfall were higher than that of 24-hour extreme rainfall. This can be explained by the insuf?cient duration of the 1-hour rainfall to achieve the minimum suction value. Upon achieving the minimum suction value, further in?ltration would cause deeper propagation of wetting front. In general, the depth of wetting front was positively correlated with the soil particle size. For instances, with the same applied intensity and duration of rainfalls, the sand-gravel has a deeper wetting front than those of the silty gravel and sandy silt. This was caused by the low water retention ability and the large differences in unsaturated permeability of the coarse-grained soil within a small suction range. For silt (kaolin), it was found that the response of suction variation to the rainfall in?ltration was unexpectedly quick despite of the low saturated permeability obtained from the falling head permeability test on an identical soil density. This observation can be explained by the relatively low initial suction condition (70 kPa) of the soil compared to its air entry value (approximately 90 kPa obtained from the SWCC of silt). As observed in the SWCC of silt (Fig. 9), little changes in volumetric water content within the suction range of 0 to 70 kPa would induce large suction variation. Besides, the tendencies of silt to shrink and crack have caused the in?ltration capacities far in exceedance of the expected saturated permeability. This ?nding was supported by the observations of the desiccated surfaces and cracks in the silt (kaolin) model.
Fig. 12. Suction distributions as the results of 24-hour extreme rainfalls for the two layered soil system, i.e. sandy silt underlain by silty gravel.

7. Conclusions Two laboratory models, i.e. one-dimensional soil column, and twodimensional slope model were constructed to investigate the in?ltration characteristics of four different types of unsaturated soils. The following conclusions can be drawn from the results of the laboratory studies:

soil column. These observations can be explained by the mechanism of lateral ?ow in the two-layered soil system. When a less permeable soil (sandy silt) is underlain by a more permeable soil (silty gravel), the water tends to ?ow along the interface of upper layer and accumulate at the toe of the slope. The water will start to in?ltrate freely into the bottom layer after a total breakthrough suction value is achieved after prolonged rainfall (Ross, 1990). The phenomenon has been demonstrated by numerous researchers as the capillary barrier effect (Hillel and Baker, 1988; Stormont and Anderson, 1999; Fala et al., 2005; Parent and Cabral, 2006). In conclusion, the twodimensional slope model should be employed if the suction distributions in soil were to be governed by the lateral ?ow mechanisms of the two-layered soil system.

6.2. Effect of rainfall intensity and duration on suction distribution Fig. 13 shows the suction distributions in sand-gravel, silty gravel, sandy silt, and silt (kaolin) as the results of 1-hour and 24-hour extreme rainfalls (experiments C1 to C8). During rainfall in?ltration, the suction decreased gradually until a minimum suction value was achieved. Minimum suction value is de?ned as the lowest achievable suction in soil at any depth under certain rainfall condition, which is governed by the intensity and duration of rainfall applied as well as the saturated permeability of the soil concerned (Gofar and Lee, 2008). In general, the higher the intensity of the rainfall, the lower the minimum suction value (as observed in Fig. 13a), provided the rainfall intensity has not exceeded the saturated permeability of the soil and the duration of the rainfall is suf?cient to achieve the minimum suction value. For silty gravel (Fig. 13b), the soil has the same minimum suction values for both 1-hour and 24-hour extreme rainfalls despite the intensity of the 1-hour rainfall being higher. This is because the intensities for both rainfalls have exceeded the saturated permeability of the soil, thus it was the saturated permeability of the soil that controlled the minimum suction value. As for the sandy silt (Fig. 13c) and silt (Fig. 13d), the minimum suction

(i) Both one-dimensional soil column and two-dimensional slope model can be used to quantify the rainfall in?ltration characteristics, and suction variations in unsaturated soils. For single-layered homogenous soil, the simpler one-dimensional soil column could give an equally good result with the twodimensional slope model. The two-dimensional slope model, however, should be employed for the two-layered soil system because of the dominant effect of horizontal ?ow mechanism. (ii) The capillary barrier effect was observed when a less permeable soil layer (sandy silt) is underlain by a more permeable soil layer (silty gravel). The in?ltrated water tends to ?ow along the interface of upper layer and accumulate at the toe of the slope. The water will start to in?ltrate freely into the bottom layer after a total breakthrough suction value is achieved. (iii) The minimum suction value in a soil is governed by the rainfall intensity, rainfall duration, and the saturated permeability of the soil. Upon achieving the minimum suction value, further rainfall in?ltration will lead to deeper propagation of wetting front. In general, the depth of wetting front was positively correlated with the soil particle size. (iv) The ?ow of water in the ?ne-grained soil that subjected to shrink and crack is independent of the mass permeability of the soil matrix. The preferential ?ows may cause the in?ltration capacities far in exceedance of the saturated permeability of soil, as observed in this laboratory study. Acknowledgements This project is funded by the Fundamental Research Grant Scheme (FRGS), Ministry of High Education Malaysia. The authors acknowledge the useful comments of the reviewers.

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Fig. 13. Suction distributions as the results of 1-hour and 24-hour extreme rainfall for the homogeneous soils, i.e. (a) sand-gravel, (b) silty gravel, (c) sandy silt, and (d) silt (kaolin).

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