Expansive Residual Soil Stability Behavior During Wetting and Drying Process

2.1 Soil water characteristic curves (SWCC)

Based on the geotechnical point of view normally, aside from the previous studies, some principles or theories, including the complicated apparatus or unsaturated soil mechanics devices laboratory works, are necessary to evaluate the best-fit of soil water characteristic curves (SWCC) and to define the permeability function [15].

In unsaturated soil theory, the difference between air and hydrostatic water pressures can represented by (u_a – u_w). In the actual conditions, air pressure has always zero at the field or can be mentioned by the continuous air voids; this suction parameter can be stated in the form of (– u_w) with hydrostatic pressure, which moves to the left side of soil stress cross-section [16]. If occurred the infiltration process in the rainy season, the zone of unsaturated soil can be filled out in one part by the water and form a perched water table with little movement of hydrostatic pressure to the right side or can be represented by (+ u_w) in stress cross-section which is depended on the depth zone of wetting front.

In unsaturated soil analysis, SWCC defines the relation between water content and suction [17]. The SWCC describes the behavior and soil capability in conducting infiltration and/ or groundwater storage processes, which depends on the grain size distribution model according to the soil type. The SWCC model consists of 3 (three) conditions to represent the desaturation zone in a soil type, including the boundary, transition, and residual conditions, as shown in Figure 1 [18].

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Figure 1. The phases of SWCC modeling [16]

There are some parameters to perform a best fit in modeling SWCC; among them are saturated water content (θ_s), which describes the soil behavior and maximal capability to infiltrate and storage the water during the wetting process, especially during the rainy season. Suppose the air pressure has filled out partially soil pore to reach the ultimate condition with signed by the difference of a quite large pressure between air and water. In that case, this condition can be meant as the magnitude of air entry absorbed by soil pore represented as (ψ_aev) [19]. When the increasing of suction occurs in a short time, which correlates to the desaturation velocity (δ_s), and it is influenced by the pattern of grain size distribution on a type of soil, so that the curve tends to decrease drastically and has the certain gradient at the middle of the curve, it can be defined as the inflection point (ψ_m). Unfortunately, the distribution of data recorded in the actual measurement of the permeability coefficient at the laboratory work has some errors represented by a geometric standard deviation ($\sigma$)​ modeling process of SWCC in determining the best-fit condition. However, it can be predicted by using the equation proposed in the previous research in determining the coefficient of permeability [20]. When the increasing of suction further occurred, which has failed to perform in removing many water molecules at the soil layers, it can be represented by residual point (ψ_r). The minimum capability of the soil to perform infiltration and storage during the drying process, especially during the dry season, can be defined as residual water content (θ_r). In processing of best fit when the curve modeling of SWCC unimodally according to 6 (six) parameters (θ_s, ψ_aev, ψ_m, σ, ψ_r, θ_r), it can be analyzed by using the non-linear regression as the permeability function. Analysis of validation of best-fit condition of SWCC uses Eq. (1) for water content (w_c) in unsaturated conditions compared with laboratory investigation results [21, 22].

$\mathrm{w}_{\mathrm{c}}=\left(1-\frac{\ln \left(1+\frac{\psi}{\psi_{\mathrm{r}}}\right)}{\ln \left(1+\frac{10^6}{\psi_{\mathrm{r}}}\right)}\right) \cdot\left[\theta_{\mathrm{r}}+\left\{\left(\theta_{\mathrm{s}}-\theta_{\mathrm{r}}\right) \cdot\left(1-(\beta) \operatorname{erfc}\left(\frac{\ln \left(\frac{\psi_{\text {aev }}-\psi}{\psi_{\text {aev }}-\psi_{\mathrm{m}}}\right)}{\sigma}\right)\right)\right\}\right]$                (1)

2.2 Estimation of permeability function

In the soil mechanics of unsaturated soil, the coefficient of permeability is almost constant for saturated soil (k_sat); however, for unsaturated soil, the coefficient of permeability (k_unsat) is a function of the water content (w_c) [23-26]. The permeability function describes the soil behavior and soil capability in conducting water flow in the soil layer, and it depends on grain size distribution according to soil type [27]. The estimation of the permeability function is based on 3 (three) parameters (θ_s, ψ_aev, θ_r) in the best-fit result in defining the SWCC [28]. Normally, these best-fit of SWCC equations are mathematically continuous and governed by a few fitting parameters. The study uses normalized water content to predict permeability function (k_w) to obtain the SWCC using Eq. (2).

$\mathrm{k}_{\mathrm{W}}=\mathrm{k}_{\mathrm{sat}} \cdot \frac{\int_{\ln (\Psi )}^{\ln \left(\mathrm{\Psi}^r\right)} \frac{\theta_{\mathrm{s}}\left(\mathrm{e}^{\mathrm{y}}\right)-\theta_{\mathrm{s}}(\Psi)}{\mathrm{e}^{\mathrm{y}}} \theta_{\mathrm{s}}^{\prime}\left(\mathrm{e}^{\mathrm{y}}\right) \cdot \mathrm{dy}}{\int_{\ln (\Psi \mathrm{aev})}^{\ln (\Psi \mathrm{r})} \frac{\theta_{\mathrm{S}}\left(\mathrm{e}^{\mathrm{y}}\right)-\theta_{\mathrm{S}}\left(\Psi_{\mathrm{aev})}\right.}{\mathrm{e}^{\mathrm{y}}} \theta_{\mathrm{s}}^{\prime}\left(\mathrm{e}^{\mathrm{y}}\right) \cdot \mathrm{dy}}$               (2)

4.1 Environmental and topography condition

The area of South Tangerang City is 147.19 km². The study location is very close to Jakarta, around 26 km. The study area is a low terrain with soft and low hilly areas. These areas exist at the elevation ± 26 – 50 meters above sea level at the northern part of the Java Sea with a slope gradient of less than 7%. The area has an average air temperature of around 26.4°C-28.2°C, and average air humidity is 98%. The highest rainfall is 526.8 mm, while the average rainfall is 225.9 mm every year. The highest rainfall normally occurs in January, with a duration of 25 days. The average wind speed in a year is 4.0 m/sec. Ocean areas can contribute significantly to the tropical water's condition and play an important role in the formation of alluvium deposits, especially when expansive residual soil is unsaturated, followed by negative pore-water pressures. These layers are often found in the study location, such as the Cisadane River deposit.

4.2 Geological condition

The geological conditions in this area include alluvium-diluvium deposits, tuffaceous siltstone, sandstone, and argillaceous material [40]. From a geotechnical point of view explained that the weathered process from the rock layer could result in a residual fine-grained soil layer, such as silty clay, clayey silt, sandy silt until 20 m depth, and silty sand that formed the ground surface to a depth larger than 20.0 to 30.0 m. Generally, the residual fine-grained soil originates from the weathering process of granite and/ or sedimentary rocks; the texture of residual soil depends on environmental conditions along the Cisadane River, such as the alluvium deposits at the beginning of formation. Characteristics of swelling-shrinkage are that the expansive soil is more produced by the weathering process of basalt rock. Basalt rock can produce the montmorillonite fraction in soil.

Every year during the rainy season, the alluvium-diluvium deposit sedimented by the flow of the Cisadane River is very susceptible to the transformation of water content. The deposits can be classified as fine-grained soil classified as very to medium class of landslide susceptibility, and this vadose zone can be categorized as a shallow landslide potential reported by the local office of Vulcanology & Mitigation of Geology Disaster Center (PVMBG-Tangerang City). The swelling potential of expansive residual soils significantly depends upon wetting and shrinkage. Montmorillonite content shown by the plasticity index (PI) also plays an important role in determining SWCC and permeability function parameters. Instability or failure of expansive residual soil is primarily due to water infiltration due to high rainfall at the Cisadane watershed during the wet or rainy season. Furthermore, it is very difficult significantly to define some points in the transition phase in describing the best-fit SWCC as well, especially in determining air-entry value, inflection, and residual section point.

4.3 Physical and mechanical properties data

The standard procedure for laboratory and field soil investigation used D1586/D1586M − 18 [41] and SNI 8460 (2017) [42]. Figure 5 shows the resulting test conducted at the study area consisting of Standard Penetration Test (SPT)-borehole (BH) or boring log for sampling and Cone Penetration Test (CPT). 5 (five) BH points were performed with 6 (six) position of CPT nearly location with borlog-SPT area.

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(a) Typical results of borlog-SPT

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(b) Typical results of CPT

Figure 5. Typical of field test results (SPT and CPT)

Figure 5 (a) shows the typical soil layers from the SPT-BH investigation where topsoil constitutes soft sandy silt layers in brown colour from the ground surface to 1.0 m depth. At a depth of 1.0 to 6.0 m, it shows the brown elastic to plastic soft to medium stiff sandy and/ or silty clay, then it continued the medium stiff to very stiff plastic clay at a depth of 6.0 to 10.50 m. The next layer indicates hard sandstone and very stiff plastic clay, intermittently until 30.0 m depth. The interpretation of soil layers from CPT, as shown in Figure 5 (b), is determined by the graphical correlation proposed by the Robertson method [43], and the results are not so far from the results of the SPT-BH and herein the CPT test considered had failed to penetrate deeper and cannot be continued (b). The vadose zone exists from the ground surface to 5.0 or 6.0 m depth and has the number of blow (N) from 12 and 14 and cone resistance (q_c) from 0.4 to 7.0 MPa.

Table 1 shows the summary of standard laboratory work results using the macro investigation. The soil sample classification is an anorganic clay with high plasticity (CH) or A-7-5 from the ground surface to a depth of -5.00 m. It is assumed to be an unsaturated or vadose zone. In the existing condition, the river water level influences the groundwater table's fluctuation. Furthermore, PI > 30% can be assumed to be an expansive soil and create additional problems at the sensitive zone for shallow slope stability.

Table 1. Summary of physical and mechanical properties at prediction of vadose zone (-5.0 m depth from ground surface)

4.4 Estimation of SWCC curve

SWCC curve is the mechanical behavior of expansive residual soil represented by soil suction measurements; this study used the filter paper method. The method was selected as a conventional method of soil matric suction measurement since this method was simple for laboratory work [44]. It has been conducted the plotting for the SWCC curve unimodally during modeling simulation repeatedly based on data processing of water content (w_c) and soil suction (ψ_s), as shown in Figure 6. Simulation of the best-fit condition was started with the appropriate initial values, including the 6 (six) parameters (θ_s, ψ_aev, ψ_m, σ, ψ_r, θ_r). Figure 6 also indicates that the water content (w_c) is very influenced by temperature and humidity factors in the existing environment. This figure shows clearly that the matrix suction had the magnitude of immediately increasing during the transition condition, which was correlated with the velocity of the desaturation process (δ_s). Then, this point could be an inflection point of suction (ψ_m). The grain size distribution significantly influenced the magnitude of soil suction on expansive residual soil. A large fine-grained size would increase the soil's capability to infiltrate or store the water in the soil layers. These conditions would increase the magnitude of soil suction.

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Figure 6. Plotting data results for best fit on the SWCC curve

It is possible to use the big data to determine unsaturated soil parameters in the SWCC best-fit curve model during the study; therefore, conventional probability analysis is required to determine the boundary range or condition for these parameters. Physical properties and permeability test results are very decisive in this matter. Table 2 is very helpful in plotting all values from soil suction at air-entry value, inflection, and residual section points, respectively, when the phase changes from saturated to unsaturated conditions.

Table 2. Representation values based on the SWCC best-fit curve and simplified model of laboratory saturated permeability test

4.5 Estimation of permeability function

Estimating the function of permeability (k_w) could be conducted by simulation to determine the best fit in the SWCC curve, as shown in Figure 7. Permeability function reflected the permeability coefficient correlations between saturated (k_sat) and/ or unsaturated (k_unsat) conditions and soil suction (ψ_s). The measurement of the coefficient of permeability was done using the laboratory falling head apparatus. The estimation of the permeability function is predicted by 3 (three) important parameters, such as θ_s, ψ_aev, and θ_r, in the best-fit SWCC curve simulation.

The permeability function is a determination factor in predicting every groundwater flow, and the behavior is influenced by grain distribution size. More fine-grained size would lower the capability of the soil to conduct groundwater flow and create a higher magnitude of soil suction. Table 2 shows the parameter range value of the permeability function (k_w) for expansive residual soil.

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Figure 7. Permeability function and soil suction

4.6 Seepage analysis and pore water distribution

Seepage analysis forms an important and basic part of geotechnical engineering. Seepage analysis may be required in volume change prediction and fundamental changes to the wall slope. It is very difficult to determine deformation and soil stability related to the flow model. The selection flow model of unsaturated soil plays a primary and important role, which is coupled with the change saturation-desaturation process. The saturation-desaturation process depends on microclimate and can be characterized in terms of factors such as temperature, relative humidity, wind velocity, infiltration, etc. The seepage problem, according to microclimate conditions will be related with the change of groundwater table and/or impervious boundary, respectively when the soil is saturated. However, in the case of unsaturated soil, the pore water pressure is negative in response to the imposed microclimate condition at the ground surface.

In the last two decades, the development and application of the computer to solving complex problems has been extensive. An unsaturated soil problem involves highly non-linear soil properties, such as coefficient of permeability and water storage functions. Manual analysis can be difficult because the partial differential equations to be solved are highly non-linear. This has given rise to the use of general partial differential equation solvers that are designed to solve equations from many areas of soil mechanic engineering. The solution of transient, unsaturated soil systems requires two non-linear soil property functions; the coefficient of permeability function and the water storage function. Both soil property relationships regarding the negative pore-water pressure or soil suction can be written.

Conventional analysis for groundwater flow through saturated and unsaturated soils is governed by Darcy’s law. However, two major differences exist between the water flows in saturated and unsaturated soils. First, the ability of the unsaturated soils to retain water varies with soil suction. Second, the water permeability coefficient is not a constant in unsaturated soils but a function of soil suction. Thus, it is essential to define all parameters from the soil-water characteristic curve (SWCC) as shown in Figure 8 before starting seepage analysis in unsaturated soil. The seepage analysis will define the relationship between the soil suction and either the water content or the degree of saturation. The water permeability function varies with soil suction for simulating transient seepage in unsaturated soil slopes. Currently, permeability function for seepage analysis is obtained from a measured saturated and unsaturated water permeabilities in wet and drying conditions, respectively of the existing SWCC as shown in Figure 9.

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Figure 8. Parameter SWCC curve (w_c, θ_s, k_w, and ψ_s) on seepage analysis

Hereinafter, simulation using was conducted to model the seepage phenomenon and pore water distribution based on the laboratory measurement for water content (w_c), saturated water content (θ_s), permeability function (k_w), and soil suction (ψ_s) from the approximation of SWCC the best-fit curve and permeability function parameters from Figures 8 and 9, respectively. The simulation purpose was to define the seepage model and the distribution of pore water pressure according to time magnitude, especially in the overview of every depth from the top of the slope due to the infiltration process during with and without the precipitation in dry and/ or rainy seasons.

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Figure 9. Permeability function in seepage analysis

Figures 8 and 9 show the existing SWCC curve after data plotting on the finite element analysis using the Seep/W, so that the analytical approach results from Figures 6 and 7 must be matched with Figures 8 and 9, respectively each other before they are used as input data on seepage analysis and pore water distribution. Figure 8 and 9 also indicates that the parameter of permeability function (k_w), soil suction (ψ_s), and saturated coefficient permeability (k_sat) from estimation of permeability function as the input data were used in analyzing of seepage phenomenon in steady state and transient on expansive residual soil. The seepage analysis in transient conditions was observed during the maximum intensity of rainfall or precipitation data in the rainy season in the study area [45, 46].

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Figure 10. Intensity of maximum rainfall model as the input data on finite element simulation during seepage analysis

The rainfall data were obtained from Meteorological, Climatological, and Geophysics (BMKG-Tangerang City). Research on the pattern and intensity of rainfall in the city of Rainfall data consists of the pattern and intensity of rainfall. This study conducted pattern analysis and intensity of rainfall in the years 2020 to 2025; there is one data used in this analysis, including the rain observation data at station officials, which is close to the study area.

Figure 10 shows that the seepage analysis was conducted with a groundwater depth of (–) 1.20 m at the study location around Jalan. JLS (Jalan Lingkar Selatan) and (–) 1.50 m at near of Cisadane River. Rainfall intensity in the Tangerang District area was around 13.528 mm/day or 1.566 x 10^-7 m/s. During the wetting condition, seepage analysis was calculated for 2 (two) days from the first rainy day, and during the drying condition, it was reviewed 3 (three) on the next day.

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(a) Contour modeling of pore water pressure (PWP) for t = 0.00 days

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(b) Contour modeling of pore water pressure (PWP) for t = 0.13 days

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(c) Pore water pressure distribution (PWP) during t = 0.00 to 0.13 days

Figure 11. Modeling of contour and pore water pressure (PWP) distribution (t = 0.00 to 0.13 days) (Seep/W analysis)

Figure 11 shows that the rainwater precipitation performed a desaturation process to the unsaturated soil layer during 0.00 to 0.13 days (11,034 seconds or 3.07 hours) through the gap and/ or soil pore. Negative pore water pressure was also conducted in closing to equilibrium hydrostatic line condition with the tension from (–) 0.00070 kPa to (–) 12.403 kPa.

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(a) Contour modeling of pore water pressure (PWP) for 0.13 days

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(b) Contour modeling of pore water pressure (PWP) for t = 0.17 days

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(c) Pore water pressure distribution (PWP) during t = 0.13 to 0.17 days

Figure 12. Pore water pressure distribution (PWP) during (t = 0.13 to 0.17 days) (Seep/W analysis)

Figure 12 indicates that the water rainfall was successful in the desaturation process of all ground surfaces at the unsaturated zone for 0.17 days (14,871 seconds or 4.13 hours), and tension of cross-section had reached the equilibrium hydrostatic condition with negative pore water pressure of (+) 0.00025 kPa in the stress cross-section.

In this study, practical applications of unsaturated soil mechanics are time-consuming and expensive unsaturated soil laboratory tests in determining engineering properties. The accuracy of the prediction of the SWCC best-fit curve and permeability function really produces a good result and is validated by shallow slope conditions in the field. Figure 12 is the first simulation analysis due to infiltration during rainfall here; three predictions can be drawn from this work such as high-intensity rainfall can rapidly transform soil surface features of shallow bare soil; in the vadose zone, shallow slopes are more easily eroded than nearby steep slopes, and there is a more pronounced lowering of the ground surface due to compaction and denser microlayers on gentler slopes.

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(a) Contour modeling of pore water pressure (PWP) for 0.22 days

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(b) Contour modeling of pore water pressure (PWP) for 1.97 days

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(c) Pore water pressure distribution (PWP) during 0.22 to 1.97 days

Figure 13. Pore water pressure distribution (PWP) during t = 0.22 to 1.97 days (Seep/W analysis)

Figure 13 shows that the water rainfall has already finished the desaturating process from all pores at saturated soil layers for 0.22 days (18,790 seconds or 5.22 hours) to 1.97 days (170,299 seconds or 47.31 hours). Infiltration capacity and water storage at the ground layers are already in perfect saturation condition. Finally, the water rainfall could overflow to the low level of the Cisadane River, with each negative pore water pressure of (+) 0.00037 kPa and (+ 0.00243 kPa), respectively.

Figure 14 indicates that at the time of 2.06 days (177,554 seconds or 49.32 hours) to 5.00 days (432,000 seconds / 120.00 hours). The negative pore water pressure has been trying to move away from the equilibrium hydrostatic line of the cross-section or move back towards the initial hydrostatic condition because the rainfall has been stopped, and occurred the water rainfall during the evaporation process in unsaturated soil layers, finally negative pore water pressure increased significantly with each of (–) 0.330 kPa and (–) 5.968 kPa in stress cross section.

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(a) Contour modeling of pore water pressure (PWP) for 2.06 days

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(b) Contour modeling of pore water pressure (PWP) for 5.00 days

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(c) Pore water pressure distribution (PWP) during 2.06 to 5.00 days

Figure 14. Pore water pressure distribution (PWP) during t = 2.06 to 5.00 days (Seep/W analysis)

Based on Figure 14, it can be concluded that a decrease in the swelling ability of unsaturated expansive clays corresponds to a reduced water absorption capability when the soils were alternately wetted and partially shrunk. On the other hand, an increase in the swelling potential was noted when the soils were fully shrunk. In either case, equilibrium can be attained after several cycles. As the number of cycles increases, further destruction of large and disorientation of particle structure.

4.7 Slope stability and deformation analysis

Slope stability and deformation analysis were conducted using finite element analysis using the Geostudio software program. This modeling aims to determine the magnitude of stability (safety factor) and deformation on slopes analyses of expansive residual soil due to wetting and drying conditions. The simulation slope stability and deformation results are shown in Figure 15. The soil water characteristic curve (SWCC) as the best-fit curve should be selected to investigate the hydraulic properties of unsaturated soil because it is closely related to the matric suction and the water content of the unsaturated soil. The best-fit curve representing the volumetric water content verse matric suction results from the hydraulic parameter of SWCC before landslides occurred. From Figure 15, it would be indicated that the positive pore water pressure changes gradually increase during the rainy season and vice versa to negative during the dry season.

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(a) Simulation stability model during the rainy season

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(b) Simulation stability model during the dry season

Figure 15. Analysis results of stability analysis during rainy and dry seasons (Slope/W analysis)

Figure 15 (a) also shows that the results of finite element simulations indicate that the critical safety factor (SF) during the wetting condition where the infiltration process occurred in the rainy season obtained SF = 0.152. Drying conditions or evaporation processes, as shown in Figure 15 (b), that occurred during the dry season resulted in SF = 1.265. The magnitude of deformation could be obtained at 4.304 cm or 43.040 mm (according to the y-axis direction) and 3.836 cm or 38.360 mm (according to the x-axis direction) during wetting conditions or infiltration process. During dry conditions or evaporation, it could indicate 0.471 cm or 4.710 mm and 0.214 cm atau 2.141 mm according to y and x-axis directions, respectively.

The magnitude of deformation could be obtained at 4.304 cm or 43.040 mm (according to the y-axis direction) and 3.836 cm or 38.360 mm (according to the x-axis direction) during wetting conditions or infiltration process. During dry conditions or evaporation, it could indicate 0.471 cm or 4.710 mm and 0.214 cm or 2.141 mm according to y and x-axis directions, respectively.

Soil behavior and the properties of unsaturated residual soils can have swelling potential and shrinkage potential. In the area study, these layers exist from the ground surface to a depth of -5.00 m. Based on the theory and practice of unsaturated soils, several efforts must be made to maintain the water content (w_c) under infrastructure construction through the simulation of slope stability and deformation analysis of the road. The topographic conditions located next to the river mean several drainage control factors must be considered, including the drainage system, settlement, groundwater infiltration, erosion by river flows, and upward pressure (up-lift). An integrated analysis should be elaborated through rainfall, laboratory and field tests, and numerical analysis.

The research study's purpose was to determine the magnitude of deformation and stability conditions at soil slopes due to wetting and drying conditions, and curve (SWCC) was considered during the simulation. The simulation results indicated that the dry season was more stable than the rainy season. Deformation during dry conditions was smaller than 1.0 inch (25,4 mm), with the safety value (SF) of slope sliding larger than 1.20. The opposite would occur during wet conditions or the rainy season.

Assessment and analysis of SWCC and the selection of permeability function were very crucial in stability analysis on unsaturated expansive residual soil [47]. Existing conditions and field observation must be considered carefully to validate the simulation results. Rigorous laboratory analysis, especially during the measurement of the moisture water content (w_c), saturated water content (θ_s), permeability function (k_w) and soil suction (ψ_s), and the estimation for unsaturated and saturated permeability (k_unsat; k_sat) respectively must be performed to avoid some errors in assessment SWCC and permeability function (k_w) [48]. Some errors in the determination of SWCC and permeability function (k_w) would cause some mistakes in defining the safety factor (SF).

Another point obtained from the research field observation is related with the characteristic of expansive soil, which expands when the moisture content of the soil increases. The clay mineral of montmorillonite is mainly responsible for swelling and shrinkage potential. Expansive soils are generally residual soil left in the place of their formation after the chemical decomposition of the rocks, such as Basalt found in this study area; expansive soils could be saturated and/ or unsaturated condition. These lands are generally dry because the outflow of water from springs are quite large, as can be seen on the Cisadane River dyke or embankment. During the rainy season, they become wet. The swelling – shrinkage as the water content fluctuation- normally begins from small cracks to fractures. Severe movement of the soil mass may occur. Structures built on such soils may experience cracking & damage due to differential heave or instability with the lowest safety value (SF) during the rainy season. In yearly, repairs to the damages of road body structures are always carried out in this area because they are not taken seriously in studying behavior of unsaturated expansive residual soil. The average active zone at this study area is less than 5.0 m depth, as well as the plotting the liquidity index (LI) and/ or plasticity index (PI) against depth. From observation, normally, the initial movement of unsaturated expansive soil material was occurred at the one third to the maximum depth of these layers. It could be deduced that the swelling process was triggered by the sliding potential at the study location in heavy rainfall.

The authors would like to thank P3M Politeknik Negeri Jakarta for funding this research through the Basic Research Grant No.: 374/PL3.A.10/PT.00.06/2024.