Experimental Insights Towards Understanding the Possibilities of Using Chloride Substances in Landslide Stabilization

Abstract

This study explores the effect of cation adsorption on the shear strength and mineralogical characteristics of smectite-rich landslide clay collected from the Nishinotani landslide in Ehime Prefecture, Japan. Laboratory experiments were conducted using aqueous solutions of calcium, magnesium, and potassium chlorides at concentrations of 1000, 6000, and 12,000 mg/L. Ion chromatography, X-ray diffraction (XRD), and ring shear tests were conducted to evaluate the interaction between ion uptake and its influence on the change in shear strength. The results showed that calcium and potassium ion adsorption increased with both concentration and time, leading to enhanced residual shear strength and crystallinity, primarily due to stronger Coulombic interactions and favorable ionic size compatibility with smectite. Conversely, magnesium ions exhibited adverse effects, including reduced strength and mineral ordering, attributed to calcium leaching and weaker interparticle bonding. The findings indicate that selective cation exchange can be an effective, sustainable alternative to conventional landslide stabilization methods, especially in fine-grained, expansive clay systems. This work contributes to the development of geochemically engineered landslide mitigation strategies based on microstructural and mineralogical reinforcement.

1. Introduction

Landslides are one of the most critical natural hazards, typically initiated when slope-forming materials lose stability due to interactions among lithology, geological structure, topography, and weathering, and are further accelerated by external triggers such as intense rainfall, rising groundwater levels, snowmelt, earthquakes, volcanic activity, or human disturbances. In most mountainous areas with tectonic influence, the existence of large-scale landslides, mostly in a creeping state of displacement, is an added disaster risk to mountain populations. Stabilization efforts of such landslides include various structural as well as non-structural measures including drainage-assisted water table lowering, and stability is assured by evaluating the residual shear strength of the sliding zone clayey material.

The shear residual strength of a clayey material represents the minimum constant shear resistance mobilized along a well-defined slip surface, typically after significant displacement under relatively low shear strain rates, during which clay particles are reoriented into a stable configuration [1]. This parameter plays a critical role in evaluating the long-term stability of both first-time and reactivated landslides [2]. In many cases, landslide slip surfaces are located within clay-rich soils that have experienced previous shearing, thereby reaching a residual state [3,4]. Consequently, accurate assessment of the residual strength of such soils is essential for reliable hazard analysis and the design of effective slope stabilization measures. Residual shear strength mainly depends on the mineral composition rather than plasticity or grain size. Soils rich in massive minerals such as quartz, feldspar, calcite, and mica exhibit higher residual strength, whereas those dominated by clay minerals, including montmorillonite (or smectite group), show much lower strength [5].

Conventional landslide stabilization or prevention strategies primarily rely on engineering solutions, including subsurface drainage systems and structural reinforcements. Despite their effectiveness, conventional stabilization methods are often costly, time-consuming, and environmentally intrusive, limiting their applicability in large-scale or resource-limited settings. This motivates the exploration of alternative approaches that can enhance slope stability with lower economic and social impacts.

So, one promising direction towards a new landslide stabilization technique involves investigating the influence of chemical interactions between soil minerals and groundwater composition. Several studies, e.g., refs. [6,7,8,9,10,11,12] have reported that increasing the concentration of salt solutions enhances the shear strength of soils. Di Maio and Fenelli [13] demonstrated that replacing distilled water with alternative fluids significantly alters the mechanical behavior of clayey soils. Abood et al. [14] further showed that adding chloride salts such as NaCl, MgCl₂, and CaCl₂ to silty clay reduces the liquid limit, increases the plastic limit, lowers the plasticity index, and enhances the maximum dry density and compressive strength. Additionally, Rassoul Ajalloeian et al. [15] found that higher water salinity decreases Atterberg limits and compressibility indices, while increasing shear strength parameters. Collectively, these studies confirm that chloride salts have a positive influence on the engineering properties of fine-grained soils. Furthermore, Shinichiro Matsuo [16] has found that the stability of the slope was disturbed by the loss of calcium ions from the slope through seepage water. Ouhadi et al. [17] have also deduced that pH and ion exchange are two different phenomena responsible for overcoming soil dispersity.

Despite all these findings, limited research has addressed how chemical-induced mineralogical changes influence the strength characteristics of landslide-prone clays. In particular, understanding the effect of cation adsorption changes on shear strength remains underexplored. To understand how geochemical changes in clay mineralogy could influence the frictional resistance of clay particles, this study investigates the role of ion adsorption from calcium chloride, magnesium chloride, and potassium chloride solutions on the mineralogical and mechanical properties of landslide soils.

This approach of modifying soil properties through artificial cation exchange represents a paradigm shift from conventional slope or landslide stabilization techniques. Since groundwater chemistry directly affects slope stability, controlling cation concentrations can be considered to offer an innovative, cost-efficient, and sustainable strategy for landslide stabilization. A schematic diagram of a possible landslide prevention strategy using chemical substance injection into the landslide slip surface or weak clay layer zone is presented in Figure 1. While practical applications may still be limited due to data constraints, emerging evidence highlights that cation adsorption and exchange significantly influence the residual strength enhancement and microstructural stability, opening avenues for novel slope stabilization techniques. The conceptual basis of chemically modifying clay behavior through pore-water chemistry builds upon earlier laboratory and field investigations by Di Maio and co-workers, including long-term monitoring at the Costa della Gaveta earthflow [18] as well as field-scale potassium chloride stabilization studies reported by Helle et al. [19]. This study complements these landslide stabilization efforts by providing controlled laboratory quantification of ion-specific adsorption effects on drained residual shear strength.

With the above background information and issues in landslide stabilization techniques, the primary objective of this study is to conduct fundamental experimental work towards proposing chemical-based countermeasures and establishing long-term stability management strategies for landslide-prone regions. For this, controlled laboratory experiments were conducted using aqueous solutions with varied concentrations of chloride compounds to examine their effects on the shear strength of landslide clay and mineralogical transformations. The landslide clay samples for experimental use were obtained from a real landslide site in Ehime Prefecture, Japan.

2. Practical Considerations and Limitations

The practical implementation of chemical-based landslide stabilization through artificial cation exchange involves many challenges. For example, reaching landslide slip surfaces is inherently complex due to their irregular geometry and spatial variability. However, existing investigation and remediation techniques, such as inclined drilling, drainage wells, and subsurface galleries, can be used to access weak clay layers identified through borehole logging, inclinometer measurements, and geophysical surveys. The proposed approach is therefore intended as a localized or targeted treatment applied to identified shear zones (as also illustrated in Figure 1) rather than a uniform treatment of the entire landslide mass.

Moreover, the permeation of chemical solutions through fine-grained soils is limited by low hydraulic conductivity. In landslides, however, slip surfaces are typically composed of mobilized or heavily sheared clayey material with reoriented clay particles, micro fissures, and shear-induced fabric, which can locally enhance permeability relative to intact clay. Ion exchange is therefore expected to occur preferentially in these zones. Low-pressure injections or controlled infiltration methods are required to promote chemical interaction while avoiding hydraulic fracturing or reactivation of slope movement.

The experimental results presented in this study are derived from controlled laboratory conditions and should be interpreted accordingly. While the observed changes in mineralogical characteristics and residual shear strength provide valuable insight into governing mechanisms, direct extrapolation to field-scale landslides is not straightforward. Natural slopes exhibit complex stress paths, heterogeneous mineralogy, and variable groundwater regimes that cannot be fully replicated in laboratory testing. Consequently, the results should be regarded as mechanistic evidence rather than direct design values.

Furthermore, groundwater circulation represents an additional controlling factor in field applications. Groundwater flow may dilute the introduced or injected salts, transport the ions away from the treatment zone, or gradually reverse cation exchange through mass-action effects [20,21]. These processes suggest that chemical stabilization may require repeated application, recharge, or integration with drainage measures to regulate pore-water chemistry. Long-term effectiveness would therefore depend on site-specific hydrogeological conditions, monitoring of groundwater chemistry, and evaluation of ion persistence over time.

3. Materials and Methods

3.1. Landslide Soil Sample

The landslide soil samples for laboratory experiments for this study were obtained from the Nishinotani landslide area in Kumakogen Town, Ehime Prefecture, Japan (Figure 2). The landslide is a creeping-type mass movement and has a few centimeters of displacement per year. It is situated near the boundary between Ehime and Kochi Prefectures, to the north of National Route 33, which connects Matsuyama and Kochi cities. The affected area lies on a southeast-facing slope along the left bank of the Niyodo River. The slip surface is interpreted to form within a landslide mass composed of colluvial deposits of approximately 10 m to 15 m thickness.

The clay samples used in this study were collected from the clay-rich zone interpreted as the active slip surface of the Nishinotani landslide, based on borehole logging data, field observations, and previous geotechnical investigations conducted as part of the ongoing landslide mitigation project. The sampling depth corresponds to the shearing zone where long-term displacement was confirmed, and the shear zone material was at a near-residual state of shear. Although the samples were collected from a limited number of locations, the mineralogical composition and physical properties of the material are consistent with those typically observed in slip surface clays of creeping landslides in the region. To minimize the influence of local heterogeneity, the collected material was thoroughly remolded prior to testing, following standard practice for residual shear strength evaluation, which is known to be independent of initial soil fabric.

Owing to the large scale of the movement and the potential for extensive damage, a landslide prevention project supported by the Ministry of Land, Infrastructure, Transport, and Tourism (MLIT) is still underway. The countermeasure techniques include installation of drainage wells, horizontal boreholes, surface drainage channels, and anchor systems in selected areas.

To find out the mineralogical composition of the collected landslide clay samples, X-ray diffraction (XRD) tests were conducted according to existing and well-accepted standard procedures. The XRD patterns of the powder method (with particle size below 0.075 mm) and subsequent sediment and ethylene glycol treatment (sediment method with particle size below 0.002 mm) revealed that the landslide soil primarily contains feldspar and mica, along with a significant proportion of mixed-layer clay minerals, predominantly chlorite and the smectite, as typically indicated in Figure 3 (powder method X-ray diffraction pattern) and Figure 4 (ethylene glycol-treated sediment method X-ray diffraction pattern). Moreover, in

Table 1. The basic physical properties of the tested soil prior to any treatment.

Parameter	Value
Soil solid density (g/cm3)	2.75
Liquid Limit LL (%)	51.82
Plastic Limit PL (%)	33.66
Plasticity Index (PI)	18.16

, the basic physical properties of the tested clay material prior to any treatment are summarized, which were obtained in accordance with the Japanese standards for soil testing methods [22].

3.2. Characteristics of Smectite

The smectite group of clay minerals comprises species whose structure is built from stacked, negatively charged 2:1 layers. Each 2:1 layer consists of two continuous tetrahedral sheets (T), in which [SiO₄]⁴⁻ tetrahedra share three corners with adjacent tetrahedra to form a hexagonal, two-dimensional network in the single plane. Sandwiched between these tetrahedral sheets is an octahedral sheet (O), which may be dioctahedral (with divalent cations, such as Mg²⁺ or Fe²⁺, occupying two out of every three octahedral sheets) or trioctahedral (with all octahedral sites filled). The negative charge generated by isomorphous substitution in the 2:1 layer is balanced by hydrated, exchangeable cations—primarily Ca²⁺, Mg²⁺, and Na⁺—present in the interlayer spaces [23]. Smectites are expansive clay minerals frequently found in a wide range of surface and subsurface geological settings, including soils, sediments, and aquatic environments [24]. These sheet-like minerals are distinguished by their large specific surface area [25] and cationic exchange capacity due to isomorphic substitutions in their crystalline structure. Due to these properties, these compounds have a high potential to absorb inorganic cations, such as the major cations found in natural waters (Na⁺, Ca²⁺, Mg²⁺, and K⁺) [26]. Naturally occurring smectite clays typically contain both sodium (Na⁺) and calcium (Ca²⁺) ions as exchangeable cations that compensate for the negative charge on their 2:1 layered structure [27].

3.3. Sample Preparation

The soil samples collected from the Nishinotani landslide were sieved through a 425 μm sieve and oven-dried to achieve 0% water content before testing. Test samples were prepared to evaluate ion adsorption, residual shear strength, and mineralogical changes through ion chromatography, ring shear tests, and X-ray diffraction (XRD) tests, respectively. Initially, the effect of calcium chloride (CaCl₂) solution on strength and mineral properties was examined, followed by similar tests using magnesium chloride (MgCl₂) and potassium chloride (KCl) solutions. For each test, 200 g of soil was mixed with 120 mL of distilled water containing the respective salt at concentrations of 1000 mg/L, 6000 mg/L, and 12,000 mg/L. The mixtures were left to soak for 2 and 7 days, including deairing time, to allow sufficient ion adsorption. Calcium and magnesium, both divalent cations, were used to study intra-valence effects, while potassium, a monovalent cation common in clays, enabled comparison across different valence states.

3.4. Experimental Procedures

To evaluate the effectiveness of chemical treatment for improving soil shear strength, ring shear tests were performed on soil samples mixed with calcium chloride (CaCl₂), magnesium chloride (MgCl₂), and potassium chloride (KCl) solutions. The chemical substances used to make aqueous solutions are shown in

Table 2. Chemical substances used to make aqueous solutions.

Compound	Chemical Formula	Cation	Charge
Calcium Chloride	CaCl2	Ca2+	+2
Magnesium chloride	MgCl2	Mg2+	+2
Potassium chloride	KCl	K+	+1

.

The number of ions adsorbed at the end of soaking was determined using ion chromatography tests, and additionally, X-ray diffraction tests were also performed to compute the degree of crystallinity by observing changes in peak diffraction intensities. The details of the experimental procedures are presented in the subsequent section.

3.4.1. Ion Adsorption Tests

In this experiment, ion chromatography was used to measure the concentrations of ions adsorbed onto and released from the soil samples. The technique separates ions using a dilute solution (eluent) that flows through a column containing an ion-exchange material. As the sample passes through, different ions move at different speeds and are detected automatically by an electrical conductivity or ultraviolet detector.

Five cations (Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺) and four anions (Cl⁻, NO₂⁻, NO₃⁻, and SO₄²⁻) were analyzed. However, because the changes in Na⁺, Ca²⁺, Mg²⁺, and K⁺ were more significant than those in other ions, the discussion focuses on these four.

Eluent solutions for both cations and anions were prepared, and small volumes of the test solutions at various concentrations were filtered before analysis. The same procedure was applied to water samples collected after compaction and after different soaking periods. The eluent was placed in the ion chromatograph, with the flow rate set to 1.0 mL/min for cations and 1.2 mL/min for anions. A standard solution was first injected, and a calibration graph appeared after about 25 min. Then, the filtered samples were injected one by one, taking care to avoid air bubbles. Ion concentrations in the samples were determined by comparing the peak areas of the standard and sample graphs.

3.4.2. X-Ray Diffraction Analysis and Determination of Crystallinity

We used X-ray diffraction (XRD) tests to identify the clay and non-clay minerals in the clay samples and to see how their crystal structures change. When X-rays hit a solid with a regular atomic layout, the scattered rays interfere and make a pattern that tells us the crystal structure. In this study, we followed three routine procedures: (1) the powder method, in which, the sample was oven-dried, finely ground using an agate mortar, placed on a glass slide (powder holder), and scanned over a 2θ range of 2° to 50° at a rate of 2°/min to identify all mineral phases; (2) the sedimentation method, in which, a fixed amount of sample was dispersed in distilled water and allowed to settle so that the coarse particles (i.e., composed of quartz, feldspar, pyroxene, etc.) settled at the bottom and the clay particle-rich suspension in the upper portion was collected and placed on a glass slide in an oriented condition, which after drying on a hot plate was scanned over a 2θ range of 2° to 22° at a rate of 2°/min to emphasize the clay mineral peaks; and (3) the ethylene glycol method, in which, the oriented clay particle sample was exposed to ethylene glycol vapor at 60 °C, allowing the glycol molecules to penetrate the interlayer spaces, leading to expansion of smectite by approximately 2 Å but no change in chlorite, which enables the identification of smectite–chlorite mixed layers (as also illustrated in Figure 4). The scan range and speed are identical to those used in the sedimentation method. These X-ray diffraction procedures were carried out using equipment and protocols from Rigaku Electric [28] and Kato S. [29].

The height of a diffraction peak depends on how the atoms are arranged, which elements are present, their thermal vibrations, any defects in the crystal, and the instrument’s performance and settings.

The peak intensity in X-ray diffraction depends on factors such as atomic arrangement, atomic type, thermal vibration, structural defects, and instrument characteristics. Crystallinity represents the proportion of crystalline material relative to the total sample when both crystalline and amorphous phases are present. The degree of crystallinity is the ratio of crystalline minerals to whole mineral substances. The areas of crystalline peaks and amorphous material are shown in Figure 5.

In crystalline analysis, the crystalline percentage is calculated for a material made up of crystalline and amorphous parts. Crystallinity $X_c$ is calculated by separating the multiple peaks into scattering due to the crystalline part (normally sharp peaks) and scattering due to the amorphous part, and substituting each integrated intensity into Equation (1) [24].

$$X_c={\displaystyle \frac{I_c}{(I_c+I_a)}}\times 100 (\%)$$

(1)

where $X_c$ is crystallinity (%), $I_c$ is the area of crystalline peaks, and $(I_a+I_c)$ is the area of amorphous and crystalline peaks.

This formula shows that higher crystallinity means a larger share of well-ordered crystal material. In clays, this indicates a better-developed crystal structure and is closely tied to their physical behavior, so clays with high crystallinity generally have higher strength [29].

3.4.3. Ring Shear Tests

Finally, ring shear tests were conducted on the soil samples passing a 0.425 mm sieve. Initially, approximately 150 g of clay passing the 0.425 mm sieve was mixed with distilled water until it reached a slurry state and freely flowed on a metal plate. The slurry was then deaired in a vacuum chamber to ensure near 100% saturation, which was because all tests were to be conducted with fully saturated samples to avoid any unknown effects of the state of unsaturation (or metric suction) on the shear resistance. Two ring-shaped filter papers (outer diameter 12 cm, inner diameter 8 cm) were cut and used on the bottom and top of the ring shear apparatus to ensure the shearing condition was fully drained. The deaired slurry was poured into the apparatus, and vertical pressure was applied to consolidate the sample in stages until the target vertical stress of 100 kPa was achieved.

The ring shear apparatus used for shear strength tests on the soil samples treated with various chloride concentrations is a self-made, well-calibrated machine based on the concept of Bishop et al. [30]. A schematic diagram of the ring shear device employed in the tests is illustrated in Figure 6, together with the data presentation method and test specimen shape and dimensions. A ring shear apparatus has been widely used to measure the shear strength of soils. It has two major advantages: the area of the shear does not change during shearing, and unlimited continuous shear deformation can take place in the shear plane.

As also indicated in Figure 6, the specimen container has an inner and outer diameter of 8 cm and 12 cm, respectively, and a depth of about 3.2 cm. The specimen is sheared through a level of 0.7 cm above the base of the lower plate. It has already been proven by Bishop et al. that the ultimate residual strength is unaffected by the initial structure of the soil [31], so a remolded soil was used for all shear strength tests. Consolidation was performed in stages, and once the required vertical pressure of 100 kPa was achieved, the sample was prepared for shearing. After consolidation, a shear rate of 0.16 mm/min was adopted, which is sufficiently low to minimize the influence of shear rate on soil behavior, following the findings of Bhat et al. [32] for this particular ring shear apparatus. Then, the residual shear strength was determined after ensuring that shear stress had stabilized and reached the minimum attainable value. The ideal shear stress-displacement and volumetric strain-displacement curves are shown in Figure 6b.

Interpretation of the residual shear strength was made using the Mohr–Coulomb failure criterion, $\tau =c+\sigma tan \varphi$, originally proposed by Coulomb [33] and later refined by Mohr [34] and Terzaghi et al. [35]. Since fully saturated, remolded samples were used in the tests and in the residual state of shear, the influence of grain interlocking and interparticle force of adhesion can be completely ignored, the cohesion in the Mohr–Coulomb equation can be taken as zero, which leads the Mohr–Coulomb equation to Equation (2).

$${\varphi }_r={tan}^{-1}{\displaystyle \frac{{\tau }_r}{\sigma }}$$

(2)

where ${\varphi }_r$ is angle of residual internal friction; ${\tau }_r$ is residual shear stress, representing the minimum constant shear resistance measured for the test samples with different adsorbed ions; and $\sigma$ is the normal stress applied during the shearing, which in our test conditions was fixed at 100 kPa only, considering that the Mohr–Coulomb failure envelope remains a straight line and any amount of normal stress would yield the same angle of residual internal friction.

To investigate the influence of different chloride compounds on the residual strength of the soil, another 150 g of clay (passing through a 0.425 mm sieve) was mixed with aqueous solutions of CaCl₂, MgCl₂, and KCl at concentrations of 1000, 6000, and 12,000 mg/L. The mixtures were prepared into slurry form, deaired in a vacuum chamber to achieve near 100% saturation, and placed into the ring shear apparatus. Consolidation was again carried out under the vertical pressure of 100 kPa, and the specimens were then sheared, using the same procedure as for the distilled-water sample. After attaining the residual state of shear, each specimen was taken out of the ring shear box, and soil samples were collected for ion adsorption and XRD analyses.

Leachates collected after 2 and 7 days of soaking after consolidation were subjected to ion chromatography tests, which revealed the presence of calcium and sodium. Notably, sodium leaching occurred after 7 days at the highest MgCl₂ concentration (12,000 mg/L), while calcium leaching was observed at lower MgCl₂ concentrations.

4. Results and Discussion

4.1. Summary of the Test Results

The results as interpreted from the test data obtained from the XRD analyses (for mineral identification), ion chromatography (for quantifying ion adsorption), and ring shear tests (for residual strength), as already detailed in Section 3 are summarized in

Table 3. Summarized results of the tests performed on the landslide soil samples mixed with chloride compounds.

Pore Water Solution	Concentration (mg/L)	Ion Adsorption (mg/L)		Residual Internal Friction Angle (ϕr)		Peak Diffraction Intensity Smectite/Chlorite Mixture (cps)		Degree of Crystallinity (%)
		2 Days	7 Days	2 Days	7 Days	2 Days	7 Days	2 Days	7 Days
CaCl2	1000	69	76	21	22	2088	2956	27	31
	6000	689	802	24	24	3055	3125	34	34
	12,000	2378	2612	25	25	3640	3407	37	35
MgCl2	1000	18	25	24	22	3253	3050	41	39
	6000	88	160	23	20	2913	2013	40	38
	12,000	633	705	20	23	1778	2005	31	32
KCl	1000	458	475	22	22	1766	1813	25	34
	6000	2832	2874	22	22	2152	2174	36	37
	12,000	5850	5918	23	23	2531	3189	40	41

. Although it is difficult to interpret the understandings from these data in this table, it gives a quick picture of the concentrations of chloride solutions used in the tests, change in the amount of adsorbed ions in 2 and 7 days, change in the residual internal friction angle with the chloride concentration and period of soaking, and change in the degree of crystallinity. A detailed analysis of these changes and associated discussion will be performed in the subsequent sections.

4.2. Chloride Ion Uptake vs. Concentration

Figure 7 summarizes the relationship between cation adsorption and the concentration of chloride-based aqueous solutions. In general, the results indicate that ion adsorption increases with higher porewater salt concentrations for all three cations. However, extending the soaking period from 2 days to 7 days does not lead to a significant increase in adsorption, except in the case of Mg²⁺ at lower concentrations, where a modest increase is observed. This suggests that ion adsorption occurs relatively quickly after exposure to saline porewater, and prolonged soaking offers limited additional benefit. Therefore, enhancing ion adsorption is more effectively achieved by increasing the salt concentration in the porewater rather than extending the equilibration time at lower concentrations.

Cation adsorption capacity is governed by both ionic charge and charge density. Although divalent cations like Ca²⁺ and Mg²⁺ possess higher charges (valence = 2), their larger charge density hinders their effective penetration into the interlayer spaces of smectite. In contrast, the monovalent K⁺ ion (valence = 1) exhibits a smaller charge density, enabling it to access and fit into the negatively charged area within the smectite structure, thereby enhancing its adsorption within the clay matrix more easily [36]. Similar findings have been observed in our test results; the potassium chloride shows higher adsorption capacity compared to Ca and Mg.

4.3. XRD Response to Ca, Mg, and K Chloride-Mixed Clay Samples

The figure below summarizes the data obtained by XRD testing using the sedimentation method on samples taken after shearing when calcium chloride, magnesium chloride, and potassium chloride aqueous solutions were added, respectively.

As shown in Figure 8, Figure 9 and Figure 10, the tremolite peak near 2θ ≈ 10° remained nearly unchanged across all treatments, indicating its structural stability and limited participation in ion exchange. In contrast, the smectite–chlorite mixed-layer peak near 2θ ≈ 6° showed marked variation with different cations. Calcium and potassium adsorption increased peak intensity, reflecting enhanced structural ordering, whereas magnesium caused a decline, suggesting lattice disturbance. These trends demonstrate that the smectite layers are more sensitive to ionic type and hydration behavior than tremolite. Overall, Ca²⁺ and K⁺ contributed to structural stabilization, while Mg²⁺ produced slight disorder within the clay lattice.

Figure 11 and Figure 12 summarize the relationship between peak diffraction intensity and crystallinity with calcium, magnesium, and potassium ion adsorption.

The results presented in Figure 11 and Figure 12 indicate that peak diffraction intensity tends to increase with greater calcium ion adsorption, accompanied by a corresponding enhancement in crystallinity. This suggests strengthening the mineral structure due to calcium incorporation. In contrast, Figure 11 and Figure 12 reveal that as the amount of magnesium ion adsorption increases, both peak diffraction intensity and crystallinity exhibit a declining trend, likely due to the disruptive effect of magnesium on the clay’s layered structure. Zhang et al. [37] identified a detrimental effect of magnesium on both soil aggregate stability and hydraulic conductivity. They concluded that this was attributed to magnesium’s high hydration energy and larger hydration shell, which promoted greater clay swelling. The soils investigated in their study comprised minerals such as smectite, kaolinite, vermiculite, and illite. These observations are consistent with our experimental findings for samples treated with magnesium chloride.

Meanwhile, Figure 11 and Figure 12 demonstrate that potassium ion adsorption leads to an overall increase in diffraction intensity and crystallinity, indicating that potassium contributes positively to the structural ordering of the clay minerals. The size of the potassium ion, unlike the calcium ion, fits perfectly into the clay lattice structure and thus greatly reduces hydration and swelling of clays [38]. When the diffuse double layer between interacting clay particles shrinks sufficiently, van der Waals attractive forces begin to dominate over electrostatic repulsion, leading to particle aggregation that enhances soil stability and suppresses clay swelling [39]. The presence of potassium ions at sufficient concentrations may reduce the thickness of the diffused double layer, which reduces the expansion tendency of clays [40]. The governing mechanisms behind these changes were attributed to electrostatic (Coulomb) forces and intermolecular (van der Waals) forces. Specifically, the valence, molecular weight, and ionic radius of the exchangeable cations significantly influenced the interparticle bonding [41]. Calcium ions, with relatively high valence and moderate ionic radius, promoted stronger attraction between particles, thereby enhancing soil strength. Although magnesium ions possess a higher charge density, their smaller ionic radius and higher electronegativity resulted in weaker intermolecular forces, ultimately reducing strength [42,43]. Potassium ions, on the other hand, while exhibiting lower valence, fit precisely into the smectite structure due to their ideal ionic radius (~1.38 Å), resulting in enhanced structural stability and increased shear strength [44,45].

4.4. Residual Strength of Cation-Adsorbed Clays

The ring shear test results for the respective chloride compounds are summarized in Figure 13, Figure 14 and Figure 15.

Residual shear strength and the residual friction angle by the adsorption amount of different chloride solutions are summarized in Figure 16.

We observed that the soil mixed with an aqueous solution of calcium chloride and potassium chloride showed a peak increase in strength during the shear test because, as seen from higher diffraction intensity, the sample might be flocculated [46] and flocculation results in higher strength [47]. It was found that calcium ions are particularly effective in enhancing particle cohesion and reducing the swelling potential of clay soils by contributing to structural stabilization [48].

As illustrated in Figure 16, the residual friction angle exhibits a clear increasing trend with higher levels of calcium ion adsorption. Anson and Hawkins [49] reported a significant increase in the residual shear strength of sodium montmorillonite when the sample was treated and tested with 80 mg/L of Ca²⁺, compared to that tested with deionized water. However, further increases in calcium concentration resulted in only a marginal improvement in residual strength. This trend aligns closely with our findings, where clay samples mixed with calcium chloride solution exhibited a marked initial gain in residual strength, followed by a plateau at higher Ca²⁺ concentrations. Due to the relatively high valence and larger ionic radius of calcium, the degree of hydration was lower, which reduced basal spacing and limited smectite expansion [43,44].

Conversely, Figure 16 shows that increasing magnesium ion adsorption leads to a reduction in residual strengths. This behavior is attributed to ion exchange between magnesium and pre-existing calcium ions within the clay matrix, which appears to weaken interparticle bonding. Calcium leaching was observed at a lower MgCl₂ concentration. The preferential leaching of calcium observed during magnesium chloride treatment can be explained by competitive cation exchange and differences in hydration behavior between Mg²⁺ and Ca²⁺. Magnesium ions possess a higher hydration energy and a more strongly bound hydration shell than calcium ions, which limits their ability to form effective interparticle bridges despite their divalent charge. When Mg²⁺ is introduced into a clay system initially containing exchangeable Ca²⁺, magnesium competes for exchange sites on smectite surfaces and can displace calcium ions into the pore water. However, the newly adsorbed Mg²⁺ ions, due to their stronger hydration and smaller effective radius, produce weaker interparticle attraction than Ca²⁺. As a result, calcium leaching is accompanied by a net reduction in interparticle bonding, increased diffuse double layer thickness, and a decrease in residual shear strength. This mechanism explains why strength reduction is observed despite continued divalent cation exchange during MgCl₂ treatment.

The hydration energy of magnesium is higher than that of calcium, and the hydration ratio is also greater [50]. This causes a large separation distance between clay layers and less attraction between them to cause flocculation, causing strength loss in the clayey soil. Notably, after 7 days of immersion in a 12,000 mg/L magnesium chloride solution, the release of sodium ions was detected. This ion exchange is a contributing factor to the subsequent recovery or slight increase in residual strength, possibly due to reconfiguration of the soil structure. Magnesium chloride (MgCl₂) enhances the strength and stability of clay by initiating cation exchange, wherein divalent magnesium ions (Mg²⁺) replace monovalent sodium ions (Na⁺) on the clay surfaces, thereby reducing repulsion and improving particle bonding [51].

In the case of potassium ions, as depicted in Figure 16, the residual friction angle increased with greater potassium ion adsorption. Helle et al. [52] have reported that adding KCl to quick clay significantly improved its mechanical properties, increasing the undisturbed shear strength from less than 10 kPa to 25–30 kPa and the remolded shear strength from below 0.5 kPa to over 6 kPa, effectively eliminating its quick clay characteristics. This indicates the strength improvement of clayey soil using the potassium chloride compound, similar to the result obtained by our lab tests.

Although the results demonstrate that modifying pore-water chemistry can enhance residual shear strength, the persistence of this improvement depends on the long-term chemical boundary conditions. If groundwater chemistry later becomes less saline, the driving force for maintaining the exchanged cation population may decrease, and the physicochemical strengthening could be reduced through mass-action–controlled redistribution of ions. This reversibility has been highlighted in prior work on bentonite exposed to salt solutions, where changes in behavior were shown to evolve when the pore fluid was replaced by distilled water [20], and the broader implications for non-permanent cation effects were subsequently discussed [21]. Therefore, field implementation should consider not only the initial ion delivery but also the likelihood of post-treatment dilution, flushing, and chemical cycling.

4.5. Implications for Practical Application

4.5.1. Field Applicability

From a practical perspective, the application of chemical stabilization through artificial cation exchange requires careful consideration of field implementation constraints. Injection of chloride-based solutions into landslide slip surfaces would need to be performed using low-pressure grouting or permeation techniques similar to those currently employed for chemical grouting and groundwater conditioning, in order to avoid disturbance of the soil fabric or triggering additional deformation. Injection pressures must remain below the in situ effective stress to prevent hydraulic fracturing or reactivation of the slip surface. The required injection volumes would depend on the thickness and lateral continuity of the weak clay layer, its permeability, and the targeted ion concentration; however, the laboratory results suggest that strength enhancement is primarily governed by pore-water ionic concentration rather than prolonged exposure time, indicating that relatively limited volumes may be effective if adequately distributed.

Moreover, cost considerations represent both a potential advantage and a limitation of this approach. Chloride salts such as calcium and potassium chlorides are inexpensive and widely available compared to conventional structural countermeasures; however, costs associated with drilling, injection infrastructure, monitoring, and repeated treatments must be accounted for in large-scale applications. Previous field trials indicate that salt-based stabilization can achieve substantially lower unit costs than conventional measures when installation procedures are optimized, and treatment zones are appropriately spaced, suggesting a potentially favorable cost–benefit ratio under suitable site conditions [19]. Environmental impacts also require careful evaluation, as the introduction of chloride solutions into groundwater systems may increase salinity and potentially affect downstream ecosystems or water use. Consequently, field applications require controlled dosing, site-specific hydrogeological assessment, and post-treatment groundwater monitoring to ensure compliance with environmental regulations.

With respect to timeframes, the laboratory results indicate that ion adsorption and associated changes in residual shear strength occur rapidly, typically within a few days, suggesting that mechanical stabilization effects could develop over short timescales following treatment. Nevertheless, long-term performance under natural groundwater flow conditions remains uncertain, particularly due to potential ion dilution, leaching, or reversal of cation exchange through mass-action effects. Therefore, while chemical stabilization through cation exchange shows promise as a supplementary or localized landslide mitigation strategy, field-scale trials and long-term monitoring are essential to evaluate durability, environmental compatibility, and realistic stabilization timelines under natural conditions.

4.5.2. Environmental Consideration and Future Research Needs

While the present study focuses on laboratory-scale evaluation of the effects of chloride-based cation adsorption on the residual shear strength and mineralogical behavior of smectite-bearing landslide clay, the authors acknowledge that field application of chemical treatments requires a rigorous environmental and hydrogeological assessment. In this work, chloride concentrations were selected to systematically investigate mechanistic trends in ion adsorption, rather than to define environmentally permissible limits. Accordingly, quantitative threshold concentrations based on regional or international water-quality standards were not explicitly evaluated and should be addressed in future site-specific studies.

Potential migration of injected ions beyond the treatment zone may occur through dispersion, or episodic flushing driven by rainfall or groundwater recharge. The time scale and extent of such migration depend strongly on local hydrogeological conditions, including hydraulic conductivity, groundwater gradients, and the continuity of the shear zone. These processes were not examined in the present laboratory study but represent an essential component of future work, which should include hydrogeological characterization, tracer-based monitoring, and numerical transport modeling prior to field implementation.

The cumulative effects of repeated chemical treatments also warrant careful consideration. Conceptually, chloride substances may be introduced in solid or aqueous form through piezometric holes or purpose-designed boreholes arranged in a grid pattern along the slip surface. However, optimal injection spacing, dosage, and frequency remain uncertain at this stage. In practical applications, chemical treatment should be coupled with real-time displacement monitoring and groundwater-quality measurements, enabling adaptive control of injection volumes and intervals to avoid excessive salinity buildup while maintaining mechanical effectiveness.

The persistence or reversibility of chemically induced soil modifications is another critical uncertainty. Cation exchange and associated microstructural changes may be partially reversible under changing pore-water chemistry due to dilution or mass-action effects [20]. Consequently, the long-term effectiveness and environmental footprint of chemical stabilization can only be assessed once injection methods, treatment frequency, and spatial distribution are clearly defined. Compared with conventional stabilization measures—such as large-scale excavation, retaining structures, or cement-based grouting—chemical treatment may offer reduced physical disturbance and material use, but it requires strict regulatory oversight, monitoring, and validation through field-scale trials.

Overall, although the present experimental results demonstrate the potential of chloride-induced cation exchange to enhance residual shear strength, the proposed technique must be evaluated holistically at the field scale. This includes environmental risk assessment, regulatory compliance, and comparison with existing stabilization methods. The current study, therefore, can be viewed as a foundational step toward a conceptual field application strategy to be refined and validated through future pilot-scale and full-scale investigations, including those already reported in recent field studies by Di Maio [18] and Halle et al. [19].

5. Conclusions

Through integrated ring shear testing, ion chromatography, and X-ray diffraction, this study clarified how controlled cation environments govern the mechanical and mineralogical responses of smectite-rich clay from the Nishinotani landslide. Calcium chloride treatments, especially at higher concentrations and longer soaking durations (2 and 7 days), promoted greater Ca²⁺ adsorption, which translated into higher residual shear strength and residual friction angle, alongside increased XRD peak intensity and crystallinity. These improvements are consistent with enhanced interparticle bonding driven by Coulombic attraction between Ca²⁺ and negatively charged clay surfaces. In contrast, MgCl₂ solutions increased Mg²⁺ uptake but induced Ca²⁺ leaching, leading to measurable declines in both strength parameters and crystallinity. This weakening is attributable to Mg²⁺’s smaller effective radius and stronger hydration characteristics, which diminish interparticle bonding relative to Ca²⁺. Potassium chloride treatments showed the opposite trend: increasing K⁺ adsorption improved residual strength and friction angle, with XRD patterns indicating parallel gains in peak intensity and crystallinity, underscoring K⁺’s compatibility with the clay structure.

Overall, the results demonstrate a clear relationship between ion adsorption behavior and both soil strength and mineral structure. Calcium and potassium ions improve the stability of smectite-rich clay, whereas magnesium ions reduce it. Effective chemical stabilization, therefore, requires careful selection of exchangeable cations and consideration of the mechanisms governing ion exchange, aggregation, and microstructural organization. Calcium acts as an efficient stabilizer for smectite-bearing clayey soils, while potassium shows strong potential for long-term improvement. In contrast, magnesium should be applied with caution due to its destabilizing influence. These findings highlight a geochemically based and cost-effective approach for landslide mitigation and emphasize the need for field-scale studies to evaluate durability, ion transport, and long-term performance under variable hydrological and chemical conditions. Accordingly, they should be interpreted as laboratory-scale mechanistic evidence supporting chemical stabilization concepts, while comprehensive environmental assessment and field-scale validation remain essential prerequisites for practical implementation.

Also, certain limitations do exist in this experimental method. It is considered that coagulation by cations occurs at the same time as ion exchange. In this study, shear tests were conducted after ion exchange and subsequent coagulation. The ion exchange on-site must take place while the shear is ongoing. In other words, an aqueous solution is poured into the clay sample, and the cations are allowed to be adsorbed. During this process, coagulation may cause an increase in particle size, as well as changes in density and void ratio. Experiments must be conducted to determine how much these changes affect the shear strength with a view towards preparing for practical applications. Therefore, future work should include repeated testing and statistical evaluation to quantify experimental variability and further strengthen reliability in the observed ion-specific trends.