Stabilization of Weathered Clay Shale Using Propylene Glycol and Potassium Chloride as An Embankment Material Alternative

The strength value of unweathered clay shale stacked beneath the soil surface is high. Clay shale will deform and lose its strength if exposed to the surface and changes in weather conditions due to brittleness caused by weathering. Thus, it would be dangerous if used as embankment material due to its sensitiveness. This study analyses the effect of adding propylene glycol and potassium chloride to weathered clay shale. Propylene glycol and potassium chloride are commonly used as a drilling fluid to prevent the degradation of the shale layer. They can increase the stability of clay shale by making it more durable when in contact with the wetting and drying process. The results from this study show that propylene glycol, added to clay shale, can increase the value of soaked CBR to twice the initial value and decrease the swelling percentage. Adding potassium chloride to the clay shale can also increase the value of soaked CBR to twice the initial value and reduce the swelling percentage. However, the combination of Propylene glycol and potassium chloride cannot increase the value of soaked CBR and could even shrink the sample and reduce the optimum water content.

Clay shale; CBR Value; Potassium Chloride; Propylene Glycol; Stabilization

The engineering behavior of shale is a highly complex subject. Clay shale is firm and stable in undisturbed conditions or when stacked underneath the ground surface, thus showing high undrained shear strength. If it is exposed to the open air, sunlight, and water, it will be weathered quickly and transform from hard rock to soft clay (Alatas et al., 2015) an change properties from high shear strength value to low shear strength value (Adisurya & Makarim, 2022; Lee et al., 2001). It can be caused by shale's brittleness and its low durability, which is its primary characteristic (Widjaja, 2008), especially when it has contacted to water or if it is exposed to the surface, as shown in Figure 1 (Sadisun et al., 2010).

Occasionally, the soil near the location of embankment construction did not meet the specifications of materials. There are some options to solve this situation, including transporting soils from other locations that are further away from the location or using additives to improve the local soil material (Yusuf & Zava, 2019). Due to the scarcity of material around the embankment location in several parts of the globe, including the United States of America, shale is often used as the embankment material. Yet, the usage of shale as embankment material usually comes with a high cost of maintenance and repair due to the condition of the shale that can deteriorate following the degradation of physical and mechanical properties caused by the characteristic of the shale itself. Therefore, the most common issues discovered when using shale as an embankment material are excessive settlement and instability as the embankment material degrades due to weathering. A more serious problem will occur, especially in wet areas where the embankment is subjected to repeated wetting and drying conditions. This condition will lead to faster shale degradation and embankment stability issues (Gomez-Gutierrez, 2013).

Figure 1 Illustration of physical disintegration of clay shale (Sadisun et al., 2010).

    According to the geological map of Indonesia, there is a pretty large clay shale formation in Sentul, West Java, and some of them are exposed to the surface. Figure 2 depicts the Jatiluhur formation (shown in green color). Some construction problems have been documented related to the clay shale deposits near the Sentul area, e.g., a landslide during the construction of Wisma Atlet Hambalang (Alatas et al., 2015), as well as some local problems such as crack and collapse road, and slope failure in the Sentul Area (Nuryanto, 2021) that give disadvantages for the surroundings. It is assumed that these problems are related to the degradation of the clay shale layer.

Figure 2 Jatiluhur clay shale formation in Sentul, West Java (Systematic Geological Map, Indonesia)

Referring to the conducted research (Widjaja, 2008; Oktaviani et al., 2018), clay shale from Sentul, West Java, has a range of durability values ranging from low to high depending on the samples taken. The XRD result shows that clay shale in this formation, especially for the minerals that comprise the Clay Shale in Sentul Area, shows less potential for swelling than quartz minerals. However, it also contains montmorillonite and illite (Oktaviani et al., 2018).
    Most shale layers are also stacked beneath the ground surface and discovered during the drilling process. Because of the shale's water-sensitive nature, this usually results in the boring well-becoming unstable. An important consideration in the drilling process is protecting the water-sensitive shale and reducing problems caused by water absorption (Patel, 2009). Drilling fluid is one of the solutions that can be implemented to prevent shale formation degradation during the drilling process. Water-based drilling fluids, such as inorganic salts and organic polymers, are widely used due to their relatively low environmental impact (Lee et al., 2001). The presence of salts in clay shale materials reduces swelling potential while increasing shale strength (Lyu et al., 2018). One example of inorganic salt is potassium chloride, which is known as one of the inhibitors for preventing shale layer degradation. High concentrations of potassium salts, such as potassium chloride, are mostly used due to the effective inhibition level among solutions (Zhong et al., 2016). The concentration of such additives between 2% to as high as 37% is frequently recommended in treating drilling fluids to minimize swelling of the sensitive clay. Salt will prevent swelling through various mechanisms (Patel, 2009). Potassium chloride is also a common inhibitive agent to prevent the shale from swelling. When it interacts with clay minerals, it has higher bonding energy, preventing water from inhibiting the clay mineral more than another cation. It also requires only minor heat to form a non-expandable compound (Ghaleh, et al., 2020). According to the Unified Soil Classification System (USCS), KCl or potassium chloride can also increase the consistency of clay soils and change the clay class from clay with low plasticity to silt with low plasticity (Arasan & Yetimoglu, 2007).

In addition to inorganic salts, polymer additives are commonly used to reduce shale hydration. In addition to inorganic cations, polymer additives and water-soluble organic polar compounds such as glycerol, glycol, and sorbitol, can also provide shale stabilization and swelling inhibition. Due to their slightly moderate viscosity, hygroscopicity, colorlessness, and odorlessness, glycols with a molecular weight of less than 200, such as ethylene glycol, propylene glycol, butylene glycol, are suitable for this purpose (Lee et al., 2001). Meanwhile, intercalation is the process by which an organic polymer compound inhibits clay minerals. This intercalation process will prevent the hydration of clay minerals when in contact with water that may cause disintegration and reduction of durability (Zhang, et al., 2016)

Because clay shale has a tendency to soft soil behavior after being exposed to the surface, it must be stabilized to improve its strength. One solution is adding cement to the soil (Damoerin et al., 2015). This research uses potassium chloride and propylene glycol as additives to stabilize clay shale and prevent its degradation when used as embankment material. The experiment aims to observe the change of characteristics of clay shale. 

    Sentul Clay shale is composed mainly of clay minerals with low plasticity and swelling potential. Nonetheless, degradation of clay shale still occurs due to the primary characteristic of shale that degrades when exposed to water on the surface. The initial CBR result of the unsoaked condition of compacted clay shale is 15%, but the CBR result for the soaked sample has a significant difference from the unsoaked condition, which is 2.5%. Propylene glycol and potassium chloride can stabilize clay shale minerals by preventing them from coming into contact with water, which causes clay shale to degrade. When mixed with clay shale, propylene glycol can increase the value of soaked CBR by up to twice that of the first set of soaked samples and reduce swelling percentage from 2.5% to 0.3%. Adding potassium chloride to clay shale may also increase the value of soaked CBR twice as much as the first set of soaked samples and reduce swelling potential from 2.5% to 0.08%. The mixture of propylene glycol and potassium chloride will cause the change in optimum moisture content from the sample. The mix of these additives may also be ineffective at increasing the value of unsoaked and soaked CBR in clay shale samples. When combined, these additives cause a negative swelling number in the clay shale sample, causing it to shrink.

The Authors would like to deliver gratitude to Soil Mechanics Laboratory, Civil, and Environmental Engineering Department, Universitas Indonesia, for supporting this study, and Mr. Idrus M. Alatas for the great discussion related to clay shale topics.
   The authors also wish to thank the PUTI Grant of Universitas Indonesia with the following contract NKB-3339/UN2.RST/HKP.05.00/2020 for supporting this research.