## 1 Introduction
According to [PERSON] [23] and based on common observations of geotextile performance, nonwoven geotextiles tend to be more permeable, be better separators, and be more flexible than woven geotextiles. In woven geotextiles, the mechanical strength, durability, and effectiveness for erosion control are enhanced compared with nonwoven geotextiles. Despite the enhancement in properties, woven geotextiles tend to be less flexible and offer less filtration than nonwoven geotextiles. In this research, we used nonwoven geotextiles because they are cheaper and faster to produce, making them suitable for limited-life geotextiles made from coir. Moreover, nonwoven geotextiles present biaxial tensile properties [24] ideal for slope reinforcement. Most previous studies on coir geotextiles have focused on their performance for soil erosion control. This study on coir geotextiles was conducted to observe their performance when utilized as soil reinforcement for slope stabilization. Since coir is made from natural fibers, it is expected to have lower mechanical strength and shortcomings relative to synthetic fibers when it comes to durability due to degradation. Accordingly, we explored the mechanical performance of coir geotextiles under various chemical treatments and exposures to evaluate the viability of the product when used as an alternative material for slope reinforcement. The present study focused on addressing the weaknesses of coir geotextiles and improving their mechanical properties in slope stability applications by applying two treatment methods: alkaline treatment, i.e., mercerization, and alkaline peroxide treatment, i.e., bleaching. Since geotextiles are commonly placed on slopes or in contact with the ground, they are exposed to numerous factors that affect their strength and durability. In this study, we also aimed to evaluate the strength performance of coir geotextiles when exposed to environmental conditions such as field and groundwater exposure. Additionally, the effectiveness of coir geotextiles in slope stability applications was verified through slope stability analysis simulations using Rocccience-Slide2 software. The findings of this study can help to identify the most suitable treatment method to improve the strength of coir geotextiles exposed to environmental conditions leading to biological and chemical degradation. It is expected that this study can contribute to enhancing the performance of coir geotextiles, making them a viable alternative to synthetic geotextiles. Coir fibers are considered waste material in the agricultural sector. Coconut fiber is one of the most famous forms of waste from coconut production [25]. The world produces around 30 million tons of coconuts, mostly coming from tropical countries in South America or Southeast Asia. Coconuts are abundant in the Philippines, and according to [PERSON] et al. [26], the Philippines uses 14.69 billion coconuts per year for different purposes; however, when coconut fiber is extracted, around 9 billion husks from these coconuts are left out or burnt in the field. As such, using coir geotextiles to solve geotechnical engineering problems can be regarded as a sustainable solution to address the waste management problem of the agricultural industry, in addition to reducing the cost of slope stabilization methods. The use of natural fiber geotextiles conforms to green concepts in engineering, promoting environmental benefits.