# Shear Strength Improvement of Clay Soil Stabilized by Coffee Husk Ash
## 1 Introduction  
Natural soils, especially clay soil, usually lack the mechanical and geotechnical properties required for construction projects and, hence, require treatment to achieve geotechnically acceptable conditions [1, 2]. Clay soils present some difficulties for geotechnical and civil engineers due to their weak strength, high sensitivity to moisture, and excessive swelling. Moreover, when clay soils are found above the groundwater table, it becomes problematic because it swells with an increase in moisture content and shrinks with a reduction in moisture content. In addition, significant changes in groundwater occur when rainfall lasts for a long period [3, 4]. Due to their intrinsic mineralogical behavior, these types of soils are well-known for their volume change behavior in response to moisture changes. Expansion of the soil results in cracks and structural failures, which include expansion in the pavements, fissures in the floors and walls, and distortions in the window and door frames [5]. Soil swelling causes cracks and construction collapse depending on the level of swelling; these distortions might be slight, moderate, or serious [6]. Waste disposal is becoming a major concern for the majority of countries around the world. The massive generation of these byproducts is producing environmental and financial difficulties [7, 8, 9]. In general, it is preferable to recycle waste materials rather than to allow them to decompose into the environment [10]. To limit their environmental impact, the utilization of these wastes as soil stabilization can be an excellent solution. To accomplish this goal, one strategy is to alter the physical and mechanical parameters of the clay soil using what are recognized as soil stabilization techniques [11, 12, 13, 14, 15]. The most notable approaches for resolving undesired soil behaviors and enhancing claysoil parameters include compaction, reinforcement, and additive stabilization [2; 16; 17]. Furthermore, the requirement to use locally sourced materials, to have low costs, and to make technological advancements have increased the competitiveness of soil-additive treatment [18; 19; 20; 21; 22]. Additionally, engineers can now use nanoscale stabilizers as additives to enhance the properties of soils as a result of technological advancements [14; 23; 24; 25; 26; 27]. The unique characteristics of these nanomaterials are due to the number of molecules and atoms present on their free surface, and the consequences that this has on the surface properties from the perspectives of physical, chemical, and reactivity considerations are evident [28]. Lime and cement are the most regularly applied additives in the building and road construction industry for unstable clay soil stabilization [16; 29; 30; 31]. However, conventional cementitious stabilizers such as cement are being questioned not only for their negative impact on the environment during manufacturing but also for their high costs [32]. From a geotechnical viewpoint, these ground improvements based on soil-cement and lime mixtures are related to the following basic reactions: flocculation, particle agglomeration, cation exchange, pozzolan reaction, and carbonation [33]. In reality, engineering properties can be rapidly improved by adding small amounts of lime or cement to clay soils. However, the effectiveness of this treatment is dependent on both the amount of addition used and the mineral content of the soil. The use of locally sourced materials is necessary for sustainable and cost-effective construction projects such as buildings, roads, and trains. Over the last couple of years, substantial research has been conducted on the use of agricultural waste in the manufacture of sustainable construction products. Due to the proper geotechnical and mineralogical analysis of waste materials as well as their interaction with soil, large-scale application is conceivable [11; 34; 35]. One of the waste materials in question is the byproduct of the coffee industry.