# Effect of Mineral Additives on the Behavior of an Expansive Soil for Use in Highway Subgrade Soils
## 1 Introduction  
With the fast development of infrastructure construction in Kenya, many highways, municipal roads, and railway lines have been constructed in recent years, such as the Nairobi-Mombasa railway line and Southern Bypass in Nairobi. The construction of the subgrade frequently encounters geotechnical engineering issues caused by black cotton soil (BCS). BCS is a type of clay with high swelling potential when exposed to water [1, 2] and is mostly found in the plain region of neighboring Nairobi, in lowlands on the northeastern coast, and in the Victoria lake region of Kenya [3]. The major components in the BCS were found to be clay minerals including montmorillonite, chlorite, vermiculite, and illite, etc. [4]. Such a high content of clay minerals is the main reason for the high expansion of the Nairobi BCS [5]. The water swelling behavior causes serious problems in highway construction if BCSs are directly used for the foundation [6]; otherwise, the BCSs have to be excavated and thrown away. A case in point found in Kenya is that the BCSs were always piled up by the roadside due to the fast development of highway constructions. The increase of the discarded BCS results in serious environmental disaster. To overcome these problems, a facile but low-cost strategy has to be found to make use of such clay in highway construction. Soil stabilization is the best-known method of making use of clays and can decrease the volume change of such clay due to change in water with the help of stabilizers. Such stabilizers are mixed withthe soil to reduce the swelling potential and plasticity of expansive clay, improving the durability and strength of the soil [7; 8; 9; 10; 11; 12; 13]. The most commonly used stabilizers for soil modification are hydrated lime, ordinary Portland cement, petroleum sulfonate, asphalt, and certain resins [14; 15; 16; 17; 18]. [PERSON] et al. [19] studied soil stabilization using the Atterberg limits, swell percent, and swell pressure tests, and found that lime significantly improved the physical-mechanical properties of expansive soil. pH, electrical conductivity, and X-ray diffraction (XRD) tests were conducted on an expansive soil treated with lime, and [PERSON] [20; 21] researched the lime-soil reaction over a short term and the progression of the pozzolanic reaction over a longer term. Industrial byproducts/wastes such as fly ash, cement kiln dust, and rich husk ash can bind soil particles together and reduce water absorption by clay particles due to their pozzolanic properties [22; 23; 24; 25; 26; 27]. Atterberg limits, compaction, California bearing ratio (CBR), and unconfined compressive strength (UCS) tests were performed on expansive soil, and [PERSON] and [PERSON] [28] found that the optimum physical-mechanical properties of the expansive soil were obtained with the addition of up to 8% pyroclastic rock dust content. However, the heavy use of these stabilizers will cause a sharp increase in construction cost. A local natural material which is a vast resource and easy for manufacturing needs to be found to decrease the use of conventional stabilizers. Natural volcanic ash (VA), which is composed of mainly rock minerals with minor amounts of volcanic glass fragments, is found abundantly in Kenya. Its pozzolanic properties make VA able to bind clay particles together and reduce the water swelling of clay. As a result, VA-stabilized soil presents a decrease in the plasticity index and expansive potential and an increase in the strength and durability [29]. Unfortunately, the insufficiency of calcium oxide means that VA cannot obviously improve the strength of clay [30]. The addition of natural pozzolana has a minor effect on the plasticity index of the clays. Natural pozzolana is unsuitable for use alone in soil stabilization. In limited research, VA was used to stabilize clay soil in association with other stabilizers such as ordinary Portland cement and hydrated lime [31].