**1.1 Ground improvement methods**

Several ground improvement techniques are currently in use to improve the engineering properties of soils. These modification techniques have been divided into several categories [1].


The above mentioned soil modification techniques are not limited to any particular type of soil. It can be adopted for any soil depending on their suitability and ease of field applicability, economic constraints along with the availability of resources for their implementation on any particular site.

### **1.2 Soil reinforcement**

Among several ground improvement methods, soil reinforcement is an effective and dependable method for upgrading the strength and stability of various civil engineering construction practice including pavement, embankment, retaining structures, foundations and slopes. Reinforced soil is a composite mass in which tension resisting elements in different forms (geosynthetics, fibers etc.) are embedded to increase the strength, stiffness, compressibility and permeability of soils. After the earliest reinforcement in the form of galvanized steel strips of high tensile modulus, use of synthetic materials named as geosynthetics in different forms (geogrid, geotextile, geocomposite etc.), and of natural products (bamboo, jute, and coir) are being adopted in the form of sheets or meshes. In most applications, the conventional method of soil reinforcement is in a continuous planer form introduced within the soil mass in a definite pattern, resulting in the systematically reinforced soil [2].

The one-dimensional orientation of reinforcement is installed sequentially in alternating layers as per the design requirements of the structure.

### **1.3 Fiber-reinforced soil**

Fiber-reinforced soil has gained popularity in around last 35–40 years [2] where flexible, discrete fibers are being mixed within soil mass. Fibers act like tension resisting element which cause significant amendment in the various engineering aspects of soil including strength, stiffness, compressibility, permeability. Unlike conventional soil reinforcement methods, fiber-reinforced soil maintain strength uniformity within the soil mass by evading the generation of any weak plane during field placement. Fibers are available in abundance in natural and waste form, and also manufactured in desired properties known as synthetic fibers. Utilization of waste fibers (tyre derived fibers, plastic waste fibers etc.) for civil engineering work can help in solving disposal problems which will be cost effective and also help in enriching the environment.

The method of fiber reinforcement in soil is being used from ancient times where natural fibers in the form of straw were mixed in the soil brick to provide integrity by arresting the crack development [3]. The curiosity of fiber-reinforced soil in last century started by Waldron [4] when he investigated the effect of roots of plant and tree on the earth slope stability. With increasing attention, fiber reinforced soil is increasingly providing an option of its use behind retaining structure as backfill material [2], construction of embankments [5–7], slopes stabilization [8], earth retaining constructions [9] and clay liners [10].

The use of fibers in natural and synthetic form like coir, jute, wool, steel, nylon, polyester, polypropylene, and fiber glass as tension elements for various soil have been reported by other investigators by means of unconfined compression, CBR, direct shear and triaxial compression tests in the last 35–40 years [2]. However, the preliminary works was largely on fiber-reinforced sand where the influence of the key aspects such as fiber concentration, fiber aspect ratio, soil compaction level and testing environments on the overall performance of fiber-reinforced sand was studies [11–15].

The effects of fiber inclusion on clayey soils have been explored by direct shear tests [16–18], triaxial compression tests [19–22], unconfined compression tests [23–29], tensile strength tests [30], fiber pullout tests [31] and CBR tests [32–34]. The common findings of the past investigations on fiber-reinforced soil are that the fiber inclusion increases the stress–strain responses, UCS, soil ductility and CBR, and modify the post-peak strength reduction of soil. The inducement of shear strength happens up to some controlling fiber concentration and fiber length.

The fiber benefits depend on the bond strength and surficial interaction between soil and fiber [17]. The soil particle size also influences the shear strength of fiber reinforced soil [35]. Fiber reinforcement had also effectively reduced the amount and degree of desiccation and tension cracks development, suppressed the swelling potential, and increased the permeability of clay soils [36–38]. It has been noted that the compressive strength of fiber-reinforced soil is highly controlled by the size of specimen [39] and compaction state [28, 29].

In this present study, an attempt has been made to investigate the effect of glass fiber inclusion on the strength aspect of a clayey soil for its possible suitability for road pavement construction. The investigation has been carried out by conducting compaction, UCS, CBR and triaxial compression test by varying fiber content.
