## 2 Literature Review  
### Retaining Walls  
Retaining walls are constructed to sustain the lateral pressure of the earth behind them. They are structures used to contain soil or other loose materials when their natural steeps are undesirable, in the case when building linear infrastructures such as railways or roads [5]. Retaining walls are often an overlooked critical asset of infrastructures because they are constantly around us. Each year, globally, millions of square meters of retaining walls are constructed for private and public projects. Retaining walls save space, reduce impacts, and allow owners to get the most out of a given property or right-of-way. Thus, retaining walls are an important part of development projects today [8]. There are many kinds of retaining walls, with different forms and structural characteristics according to the dimensions and location [9]. Over the last three to four decades, due to the development of materials and enhancement in technical understanding of geotechnical engineering, different types of soil retention systems have evolved [10]. There are several classifications of types of earth retaining structures. In general terms, these structures may be classified into two groups, externally stabilizes walls and internally stabilized walls. The examples of first category are gravity walls, reinforced concrete cantilever and reinforced concrete counterfort walls. These walls are essentially characterized by the concept that the lateral earth pressures due to self-weight of the retained fill and accompanied surcharge loads are carried by the structural wall [10]. The construction sequence of these walls involves the casting of base and stem followed by backfilling with specified material. This requires a considerable amount of time as concrete has to be adequately cured and sufficient time spacing has to be allowed for concrete of previous lift to gain strength before the next lift is cast [10]. The internally stabilized walls include metal strip walls, geotextile reinforced walls and anchored earth walls. These walls comprise of horizontally laid reinforcements which carry most or all of the lateral earth pressure via soil-reinforcement interaction or via passive resistance from the anchor block [10]. This reduces the volume of concrete and steel reinforcement in the wall significantly, thus its construction is relatively fast speed. Retaining walls with relief shelves can be considered as a special type of retaining wall [11]. Another classification establishes three different groups: gravity walls, embedded walls and hybrid walls [2]. * Gravity walls: The purpose of a gravity wall is to avoid sliding and overturns throughout its own weight and by the friction of its base with the land. They are generally built over flat land before being refilled behind the wall. Given their characteristics, they are generally used in high lands, as per example, for retaining embankments in roads and railways. They can also be used in order to support excavations, which are under the natural level of the land. For these situations, the excavation is made in open air and the refilling is done behind the wall. These walls require an additional excavation and refill, as well as the occupation of the land during construction. One of the basic characteristics of the gravity walls is that the drainage is done through drain tubes located behind the wall with the purpose of reducing the water pressure. * Embedded walls: An embedded wall is the one which base penetrates in the land to achieve side support of the passive resistance of the land on the lower front part of the wall. This resistance can be used as a sole resistance to provide stability to the wall or it can also be used with moorings, supports or other support sources of side support to increase the stability. In this case, they are considered anchored walls or underpinned walls. Unlike with gravity walls, building embedded walls requires specialized equipment and in-situ operations. These walls need to be built before starting the excavation.