## 1 Introduction  
Landfill engineering mainly deals with preventing or mitigating the effects of environmental threats and nuisance. One of the major concerns when designing an embankment-type landfill or preparing a reclamation plan is the geotechnical safety of the structure. When analyzing the overall stability of a landfill, the main task is to precisely determine the mechanical and elasticity parameters of the landfill to evaluate potential displacements and to analyze the scenarios of waste-built slope failures [1]. Decomposition of landfill waste and its characteristics depends upon many factors such as waste composition, compaction, initial moisture content, inhibition, rate of moisture transport, amount of oxygen available, changes in a wide range of pH values, temperature, and other factors [2, 3]. The rate of chemical and biological reaction in a landfill generally increases with temperature and moisture. Waste decomposition in landfills tends to take place slowly over a very long period of time [1, 2, 3, 4]. Municipal waste deposited on landfills is a highly geotechnical and diverse material, depending on the morphological composition, age, and state of compaction. For all these reasons, when it comes to evaluating or modeling the geotechnical behavior of the entire structure, the task becomes very much challenging. This refers mainly to deformation and slope stability analyses of embankment-type landfills. Due to continuous operation of the landfill, the main challenge in the present study was to control the entire process of geometry engineering to assure full safety of the entire structure. For this purpose, the pioneering solution at that time was the introduction of observational methods as an approach allowing the necessary modifications to a development plan at every stage of the geotechnical work [5; 6]. This method allows the introduction of continuous amendments to design solutions during the construction, exploitation, and reclamation process at landfills. At that time, and even nowadays, the approach of applying similar solutions is not usually used for contaminated sites. Predicting the behavior of a landfill body and the subsoil is a very challenging task. It is mainly due to difficulties in the precise determination of waste material parameters and adopting accurate computation models. Observational methods also allow back-analysis to be used to investigate the material properties and perform validations of predicted behavior of the landfill subsoil and structural elements [7]. The present study concerns an old municipal landfill with geometry that needed to be adjusted and engineered due to waste capacity expansion requirements, for a very limited area. For this purpose, a number of slope improvement techniques were used. They mainly involved geosynhetics applications, a dewatering system design, and retaining wall and berm construction. Geosynhetics have recently found wide application in the design and construction of embankment-type landfills. This application has been triggered by the economic and technical advantages that geosynhetics can offer when compared to more traditional materials [8; 9; 10]. The geogrids used in the construction process should maintain their performance characteristics up to 100 years, according to the standards. Due to the ageing process, present especially at such sites as landfills, the life design of the geogrid would depend on the exploitation time [11; 12]. The short-term effects such as installation damage that reduce the maximum tensile strength decreases the allowable tensile strength of the material. A creeping process, aging by oxidation and abrasion, is also of major concern as it results in long-term strength loss [13; 14; 15; 16]. One of the main challenges on landfills is to secure the slope stability. However, given the conditions prevailing at waste landfills, the geosynhetics are exposed to a very hazardous environment.