## 1 Introduction  
In the first decade of the 21 st century, migratory movement from the countryside to the city has been prevalent, especially in recent years, due to the economic crisis. This migratory pressure is more noticeable in large cities within Europe, such as the city of Madrid (Spain). To provide urban services and housing to meet population demand, it is necessary to occupy terrains characterized as problematic that were previously discarded due to their high content of potentially expansive clays. In general, this type of deposit is found in regions with arid and semi-arid climates, such as the Mediterranean strip, South Africa, southern North America, northern South America, etc. It is estimated that 28% of the existing geological formations contain clays with expansive capacity forming the subsoil of large cities such as London, Madrid, Cape Town, etc. A common solution would be to remove the clays to landfill, or to use loans of other materials with better geotechnical characteristics, which would require great economic resources and potentially cause environmental problems.  
The growing demand for urban land in the city of Madrid in recent years has made it necessary to use geotechnically problematic areas which have historically been avoided [1; 2; 3]. Without any doubt, the lands in the southeast of Madrid are the most relevant in this aspect, not only because they are the new and future areas of expansion of the capital city of Spain, but also because they pose a barrier to the social and economic growth of the city due to their clayey and potentially expansive nature [4].  
The construction of infrastructure in this area: roads, railway lines, and macro-housing developments (meaning urban actions with an occupation of 7,000,000 m\({}^{2}\) or more), has made it necessary to improve a series of design and construction procedures that take into consideration the presence of volumetrically unstable clay in an effort to reuse excavated material to minimize the environmental impact of creating new landfills and borrow pits [5; 6].  
There are no inert landfill areas in the area of new urban developments in the southeast of Madrid and its contiguous towns. This, in addition to the fact that the borrow pits of traditionally used materials are far from the area of action, makes the expansion of the urban network economically unfeasible. In order to overcome this problem, construction methods and procedures have been developed to make use of materials which, in accordance with the General Technical Specifications for Road and Bridge Work (PG-3) of the Spanish Ministry of Public Works, would be defined as marginal materials.  
This work aims to propose a constructive procedure that allows the reuse and implementation of these clay materials, improving their geotechnical properties. The proposed procedure is applied to a case study, the new urban extensions of the city of Madrid. The results show that it is possible to apply the proposed methodology in a generalized manner in the construction sector. Additionally, when using existing materials, the environmental impact associated with the expansion of these areas is drastically reduced, with fewer transport costs and less collection of materials in landfills. In parallel, the need for consumption of raw materials associated with this activity decreases.