## 7 Conclusions  
In traditional injection techniques, a mixture of water and cement with additives, or non-expanding resins, is generally used. The design objectives of these treatments are controlled by the nature and quantity of the components present in the mixture and the injection pressure, which determines the area of influence of the consolidation [14].  
The injection of polyurethane resins has a different behavior, due to the chemical nature of its components, polyurethane. In fact, they do not require any injection pressure and during the synthesis reaction a certain amount of carbon dioxide is generated which causes the volumetric expansion of the mixture (Figure 1). The solidified resin reaches the natural balance with the surrounding soil, when the pressure of swelling of the resin, resulting from the reaction, coincides with the average pressure of the confinement of the ground.  
The capacity of propagation of the resin in the different types of soil, can be established according to the following criteria. In gravels and sands the propagation is, mainly, by impregnation of the intergranular spaces. In clays, the voids and fractures present in the macrostructure of the soil are saturated by the resin, or compressed by the swelling pressure during the expansion of the mixture.  
The injection of expanding polyurethane resin into the soil is a technology for soil improvement that is also used for correct differential settlement in building. It is noteworthy that, because is a non-destructive method, this technology is especially suitable for historic buildings which are part of the country's architectural heritage.  
The principle of the technology is based on the injection of a certain volume of expanding polyurethane resin into the soil, below the foundation bearing surface which then expands, displacing the soil around the injection site [15; 16]. The expansion is accompanied by a reduction in swelling pressure and an increase in the average confining stress. The process stops when it reaches the equilibrium pressure.  
The resin penetrates into the terrain to be treated, increasing in volume and compressing the soil in all directions (radial expansion) favoring the pathways that offer the least resistance [3; 4]. The resin continues expanding until the soil prevents any further radial compression. At that point, the only possibility for expansion is an upward displacement.  
This process continues at each injection point until the lifting of the structure is first observed. When the initial lifting is observed, it means that the consolidating action is being directed vertically upward and that this is the direction that offers the least resistance, while the surrounding soil offers greater resistance in respect of the decrease in the structural load. This shows that the foundation soil has been compacted enough to withstand not only the increase of static loads, but also the loads developed as a result of the lifting [16].  
Due to the recent implantation in Spain of this technology, it is not possible to prove the efficacy of long-term treatment (more than 10 years).  
But new dynamic penetration tests and supervisions of the stabilized building are demonstrating that the mechanical properties of the treated ground continue being stable and that they have not diminished in the three years passed since the treatment.  
[PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON] and A.M.d.l.S. analyzed the data. All authors collaborated on the interpretation of the results and on the preparation of the manuscript.  
This research was funded by URETEK.  
The authors would like to thank URETEK for funding the research project titled \"Method Injecting Expanding Polyurethane Resin in Different Types of Soil\" carried out at the School of Civil Engineering at the Polytechnic University of Madrid, as well as the technical documentation provided.  
The authors declare no conflict of interest.