Before the conquest, the Inca Empire consisted of some 10 million inhabitants. Political unification required a superb highway system that included at least 40,000 km of serviced roads. When an Inca road followed the contour of a mountain, earth embankments needed to be constructed. For stability, these earth embankments required the building of retaining walls. The Incas also built retaining walls for their agricultural terraces. These terraces were located in steep rock slopes. This study involves the stability analysis of the retaining walls built by the Incas. This analysis was made with respect to static and earthquake loads. The Inca walls were made by piling up rock pieces made of granite, a very durable material. The size of the rocks used varies between 10 cm and 2 meters. The source of the stability of the walls rests on the frictional resistance between the rock pieces. This frictional resistance was found to be greater than the pressures exerted by the soil abutting the walls. Since no cement was used in the building of the walls, these are freely drained. These retaining walls have been stable for more than 500 years. In light of this remarkable achievement in sustainable engineering, it is undoubtedly very important to investigate the reason for their long-term stability so that this knowledge can be applied to modern engineering design problems. The stability and sustainability analyses of the Inca walls were made using field, theoretical and numerical investigations.
Pre-Columbian civilizations are sometimes thought to be primitive, inferior societies in comparison to modern-day civilization. Some of the great achievements of pre- Columbian societies have even been credited to extra-terrestrial beings because they appear to be too advanced for such "primitive" cultures to construct. In his book, Chariots of the Gods, Erich Von Daniken  hypothesizes that our planet was visited by extra-terrestrial beings during ancient times. Von Daniken surmises that these visitors were welcomed as gods (as the Spanish Conquistadors were welcomed to Mexico by the Aztecs).