###### Abstract  
For thousands of years, the volcanic activity present along the Andes Mountain range has generated a large amount of pyroclastic material. As a result, around 60 percent of the soils present in Chile have a volcanic origin, of which, we can find soils derived from volcanic ash. These correspond to soils whose origin is the weathering of volcanic ash, which generates minerals such as allophane, imogolite, and halloysite. The presence of these minerals gives these soils unique geotechnical properties, such as high plasticity, low dry unit weight, and a unique internal structure. Subjecting these soils to extreme temperatures like those needed to perform standard laboratory tests produces changes in their structures, and thus in their geotechnical behavior. These changes are important to be aware of with respect to slope stability problems, embankment conformation, surface foundations, etc. In the present study, a type of soil found in Chile originating from the weathering of volcanic ash and locally named Trumao was studied. Due to its age and formation processes, the main minerals found in the soil are allophane and imogolite, and hence it belongs to the allophanic soil type. The material was studied in its natural state (undisturbed) and, after being oven-dried using common geotechnical tests, the behaviors of both samples were compared. The study shows that some properties are affected significantly by the oven-drying process, and thus it is not recommended to expose the material to high temperatures during geotechnical laboratory testing to avoid misleading results.  
residual soils; volcanic ash; geotechnical characterization; volcanic soils +
Footnote †: journal: Appl. Sci.  
Accepted: 5 December 2021  
Published: 9 December 2021  
In general, the geotechnical properties of soils derived from allophanic volcanic ash can be correlated to a variation in the temperature during the formation process and the internal structure of the soils [6]. Trumao changes the internal structure of its particles when exposed to high temperatures. For example, in preparing soils for geotechnical laboratory tests, in most cases, the samples achieve temperatures above 100 \({}^{\circ}\)C. Therefore, the preparation process could change the original structure, mechanical behavior, and physical characteristics of some soils. Thus, caution must be considered in studying the natural behavior of these soils when studied in the laboratory in order to prevent misleading results. Due to the high occurrence of these soils in Chile, they are commonly the foundation material of a wide variety of constructions and civil infrastructures. However, due to their geotechnical characteristics, these soils are not commonly used as embankment construction material. Furthermore, it is reported their usage as a filter material for water treatments [7; 8]. For these applications, the changes in the physical properties after oven-drying could affect the capacities of the filters and should be taken into consideration.  
The present research compares the geotechnical behavior of two samples of Trumao allophanic soil; one, in an undisturbed state (without physical alteration or heat exposure) and the other, in an oven-dried state (the sample achieves 110 \({}^{\circ}\)C in the oven and is remolded if the test requires it). The samples were collected in the Nuble Region of Chile. The comparisons were made on common civil engineering properties, such as Atterberg limits, specific gravity, modified Proctor, California Bearing Ratio (CBR), unconfined compression strength, sensitivity, deformation properties, and Mohr-Coulomb failure criterion parameters.