Microstructural analysis of collapsible soil before and after collapse and with loading and unloading cycle

¹ UFPE, Civil Engineering Department, 50740–550 Av. da Arquitetura, Brazil
² UFPE, Nuclear Energy Department, 50740–545 Av. Professor Luiz Freire, 1000, Brazil

^* Corresponding author: iltonalves@gmail.com

Abstract

Aspects related to the volume variation of collapsible soils due to the change in water content are influenced by several factors, including the microstructure. The microstructure analysis of the soil is especially important in order to understand these aspects. This paper shows a comparative analysis of the microstructure of three samples of a collapsible Yellowish Red Sand from the district of Petrolina (northeast of Brazil), carried out by means of two techniques: Scanning Electron Microscope (SEM) and X-Ray Computed Tomography. Three undisturbed sample were used: the first no pressure was applied, in the second an one-dimensional compression of 160 kPa was applied and then the sample was unloaded, and in the third, between the compression and decompression, the sample was flooded with the occurrence of structural collapse. In the first sample, the soil microstructure is a matrix with a predominance of grains of sand clothed with iluvial clay with interconnected pores that confers instability. The second sample showed a reduction in macropores after loading. In the third sample the structure remained with a simple packing arrangement, however, there is a greater packing between the grains, in addition to the wetting-induced softening of the clay particles that fill in the voids. It is also observed, through the radiodensity analysis that at the top and bottom of the second and third samples there was an increase in volume due to the stress relief that is not observed in the central section of the samples. The two microstructure analysis techniques are complementary.