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.
Highlights
The study of the behavior of soils has had many advances due to the development of techniques and equipment capable of carrying out studies on increasingly smaller scales
Vilar and Rodrigues (2015) [03] carried out a survey of the collapsible soils published in different places around the world, and it seemed that 36 % of the analyzed soils were sandy, 38 % silty and 26 % clayey
The soil microstructure was a matrix with predominance of sand grains clothed with illuvial clay, which gives the character of instability
Summary
The study of the behavior of soils has had many advances due to the development of techniques and equipment capable of carrying out studies on increasingly smaller scales. This greatly improved the understanding of the behavior of metastable soils. The study of mechanism of collapse phenomenon showed that it was due the local failure between grain or peds and was best described in terms of two separated effective stresses, applied stress and suction (Barden et al 1973 [01]). Collapsible soils are quite problematic, as they develop large variations in volume due to reduction of suction. The mechanism of collapse applies to different types of soils, not just in sands
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