The geotechnical characterization of residual soils is a complex matter and is not always successful because current interpretation methodologies dedicated to sedimentary soils do not adequately respond to the behavior of this type of soils. The problem has been under scope by several Portuguese and international institutions. The work carried out in the experimental Site of the Polytechnic Institute of Guarda (IPG) since 2003, constituted by residual soils and decomposed rocks of the local granite massif, is highlighted herein. The work was strongly supported by MOTA-ENGIL (Portuguese construction company) and the Laboratory of Math Engineering (LEMA, Polytechnic Institute of Porto). The characterization of the test site and the respective research work is presented. The research work involved interpretation of in situ tests (SDMT, SCPTu, PMT, SPT, DPSH, and geophysical tests), tests in controlled chambers (DMT, geophysical, and suction tests), and laboratory tests (oedometric tests, direct shear tests, and triaxial tests with several stress paths). The tests were performed on natural structured soils, artificially cemented mixtures, and unstructured soils. Advanced math and statistical analysis were applied in the development of new correlations to obtain geotechnical parameters representative of these soils. Furthermore, the work also allowed to recognize the physical characteristics of the materials and better understand their mechanical behavior.

Triaxial tests are performed for remolded, artificially isotropic, and anisotropic structured samples under undrained conditions at confining pressures of 25, 100, and 200 kPa. Based on these test results, a binary-medium constitutive model is formulated based on homogenization theory and a breakage mechanism to describe the behaviors of structured soils. In this model, the binary-medium material is idealized as a representative volume element (RVE) composed of bonded elements, whose mechanical behaviors are expressed by the linearly elastic model, and frictional elements, whose mechanical behaviors are described by the double-yield surfaces constitutive model. The parameters of the bonded and frictional elements are determined from the test results of structured and remolded samples, respectively. The expressions for the breakage ratio and local stress coefficient matrix are introduced, and their parameters are provided. The computed results are compared with the test results, demonstrating that the model can reflect the main deformation features of structured soil relatively well, including the influence of anisotropy, gradual damage to particle bonding, and pore development.

Structured soils exhibit significantly different mechanical behaviors than reconstituted soils because of the influence of their structure. A theoretical study of the structured soils is carried out in this paper. A newly defined variable-relative structure degree-was used to quantify the integrity of the soil structure during compression based on intrinsic compression curves of the intact structured soils. Also, a new volume change equation for structured soils was developed by using effective stress and relative structure degree as variables. The volume change equation provides the interpretation of the nonlinear compression curve of structured soils in the space of void ratio against logarithmic mean effective stress. The proposed approach for structured soils was extended to the triaxial stress state by introducing equivalent, current, and normal yield surfaces, so that the influence of stress history and soil structure could be considered in the model. The characteristics of the proposed model were illustrated through simulations of the influence of soil structure and stress history. The proposed model was validated by making comparisons between experimental data and model predictions.