A new approach in studying the engineering behavior and mechanical properties of artificial bonded soils in the laboratory

Abstract

The construction of structures on structured soils or the exploitation of such materials for construction purposes, such as in road pavement projects, has gained more importance with time. In some parts of the world, their study has become a necessity. Such soils, like residual soils, are widely encountered in tropical and subtropical regions. Even though their names may vary according to local culture or their morphology, they have all in common the bond structures. This property is a key parameter of those soils. However, to better study their behavior, the use of the artificial bonded sample in the laboratory has been adopted, offering an effective simulation. In the present study, the behavior of residual soil-like has been investigated under undrained conditions in triaxial equipment by using a large number of artificial samples made in the laboratory. The artificial bonded and unbonded samples were made from a mixture of sand, kaolin, and water. A thermal process was applied for the bonded specimens, whereas the unbonded samples were not fired. A preliminary investigation was carried out on four different particle size distribution curves. In those gradation curves, the dry ratio of kaolin/sand, and the kaolin particle size distribution paths, were kept the same, only the sand grain size distribution was varied. The study was conducted on the chosen best-fitted gradation curve of sand-kaolin. Besides the triaxial tests, direct shear box apparatus was also used, for comparative purposes. For every type of the tested material, three different initial effective confining pressures or normal stresses were applied. Throughout this process, five different bonding levels were used. Several properties of such soils were examined, among them: the stress-strains, the pore water pressure evolution, the stress ratio, other strength parameters, and so on. The equivalent artificial bonded specimens, but in an unbonded state, were used to gain a better understanding of their mechanical characteristics. A novel approach was investigated and established, based on a new parameter called bonding index (B_i). This parameter was set from the bounding surface, which is one of the most important features of bonded soils studied under triaxial tests. The proposed method was evaluated as an effective and practical one. The strength parameters of the bonded soils such as the cohesion intercept, the angle of internal friction, the peak strength, and the stress ratio, were found to be straightly related to B_i. The latter asserted well the enhancement of bonding. Furthermore, B_i would be used to define the confining stress level, from which a B_i close to zero value implies the highest stress level for the artificial bonded soils. However, independent of the stress level, all unbonded soils display a B_i equal to zero value. The coupled effect of B_i and the confining pressure was grouped in three main stages. The first stage, at lower confining stresses, where a remarkable high value of B_i is recorded. The second stage is a step of moderate stress and, the third stage, as where the smallest B_i value was observed. Every stage was associated with a particular behavior of those soils according to the bonding level in presence. It is worth pointing out that a soil sample of higher B_i was found to be less ductile. The suggested method was observed to be an appropriate alternative means for the geotechnical evaluation and analysis of the behavior of structured soil materials. Comparison from the results of both CIU tests and DST revealed a good agreement for weakly and unbonded samples, particularly for strength parameters, the cohesion intercept, and the angle of internal friction. However, for highly bonded materials important divergence was observed, with an overestimation from the DST results. A study of the debonding process was carried out through a new approach. This method was constructed from the deviatoric stress increment (∆q) against the axial strain (ε_a) curves, drawn in a natural scale. Six important features, points, were found to be typical of bonded soils, while only two of them were observed for unbonded samples. The first yield was identified at the initial point, after which the slope of ∆q decreased significantly coupled with the maximum pore water pressure increment 〖d∆u〗_max. This point revealed the debonding process starting point. The second point is at 〖∆q〗_max, at the second yield, a point of major loss of strength. The third and fourth points were at d∆u=0 and ∆q=0 (q_max), respectively; while the fifth point was identified as where 〖∆q〗_min. The last point was at the critical state or the equivalent state. Every point represented a particular behavior state of bonded soils. Throughout the study, it was observed that confining pressure influences considerably the response of bonded soils. For example, the aforementioned six features, specific to bonded soils, were found to be reduced to only two points, particularly for weakly and moderately bonded materials, with the increase of σ_3 from 30 kPa to 700 kPa. Furthermore, a bigger value of the bonding index was achieved at lower confining stress. Therefore, it is recommended, for a better understanding of the behavior of the bonded soil materials, to conduct such investigations at lower initial effective stress, especially for the analysis of the debonding process.