A permanent deformation predictive model for fine tropical soils considering the effects of the compaction moisture content on material selection
Introduction
In addition to the geotechnical properties, the mechanical responses of soils and granular materials used on base, subbase, and subgrade layers must be considered in pavement design. The experimental characterization of these properties considering the elastic and the plastic behaviors of the materials has been carried out by means of two tests: resilient modulus (RM) and permanent deformation (PD), respectively. These tests have been performed on repeated load triaxial (RLT) devices to simulate the cyclic loading of the vehicles in the field.
Different factors affect the behavior of the materials in RM and PD tests and the pavement structural responses in the field. Those include gradation, moisture content, traffic speed, magnitude, and history of loading cycles [1], [2], [3].
Regarding the moisture content, studies generally take either the post-compaction [4], [5], [6], [7] or the compaction moisture content [8], [9], [10], [11], [12] into account. The use of inadequate water content during compaction in the field can lead to the premature failure of the pavement system [11].
In general, the equilibrium moisture content for geotechnical materials of Brazilian pavement layers (subgrade, subbase, and eventually even the granular base) does not vary significantly throughout the year due to the climatic characteristics of the country, i.e., wet summer and dry winter. In both seasons, the incidence of high insolation is observed. The pavement layers tend to keep their optimum moisture content or dry slightly. This leads to an increase in suction, which in turn increases RM and decreases PD.
Within this context, it is appropriate to proceed with the pavement design by considering the parameters obtained in the optimum moisture content (OMC) condition. Thus, RM and PD tests have been performed in Brazil considering the OMC for pavement design. Hence, the PD predictive model considered in the newly released Brazilian mechanistic-empirical (ME) pavement design guide assumes that the test specimens are compacted at the OMC or close to it. In addition, the test standards generally allow the variation on the moisture content of the samples of about 0.5% for soils and 1.0% for granular materials (e.g., gravel and crushed rocks). However, the Brazilian practice is to admit variations of up to 2.0% with respect to the OMC in the field.
Among the types of soils used in the construction of pavement structures in Brazil, the fine tropical soils deserve attention. They are widely found in the Brazilian territory and have been often regarded as inadequate to be used as pavement materials when traditional analyses based on parameters obtained from the California Bearing Ratio (CBR) are adopted. However, it is well known by the pavement research community that CBR does not necessarily represent adequately the caracteristics of this type of soils. Instead, a methodology such as the Miniature, Compacted, Tropical (MCT) tests, which was developed specifically to consider the inherent characteristics of fine tropical soils, must be adopted. The newly released Brazilian mechanistic-empirical design guide (MeDiNa) no longer takes CBR into account for material selection, but rather the mechanical characteristics by the RM and PD determined from RLT tests, as well as the MCT classification.
In this sense, Lima et al. [12] analyzed the mechanical responses of pavement structures for four different fine tropical soils and observed that the compaction moisture content is a key and sensitive factor that may result in significantly high permanent deformations if an excessive amount of water above the optimum content is adopted. They also concluded that it may not be adequate to establish a universal allowable range for moisture content, such as the 2.0% adopted in Brazil. Instead, the individual material mechanical characteristics should be taken into account to indicate the allowable moisture content ranges.
Only a few PD models have been proposed and used in the literature [9], [10] to predict the plastic behavior of soils considering the moisture content factor. In general, the focus has been the evaluation of the effects on the RM. Thus, this paper aims to evaluate two PD models existing in the literature and to propose a new model that takes the effects of moisture on the mechanical behavior of fine tropical soils into account. This model can be potentially used as a tool to optimize the process of material selection. In addition, it can also be eventually incorporated in the Brazilian M-E design guide to provide more realistic pavement structural response predictions.
Section snippets
Experimental investigation
Six soils were subjected to PD tests with three diferent moisture contents. A statistical analysis was conducted to identify the factors that most influenced the PD results considering four of these materials. As shown in Fig. 1, the experimental program of this research was divided in two groups of tests: physical and mechanical characterizations.
Physical property tests were performed to characterize the gradation, the Atterberg limits (Liquid Limit - LL and Plastic Limit - PL), and the
Results and analysis
The PD as a function of the loading cycles obtained from the tests at the three moisture conditions specified in Table 1 are shown in Fig. 3. As expected, PD increased with the compaction moisture content. Moisture contents below the OMC resulted in low PD values. In addition, for all moisture conditions, the application of larger deviator stresses increased the PD accumulation.
Fig. 3 also indicates that the soil classified as lateritic by the MCT methodology, i.e., S1, presented lower PD at
Conclusions
This study evaluated the effects of the compaction moisture content on the permanent deformation (PD) of fine tropical soils. For that, physical and mechanical repeated triaxial load tests were performed. Statistical analyses were also conducted to identify the correlations and effects among factors that could significantly influence the accumulation of PD. Two models found in the literature that take the moisture content into account were evaluated and a new model was proposed in order to
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. The authors are also grateful for the financial support received from CENPES/PETROBRAS.
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