Evolution of Lateritic Soils Geotechnical Parameters during a Multi-Cyclic OPM Compaction and Correlation with Road Traffic ()
Gravel lateritic soils are intensively used in road geotechnical engineering. This material is largely representative of engineering soil all around the tropical African Countries [1,2]. Gravel lateritic soils from parts of Burkina Faso and Senegal (West Africa) are used to determine the evolution of the geotechnical parameters from one to ten cycles of modified Proctor compaction. This test procedure is non-common for geotechnical purposes and it was found suitable and finally adopted to describe how these problematic soils behave when submitted to a multi-cyclic set of Modified Proctor compactions (OPM) [3,4]. On another hand, we propose a correlation between the traffic and the cycles of compaction considered as the repeated load. From that, this work shows the generation of active fine particles, the decrease of the CBR index and also the mechanical characteristics (mainly the Young Modulus, E) that contribute at least to the main deformation of the road structure.
1. Introduction
This paper is primarily intended to demonstrate that under unpredicted traffic and repeated loading, properties of gravel lateritic soils used as pavement layer can significantly change. According to [5-10], gravel lateritic soils are very sensitive to an exceptional variation of stresses under which they are subjected in a pavement structural fill. Thus, it is expected that most of the physical and mechanical properties of gravel lateritic soils evolves during the design life.
It is then important to find an adequate method of testing that can deal with such behavior already known in the literature. It is then necessary to perform usual characterization tests on these kinds of materials by studying the evolution of their main properties under traffic such as gradation, plasticity, CBR (Californian Bearing ration), Los Angeles loss, Shear strength (UCT), etc.
To do this, tests are conducted so that they can simulate multi-cyclic axial loading generated by traffic loads. The first cycle of OPM compaction (cycle 1) corresponds to the specifications that are led to the initial design of pavement:
• Compaction at the Optimum Modified Proctor (OPM) and determination of the initial CBR value of the material that will have to support traffic.
• Determination during the same initial state of all physical and mechanical characteristics of materials, as reference values such as gradation, Atterberg limits, CBR, Los Angeles loss, Shear strength as Unconfined Compression Test characteristics (UCT), etc.
• And finally, perform multi-cyclic compaction procedure to determine soil characteristics at each cycle of compaction.
2. Test Procedure and Material Properties
After complete characterization of a gravel lateritic specimen from Burkina Faso (between Boromo and Bobo Dioulasso mainly used for the design of this West African International Road) and Senegal (in the western part of the country, as Yenne and Thiès), (sieve and hydrometer analysis, Atterberg limits, methylene blue, etc.), soils are compacted and subjected to mechanical tests at the Optimum Modified Proctor (OPM). Theses mechanical tests are essentially CBR tests, unconfined compression test and resistance to degradation by abrasion and impact in the Los Angeles machine. After the first cycle, the remaining material is used to perform exactly the same tests during the subsequent cycles (2nd, 3rd, …, 10th cycles) (Table 1). The purpose of these tests is to compare the evolution of main properties (particle size distribution, CBR, Young modulus, etc.) with repeated cycles of compaction. Tables 2(a) and (b) below summarize the overall results:
Table 1. Values of material properties at cycle 0 (raw material), (The main Lateritic Soils used in this paper are sampled from Burkina Faso between Boromo and Bobo Dioulasso).
Table 2a. Summary of the test results depending on the soil provenance and the cycles of compaction.
Table 2b. Summary of the test results depending on the soil provenance and the cycles of compaction. * (Empty cells indicate insufficient quantity of materials for further testing. Multi-cyclic compaction uses a large amount of material per cycle. In this case, several samples were compacted at the same water content in order to provide enough amount of material for each cycle).
3. Interpretation of Results
3.1. Generation of Fine particles and Changing in Characteristics of Consistency
As shown by figures below, the transition between first to 10th cycles contributes to a strong generation of fine particles, as well as a gradual increase of plasticity (Figure 1). The amount of fines particles (% < 80 μm) increases from 17% (which is the limit generally accepted for such materials) for the first cycle and reaches 46% for the 10th cycle. From the first to the 10th cycle, plasticity of materials also changes from 21% to 31% for the sample of Pk 247 and from 29% to 40% for the sample of Pk 272 + 600.
The Figure 2 gives the results of Los Angeles tests performed on gravel lateritic soils samples. The test was conducted in a particular procedure that is “unconventional”. In the case of the strict application of the standard, the test is performed in the fraction 10/14 with a mass of test sample of 5 kg. For our purposes, we took care to fill the hollow steel cylinder with the total fraction of the material without any selection. This procedure allows testing the total mass of the initial material without any selection and therefore allows completing Figure 3 showing the generation of fine particles and changes in plasticity. Since the test measures the resistance to degradation by abrasion and impact of the material in a rotating steel drum containing a specified number of steel balls, results show a strong increase of percent loss by abrasion and impact as the number of cycle increases. In this sense, both coarse and fine aggregates fragment extensively during the test. This further demonstrates the problematic behavior of all gravel lateritic soils related in the literature [10].
3.2. Comparison with the Specifications in the Western African Area (West African Standards―WAS)
From Figure 3 we can remark that, at the end of compaction cycles, materials tested are outside of specifications for the plasticity index and the amount of fine particles (<80 mm) as required by specifications.
Figure 1. Evolution des fines (% < 80 m) et de la plasti-cité (PI) (Pk 272 + 600).
Figure 2. Evolution of percentage loss by abrasion.
Figure 3. Comparison between results (<0.08 mm et PI (%)) and specification of the WAS.
Figure 4 shows the variation of CBR values with the cycles of modified Proctor compaction. Table 3 below reminds technical recommendations contained in current textbooks approved by the CEBTP, the BCEOM and the LCPC [11] for the use of gravel lateritic soils as base courses and in the case of a T1 to T2 traffic level: