Soil improvement or stabilization is a process of altering the physical characteristics of soils to improve their rheology, namely their shear strength characteristics. This process is beneficial when the readily available soils for road construction, is unable to withstand the projected traffic loads. Whether the improvement or stabilization focuses on the base layer or the subbase layer, or both, the bearing capacity of the whole pavement is increased, which results in meaningful technical due to added strength, economic due to reduced costs and environmental consequences because of lesser transport associated pollution.
It is well documented that Natural Desert Sand (NDS) referred to as dune sand is unfit for road construction. NDS is made up of fine and rounded particles, and therefore, it lacks the coarseness and angularity found in mechanically controlled and Crushed Fine Aggregate (CFA) that inherently creates an interlocking mesh, resulting in a stronger aggregate skeleton.
As a result, bases or subbases made of NDS do not provide adequate support for even lowvolume roads, compared to bases or subbases made of CFA. Therefore, adequate road construction material needs to be hauled, imported and even shipped over long distances in African countries like Senegal, Mali, Benin, Chad, Cote d’Ivoire, Algeria and Libya.
Ordinary Portland Cement (OPC) has however been used for many decades in small quantities to improve soils performance in compression, or in larger quantities to stabilize and increase flexural strength by increasing the rigidity of the soils.
In addition to the recourse to OPC, recourse in reasonable quantities to cheaper and readily available NDS, adjacent to the projected road alignment, saves on extracting, crushing, importing and hauling or shipping over long distances, good quality materials. This ultimately saves valuable and scarce aggregate resources which would have otherwise provided the needed coarseness and angularity.
This thesis investigates the improvement (flexible behaviour of the pavement in compression) and even the stabilization (rigid behaviour of the pavement in flexure) of NDS with OPC and with a combination of OPC-fly ash (FA), for bases and subbases of low-volume roads crossing desert land and subject to overloaded trucks. By increasing the base and subbase strength with these cementitious binders, the thickness of the asphalt concrete (AC), base and subbase layers can be substantially reduced. This is due to the stiffer support, resulting in lower tensile strain at the bottom of the AC layer, and lower compression strain at the top of the subbase and the subgrade, thereby reducing the effect on the native soil, the result of which is an increased structural life of the pavement and a reduced environmental life cycle cost.
The value of also improving or stabilizing the subbase, to increase the structural life is equally investigated in this thesis, in order to promote more sustainable roads.
To assess the increase in structural characteristics of stabilized soils for use as base and/or subbase layers with the addition of OPC and OPC-FA, several tests were performed such as compaction, unconfined compressive strength (UCS), resistance to deformation (CBR), permeability, and triaxial tests.
These tests were adequately proportioned and repeated on different samples in order to show how to draw relevant conclusions and recommendations leading to the least cost structural design, in economic and environmental terms.
The findings for instance with Libyan dune sand (NDS) are such that changes in the physical properties of the base or subbase, significantly affect their mechanical behavior. It is found that the optimal composition depends on three factors: the ratio of NDS and CFA, the type of stabilizer (OPC and/or FA), and the water / cement ratio.
Samples were mixed with two different ratios of aggregates (50:50 CFA: NDS and 30:70 CFA: NDS). Each sample was respectively mixed with the four different percentages of OPC (0% control, 3%, 5%, and 7%) and four different percentages of FA (0% control, 3%, 5%, and 7%) for a total of 32 samples tested.
The result show that the optimal mix would be composed of 30:70 CFA: NDS for the aggregate portions, with an optimal binder content of 7% of OPC and 7 % of FA. An economic analysis is performed for various percentages of cement and combinations of cement and fly ash. It is shown that despite high binder contents, the recourse to locally available NDS results in lesser consumption of non-renewable resources, lesser extraction, crushing and transport associated energy consumption and therefore, in social and environmental benefits from reduced noise, lower pollution emission and dust due to hauling. Additional and meaningful economic benefits also stem from reduced life cycle costs to the agency and also to the users, as a result of the increased structural capacity and therefore improved pavement performance.
In summary, cement and/or fly ash stabilization, whilst increasing the pavement strength, reduces the asphalt concrete, base and subbase thicknesses and associated construction and future maintenance costs.
This is in line with sustainable development objectives of the Paris Agreements, and their world reach, which require a balance between the economic, the social and the environmental aspects, and capacity building in Southern countries based on sound engineering principles.
In a strategic perspective, this research is in line with the above stated sustainable development objectives and is a demonstration on how to practice sustainable engineering in today’s civil engineering design in areas where good construction materials are scarce, a harsh reality in Northern Africa (Sahara) whereby the approach is validated, but also, albeit to a lesser extent, in Sub-Saharan Africa (SSA).
In an empirical and practical perspective, this research provides a methodology to use poor material to build sustainable roads by substantially reducing hauling over long distances and saving scarce and good quality materials. In fact, it is not so much the proportions found hereafter that are important but the methodology by which these findings are obtained. It is therefore hoped that this methodology is conducted as part of the preliminary design of projects of magnitude in areas suffering from scarce materials, as is too common in Africa.
Another contribution of this thesis is to highlight the cost-effectiveness of desert sand improvement or stabilization, by reducing the amount of new and imported material and optimizing the thickness of the asphalt concrete course.
| Date | 3 Dec 2021 |
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| Original language | American English |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Gabriel J. Assaf (Supervisor) |
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