Optimisation of Weak Soil Stabilisation with a Hydraulic Binder for Road Construction Subgrade

Road pavement construction is designed to be durable and to ensure the safety of the vehicles using that road. Road construction can be designed in basic ways, depending on the way it will behave after applying the load. It can be designed as rigid construction, where the basic material used for the construction and road surface layer is cement concrete. The pavement construction can also be designed as flexible, in which the top layers are made of asphalt with bituminous binder. Regardless of which type of road pavement construction is chosen, it is vital to properly take care of the subgrade to ensure durability of the road construction.

The road pavement is a linear object, which passes changeable terrain. It affects the subgrade quality, which can be made of weak or expansive soils. Those soils require improvement to serve as the road pavement construction subgrade. There are several possibilities for subgrade strengthening. The simplest method is soil replacement where the weak soil is replaced by better. The replacement can be done with the use of recycling materials [1].

Another easy and inexpensive method of soil improvement is treatment with different types of binders. Currently, one of the most widely used binders for soil treatment is Portland cement. However, different hydraulic road binders are gaining in popularity [2]. Such road binders contain high amounts of supplementary cementitious materials and by-products like fly ashes or slags [10]. Due to the reduced amount of cement clinker, they are considered environmentally friendly, reducing carbon footprint and enabling to reduce the use of raw materials for clinker production. Currently, cement production contributes about 7% [14] of total worldwide CO₂ emissions. This creates space for significant change in total emissions by affecting the cement industry, involving additional binders.

Soil stabilization can also be done with the use of lime, which increases the soil parameters including California bearing ratio (CBR) and reduces plasticity index [4]. The use of polymer agents is another way to chemically treat clayey soil. Their interaction with soil particles leads to changes in water attraction. Consequently, water is expelled during the compaction process which results in increased bearing capacity and other mechanical properties of the treated soil [7]. In the case of chemical treatment, it is also important to verify if the soil can be treated with chosen agents. Some soils may not cooperate with binders well so this kind of soil properties enhancement needs to be designed for each soil individually as it was done in the research [9] for weathered granite soil.

Soil treatment was also reported to be done in a non-chemical way. Mixing weak or expansive soil with sand or gravel-size aggregate may lead to a significant increase in the resilient modulus of the treated soil. The material used for non-chemical enhancement of soil characteristics can be obtained from recycling, e.g. sand-size recycled glass [5]. It was also reported that fly ash enhanced soil parameters improve while adding fibre (also those coming from recycling processes like polyethylene terephthalate (PET) [6]. Non-chemical soil treatment way also leads to a decrease in carbon footprint, decreasing the use of cement binder. If the soil enhancement is done with recycled materials not only it decreases carbon footprint but also it contributes to a circular economy, decreasing the use of raw materials and preservation of natural resources, which was highlighted e.g. in [15].

Another possibility to strengthen the road subgrade is the use of geotextiles. It was reported in the paper [3] that the use of unscreened gravel with geocell enabled to decrease dynamic deformation by over 25% when used to treat soft rock subgrade. The effectiveness of the geocells was confirmed in other studies [8].

Other types of subgrade reinforcements include more advanced technology involving heavy machines. Those are done if the soil is too weak to be chemically treated, geotextiles are not sufficient and there is too of weak soil for soil replacement. The advanced subgrade strengthening technologies belong among others different types of piles and columns.

This paper describes a case study of designing and optimising soil treatment with Portland cement CEM I 42,5R and with aggregate. The aim of the research was to find economical and environmentally friendly solutions for soil improvement for road construction.

In this study four materials were used to obtain optimal parameters of the subgrade material. The first material was the original soil. The soil used in the research was obtained from the virgin area and it was supposed to act as a subgrade of the road pavement construction. However, due to frost heave susceptibility and to expected low bearing capacity of the material it was decided to be treated with cement binder and with aggregate to enhance the cement stabilisation effectiveness. The soil was classified as silty sand (siSa). The particle size distribution test was carried out according to the standard EN 933-1 [11]. The result of the test for the soil is presented in the Fig. 1.

Figure 1.

The second material was the aggregate 0–63 mm used to enhance the bearing capacity of the soil in a non-chemical way. It was also used to improve effectiveness of chemical treatment with cement, as the course aggregate should create with the original soil a structure enabling to achieve much higher compressive strength of the material. The aggregate particle size distribution is also presented in the Fig. 1.

Those two above-presented materials were mixed to achieve better compressive strength of the cement-stabilised road subgrade material. The materials were mixed in proportion 20% aggregate with 80% soil and 40% aggregate with 60% soil. The particle size distribution graphs of these mixtures are presented in Fig. 2.

Figure 2.

Apart from the use of aggregates, the soil was chemically improved by the addition of different amounts of cement CEM I 42,5 R. Different proportions of different ingredients were tested in terms of compressive strength in accordance with EN 13286-41 standard [12]. The amounts of Portland cement used for chemical soil enhancement were 6%, 7% and 8% by mass. Finally, one of the combinations was tested in situ. The chosen type of soil stabilisation was applied in the trial section to compare the results achieved in the laboratory with the final parameters of enhanced subgrade in full scale.

Laboratory samples were prepared in the number of 3 for all combinations. The results of the compressive strength test done after 28-days of strength build-up for all combinations are presented in Fig. 3.

Figure 3.

Cement-enhanced laboratory samples’ results of compressive strength test reveal that the addition of the aggregate to change particle size distribution within the soil leads to a significant increase in the final compressive strength of the samples. For 100% silty sand soil the final compressive strength was close to 1.0 MPa for all three dosages of cement – 6%, 7% and 8%. The addition of 20% of aggregate enabled to achieve of 2.0 MPa with 8% of cement, while the addition of 40% of aggregate led to the achievement of 2.0 MPa with just 6% of cement and 2.5 MPa with 8%.

After the laboratory stage of the research one composition was taken for field tests. It was decided to compare soil enhanced with 40% of the aggregate with 7% of CEM I 42,5R. The field samples were taken from four separate sections within one of the road constructions done by Eurovia Polska S.A. company.

All sections were prepared in the same way with the same materials. Virgin soil on the whole construction site was visually homogenous. Soil moisture content was verified so that the moisture content of the final mix was the same as in the laboratory, close to the optimal moisture content obtained in the Proctor compaction according to standard EN 13286-2 [13]. In the next step the aggregate and the cement binder were spread on the subgrade surface in the amount that guarantees a proportion of 7% cement, 37.2% of aggregate and 55.8% of soil (silty sand) after mixing to create a 40 cm thick layer after the compaction process. The mixing was done with the recycler presented in Fig. 4.

Figure 4.

After the mixing process, the samples of the soil-aggregate-cement mixtures were taken for preparation of samples for compressive strength test. The samples were tested after 28 days of their preparation process. The results of the test carried out for the 4 tested sections are presented in Fig. 5 in comparison to the results obtained from the samples prepared in the laboratory mixing process.

Figure 5.

The results reveal that mixing done by heavy machine such as a recycler enabled to achieve homogenous mix (all testes sections had similar compressive strength of between 3.25–3.50 MPa). The mixes were also of a higher quality than those done in the laboratory giving the mean value of compressive strength of 3.35 MPa for construction site samples and 2.35 for those made in the laboratory.

The research results presented in the paper are important in terms of decision-making about the designed solution for construction in terms of geotechnics and road engineering. The pressure for both high quality and for ecology is omnipresent in the current world. Especially important are aspects of circular economy and carbon footprint limitation. Obtained results help to optimize weak soil enhancement so that it has good quality and is also attractive from economic and ecologic points of view. The results reveal that chemical treatment of soil has its limitations unless there is additional material used for non-chemical modification, in this case aggregate 0/63. The aggregate used for this purpose can come from the recycling process. The optimal content of the aggregate enables to visible decrease in the amount of cement binder used for soil stabilisation together with increasing the compressive strength.

The research also confirmed that the machines used at the construction site are sufficient to achieve homogenous mix of high quality. The recycler used in the research resulted in the production of a material that had higher compressive strength in comparison to the mix prepared from the same materials in laboratory conditions. It confirms that the technology can be safely applied to serve as a subgrade for road pavement construction.

As the technology of involving recycled granular materials in soil stabilisation gains popularity it will limit the use of cement leading to a significant decrease in carbon footprint, as the cement production industry is responsible for a large contribution to global CO₂ production. The use of recycled materials also leads to the preservation of natural resources, keeping them for future generations which apart from economic reasons is the most important advantage of presented in this paper technology.