Methodology

A device similar to that described by Tamarakar et al. (2005) was constructed for the experiments (Figure 1). Four types of materials were used for the experiments: natural soil, bentonite, lime, and cement. The soil used is classified as CL according to the Unified Soil Classification System. The bentonite used in this study has a plastic limit of PL=36 and a liquid limit of LL=155. Also, hydrated lime and Type 2 Portland cement were used for soil stabilization. Specimens prepared include: natural soil, soil-bentonite with 5%, 10%, and 15% bentonite, soil-lime mixture with 2%, 4%, and 6% lime, and soil-cement-lime mixture according to Table (1). To ensure the repeatability of results, two samples were prepared and tested for each experiment.

Fig. 1- Different parts of the tensile device and their connection to the direct shear apparatus lever: 1) Holding clamp, 2) Fixed mold, 3) Movable mold, 4) Horizontal platform, 5) Placement on the direct shear apparatus platform, 6) Connection of the movable mold to the force application lever, 7) Rammer for compaction of specimens

Table 1- Weight percentages of lime and cement in samples made of soil-cement-lime mixture

The specimens were prepared at 15%, 18%, 20%, and 24% moisture contents and cured for 28 days. Also, the natural soil specimens were prepared at dry densities of 1.58 g/cm³, 1.62 g/cm³, and 1.66 g/cm³. But the treated soil specimens were prepared at a dry density of 1.62 g/cm³ only. First the soil and the additive were mixed and water was added. The mixture was cured for 28 days and then poured into compaction molds in five layers, each compacted with 4 blows of the standard compaction hammer. The tests were conducted at different strain rates of 0.06, 0.12, 0.24, 0.48, 0.72, and 1.44 mm/min.

Results Analysis

Figure (2) depicts the failure mode of an specimens. It can be observed that the tensile cracks are nearly straight and perpendicular to the direction of applied force, indicating the movement of the shear molds in the direction of tensile force application. In stiffer specimens, cracks appeared almost instantaneously across the entire surface of the specimen, whereas in softer specimens, cracks initially appeared at the edges and propagated towards the center, consistent with other studies (Tamrakar et al., 2005, Wei et al., 2022). 

The experiments conducted on soil with 20% moisture content and a dry density of γ_d = 1.62 g/cm³ showed that initially the tensile strength increased as strain rate was increased, reaching its peak at a strain rate of mm/min 0.24. Consistent with Xiu et al. (2021) and Wisetsaen et al. (2015).

Also, it was observed that the tensile strength initially increases with increasing moisture content, peaks at 18% moisture, and then decreases at higher moisture contents. These results are consistent with the theory of unsaturated soil mechanics (Pande & Pietruszczak, 2015).

The effect of addition of bentonite on the tensile strength of soil is nearly linearly, where each 1% increase in bentonite increases the tensile strength by approximately 1.5 kPa. It appears that the mineralogy of clays significantly influences the tensile strength of compacted soils.

Lime stabilized specimens were tested after curing periods of 3, 7, 14, and 28 days. The soil strength increases primarily due to the occurrence of pozzolanic reactions between lime, water, silicates, and aluminates present in the clay minerals. The reactions are time-dependent and gradually increase the soil's strength over time.

It appears that for soils stabilized with 6% lime, the increase in tensile strength due to longer curing time is higher than the same for 2% and 4%. Also, the tensile strength of samples with the same curing time increases by 10% for each 1% increment in lime content.

The tensile strength of the specimens treated by different amounts of lime and cement exhibits a fairly consistent trend with curing time. Also, the tensile strength of samples with the same curing time increases by an average of 5% for each 2% increment in cement content. However, there is a noticeable difference for samples stabilized with 6% lime and 7% cement, which show a steeper increase.

Conclusions

Based on the experiments and observations conducted, the following conclusions can be drawn:

• The performance of apparatus used for testing was satisfactory and reliable.
• The cracking pattern in softer samples initially appeared at the edges and propagated towards the center, whereas in denser samples, cracking occurred suddenly throughout the specimen. Samples with higher density exhibited larger crack openings.
• Increasing strain rate initially increases tensile strength, but up to a certain point., it decreases. This is due to the inability to form cohesive bonds between particles.
• Moisture content significantly affects the tensile strength of compacted soils. In general, at a constant dry density, increasing moisture content initially increases tensile strength, followed by a decrease. This behavior is attributed to transitioning from low to higher degrees of saturation beyond the threshold moisture content.
• Adding bentonite significantly affects tensile strength due to the increase in clay minerals in the soil.
• Increasing curing time results in higher tensile strength. This is attributed to the progressive pozzolanic reaction over time. However, it is expected that the rate of increase in tensile strength may decrease over time.

Acknowledgments

This article is extracted from the dissertation of the Msc course in the Faculty of Civil Engineering and Architecture of Shahid Chamran University of Ahvaz. The financial support of the Vice Chancellor for Research of Shahid Chamran University of Ahvaz in the form of grant GN: SCU.EC96.824 is hereby thanked and appreciated.