Investigating Calcareous and Silica Sand Behavior at Material Interfaces: A Comprehensive Study

As illustrated in Figure 1, two distinct types of sand, namely silica and calcareous sand, were utilized in the study, both adhering to a specified grading curve. The investigation aimed to evaluate the interface shear strength between these sands and steel or concrete surfaces. Table 1 provides an overview of the physical and geotechnical properties of the sand materials employed. Both silica and calcareous sand fall under the classification of poorly graded sand (SP). Notably, the visual examination of these sands reveals that calcareous sand exhibits a more angular particle morphology compared to silica sand. Table 2 presents the chemical properties of calcareous sand, which was sourced from Kish Island, situated in the Persian Gulf in southern Iran. X-ray fluorescence (XRF) analyses confirmed that Calcite is the principal component of Kish sand.

Figure 1:

Grain size distribution curves and photographs of calcareous and silica sands.

For the experimental setup, concrete and steel plates with varying degrees of Rn were utilized. Figure 2 provides a visual representation of steel plates with different Rn values. Rn denotes the ratio between the groove depth of the steel or concrete plate and the D50. The concrete plates, as seen in the bottom row of Figure 2, were prepared using a mixture consisting of fine silica sand, ordinary Portland cement, and water, in a ratio of 1.8:3:1. These concrete plates underwent a curing period of 28 days prior to the interface shear tests. The surfaces of both the steel and concrete plates were intentionally grooved prior to the direct shear tests. Each trench on the plates assumed an isosceles triangular cross-section, featuring varying heights in correspondence to the assigned Rn values and an apex angle of 45 degrees. It’s worth noting that the concrete plates exhibited localized damage following the direct shear tests and were used in just one test each. On the other hand, the steel plates remained undamaged throughout the testing. To achieve an Rn value of 0 for the concrete surface, a sanding process post-casting was implemented. This involved carefully sanding the surface of the concrete to significantly reduce its porosity and smooth out any roughness. The result was an almost smooth surface, comparable to that of steel, which allowed the Rn = 0 condition to be met for the experiments.

Figure 2:

Steel and concrete plates with various Rn values.

The experimental approach encompassed a series of 10 × 10 cm direct shear tests conducted on prepared samples. To assemble the sand samples, dry sand was compacted within a box using tamping techniques to achieve the desired relative density. The tamping method employed a manual hammering device with a base area matching the sample surface (10 × 10 cm). This device featured a 1-kilogram weight attached to the base via a rod. The weight was lifted to a height of approximately 30 cm and then released, allowing it to hit the metal surface above the sample due to gravity. The entire amount of dry sand was poured into the mold and then compacted in one layer, considering the low thickness of approximately 3 centimeters in the upper part of the mold. The uniformity of the compacted sample was ensured by maintaining consistent tamping procedures, resulting in an almost uniform density. The relative density was recalculated after applying the normal stress, and if the density change exceeded 5% relative to the desired relative density, the test was rejected, and the sample was remade to ensure accuracy.

For the interface tests, concrete or steel plates were positioned at the bottom section of the box, while the sand was compacted into the top section. It was essential to ensure precise alignment of the border between the plate and the sand on the shearing line. Following sample preparation, the specimens were subjected to a constant horizontal displacement rate of 1 mm/min in accordance with ASTM-D3080. To ascertain the shear strength parameters of the samples, direct shear tests were carried out under three different normal stress values: 100 kPa, 250 kPa, and 500 kPa.

Grain crushing can significantly impact calcareous sand, even at relatively low stress levels, potentially altering soil granulometry after shearing tests (Datta et al., 1979; Hakimelahi et al., 2023; Li et al., 2021b; Tavakol et al., 2023). While intending to assess grain crushing in the early stages of the experiments, the small size of the mold and the limited volume of sand used made accurate measurement challenging. To maintain consistency and reliability in the results, the sand was replaced with fresh, uncrushed samples after each shear test. This approach ensured that each test started with sand of consistent granulometry, mitigating the effects of particle crushing on the findings.

In the current study, calcareous and silica sands were employed at two distinct relative densities, 30% and 90%, to explore their interface characteristics with steel surfaces. Figure 3 illustrates the shear strength envelopes for both calcareous and silica sands, along with the corresponding friction angles. The findings reveal that calcareous sand exhibits friction angles approximately 17% and 32% higher than those observed with silica sand at relative densities of 30% and 90%. This disparity can be attributed to the inherent differences in particle morphology and surface texture between the two sands, with calcareous sand particles displaying a more angular and rougher surface, resulting in an elevated friction angle.

Figure 3:

Shear strength envelopes for the tested soils at Dr of 30 and 90%.

Within this section, the focus is on delving deeper into the mobilized friction angle between calcareous and silica sands and steel plates, particularly emphasizing tests conducted with calcareous and silica sand at a relative density of 90% and various Rn. Figure 4 presents the interaction shear strength envelopes for both sand types, showcasing the influence of changing Rn on the steel plates. It’s notable that the variations in slope due to Rn changes in both sand types exhibit a similar trend. However, for equivalent Rn values, the shear strength envelope for calcareous sand is slightly higher than that for silica sand. In Figure 4, the symbol ϕ represents the mobilized friction angle at the interface between the soil (calcareous or silica sand) and the steel plates under the specified conditions when used after Rn. However, it indicates the internal friction angle when used after silica or calcareous sand.

Figure 4:

Shear strength envelopes for the interaction between (a) calcareous sand and steel plates, and (b) silica sand and steel plates, with different Rn values (Dr = 90%).

Moving on to the maximum shear strength values, Figure 5 provides insights into the differences between calcareous and silica sands at various Rn values of the steel surface. The results highlight that calcareous sand consistently demonstrates higher shear strength compared to silica sand for a given Rn value. Moreover, this difference between the two sand types becomes more pronounced with increasing normal stress and Rn values.

Figure 5:

Maximum interface shear strength values between calcareous or silica sand and steel plates with various Rn values, under various normal stress levels and Dr = 90%.

The changes in the friction angle ratio were also scrutinized, representing the ratio of the mobilized friction angle at the sand-steel interface to the friction angle of the sand. Figure 6 demonstrates that this ratio remains relatively consistent, regardless of the sand type. Figure 6 provides insight into the variation of the friction angle ratio (фi/фs) which represents the ratio of mobilized friction angle at the calcareous sand-steel interface (фi) to friction angle of calcareous sand (фs). However, it does exhibit an increase with rising Rn values for both calcareous and silica sands, indicating the influence of surface roughness on the interface behavior.

Figure 6:

Friction angle ratios for both calcareous and siliceous sand (Dr = 90%).

To comprehensively examine the impact of relative density on the behavior of the calcareous sand-steel interface, the tests were conducted at two distinct relative densities, 30% and 90%, while varying the roughness of the steel surfaces (Rn values of 0, 0.75, 1, and 1.5). The outcomes, as depicted in Figure 7, showcase a noticeable increase in the slope of the shear strength envelopes as Rn values rise in both sets of tests. This trend underscores the heightened influence of Rn in cases of lower relative density, with the lines representing lower relative density exhibiting greater separation.

Figure 7:

Shear strength envelopes for the calcareous sand-steel interface under (a) Dr = 30% and (b) Dr = 90% with various Rn values.

For a more detailed analysis of the interplay between relative density and Rn, the maximum shear strength values of the sand-steel interface are presented against Rn in Figure 8. Here, it becomes evident that the effect of Rn becomes more pronounced with increasing normal stress. Specifically, the lines representing a normal stress of 500 kPa exhibit steeper slopes compared to those corresponding to normal stresses of 100 and 250 kPa. Moreover, the discrepancy between the results of samples with relative densities of 30% and 90% widens as the normal stress value escalates from 100 to 500 kPa. This suggests that an increase in Rn leads to an enhanced maximum shear strength, a value intrinsically linked to normal stress and relative density.

Figure 8:

Maximum shear strength of the calcareous sand-steel interface versus the Rn under various normal stress levels.

Figure 9 provides insight into the variation of the friction angle ratio (фi/фs) which represents the ratio of mobilized friction angle at the calcareous sand-steel interface (фi) to friction angle of calcareous sand (фs). Notably, all the recorded values are less than one, indicating that the friction angle at the calcareous sand-steel interface consistently remains smaller than that of calcareous sand. It is important to highlight that, for a given Rn value, the friction angle ratio is notably higher when the relative density is 90% compared to 30%. This underscores the significant impact of relative density, especially when Rn values are at the lower end of the spectrum. Recent studies, such as Feng et al. (2024), have explored the mobilized shear strength at the interface of silica sand and steel surfaces. Their findings indicated that with increasing Rn, in some cases, the mobilized friction angle at the sand-steel interface is higher than that of pure sand. This suggests that the ratio might indeed approach 1 or exceed it.

Figure 9:

Variation of the friction angle ratio of calcareous sand-steel interface versus the Rn under Dr 0f 30 and 90%.

In an effort to discern the variations in how carbonate sand interacts with steel and concrete surfaces, the tests were done on the calcareous sand-concrete interface at a consistent relative density of 90%. The Rn for the concrete plates matched that of the steel plates. The shear strength envelopes for the calcareous sand-concrete interface are depicted in Figure 10. Much like the findings in the calcareous sand-steel plate interface, an increase in Rn and normal stress elevates the maximum mobilized shear strength. In this case as well, heightened Rn values contribute to steeper slopes in the shear strength lines, mirroring the behavior observed in the calcareous sand-steel plate interface.

Figure 10:

Shear strength envelopes of the calcareous sand-concrete interface.

To further dissect the influence of steel and concrete plates, the maximum shear strength values for both plate types under various normal stress conditions are presented in Figure 11. These results unveil a nuanced relationship: for a constant normal stress and Rn values exceeding 0.75, the mobilized shear strength at the sand-steel plate interface surpasses that of the sand-concrete plate interface. When Rn values are set at 0 and 0.75, the disparities between the results are negligible, and the outcomes remain largely independent of the plate material. However, as normal stress escalates, the distinctions between results obtained with Rn values of 1 and 1.5 become more pronounced, signifying the intricate interplay between surface roughness and plate material.

Figure 11:

Maximum mobilized shear strength values at the interface of calcareous sand and steel or concrete plates under various normal stress values (Dr = 90%).

The variation of the friction angle ratio for the calcareous sand-concrete or steel interface at a relative density of 90% is illustrated in Figure 12. At an Rn value of zero, the friction angle ratio is marginally higher in the case of the concrete plate when compared to the steel plate. This divergence can be attributed to the unevenness of the concrete surface in contrast to the steel surface when Rn equals zero. However, for Rn values exceeding zero, the friction angle ratio for the steel plate exceeds that of the concrete plate. This shift can be ascribed to the sharper edges found on the steel surface in comparison to the concrete surface. The most significant discrepancy in the friction angle ratio is evident at the maximum Rn value, highlighting the importance of surface roughness in governing interface behavior.

Figure 12:

Friction angle ratio for calcareous sand-concrete or steel interface at Dr of 90%.

In this section, the effects of shearing direction on the interaction between calcareous sand and steel or concrete surfaces were examined through two sets of direct shear tests. Unlike previous tests where the angle between the shearing direction and grooves (β) was fixed at 90 degrees, β was adjusted to zero degrees, as depicted in Figure 13, for these particular tests. The tests with plates in the perpendicular direction were conducted using sand with a Dr of 90%.

Figure 13:

Shearing angles of 0° and 90°.

The results, presented in Figure 14, illustrate the impact of altering the shearing angle from 0 degrees to 90 degrees on the shear strength envelopes at the interface between calcareous sand and concrete and steel plates. In Figure 14, the symbol ϕ represents the mobilized friction angle at the interface between the calcareous sand and the steel or concrete plates when written after steel or concrete. It represents the internal friction angle of calcareous sand when written after calcareous sand. Notably, an increase in the shearing angle leads to a steeper slope in the shear strength envelope. In all cases, the interaction shear strength envelopes fall below the shear strength envelope observed for pure sand. Furthermore, the discrepancy between the results at shearing angles of 0 degrees and 90 degrees becomes more pronounced as the normal stress increases. Additionally, it’s worth noting that the influence of the shearing angle on the interaction shear strength envelopes is more pronounced for steel plates than for concrete plates. This observation underscores the significance of shearing direction in governing the interface behavior, with implications for different materials.

Figure. 14:

Effect of shearing angle on the shear strength envelopes at the interface of calcareous sand and concrete or steel plates (Dr = 90%).