3.1. Influence of stress state on dynamic rebound modulus and penetration depth
It can be seen from Fig. 5 that the dynamic rebound modulus of coarse-grained chloride saline soils under different confining pressures shows a decreasing trend with the increase of deviatoric stress at the optimal water content w equal to 5.1%, and the decreasing trend gradually slows down with the increase of deviatoric stress. At the same time, the dynamic rebound modulus also increases with the increase of confining pressure. The reduction amplitude of the dynamic rebound modulus with the increase of deviatoric stress is shown in Table 4. The higher the salt content and confining pressure of coarse-grained chloride saline soil, the more significant the influence of deviatoric stress on the dynamic rebound modulus. The maximum decrease in dynamic rebound modulus is 33.3% when the salt content Z is 8.0% and the confining pressure is 80 kPa (Fig. 5(d)). This is because negatively charged soil particles can adsorb positively charged sodium ions to form a diffusion double layer. Before the soluble salt in the soils reaches saturation, the increase in salt content increases the concentration of dissolved ions, increases the thickness of the double layer, and the lubrication effect of sodium ions gradually strengthens. The shear strength of chloride saline soils gradually decreases. In the later stage of continuous loading, the shear effect caused by the increase of deviatoric stress is greater than the shear strength and the lateral effect caused by confining pressure, leading to the continuous displacement and rearrangement of soil particles. During this process, the plastic strain gradually increases, and the dynamic rebound modulus gradually decreases.
Table 4
Reduction amplitude of dynamic rebound modulus with increasing deviator stress under different confining pressures and salt content
Confining pressures | Salt content |
Z= 0.0% | Z = 2.0% | Z = 5.0% | Z = 8.0% |
σ3 = 80kPa | 2.3% | 8.2% | 11.7% | 15.4% |
σ3 = 60kPa | 5.6% | 11.5% | 15.5% | 16.7% |
σ3 = 45kPa | 7.2% | 17.4% | 19.5% | 22.3% |
σ3 = 30kPa | 9.9% | 22.8% | 22.2% | 26.1% |
σ3 = 15kPa | 12.7% | 28.6% | 26.7% | 33.3% |
Figure 6 indicates that the dynamic rebound modulus of coarse-grained chloride saline soils with different deviatoric stresses continuously increases with the increase of bulk stress under different salt contents at the optimal water content w = 5.1%. The amplitude of the dynamic rebound modulus increase under different deviatoric stress is shown in Table 5. The smaller the deviator stress, the greater the increase in dynamic rebound modulus caused by the increase in bulk stress. At the same time, the higher the salt content of coarse-grained chloride saline soils, the more significant the effect of bulk stress on dynamic rebound modulus. The maximum decrease in dynamic rebound modulus is 107.7% when the deviatoric stress is 0.5σ3 at a water content of 8.0%. When the deviatoric stress is given a certain value, the increase in bulk stress is actually an increase in the constraint of confining pressure on the lateral deformation of the sample, resulting in an increase in the stiffness of the sample and a gradual increase in dynamic rebound modulus.
Table 5
Reduction amplitude of dynamic rebound modulus with increasing bulk stress under different deviator stress and salt content
Deviator stress | Salt content |
Z= 0.0% | Z = 2.0% | Z = 5.0% | Z = 8.0% |
σd = 0.5σ3 | 40.3% | 59.1% | 76.5% | 107.7% |
σd = 1.0σ3 | 32.8% | 40.0% | 62.5% | 91.7% |
σd = 2.0σ3 | 25.4% | 28.9% | 46.7% | 63.6% |
It can be seen from Fig. 7 that the penetration depth of coarse-grained chloride saline soil with different salt contents continuously increases with the increase of unit compressive stress under the same water content conditions. The penetration depth increases approximately linearly at high salt content (Z > 2.0%), whereas the penetration depth increases nonlinearly at low salt content (Z ≤ 2.0%). When the unit compressive stress is constant, the penetration depth of samples with the same humidity soaked by water increases with the increase of salt content. When the salt content Z increases from 2.0–8.0% and the penetration depth is 5.0mm, the vertical local compressive stress applied to coarse grained chloride saline soils under different water contents gradually decreases, and the reduction amplitude of the vertical local compressive stress is shown in Table 6. The penetration depth of coarse-grained chloride saline soils with lower salt content is less affected by unit compressive stress. As the salt content gradually increases, the influence of unit compressive stress on the penetration depth is gradually evident, especially for the samples with high salt content. This indicates that the unit compressive stress has a greater impact on the penetration depth of samples with high salt content, namely, the higher the salt content, the smaller the ability of coarse-grained chloride saline soils soaked by water to resist local compressive stress. This is because the salts in the coarse-grained chloride saline soils are easily soluble in water, leading to an increase in soil voids, a decrease in soil compactness, an increase in compressibility, and a decrease in deformation resistance.
Table 6
Reduction amplitude of local compressive stress under different salt and water content
Salt content | Reduction amplitude of local compressive stress |
ω = 4.0% | ω = 5.1% | ω = 6.0% |
Z = 0→2% | 13.9% | 16.4% | 21.4% |
Z = 2→5% | 19.1% | 23.9% | 29.6% |
Z = 5→8% | 22.1% | 27.5% | 39.0% |
3.2. Effect of water content and salt contents on dynamic rebound modulus
As shown in Fig. 8, when the confining pressure is 45 kPa, the dynamic rebound modulus of coarse-grained chloride saline soils with different salt contents gradually decreases with the increase of water content under different deviatoric stress. The water content increases from 4.0–6.0%, and the reduction amplitude of the dynamic rebound modulus MR of coarse-grained chloride saline soils with different salt contents is shown in Table 7. When the salt content is given a certain value, the influence of water content on the dynamic rebound modulus of coarse-grained chloride saline soils becomes more significant with the increase of deviatoric stress, and the decrease in dynamic rebound modulus becomes greater. The maximum decrease reaches 13.5% when σd is 90 kPa and the salt content Z is 5.0%. In addition, as the water content increases, the reduction amplitude of the dynamic rebound modulus of coarse-grained chloride saline soils increases with the increase of water content under different stress conditions. This is because the increase in water content promotes the formation of water film between soil particles, and the dissolution of salt in water destroys the salt skeleton in coarse-grained chloride saline soils, especially under high salt content conditions. As the water content increases, the lubrication effect of the salt solution on the contact behavior between soil particles increases, which weakens the friction strength between coarse particles, leading to a gradual decrease in the dynamic rebound modulus.
Table 7
Reduction amplitude of dynamic rebound modulus under different deviatoric stresses and salt contents
Salt content | Deviatoric stress |
σd = 23kPa | σd = 45kPa | σd = 90kPa |
Z = 0% | 6.4% | 9.7% | 12.6% |
Z = 2% | 7.5% | 10.1% | 12.2% |
Z = 5% | 8.6% | 13.3% | 13.5% |
Z = 8% | 9.3% | 12.0% | 12.8% |
As shown in Fig. 8, the dynamic rebound modulus MR of coarse-grained chloride saline soils with different water contents gradually decreases with the increase of salt content under different deviatoric stress conditions when the confining pressure is 45 kPa. When the salt content Z increases from 0.0–8.0%, the decrease in dynamic rebound modulus MR is shown in Table 8. It can be seen from Table 8 that the effect of salt content on the dynamic rebound modulus becomes more significant with the increase of deviatoric stress when the water content is given a certain value. The decrease in the dynamic rebound modulus gradually increases, and the maximum decrease reaches 43.3% when the deviatoric stress σd is 90 kPa and the water content w is 5.1%. At the same time, under different stress conditions, as the salt content increases, the reduction amplitude in the dynamic rebound modulus of coarse-grained chloride saline soils tends to increase with the increase in water content, and the maximum decrease reaches 2.1%. This is mainly because the increase in sodium chloride strengthens the salt skeleton of soils. After the soils encounter water, a part of the salt skeleton can be dissolved, increasing the gaps between soil particles and enhancing the compressibility of the soils. The increase in salt solution concentration promotes the lubrication between soil particles, making it easier for soil particles to slip or rearrange under cyclic loading, resulting in plastic deformation and a decrease in the dynamic rebound modulus of the soils.
Table 8
Reduction amplitude of dynamic rebound modulus under different deviatoric stresses and water contents
Water content | Deviatoric stresses |
σd = 23kPa | σd = 45kPa | σd = 90kPa |
ω = 4.0% | 31.2% | 35.5% | 43.0% |
ω = 5.1% | 32.2% | 37.2% | 43.3% |
ω = 6, 0% | 33.3% | 37.1% | 41.9% |
3.3. Effect of water content and salt contents on CBR
It can be seen from Fig. 9 that the CBR of coarse-grained chloride saline soils with different penetration depths decreases with the increase of water content under different salt content conditions, and the CBR of the penetration depth of 5mm is greater than that of 2.5mm. Therefore, the CBR corresponding to the penetration depth of 5mm is used as the bearing ratio of the samples tested in this study. As the water content increases from 4.0–6.0%, the CBR of the samples decreased from 45.4%, 39.1%,31.5% and 24.7–35.2%, 27.7%, 19.6% and 12.0%, the CBR values of coarse-grained chloride saline soils decrease by 22.5%, 29.2%, 37.8%, and 51.4% under low to high salt content conditions, respectively. This implies that the larger the salt content of coarse-grained chloride saline soils, the more significant the impact of water content on CBR. When the salt content increases from 2.0–8.0% (Fig. 9 (b), (c) and (d)), the difference between the CBR with a penetration depth of 2.5mm and the CBR with a penetration depth of 5mm tends to decrease as the water content increases. This indicates that the CBR of coarse-grained chloride saline soils is significantly affected by the initial water content, namely, the higher the water content, the smaller the CBR of coarse-grained chloride saline soils immersed by water. The above experimental phenomena, on the one hand, originate from the double layer theory. The thickening of the double layer weakens the cohesion and effective connection between soil particles, reduces the shear strength of the soil, and leads to the failure of the sample to achieve the expected compactness after compaction; On the other hand, are mainly due to the re-destruction of the soil skeleton subjected to the water and salt migration after the sample is immersed by water, and the plasticity of coarse-grained chloride saline soils is further enhanced. Therefore, the higher the water content, the more significant the strength attenuation of coarse-grained chloride saline soils immersed by water.
As shown in Fig. 10, when the compaction is 96%, the CBR of coarse-grained chloride saline soil samples prepared under different water content conditions decreases with the increase of salt content after immersion. With the increase of salt content in the formed samples, the CBR of the samples decreased from 45.4%, 38.4%, and 35.2–24.7%, 17.6%, and 12.0%, the CBR values of coarse-grained chloride saline soils decrease by 45.6%, 54.2%, and 65.9%, respectively, when the water content gradually increases. With the increase of water content, the reduction amplitude of the CBR of coarse-grained chloride saline soils caused by the increase of salt content gradually increases, and the reduction amplitude is 44.4%. In addition, as the salt content increases, the difference between the CBR with a penetration depth of 5.0mm and the CBR with a penetration depth of 2.5mm under different water content conditions is shown in Table 9. The difference increases first and then decreases. This result once again verifies the conclusion that the dynamic rebound modulus is affected by the chloride salt content. In other words, as the salt content increases, some or all of the chloride salts in the soils dissolve in water when the soil is immersed in water for a short time, causing the salt skeleton to be damaged and the gap between soil particles to increase. In this case, the rate of settlement deformation generated under the self-weight and load of the soil gradually increases. When the coarse-grained chloride saline soils are immersed in water for a long time and have a large amount of water, the chloride salt fully dissolves and the seepage of water in the coarse-grained chloride saline soils can take away some soil particles in the soils, causing the corrosion, which further increases the voids in the soils. In this case, the settlement deformation of the soil gradually increases, and the strength of the chloride saline soil gradually decreases.
Table 9
Difference between CBR5.0 and CBR2.5 under different water contents
Salt content | Difference between CBR5.0 and CBR2.5 |
Z= 0.0% | Z= 2.0% | Z= 5.0% | Z = 8.0% |
ω= 4.0% | 1.0% | 4.1% | 5.0% | 3.9% |
ω= 5.1% | 0.8% | 2.9% | 3.8% | 2.5% |
ω= 6.0% | 2.2% | 3.2% | 2.5% | 2.0% |