To study the dynamic properties of nano-MgO-modified soil, the contents of nano-MgO which were 0%, 1%, 2%, 3%, 4% and 5% of the dry weight for soil were added into the soil. The water content of soil samples were 16% and 22%. The variation of shear modulus and damping ratio of soil was analyzed through the resonant-column test of the modified soil. The results show that with the increasing of shear strain, the shear modulus of soil decreases and the damping ratio of soil increases, the shear modulus for nano-MgO-modified soil reaches the maximum when the content of nano-MgO is 5%. The maximum shear modulus of soil with water content of 22% is smaller than that of soil with water content 16% under the same confining pressure, the shear modulus of soil increases linearly with the increasing of nano-MgO content. The reinforcement mechanism of the nano-MgO-modified soil was analyzed such as pore filling effect of nano-MgO, effect of water content, effect of vibration compaction, interaction between a nano-MgO group and a soil particle, interaction between several nano-MgO groups and soil particles.
The dynamic shear modulus and damping ratio are two important parameters of the dynamic properties for soil, which are essential parameters to seismic response analysis and seismic safety evaluation. The reasonability of parameters will directly affect the safety of engineering construction, these should be obtained for the engineering design. Conti and Viggiani  proposed a method to compute the shear modulus and damping ratio using the nonlinear fit of experimental transfer functions. The dynamic properties of most natural soil are insufficient to the anti-seismic needs of important engineering project, so some soil mixed with special material can be used to improve the dynamic shear modulus and damping ratio of soil. Yang  studied the influence of different confining pressures and different cement contents on dynamic properties of cement modified soft clay, and the variation law of small strain ranges is obtained. The results show that no simple and effective method has been found to improve the dynamic shear modulus and damping ratio of soil.
To improve the mechanical properties of soil, many materials were selected to add into soil in previous studies. Leong and Eriktius  used fly ash to improve peat soil. The results show that the shear strength of peat soil can be increased several times by adding fly ash, but the improved peat soil mixed with fly ash can easily pollute the environment; Oluremi et al.  has effectively improved the plasticity of mixed Laterite by mixing laterite in Nigeria with different sand and stone admixtures. Nano-materials is the superfine materials that it has the special structural features such as small size, size effect, quantum effect and interface effect, and it is applied to many areas [5,6,7]. The strength and deformation of the soil mixed nanomaterial are studied by some scholars [8,9,10,11,12,13]. Taha M. and Taha O. conducted some experiments to study the influence of nanomaterial on the properties of expansion soil and contraction soil, the results showed that the nanomaterials can reduce the development of surface shrinkage cracks of compacted soil without reducing the coefficient of permeability. Anggraini et al.  selected coir fibers, Ca(OH)2 and Mg(OH)2 nanoparticles to improve the mechanical properties of lime-treated marine clay, it showed that they can enhance the effective internal friction angle and the cohesion of soil. There are few researches about the reinforcement mechanism of nano-modified clay. Zhou et al.  added nano-alumina into the soil and conducted the unconfined compression strength test of nano-modified clay, the reinforcement mechanism of nano-modified soil was discussed. It can be found that the dynamic property of nano-modified soil is still need to study. It is very significant to conduct the dynamic test of nano-modified soil.
Nano-Mgo is a kind of high-function material with the development of nanotechnology. It has small size effect, surface effect, interface effect and other physical and chemical characteristics different from traditional magnesium oxide . These special effects of Nano-MgO make it show good application value and broad development space in materials, energy, information, biology and other fields . At present, the research and application of nano-materials in civil engineering mainly rely on cement-based materials and related building materials, while the research on natural soil is less. It is necessary to study the strength characteristics of nano-modified soil, especially the dynamic strength characteristics.
To improve the dynamic properties of nano-MgO (NM)-modified soil, a number of resonant column tests were carried out for nano-MgO-modified soil with different NM content, confining pressure and water content. The variation of shear modulus and damping ratio for modified soil affected by these parameters was studied. Based on the analysis of NM–soil interaction, the reinforcement mechanism of NM modified soil was discussed.
Materials and Methods
The NM used in this test was produced by Xuan Cheng Jing Rui New Materials Co. China. The physical parameters of NM are listed in Table 1.
The soil was taken from a construction site in Nanjing, China. The physical properties are shown in Table 2. The grain-size curve (based on a grain-size analysis test) is shown in Fig. 1. It shows that more that 50% of the particles have a particle size greater than 0.075 mm. After analyzing, the soil belongs to the low liquid limit silty clay soil according to Standard for soil test method, Ministry of Construction, P. R. China. GB/T 50,123–1999 .
The preparation process of soil samples: the soil after natural weathering was crushed (water content is 3.45%), the water contents (ω) of samples prepared in the laboratory were 16% and 22%. The NM contents selected were 0, 1, 2, 3, 4, and 5% of the total dry weight for soil. When the sample was prepared, water was first added to dry soil to make soil samples with corresponding water content, and then the NM with corresponding content were added to the soil samples to obtain the test samples. The related parameters of soil samples are shown in Table 3. The samples were compacted in five layers to produce a cylindrical sample with a diameter of 50 mm and a height of 100 mm. The NM-modified and unmodified compacted soil samples used in the test were prepared by hand-mixing the dry soil, NM and water. The soil samples were saturated by vacuum suction saturation method, the saturation time of vacuum suction was not less than 12 h. The relevant parameters of samples are listed in Table 3.
The resonant-column test has been used extensively for the dynamic characterization of soil under different confining pressures and shear strain conditions . The resonant-column test is conducted to study the dynamic shear modulus and damping ratio of soil sample at every exciting force in a certain consolidation pressure (100, 200, 300, 400 kPa). Due to the small amplitude of shear strain from 10–6 to 10–4, the resonant column test is carried out, and the damage of soil sample is very small. The results of resonant column test have little discreteness and good reproducibility. The instrument used in this test is GZZ-50 resonant column instrument produced by Liyang Yongchang Engineering Instrument Co. the instrument is based on the transverse-free vibration method, and the vibration frequency of test is set to 50 Hz.
Relationship Between the Shear Modulus and Shear Strain
The shear modulus and shear strain with different NM contents and 22% water content under the confining pressure of 100, 200, 300 and 400 kPa are presented in Fig. 2. It can be observed that the shear modulus remains unchanged with the increase of shear strain when the shear strain is small from 10–6 to 10–5 in the same confining pressure, and the stress showed a linear relationship with the strain when the shear strain was small. When the shear strain was larger than 10–5, the shear modulus decreased with the increasing of shear strain which showed the nonlinear of soil deformation. It can be seen from Fig. 2 that six groups of soil samples with different NM contents have shown the same law under different confining pressures. The shear modulus of soil without NM is the smallest of all, and the shear modulus of soil increases with the increasing of NM content.
Relationship Between the Damping Ratio and Shear Strain
Figure 3 shows that the damping ratio and shear strain with different NM contents and 22% water content under confining pressures of 100, 200, 300 and 400 kPa, the damping ratio of soil samples kept near 0.01 when the shear strain was small (10–6 ~ 10–5), and then began to increase with the shear strain. A small positive effect can be observed on the damping ratio of soil with the increasing of NM content, the increasing rate of damping ratio is low with the increasing of NM content at the same shear strain. In addition, the increasing rate of the damping ratio became stable gradually with the increasing of confining pressure.
Maximum Shear Modulus with Different NM Contents under Different Confining Pressures
The results showed the maximum dynamic shear modulus of soil with the increasing of NM content under different confining pressures. It can be found from the results that the dynamic shear modulus of soil has been strengthened obviously with the increasing of NM content under various confining pressures of 100, 200, 300 and 400 kPa. It showed that the maximum shear modulus of soil sample C12 with 5% NM content under four kinds of confining pressures are 119 MPa, 157 MPa, 191 MPa and 216 MPa. Compared with the maximum shear modulus of soil sample C7 without NM, the maximum shear modulus increased, respectively, by 118.8%, 87.1%, 73.6% and 60.4%, and the maximum increasing proportion can reach to 118.8%. It is obvious that nanomaterials can improve the mechanical properties of soil.
Effects of NM Content and Water Content on the Maximum Shear Modulus
Figure 4 shows the relationship between Gmax and NM content under different confining pressures, and Fig. 5 shows the relationship between Gmax and NM content with different water contents. On the condition of the same water content (22%), it can be seen from Fig. 4, the maximum shear modulus of soil samples have nearly shown a linear increase with the increasing of NM content. It means that the maximum shear modulus of soil was significantly affected by NM. It can be seen from Fig. 5, the maximum dynamic shear modulus of soil with water content 16% is larger than that of soil with water content 22% under the confining pressure of 400 kPa, the increasing trend was very close for two different kinds of water contents. The increasing rate of the maximum dynamic shear modulus of soil with water content 16% began to decrease when the NM content surpass 4%. The same thing does not happen when the water content is 22%.
The experiment results showed that NM in the soil can make an obvious increasing of the dynamic shear modulus of soil. The increasing proportion can reach to the maximum of 118.8% with water content of 22% and NM content of 5%. It can be easily concluded that the small amount of NM can improve the maximum shear modulus significantly. It shows the effects of NM on the mechanical properties of soil. The future of nanomaterial application is worth looking forward to in the geotechnical engineering. There are five aspects about the reinforcement mechanism of nano-modified soil.
Pore Filling Effect of NM
Figure 6 is a diagrammatic sketch of NM group filling up the pores between soil particles. Figure 7 is the sample diagram of soil sample with different Nano-MgO content. The average diameter of NM particles is much smaller than that of the soil particle. NM particles would fill up the pores between soil particles when NM mixed with wet soil, which has a certain effect on the reduction of amount for soil intergranular pores and size. It makes the pores smaller and improves the distribution of pores, the contact between the particles is changed to make the soil compactness and the strength of clay soil is enhanced. The soil particles become denser with the increasing of NM content, the shear modulus is enhanced with the increasing of NM content. The soil appears to be fully elastic when the shear strain is small, and it consumes little energy when the dynamic load stress wave is spreading in the soil. While the shear strain becomes larger, the probability of occurrence of relative motion between soil particles will increase. Because the average diameter of nanoparticles is much smaller than that of soil particles, the filling effect of nanoparticles on the pore space between soil particles makes the contact area between them larger. The spreading resistance of dynamic load stress wave becomes larger, which cause the increasing of energy consumption. The dynamic damping ratio of soil increases with the increasing of shear strain and NM content. NM content has little influence on the dynamic damping ratio when the confining pressure is very large. The contact between the soil particles and NM particles becomes closer under high confining pressure. The probability of relative motion becomes lower and the energy consumption tends to stable, the damping ratio tends to stable.
Effect of Water Content
Due to the hydrophilic properties of nanomaterials, the NM particles will absorb water in the pores, which makes a lot of NM particles agglomerate together to form a small group during the process of NM and soil mixed with water. As is shown in Fig. 6, a large amount of free water between soil particles is adsorbed on the surface of nanoparticles, which makes the direct contact between soil particles and water less, it can be seen that nano-materials have some hydrophilicity and strong adsorption. The effect of water content on soil strength becomes less. The liquidity of soil becomes lower when the pore water is absorbed by NM group, the soil particle movement is limited. The maximum shear modulus increases significantly with the increasing of NM content under the condition of high water content. In practical engineering, the advice of mixing nanomaterials into soil could be considered for decreasing the water effect on engineering properties of modified soil.
Effect of Vibration Compaction
The relative dislocation and realignment always happen between the soil particles when the soil sample is under dynamic loading [21, 22]. At this time, nanomaterials will put into smaller pores with soil particles to form a more stable space force system. It performs more perfect pore structure and closer contact between soil and nanomaterials shown in Fig. 8. When the pores become smaller, the effect of NM group becomes more prominent, the interactions between soil particles and nanomaterials become stronger due to their closer contact.
Interaction Between Single NM Group and Single Soil Particle
There is the strong force between NM particles, because the NM particles are the small size, large specific surface area and high surface activity. Because of the uneven surface of nanoparticles, the friction force on the surface and the mechanical occlusion force between the particles are easily produced when the particles contact and move with each other. These forces make the nanoparticles aggregate easily. The NM particles have close contact with soil particles due to its large specific surface area. The force between NM particles and soil particles is very strong. The soil particle would produce relative motion or the tendency of relative motion with other soil particles when it is subjected to twist shear force [23,24,25,26,27]. Figure 9 shows that the NM group would produce restriction force on soil particles. It will limit the motion and deformation of soil particles, the contacting pattern is produced between single NM group and single soil particle when the relative motion happened. The contact of these would be closer when they are subjected to compression under the condition of dynamic loading. The restriction would be strengthened. The restriction is very obvious for the high stiffness of NM group. The unit which is made up of a NM group and a soil particle produces less strain, the shear modulus is larger compared to single soil particle under the same stress condition.
Interaction Between Various NM Groups and Soil Particles
Many NM groups have contact with soil particles after mixing them together. NM groups would fill up the pores between soil particles, and every single NM group would be surrounded by several soil particles. Therefore, the nano-group has a certain spatial restraint effect on all soil particles in contact with itself. Accordingly, the soil particles are also surrounded by many NM groups. These NM groups constitute a space grid, and there exist the gravitation between NM groups. Each NM group has a certain appeal to the surrounding NM groups, forming a spherical force space, so they are located in both the center of the sphere and the surface of another sphere. The soil particles are located in the spherical force space. In this way, every NM group is attractive by many other NM groups. The magnitude of force between two NM groups is closely related with the size of NM groups and distances between them. The increasing of NM content increases the volume of NM groups and decreases their distance, which increases the force between NM groups. These interactions can be influenced by each other. The soil particles can transfer the force from dynamic loading to the space structure formed by NM groups effectively, and the space structure would become more stable through adjusting based on the movement and deformation of soil particles. It is shown in Fig. 10, NM groups and soil particles are interacted in such an effective way. The shear modulus of the soil particles surrounded by NM groups increases as the soil is constituted of countless groups.
Through the resonant column tests, the dynamic properties of nano-MgO-modified soil were analyzed, the following conclusions can be obtained:
NM can enhance the dynamic shear modulus of soil significantly. The dynamic shear modulus increases with the increasing of NM content. The dynamic shear modulus reaches the maximum value when the NM content is 5%.
NM can also enhance the damping ratio obviously, especially under the condition of small shear strain. The damping ratio increases with the increasing of NM content at the same shear strain. The damping ratio of NM modified soil is larger than that of soil without NM.
The variation of shear modulus of soil under different water contents was analyzed. The results showed that when water content was higher, the shear modulus and damping ratio of soil with NM would become smaller, the strength increasing was more obvious under higher water content.
Based on the analysis of NM–soil interaction, the reinforcement mechanism of NM modified soil was discussed. There are five aspects about the reinforcement mechanism of nano-modified soil: pore filling effect of NM、effect of water content effect of vibration compaction interaction between single NM group and single soil particle and interaction between various NM groups and soil particles. Through the reinforcement mechanism of the above five aspects, nano-materials effectively strengthen the dynamic strength characteristics of modified soil and improve the mechanical properties of the soil.
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Financial support comes from the National Natural Science Foundation of China (Grant No. 51508159), the Fundamental Research Funds for the Central Universities of Hohai University (No. 2019B12914), the Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University (No. GHXN201904) are gratefully appreciated.
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Gao, L., Luo, Y., Ren, Z. et al. Experimental Study on Dynamic Properties of Nano-MgO-Modified Silty Clay. Int. J. of Geosynth. and Ground Eng. 6, 27 (2020). https://doi.org/10.1007/s40891-020-00210-5
- Modified soil
- Shear modulus
- Damping ratio
- Resonant-column test