Journal of Engineering Geology

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Introduction
The dispersivity phenomenon occurs due to the dissolution of some of the ions in clay soils or against the shear stress of normal water flow in cohesion-less soils. Water surface flows in low slopes cause surface erosion of dispersive soils. Dispersivity in the soil starts from a point and gradually expands; the starting point can be the holes from the activity of the animals, the existing cracks or the growth path of the roots of the plants. There is a lot of field evidence to recognize the dispersivity of the loess soils. In field investigations, soil dispersivity can be detected according to the following parameters: geological origin of the loess soil, mineralogical composition, gradation, drainage pattern, slaking of agglomerates, specific morphology, high permeability, geographical area (length and width relative to origin), soil color, relationship between slope and soil erosion, precipitation, erosion of column cracks, heeling, mud flowing runoff and the presence of salt crystals in loess soils. In terms of sedimentological characteristics and engineering geological properties, Golestan loesses have been dispersed in three areas 1, 2 and 3, which are consistent with the loesses of clay, silt, and sand types, respectively.
Material and methods
Loess soils in three regions of east and northeast of Golestan province were sampled. Sampling was conducted in two forms of wax-coated agglomerates and metallic cylindrical tubes. Depth of sampling follows the foundation of the buildings located on the Mehr Housing site and the Cheshme Lee village, varying from 0.5 to 2 meters. On the path of the Beqqeje Bala village, sampling was carried out from the path trench. After transferring to the laboratory, samples were subjected to gradation testing, Atterberg limits test to determine the unit weight of the volume and density.
The pinhole test was done on samples with the unit weight of normal volume (g_n) and maximum volume (g_dmax) and its rate of dispersion was determined. The research background, field evidence and the results of laboratory experiments indicate the dispersion of soil sampling areas. The results show that soil compaction reduces the severity of dispersion and decreases the flow rate, so that the flow rate has decreased in the Maravehtapeh sample by 38%, in the Cheshmeli sample by 13% and in the Beqqeje Bala sample by 43%. Compaction cannot eliminate the dispersion of soil. Adding nanoclay decreases the severity of soil dispersion and eliminates its dispersion properties in most cases.
In order to evaluate the effect of nanoclay on severity and to decrease the dispersion property of soil with ratios of 0.5, 1, 2, 3, 4 and 5 wt%, of Montmorillonite Nanoclay was added.
The nanoclay used in the present research was selected from the Sigma-Aldrich America Company called montmorillonite nanoclay and was purchased from its domestic representative, i.e. Iranian Nanomaterials Pioneers Company. The product has a density of 300 to 370 kilograms per cubic meter and a particle size of between 1 and 2 nm. The specific surface area of the nanoparticle is about 250 square meters per gram. Its color in normal light and in 1 to 2% moisture is yellow to yellowish buff.
Results and discussion
The rate of dispersion of samples with nanoclay was measured in Pinhole Test Apparatus. Also, the method of mixing nanoclay with dispersive soil shows different behaviors in severity of dispersion and its reduction. Given that the specific surface of nanoclay is high and this property can include the whole surface of soil grains as a sticky coating and increase soil cohesion, the mixing method is practically one of the most important steps in examining the effect of nanoclay on soil stabilization. At ratios of 0.5, 1, 2, 3, 4 and 5 wt% of nanoclay, nanoclay was mixed with soils of sampling regions by four methods:
In the method A, they were completely mixed with the preparation of a homogeneous mud from soil and nanoclay via an electric mixer.
In the method B, mixing of loess soil with nanoclay was performed in optimum water content.
In the method C, mixing of loess soil with nanoclay was conducted in the form of dough by hand mixer. In the method D, mixing of loess soil with nanoclay was carried out in the form of vibration dry by grading sieve shaker.
After mixing with nanoclay in the desired method (four methods A, B, C, D), the samples were first stored in sealed plastic containers for 24 hours. Then, the samples containing nanoclay were reconstructed in cylindrical mold of the pinhole device with the unit weight of maximum dry volume and moisture of two percent higher than the optimum moisture content and a hole was created in the middle of it. The samples remained in this position for 24 hours, and then the test was performed. Testing was carried out on each sample according to the standard D4647-93, and flow rate reading was done over a period of two minutes to 18 minutes.
Conclusion
The conclusion of this study shows that the three loess samples taken have a dispersivity potential and the flow rate is low in the unit weight of maximum volume, but the dispersivity potential does not eliminate. Adding nanoclay with any weight ratio reduces the flow rate and eliminates the soil dispersivity potential.
The results of this survey showed that 1% nanoclay weight ratio is technically and economically the most appropriate mixing ratio. With this weight ratio, the method of preparing homogeneous mud with an electric mixer (method A) produces the lowest flow rate, so that the flow rate from 1.3 ml per second in pure soil to 0.3 ml per second in the soil containing nanoclay is reduced by 50 mm. Therefore, it can be said that this method is more suitable, but it is not operationally efficient and the method B is more appropriate. In the method B, the flow rate reaches from 1.3 to 0.55 ml per second.

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Introduction
The existence of problematic soils due to their geotechnical properties, such as low strength and stability, high compressibility, and swelling, is one of the most important issues and challenges that geotechnical and civil engineers are faced in urban environments, especially in metropolises. Various methods are used to stabilize and to improve the behavior of problematical soils such as compaction, consolidation, stone columns, jet grouting, biological procedures, and additive materials including nanomaterials. Because of their high specific surface, the use of nanoparticles is very effective to increase the shear and mechanical strength parameters of soil. Mashhad city is located on alluvial deposits of Mashhad Plain. A wide area of this city, especially the central and eastern areas where the Imam Reza holy shrine is located, has been built on weak and fine-grained deposits. Considering constructing high-rise buildings such as hotels and commercial complexes in these areas, as well as the need for restructuring the urban decay, the soil improvement will be inevitable. Given the significant application of these nanoparticles, the purpose of this study is to investigate the effects of nanoclay and nanosilica on each other and to find their optimal composition as a suitable alternative for traditional materials to improve the weak and problematic soils. This not only increases the bearing capacity and strength properties but also reduces the cost and time of project implementation.
Method and Materials
To achieve a hybrid with maximum strength and bearing capacity in executable projects, laboratory tests were performed on the soil picked up from the vicinity around Razavi holy shrine in Mashhad mixed with nanoclay and nanosilica. The type of soil is classified as CL-ML based on sieve and hydrometer tests. The nanoclay used in this research is the type of montmorillonite- K10, and the nanosilica is as a powdered shape with 99% purity.
At first, nanoclay and nanosilica were mixed independently with soil in six different weight ratios (0%, 0.1%, 0.5%, 1%, 2.5%, & 5%) and (0%, 0.1%, 0.25%, 0.5%, 0.75%, & 1%). Soil mechanical and strength properties, including compressive and shear strength, settlement, plasticity index, and swelling, were studied by standard laboratory tests on all specimens. After determining the optimum ratio of each nanoparticle, four hybrids consisting of nanosilica and nanoclay were made in four different combinations and then the effects of these four hybrids were investigated on the soil in the laboratory scale (Table 1).
Table 1. Composition of hybrids made with different percentages of nanomaterials

Nanomaterials composition	Hybrid Name
5% Nanoclay + 0.25% Nanosilica	5NC + 0.25NS
5% Nanoclay  1% Nanosilica	5NC + 1NS
2.5% Nanoclay + 0.25% Nanosilica	2.5NC + 0.25NS
2.5% Nanoclay + 1% Nanosilica	2.5NC + 1NS

Conclusion
The results of the Atterberg limit test on improved and pure soil indicate that the addition of nanoclay and nanosilica and the optimized ratios of these nanoparticles hybrid to increase the soil resistance parameters did not change the soil swelling index.
Evaluation of shear strength test results showed a significant synergistic effect of these nanoparticles on increasing the shear strength parameters. The nanoparticles hybrid of 2.5% nanosilica and 1% nanosilica increased the cohesion up to 106% and also hybrids of 5% nanosilica and 1% nanosilica increased the internal friction angle of soil up to 32%.
Examination of unconfined compressive strength tests presented a 134% increase in the compressive strength of the specimen improved with 2.5% nanoclay and a 620% increase in soil improved with 1% nanosilica. The optimum hybrid compositions of 5% nanoclay and 1% nanosilica increased significantly the compressive strength of the studied soil up to 785% and reduced the settlement of the soil by 60% compared to pure soil.

1. Laboratory studies of electron microscopy examination on ​​pure and improved soil samples with nanoparticle hybrid revealed the presence of these particles in pores of the improved soil. On the other hand, the high specific surface area of ​​the nanoparticles increased the interaction of the soil particles, and the effect of adding these nanoparticles on the refining process is observed in compressive strength increase.

As the nanoclay, nanosilica, and hybrid of nanoparticles are the results of soil processing, these particles are very effective to solve the environmental problems because of good compatibility with soil environments. In addition, low volumes of nanoclay, nanosilica, and hybrid in these nanoparticles are necessary to increase the compressive strength and decrease the settlement of soil. Therefore, using these nanoparticles at the project site reduces significantly the cost and execution time of the project.