Journal of Engineering Geology

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(Paper pages 523-542) This paper presents a rigid circular footing model with specified properties and dimensions on a sandy-clay soil with Mohr-Coulomb material. This model is analyzed dynamically with finite difference 2D FLAC software under vertical component of ground excitations. Then the soil is improved with cement grouting and analyzed again. Consequently, the load-settlement curves under a circular footing, due to vertical component of ground accelerations through the underlying soil, are plotted. Also the dynamic bearing capacity of natural and soil cemented foundation is presented and discussed. The analysis results show that adding 2, 4 and 6 percent of cement, with certain conditions, cause 2.7, 4.2 and 7.0 times increase in dynamic bearing capacity, respectively, in comparison to normal soil.

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(Paper pages 757-772) Jet grouting is a method for improving of soil properties and its physical characteristics. However, in this method due to high pressure and velocity of cement slurry the soil structure has been damaged as some parts are moved from the borehole replacing with cement slurry. The grains, which are remained in the borehole, mixed with slurry (cement) and create an improved mass of soil. This mass is named “Soilcrete”. Soilcrete mass has special characteristics such as high strength, low deformability and very low permeability. In this paper, principles governing to jet grouting and effective parameters have been explained. Then the test results obtained from Soilcrete column have been analyzed and discussed. Based on the results, jet grouting has led to increase and improvement of physical and mechanical characteristics of soils, i.e. uniaxial compressive strength, cohesion and internal friction angle. Finally the values of jet grouting parameters are recommended in order to achieve larger diameters in the mentioned site based on trial grouting results

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The collapsible soils are usually known as soils with open structure and weak bonding between particles. The behavior of such soils is similar to very loose soils. These types of soils, when saturated without any changes in loading or subjecting vibratory loads, experiences huge settlements. The present research deals with investigation on collapsible soils located in the North East of Mashhad. The results of laboratory and in situ tests show that collapsible soil in this region was very sensitive to the increasing of moisture content. This means that an increase in soil moisture content, significant excess settlement occurs during a short time. This indicates that the soil in this region suffers from high potential collapsibility. The huge soil settlement will lead to the stability of existing structures to be at risk. It is, therefore, necessary for the collapsible soil in the region to be improved. For soil improvement, many techniques including moisture mitigation and soil replacement or compaction may be employed. Also stabilization of soil with lime, cement or coarse aggregates are practical methods. Which The results of the present research indicate that stabilization of soil lime is the most appropriate method for increasing bearing capacity of soil and reducing structural settlement.

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Introduction
The design engineers usually follow a specific decision-making process for optimal selection of the type of required foundation and its design. In this state, in case the surface foundation is not appropriate for the project conditions, before making any decisions about the use of deep foundations, the proper methods for optimization of the liquefied soil should be evaluated in order to compare the advantages and disadvantages of each of them with those of deep foundation, in terms of efficiency, implementation problems, costs, and finally to select the best choice. One of the best methods of soil improvement is the use of stone columns. The rationale behind the use of stone columns is the high shear strength of materials and the provision of lateral grip by surrounding soil. Therefore, the stone column can receive the load from the structure, and transfer it to the resistant layers. In the soils with low shear resistance, the lateral constraint crated by the surrounding soils is not enough for preventing the sideway buckling of the column under which is subjected to the loads. Thus, special measures should be considered for the use of stone columns in these soils. One of these methods is the use of reinforcement shelves such as geogrid and geotextile. Investigating the previous studies, the lack of evaluation of the design parameters such as the settlement ratio of the soil improved by the reinforced stone column to the geogrid, and provision of design graphs in this regard, has been revealed. Therefore, by extension of the studies conducted by Chub Basti et al. in 2011, the design graphs were provided in this regard.
Material and methods
The PLAXIS V8 Software was used for modelling the soft soil improved by the stone column. For increasing the precision of the results, the 15-knot element was used in the current study. The fine mesh was used in the models made for the analysis of the problem. For simulation of the improved soft soil with the stone column in a single cell, the modelling was implemented in a two-dimensional environment in axial symmetry conditions. In the current study, it was assumed the rigid foundation is on the improved bed. Thus, for analysis of the simulated model, a vertical strain up to 2% of the soft soil height has been applied on the ground. Also, for simulation of the soil behavior, an appropriate model of soil and parameters proportional to the materials should be allocated to the construct geometry. The non-linear stress-strain of the soil in different levels of the problem can be simulated. The number of model parameters increases with the level of problem rupture. For precise simulation, we need the proper parameters of the materials. For modeling of soft soils and stone columns, elastic-plastic model with Mohr-Coulomb rupture criterion was used. In the current study, it was assumed the soft bed is located on a very hard layer of soil. Therefore, the vertical deformation was prevented on this horizontal boundary. Also, the horizontal deformation in two vertical edges was prevented and only deformation in vertical direction was allowed. The soft bed close to saturation was considered without the determined free water level. For models with stone columns, the element of interface between the stone column and soft soil, has been used. The reason behind using this element is that the stone column rupture is of shear form and due to this, a significant shear stress is created on the common surface between the stone column and soft soil. The percentage of the replacement area is defined as the ratio of the total area of the stone columns to the total area of the non-improved area. In the current study, the percentage of the replacement area is utilized between 10 to 30%, which is used in implementation. Also, the diameter of the stone columns is from 0.6 to 1.2, in the analyses.
Results and discussion
The results of the numerical study were compared with the existing theoretical relationships provided by Poorooshasb and Meyerhof (1997), and Pulko et al. (2011). Figure 1 shows the comparison of the replacement percentage (RP) and settlement ratio (SR) in the non-reinforced state in the current study as well as theoretical relationships proposed by the previous researchers. Based on this figure, there is a difference between the results of the current study and those of Poorooshasb and Meyerhof (1997), and Pulko et al (2011). Poorooshasb and Meyerhof (1997) calculated the settlement ratio in their proposed material with the assumption of linear elasticity of the materials without consideration for plastic settlement. Therefore, the settlement of the improved soft soil with stone column, calculated by Poorooshasb and Meyerhof, would not show the real amount. However, Pulko et al. (2011), with consideration for the elastoplastic behavior of the materials, the lateral expansion of the stone column, and the primary stress of the soil around the column, provided more realistic results that correspond closely with the present study. Also, for designing the stone column, the results of its reinforcement have been also provided in the graph presented in Figure 2. Thus, by the use of these graphs, the ratio of settlement reduction can be obtained for each distance between columns and with different percentages of alternatives../files/site1/files/124/2jalili%DA%86%DA%A9%DB%8C%D8%AF%D9%87.pdf

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Introduction
In many areas of the world, the mechanical properties of soils for utilization of land are not sufficient. For improvement of these lands, soil stabilization such as compacting, installation of nails, elders of piles, mixing soil with lime or cement before or during constructions on the surface or inside of the ground can be useful. Microbially induced carbonate precipitation (MICP), due to its versatility and stable performance, has been recently attracted the attention of many researchers in the field of the geotechnical engineering around the world. MICP is a biological technique that is naturally caused to create a cementation agent, which is known as calcium carbonate or calcite by controlling the metabolism of bacteria. Although there are many biological processes that can be lead to MICP, but the using of urea hydrolysis by bacteria is commonly used more. In this method, aerobic bacteria with the enriched urease enzymes inject into the soil. Hydrolysis of urea occurs when the bacteria speeds up the hydrolysis reaction to produce ammonium and carbonate ions. In the presence of soluble calcium ions, carbonate ions are precipitated and formed the calcium carbonate crystals. When these crystals are formed on a grain of soil or like a bridge between them, they prevent the movement of grains and thus improve the mechanical and geotechnical properties of the soil.
Material and methods
In the present study, the effect of increasing fines on the improvement of Anzali sandy soil, and soil resistance parameters for improving the clean sand and its mixtures with a fine grained cohesive soil and a fine grained cohesionless soil separately in a percentage weight of 30 by MICP and using a small scale of direct shear test (6×6) have been investigated. In the present study the sandy soil was collected from the coast of Bandar Anzali Free Zone and for the preparation of samples of clayey sand and silty sand, Kaolinite clay soils and Firouzkooh broken silt were used, respectively. Anzali sand is poorly graded and had a rounded corner with an average particle size of 0.2 mm, somewhat, sharpening cores are also found in its granulation. In addition, its fine grained content is very small (less than 1%). The Kaolinite clay is also labeled with a liquid limit of 40, a plastic limit of 25, and a plasticity index of 15 as an inorganic clay (CL). The used microorganism in this study is urease positive Sporosarcina pasteurii, which is maintained with the number of PTCC1645 at the Center Collective of Industrial Microorganisms of Iran Scientific and Research Organization. The bacterium was cultured in a culture medium containing 20 g/l yeast extract and 10 g/l ammonium chloride at pH 9 under aerobic conditions in incubator shaker machine at 150 rpm and temperature of 30 °C. The organism was grown to late exponential/early stationary phase and stored at 4 °C before injection in samples. A solution of calcium chloride and urea with a molar ratio of one is also used as a cementation solution. With the direct shear test (6cm×6cm) as a benchmarking of the shear strength in the before and after improvement steps, molds fitted with a shear box made of the galvanized sheet with a thickness of 0.6 mm and it consists of two main parts, the body,  in the middle of which an exhaust pipe was embedded in the injector waste fluid. At the bottom of the samples, a layer of filter paper was placed in order to prevent soil washes, and then all samples with a thickness of 2 cm, with a relative density of 30% at the same weight and height were pressed. In the upper part of the samples, a layer of filter paper is similarly used to prevent the discontinuity of soil particles when injected biological materials are used. Biological solutions are injected from the top to the specimens and allowed to penetrate under the influence of gravitational and capillary forces in the sample and discharge the inhaled fluid from the exhaust pipe. The criterion for determining the volume of the solution to inject into each sample is the pure volume (PV) of soil. The preparation process of the samples was initiated by injection of a PV water unit, followed by a two-layer mixture of bacterial suspensions and cementation solutions, each with a volume of one PV, and then for biological reactions, 24 hours to the sample at laboratory temperature (25 ± 2) is given. After the time of incubation, the solution of cementation is injected into the sample for a period of three days and every 24 hours. The processing time of samples is also considered 28 days. In this study, optical density (OD) was selected as a benchmark for estimating the concentration of bacterial cells in the culture medium, and in all stages of development, and precisely before injection of bacteria suspension into soil samples, it was measured by a spectrophotometer device at 600 nm (OD₆₀₀) wavelength, which was obtained for all bacterial suspensions in the range of 1.7 to 2 before the injection. To determine the activity of urea bacteria, 1 ml of bacterial suspension was added to nine milliliters of 1.11 molar urea solution, and by immersing the electrode of the electrical conductivity in the solution, its conductivity was recorded for 5 minutes at 20 ± 2 ° C. The rate of urea activity in the pre-treatment stage for all specimens was in the range of 0.8 to 1.23 mS min^-1. In order to evaluate the shear strength parameters of soil samples, before and after the improvement operations, a direct shear test was used based on the ASTM D3080 standard. This test was performed for all samples under stresses of 50, 100 and 150 kPa in undrained conditions at a loading speed of 1 mm/min up to a strain of 15%. Also, samples of soil with a moisture content of 7% and a relative density of 30% (as already mentioned) have been restored. SEM analysis was carried out to determine the distribution of sediment between soil particles and EDX analysis in order to identify carbonate calcium sediment formation elements in improved soil samples, by scanning electron microscopy on Anzali sandy soil samples in before and after improvement conditions.
 
 
Conclusions
The effect of the increasing cohesive and cohesionless fines on the bio-treated process of sandy soil is the main subject of this research. For this purpose, three samples of clean sand, sand containing 30% clay and sand mixture with 30% silt in a relative density of 30% were treated with MICP method and their shear strength parameters were evaluated by direct shear test after 28 days of processing. Using the direct shear test and analyses of SEM and EDX data, the results are represented as below:
1.  The microbial sediment of carbonate calcium has greatly improved the resistance properties of all three soil samples.
2. A sample of clayey sand, in spite of a higher improvement compared to the other samples with an average shear strength of 113.7% in comparison to to its untreated state, it has the lowest shear strength among the three improved samples.
3. Increasing the clay content of 30% increases the soil voids. On the other hand, it reduces the friction angle and shear strength of the soil in the pre-treated state and also facilitates easier movement of the bacteria between the pores in the soil. More favorable distribution of sediment calcium carbonate was occurred and, as a result, increased adhesion between soil particles.
4. The increase of cohesionless fine particles creates more bonding points between sand particles and, therefore, calcium carbonate crystals form shorter distances between the soil bridges. As a result, with the end of the improvement process, the shear strength parameters of the sandy soil containing 30% of the silt compared to the clean sand have a higher value.
5. SEM images of the clean sand in both before and after improvement show that the calcium carbonate precipitation occurred with a uniform and thin layer that surrounds sand grains and another part of the sediments formed in the joint of grains.
6. Cube-shaped crystalline sediments confirm that the sediment formed in the soil is a stable type of calcite and that the relative increase in the friction angle of the improvement samples can be attributed to solid particles and multifaceted sediments. Also, the elements of carbon, oxygen, and calcium, which are the main components for the formation of calcium carbonate deposits, have been found in the EDX analysis of improvement sand samples../files/site1/files/124/7sohrabi%DA%86%DA%A9%DB%8C%D8%AF%D9%87.pdf
 

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Introduction
Microbial induced calcite precipitation (MICP) is one of the environment-friendly soil improvement methods that uses urease activity of the microorganisms to bound soil grains.
This method is based on three following steps:
1. Urea hydrolysis by urease activity of microorganisms and formation of ammonium and carbonate ions:
(2)
2. The reaction between carbonate and calcium ions and formation of calcium carbonate:
(2)
3. Bonding the soil particles by calcium carbonate.
One of the main challenges in use of MICP for soil improvement is the selection of proper injection method. An efficient injection method should lead to the construction of a homogeneous specimen beside of less used materials. In this study, a new method based on the theory of convection of liquids, for injection of bacteria and cementation solution is introduced.
Specimens are made according to the new injection method and their strength and homogeneities are tested. The obtained results are compared with the specimens which are made based on common injection method. Eventually, the success of the proposed injection method is investigated.
 
Material and methods
Gram-positive microorganism Sporosarcina Pasteurii No. 1645 (DSM 33) is provided from Persian type culture collection (PTCC). To make sand columns, Poly Vinyl Chloride (PVC) tubes were used with an internal diameter of 5cm and length of 12cm. Molds were placed vertically and a scouring pad and approximately 1 cm gravel as a filter are placed at the bottom of the column. Then the column packed with pure silica (Table 1). Finally, a scouring pad and approximately 1 cm gravel as a filter are placed at top of the column and mold were closed with a threaded Polypropylene layer on top and bottom with a hole for injection of bacteria and cementation solutions.
Table 1. Sand properties used in this study

Soil Type	Gs	γd	e	D10	D30	D60
Sw	2.6	1.84	41%	0.11 mm	0.43mm	0.85mm

In this study, a new multi-step method of injecting bacterial and cementation solutions is introduced. Injection of solutions is done after washing the sand column with distilled water. At the first step, 0.25 times of the void volume of soil, the bacterial solution is injected into the sand column. The bacteria allowed resting in the sand for 2 hours before the cementation solution was injected. After 2 hours, cementation solution is injected into the sand column by the amount of 0.25 times of pore volume of soil. The cementation solution consisted of 1.5 M urea and 3 M Calcium chloride. Again after 2 hours delay, bacterial solution and cementation solution are injected into sand column both by the amount of 0.25 times of pore volume of soil, same as aforementioned steps. In order to provide a comparison between the proposed injection methods of this study with conventional injection method, specimens are also made by the conventional method. In these specimens, bacterial solution and cementation solution are injected into the soil both by the amount of 1.5 times of pore volume of soil.
Results and discussion
To evaluate the homogeneity of the biologically improved sand specimens, the specimen is divided into 6 equal parts and the amount of calcium carbonate in each part is measured. It is found that calcium carbonate crystals are formed more homogenous in parts of specimens which are improved by new injection method (Figure 1). While specimens improved with conventional injection method are not homogeneous. The new injection method used in this study is based on the theory of convection in cementation and bacterial solution. Since the specific gravity of used cementation solution (3M urea and 1.5M calcium chloride solution) is 1.120 gr/cm3 and the specific gravity of ammonium chloride (which is the result of reaction between ammonium and chloride ions) is 1.031 gr/cm3, therefore a convection flow occurs in cementation solution after urease reaction (reaction 1) because of difference in specific gravity of two mentioned solutions. This convection flow causes a sustainable contact between cementation and bacterial solution in entire height of specimen.

Figure 1. Amount of calcium carbonate deposition along improved specimens by new and conventional injection method
To examine the efficiency of newly suggested injection method in this study, uniaxial compressive strength test (UCS) is performed on biologically improved sand specimens. Figure 2 shows stress-strain curves of specimens. The peak strength of specimens with conventional injection method is about 0.6 MPa. While the peak strength of biologically improved specimens prepared by new injection method is about 1.6 MPa. The reason for this difference in the obtained results is that when the volume of bacterial solution is more than the pore volume of soil, a part of bacteria solution in the first step of injection is removed. Then with an injection of cementation solution, more amounts of bacteria removes from the specimen before efficient placement of bacteria between soil particles. However, in new injection method the total volume of injection solutions (bacterial and cementation solutions) are equal to the pore volume of soil and this prevents the removal of bacteria from a porous medium.

Figure 2. Uniaxial stress-uniaxial strain curves of biologically improved specimens
Conclusion
In this study, the feasibility of using a new injection method for biological soil improvement is investigated based on the theory of convection with the aim to decrease the volume of bacteria and cementation solution. In this method, the final volume of bacterial and cementation solutions are reached the soil void volume in 4 consecutive injection steps. Specimens are made to investigate the efficiency of the proposed injection method. Also, specimens are made base on conventional injection method to provide the comparing possibility. Studying the precipitated calcium carbonate along the specimens show more homogeneity in ones prepared by proposed injection method in comparison to the specimens made by the conventional method. The obtained results of UCS tests are also showed that specimens made by new injection method have the more uniaxial strength (1.6 MPa) while the conventional method specimens are presented the strength of 0.6 MPa. Eventually, the proposed injection method of this paper implies less amount of bacterial and cementation solutions in a proper and efficient manner to bond the soil particles which leads to specimens with more strength, stiffness and homogeneity../files/site1/files/131/2Extended_Abstract.pdf
 

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Introduction
Pavement layers as a part of road structure play an important role and provide a flat and secure surface. Subgrade layer could act as a compacted embankment, natural or stabilized ground. Subgrade is a foundation of pavement layers, and it withstands all of loads due to vehicles that are transferred from upper layers (i.e., subbase, base and asphalt layers).Therefore, constructing pavements with bearing capability, high durability, quality, and maintenance in proper operating conditions is very important. However, suitable materials for constructing pavement layers are not available, and improvement techniques should be employed for them. Generally, different methods such as mechanical or chemical are available for improvement. Nowadays, geosynthetic materials such as geotextile and geogrid are used to optimize and enhance the bearing capacity of pavement layers. The present study is aimed to investigate the effects of geotextile applications on bearing capacity of clay-gravel mixtures in pavement layers.
Material and Methods
In this research, materials were prepared from Barandouz area. Clayey soil was mixed with gravel in 25, 50 and 75 percentages (by weight). Geotextile was woven and made of polypropylene (with commercial name Fibertex-F-32). Geotextile effects in mixture were evaluated in two conditions. Position number one indicates the arrangement of geotextile.  This means, at first, one geotextile layer was embedded in the middle of materials. Then, two and three geotextile layers in equal depths from each other were used in soil mixtures. Position number two shows the mixing pieces randomly. This means that geotextile pieces in 1×1 and 5×5 cm² were prepared and were randomly mixed with materials in 1, 2 and 3 percentages (by weight). For evaluating geotechnical behavior of improved clay-gravel mixtures, compaction and California bearing ratio test (CBR) (in dry and saturate conditions) based on ASTM were performed.        
It should be noted CBR test in dry and saturate conditions were carried out in three different compaction energies (i.e. 10, 25 and 56 blow count for per layer). Moreover, CBR was evaluated for piston penetration at 2.5 and 5 cm in the specimen.
Results and discussion
The findings of this study could be summarized as:
1. Results of compaction test showed that, in the unimproved position, with increasing gravel content in clay, maximum dry unit weight (γd_max) has been increased, while simultaneous optimum water content (w_opt) decreased.
In the improved position, in the first mode, when a geotextile layer was embedded in the middle of the specimens, γd_max reached to its upper value, whereas w_opt reached to its minimum value. On the other hand, with an increase in the number of geotextile layers in clay-gravel mixtures, dry density has been decreased, but optimum water content increased. Furthermore, in the second mode, when geotextile pieces with 1×1 and 5×5 cm² were randomly mixed in the specimens, the findings revealed that geotextile pieces with 1 cm² areas and 1% by weight in clay-gravel mixtures increases γd_max and reduces w_opt.
2. In dry and saturate conditions, California bearing ratio (CBR) test result displayed that in the unimproved condition, with an increase in gravel content in the clay, CBR value has been increased. In the improved situation, in the first mode, when a geotextile layer was embedded in the samples, CBR had a maximum value in all of the compaction energies even though it is reduced as the number of layers increased. In the second mode, when geotextile pieces in 1×1 cm dimensions with 1% (by weight) were randomly mixed with the specimens, CBR value reached at high.  In contrast, with increasing dimensions of pieces and percentages in the presence of geotextile in clay-gravel mixtures, CBR values declined.  Therefore, it can be concluded that, according to Code 234 (Iran Highway Asphalt Paving Code), the application of one geotextile sheet in the middle of materials or geotextile pieces in 1×1 cm dimensions with 1% (by weight) random mixing  is suitable for subbase and base layers in pavement design.
3. CBR test results in the saturate condition in clay-gravel mixtures illustrated that, in the non-reinforced condition, with an increase in clay content in specimens, swelling value keeps rising sharply. On the contrary, in the reinforced position with embedding a geotextile layer in the middle sector of samples or through adding geotextile pieces (1 cm²) with 1 % content  (by weight) to the specimens, the rate of swelling significantly decreased.   
Conclusion
To sum up, the main objective of the present study was to investigate the impact of geotextile applications on bearing capacity of clay-gravel mixtures in pavement layers. The findings demonstrated that when geotextile as a layer was embedded in the middle part of specimens or as pieces with 1×1 cm dimensions and 1% content (by weight) was randomly mixed with the mid materials, the bearing capacity of the reinforced specimens was enhanced.  In contrast, in the saturate condition, swelling potential significantly was reduced. It is noteworthy to mention that 1 cm² pieces of geotextile is more effective than the layers. This is due to the fact these pieces make aggregates closer to each other. Thereby, minimum void ratio (e_min) reaches its least value, the structure of grading improves, and the contacts between particles and geotextile pieces rise. As a suggestion for further research, it looks promising to evaluate the dynamic properties and the behavior of the improved materials with other geosyntheticses.
 

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Introduction
Improving the geotechnical characteristics of soils including superficial or deep soils has always been a challenge to geotechnical engineers. Therefore, various physical and chemical methods are used to improve different types of soils. In general, any physical, chemical, biological or combination of methods are used to change the characteristics of natural soil mass in order to achieve engineering goals which is defined in the "soil stabilization." Among different types of additives for soil stabilization, the use of pozzolans has been investigated by researchers because of their chemical compatibility with the environment and the cementation products due to chemical reactions. Todays, a lot of researches has been done on the use of natural or artificial zeolites as pozzolanic materials for the production of cement mixtures. This material, as a pozzolan, increases the speed of the pozzolanic reactions and reduces the density of cement products. However, many studies have been done to investigate the effect of zeolite and sepiolite on the resistance of cement products such as concrete, but so far, the use of these additives has been less considered for soil improvement. On the other hand, because of the compatibility of zeolite and sepiolite with the environment and their unique physiochemical properties, it is necessary to pay attention to these additives in order to improve the soil. Therefore, in this research, the effect of zeolite and sepillot additives with different percentages at different treatment times have been investigated to determine the elasticity modulus and hydraulic conductivity with focus on soil microstructure behavior.
Materials and methods
1. The properties of the soils
In this research, two types of soil including clayey sand (with 20% clay) and sandy clay (with 51% of clay) were used. The studied soils were a mixture of clay and sand of Firoozkouh (a typical type of sand located in north of Iran). Some physiochemical properties of zeolite and sepiolite are presented in Table 1.
Table 1. Physiochemical properties of zeolite and sepiolite used in this study

L.O.I.	Na2O	K2O	MgO	CaO	Fe2O3	Al2O3	SiO2		Chemical component
25.11	0.02	0.01	15.73	0.01	o.61	0.3	55.3		Sepiolite (%)S
11.94	0.13	-	0.87	2.45	1.26	13.54	69.74		Zeolite (%)

2. Experiments
The uniaxial compressive strength tests were performed at 0.1 mm/min according to ASTM D2166 standard. The stabilized soil samples were compacted at percentages of 0, 5, 10, 15, 20 and 25 in cylindrical molds (38mm × 76mm) in five layers to achieve the desired density. In order to investigate the effect of curing time, the samples were placed inside sealed containers and underwent the test at instantaneous, 7, 14, and 28 days and at the desired additive percentages. To investigate the effect of additives on the soil hydraulic conductivity, clayey sand soil with additives 5, 10, 15, 20, and 25% was prepared using dry mixing method. Then, the prepared mixture was poured from a specific height into the permeability mold with a height of 8.65 cm and diameter of 5 cm. In this way, the specific dry unit weight of all samples was obtained as 1.47 g/cm³, close to the minimum specific dry unit weight. In this research, concerning the considerable effect of fine-grained soils on hydraulic conductivity, falling head test was used to determine the permeability coefficient.
In order to the morphology of the clayey sand soil without additives and stabilized with additives 15% was examined through SEM test.
Discussion and results
1. Modulus of elasticity
In this study, after uniaxial tests in different percentages and ages, the stress-strain graphs were plotted and then the elasticity modulus was calculated. The results showed that, with increasing zeolite content, the modulus of elasticity has been increased and, with increasing curing time, except for a slight decrease, after 7 days, the modulus of elasticity increased. During the initial treatment (7 days), the hardness of the sandy clay soil decreased and then increased with increasing time. In general, hardness in both soils in the high percentages of zeolite is significantly is increased.
Also, the effect of sepiolite on the modulus of elasticity has been studied. The results indicate that with the increase in the percentage of additive and lengthening the curing time, the modulus of elasticity is increased. This increase in the stabilization of both sandy clay and clayey sand soil is almost the same. In addition, in the case of sepiolite modification, the elasticity of sandy clay and clayey sand is approximately equal to 5 times in comparison to the initial value of unstabilized soil. However, in zeolite, the modulus of elasticity in clayey sand soils is almost 2 times, and sandy clay is nearly 5 times higher.
2. Permeability
To investigate the effect of additives on the soil hydraulic conductivity, clayey sand soil with additives 5, 10, 15, 20, and 25% was prepared using dry mixing method. The samples were saturated in a short period and permeability test was carried out immediately. Permeability coefficient changes were mostly influenced by physical factors. Therefore, due to the fineness of both types of additives, the hydraulic conductivity decreases with increasing additive content. The amount of reduced hydraulic conductivity in sepiolite stabilization is greater than zeolite due to the structure of the sepiolite (fiber-shaped) compared to zeolite.
 
 
3. SEM imaging
In this study, attempts were made to examine the reasons behind the obtained results more carefully through SEM imaging.

c                                     b                              a
Figure 1. SEM image of non-stabilized clayey sand soil (a) soil stabilized with zeolite 15% (b) soil stabilized with sepiolite 15% (c) during the curing time of 28 days at magnifications 10000X
Figure 1a displays the SEM image of non-stabilized clayey sand soil. As can be seen in the figure, the soil structure is clear as layered and clay scales can be seen as laminated. Figure 1b demonstrates the SEM images of clayey sand soil stabilized with zeolite 15% during the curing time of 28 days. The sample has lost its layered structure in response to stabilization with zeolite during the curing time and changed into an integrated structure. This can be due to incidence of chemical reactions such as ion exchange and pozzolanic reactions in response to adding zeolite. Figure 1c demonstrates the SEM images of clayey sand soil stabilized with sepiolite 15% during the curing time of 28 days. As shown in the figure, the sepiolite has a fibrous-shaped structure that is longitudinally twisted. Also, with  curing time increase, complex structures have emerged that could be due to the occurrence of chemical reactions.
Conclusion
This study examined the effect of zeolite and sepiolite additives on strength parameter of clayey soils. Accordingly, uniaxial compressive strength test was performed on clayey sand and sandy clay soil at percentages of 0, 5, 10, 15, 20 and 25% of zeolite and sepiolite with instantaneous curing times of 7, 14 and 28 days. Further, permeability test was conducted at different percentages on stabilized clayey sand soil. Also, to investigate the effect of these materials on soil microstructure, SEM imaging was performed at 28 days. The results show that both additives increase the elastic modulus of clayey sand and sandy clay soils. Also, the results indicate a steady increase in the stiffness of the cured soil with sepiolite during processing time. However, reducing soil hardness can be seen in stabilizing with zeolite at lower rates and lower percentages. In permeability test, hydraulic conductivity decreases with increasing additive content. The rate of permeability reduction in sepiolite is higher than zeolite. SEM images show that chemical reactions create an integrated structure that ultimately increases uniaxial compressive strength and modulus of elasticity. Also, SEM imaging depicts physical changes along chemical reaction in soil stabilized with sepiolite. Ultimately, increasing soil strength resulting from additive alongside environmentally friendliness is recommended in superficial and deep improvement of soil../files/site1/files/144/Rajabi.pdf
 

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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.
 
 

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The soil of the Arvand free zone in the north of Khorramshahr is fine cohesive and cannot be used in earth works. On the other hand, suitable materials for this purpose (coarse-grained soils) are located at the farther distances which a considerable cost requires. In this regard, it is trying to improve the soil with lime and furnace steel slag. This study is focused on improvement of the fine-grained soil by adding various contents of lime and furnace steel slag. For this purpose, after sampling and performance of compaction tests, different amounts of slag (10, 20 and 30% by weight of dry soil) and lime (2, 4 and 6% by weight of dry soil) were added to the soil and after curing for 28 days, the effect of additives on the physical and mechanical properties of soil was investigated by using several tests such as Atterberg limits, compaction, uniaxial compressive strength (UCS) and CBR as soaked and unsoaked. Based on USCS classification the study soil is CL, its plasticity index is about 25% and sulphate ion content is more than 0.5%. Experimental results show that by adding slag and lime at different contents to soil, mechanical properties of soil improve dramatically, so plastic index of soil decreased and UCS and CBR has been increased. Also, the maximum dry unit weight of soil increases and the optimum moisture content decreases. The test results also indicate that the effect of lime on soil is higher than slag and the effect of slag for less than 35% is not considerable, however the test result of unsoaked CBR show that the bearing of soil increase in the more than slag content 20% is significant. According to the previous studies, due to the relatively high sulphate ion content in the soil, the use of lime alone is inappropriate and the slag can only physically improve soil conditions but also chemically prevent the formation of large volume minerals (like Ettringite) by the reaction of lime with soil sulphate ion../files/site1/files/152/%D8%AD%D8%B3%DB%8C%D9%86_%D8%B2%D8%A7%D8%AF%D9%87.pdf

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In coastal industrial areas, in addition to the presence of loose soil, sulfate attack on soil improvement elements, such as soil-cement, is a double problem. Generally, the use of type V cement is recommended as one of the methods to reduce the detrimental effects. Considering the limited resources of this type of cement, firstly to determin the relationship between the cement content and the strength obtained in sulfated environments is one of the important engineering question in this field and secondly, as an alternative option, the use of type II cement which is more available, is suggested to use in combination with suitable additives. The present study pursues the above two goals by making cylindrical soil-cement specimens with sand, water and Portland sulfate resistant cements. Sodium sulfate is used as the sulfate in soil and water. In the research, first of all, the relation between type V cement content and unconfined compressive strength of soil-cement is obtained at 0% to 5% sulfate concentration, which results in a cement content of 400 kg/m³ completely limited the sulfate attack effects in a sulfate concentration of 2%. Secondly, the combination of type II cement with barium chloride and hydroxide was tested. The related results show that the combination of type II cement with barium chloride and hydroxide had higher strengths, about 2.7 to 3.3 times, respectively (in 362 days), than the soil-cement containing type V cement../files/site1/files/152/%D8%A2%D8%AA%D8%B4_%D8%A8%D9%86%D8%AF.pdf
 
 

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Soil stabilization and reinforcement has long played an important role in civil engineering, especially in geotechnics, and over time and the need for a more robust and stable ground to withstand gravity and higher shear forces, has become particularly important. Also, in recent years, with the entry of the environment into the construction industry, with the aim of reducing the adverse effects of industrial waste and construction waste on people's living environment and preserving the environment for the future, in many cases reduces the economic costs of projects. In this research, granular soil is reinforced in two loose and semi-dense states using a waste material called ethylene-vinyl acetate (EVA). The experiments were performed without adding moisture, by weight percentage method and using CBR device. The results show that soil resistance increases significantly with the use of these additives and its effect on soil increases with decreasing soil specific gravity. Also, the optimal amount of additives in loose and semi-dense state is 2% additive and 1% additive, respectively.


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The goal of this study was improvement of sandy soil using a combination of polystyrene foam container waste and Portland cement. For this purpose, Babolsar sand was used as the base soil. Strips of disposable polystyrene foam container waste in “chips” of 50 ´ 5 mm and 50 ´ 10 mm were added to the soil at 0.0%, 0.1%, 0.2% and 0.3% by weight along with 3% Portland cement at a relative density of 70%. All samples were cured for 7 days under saturated conditions and then tested using a large-scale direct shear apparatus. The results showed that, in both cemented and uncemented samples, the addition of foam chips increased the cohesion and internal friction angles, which increased the shear strength of the soil. At higher percentages and using larger-sized foam chips, the shear strength increased even more. In uncemented samples, the stiffness did not change with the addition of foam chips, yet the final dilation of the samples decreased. In cemented samples, both the stiffness and softening behavior after the peak strength point decreased. The final dilation of the cemented samples increased at higher foam chip contents and for the larger sized chips. The results of numerical analysis showed that the use of foam chips increased the safety factor of a slope improved in this manner. It also was found that the foam chips with a lower length-to-width ratio had a greater effect on increasing the safety factor of the tested slopes.

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