Journal of Engineering Geology

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Saline soils are of challengeable soils that may cause many problems in civil engineering projects. In this study, volume change behaviour of saline soils and also the effect of improvement and reinforcement on them have been investigated using laboratory tests as well as consolidation test, swelling pressure test and free swelling test. The case study is Amirkabir Highway which connects the cities Qom and Kashan.  Fifty four kilometer of this highway was deformed like waves due to existence of saline soils. The laboratory investigations showed that the studied soil has a considerable swelling potential which appears to be the main cause of damage to the highway pavement, therefore it is decided on improving the subsoil condition. The research program comprises of studying volume change behavior of saline soil, stabilized with lime and epoxy – resin polymer and reinforced with polypropylene fiber. Afterwards, results for two cases of stabilized and non-stabilized samples have been compared. According to the results, the main cause of swelling is soil disturbance and structure destruction of initial soil composition. Considering all of test conditions, it is appeared that, although lime is a traditional stabilization material but is economic for the most geotechnical projects and usage of polymer is suggested only in special applications due to its rapid setting

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Desiccation cracking commonly occurring in compacted clayey soils typically used as landfill liners can result in poor hydraulic performance of the liner. In this research, a simplified image processing technique was developed in order to characterize desiccation cracking intensity in compacted clayey soils. Three pairs of compacted clayey soils were studied in a relatively large scale experiment to evaluate the effect of geotextile cover on desiccation cracking under real-time atmospheric conditions. Digital images were taken from the surface of soils at certain time intervals for 10 months and were analyzed to determine crack intensity factor (CIF). The key parameter in identification of cracks as accurately as possible was found to be sensitivity. Calibration process was based on using %20 of the images with different crack intensities whose crack dimensions and therefore CIF values have been already measured to compare to program output. A calibration coefficient for sensitivity was accordingly determined based on the average difference between the sensitivity introduced by the program and the actual sensitivity calculated based on an overlaying process. Result of verification of this methodology indicated that it can be reliably used to determine CIF of compacted clay soils in a simple yet accurate manner.

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Marl rocks are from weak rocks which cause some problems due to high swelling and efflorescence capability, low resistance and durability in construction of engineering structures. Creation of these problems is due to inadequate recognition of engineering geology properties of these rocks. Hence, in this research for determination of the physical and mechanical parameters of marl rocks of Safa dam site, Aterberg limits, density, porosity and moisture percent, uniaxial compressive strength (UCS), direct shear, swelling and three axial compressive strength tests and X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses were performed on available samples from excavated bores. Results of experiments indicate that the type and contents of minerals and percentage of calcium carbonate in marl rocks, especially clay minerals are effective factors on engineering geology properties of these rocks

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One way of reduction of leakage from beneath of earth dams is using of one contact clay layer with very low permeability and intermediate to high plastisity and connectig it to core of dam. Since, most of fine-grained soil in environtment of dam have low plastisity and preparing it from another place is not economic, use of bentonite in order to improvement of engineering characteristic of borrowed clay is suitable way.
In this search effect of bentonite on geotechnical properties of fine-grained soils with low plastisity are evaluated. Results of this research show that hydraulic conductivity, consolidation coefficient, dry density, colifornia bearing ratio (CBR),.....are decreased with increase in bentonite content but optimum moisture , Aterberg limits , cohession and so on are increased with bentonite addition. Finally, with analysis of obtained result, optimum percent of bentonite is offered in order to improve of engineering properties of used clay in contact region.

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This paper aims to present an experimental and numerical study on the effect of wetting-drying cycles and plasticity index on the California Bearing Ratio (CBR) of lime stabilized clayey soils. The numerical analysis was carried out based on finite element method for comparison between results of experimental and numerical studies. Three clays with different plasticity indices were mixed with various amounts of hydrated lime and compacted at optimum water content. The CBR tests were conducted to the soils and admixtures after specified curing time and various numbers of wetting-drying cycles. The experimental results indicate that addition of lime content up to 4% causes significant increase in the CBR values. Based on the obtained results the CBR decreases during the wetting phase and increases during the drying phase of each cycle. After 3 cycles the CBR values of lime stabilized clayey soils are increased. Also, for stabilized clays by increasing the plasticity index, the CBR values resulted by increase of lime content are decreased. The comparison between numerical and experimental analyses indicates a good agreement between results.

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One of the major problems associated with the clayey soils is the swelling potential due to moisture absorption, which results in applying high pressure on the superstructures, and may cause failure or large deformation of the structures. Among the solutions to mitigate the swelling problem of clayey soils is their stabilization using additives. This study aims to compare the effects of three types of additives on the reduction of swelling potential of two types of clayey soils, with two different plasticity indexes. The additives used in this research include two traditional additives namely, cement and lime, and one type of nontraditional stabilizer namely, CBR PLUS nano polymer. These additives were added to the soils in different contents, and the Atterburg limits, and the swelling of the soils were measured at different times after addition of the additives. The results show that the CBR PLUS is more effective in reducing the swelling potential of the soil with high plasticity index, by which, the swelling was reduced by 1500%, while the addition of  lime and cement reduced the swelling about 1000%. For the soil with low plasticity index, the cement is found to be more effective than the lime and CBR PLUS in reducing the swelling potential. The addition of 7% of cement resulted in 1400% of reduction in swelling, against 600% reduction for the addition of the same content of lime. In addition, it is found that the CBR PLUS and cement are, respectively, more effective in reducing the plasticity index of the soil with high and low plasticity index

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Consolidated-drained triaxial compression tests were conducted to compare the stress-strain and volume change response of sands and clayey sands reinforced with discrete randomly distributed poly-propylene fibers. The influence of various test parameters such as fiber content (0.0%, 0.5% and 1.0% by weight), clay content (0%, 10% and 20% by weight), relative density (50% and 90%) and confining pressure (100 kPa, 200 kPa and 300 kPa) were investigated. It has been observed that addition of clay particles to the sands decreased the shear strength of samples. Also, increase in clay content reduced dilation and increased compressibility of the mixed soil. Addition of the fiber to both sands and clayey sands samples improved the shear strength and increased ductility and axial strain at failure point. 

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Geotechnical engineers, in many cases face with low strength or high swelling potential of clayey soils. Stabilization methods are used to improve the mechanical properties of this type of soils. Lime and cement are the most popular materials used in chemical stabilization of clayey soils. If sulphate exists in the stabilized clayey soil with lime, or if soil is exposed to sulphates, problems such as strength reduction and swelling increase will occur. Reuse of industrial residual such as Rice Husk Ash (RHA) can be beneficial from the economy point of view. RHA includes a proper amount of silica with high specific surface area which is very suitable for activating the reaction between the soil and lime. In this paper, chemical stabilization of gypsum clays using lime and RHA is addressed Sulphates exist in the constitution of the soil. Unconfined compression strength and swelling potentials of the stabilized soil are evaluated. The results of this study indicate that RHA has positive impacts on improving mechanical properties of the gypsum clays stabilized with lime. From the view point of strength and swelling characteristics, and economy, addition of 6–8% lime and 8-10% RHA as an optimum amount is recommended.

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Introduction
One of the most important 1-ring aromatic organic pollutants is phenol and its related compounds. These compounds are classified as hazardous wastes base on U.S.EPA primary contaminates list. The phenolic compounds are very poisonous and these are harmful for human health and also for other biota.
To control the movement of such hazardous organic waste in a contaminated soil, solidification/stabilization (S/S) process can be an effective alternative.  Due to the negative impact of organic compounds on the cement hydration, the cement-based S/S may be not effective for controlling the movement of such pollutants. To avoid these effects, using some additives during solidification period has been recommended. One of the proposed of such compounds is organophilic clay that is the modified montmorillonite by quaternary ammonium salts (QAS). There are several researches to evaluate the organophilic clay effect on adsorption and stabilization of organic compounds during S/S process. The effectivity of S/S process can be examined by several tests such as leaching test, durability, unconfined compressive strength (UCS), etc.
In this study, efficiency of ordinary and organophilic clay was evaluated in the solidification and stabilization process based on unconfined compressive strength of a phenol-contaminated soil.
Material and methods
In this study, an artificially phenol contaminated sand was considered to evaluate the effectivity of the white cement based S/S process by using two different additives of ordinary and organophilic clay.
The contaminated sand contains 2000 ppm of phenol. S/S process was conducted on 14 samples with different amounts of white cement (15 and 30 wt%) as binder and ordinary/organophilic clay (0, 8, 15, and 30 wt % for each of them) as the additives. Two zero percent additive samples are considered as control samples.
All samples were cured for 28 days and then UCS test was conducted for all of them.
Results and discussion
Unconfined compressive strength of all examined samples were ranged from 2226 to 6999 KPa. In the samples with equal amount of cement, th higher UCS values can be observed in blank samples (without any additives and phenol). By adding phenol in the examined sand, UCS of the solidified sample reduces 3 -3.5%.Moreover, results showed that UCS was reduced by increasing the amount of clays. The reduction of the samples containing organophilic clay was higher than samples containing ordinary clay. Unconfined compressive strength values of all samples met the minimum standards indicated by France, Netherlands, Britain and America for disposal in a sanitary landfill. The sample with 30% white cement and 8% bentonite was the maximum amount of UCS (4856 KPa) and the sample with 15% white cement and 30% organophilic clay was the minimum one (2226 KPa). In this study, the average cost of organophilic clay-based solidified samples was 2.3 to 2.8 times more than the average cost of the bentonite-based solidified samples.
Conclusion
In this study, the strength of the cement-based solidified samples contaminated by phenol was investigated. The summary of the findings of the research is as follows:
1. By adding the phenol to pure sand, the UCS of the samples can be reduced 3-3.5 %.
2. Addition of organophilic clay reduces the UCS of the samples more than the ordinary clay (bentonite) in the same amount.
3. All samples met the recommended UCS level for the S/S process. The minimum UCS level is for the sample with 15% of cement and 30% of organophilic clay.
The cost of S/S process is between 23 and 650 $/ton of contaminated soil depending on the amount of used additives and binder. The samples containing organophilic clay has a higher cost than the similar sample containing ordinary clay.
4. To evaluate the S/S process effectivity, a leaching test of phenol (such as TCLP) is recommended  ./files/site1/files/0Extended_Abstract2.pdf

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Introduction
Contaminants can be categorized into organic and inorganic groups. Organic contaminants are carbon based, and their presence in waste forms may be as a single contaminant associated with inorganic contaminants, or a suite of complex mixtures which may be toxic at very low concentrations. Organics of greatest environmental concern are usually refined petroleum products, chlorinated and non-chlorinated solvents, manufactured biocides, organic sledges and substances from manufacturing processes. Most contamination due to organics are associated with accidental spills and leaks, originating from equipment cleaning, maintenance, storage tanks, residue from used containers and outdated material (Yong and Mulligan, 2004). Transport and fate of organic contaminants are important. Organic contamination migrations are due to advection (by fluid flow through soil) and diffusion, but other forms of transport e.g. infiltration may also contribute to migration (Environment Agency, 2002). The response of the soil to a contaminant depends upon the type of soil and the nature of the contaminant. The sensitivity of soil to contaminants depends upon the type of soil (such as particle size, mineral structure, bonding characteristics between particles and ion exchange capacity) and the nature of contaminants. Fang (1997) defined a sensitivity index (ranging from 0 to 1) to different types of soil. Sensitivity of sand and gravel (0.01 to 0.1) is much lower than clay particles (0.6-0.9). There are a number of techniques for remediation of contaminated land. These include physical (washing, flushing, thermal, vacuum extraction, solvent extraction), chemical (stabilization and solidification) and bioremediation techniques. However, the applicability and feasibility of different methods for remediation are dependent on many factors such as soil characteristics (soil type, degree of compaction and saturation), site geology, depth of contamination, extent of contaminant in lateral direction, topography, surface and ground water and the type and amount of contaminants. Thermal treatment and using surfactants are the most popular methods for remediating the soil contaminated with petroleum compounds. In this research remediation of a soil contaminated with different percentages of gasoline was studied through physical techniques in laboratory. The applied physical techniques were thermal technique and use of two different kinds of surfactants. The obtained results were compared with each other and discussion was performed.
Material and methods
Soil, gasoline and surfactant are the basic materials that were used in this work. The soil that was used in this testing program was a clayey soil. Two different types of ionic and nonionic surfactant, namely Tween 80 and SDS, were used in this work for remediating soil, contaminated with gasoline. Contaminated soil was prepared by adding 5 and 10 % weight (to air dried soil) of gasoline. 6 kg air dried soil was selected and the desired amount of gasoline was weighted, then it was sprayed on the soil and thoroughly mixed by hand for about 2 hours. The prepared mixture was kept inside a covered container for a week in order to come to equilibrium with the soil. For thermal remediation the contaminated soil with a specific percent of gasoline was kept inside a constant convection oven at 50, 100, and 150^oC for about one week to desorb the contaminating compound. Tween 80 and SDS were used for remediation of the contaminated soil. The amount of used Tween 80 was 25% weight of contaminating compound and selection of SDS amount was based on 50% weight of contaminating matter. The samples for the main tests were prepared by static compaction according to the optimum water content and maximum dry unit weight that were obtained from standard compaction tests. Atterberg limits, grain size distribution, compaction and unconfined compression tests were performed on samples of natural, contaminated and remediated soil according to the ASTM standard.
Results and discussion
The results of Atterberg limits (LL, PL and PI) for the contaminated soil (with 5 and 10 % gasoline) indicated that the values of them are increased with increasing the percent of gasoline. These values are nearly the same as natural soil after remediation with thermal method and surfactants. The grain size distribution curves were determined for the natural soil, contaminated soil with 5% and10% gasoline and soil remediated by thermal and surfactant techniques. The results showed that by using thermal technique the percent of clay is decreased and the percent of sand and particularly silt is increased by increasing temperature. The results of grain size distribution for the soils remediated by SDS and Tween 80 showed that the percent of clay is reduced but the percent of silt and sand are increased. Comparing the results of the two surfactants shows that the effect of Tween 80 in reduction of the percent of clay is more than SDS. The results showed that after thermal treatment, the maximum dry unit weight decreases and the optimum water content increases. For the contaminated soil with gasoline a reduction in maximum dry unit weight is observed compared with natural soil. The effect of SDS and Tween 80 on soil remediation is reduction in maximum dry unit weight and optimum water content. The results of compression strength showed that adding gasoline to soil causes a reduction in final strength and this reduction is a function of gasoline percent. The results also indicated that the strength of remediated soil by thermal or surfactant techniques, is reached nearly to the strength of natural soil. Scanning electron microscopy (SEM) tests were performed on the samples in order to observe the microstructure of the samples in different conditions (natural and contaminated with different percent of gasoline). The results of SEM showed that the structure of soil is changed by contamination to gasoline. It can be said that the gasoline causes reduction in the thickness of DDL because of low dielectric constant and hence a flocculated structure is formed. In the flocculated structure due to attractive forces, the fine particles paste to each other and form coarse particles. Therefore, variations in the Atterberg limits and compaction parameters can be resulted from forming new structure by adding gasoline. These results of compression strength are not in agreement with the theory of diffuse double layer (DDL). The reduction in dielectric constant would cause a flocculated structure in soil and the strength of the contaminated soil should be increased in comparison with the natural soil. It can be said the viscosity of gasoline cause reduction in the strength of contaminated soil.
Conclusion
In this experimental work a cohesive soil was contaminated with 5% and 10% of gasoline. The experimental tests showed that the properties of contaminated soil are different from natural soil and the change in the properties is a function of gasoline percent. The contaminated soil, was remediated by thermal treatment and also using two surfactants. The results also showed that using surfactants is more effective than using thermal method in soil remediation, and can treat the soil nearly to its original condition.
-Base on the SEM analysis results, adding gasoline to the soil, will change the soil micro structure to a flocculated one.  
-The gradation curves show that adding gasoline to the soil will change the gradation from finer to coarser.
- Contamination to gasoline will change the compaction parameters of the soil, and will reduce the soil final strength significantly.
- The results show that using thermal method and surfactants is effective in remediating the soil, but it is more effective to use surfactants. 
References
Yong, R.N., Mulligan,. “Natural attenuation of the contaminants in soil”, CRC press, Boca Raton, FL (2004).
Fang, M.Y. “Introduction to Environmental Geotechnology”, CRC Press,FL.USA, (1997).

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Introduction
Many studies have shown that the lime stabilization method can increase the strength and hardness of cohesive soils. Increasing these parameters is dependent on several factors such as curing time, lime content, clay minerals, soil particle size and moisture content.
When lime is added to moisture clay soils, a number of reactions occur to improve soil properties: 1- short-term and 2- long-term reactions. The short-term reactions include cation exchange, flocculate and carbonation; whereas, the long-term reactions include pozzolanic reactions. Since adding lime changes clay particles structure, it can change shear strength parameters.
Using geogrids as reinforcement in soil mass creates a composite system in which the soil tolerates compressive stresses. The elements of the reinforcement are also responsible for tensile stresses and interaction the reinforcement elements and soil increases the strength and ductility. The mechanism of stress transfer is based on interaction between soil and reinforcement. Accordingly, one of the most important issues in the analysis and design of reinforced soil structures is determination of frictional resistance parameters in soil-geogrid interface (adhesion and friction angle) which is discussed in this paper.
Stability and performances of reinforced earth structures significantly depend on the shear behavior of interface soil-geogrid in different weather conditions. Factors such as rainfall, seepage of groundwater and seasonal changes influence on soil moisture content. Changes in moisture content or soil dry density change interface soil-geogrid resistance. Increasing moisture content reduces the shear strength of reinforced soil and sometimes leads to large deformation or failure of system.
In this study, clayey soil with low plasticity (CL), hydrated lime for soil stabilization and two types of geogrid with different aperture size for reinforcing were used. In order to improve the brittle behavior of lime stabilized soils and to increase ductility of the samples, in the present study, lime stabilization and geogrid reinforcement was investigated, simultaneously. The interface shear strength parameters of treated soil with different lime content-geogrid and reinforcement coefficient were determined by direct shear tests. In addition, to study the effect of moisture content on interface shear strength soil-geogrid, all samples were subjected to shear in optimum and higher moisture content because the long-term performance of reinforced cohesive soils exposed to seasonal variations is evaluated.
Material and methods
The selected soil for the study is clayey soil from south region of Tehran, Iran. According to Unified Soil Classification System (USCS), the soil was classified as CL (clay of low plasticity).
In this study, three series of specimens were prepared and tested as follows:

• Stabilized samples with 0, 2, 4 and 6% lime for 7 days curing time
• Reinforced samples by geogrid (with and without transverse ribs of geogrid)
• Reinforced stabilized samples with different lime contents (0, 2, 4, 6 and 8%) by geogrid (with and without transverse ribs of geogrid) for 7 days curing times

To investigate the effects of bearing resistance provided by the transverse members of the geogrid and their contribution to the overall strength for reinforced soil sample, numerous tests were conducted with the geogrid without transverse members (all the samples had the same number of longitudinal members of the geogrid).
Direct shear tests were carried out on specimens based on ASTM D5321 at constant horizontal displacement rate of 1 mm/min.
Results and discussion
The results reveal that the shear strength of the stabilized soil increased and there are maximum values in an optimum lime content which is about 4%. Increasing lime content to an optimum lime content of clay caused the maximum changes in clay minerals because of cementitious and pozzolanic reactions and increases the strength of the clayey soil. Reduction of strength by adding lime to the soil more than the optimum content may be caused by the following reasons:
1. Stopping pozzolanic reactions because of finishing reactance during reaction
2. Making difficult the release of limewater (Ca OH ₂) in the cementitious context of soil.
Until SiO₂ and AL₂O₃ are not finished, pozzolanic reactions continue and produce cementitious product, thus the shear strength increases and improves the long-term performance of the stabilized soils.
Reinforced soil samples have higher shear strength relative to samples without reinforcement subjected to the same normal stress. This increase in shear strength is mainly attributed to the interlocking of soil particles that penetrate through geogrid apertures. In addition, geogrids restrain particles´ movement and thus increase the mobilized frictional resistance at particle contact points.
Increasing in lime content to 4% (optimum lime content in this study) has significant effect on the development of adhesion and then decreases gradually with increasing of lime content from 4 to 6%, while friction angles remain constant approximately.
Adhesion and friction angles decrease with increasing moisture content.
The results show that the reinforced stabilized specimen with 4% lime has the maximum value of reinforcement efficiency. The increase in moisture content can significantly reduce the reinforcement efficiency.
It is clearly observed that the reinforcement coefficient of reinforced stabilized sample by geogrid that has smaller aperture opening size (4Í4 mm) is higher than reinforced stabilized sample by another geogrid (10Í10 mm) in optimum and higher than optimum moisture content.
Conclusion
One hundred and twenty samples in 3 specimen categories including lime treated, reinforced and reinforced treated samples were prepared for the current study for 7 days curing time in optimum content and higher than optimum content. The main results can be concluded as:
The test results indicate that the shear strength of stabilized clayey samples increases after 7 days curing time due to pozzolanic reactions.
The results show that reinforced samples have higher shear strength relative to unreinforced samples.
Adhesion and friction angles and reinforcement efficiency decrease with increasing moisture content.
The reinforcement coefficient of reinforced stabilized sample by geogrid 1 that has smaller aperture opening size is higher than by geogrid 2. In general, interaction between particles and geogrid with smaller mesh size is stronger because of matching the size of soil particles and meshes../files/site1/files/123/8Extended_Abstract.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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This study numerically investigates the bearing capacity of drilled shafts (bored piles) in clay using FLAC^2D. The results obtained in this study are compared with centrifuge test results. The results of the empirical relationships available in the literature are compared with the results of the present numerical study. A series of analyses is also conducted to assess the effects of various soil and pile parameters on the magnitude of tip and side resistance of bored piles embedded in clay. These parameters include the soil elastic modulus, pile length and diameter, undrained shear strength, unit weight, and Poisson’s ratio of soil. Furthermore, the coupling effect of soil undrained shear strength and elastic modulus of soil on tip resistance are investigated. The results show that the lower value of soil elastic modulus results to lower effect of soil undrained shear strength. The effect of soil undrained shear strength on tip resistance is approximately constant (about 83% for a change of soil undrained shear strength between 25 to 200 kPa) for the range of elastic modulus between 20 and 180 MPa. Also, a new equation is proposed to estimate the bearing capacity factor of N^*_c.
 

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Introduction
Bearing capacity is very important in geotechnical engineering, which depends on factors such as footing shape, stress distribution under footing and failure mechanism of soil. Construction of the footing near a slope affects the behavior of footing and reduces the bearing capacity. Also, construction of structures on soft soil usually involves problems such as excessive settlement, deformation and stability problems. In order to increase the bearing capacity, especially in soft soils, one method is adding stone columns to soils. In this method 15 to 35 percent of unsuitable soil volume is replaced with appropriate material. In this research, the bearing capacity and settlement of a strip footing on a clayey slope reinforced with stone columns is investigated. For this purpose, a series of small-scale model tests was performed on the slope reinforced with both types of ordinary and vertical encased stone columns. The effects of length of stone column and location of stone column on the behavior of footing was studied and the optimum length of column and best location for column were determined. Also, some tests were performed on the effect of group stone columns on the footing and the efficiency of columns was investigated.
Material and methods
In order to determine properties of clay soil, stone column and encasement material, some preliminary standard tests were performed. The stone column material was selected with aggregate size ranging from 2-10 mm considering the scale effect. The performance of stone column depends on the lateral confinement provided from the surrounding soil and this lateral confinement represents undrained shear strength of the soil. In very soft soils (cu<15 kPa), the lateral confinement is not adequate and the stone column cannot perform well in carrying the required bearing capacity. For this reason, a series of undrained shear strength standard tests were carried out on clay samples with different water contents. According to these tests, the amount of water content of clay related to cu-15kPa was equal to 25%; while the natural water content of the clay was 4%. Therefore, the additional amount of water was weighted and added to clay. The apparatus of this research was consisted of two main parts including a test box and a hydraulic loading system. The test box dimensions should be such that for all states of the tests, the stress in the soil applied from the loading would be almost zero at all boundaries of the box. Thus, a box was built to accommodate the clay slope with 150 cm×120 cm×30 cm dimensions. The test box was built using steel material and steel belts were welded around it to prevent the deformation at high loads. The front side of the box was made from two pieces of tempered glass and a 10 cm×10 cm grid was drawn on them, for making the slope during construction and observation of deformations during the loading easier. The model strip footing dimensions were 29 cm length, 10cm width and 4cm height and it was made from steel to have no deformation during the loading. The displacement of the footing was measured using two dial gauges with accuracy of 0.01 mm.
The clay was filled in the test box in 5 cm thick layers and compacted with a special 6.8 kg weight tamper. All model stone columns were constructed using the replacement method. In this method, a 10 cm diameter open ended steel pipe was inserted into the soil and the clay within the pipe was excavated. Then the stone column material charged into the hole in 5 cm layers and each layer was compacted using a 2.7 kg special circular steel tamper with 10 blows. The 5cm compactions were repeated until the construction of ordinary stone column was completed. For construction of vertical encased stone columns, the cylindrical encasement mesh should be constructed first. Then, after excavating the hole, the prepared encasement mesh was placed inside the hole and the aggregates were charged into the hole in 5 cm layers and compacted.
Results and discussion
The loading method used in all tests was a stress control method. Bearing capacity values were determined from pressure-displacement diagrams using tangent method. All test results show that when any type of stone columns was added to slope, the bearing capacity of adjacent footing was increased. Vertical encasing of stone columns leads to a further improvement in the behavior of the footing. Influence of length of ordinary stone columns on the behavior of strip footing near clayey slope, was studied for four different lengths. Results show that, the optimum length of stone columns giving the maximum performance is about 4 times their diameter. Also, the location of column for both ordinary and vertical encased stone columns was studied using a series of laboratory tests and results show that the best location for the stone column is right beneath the footing. Also, group stone column tests resulted that for both ordinary and vertical encased types of stone columns, the group of two columns had a better efficiency than the group of three columns.
Conclusion
In this investigation, some model tests with 1/10 model scale on a strip footing near a clayey slope reinforced with stone columns were performed and the effects of different parameters such as stone column length and location were studied. Based on results from experiments on different states of stone columns, the following concluding remarks may be mentioned:
- The maximum encasement influence was observed when the encased stone column is placed under the footing.
- The optimum length of ordinary stone columns which are placed beneath the strip footing gives the maximum performance more than 4 times to their diameter.
-Bulging failure mode governs when the stone column is placed under the footing. When stone column is not beneath the footing, the failure mode was lateral deformation.
- Comparing the different locations of stone columns in the slope shows that for both ordinary and vertical encased stone columns, the best location having the most influence on the strip footing is under the footing and with increasing the spacing between column and footing, the bearing capacity is reduced.
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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 standard penetration test (SPT) is one of the most common tests in geotechnical investigations. The results of this test are known as a simple, inexpensive, and tangible criterion in geological and geotechnical engineering. Many computational methods and engineering judgments depend on the results of this test. In this research, estimation of physical and engineering properties of clay soils was carried out using statistical methods based on standard penetration test results. The scope of this case study is related to a variety of clayey soils in Tabriz (the northwest of Iran). The existing relationships were confirmed based on database of this study. After statistical analysis of the database, eight relationships including single and two-variable associations have been proposed to estimate the physical and engineering properties with better performance using nonlinear regression.
Material and Methods
Different types of clayey silt and marl layers spread in Tabriz were included for the purpose of this study. The geological age of these layers dates back to the Miocene and Pliocene era. This research was conducted in two sections of the field and analysis. Two machine boreholes were drilled, and, based on ASTM, a standard penetration test with other laboratory tests were performed on the soil specimens in order to determine the physical and plasticity properties. According to the results of this study and the existing data, a total of 107 series were prepared. Based on the soil properties, 11 variables were selected including the fine grain percentage (FGP), liquid limit (LL), plastic limit (PL), percentage of clay particles (C), plastic index (PI), consistency index (CI), activity (A), dry unit weight (γ_d), natural moisture content (w_n), initial void ratio (e_o), and effective vertical stress (σ'_v). The standard penetration tests were run for each meter in drilled boreholes. The results of this test were corrected according to NCEER method. The correlation between the variables and corrected standardized penetration test results (N₆₀) were studied by Spearman ranking coefficient. Verifications of the existing eight experimental relationships between  standard penetration and other soil properties, proposed by Kayabasi (2015) and Hoshmand et al., (2012), were checked out using the findings and data of the present study. The linear, exponential, logarithmic, and exponential regressions between each variable and N₆₀ were investigated using SPSS software, version 16. The best regression with the highest R² for each variable was selected. Eight new relationships were proposed. Performance of the suggested relationships was compared with the existing relationships.
Results and Discussion
The findings of the current study could be summarized as:
1. The clay soils of the studied area in Tabriz were classified into four categories including CH, MH, CL, and ML according to USCS classification. The range of changes in plastic index and liquid limits of the samples were 9.19 ~ 45% and 29 ~ 77%, respectively. The corrected standard penetration test results (N₆₀) changed from 9 to 28 showing that soil compression was low to high.
2. The highest positive and negative Spearman correlation coefficients were related to the consistency index (+0.772) and moisture content (-0.759), respectively.
3. The existing empirical relationships, based on the database of this study, were found to have better statistical coefficients in terms of consistency index, activity, moisture percentage, and fine grained percentage. In term of sample depth, the experimental relationship, showed the lowest statistical coefficient.
4. Four single-variable and two-variable relationships were proposed by nonlinear regression analysis. Using these relationships, clay soil properties including activity, moisture content, fine grain percentage, and consistency index were estimated based on N_60. In addition, two relations were proposed between sample depth (D) and vertical effective stress (σ'_v) with N₆₀. The statistical coefficients of the suggested relationships were better than the existing empirical relationships. The proposed relationship of estimating the consistency index with coefficient (R²) of 0.673 and regression line slope of about 1 had the best performance.
Conclusion
In general, the main objective of this study was to investigate the correlation between physical and plasticity properties of clay soils and N₆₀ on Tabriz clayey soils. Clay soils of the present study included various silty and marl layers. Sufficient correlation was observed between the physical and engineering properties of clay soils and N₆₀. The validation of the existing experimental relationships based on A, w_n, FGP, and D resulted in weak statistical coefficients (R² <5) employing the database of the current study. Six new experimental relationships were proposed to estimate A, FGP, w_n, and CI as well as two correlations of N₆₀ with effective stress and sample depth. Generally, the results have been revealed that the statistical coefficients of the proposed relationships were improved compared with the existing relationships. The most suitable relationship was the estimation of soil consistency index (R²~70) and root mean square error (RMSE=129). Finally, due to the novelty of this research topic, verification and development of the proposed relationships for the soils has been recommended in other areas.

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Sandy soils usually contain different amounts of fines like silt and clay, causing some changes to their shear strength and dilation characteristics. Bolton [1] conducted  some experiments on the different sands and suggested a relation between the parameters of the soil shear strength. In this paper, some experiments were performed on fine contained sand and the extended Bolton's relation was has been proposed. In this paper, shear strength and dilation behavior of a pure sand mixed with different amounts of silt or clay fines were studied using direct shear test device (100*100*30 mm), and a total of 96 tests were carried out. The samples were prepared separately using clay and silt contents of 0, 10, 20 and 30% in different relative densities of 70, 80, 90 and 100%. They were tested under three surcharge pressures of 90, 120 and 150 kPa, under particle crushing threshold. Variations in shear strength, maximum friction angle, critical state friction angle and cohesion, as well as dilation angle were investigated by increasing in the mentioned amounts. The results demonstrate that shear strength, dilation angle, maximum friction angle decreased by clay content increase, however, they increase with increase in silt content. In addition, a new form of the Bolton's relation for fine contained sandy soils was presented.

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Introduction
The use of various additives to improve the properties of soils from past years have been studied by different researchers. Such additives are lime, cement, fly ash and fiber which have been used frequently in combination with soil. Lime is one of the oldest additives that it is utilized with different types of soils. Lime has positive impact on geotechnical properties of soil that alter some of the soil characteristics. Adding lime causes to reduce plasticity ranges, enhanced efficiency, strength and shrinkage of the soil. Extensive researches in the field of sustainability of clay with lime indicate that the optimum percentage of lime in the soil modification is between 1 to 3% by weight of the soil. But some researchers believe 8% by weight of lime are effective for soil stabilization. The presence of lime in clay soil yiels to occur some reaction, that it improves the soil properties. Reactions are included cation exchange flocculation, carbonation and pozzolanic reactions. Cation exchange between the clay cations and calcium cations takes place in lime. Cation exchange causes clay particles to get closer to each other creating complex structures in the clay soil and this improves the   clay soil features. In recent years the use of nanoparticles is considered in civil engineering field. The investigations have demonstrated that the use of nanomaterial increases cement reactivity and also improves density because it is filled with particles. Recent research has shown that the use of montmorillonite nano-clay soils to control swelling and to reduce failure potential in the soil. A number of researchers have expressed the use of nanoparticles causes to decrease the hydraulic conductivity of soils. In this paper, the effect of nano-clay and lime on the important soil parameters is evaluated. For this purpose, lime at 2 and 4 percentage and nano-clay at 0.5, 1 and 2 percentages have been added to clay soil and their impact on parameters such as optimized moisture, Atterberg limits, unconfined compressive strength and self-healing properties of soil is evaluated. Self-healing properties is one of the features, to repair damages due to internal erosion in the clay which is very efficient and important.
Materials and experimental methods
In the present research, the effect of lime and montmorillonite nano–clay to soil strength is evaluated. For this purpose, samples of clay soil (CL) has been used. In the experimental study, the percentages of additives mixed with the dry soil and then the optimum moisture and maximum specific weight of soil are determined with different percentages of additives. Soil Atterberg limits based on the ASTM D4318 standard have been determined.   Dry samples have been mixed together and then the water is added and mixed well with each other. Then the sample has been prepared in the form of a steel cylinder (cylindrical specimens) with a diameter of 50 mm and a height of 100 mm. Specimens were molded immediately and the weight and dimensions were carefully measured and then placed in plastic to prevent moisture loss and put them at 20 °c and 90%  moisture curing room.
Results and discussion
In this study, the percentage of lime is between 0, 2, 4 percent by weight and nanomaterials percentage is between 0.5 and 1 and 2 percent that can be varied in order to analyze the effect of various additives on the properties of the soil samples. The results indicate that increasing the nano-clay and lime percentage can enhance the optimum specific gravity of soil. The optimum moisture content of sample without any additive is equal to 19.5%. However, samples contain 2% nano-clay and 4% lime, the optimum moisture content increases to 23.5%. But the presence of lime reduces the maximum dry density of soil while adding nano-clay increases this amount. In samples with 4% lime and with no nano-clay, maximum dry density is 17  but in case of lime with 4% and nano-clay with 2% it is increased to 17.5 . In addition, adding lime without the presence of nano-clay only increases strength of soil. When 2 percent of lime is added, the strength of soil increases about 39 percent. As mentioned before, the effect of lime and nano-clay on increasing of unconfined compressive strength is almost the same which means by adding 2% of lime or nano-clay the strength of the soil increases about 40 percent. Using both lime and clay nanoparticles simultaneously (each 2%), a significant increase in strength of soil occurs in approximately 77 percent.
Conclusion
The use of nano-clay and lime improves soil strength parameters. But economically lime is more affordable than nano-clay. Therefore, if you need to increase only unconfined compressive strength, then the nano-clay is not recommended.
When it comes to self-healing in clay, the nano-clay can improve resistance rupture of the soil. By adding 2% of nano-clay in soil, healing of soil resistance after the break and after 24 hours can reach up to 60% of the ultimate strength of the soil. This property can be used to repair of locations that are subjected to internal erosion and scouring.
 
 

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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
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.