Improvement of collapsing problematic soils on the sabzevar-mashhad railway route (northeast of Iran) using traditional additives

Collapsing soils are among the problematic soils in geotechnical engineering. These soils have a relatively good resistance in their natural state with low humidity, but in case of saturation without change in the incoming load or subjected to vibration force, they settle a lot and in many cases, it will cause cracking or destruction of foundations, roads, railway lines, dams, pipelines and other existing structures. Collapsing soils are scattered in many regions of the world, in Iran these soils are mostly observed in the central and eastern regions. In this research, according to the samples taken from Khorasan-Razavi province in the area of Sabzevar-Mashhad, these samples have been shown to be in the category of collapsing soils based on the tests performed according to ASTM standards. In order to improve this soil, cement and lime additives with percentages of 5 and 10 percent have been used, which has improved the shear properties of the soil due to the addition of lime and cement, so that it causes an increase in the friction angle in the soil, which indicates an improvement in the soil parameters. The results obtained with regard to the addition of cement and lime to the compacted soil show that the index of compaction in the addition of lime is better than that of cement and the results show that with the increase of lime, the soil has a 68% and 78% decrease in the soil compaction index, respectively, but when cement is added to the soil, it causes the percentage of raking index to improve by 34 and 37%.

acuerdo con las normas ASTM. Para mejorar este suelo se han utilizado aditivos de cemento y cal con porcentajes de 5 y 10 por ciento, lo que ha mejorado las propiedades de corte del suelo por la adición de cal y cemento, de forma que provoca un aumento del ángulo de fricción. en el suelo, lo que indica una mejora en los parámetros del suelo. Los resultados obtenidos con respecto a la adición de cemento y cal al suelo compactado muestran que el índice de compactación en la adición de cal es mejor que el del cemento y los resultados muestran que con el aumento de cal el suelo tiene un 68% y 78% de disminución en el índice de compactación del suelo, respectivamente, pero cuando se agrega cemento al suelo, hace que el porcentaje de índice de rastrillado mejore en un 34 y 37%.

INTRODUCTION
Collapsing soils are among the problematic soils in geotechnical engineering. These soils have relatively good resistance in their natural state with low moisture, but in the case of saturation without change in the applied load or subjected to vibration force, they settle a lot and in many cases it will cause cracking or destruction of foundations, roads, railway lines, dams, pipelines and other existing structures. Collapsing soils are scattered in many regions of the world, in Iran these soils are mostly observed in the central and eastern regions. These soils are mostly composed of young sediments deposited in arid and semi-arid areas. These deposits include; • Materials accumulated due to gravity or washing at the bottom of slopes (deposits) • Accumulation of sediments in rivers or large floods (alluvium) • Wind deposits caused by dust, silt and fine sand of Dante (wind drift) These soils have low dry specific gravity and are mostly loam, fine-grained silty soils (ML), silty clay (ML-CL) and clay with low plasticity (CL). These soils have low specific gravity and are sensitive to changes in humidity.
Damages to construction projects are happened due to landslides in loess; This phenomenon has become one of the most dangerous geotechnical phenomena in the last century (Jefferson et al. 2005;Sharifi Teshnizi et al. 2022). Collapsing soils are those problematic soils that are dry, stable and strong in their normal state; However, with the increase of moisture percentage and loading of large deformations, rapid settlement and a large decrease in porosity ratio are observed in them (Jefferson et al. 2008a;Al-Obaidi et al. 2013). Large areas of the world -a surface equivalent to 10% of the continental masses are covered by these soils. These areas include continuous deposits from North China to Southeast Europe and scattered areas from North and South America and New Zealand (Jefferson et al. 2008b;Abbeche et al. 2010). In Iran and its surrounding areas, there are thick deposits of silty materials, which are described as loess. The distribution of Los in Iran can be divided into two parts: desert loess and northern loess. The loess of northern Iran has an area equal to 388 thousand hectares; 74 thousand hectares of this area belong to the border areas of Iran and Turkmenistan (Babanejad et al. 2022;Sharifi Teshnizi et al. 2022).
These soils are the cause of many geotechnical challenges and structures in different regions of the world. The collapse phenomenon occurs not only in natural deposits, but also in artificial embankments that have been compacted with a moisture percentage lower than the optimum moisture percentage. The collapse phenomenon occurs not only in natural deposits, but also in artificial embankments that have been compacted with a moisture percentage lower than the optimum moisture percentage. With the expansion of the construction industry, many structures and roads are built on collapsing soils. If these soils are not improved, the resulting settlement will impose many costs on the project (Rollins and Kim 2010;Al-Obaidi et al. 2013).
During the improvement process, suitable conditions are created for the implementation of the project on the ground. The result of improvement is the reduction of construction cost and project implementation time. Although the history of land improvement dates back to Babylonians and ancient Greece, this technology was considered a new technology until 1960 (Munfakh 2003). For construction on collapsing soils, the structure must be designed in a resistant way to be stable against large settlements; Otherwise, the soil should be improved in some way to eliminate its sensitivity to moisture (Abbeche et al. 2010).
The extent of the areas covered by these soils, the high costs of replacing them with suitable soil, and the increasing cost of materials, have increased the need for more investigations about the available materials, as well as investigating the possibility of using waste in soil improvement (Sariosseiri and Muhunthan 2008). The improvement methods of these soils include pre-construction improvement and postconstruction improvement methods. The choice of remediation method depends on the needs of a project and the characteristics of the land, the most important of which is the amount of collapse and the depth of the region with the potential for erosion (Jefferson et al. 2008a;Karimizad et al. 2020;Sharifi Teshnizi et al. 2022).
In some areas, it is possible to use improvement methods before construction; In this way, either the collapse of the soil is completely prevented or by applying conditions, the soil collapses before the construction of the building (Murthy 2007;Cui et al. 2019). One of the common methods of designing a foundation on collapsed soils is soil removal and replacement of these soils. In this method, the soil is excavated to a depth that affects the foundation. Then it is replaced by a non-collapsed soil (Houston and Nelson 2012). Premoistening means flooding or moistening the soil before the construction of the structure; In this way, the collapse of the soil after the construction of the structure is reduced to the minimum possible amount. Moistening of the soil is done by means of ditches and ponds dug in it.
This method is suitable for channels and communication paths that are not under the effect of very large loads; Because pre-moistening leads to the settlement of the foundation under the existing overhead and applying more load will involve the subsequent settlement of the foundation; Therefore, pre-moistening without pre-loading is not a suitable method to prevent foundation settlements. Another way to improve the soil is to prevent the entry of moisture. This method includes measures such as controlling irrigation conditions, building sealed areas, limiting areas with vegetation near the structure, using geomembrane and building suitable drainage structures. One of the most effective and practical methods of reducing soil collapse is the use of compaction. This method is used for both shallow reaming soils and deep deposits. The most effective efficiency in this method is observed in the first five meters. The design of deep soil mixing 5 is aimed at improving the properties of unsuitable soils located in the upper layers and transferring the foundation load to hard clay layers in the depths. Deep mixing columns are implemented in a network to improve the properties of the soil as well as to apply remote stress to the soil. The distance between the columns and their density is determined according to the load from the foundation. Making changes in the soil in order to increase the carrying capacity and resistance to physical and chemical stresses caused by the environment is called soil stabilization. This process is done with or without additives. By means of various methods of stabilization, the characteristics of the soil can be changed, such as resistance, hardness, compressibility, permeability, efficiency, swelling tendency, and sensitivity to water and tendency to volume change. The scope of these methods can include changes in soil granularity and density to the use of various compounds or thermal and electrokinetic methods (Bell et al. 2007;Karimiazar et al. 2020Karimiazar et al. , 2021Mirzababaei et al. 2021;Johari et al. 2022;Opukumo et al. 2022;Seiphoori and Zamanian 2022;Khodabandeh et al. 2023).
In the process of soil stabilization, the bonds between particles are changed. This change leads to the creation of a new structure that has more adhesion than the previous soil structure. In some of these methods, the previous structure of the soil is broken, but in others, by injecting materials, this structure is strengthened and its resistance increases. Soil stabilization can be done on the surface or in depth. In the fixations that take place on the surface, the soil is combined with the desired materials on site using mixing equipment; After that, the desired compaction is obtained by using road-making rollers, small drums or other vibrating equipment. Stabilization with cement, lime and other waste products is widely used in road construction. Also, researches have been done on compounds obtained from soil with salt, acid and base. The obtained results showed that these materials lead to the reduction of leakage in the soil (Evstatiev 1988;Bell et al. 2007;Zhang et al. 2021).
The history of bed soil improvement with various compounds such as lime and cement goes back decades. Nowadays, improvement based on cement compounds is widely used all over the world. Also, some researches have been done on some salts and their effect on reducing the ability to grind. In most of the researches conducted in the field of soil stabilization, the samples have been reconstructed with optimal moisture percentage and maximum dry weight; then, the changes in resistance, optimal moisture content and maximum dry weight of the handmade samples have been investigated. The main goal of this research was to investigate the amount of changes in compaction index in the case of stabilization of compacted soil, to investigate the effect of cement and lime in weight percentages and different density ratios. In addition, the effect of applying suitable processing conditions on the smoothness index of samples stabilized with lime and cement is investigated. Due to the importance of this problematic soil on the railway line, investigating the effects of improving this soil will be of great importance to reduce the risks of the route.

Physical and mechanical characteristics of collapsing soil
Collapsing soils are among the problematic soils, which cover large areas of the earth's surface equivalent to 10% of continental masses. These areas include continuous deposits from North China to Southeast Europe and scattered areas from North and South America and New Zealand (Jefferson et al. 2008a). Due to the presence of weak cement joints and loamy structure, these soils, especially in the unsaturated state in dry or semi-arid regions, naturally have a specific weight and a low moisture content. In addition, large deformations, rapid settlement and a large decrease in porosity ratio are observed in unstable soils during the increase in moisture content and loading. These soils may be seen in various forms both naturally and artificially made by humans. In order to reduce the effects of these soils, it is necessary to collect information about the locations of these types of soils, the geological characteristics of these areas, as well as the geomorphology of these areas (Al-Obaidi et al. 2013). In Iran and its surrounding areas, there are thick deposits of layered materials that are described as loess (Rahimi et al.;Babanejad et al. 2022;Sharifi Teshnizi et al. 2022).
Collapsing refers to the sudden fall of soil due to the loss of resistance of the binding agent of soil particles, and the amount of looseness created depends on the initial porosity of the soil. They are when saturated (moistened). The main features that make soils show collapse characteristics are (Pells et al. 1975;Sharifi Teshnizi et al. 2022): • High porosity (more than 40%) • Low degree of saturation (below 60 percent) • High amount of litter (more than 30% and sometimes up to 90%) • Fast softening in water (opening time of windblown samples with high collapse is less than 1 minute).
Collapsing soils are spread in many parts of the world, including Iran. According to the available statistics, collapsing soils occupy approximately 16% of the total area of the earth. Figure 2-3 shows the approximate map of the spread of this type of soil in Iran. Since collapsing soils mostly have a loose and semi-dense or non-dense structure, the existence of these types of soils should generally be expected in areas covered with wind deposits. Experiences from geotechnical studies conducted in different regions of Iran also confirm this and indicate the presence of silty soils in many plains covered with wind-blown sediments such as Gorgan, Mughan, Saveh, central regions and southeastern plains of the country (Fig. 1). Collapsing soils are known as loess soils. The word loess comes from the German word meaning loose, which was first used by von-Leonhard to describe a friable layered deposit along the Rhine Valley near Heidelberg. Pye (1995) listed ten criteria to identify loess as follows (Gibbs and Bara 1962;Pye 1995).
A loose deposit with a high percentage of layer particles, without layering, porous, permeable, forming stable vertical walls, easily eroded by water, pea-yellow color (due to scattered particles of limonite), Quartz as the main constituent mineral (40 to 80 percent), feldspar as a secondary mineral, varying percentage of clay (5 to 20 percent) and carbonates (1 to 20 percent). According to him, for most scientific purposes, it is enough to define loess in the following simple way: A detrital deposit, consisting of layer-sized particles, necessarily formed by the accumulation of wind-borne particles (Clevenger 1958).

• Granulation of collapsing soil:
The particle size distribution curve for loess soils shows that loess are layered soils (lay percentage between 50 and 90%) which also has a small percentage of fine sand and clay.
Loess can be divided into true loess, pseudo-loess, and sandy loess, or in another classification, they can be classified into layered loess, sandy loess, and flat loess (Clevenger 1956). Based on the research conducted on the results of granulation and hydrometric tests on the loess of Sabzevar province, it has been reported that the percentage of silt is between 42 and 85%, which sometimes has a small percentage of fine sand and clay.

• Permeability of collapsing soils:
The permeability of loess in the vertical direction is much higher than its permeability in the horizontal direction, and the reason for this is the increase in the presence of long vertical holes in the structure of loess, which are considered as traces of fossilized plant roots. Loess deposits are better than real clay deposits in terms of drainage (their permeability varies from 10-8 to 10-5 m/s). Unlike clay, loess is not susceptible to frost. The reason for this is their permeability to clay, but like clay, they can show rapid conditions and its compaction is very difficult if not impossible (Clevenger 1956).

• Density of solid particles of collapsing soils:
In loess soils, the range of solid particle density values is limited. For example, loess of this density varies from 2.65 to 2.7 (Bell 2013).

• Specific dry weight of collapsing soils:
The initial dry specific gravity is an effective factor on soil collapse, and with the increase of the initial dry density, as seen in Fig. 6-2, the collapse potential decreases.
The range of dry density varies from very low to low (for example, in Los Camlapes, British Columbia, dry density ranges from 1.4 to 1.46 grams per cubic centimeter), the low density is reflected in the ratio of porosity. Lunger found that the density of undisturbed loess in the Missouri Basin ranges from 1.2 to 1.36 g/cm3. Also, if these materials become wet or dense, the density increases and sometimes reaches 1.6 grams per cubic centimeter (Clevenger 1956(Clevenger , 1958.

• Porosity ratio:
In the case of some Chinese loess, the ratio of porosity varies from 0.81 to 0.89 and porosity from 45 to 55% (Thornton 1972).

• Paste properties:
In loess soils, the pasty property varies from low to medium, the pasty property in these soils increases with the increase in clay percentage. The limits of flow range from 25 to 35% and exceptional values for clay loess vary up to 45%. The reported pulp index values are between 5 and 22%. Fig. 7-2 shows the pasty properties of Missouri Basin loess (Clevenger 1956(Clevenger , 1958.

• Fluidity limit:
The fluidity limit of soil was first defined by Etterberg as follows: The fluidity limit is the amount of moisture at which the soil moisture changes from a semi-solid state to a liquid state. A more recent definition of the soil's fluidity limit is the degree of humidity at which the soil particles maintain their natural state, but there is no binding force between the particles. The new definition better explains what is happening in collapsing soils. In general, in collapsing soils, especially in loess with low clay percentage, if the natural soil moisture is close to the psychological soil moisture, it will cause the soil skeleton to collapse and lose its strength. If the soil moisture is more than the psychological limit, the soil will be subjected to an external load and will undergo compaction. This type of soil instability can be explained by the psychological index (Gibbs and Bara 1967;Handy 1973). In this case, when LI is equal to or greater than unity, the soil will be compacted. A criterion has been suggested for areas where the thickness of the soil is more than ten meters (Clevenger 1958;Gibbs and Bara 1967).

• Shear strength:
The shear resistance of loess is significant in the percentage of natural moisture, so that even if their volumetric weight is low, they can remain stable as a vertical wall at a height of 15 to 24 meters without support. At low humidity, loess has high shear resistance coefficients due to the locking of soil particles and cementation (Clevenger 1958;Gibbs and Bara 1967). The apparent stickiness of the soil cannot reach kg/cm2, which decreases to 0.75 kg/cm2 due to wetting.

The study area
The sampling site was in Semnan province, in the area between Damghan and Sabzevar, near the railway tracks. Fig. 2 shows the location of sampling in this research. According to the visits made, this area was selected as the study area in this research. Fig. 3 shows the condition of the soil in the region and the grooves caused by the effects of the soil, and Fig. 4 shows the location of the sampling site and the position of the railway station in relation to the position of the problematic soil.
It is worth mentioning that the sampling was done in two ways, sampling in the form of disturbed soil, which was used using a shovel to determine the physical characteristics of the soil, and sampling in the form of intact soil, that this method was used using 30 x 30 x 30 cm boxes. In the intact sampling method, sampling was done in such a way that the percentage of soil moisture did not decrease or change and the soil texture was not damaged, so these samples were used for mechanical and shear tests as well as for improvement time. It is worth mentioning that aluminum foil along with paraffin was used around this sample, the purpose of this work was to prevent the change of the moisture percentage of the sample.

Additive materials
Additives include lime and cement, whose characteristics are discussed below.

• Lime
The lime used in this research is the type of shale that has been passed through sieve number 100 and its pH is about 12.81. Chemical analysis of lime using XRF test is shown in Table 1.

• Cement
In this research, type II cement from Bukan cement factory in the northwest of Iran was used, and the results of its physical and chemical properties are shown in Table 2.

• Water
The water used in this research was distilled water for all the experiments. 1 Loss on ignition 3. RESULTS

Base soil test results
After sampling, the samples were transferred to the laboratory for laboratory tests, and according to ASTM and ASSHTO standards, relevant tests were performed to perform physical and mechanical properties. At first, to determine the characteristics of the base soil, relevant tests were conducted on it, and the results are discussed below. Fig. 6 shows the granulation status of the used materials, based on the granulation done, and the percentage of different sizes of materials is shown in Table 3. In this research, it is considered from the fine-grained soil sample for improvement that this soil sample is of ML type, which is shown in Fig. 6 of the graining curve of the soil sample used in this research. Table 4 shows the characteristics and size of soil particles used in this research. In Fig. 7, the location of the soil sample used in this research is shown in the plasticity diagram. Based on the USDA soil classification system used in this research, it is classified as Silt Loam.

• Consolidation parameters
In addition to the physical parameters, the consolidation parameters were examined, which was done based on the ASTM D5333-03 standard, which is used to measure the subsidence potential of soils.
In this test, it is done by using the consolidation test with the use of an oeometer device, taking into account the vertical pressure until reaching the real pressures of the ground. In this test, the soil is subjected to the influence of water and becomes saturated, and at this stage, the possible shapes due to the subsidence of the soil were recorded per square centimeter loaded in natural humidity and then it is done with weights of 1, 2, 4 and 8 kg/cm2. Fig. 8 and Table 4 show the characteristics and results obtained from double consolidation on natural soil.

• Consolidation parameters
In addition to the physical parameters, the consolidation parameters were examined, which was done based on the ASTM D5333-03 standard, which is used to measure the subsidence potential of soils.
In this test, it is done by using the consolidation test with the use of an oeometer device, taking into account the vertical pressure until reaching the real pressures of the ground. In this test, the soil is subjected to the influence of water and becomes saturated, and at this stage, the possible shapes due to the subsidence of the soil were recorded per square centimeter loaded in natural humidity and then it is done with weights of 1, 2, 4 and 8 kg/cm2. Fig. 8 and Table 4 show the characteristics and results obtained from double consolidation on natural soil.

• Triaxial CU test
This test is the parameters of soil shear strength. The triaxial test is a more complex and reliable method than other tests to determine the shear strength of soils. One of the methods for determining the shear parameters of the soil is the triaxial test using the CU method, which is based on the ASTM D2850 standard. In this method, the drained type is under hydrostatic pressure during consolidation, and at the time of the test, drainage is not done by closing the drainage valves until the soil breaks. 5 parameters obtained from the triaxial test are shown. Fig. 9 shows the amount of changes in the stress-strain curve in the triaxial test in the studied soil.

Soil test results with additives
In this research, according to the obtained results, it shows that the soil of the studied area is of collapsing type, which can cause changes in soil parameters due to saturation and cause many problems, so it needs to be improved with Paying attention to the evidence in the region and quick access to traditional additives, traditional lime and cement additives were used. The results of these additions are fully explained below.

Effects of additives on consolidation parameters • The effect of lime additive percentage on consolidation parameters
In order to improve the consolidation parameters, in this research, 5 and 10% lime percentages were used to improve the parameters. Fig. 10-4 shows the amount of changes made in the consolidation chart at different percentages of lime. Table 7   According to the results, it shows that by adding lime, it is effective in controlling and increasing the parameters of soil compaction. So that at the time of adding 5% additive to the soil, it settled about 0.036 cm, and after adding 10% additive, the amount of soil settlement was about 0.03 cm, which shows the improvement in the amount of soil settlement when the soil is saturated. According to the results, it shows that the collapsing soil with the addition of 5 to 10% of lime controls the Collapse and the collapsing index has changed to a low level according to ASTM.

• Cement additive percentage in improving consolidation parameters
In order to improve the consolidation parameters, in this research, 5 and 10% cement percentages were used to improve the parameters. Fig. 13 shows the amount of changes made in the consolidation chart in different percentages of cement. Table 8 and Fig.s 14 and 15 show how to improve consolidation parameters in soil added with different percentages of cement.  According to the results, it shows that by adding cement, it is effective in controlling and increasing the parameters of collapsing soil, that the more the amount of cement added to the soil increases, the reduction of collapsing index is added, and the amount of soil settlement is also reduced. So that at the time of adding 5% additive to the soil, it settled about 0.062 cm, and after adding 10% additive, the amount of soil settlement was about 0.038 cm, which shows the improvement in the amount of soil settlement when the soil is saturated.
According to the results, it shows that the collapsing soil with the addition of 5 to 10% of cement controls the compaction and the compaction index has changed to the medium category according to ASTM.

The effect of additives on the triaxial shear parameters of CU
Below, we explain the effects of additives on the shear properties shown in the triaxial CU test.

• The effect of lime additive percentage on shear parameters
In order to improve the shear parameters in this research, at first, 5 and 10% lime percentages were used to improve the parameters. Fig.s 16 and 17 show the changes in adhesion and internal friction angle of the soil in different percentages of lime. According to the obtained results, it shows that the addition of lime increases the shear parameters of the soil so that the friction angle increases in both cases, which indicates the increase of the shear parameter in this type of soil obtained by adding lime. According to the increase in the friction angle, it shows that in this case, there is a decrease in adhesion in the soil, which indicates an improvement in the behavior of the soil after the desired addition to the soil.

• The effect of cement additive percentage on shear parameters
In order to improve the shear parameters in this research, 5 and 10% cement percentages were used to improve the parameters. Fig.s 18 and 19 show the changes in the adhesion and internal friction angle of the soil in different percentages of cement. According to the obtained results, it shows that the addition of cement percentage increases the shear parameters of the soil so that the friction angle increases in both cases, which indicates the increase of the shear parameter in this type of soil obtained by adding cement. According to the increase in the friction angle, it shows that in this case, there is a decrease in adhesion in the soil, which indicates an improvement in the behavior of the soil after the desired addition to the soil.

SUMMARY
According to the results obtained, the base soil was evaluated based on different criteria, the results of which are shown in Table 9. Based on these relationships and criteria, it shows that the said soil is in the category of medium to high leveling, so it will need to be improved. According to the obtained results, the desired soil should be evaluated. which was done in this research to improve traditional lime and cement additives. Indian criterion High collapse 7 Eyadat and Balvaheri criterion The possibility of collapse 8 Eyadat and Oli criterion The soil is susceptible to collapse 9 Eyadat and hana criterion Collapsing soil 10 Abelf criterion High collapsing soil 11 Jennings and Knight criterion Intense collapsing soil 12 ASTM D5333 standard Relatively intense collapsing soil The results obtained regarding the addition of cement and lime to the compacted soil show that the compaction index in the addition of lime is better than that of cement, which is shown in Fig. 20. With the increase of lime, the soil has a 68% and 78% decrease in soil compaction index, respectively, but when cement is added to the soil, it causes a 34% and 37% improvement in the compaction index.

RESULTS
The obtained results show that the soil of the studied area is of the type of soil and the indices of the two criteria of Jenning and Kinght (1975) and ASTM (2003) have a more appropriate assessment of the state of the area. Therefore, according to these conditions, other soils of this region can be evaluated from this criterion. According to the obtained results, it shows that with the traditional additions of lime and cement, it is possible to improve the collapse index of these soils. According to the obtained results, it shows that lime can increase the parameters of the soil and especially the index of soil compaction compared to cement due to the greater reaction of this soil. With the addition of lime and cement, the shear properties of the soil have been improved so that the friction angle in the soil has increased, which indicates the improvement in the soil parameters.