Introduction

Soft clay soils can significantly damage the structure (building, road, bridge, dam, etc.) on which it is planned to be built due to their low bearing capacity, high water absorption capacity, large and different deformation. To avoid such situations, either the soft soil should be removed from this area or the soil should be improved by various methods. Removal of soft soils will cause great damage to the environment due to the thickness of the layer and will be costly. Considering the economic and environmental damage in soft clay soil, on-site improvement is preferred.

There are two methods for the stability of soils. The first of these is mechanical stabilization. This method’s purpose is to compact the soil without mixing any additives to the soil with insufficient engineering properties, without creating any chemical reaction, or to make the soft soil suitable for the purpose by mixing it with other soils. Stone columns (Şahinkaya et al. 2017; Nazariafshar and Aslani 2021), vertical drains (Lee et al. 2021; Ayeldeen et al. 2021), dynamic compaction (Scott et al. 2021; Caı et al. 2022) can be given as examples of soil improvement with mechanical stabilization. The second method is to improve the ground by mixing an additive such as cement (Wei and Ku 2020; Thomas and Rangaswamy 2019), lime (GhavamShirazi and Bilsel 2021; Wang and Korkiala-Tanttu 2018), bitumen (Lınsha et al. 2016; Batra and Arora 2016) and fly ash (Karami et al. 2021; Andavan and Pagadala 2020) into the soil.

Nowadays, the use of additives, including waste materials, for soil improvement or soil reinforcement is on an increasing trend worldwide (Zorluer and Usta 2003; Taspolat et al. 2006; Gökalp et al. 2012; Harichane et al. 2012; Çimen et al. 2014; Ontürk et al. 2014, Sivrikaya et al. 2014; Vekli et al. 2016; Yi et al. 2016; Amini and Ghasemi 2019; Diallo and Ünsever 2019; Liu et al. 2019; Yılmaz 2020). The main reason for this approach is that waste materials such as MP and PP create environmental problems and require very large areas for storage.

In the study, MP accumulated during cutting and cleaning was used as a stabilization material. The proportion of marble accumulated as waste during block production in the quarries equals 40–60% of the total production volume (Çelik 1996). Approximately 5.2 million m3 of the 15 million m3 marble reserve in the world is in Turkey. Marble is produced in about 50 countries around the world (Rai et al. 2020). The amount of waste from marble production is, the damage it will cause to the environment, and storage problems are obvious with numbers. The most important reason to use MP on soils is Ca2+ ion, which is the most resistant material to sulfate in its structure and helps the stability of the soil (Li et al. 2018).

PP was used as the other stabilization material in the study. PP is generally used in construction, agriculture, and dentistry. Therefore, particles larger than 3 mm are used in these areas. PP is generally a light-colored, highly vesicular glass consisting of SiO2, Al2O3, and Fe2O3 (Day 1990). In the study, particles smaller than 3 mm remaining in the field were used as improvement material. Turkey has approximately 40% of the world’s pumice reserves, which is around 18 billion m3, and has approximately 7 billion cubic meters of pumice reserves (BSD 2006).

The study aimed to improve the soil and reduce the lateral load by adding PP, which is a relatively light material, to the soils that are not suitable, especially for appropriate filling, etc. In this study, MP and PP were used as additives in soft clay soil. The basic physical soil properties and strengths of PP and MP, which are added to the soil in certain proportions (5%, 10%, 15%, 20%) by weight, have been examined. In addition, their strengths were examined again at certain curing times (0, 7, 14, and 28 days), and the soft soil improvement rates of the additives were examined in detail. As a result of the study, it is aimed to use a very significant amount of waste to be discharged into the environment as soil improvement material.

Materials and methods

Soft clay

In the study, clay is taken from the Yozgat Divanlı Village; High-Speed Train Tunnel construction site was used as the experimental clay, which was taken from a depth of approximately 2 m. Experiments were done with undisturbed soil samples taken from the field. The test instruments used in experimental studies and chemical analyses were calibrated according to the relevant specifications and standards. The plasticity and liquidity properties of clay and grain density, permeability, compaction properties, and grain density, permeability and compaction properties were determined according to ASTM. The strength parameters of the clay were also determined. The chemical properties of clay were determined by XRF analysis.

Pumice powder (PP)

Pumice powder (Sieve No. 40) used as an additive in the study is obtained by breaking down a porous, siliceous volcanic rock known as pumice, formed as a result of volcanic events, which is light-colored, spongy, resistant to physical and chemical factors. In addition, according to TS 2823 (1977) standards, pumice is a volcanic natural lightweight aggregate with non-interconnected voids, sponge-like silicate-based, unit volume weight generally less than 1 g/cm3, hardness about 5.5–6.0 according to Mohs scale and showing a glassy texture. It also contains silicon dioxide up to 75% chemically. Pumices contain different types of minerals in their structures. These are mainly amphibole, pyroxene, biotite, plagioclase and opaque minerals. Approximately 70% of the aggregate of pumice contains voids. There are many pumice quarries in the region of Nevşehir City between Ürgüp, Derinkuyu, and Acıgöl. The pumice powder used as an additive in the study was obtained from a quarry near Şahinefendi Village. The specific gravity values of Pumice obtained in Nevşehir region vary between 2.33 and 2.35, and dry unit weight is between 830 and 748 kg/m3 for 0–4 mm, 594 and 561 kg/m3 for 4–8 mm, and 502 and 516 kg/m3 for 8–16 mm, depending on aggregate size, while water absorption values are 22.75% and 26.20% for 0–4 mm, 33.58% and 35.40% for 4–8 mm, and 39.40% and 40.30% for 8–16 mm, depending on the aggregate size, and the frost resistance changes between 0.92% and 1.33% (Gündüz 2005). In addition, the pumice void rate and porosity are quite high, varying between 2.3–3.1% and 69–71%, respectively.

Marble powder (MP)

Marble powder is the smallest size marble waste. They are marble grains formed during the cutting of blocks and slabs in marble plants, most of which are less than 1 mm (Zorluer and Usta 2003). According to previous studies, it has been calculated that approximately 30% of the marble processed in marble factories is thrown as dust (Ontürk et al. 2014). There are about 5.2 billion m3 reserves in Turkey. This reserve is approximately 40% of the world’s marble reserves. While 25% of processed marble is obtained from marble quarries, around 75% of waste material (broken marble, marble dust), that is a large amount, is produced (Erkek and Özdemir 2011). The marble dust used as an additive material in the study (Sieve No. 40) was obtained from a company named “İlktaş Maden” in Nevşehir. The average specific surface of the supplied marble powder is 5625 cm2/g, and its density is 2.78 g/cm3.

In addition to these physical properties, XRF analysis was performed to determine the chemical properties of soft clay, pumice powder, and marble powder used in the study, and the analysis results are given in Table 1. In addition, the appearance of clay, PP, and MP samples used in the experiments is given in Fig. 1.

Table 1 XRF results of clay and addition agents used in the experiment
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Fig. 1
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Sample of clay, PP, and MP

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Method

In the study, first of all, the water content, specific gravity, soil particle distribution, liquid limit, plastic limit, compression parameters, and impermeability coefficient and strength properties (free compressive strength) of the clay sample were investigated in accordance with ASTM standards (ASTM D 854-02 2003; ASTM D 422-63 2003; ASTM D 4318-00 2003; ASTM D2166-00 2003; ASTM D5856-95 2007; ASTM D1557-09 1557 2009). Later, 5%, 10%, 15%, 20% PP, and MP were added to the clay soil by dry weight. Before starting the experiment, soil mixtures were left to mellow for 24 h to ensure a homogeneous distribution of moisture and compacted according to ASTM. The clay and PP-MP were mixed thoroughly, and the mixture became homogeneous. In the experimental study, the material passing through Sieve No. 4 (4.76 mm) was used for the clay sample, and MP and PP passing through Sieve No. 40 (0.420 mm) were used.

Laboratory studies

The effect of PP and MP on specific gravity

In this part of the study, the specific gravity values were determined without adding any additives to the clay soil samples prepared in accordance with ASTM 854-02 standards. Then, specific gravity values were determined by adding 5%, 10%, 15%, and 20% PP and MP by dry weight to the clay soil sample. Results are given in Fig. 2. When the graph in Fig. 2 is examined, it is seen that as the additive rate of MP increases, the specific weight of the added clay soil increases, and as the additive ratio of PP increases, the specific weight of the added clay soil decreases. In the same proportions, it is seen that marble powder increases the specific gravity of the clay soil more than the pumice powder.

Fig. 2
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Specific gravity PP–MP addition in rates of 5–10–15–20%

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The effect of PP and MP on granulation

How PP and MP were added to the clay soil in certain proportions (5%, 10%, 15%, 20%) changed the grain distribution of the soil was examined. In the study, the samples were subjected to the washing sieve analysis for the coarse particles and the hydrometer test for the fine particles in accordance with ASTM standards (ASTM D 422-63). Sieve analysis results are given in Figs. 3, 4. When the sieve analysis graphs are examined, it is seen that the fine-grained material ratio (Sieve No. 200) of the soil decreases with the addition of PP and MP to the clay soil. However, a linear increase or decrease did not occur due to the proportion of additives added to the clay soil. This indicates that both additives obtained from the No. 40 sieves do not have a uniform grain distribution.

Fig. 3
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Sieved percentage—PP addition in rates of 5–10–15–20%

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Fig. 4
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Sieved percentage—MP addition in rates of 5–10–15–20%

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The effect of PP and MP on consistency limits

With consistency limits, it can be determined how soil will behave at different water content. However, what is more important is that the consistency limits also give an idea about the bearing capacity of soil (Bilgen et al. 2012). Also, in other previous studies (Skempton and Northey 1953; Wroth and Wood 1978; White 1982; Sivapullaiah et al. 2000; Bilgen et al. 2012), it can be concluded that the plasticity index calculated according to the liquid limit and plastic limit values should have a smaller value because it has an effect on the strength of the soil and increases the soil workability. This situation is related to the direct effect of water in the soil on the soil behavior. Therefore, in this part of the study, in accordance with ASTM standards (ASTM D 4318-00), taking into account the dry weight of the clay sample, the liquid limit, plastic limit, and plasticity index values of the samples prepared by adding PP and MP materials at the rates of 5%, 10%, 15%, 20% have been found. The values found are given in Table 2. Also, liquid limit and plastic limit change are given in Fig. 5. When Table 3 and Fig. 5 are examined, it is seen that the liquid limit and plastic limit calculated based on these values decrease with the addition of PP and MP additives to the clay soil in increasing proportions. On the other hand, when comparing the effect of PP and MP waste materials added to the clay soil in the same proportions on the consistency limits of the clay soil, it was observed that the consistency limits of the MP additive material, thus the plasticity index, decreased even more.

Table 2 Consistency limits clay—PP and MP addition in rates of 5–10–15–20%
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Fig. 5
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Consistency limits—PP and MP addition in rates of 5–10–15–20%

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Table 3 Compaction parameters clay—PP and MP addition in rates of 5–10–15–20%
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The effect of PP and MP on compaction parameters

It is necessary to know the optimum water content (wopt) and the maximum dry density (γmax) parameters of the soil, which are also defined as the compression parameters, to obtain a desired level of compression and therefore suitable stability in soil. To determine these parameters, the samples prepared by adding PP and MP additives (5%, 10%, 15%, 20%) to the clay soil in certain proportions were subjected to the standard Proctor test in accordance with ASTM standards (ASTM D1557-09). Compression parameter changes found as a result of the study are given in Table 3 and Figs. 6, 7. When Table 3, Figs. 6, 7 are examined, it is seen that the optimum water content (wopt) of the soil formed with PP and MP additives added to the clay soil at increasing rates decreased, and the maximum dry density (γmax) value increased. On the other hand, when the additives added to the clay soil were compared within itself, it was observed that the maximum dry density value of MP compared to PP increased even more, and the optimum water content value decreased even more.

Fig. 6
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Compaction parameters—PP addition in rates of 5–10–15–20%

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Fig. 7
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Compaction parameters—MP addition in rates of 5–10–15–20%

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The effect of PP and MP on permeability coefficient

Another important issue in soil stabilization is the permeability of the soils. Although the permeability of fine-grained soils is very low compared to coarse-grained soils, it may cause the soil to lose its stability after a stable flow that becomes saturated over time. Considering this situation, at this stage of the study, a permeability test was carried out by compression at the descending level on the plain clay soil and clay soils with PP and MP additives at the specified ratios (5%, 10%, 15%, 20%) at optimum water content determined by Proctor test in accordance with ASTM standard (ASTM D5856-95). In the study, the average permeability coefficient was determined for each soil sample (Table 4). The values found are given in Fig. 8. When the test results presented in Table 4 and Fig. 8 are examined, it has been observed that PP and MP additives added to the soil at increasing rates decrease the permeability coefficient of the soil. On the other hand, it is seen that MP additive material further reduces the permeability of clay soil compared to PP material.

Table 4 Permeability coefficient clay—PP and MP addition in rates of 5–10–15–20%
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Fig. 8
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Permeability—PP and MP addition in rates of 5–10–15–20%

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The effect of chrome and iron slag on unconfined compressive strength

The strength properties of fine-grained materials such as clay and silt can be determined by experiments such as the triaxial pressure test and the free pressure test in the laboratory. In simple terms, the strength of clay soil can be determined by the free pressure test. This test is used more widely since it is simpler than the triaxial pressure test (unlike the triaxial test, any soil sample is tested uniaxially without any lateral pressure (cell pressure) and since the strength properties for cohesive soils give faster results). In this study, PP and MP additives were added to the clay soil in proportions based on dry weight (5%, 10%, 15%, 20%), and the samples prepared in accordance with the standards (ASTM D2166-00) were subjected to 0-, 7-, and 14-day curing times and free compressive strengths have been found. The values found are shown in Table 5 and Fig. 9. When the results were examined, it was seen that PP and MP additives added to the clay soil at increasing rates during all curing times increased the free pressure strength of the soil. In addition, the free pressure strength increased with the increase in PP and MP ratios added to the clay soil, but the MP additive material was more effective on the free pressure strength of the clay soil compared to the PP material.

Table 5 Unconfined compressive strength—PP and MP addition in rates of 5–10–15–20%
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Fig. 9
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Unconfined compressive strength—PP and MP addition in rates of 5–10–15–20%

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Results and Discussion

The following results were obtained from the experiments on soil samples prepared with PP and MP waste additives added in proportions 5%, 10%, 15%, 20% by dry weight to the clay soil, which was determined to be high plasticity clay.

Considering the detailed chemical analysis results (XRF) for the additives PP and MP in Table 1, it is believed that the high content of calcium oxide (CaO) contained in the additive MP reacts with the silicate in the soft clay to form a very stable calcium silicate. This chemical change, which is a pozzolanic reaction, is very important in increasing the strength of the soft soil. With the addition of MP, the bearing capacity of the soft soil is increased significantly.

It was observed that the specific gravity of PP, which was added to soft clay soil, decreased, while MP, which was added to soft clay soil in the same proportions, increased the specific gravity of soft clay soil. This indicates that PP can be easily used in areas that need to be lightly backfilled after improvement.

When comparing the sieve analysis results of PP added soil and MP added to soil, it was found that the fine particle ratio of MP added soil decreased even more. From this, it can be seen that the use of additives with finer grain size than clay soil can accelerate the reaction time, increase the strength, decrease the void ratio, and thus achieve better compaction and decrease permeability.

With the addition of PP and MP additives, the liquid limit, plastic limit, and the plasticity index calculated based on these values decreased. According to the result, it can be said that PP and MP added to the soil have the effect of lowering the boundary water content of the soft clay soil. In addition, considering the previous studies, it can be said that PP and MP additives have a positive effect on the strength of the clay soil. On the other hand, when the effects of PP and MP waste materials added to the clay soil in the same proportions are compared on the consistency limits of the clay soil, it can be said that the MP additive material is more effective in improving the consistency properties. Again, it can be thought that this situation may be due to the high rate of calcium oxide in MP content. From these results and evaluations, it can be deduced that both additives can have a significant effect on the bearing capacity of the soil by reducing the plasticity index depending on the consistency limits of the soft clay soil in terms of geotechnics.

Examination of the Proctor test results shows that the optimum water content (wopt) of the soil prepared with the additives PP and MP, which were added to the clay soil in increasing amounts, decreased, while the value of maximum dry density (γmax) increased. At the same additive ratios, MP increased the value of maximum dry density more than PP, but further decreased the value of optimum water content. Moreover, the results show that the most effective PP and MP additive ratios are 20% in compaction. In general, the increase in maximum dry density (γmax) with PP and MP is the increase in their ability to compact with the water they absorb by deposition between fine clay grains.

By studying the falling soil permeability test results, it was found that the additives PP and MP, which are added to the soil at an increasing rate, decrease the permeability coefficient of the soil. On the other hand, it is found that the additive material MP further decreases the permeability of the soft clay soil as compared to the material PP. Therefore, it can be concluded that by adding PP and MP additives to the clay soil, the permeability can be further reduced so that there is no permeability or these additives can be effectively used in similar areas where the permissible impervious curtain and/or soil permeability is low.

In examining the results of the unconfined compression test, it was found that the unconfined compressive strength of the soil increased with increasing amounts of PP and MP additives at all curing times. This results from the increasing interaction of PP and MP additives with the soil in parallel with increasing curing times. The additive MP has a stronger effect on the unconfined compressive strength of the clay soil than the additive PP. The most suitable ratio of MP layer is 15% in terms of curing times and free compressive strengths. The reason the additive MP has a greater effect on the unconfined compressive strength than the additive PP, is the effect of the high amount of calcium oxide compound in the content of MP.

The resulting data are very important because it can eliminate the environmental pollution caused by marble and pumice waste, which has a large application area, and eliminate the need for additional landfill area. Based on the experimental results, it was found that PP and MP can be used as alternative additives to increase the bearing capacity of soil in areas with soft subsoils, such as airports, roads, and light structures, and reduce settlement to some extent. Therefore, it is considered that PP can be readily used by application engineers to improve soft soils, especially in stabilizing access embankments, and MP in improving soft soils, especially infill areas where soft soils are preloaded. Future studies may investigate whether PM and MP wastes can be mixed and in certain proportions to improve soft soils, or what changes in engineering and strength properties of soft clay soils can be achieved with a third waste material mixture of MP and PP.