Introduction

Freeze-thaw cycles are a significant issue in cold climates in regional engineering. In addition, it was recognized that freeze-thaw cycles have a major impact on the structure and thus on the geotechnical engineering properties of soil. The main reason for freeze and thaw is the presence of permafrost in old regions. Challenging geotechnical engineers strive to design what is most efficient and economical. A soil treatment approach that considers the underlying soil type, treatment depth and location needed for treatment, desired level of improving availability of qualified personnel, and materials processes environmental friendliness and project cost (Fatehi et al. 2021; Omoregie et al. 2019). Failure in the soil also causes settlement in building foundations which also has a lot of effects on human lives (Steiner, Vardon, and Broere 2018). As the temperature in the region increases, the thawing process starts from the top to the bottom at the same interval. The moisture released due to thawing rests on the frozen layer and acts like a saturated zone separated from the main body of soil. Thawing the moisture content under the frozen zone is higher than it was throughout the pre-frozen phase (Zhang and Wang, 2001; Calvo 2009). Due to the rise in temperature, the soil moisture defrosts which results in the formation of empty spaces, which gradually become filled under the action of loading and self-weight, known as the thawing effect, and it leads to a reduction in the compressive strength of the soil (Firoozi et al. 2015). The purpose of this paper was to examine various investigative techniques in developing an understanding of the effect on mechanical and physical properties of soils due to the impact of freeze and thaw, and increase in compressive strength of the soil by using terrazyme was analyzed. It has been concluded that freeze and thaw affect the structure of soil on a microscopic level, and in turn, the geotechnical properties of soils also changed (Chen et al. 2004). For construction in a cold region, highway pavements are left unpaved to an extent to harbor the freeze-thaw action (Eigenbrod, Knutsson, and Sheng 1996). The soils subjected to the freeze and thaw cycle have a twofold effect on the density, i.e., dense soils tend to loosen, and loose soils show densification (Konrad 1989). Hydraulic conductivity continues to increase regardless of changes in soil density (Chamberlain and Gow 1979; Othman and Benson 1993; Viklander 1997).

Fatehi et al. (Fatehi et al. 2021) reported a detailed investigation of biopolymers and their effect on the properties of the soil. They mentioned that the use of biopolymers in soil improvement is not only environmentally friendly but also has a positive influence on the performance of soil. The authors further mentioned that the utilization of biopolymers in the soil as a binder improved both the mechanical and durability properties of soil. Wei et al. (Wei et al. 2021) performed freeze and thaw cycles to check shear behavior and structural strength of artificially structural loess. The cylindrical specimens having a diameter of 61.8 mm and a thickness of 20 mm were prepared in cutting rings. The water content was 7%, 16%, and 30% while the cement content was 0.1%, 2%, and 5% respectively. The sample was then placed in the freezing cabinet at −15 °C and was frozen quickly for 24 h; then, the sample was thawed quickly for 24 h at room temperature in a closed case. It was found that the first freeze and thaw cycle has more impact on the physical and mechanical properties of soil. Meeravali et al. (Meeravali et al. 2020) used black cotton soil and determine the changes that occur in the soil properties after freeze-thaw cycles. The samples were frozen at a temperature of −5 °C for 24 h. The samples were tested concerning 1, 3, 5, and 7 cycles. Permeability, UCS, and consolidation tests were performed to check the properties of the soil. It was concluded that the permeability increased as the number of cycles increased, and a decrease in the shear strength of the soil was observed because of an increase in the size of voids. Navale et al. (Navale et al. 2019) studied the effect of terrazyme on the index and engineering properties of black cotton soil and red soil. The properties of black cotton soil were significantly improved by stabilization with a terrazyme dosage of 200 ml/1.5 m3 for black cotton soil and for red soil by 200 ml/1.5 m3 of soil. Even unsoaked CBR in both black cotton soil and red soil has showed better improvement when treated in dry conditions at average room temperature. The free swell index of black cotton soil showed a drastic reduction with treatment from terrazyme, especially with drying. Steiner et al. (Steiner et al. 2018) studied the shear strength of illite soil against different freeze-thaw cycles. The samples were frozen at a temperature of below −2.5 °C for at least 1 h and completely thawed when the temperature reached or exceeded 3 °C for 1 h. The machine was filled with water having a temperature of 17 °C to allow the sample to thaw for at least 6.5 h. The unconsolidated un-drained (UU) triaxial compression tests of cohesive soil were performed at a pressure of 400 kPa and a loading rate of 0.1 mm/min and found that after the freeze-thaw cycle, cracks start to develop, and the shear strength of the soil starts to decrease. Zeinali et al. (Zeinali et al. 2016) used the LTU apparatus for freeze and thaw cycles according to ASTM D5918. It was found that during freeze-thaw action, the pavement heaves in winter, while thaw weakening occurs during the thawing phase. Ice lens formation occurs when there is direct access to an external source of water. Frozen soil in the subgrade increases the stiffness of the soil layers due to the ice bonding of the soil particles. During frost action, the increase in stiffness is not an issue until the soil remains in a frozen state while during the thawing phase, the decrease in stiffness causes road settlement. Firoozi et al. (Firoozi et al. 2015) studied the effect of freeze-thaw cycles on the UCS of clay soils and treated the soil with lime. Three number of cycles were applied in such a way that the soil was frozen for 24 h at −15 °C, and the thawing temperature was set at almost room temperature of 22 °C for 24 h. The test results indicate that soil compressive strength decreases with increasing number of freeze-thaw cycles. Soil treated with 5% lime showed improvement in mechanical properties. Reduction in volume and swelling, along with improvement in strength, was observed and also experienced that lime increases the brittleness index of the soil.

Enzymes are natural catalysts and are prevalent in all living life forms. They are acquired from plants and animals including microorganisms by extraction utilizing reasonably solvable (Calvo 2009). Enzymes are huge protein particles that are more productive than inorganic catalysts; the reaction rate is regularly expanded by a factor of 106 to 1012. The enzyme is a biological catalyst, made up of proteins. Bio-enzymes are chemicals (organic and liquid) used to stabilize soil and aggregate in roads and structures (Lacuoture and Gonzalez, 1995; Omoregie 2020). It also improves durability and lowers the optimum moisture content and plasticity index (PI) of the soil. The effect of terrazyme on the soil is permanent and also becomes a bio adjective in nature. These organic enzymes are also found in liquid form which is perfectly soluble in water. The addition of clay with bio-enzymes provides a significant improvement in the geotechnical properties of stabilized soils as reported by Bergmann (2000), and adding 2% of clay increases the probability of a successful stabilization with increasing content of clay up to 10–15% giving even more increase in strength. This increase in strength can be attributed to individual minerals in the clays (i.e., montmorillonite, illite, and kaolinite), and due to the presence of negative ions in enzymes, the minerals found in clays tend to attach themselves to those ions. As the binding establishes, the soil densifies and affects the mechanical and hydraulic properties (i.e., strength increase and reduction in permeability are observed). Increasing the content of clay increases its reactivity with bio-enzymes (Shukla et al. 2003). The results show both low to high values of strength improvement; this was attributed to the lower reactivity of clays with bio-enzymes where the content of clay was minimum, and bonding between clay minerals was not strong enough for adequate binding. In recent years, the use of bio-enzymes for the stabilization of soil has become more significant. Bio-enzymes have been introduced recently for soil stabilization. These are organic materials and are supplied in the form of concentrated solutions. In recent years, many researchers studied the terrazyme effect on the engineering properties of soil, but no research on the effect of freeze and thaw cycles on soil treated with terrazyme was done. In this research, along with engineering properties, the effect of the freeze and thaw cycle is also studied which is a significant part of this research.

Materials

Soil

Soil samples were collected from different cities in Pakistan, one from Yakhtangay (Shangla) KPK where snowfall in winter is significant and the temperature drops to −10 °C, and the second sample was collected from Murree where the temperature goes to −4 °C in the winter. Samples were collected from a depth of 2 m below the ground surface. The properties of the soil samples have been discussed in the “Test results” section.

Terrazyme

Terrazyme is a bio-liquid enzyme that is organic in nature and is produced from the chemical changes of fruit extracts, vegetables, sugar, and water caused due to fermentation process as shown in Fig. 1. Terrazyme is a natural, non-toxic, non-corrosive, and non-flammable liquid, produced by formulating vegetable extracts. The color of terrazyme is dark brownish molasses smell and has no other significant harmful effects (Gupta 2017). The specific gravity varies from 1 to 1.09, the vapor density is 1, and the pH value of terrazyme varies from 4.30 to 4.60 (Divya and Asha 2021). They ordinarily catalyze one specific reaction; therefore, enzymes do not deliver any adverse effects. They are temperature sensitive and work best at mild temperatures (35 °C) and lose their adequacy at a higher temperature. They tend to be pH sensitive as well and are most effective around a pH of 7 (Norris, Ryan, and Acaster 2011). Bio-enzymes are a chemical used to stabilize the soil, and the fundamental structure of the enzyme is an amino acid, in which one or more amino acids combine along a peptide to form a chain of a protein. An amino acid consists of three components attached to the central C-H bond amine group, R factor, and carboxylic group. Furthermore, it is specially prepared to enhance the engineering properties of soil (Marathe and Shankar 2021). Enzymes are attracted to the clay which is negatively charged and expel the attached water and cover the particles (Scholen 1995). Engineering properties of terrazyme-incorporated soil specimen depends upon the dosage of terrazyme used. The enzymes are liquid additives that act on the soil to reduce voids between the soil particles, minimizing the amount of water absorbed by the soil (Eujine et al. 2014). Enzymes react with organic matter (moisture) in the soil to form cementitious material. This reduces the ability of soil particles to swell, reducing permeability. In clay-water mixtures, positively charged ions (cations) are present around the clay particles, creating a film of water around the clay particles that remain attached or adsorbed to the clay surface. Adsorbed water or a double layer gives clay particles their plasticity. In various cases, the clay may swell and increase the size of the double layer which can be reduced by drying. Therefore, to really improve the soil properties, the thickness of the double layer must be permanently reduced. A cation exchange process can achieve this. Using a fermentation process, certain microorganisms are able to produce stabilizing enzymes in large quantities. Soil stabilizing enzymes catalyze reactions between organic cations and clay that accelerate cation exchange without becoming part of the final product. Terrazyme replaces adsorbed water with organic cations and neutralizes the negative charge of clay particles. Terrazyme used in this research was purchased from Nature Plus Inc., USA. The chemical composition of terrazyme is summarized in Table 1.

Fig. 1
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Terrazyme sample

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Table 1 Chemical composition of terrazyme
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Methodology

Modified proctor test

A modified proctor test was performed according to the ASTM D1557-12 (Soil and Rock 2009) standard to find out the compaction characteristics of the soil sample. An oven-dried sample of 3 kg soil passed through a #4 sieve was used in this test. A 10-lbm hammer having an 18-inch fall height was used for the compaction of the soil sample. The soil was compacted in a 4-inch diameter mold in five layers with 25 blows per layer of compaction. The test was started by adding 4% water by weight and then increased by 3% for each trial.

Specimen preparation

The sample was air-dried and passed through a #40 sieve that was used for the preparation of specimens. Remolded samples were prepared for all the tests performed. The soil was compacted into UCS mold at 95% density (obtained from compaction test). Treated samples were prepared with the addition of terrazyme in water, and for sample preparation, the optimum amount of terrazyme was used as obtained in “Optimization of Terrazyme” section. Treated and untreated samples were prepared and permitted to cure in a sealed airtight container for 48 h and then used for freeze and thaw cycles (Pooni et al. 2019).

Unconfined compression test

Unconfined compression test was performed by following the guidelines of ASTM D 2166 (ASTM 2006). The dimension of the mold used in the test was according to the standard ratio of 2:1 having a height of 8 cm and dia of 4 cm. The samples were made according to the OMC and MDD of the soil sample. Freezing and thawing action was checked on different soil specimens prepared for the UCS as shown in Fig. 2. This phase consisted of seven freeze and thaw cycles. Three soil samples were taken at each cycle, and the mean value for each cycle was used. The strain rate was 1 mm/min.

Fig. 2
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UCS test performed in the laboratory

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Freeze and thaw cycles

Each cycle began by placing the samples in a freezing cabinet at a constant temperature not warmer than −23 °C for 24 h. The test assembly was then placed in a moist room for 23 h at 21 °C and relative humidity of 100% and was removed according to ASTM standards (Standard 2016). Specimens were then brushed several times using a force of approximately 15 N. The freeze and thaw cycles were applied in such a way that the sample was first frozen to −10 °C for 24 h as shown in Fig. 3 and then allowed to thaw at room temperature of 21 °C for the next 24 h, so in this way, one freeze and cycle was completed. The same process was repeated to complete seven cycles. Meeravali et al. (Meeravali et al. 2020) also studied the effect of freezing and thawing cycles on soft soil samples and studied its effects on the engineering properties of soil in which UCS, permeability, and coefficient of consolidation tests were performed after seven cycles and concluded that UCS and coefficient of consolidation decreased with an increasing number of cycles. Zha et al. (Zha et al. 2021) used titanium gypsum to improve the properties of expansive soil, in which swelling potential, one-dimensional consolidation test, shear strength, UCS test, and X-ray fluorescence tests were performed and used 7 days curing period and concluded that the engineering properties of soil were improved.

Fig. 3
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Freeze samples

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Test results

General characterization of soil

The physical characteristics of the Shangla and Murree soils are summarized in Table 2.

Table 2 Properties of soil samples
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Gradation curves for both soil samples shown in Fig. 4 indicate the presence of fine particles in the soil.

Fig. 4
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Particle size distribution curve for Murree soil and Shangla soil

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Modified proctor test

A modified proctor test was carried out to determine the relationship between optimum moisture content (OMC) and maximum dry density (MDD). The trials were carried out by increasing the moisture content with an increment of 3%. The MDD of Shangla soil was achieved at 18.29 kN/m3 with an OMC of 13%, and the MDD of Murree soil was achieved at 18.77 kN/m3 with an OMC of 14% as shown in Fig. 5. Since Shangla soil consists of 10% sand content, it has more tendency to absorb moisture as compared to Murree soil and as shown in Fig. 5. Shangla soil reaches its peak MDD at a lower value of moisture content compared to Murree soils.

Fig. 5
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Maximum dry density vs optimum moisture content of natural soils

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Unconfined compressive strength (normal sample on OMC)

The UCS of Shangla soil comes out to be 391.19 kPa, and the UCS of Murree soil was recorded as 735.32 kPa as shown in Fig. 6. The shear strength was computed as half of the value of unconfined compressive strength.

Fig. 6
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UCS of soil samples at OMC

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Optimization of Terrazyme

Shangla soil

The quantity of terrazyme to be used in the soil sample depends upon the PI and MDD of the soil. The percentage fines, MDD, and PI of the soil were used to estimate the concentration of terrazyme as per the guidelines given by Nature Plus (Nature Plus 2004). Firstly, PI and percentage fines of the soil were used to find out the quantity of the soil measured in m3, and then, it was used to calculate the concentration of the enzyme in liters; furthermore, soil density and volume of soil were used to calculate the factor. Finally, the factor was multiplied by 1000 to evaluate the concentration of terrazyme measured in ml. Generally, 1 l of terrazyme content will be used for 32 cubic meters of soil. As a result, 1.6 factor was multiplied by 1000 to calculate the amount of soil taken in kilogram, and hence, this gave the amount of terrazyme to be used in ml. Dilution factor was taken as 1:100. A trend was developed by testing the UCS at different factors, as the selected factor is 1.6, so to get the optimum amount of terrazyme content, UCS was conducted at different factors below and above 1.6. After performing UCS and modified proctor test at 1.5, 1.6, 1.7, and 1.8 factors, the optimum value was obtained at a factor of 1.6. The terrazyme, being soluble in water, was first dissolved in a given amount of water before testing. The UCS samples were prepared and then allowed to cure properly so that the terrazyme completely reacts with the soil particles. The UCS samples were tested for a curing period of 7 days to obtain a maximum value that can be used onwards. A modified proctor tests were performed by using the factors as calculated. A trend line was set around that factor to calculate MDD and OMC. The factor which gives maximum value was selected for the UCS test. Modified compaction test values for different factors are summarized in Table 3.

Table 3 Optimization of terrazyme for MDD and OMC by different factors (Shangla soil)
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The factor (1.6) was used for the preparation of the UCS samples, as it indicates the maximum value of MDD and OMC. For further optimization, different trails were performed to evaluate the optimum content of terrazyme in which slightly above and below values from the corresponding factor were used. The factor which gives maximum strength was used for the preparation of UCS samples. Table 4 shows the different trial values after performing the UCS tests. Figure 7 shows the strength values of UCS samples at different factors. The UCS samples were crushed out after 7 days of curing. The average value of UCS obtained from the Table 4 against the given factor was used as the optimum amount of terrazyme. The UCS value was maximum at factor 1.7 and further used for the next phases.

Table 4 Optimization of terrazyme for UCS at different factors
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Fig. 7
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UCS values against different multiplication factors for calculating the quantity of terrazyme

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Murree soil

Terrazyme was used to improve soil strength, so in this phase of research, actual amount of terrazyme to be used was calculated. The amount of terrazyme to be used in the soil sample depends upon the PI and MDD of the soil. The percentage fines, MDD, and PI of the soil were used to evaluate the concentration of terrazyme for a given soil volume as per guidelines given by Nature Plus (Nature Plus 2004). Initially, PI and percentage fines were used to estimate the quantity of the soil measured in m3, and then, it was used to estimate the concentration of the enzyme in liters. Moreover, soil density and volume of soil were used to calculate the factor. Lastly, the factor was multiplied by 1000 to evaluate the concentration of terrazyme measured in ml. Generally, 1 l of terrazyme content in solution form is used for 32 m3 of soil. As a result, 1.4 factor that is to be multiplied by the amount of soil taken in kilogram gives the amount of terrazyme to be used in ml. A 1:100 dilution factor was used. For the optimum amount of terrazyme content, UCS tests were conducted at different factors below and above 1.4, and after performing UCS and modified proctor test at 1.3, 1.4, 1.5, and 1.6 factors, the optimum value was obtained at 1.5.

The terrazyme as we know is a water-soluble enzyme, so it was first dissolved in a given amount of water before testing. The prepared UCS samples were allowed to cure properly so that the reaction of terrazyme with soil particles was properly completed. The UCS samples were tested after 7 days curing period to obtain a maximum value that can be further used. The modified test was conducted for three factors, i.e., 1.3, 1.4, and 1.5 of terrazyme amount with 4% water were added for the first trial followed by 3% for each trial. Modified compaction test results for different factors are shown in Table 5. The factor which gives maximum values for MDD and OMC was then used. UCS tests of treated soil were performed at different trials obtained from MDD as shown in Table 5. Figure 8 shows the UCS values obtained at different factors. The UCS value was maximum at factor 1.5 and further used for the next phases (Table 6).

Table 5 Optimization of terrazyme for MDD and OMC by different factors (Murree soil)
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Fig. 8
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UCS test of Murree soil at different factors

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Table 6 UCS test at different trials
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Effect of freeze and thaw cycles on untreated soils

Unconfined compressive strength

Murree soils

Figure 9 shows the relationship among the number of freeze-thaw cycles and the compressive strength of the soil. The test outcomes indicate that soil strength decreases with increasing number of freeze-thaw cycles. From the results, it was concluded that UCS of 2nd, 3rd, 4th, 5th, 6th, and 7th of untreated soil was 22.19%, 36.28%, 58.57%, 64.53%, 76.73%, and 82.96% reduced concerning 1st cycle. Correspondingly, treated soil samples show a decrease in strength of 11.9%, 22.18%, 31.33%, 35.41%, 41.26%, and 46.65% compared to the 1st cycle. The decrease in UCS of soil is because of the fact that during the freezing period, the porous spaces are filled with water, which forms ice, and increases volume. The size of voids increased as the number of freeze-thaw cycles increased. This effect increases the porosity of the soil sample and reduces the UCS properties of the soil. The UCS of treated soil samples increased after the freeze and thaw cycle concerning untreated soils, and increase in the strength could be attributed to the change of the original structure from a more dispersed structure to a more flocculated structure (Muguda and Nagaraj 2019). The addition of terrazyme reduces voids between the soil particles and increases soil density after various cycles. The results are aligned with previous studies in which decrement in UCS with respect to increasing in the cycles was also reported (Meeravali et al. 2020; Xie et al. 2015; Changizi et al. 2022).

Fig. 9
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Effect of F-T on UCS on untreated and treated soil of Murree

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Shangla soil

Figure 10 shows the variation of UCS strength of Shangla soil with respect to a number of freeze and thaw cycles. The results show that the strength of soil decreases with an increase in the number of cycles. The results revealed that the UCS of untreated soil decreased by 26.12%, 29.24%, 41.16%, 45.24%, 51.62%, and 61.87% respectively at 2nd, 3rd, 4th, 5th, 6th, and 7th with reference to 1st cycle. A similar trend of decline in UCS was also observed in past work (Gowthaman et al. 2020). Furthermore, the results for treated soil (i.e., soil with terrazyme) showed that the UCS decreased by 7.47%, 15.33%, 26.48%, 26.92%, 37.08%, and 42.95% respectively at 2nd, 3rd, 4th, 5th, 6th, and 7th compared to 1st cycle. From the results, it was observed that by incorporating terrazyme in the soil, the percentage reduction in the UCS was reduced. The decrement in the loss of UCS is attributed to the addition of terrazyme which reinforce the soil. It is because the terrazyme has the ability to reinforce the soil composite by building a strong bond/adhesion between soil ingredients, thereby enhancing the density of soil samples (Gupta et al. 2017; Yusoff et al. 2017). The improvement in strength could be ascribed to the change of the original structure from a more dispersed structure to a more flocculated structure (Muguda and Nagaraj 2019). Hence, the terrazyme plays a tremendous role in declining the loss of UCS of soil along with an increasing number of cycles. A similar trend of decrement in UCS with respect to increasing in the cycles was also reported in previous studies (Meeravali et al. 2020; Xie et al. 2015; Changizi et al. 2022).

Fig. 10
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Effect of F-T on UCS on Untreated soils & treated soils of Shangla

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Variation of UCS of untreated soils

The comparative analysis of untreated soils of Murree and Shangla after a different freeze and thaw cycles shows that the strength of soils decreased with an increasing number of cycles. Figure 11 shows the variation of strength with respect to the number of freeze and thaw cycles. The results show that the strength of Murree and Shangla soils was reduced by approximately 82% and 62% respectively, after the 7th cycle with reference to the 1st cycle. However, the results indicate that the decline in the strength of Murree soil with respect to the number of cycles was more than that of Shangla soil. The increased percentage difference in the strength of Murree soil is due to the existence of finer particles which leads to significant volumetric changes compared to Shangla soil which contains sand particles as well thereby subjected to minimum volumetric changes. Hence, the decline in the strength of Shangla soil compared to Murree soil is minimal. A similar decrement in UCS with respect to increasing the cycles was also reported in past works (Meeravali et al. 2020; Xie et al. 2015; Changizi et al. 2022). Xie et al. (2015) studied the properties of soil after a different freeze and thaw cycles. The authors attributed the decrease in strength of samples after different cycles and found that a decrease in strength occurs after different cycles. The results show that 23 to 32% porosity increased after freeze and thaw cycles and cohesion between the soil particles decreased after different cycles. The physical and mechanical properties of soil are affected because of the variation of particle gaps and arrangements in the sample. Meeravali et al. (2020) explored the UCS properties of soils under different freeze thaw cycles. The authors attributed the decrement in strength after various cycles and found that due to cycles, porous space filled with water converted into ice, and volume increased this effect and increased porous in the soil sample which leads to decreased UCS of soil after freeze and thaw cycles.

Fig. 11
figure 11

Comparison of untreated soils after F-T cycles

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Variations of UCS of treated soils

Figure 12 shows the percentage variation of treated soils with respect to a number of cycles. The results show that strength decreases as the number of cycles increases. The comparative analysis of treated soils of Murree and Shangla after a different freeze and thaw cycles shows that the strength of soils reduced with an increasing number of cycles. Figure 12 shows the variation of strength with respect to the number of freeze and thaw cycles. The results show that the strength of Murree and Shangla soils was reduced by approximately 47% and 43% respectively, after the 7th cycle with reference to the 1st cycle. The reduction in UCS loss is due to the addition of terrazyme, which reinforces the soil. This is because terrazyme creates strong bonds/adhesion between soil components and has the ability to enhance soil cohesion by increasing the density of soil samples (Gupta et al. 2017; Yusoff et al. 2017). Therefore, terrazyme plays a significant role in reducing UCS loss in soil with an increasing number of cycles. A similar trend was observed in a previous study in which Dandin et al. (2014) also performed freeze and thaw cycles on treated and untreated soil with terrazyme and observed that UCS varies from 62 kPa for untreated soil to 343 kPa and 402 kPa of treated soil respectively. Furthermore, the authors reported that the percentage loss of treated soils improved by about 20–45% compared to untreated soil (Dandin and Hiremath).

Fig. 12
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Comparison of UCS values for treated soils

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Conclusion

From the above experimental results, it is self-evident that the use of bio-enzyme in soil improved the strength of soils. The following conclusions can be drawn from this study:

  • UCS characteristics declined with the increasing freeze and thaw cycles. The UCS of untreated Murree soil and Shangla soil reduced up to 82% and 62% respectively, after seven cycles. Whereas, the UCS of treated Murree and Shangla soils decreased up to 47% and 43% respectively, after seven cycles. The decrement in UCS of treated soil with respect to untreated soil is due to the presence of terrazyme which reinforce the soil sample.

  • With the addition of terrazyme, OMC of the soil decreased, and the MDD of the soil increased. The MDD of treated Murree soil increased from 18.77 to 18.97 kN/m3, whereas the moisture content decreased from 14 to 11.12% with reference to untreated soil. Furthermore, the dry density of treated Shangla soil improved from 18.29 to 18.98 kN/m3, whereas the moisture content decreased from 13 to 11.81% with reference to untreated soil.

  • The UCS characteristics with the addition of terrazyme for Murree soil improved by 35%, and Shangla soil improved by 19% respectively after seven cycles. The UCS of soils had been improved by the addition of terrazyme, and as the number of cycles increased, the freeze and thaw cycles had less effect on their compressive strength. Soils have been improved by the addition of terrazyme that has a close relationship between the UCS and freeze-thaw cycles after different cycles. When terrazyme was added to soil and soil properties were improved, the effect of this on soil samples resistance to the freeze and thaw cycle was evident, and UCS was less affected after different cycles.

Recommendation

The research study conducted was mainly focused on compressive strength behavior with that of freeze-thaw cycles. It is recommended that the admixture used in this research study was found effective for an increase in compressive strength improvement and should also be analyzed for the improvement of shear strength of soil subjected to freeze-thaw action. It is also recommended that the freeze-thaw susceptible soils are also prone to swelling which is also dangerous for the pavements, especially airports. So along with the strength properties, the swelling potential of the soil can also be checked while using the same admixture. Microstructure analysis including SEM, EDX, and XRD tests should be carried out to explore the effect/performance of terrazyme at nano level.