Strength Deterioration of Earthen Sites Loess Solidified by Calcined Ginger Nuts under Dry–Wet and Freeze–Thaw Cycles

Abstract

Earthen sites are a kind of constructure with significant historical and cultural value. However, the destruction of earthen sites caused by erosion occurs frequently. The solidification of calcined ginger nuts can improve the strength of the soil so that it can be used to protect the earthen sites. However, the strength degradation of solidified soil by calcined ginger nuts after dry–wet and freeze–thaw cycles is unclear. To reveal the deterioration pattern of solidified soil strength, the effects of its dosage and cycle number on the strength of solidified soil were analyzed through shear strength, dry–wet cycle, and freeze–thaw cycle tests. The results showed that the solidified soil strength decreased first and increased with dosage increase. With the number of dry–wet cycles increasing, the strength of the plain loess decreased rapidly and gradually turned flat. The strength loss of solidified soil was small in the dry–wet process. With freeze–thaw cycle numbers increasing, the strength of the plain loess decreased first and then tended to be flat, the strength of solidified soil decreased first and then increased slightly, and the change in the strength had a clear inflection point. With the increasing dosage, freeze–thaw cycle numbers corresponding to the inflection point were significantly reduced. These results indicate that calcined ginger nuts could enhance the resistance of earthen sites loess to dry–wet and freeze–thaw cycles.

1. Introduction

The earthen site is the oldest architectural form [1]. China’s earthen sites are widely distributed and diverse and can be divided into different types according to their functions, storage environment, and preservation forms [2,3]. Earthen sites contain various historical information about their specific historical period. Earthen sites are the witnesses and carriers of the development history of Chinese civilization and have high historical, cultural, and scientific value [4]. However, some of these earthen sites have suffered from the intervention of various natural and human factors, such as temperature, humidity, wind erosion, solar erosion, and so on, which has resulted in erosion, salt, cracks, and even the collapse of earthen sites [5,6]. Under the combined action of various factors, erosion disease is the most prominent cause [7].

Given the serious bottom erosion of rammed earthen sites, rammed roof reinforcement measures not only conform to the basic principles of cultural relics protection but are also consistent with the original construction method [8]. Therefore, rammed roof reinforcements are widely adopted in the restoration of basal erosion [9]. There are two basic methods for ramming reinforcement in the erosion area, one is to use plain soil to ram directly, and the other is to add a curing agent into the soil before ramming. Furthermore, the ramming materials affect the effect of the reinforcement of the earthen site. Reinforcement material must be non-toxic, with no pollution, and environmentally friendly [3]. The most commonly used curing agent is lime, glutinous rice slurry, or other auxiliary materials. When lime is used for reinforcement, due to the influence of lime’s composition, excessive salt crystals can be introduced. Glutinous rice slurry has good initial cohesion, but the biological macromolecules maintaining cohesion are prone to decay. However, CGN does not have the disadvantages of the above materials; it can enhance the weather-ability and strength of reinforced soil and has good compatibility with the earthen sites, while its color is closer to that of the loess site. A new material, calcined ginger nuts(CGN), is adopted to solidify loess. Li et al. [10] found that ginger nuts produce about 42% calcium oxide, about 28% β-calcium silicate, and about 20% calcium aluminosilicate at 1100°. Zhang et al. [11] found that the solidified mechanism of CGN is a hydration and carbonization reaction. Under the condition of a 2% sodium hydroxide alkaline solution, the density and surface hardness of CGN-(F + S) decreased, and the water permeability and penetration capacity increased. The calcium content of CGN-(F + S) decreased, and the change in the calcium content reflected the product of the hardening reaction to some extent. Chen et al. [12] found that the moisture content of the CGN calculus body decreased with an increase in age, but the water loss rate was slow. The internal components of CGN reacted with soil particles and water, which changed the physical and hydraulic properties of the calculus body. This calculus body has a stable structure, good integrity, and strong rain erosion resistance. CGN is a suitable grouting material for earthen sites. Zhao et al. [13] found that CGN could be used as a reinforcement material for cracks in gravel grottoes. Zhang et al. [14] found that a mixture including CGN grout, fly ash, and quartz sand had predominant durability. Except for earthen sites, Wang and Xiang et al. [15,16] pointed out that CGN could be used to reinforce murals cracks through the grouting method in wet conditions. Peng et al. [17] pointed out that CGN could improve the weatherability of rammed soil and had excellent compatibility between the reinforcement area and the original earthen site. In summary, research on the reinforcement of earthen sites by CGN mainly focused on its composition, mechanism, applicability, the grouting of repair cracks, and mechanical properties of ramming reinforcement. When using CGN-solidified soil as ramming material to reinforce the erosion area, in addition to meeting certain mechanical properties, it was also necessary to ensure its stable existence in complex environments. Therefore, further studies on the durability of CGN-solidified soils are still needed.

The properties of the soil change under the condition of repeated rainfall and air drying, which seriously threatens the stability of the site after reinforcement [18,19,20]. Hence, research needs to be conducted on the property of the soil under the dry–wet cycle. In addition, earthen sites in seasonally frozen regions experience repeated freezing and thawing [21]. The strength of the soil was reduced, which seriously threatens the stability of earthen sites. Therefore, there is a need to study the freeze–thaw cycle test for soil. However, the strength degradation of solidified soil by CGN after the dry–wet and freeze–thaw cycles is unclear. Therefore, the effects of CGN dosage, dry–wet, and freeze–thaw cycles on the property of solidified soil were investigated. This study makes some useful explorations for the durability of CGN-solidified soil. It provides guidance and suggestions for earthen site reinforcement in complex environments.

2. Materials and Methods

2.1. Materials

Due to the non-replicability and non-renewability of earthen sites, the test soil was obtained in the vicinity around the site, and it was low liquid limit clay. The properties of the soil and CGN is shown in

Table 1. Basic physical properties of soil.

Specific Gravity	Plastic Limit (%)	Liquid Limit (%)	Plasticity Index	Optimal Moisture Content (%)	Maximum Dry Density (g/cm3)
2.67	12.28	28.52	16.24	14.10	1.61

and

Table 2. Basic physical properties of CGN.

Specific Gravity	Plastic Limit (%)	Liquid Limit (%)	Plasticity Index
2.65	35.07	42.62	7.55

, respectively. The particle gradation curve of the soil is shown in Figure 1. CGN is formed after the calcination of ginger nuts; its main components are calcium oxide (CaO), β-calcium silicate (β-CS), and calcium aluminosilicate (CAS). CGN is a light-yellow powder. The mechanism of solidified soil by CGN is a hydration and carbonization reaction. Calcium oxide, calcium silicate, and calcium aluminosilicate react with water. Calcium hydroxide is then generated after the reaction of CaO and H₂O, and then carbon dioxide is absorbed to produce calcium carbonate.

2.2. Sample Preparation

According to reference [22], remolded soil was used to prepare the samples. The soils were dried and passed through a 2 mm sieve. The amount of soil and CGN were weighed according to mass fractions of 5%, 10%, 15%, 20%, 25%, 30%, and 35% of CGN. The sample with 0% CGN dosage was a plain loess sample. The optimal moisture content was chosen for all the samples. The sample size was Ø61.8 mm × H20 mm. The samples were made; firstly, the plain soil and the CGN were blended sufficiently, water was added to the optimum water content, and this was sealed in a fresh-keeping bag for 24 h to ensure uniform moisture. Secondly, the mixture was divided into three layers and put into a steel mold for static pressure forming. After the sample was formed, it was allowed to stand and was placed in the laboratory for curing. The curing age was 28 d.

2.3. Test Procedure

2.3.1. Shear Strength

According to reference [22], the shear strength test was conducted. The samples were tested after curing to a certain age: 28 d. The type of shear strength test was rapid shear, and the test equipment was a strain-controlled direct shear instrument. The specific testing process was carried out according to the following steps: (1) The shear box port was aligned, the sample was pushed into the shear box, and the fixing pin was inserted. (2) The hand wheel was rotated, and the front section of the upper box was in contact with the dynamometer. The dynamometer reading was adjusted to zero. (3) Vertical pressure was applied. The vertical pressures were 100 kPa, 200 kPa, 300 kPa, and 400 kPa, respectively. (4) After the vertical pressure was applied, the fixed pin was immediately removed. The shear rate was 0.8 m³/min until the sample was shear damaged. (5) After the shear, the hand wheel was rotated, the vertical pressure was removed, and the sample was taken out.

2.3.2. Dry–Wet Cycle

The dry–wet cycle test included two parts: dehumidification and humidification. According to Reference [23], the upper limit of the moisture content for the humidification of the sample was 20%, and the lower limit for dehumidification was 5%. To ensure the accuracy of the test, the side surface of the sample was wrapped with cling film so that the water only migrated through the upper and lower surfaces. The dry–wet cycle mode was dry first and then wet. The dehumidification process adopted a low-temperature drying method, and the sample was placed in an oven at 30 °C, with its mass measured every two hours until the moisture reached the lower limit and stopped drying. The humidification process adopted the water tank immersion method, and the sample was moved in a constant temperature water tank to absorb the water, and its mass was measured every two hours until its moisture reached the upper limit; then, moisture absorption was stopped. The number of cycles was 0, 3, 6, 9, 12, and 15.

2.3.3. Freeze–Thaw Cycle

According to references in the literature [24] and the range of climate change in the northern seasonal frozen area, the freezing temperature was −20 °C, and the melting temperature was 25 °C. The prepared samples were cured to 28 d, and then a freeze–thaw cycle test was carried out. Firstly, the samples were frozen in a low-temperature chamber for 12 h. Secondly, the samples were thawed for 12 h at 25 °C. The cycle number was the same as the dry–wet cycle.

3. Result and Analysis

3.1. Shear Strength under Different Dosage

Figure 2 shows shear strength under different vertical pressures. Shear strength curves under different vertical pressures are similar, and the shear strength increase with the increase in the vertical pressure. Figure 3 shows shear strength under different dosages. With the increase in the CGN dosage, strength decreased first and then increased, while the boundary point was around 5%. When the dosage was more than 5% and less than 25%, the strength of the solidified soil increased significantly on the basis of the original dosage. However, when the dosage was bigger than 25%, the strength increased slowly. According to the protection criteria for cultural relics and historic sites [25], reinforcement material should have sufficient strength. However, the material strength of the repair area should not be too large to avoid the phenomenon of cracking and fissures caused by the stress difference between the original and repaired region, which could threaten the preservation of the earthen site [26].

In Figure 4, the dosage of CGN ranged from 5% to 35%, the strongest growth rate of the CGN-solidified soil could reach up to 77.35%, and the reinforcement effect of CGN was more obvious at low dosages. The strength growth rate was the ratio of the difference between the strength of the high dosage and the strength of the low dosage to the strength of the low dosage, which was then multiplied by 100%. For example, during the CGN dosage from 5% to 15%, the strongest growth rate of the solidified soil was 41.5%. However, the strong growth rate of the CGN dosage from 25% to 35% is only 7.42%. This was because the porosity of solidified soils was large under a low dosage of CGN. It is easy to be damaged under the action of external forces. However, the reaction products of CGN can effectively fill the pores in the skeleton of solidified soil. The structure of solidified soil is more compact. Furthermore, the shear strength of the solidified soil improved. However, when the dosage of CGN was low, the addition of CGN made the viscous particles of the soil decrease, and the reaction products appeared to be less, so its strength was lower than that of plain loess. When the dosage of CGN was too large, CGN was not fully utilized, and a hardened structure with CGN as the main body was formed in the soil. In addition, the high dosage of CGN reduced the viscous particles in the soil. The cohesion of the solidified soil was reduced, which led to a slow increase in the shear strength of solidified soil.

3.2. Dry–Wet Cycle’s Impact on Shear Strength

The shear strength under the vertical pressure of 400 kPa was analyzed. Figure 5 shows the variation in the shear strength with dry–wet cycle numbers under 400 kPa and different dosages of CGN. The shear strength of plain loess and CGN-solidified soil decreased when the cycle number increased, but the degree of attenuation was different. The strength loss of plain loess was 47.75% in 0 to 9 cycles, 5.17% in 9 to 12 cycles, and 1.82% in 12 to 15 cycles. However, for CGN-solidified soil, the strength loss appeared to be less during the cycle. The dosage of CGN had a significant impact on the strength. Under the same cycle number, the strength loss of the sample decreased as the dosage increased.

The dry–wet cycle caused a certain deterioration in the strength of solidified soil and plain loess. Because under dry–wet cycle conditions, the sample was expanded by moisture absorption and contracted by moisture loss, this caused the deformation of the sample, such as drying shrinkage and wet swelling. After the reaction of CGN, its internal pores were smaller, and the bonding of the intergranular was higher. The wet swelling deformation caused the internal pores of the solidified soil to expand and a break in the bonding between the soil particles. The drying shrinkage deformation further increased the brittleness of the solidified soil. After repeated action, stress concentration occurred inside the solidified soil, which affected the internal structure of the solidified soil and caused the strength of the solidified soil to decrease. However, after experiencing multiple dry–wet cycles, the internal pore of plain loess expanded. Thus, there was a certain space for drying shrinkage and wet swelling deformation. The continuous decrease in strength was alleviated. Therefore, the strength loss of plain loess was large at early cycles and tended to be flat at a later stage. However, for the CGN-solidified soil, the addition of CGN made the pore structure inside the solidified soil more compact, effectively reducing the shedding of soil particles on the sample surface and thereby inhibiting the strength loss of solidified soil during the cycle.

3.3. Freeze–Thaw Cycle’s Impact on Shear Strength

Figure 6 shows a variation in the shear strength with freeze–thaw cycle numbers under 400 kPa and different dosages of CGN. From Figure 5, the strength of plain loess first decreased and then tended to be flat as the freeze–thaw cycle number increased. The strength of the solidified soil first decreased and increased slightly, and there was a clear inflection point. When the dosage of CGN was 25%, 30%, and 35%, the number of freeze–thaw cycles corresponded to the inflection point, which was three. The dosage of CGN was 10%, 15%, and 20%, and the cycle number of the solidified soil to reach the inflection point was six. However, when the dosage of CGN was 5%, and the plain loess reached the inflection point, the cycle number was nine. This indicates that with the dosage of CGN increasing, the freeze–thaw cycle number in line with the inflection point was obviously reduced. The strength of low-dosage CGN-solidified soil did not increase significantly after reaching the inflection point. In contrast, the strength of high-dosage CGN-solidified soil increased significantly. After reaching the inflection point, the CGN could considerably improve the strength of solidified soil.

The freeze–thaw cycle caused a certain degree of deterioration in strength. Because after the sample had been frozen, the porewater inside the sample underwent a phase change; it increased its volume by about 9%, and a large expansion stress was produced. This expansion stress easily produced stress concentration in its interaction with the soil, which could damage the soil structure. When the sample melted at room temperature, the water of the liquid state in the sample migrated to the damaged part of the structure, generating hydrostatic pressure [27]. Further, the internal structure of the soil was damaged, with a continuous increase in the pores of the sample. After repeated freezing and thawing, the structure of the sample changed from initial small damage and gradually evolved into damage throughout the sample. Furthermore, the sample that was produced became deformed under an external force, causing its strength to decrease. Meanwhile, when it was not frozen, according to Fick’s second law [28], it is known that the liquid water content is not consistent on the surface and interior of the soil. In the process of curing, it is easy to produce internal and external moisture differences. In the process of the freeze–thaw cycle, the liquid and solid phases are converted to each other, and the water molecules inside the soil produce endothermic and exothermic phenomena. The temperature difference is generated inside the sample, which accelerates the destruction of the internal structure.

This strength does not decrease continuously during freeze–thaw, and there is a certain threshold. The strength rebounds after exceeding this threshold. After multiple freeze–thaw cycles, pores inside the sample expand to a certain extent. The deformation has a certain space produced by freezing and thawing. The stress concentration between the solid water and soil is also significantly reduced. Therefore, the strong attenuation of the sample was gradually reduced [29]. Moreover, after the reaction of CGN, the pores inside the sample were effectively filled. A new structure was formed by wrapping and filling the internal pores with the reaction products of CGN. The compactness of the solidified soil increased. The flow of liquid water was prevented in the damaged structure so that the strength deterioration of the solidified soil decreased. The reaction products increased, and the strength degradation became less as the dosage increased. References [30,31] indicate that freezing-thawing cycles can cause the reorganization of the internal structure of the sample, and the friction between particles can increase. The reaction products of CGN were more beneficial for the exertion of the frictional effect. As a result, after a certain number of freeze–thaw cycles, the strength increased.

4. Discussion

4.1. Empirical Model of Strength Degradation

To investigate the pattern of shear strength in CGN-solidified soil and plain loess with the cycle number, the dry–wet cycle can be taken as an example. The strength loss rate was used to characterize the deterioration effect, and Formula (1) is its expression.

$${\Delta }_{ss(i)}=\frac{S_0-S_i}{S_0}\times 100\%$$

(1)

where Δss(i) is the strength loss rate of the sample under i dry–wet cycles (%). S₀ is the shear strength of the sample before the dry–wet cycle (kPa). S_i is the strength after i dry–wet cycles (kPa). The scatter plot of strength loss and the cycle number of samples is shown in Figure 7. The strength loss rate and cycle number curves of plain loess and CGN-solidified soil were fitted. The fitting curve is shown in Figure 7. The fitting formulas are shown in Formula (2).

$${\Delta }_{ss}=a{n}^b$$

(2)

where Δ_ss is the strength loss rate of the sample (%), and n is the dry–wet cycle number. Parameters a and b are the fitting parameters, the values of which are related to the dosage of CGN, and

Table 3. Fitting parameters for different dosages of the sample.

Parameter	Dosage of CGN (%)
	0	5	10	15	20	25	30	35
a	8.9893	3.6604	3.2768	3.0350	2.5218	2.0517	2.0185	1.9882
b	0.6104	0.8320	0.8632	0.9393	0.9493	1.0702	1.3231	1.4566
R2	0.9038	0.9719	0.9884	0.9839	0.9887	0.9543	0.9685	0.9529

shows the values of parameters a and b.

From Figure 7 and Table 3, the fitted curves of the samples have high correlation coefficients, which are above 0.9. The relationship curve of Formula (2) can better represent the relationship between the cycle number and the strength loss rate of the sample. The parameter a appears to decrease as the dosage of CGN increases, which also shows that increasing the dosage of CGN can effectively reduce the strength loss rate of the samples. Parameter b increases with the dosage of CGN. Parameters a and b are influenced by the dosage of CGN and have a certain regularity. Therefore, the parameters a and b were fitted with the dosage of CGN, and Figure 8 is a fitting curve. The relationship between parameters a, b, and the dosage of CGN could be expressed by different exponential relationships. The correlation degree is above 0.9, and the fitting effect is better.

From the above fitting relationship, the strength loss rate of the sample is influenced by the cycle number and the CGN dosage. From Figure 8, the dosage of CGN has a good correlation with these two parameters in the curve of strength loss rate and dry–wet cycle number. Therefore, the strength loss rate of the sample is a function of the cycle number (n) and the dosage of CGN (D_CGN), Δss = f (n, D_CGN).

The empirical model of strength deterioration was derived by function superposition under dry–wet cycle conditions. Firstly, test parameters a and b of Formula (2) were expressed as functions of the dosage D_CGN, and Formula (3) was obtained. Secondly, considering the variation in the strength loss rate and the dry–wet cycle number, Formula (3) was substituted into Formula (2) to obtain Formula (4).

$$\{\begin{array}{l}a=6.60757e^{\frac{D_{CGN}}{-3.71173}}+2.32659\\ b=0.66399e^{0.0219D_{CGN}}\end{array}$$

(3)

$${\Delta }_{ss}=a{n}^b=(2.3266+6.6076e^{-0.2694D_{CGN}})n^{0.6640e^{0.0219D_{CGN}}}$$

(4)

where Δ_ss is strength loss rate of the sample under the dry–wet cycle (%), D_CGN is the dosage of CGN (%), and n is the dry–wet cycle number.

4.2. Validation of Empirical Models for Strength Deterioration

This model was established by multiple curve fitting and function superpositions. Therefore, the accuracy of the strength deterioration model was verified. If n and D_CGN are substituted into Formula (4), the calculated value of the strength loss rate can be obtained. The calculated and measured strength loss rates were compared, and the scatter plot of the measured and calculated values is shown in Figure 9. The scattered points are distributed near the y = x. The calculated values for the empirical model of strength deterioration were closer to the measured values, and the accuracy of the model was higher. Therefore, Formula (4) could be used to predict the strength loss rate of solidified soil, and the strength characteristics of the solidified soil after multiple dry–wet cycles were evaluated.

5. Conclusions

In this study, shear strength, dry–wet, and freeze–thaw cycle tests were carried out on the plain loess and CGN-solidified soil, and the strength deterioration of CGN-solidified soil and plain loess was investigated under dry–wet and freeze–thaw cycles, and the conclusion is as follows.

(1)

The strength of solidified soil can obviously be improved when the dosage of CGN is less than 25%. When the CGN dosage exceeds 25%, the strength of the solidified soil increases slowly, and the maximum strength growth rate is about 77.35%. A low CGN dosage has more prominent reinforcement effects on solidified soil.

(2)

Under dry–wet cycles, the strength damage degree of the sample is different. The strength of plain loess decreases and tends to be flat as the number of cycles increases. During the whole cycle, the strength loss rate of plain loess was 54.74%. The strength loss of solidified soil was relatively small during the cycle. Except for the solidified soil with a dosage of 5%, solidified soils are more resistant to dry–wet cycles than plain loess.

(3)

The freeze–thaw cycle caused a certain degree of damage to the strength. The strength of plain loess decreased first and then tended to be flat as the freeze–thaw cycle number increased. During the whole cycle, the strength loss rate of plain loess was 34.41%. However, the strength of solidified soil decreased first and then slightly increased, and there was a clear inflection point. With the dosage of CGN increasing, the freeze–thaw cycle number corresponding to the inflection point of the sample decreased significantly. In addition, CGN could increase the strength of solidified soil after reaching the inflection point.

(4)

The strength loss rate of CGN-solidified soil was greatly affected by the number of dry–wet cycles. The relationship curve of Formula (4) could better represent the relationship between the number of cycles and the strength loss rate of solidified This. At the same time, parameter a decreased with the increase in the CGN dosage. It indicates that increasing the dosage of CGN can effectively reduce the strength loss rate of solidified soil.