Secondary Consolidation Characteristics of Organic Soil Modied by Biological Enzymes Based on Gibson Model

: In order to study the secondary consolidation characteristics of organic soil modified by enzyme, the secondary consolidation tests of organic soil modified by enzyme were carried out. Firstly, the consolidation coefficient C v , secondary consolidation coefficient C a and compression index C c of modified organic soil under different levels of loads and different enzyme contents were obtained according to the analysis of experimental data. Then, the parameters of Gibson rheological model were fitted respectively according to the experimental results. Finally, the relationship between rheological model parameters, C a /C c , secondary consolidation coefficient C a and loading was analyzed under different enzyme contents. The results show that: (1) The rheological model parameters of organic soil modified by enzyme are negatively correlated with the enzyme content, and positively correlated with the load value. When the enzyme content is 0.01%, some parameters reach the maximum value; (2) The secondary consolidation coefficient is related to the load. The secondary consolidation coefficient of samples with different enzyme contents shows a certain rule under all levels of load. With the increasing of load, the final secondary consolidation coefficient C a approaches a stable value; (3) The C a /C c values of the samples of the modified organic soil with different enzyme contents are between 0.042 and 0.1 under different loads. The results show that the model is suitable for describing the secondary consolidation creep characteristics of organic soil, and can be used to guide the long-term deformation prediction of organic soil foundation.


Introduction
Organic soil consists mainly of decaying plant matter and weathered rock matter (Saride et al. 2013). Organic matter in soil (the sum of various animal and plant residues and microorganisms in the soil and the materials produced by their life activities) can enter the interlayer void of soil structure, leading to soil expansion and reducing the shear strength and permeability of soil . The organic matter in soil is mainly composed of C and N organic compounds, including monosaccharides and polysaccharides with single chemical structure and humus substances with complex structure. The main component of organic matter is generally humus, and the main component of humus is humic acid. It has a porous spongy structure and is more hydrophilic and adsorbent than clay minerals. Organic matter is widely distributed in natural soil. If organic matter is divided according to the content of organic matter in soil, soil can be divided into: peat, peat soil, inorganic soil, organic soil. The organic content of organic soil is about 5%-10%.
Many experiments have been carried out to study the effect of organic matter on soil. The content of organic matter in soil can significantly affect the engineering properties of soil (Canakci et al. 2015). The presence of organic matter reduces soil compactness and peak strength values (Malkawi et al. 1999). The physico-chemical characteristics of organic matter (humic acid) are not conducive to the strength development of soil (Guo et al. 2008). There is a critical value for the influence of the mass fraction of organic matter (humic acid) on the unconfined compressive strength and failure strain of soil, and the plasticity of soil increases with the increase of the mass fraction of humic acid (Zhu et al. 2008). According to the theory of heat flow difference, the restraining mechanism of organic matter on soil strength growth is revealed (Harvey et al. 2010).
The influence of 13 kinds of organic matter on the shear strength of soil is analyzed, indicating that the existence of organic matter seriously affects the strength of soil (Tremblay et al. 2002).
Organic soils are known for their poor engineering properties, including very high compressibility and low shear strength (Saride et al. 2013). At the same time, it has the characteristics of low pH value, large porosity, high natural water content, high plasticity and low bearing capacity. Therefore, it is necessary to deal with the foundation and subgrade with higher organic matter content in the project. The traditional treatment methods commonly used are overloaded preloading method, dynamic compaction method, replacement and filling treatment method, drainage consolidation method and so on. Although the above methods have formed corresponding norms and standards, there is a large noise, high cost, low environmental protection, low efficiency and other characteristics.Based on the traditional treatment methods, scholars began to explore the traditional improvers to treat organic soil. In order to improve the poor engineering performance of organic soil, the use of modified calcium-based stabilizers such as lime, cement and fly ash to improve organic soil can effectively improve the strength of organic soil (Saride et al. 2013). Using modified magnesium thioxide cement as an improvers can effectively improve organic soil . The use of composite modifier (C-PG-CAC) to improve organic soil can effectively improve soil strength, and compared with ordinary Portland cement, this composite modifier has better improvement effect (Liu 2017). Many foreign scholars have also found better traditional amendments to improve organic soil. However, the above traditional methods have high CO2 emissions, which are not only environmentally friendly, but also have high engineering cost (Qian et al. 2015). The use of traditional modifiers such as cement and lime leads to higher environmental pollution and greenhouse gas emissions (Ramdas et al. 2020). Therefore, it is urgent to find a new improvement method. Biological methods improve the strength properties of soil by increasing its shear strength and compressibility (Ramdas et al. 2020). Enzymes are effective in improving and maintaining soil strength and hydraulic properties (Pooni et al. 2019). Enzyme is a reasonable way to improve traditional soil stabilization methods (Ramdas et al. 2020). The biological enzyme soil curing agent (hereinafter referred to as enzyme) is a new soil amelioration agent extracted from plant fermentation, which can effectively reduce the swelling and shrinkage of soil. To a certain extent, it can replace the traditional inorganic material based improves and meet the requirements of engineering standards. It has the characteristics of high temperature resistance, non-toxic, noncorrosive and environmental protection Wen and Wang 2018). Scholars have carried out relevant studies on soil improvement by water dilution biological enzymes (Shankar et al. 2009). Biological enzymes can significantly improve the mechanical properties of soil, and the greater the amount of biological enzymes, the stronger the compressive and shear resistance of soil Wen 2018). Two kinds of biological enzymes were used to improve the soil, and the shear strength of the improved soil increased a lot compared with that of the undisturbed soil (Dong 2020). The technology of soil improvement with biological enzymes has been preliminately applied in foreign countries (Shankar et al. 2009). Although the research on soil improvement with biological enzymes in China is ongoing, it is still not mature enough.
Rheology includes: creep, stress relaxation, long-term strength, elastic aftereffect and hysteresis effect (viscous effect) (Sun 2007). Creep is defined as follows: the relationship between soil deformation and time under the condition of constant effective stress. The definitions of primary and secondary consolidation are as follows: primary consolidation refers to soil compression caused by the extrusion of water and gas in pores, while secondary consolidation refers to the creep compression of soil skeleton and the rearrangement of soil particles caused by the discharge of bound water (Hu et al. 2016). The above causes two general problems: the relationship between consolidation and creep; Relationship between secondary consolidation and creep. Some scholars are inclined to the second view, that is, the secondary consolidation is creep , and the authors is inclined to this view. However, some scholars believe that it is difficult to distinguish the deformation amount of creep, the important reason is that it is difficult to distinguish the time threshold of primary and secondary consolidation ). The authors adopted the e-lgt relationship curve proposed by Bjerrum to determine the time threshold of primary and secondary consolidation (Bjerrum 1967).
In recent years, research on rheological constitutive model of soil is still a hot topic at home and abroad, and the construction of rheological constitutive model of soil is developing rapidly (Zhu et al. 2010;Jiang et al. 2008). Using the correlated flow rule and Von Mises yield criterion, a rheological model was established by introducing the damage variable (Yahya et al. 2000). The rheological mechanical properties of soil were studied through rheological tests, and the hehai model was proposed (Xu et al. 2006). However, the traditional rheological constitutive equation of soil is still widely used. Traditional methods include: experience method, element method, etc.
Among them, element method is the most commonly used method for rheological constitutive model construction because of its few parameters, clear physical meaning and simple form. In series, the nonlinear viscoplastic body and the viscoelastic shear rheological model of 5 elements were connected, and the rheological model of 7 elements was established, and the rationality of the model was verified by experiments (Xu and Yang 2005). The viscoelastic-plastic rheological model of four elements is obtained by using the software element and spring element in series and combining the viscoplastic body, and the constitutive equation of the model is described (Song et al. 2012).
The coefficient of secondary consolidation is considered to be a constant in traditional soil mechanics, including the technical code for design and Construction of embankment in soft soil foundation of highway (JTJ017-96). However, with the continuous study of scholars, it is found that this is not the case. With the increase of time, the secondary consolidation coefficient decreases (Bouchard et al. 1985), and there is a correlation between the secondary consolidation coefficient and the consolidation pressure (Nash et al. 1992;Ge et al. 2015;Miao et al. 2007;Zhang et al. 2003).
With the increase of consolidation pressure, the behavior of secondary consolidation coefficient is not the same Zhou and Chen 2006). Therefore, the relationship between secondary consolidation coefficient and consolidation pressure is the focus of future research. In addition, the non-uniform settlement and post-construction settlement deformation of buildings are related to the secondary consolidation characteristics of soil (Castro et al. 1981). Therefore, in summary, the creep test of secondary consolidation of organic soil modified by biological enzymes was carried out in laboratory. The research shows that the one-dimensional secondary consolidation process of organic soil conforms to the rheological model law of Gibson three elements, so the parameters of the rheological model of Gibson three elements are fitted. The rheological model parameters E0 (elastic modulus of Hook body), E1 (Kelvin bulk modulus of elasticity) and η (Kelvin volume viscosity coefficient) are roughly negatively correlated with the enzyme content, and E1 and η reach their maximum value when the enzyme content is 0.01%. E0, E1 and η increase with the increase of load. At the same time, the study shows that the secondary consolidation coefficient is related to the load, and the secondary consolidation coefficient of the samples with different the secondary consolidation coefficient of the samples with different enzyme contents show a certain rule under different levels of load. shows a certain rule under different levels of load. The Ca/Cc value is between 0.042 and 0.1, which is consistent with the conclusion of Mesri (Mesri 1977 The soil tested in this paper is taken from Tianxin District, Changsha city, with a depth of 1-1.5m. The soil is brown-yellow and yellow-brown, and the soil is uniform. The surface of the soil is wet, and the particles are mainly fine particles,Hands can be squeezed into a ball, the pores are small and many, After water loss, the soil grain structure will shrink correspondingly. The main physical and mechanical indexes are shown in Table 1. According to the highway subgrade design code (JTG D30-2015), the soil is determined to be clay. The biological enzyme soil curing agent (hereinafter referred to as enzyme) used in this experiment is Terra-Zyme (Wen and Wang 2018), which is developed by Nature Plus Co., LTD., and imported from The United States through Terra Lucong Technology Co., LTD.. This enzyme is a brown viscous liquid product, non-toxic and pollut-free, soluble in water, and has special smell.
The plant enzyme content is more than 60%. The pH value is between 4.3 and 5.3, and the water stability is good, as shown in Fig. 1.

Preparation of Soil Sample
Because it is difficult to measure the organic matter content in the soil, it is necessary to manually reshape the soil with different percentage of organic matter. Scholars used different methods to configure the soil organic matter, with efficient stabilizer CSCN gelled material, the humic acid (HAP) powder mixed with the original soil preparation of organic clay (Ma et al. 2018), using sodium humate as plus organic matter, reshape the different organic matter content of soil organic matter , or fulvic acid is used to make different organic matter content of soil organic matter (Xu et al. 2007). The authors uses the air-dried plant residues as the additional organic materials, and the ratio of the enzyme content to the dry mass of soil to prepare organic soil samples with the enzyme content of 0.00%, 0.01%, 0.02%, 0.03% and 0.04%, respectively. The sample preparation method is in strict accordance with the highway geotechnical test regulations (JTG E40-2007).

Test Method
The consolidation instrument used in the test is WG single lever consolidation instrument, as shown in Fig. 2. The specimen is 2cm high, with a cross-sectional area of 30cm 2 , and drainage on both sides. In the experiment, the only variable was the enzyme content. All of them adopt hierarchical loading, according to the loading ratio of 1, the primary load is 25kpa, the maximum load is 800kpa, and the loading duration of each level is 7 days. The total duration of this test is 42 days. The standard for consolidation stability of each stage load is that the deformation in the last 24h is no more than 0.005mm, and the next stage load can be added. The test data is recorded by manual reading. (1) Where, H is half of the average value of the initial and final height of the sample under a certain load; t90 is the time required for the consolidation degree of the sample to reach 90%. (1) The consolidation coefficient decreases with the increase of the load under different enzyme contents. When the load is less than 200kPa, the consolidation coefficient changes significantly with the load. When the load is greater than 200kPa, the consolidation coefficient gradually tends to be stable.
(2) Under the same load, the consolidation coefficient decreases first and then increases with the increase of enzyme content.

Coefficient of Secondary Consolidation Ca
The e-lgt relationship curve of one-dimensional secondary consolidation test of organic soil improved by enzyme with different contents is shown in Fig. 4. The e-lgt relationship curve in Fig The method proposed by Buisman to determine the secondary consolidation coefficient from the e-lgt relation curve is as follows: = −∆ /(lg 2 − lg 1 ), has been widely used. From this equation, it can be concluded that the secondary consolidation coefficient is independent of pressure (Shi et al. 2003), that is, after the completion of primary consolidation, the e-lgt relation curve of soil sample deformation tends to a straight line, and the slope of the straight line is the secondary consolidation coefficient. Due to space limitation, the authors took the sample with 0.02% enzyme content as an example, and the calculation results of Secondary consolidation coefficient were shown in Table 2.  Fig. 5. A study of Fig. 5 shows that the secondary consolidation coefficient is related to the load, and the secondary consolidation coefficient of samples with different enzyme contents show a certain rule under all levels of load. At about 100kPa, the secondary consolidation coefficient of samples with different enzyme contents reach the peak; When less than 200kPa, the secondary consolidation coefficient changes obviously with the increase of load. When the load is greater than 200kPa, with the increase of the load, the variation trend tends to be gentle and eventually approaches to a constant. When less than 100kPa, the secondary consolidation coefficient Ca increases with the increase of load. The reason is that the structure of organic soil is damaged by loads, which strengthens the creep of the bound water film, so Ca increases. When the load is greater than 100kPa, the secondary consolidation coefficient Ca decreases with the increase of the load. The reason is that as the organic soil becomes more and more compacted, the cementation capacity of adsorbing water film on the surface of soil skeleton becomes larger, which prevents the secondary consolidation deformation, and the creep capacity of organic soil becomes weaker, so Ca decreases.
With the increasing load, the structure of organic soil is damaged, and the final secondary consolidation coefficient Ca approaches a stable value. This conclusion is consistent with the research results of Zhou and Yu (Zhou and Chen 2006;Yu et al. 2007).   (Mesri 1977). This conclusion has been recognized by most scholars (Rafat and Rowshon 2018;Miao et al. 2007;Zhang et al. 2003). Mesri further pointed out that Ca/Cc=0.04±0.01 for most inorganic clays and Ca/Cc=0.05±0.01 for organic clays (Mesri 1973).
Due to space limitation, the authors took the sample with 0.02% enzyme content as an example. The values of Ca and Cc roughly meet the linear relationship (as shown in Fig. 7), the slope of which is 0.02469 (Deng et al. 2012), and the absolute value of the correlation coefficient R of linear fitting is 0.86825. The relation curve of Ca/Cc and load P was shown in Fig. 8. The Ca/Cc values of samples of organic soil modified by different enzyme contents under different levels of loads are 0.042~0.1, which is consistent with the conclusion of Mesri (Mesri 1977). Therefore, Ca/Cc values can be used to roughly estimate the Ca values of organic soil in some areas of Changsha.
After the pore pressure disappears: In the formula, t0 is the dissipation time of pore pressure, namely the completion time of main consolidation; The elastic modulus E0 of Hook body is obtained from the final settlement SV under a certain load; σ' is the incremental load.
The elastic modulus E1 and viscosity coefficient η of Kelvin can be determined by fitting the curves of strain S(t) and time t under load:

Fitting of Rheological Model Parameters of Organic Soil Modified by Enzyme
The secondary consolidation test of organic soil modified by enzyme found that the main consolidation of samples with different enzyme contents was basically completed after 1380min of different levels of loading. According to Gibson method for data processing, The y-t relationship curves of samples with different enzyme contents under different loads were obtained from the strain time curve. The study found that at the late stage of secondary consolidation, the y-t curve linear correlation is better. Therefore, linear fitting is carried out for the sub-consolidation part of y-t relation curve. according to the Eq. (4) for processing, The fitting results of y-t curve of samples with different enzyme contents are shown in Table 3-7. It can be seen from Table 3-7 that the fitting effect is good under all levels of load loading, and the correlation coefficient is above 0.88.   Fig. 9. According to Fig. 9, the rheological model parameters E0, E1 and η of the three components of organic soil modified by enzymes are roughly negatively correlated with the enzyme content. When the enzyme content is 0.01%, E1 and η reach the maximum, and the parameters E0, E1 and η increase with the increase of load. The relationship between modulus and load P, (b) The relationship between modulus E0 and load P, (c) The relationship between modulus E1 and load P.

Establishment of Rheological Model with Different Enzyme Contents
The empirical relationship between enzyme content and Gibson model parameters was established. The empirical relationship between model parameters and load under different enzyme contents ware shown in Eqs (5) The experimental values of rheological model parameters in Table (3)~(7) and the predicted values of rheological model parameters in Eqs. (5)~(9) are shown in Fig. 10. As can be seen from

Conclusion
(1) The rheological model parameters E0, E1 and η of the three components of organic soil modified by enzymes were negatively correlated with the enzyme content. When the enzyme content was 0.01%, E1 and η reached the maximum, and E0, E1 and η increased with the increase of the load.
( (4) The experimental values are in good agreement with the predicted values, which indicates that the model is suitable for describing the secondary consolidation creep of organic soil and can be used to guide the long-term deformation prediction of organic soil foundation.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.