Study on Composite Improvement of Silt Sites by Lignin and Sodium Methylsilicate and Its Micro Mechanism

Abstract

Kaifeng Zhouqiao site is located in the traffic trunk line in Gulou District of Kaifeng City. The dynamic load of urban traffic is large. In addition, the silt has poor stability of its particle skeleton structure, large porosity and poor mechanical performance. Under the action of dynamic load, the soil of the site will suffer from cracking, collapse, unstable deformation and overall stability damage. In order to enhance the stability of the soil, this paper uses sodium methylsilicate and lignin fiber to modify the site soil, evaluates the mechanical properties of the improved soil through the compression and shear tests, evaluates the durability of the improved soil through the dry wet cycle test, and reveals its modification mechanism through the micro experiment, so that the mechanical properties of the site soil can be improved, so as to achieve the purpose of repair and reinforcement. The experiment shows that the effect of improving the compressive strength of soil is the best when the content of sodium methylsilicate is 0.3%–0.5%, and the effect of improving the shear strength of soil is the best when the content of lignin is 0.5%–2%. The maximum mass loss rate of the composite modified sample after 10 dry and wet cycles is only 0.71%. The comprehensive analysis determines that the best proportion of the composite is 0.5% sodium methylsilicate and 2% lignin fiber. The modified soil has good waterproof and mechanical properties.

1. Introduction

Earthen sites are the remains of human history and culture in a certain environment. They are scientific, historical, artistic and irreplaceable. The Kaifeng Zhouqiao site is mainly composed of silt, which has the characteristics of poor stability of particle skeleton structure, large porosity and poor mechanical properties, and is prone to structural instability under the influence of the external environment. Therefore, it is urgent to study the methods to improve the mechanical properties of silt sites [1,2].

Lignin is a kind of complex natural organic polymer, mainly located between cellulose fibers. It is filled in the cellulose framework to enhance the mechanical strength of plants, and is conducive to resist the invasion of adverse external environment. At present, lignin has been widely used in soil modification tests. Through a series of indoor tests and evaluation of the test results. The results show that the use of lignin can significantly improve the frost thaw resistance of soil and can significantly control the frost heave and unconfined compressive strength of soil [3,4]. Adding a certain proportion of lignin into the soil can reduce the porosity of the clay and effectively improve the dispersion of the soil [5]. Lignin also plays a positive role in the engineering properties of expansive soil [6]. Adding lignin into the red clay can significantly improve the unconfined compressive strength of the red clay, and gradually change the failure mode of the red clay from brittle failure to plastic failure. The addition of lignin changes the compressibility of the red clay, making the structure of the soil more compact and the stability better [7,8,9]. Sodium methylsilicate is also widely used as a modifier. Sodium methylsilicate has a good effect on the strength of concrete. The microstructure of the sample added with sodium methylsilicate is more compact, thus improving the mechanical properties of concrete [10,11]. Sodium methylsilicate has also been used to improve silty sand in the Yellow flood area. Through compaction test, strength test and permeability test, it is believed that it can significantly improve the mechanical and impermeability properties of silty sand in the Yellow flood area [12,13,14]. In conclusion, sodium methylsilicate can significantly improve the cohesion and internal friction angle of soil, and play a hydrophobic role in the dry wet cycle test [15,16,17]; Lignin fiber can form an integral spatial network structure with soil particles, effectively improving the strength of soil [18].

As improvers, they perform well in improving the performance of soil, but they are seldom used in the protection and restoration of soil sites. The purpose of this paper is to improve the performance and repair effect of sodium methylsilicate and lignin composite materials on silt sites. Through unconfined compressive strength test, direct shear test, dry wet cycle test, scanning electron microscope (SEM) test and energy spectrum analysis (EDS) test, the reinforcement performance is evaluated and the micro action mechanism of lignin is revealed, providing a reliable theoretical basis for improving the overall stability of silt sites.

2. Materials and Test Methods

2.1. Materials and Sample Preparation

The materials used in this chapter are taken from the soil excavated during the excavation of Qiaozhou site in Kaifeng, Henan Province. The soil samples are brownish yellow silt. According to the soil test standard (GB/T50123-2019) [19], the physical properties of soil samples were analyzed through indoor tests. See

Table 1. Particle size table of soil samples.

Particle Size Interval Group/mm	Less Than a Certain Particle Size Mass Fraction/%
>2.0	100.00
2.0–1.0	98.58
1.0–0.5	98.13
0.5–0.25	97.66
0.25–0.15	96.25
0.15–0.075	88.57

and

Table 2. Basic physical properties of soil samples.

Soil Type	Density/g·cm−3	Initial Water Content/%	Porosity/%	Liquid Limit/%	Plastic Limit/%	Plasticity Index
Silt	1.7	6.8	33.8	23.4	18.1	8.9

for the analysis results. The debris in the soil shall be removed and sieved, and then a certain amount of samples shall be taken out and dried in the oven for 12 h to prepare the pretreated dry silt.

The soil sample was prepared in the form of single or double mixing of sodium methylsilicate and lignin. During the preparation of soil sample, it was found that when the content of lignin exceeds 2%, the fiber was easy to agglomerate, which seriously affects its uniformity in the soil. In the process of soil sample preparation, it is found that when the content exceeds 2%, the fiber is easy to agglomerate, which seriously affects its uniformity in the soil. Therefore, the content of lignin fiber used in this test is 0%, 0.5%, 1% and 2%, respectively. In order to explore the influence of different composite material content on the strength of soil samples and determine the optimal ratio, 0%, 0.5%, 1% and 2% lignin fibers were added to the modified soil with 0%, 0.1%, 0.3% and 0.5% sodium methylsilicate content. According to the different contents of sodium methylsilicate, the soil samples were divided into four groups A, B, C and D. the specific proportion is shown in

Table 3. Material mix proportion design.

Number	Sodium Methyl Silicate (%)	Lignin Fiber (%)	Number	Sodium Methyl Silicate (%)	Lignin Fiber (%)
A-0	0	0	C-0	0.3	0
A-1		0.5	C-1		0.5
A-2		1	C-2		1
A-3		2	C-3		2
B-0	0.1	0	D-0	0.5	0
B-1		0.5	D-1		0.5
B-2		1	D-2		1
B-3		2	D-3		2

.

According to the Test Code for Protection of Earth Relics (GB/T36747-2018) [20], the dimensions are Φ 39.1 mm × 80.0 mm cylindrical soil sample and Φ 61.8 mm × 20.0 mm round cake standard ring cutter soil sample, of which the former is used for compressive strength test and the latter is used for direct shear test. In order to eliminate the influence of the soil sample preparation process on its strength, the round cake shaped standard ring knife soil sample was uniformly prepared by the static compaction method, with the water content controlled at about 12.3% (optimal water content) and the pressure stabilization lasting for about 1 min The cylindrical sample shall be uniformly compacted by layers, and the water content is the same as above. When preparing the soil sample, the lignin fiber is mixed into the dry soil for 5 times and mixed evenly, and then placed in the sealed bag for 24 h. The completed soil sample is shown in Figure 1.

2.2. Unconfined Compressive Strength Testing

In order to study the relationship between the mechanical properties of composite materials with different proportions, the unconfined compressive strength test of the above soil samples was conducted using a universal material testing machine. The specific test steps are as follows: (1) after 28 days of curing, took soil samples of different proportions, placed the soil samples in the center of the loading platform of the testing machine, and tried to ensure that the loading end loads the soil samples vertically; (2) set the load and displacement of the testing machine to zero, and used a loading rate of 0.1 kn/s for continuous loading; (3) when the image curve drops and the soil sample is observed to be damaged, the test was stopped and recorded the maximum test force; (4) took three soil samples with the same proportion for test, and took the average of the three data as the compressive strength of the soil samples with the same proportion.

2.3. Direct Shear Testing

In order to determine the improvement effect of the modified material on the shear strength of the soil, four soil samples were taken from each ratio in this test for horizontal shear test under the action of unconfined and different vertical pressures P. Through the test, the shear strength under each vertical pressure can be obtained, and the Mohr Coulomb curve can be drawn according to these shear strengths, so as to calculate the cohesion c and internal friction angle $\phi$ of the modified soil with different proportions.

2.4. Dry Wet Cycle Testing

By studying the appearance change and compressive strength degradation law of soil samples under different dry and wet cycles, the performance of soil samples before and after modification to resist dry and wet cycles can be reflected. The test pieces selected for the test include soil sample A-0 and soil sample D-3.

The specific test steps are as follows: (1) Put the soil samples into the dry wet cycle test box and keep a certain distance between the soil samples. (2) Adjust the control panel, set the simulated temperature to 14 °C constant temperature according to the annual average temperature in Kaifeng, and simulate the dry and wet cycle scenario through the change of relative humidity. The relative humidity is, respectively, 80% of the maximum relative humidity and 38% of the minimum relative humidity of the year [21]. (3) Set the number of cycles N = 0, 1, 2, 3, 5, 10, and each cycle time is 12 h. (4) At the end of the cycle, observe the change of the sample and record it, then put the soil sample into the oven for drying for 6 h, and then conducted the compressive strength test of the soil sample after drying and cooling.

2.5. Scanning Electron Microscopy (SEM) Analysis

The change of microstructure in the soil can be observed by SEM test, which can reflect the action mechanism of the modified material, the instrument used is scanning electron microscope (SEM) (quanta 650, FEI, Portland, OR, USA). When observing the micro surface phase of the soil, the selected magnification is 200 times, 500 times, 1000 times and 2000 times, and the selected sample are A-0, D-0 and D-3.

2.6. Energy Dispersive Spectrometer (EDS) Analysis

Energy dispersive spectrometer (EDS, APOLLO-X, Beijing, China) is used in combination with scanning electron microscope to analyze its composition by selecting a specific range in the electron microscope image. According to the characteristics of high element plane accuracy and wide regional distribution, the element plane analysis method is selected for the test samples, the test samples are A-0, D-0 and D-3. The test is used with SEM, and the test object is the same as SEM.

3. Results and Discussion

3.1. Unconfined Compressive Strength Testing Results and Analysis

Through unconfined compression test and data processing, the relationship curve between compressive strength and material content was obtained, as shown in Figure 2. It can be seen from Figure 2a that: (1) When the content of sodium methylsilicate is fixed, the compressive strength of soil sample basically does not change with the content of lignin fiber. (2) With the increase of lignin fiber content, there was a small decrease, in which group a decreased by 6.57%, group B decreased by 4.38%, group C decreased by 1.93%, and group D decreased by 4.10%. (3) This is because the fiber itself is relatively soft and cannot play a role in bearing vertical pressure. In the meanwhile, with the addition of fiber, the workability of soil becomes poor and the compressive strength decreases slightly.

It can be seen from Figure 2b that: (1) When the content of lignin fiber is constant, the compressive strength of soil sample is positively correlated with the content of sodium methyl silicate. (2) The compressive strength increases obviously when the content of sodium methyl silicate is 0%–0.3%, and the curve is steep. When the content is 0.3%–0.5%, the curve rises slowly. Therefore, it is considered that when the content of sodium methyl silicate is 0.3%–0.5%, the effect of improving the compressive strength of soil is the best.

On the whole, sodium methylsilicate can significantly improve the unconfined compressive strength of the site soil, while lignin fiber cannot directly improve the compressive strength of the soil, but its failure mode has changed. After the soil sample is damaged, it is broken but not scattered, which reflects that the fiber provides the compressive strength for the soil. Through the comparison of unconfined compressive strength of modified soil samples, it is concluded that the average compressive strength of group D-1 soil samples is the largest, which is increased by 31.04% compared with that of plain soil samples.

3.2. Direct Shear Testing Results and Analysis

Through direct shear test and data processing, the shear strength fitting curve of each group of soil samples can be obtained, as shown in Figure 3. It can be seen from the figure that: (1) vertical pressure is an important factor affecting the shear strength of soil samples, and the shear strength increases with the increase of vertical pressure; (2) when the vertical pressure is the same, the shear strength increases with the increase of lignin fiber and sodium methylsilicate content.

It can be seen from Figure 3a that, when other factors remain unchanged (vertical pressure 100 KPA, fiber content 0%), the shear strength increases by 6.45%, 10.32% and 13.00%, respectively, when the content of sodium methylsilicate is 0.1%, 0.3% and 0.5%, indicating that the shear strength increases rapidly when the content is 0%–0.3%, and the shear strength increases gently when the content is 0.3%–0.5%. It can be seen from Figure 3b that the shear strength of soil samples increased more obviously when lignin fiber was added. When the vertical pressure and the content of sodium methylsilicate remained unchanged (the vertical pressure was 100 KPA and the content of sodium methylsilicate was 0%), the shear strength of soil samples increased by 2.95%, 14.48% and 26.24%, respectively, when the fiber content was 0.5%, 1% and 2%.

To sum up, the shear strength of D-3 soil sample is the largest. Therefore, when the content of sodium methylsilicate is 0.5% and the content of lignin fiber is 2%, the shear strength of the site soil is the best. Under the same load (taking 100 KPA as an example), it is increased by 48.15% compared with the plain soil sample.

According to the above shear strength fitting curve, the cohesion and internal friction angle of the soil sample can be calculated. According to the Mohr Coulomb theory, the intercept of the fitting curve in Figure 3 is the cohesion, and the slope represents the internal friction angle. The curves of cohesion and internal friction angle with the content of modified materials are drawn according to the obtained data, as shown in Figure 4 and Figure 5.

It can be seen from Figure 4 that: (1) With the increase of fiber content, the cohesion has been significantly improved, while the value of internal friction angle has not changed much. Fan Kewei et al. [22] also obtained similar results when studying the strength improvement effect of fiber materials on soil. (2) The cohesion increases slowly with the change of fiber in the range of 0%–0.5% and 1%–2% but increases significantly in the range of 0.5%–1%. When the content of sodium methylsilicate remains unchanged (taking 0% as an example), the content of lignin fiber in the range of 0.5%, 1% and 2% increases by 10.65%, 38.61% and 53.86%, respectively, compared with that of plain soil. (3) As the lignin fiber itself is short in length and small in diameter, when the fiber content is relatively small (0.5%), the contact area between soil particles and fibers is small, which is shown by the small increase in cohesion. However, with the increase of the fiber content (1%), the fibers aggregate and bond a large number of soil particles, sharing the external load, which is shown by the significant increase in soil cohesion. When the fiber content continues to increase (2%), The water absorption characteristics of fibers will cause some fibers to cluster and cannot be uniformly dispersed into the soil, which will reduce the increase of fiber cohesion. (4) The influence of lignin content on the internal friction angle of soil is not obvious.

It can be seen from Figure 5 that: (1) Sodium methylsilicate improves the cohesion and internal friction angle of soil samples. The 0.5% content of sodium methylsilicate in each group of soil samples improves the cohesion by 8.66%–24.25% and the internal friction angle by 12.30%–16.35%. (2) Sodium methylsilicate can effectively enhance the adhesion between soil particles, so that it can improve the mechanical properties of silt from two aspects of cohesion and internal friction angle, which is also consistent with the existing research results [23,24,25]. (3) The combination of sodium methylsilicate and fiber makes the cohesive force and internal friction angle of modified soil sample D-3 reach the maximum, which are 50.60 kpa and 22.93°, respectively, which is 67.83% and 16.81% higher than that of plain soil. (4) Combined with unconfined compressive strength test, D-3 is selected as the best proportion of composite material. The soil sample has good compressive toughness, maximum cohesion and internal friction angle.

3.3. Dry Wet Cycle Testing Results and Analysis

It can be seen from

Table 4. Record of constant temperature dry wet cycle test of soil sample A-0.

Number of Cycles	Soil Sample Quality (g)	Mass Loss Rat (%)	Apparent Phenomenon
0	93.36	-	-
1	92.81	0.59	The soil sample has no obvious change
2	92.67	0.74	There is a small amount of powdering on the surface
3	91.21	2.30	The phenomenon of pulverization is increased, with a small amount of peeling, and the surface is rough and uneven
5	88.59	5.11	A small amount of peeling occurs at the edges and corners, and the surface starts to dry and crack, and the phenomenon of powdering and peeling is obvious
10	87.37	6.42	The surface is severely peeled, powdered and cracked, and a small number of edges and corners fall off, with a small number of cracks

that the surface change of plain soil sample (A-0) is obvious after dry wet cycle. After the second drying and wetting cycle, the surface of the plain soil samples began to be powdered and peeled. After 5 cycles, the soil samples began to dry and crack, and a small amount of shedding occurred at the corners, and the powdered peeling became more obvious. After 10 cycles, peeling, pulverization, dry cracking, edge and corner shedding are aggravated, and a few cracks appeared.

It can be seen from

Table 5. Record of constant temperature dry wet cycle test of soil sample D-3.

Number of Cycles	Soil Sample Quality (g)	Mass Loss Rat (%)	Apparent Phenomenon
0	92.58	-	-
1	92.25	0.36	The surface of soil sample is smoother
2	92.27	0.33	The soil sample has no obvious change
3	92.21	0.40	The soil sample has no obvious change
5	92.11	0.51	A small amount of powder appears on the surface
10	91.92	0.71	A small amount of powder appears on the surface

that composite material modified soil (D-3) is relatively less affected by dry wet cycle. After one cycle, the surface of the modified soil sample becomes smoother, which is equivalent to that the dry wet cycle removes the attachments on the surface of the soil sample, and more shows the film-forming effect of sodium methyl silicate on the soil surface. The surface changes of soil samples in subsequent cycles are not obvious, only a small amount of pulverization occurs after the 5th cycle, and the pulverization phenomenon is obvious after the 10th cycle. It shows that the soil modified by composite materials can effectively suppress the phenomenon of soil surface powdering, peeling and cracking caused by dry and wet cycles.

It can be seen from Figure 6 that: (1) The mass loss of the plain soil sample (A-0) under the dry wet cycle is obvious. The surface of the soil sample changes little after the first two cycles, the soil falls off less, and the mass loss rate is low (below 1%). (2) When the number of cycles is between 2 and 5, the change range of the mass loss rate increases with the increase of the number of cycles. At this time, the overall stability of the soil is affected. The surface of the soil sample is obviously powdered, and the soil particles fall more. The mass loss rate reaches 5.11% at 5 cycles. (3) When the number of cycles is 5–10, the mass loss rate continues to increase slowly, indicating that the falling speed of soil particles becomes slow. After 10 cycles, the mass loss rate reaches 6.42%.

However, the composite material modified soil sample (D-3) is less affected by the dry wet cycle. After 10 cycles, the maximum mass loss rate is only 0.71%, which is about 89% lower than that of plain soil. It shows that the addition of composite modified materials can effectively reduce the spalling phenomenon of the surface soil under the dry wet cycle.

3.4. SEM Testing Results and Analysis

Figure 7 is the SEM image of sample A-0. It can be seen from the two groups of images in Figure 7a,b that there are many cracks and holes of different sizes in the natural state of the plain soil sample, which significantly reduces the compactness of the soil sample structure. In Figure 7c,d, it can be seen from the two groups of images that the surface of silt particles is rough and the gap between particles is large. The contact mode between soil particles is mainly point contact. The pores in the soil mass are mainly overhead pores with irregular shape, and there is basically no filler inside.

Figure 8 is the SEM image of sample D-0. It can be seen from the two images in Figure 8a,b that the surface of D-0 soil sample is more dense than A-0, the surface of soil particles becomes relatively smooth, the contact mode between particles is mainly surface contact, and the number of macropores decreases significantly. In Figure 8c,d it can be seen that there are many attachments on the surface of soil particles, and the lamellar structures are in parallel layers. The lamellar particles are in close contact in the form of face to face. The addition of sodium methylsilicate makes the surface of soil particles form a film, strengthening the connection between soil particles, which explains why sodium methylsilicate can effectively improve the mechanical properties of soil.

It can be seen from Figure 9 that after adding lignin fiber, lignin fiber and surrounding soil particles are closely connected to form an overall spatial network structure. With the increase of fiber content, the fiber distribution is wider, and the interaction between fibers makes the overall stability of the network structure higher. When the soil is subjected to external force, the stress received by the soil particles will be transmitted to the fiber, and the fiber will transmit this part of the tensile stress to the surrounding structure, forming a three-dimensional stress structure and improving the connection force between the soil particles [26,27].

3.5. EDS Testing Results and Analysis

Due to the high accuracy of element plane and wide regional distribution, the element plane analysis method is selected for the test. Figure 10, Figure 11 and Figure 12 show the selected representative areas.

Table 6. A-0 element content in energy spectrum area of soil sample.

Element Type	C	O	Na	Al	Si	K	Ca	Fe	-
Mass ratio (%)	7.26	57.07	2.66	7.89	20.22	1.17	1.91	1.82	A-0
Atomic ratio (%)	11.17	65.94	2.14	5.41	13.31	0.55	0.88	0.60
Mass ratio (%)	7.33	55.39	3.83	6.42	21.25	0.98	3.76	1.06	D-0
Atomic ratio (%)	11.21	62.25	2.99	4.28	16.79	0.45	1.69	0.60
Mass ratio (%)	22.21	51.94	1.43	5.51	13.57	1.30	1.58	2.46	D-3
Atomic ratio (%)	31.01	54.46	1.04	3.43	8.10	0.56	0.66	0.74

and Figure 13, Figure 14 and Figure 15 are energy spectrum analysis charts of soil samples A-0, D-0 and D-3, respectively.

By comparing Figure 11 and Figure 12 and Table 6, it can be seen that due to the addition of sodium methyl silicate, the content of Na and Si elements in the soil sample increases slightly. This is because the sodium methyl silicate solution infiltrates into the soil, reacts with water and carbon dioxide, decomposes into methylsilicic acid, and rapidly forms a polymethyl siloxane film to cover the soil surface, resulting in the retention of Na and Si elements carried by sodium methyl silicate in the soil. It shows a small increase of these two elements.

It can be seen from Figure 13 that after adding lignin fiber, the content of element C in sample D-3 is significantly increased, because the main elements contained in lignin fiber are C, h and O, so the content of element C is mainly increased.

4. Conclusions

(1)

The content of sodium methylsilicate has a positive effect on improving the compressive strength of soil samples, and can delay the appearance of cracks in the samples; Although the lignin fiber cannot directly improve the compressive strength of the soil sample, it can maintain the integrity of the soil sample when it is damaged and provide a certain toughness for the soil sample.

(2)

Sodium methylsilicate can improve the cohesion and internal friction angle of soil samples, while lignin fiber has little effect on the internal friction angle of soil, but can significantly improve its cohesion. Through comprehensive comparison, it is considered that 0.5% sodium methylsilicate and 2% lignin fiber are the best proportion of modified materials.

(3)

Composite modified materials can effectively improve the durability of soil. Compared with plain soil, the modified soil can maintain stability and integrity after undergoing dry wet cycle and anti-scouring test. In the test of the compressive strength of soil samples with different dry and wet cycles, the strength of plain soil samples is seriously reduced, while the modified soil can still maintain good mechanical properties after many dry and wet cycles.

(4)

The addition of modified materials did not significantly change the composition of soil samples. The compactness of the sample was improved by adding sodium methylsilicate. After the addition of lignin fiber, the fiber is closely connected with the surrounding soil particles, thus forming an integral structure composed of fiber and surrounding soil, strengthening the connection between soil particles, effectively limiting the sliding between soil particles, and jointly improving the overall structural stability of soil samples with sodium methylsilicate.