Experimental Investigation on Reinforcement Application of Newly Permeable Polymers in Dam Engineering with Fine Sand Layers

Abstract

The grinding reinforcement of fine sand layers is a difficult problem in dam engineering construction. As a new type of grouting material, permeable polymer with excellent impermeability and high strength is widely used in dam engineering. In this paper, a series of compressive tests were designed considering different grouting pressures, curing days, moisture content, and porosity of fine sand. The influence of grouting parameters and sand layer conditions on the strength of fine sand layers reinforced by permeable polymers was analyzed. SEM and XDR tests were conducted to analyze the microscopic characteristics of the grouting stone. The functional calculation model of the strength and the influencing factors was established to explore the main factors influencing grouting stones. The compressive strength of grouted stones increases rapidly from the 7th to the 14th day, reaching about 96% of the maximum strength. The degree of influence of different factors is grouting pressure > moisture content > porosity. The compressive strength of the grouted stones increases with the increase of grouting pressure and the number of curing days. The compressive strength decreases with the sand layer’s increasing moisture content and porosity.

1. Introduction

The safety of dam engineering has always been the focus of engineers’ attention. The fine sand layer is a common geological layer in the process of dam construction and development. The fine sand layer performs the characteristics of low bearing capacity, poor stability, and strong permeability. During the construction process and under the influence of seepage, dam collapse, ground settlement, water, and sand bursts are easily caused. Great difficulties are caused by geological disasters in the construction, operation, and maintenance of dams. The grouting method is often used in engineering to control the disease of fine sand [1,2,3]. The reinforcement effect of different grouting materials on fine sand layers has become a hot topic in geotechnical engineering.

The effectiveness of existing grouting materials for dam reinforcement is limited. The impermeability of epoxy resin grouting materials is not satisfactory [4], and the strength of cement-sodium silicate grouting materials is relatively poor [5]. Permeation grouting is an effective geotechnical consolidation technique commonly employed to improve the mechanical behavior of sandy soils. The technique consists of the injection of chemical mixtures, with very low pressure, into the voids of porous natural soils [6,7]. Many scholars conducted the effect of fine sand reinforcement by different materials [8,9]. The mechanical properties of the injected medium before and after grouting were explored, and the long-term assessment, environmental impacts, and strength of grouting stones through experiments were evaluated [10,11,12,13,14]. A functional equation was established by Maghous et al. [15,16] to calculate the reinforcement range of grouting. The combination of on-site experiments and indoor physical model tests is the commonly used method to study the optimal grouting parameters for sandy soil grouting reinforcement [17,18]. Compressive strength is the main criterion for evaluating the reinforcement effect of grouting materials on fine sand. Many scholars have investigated and proposed the methods and mechanisms of fine sand reinforcement. Effective grouting materials for fine sand reinforcement are rarely proposed.

As a new type of grouting material, permeable polymer with excellent impermeability and high strength is widely used in underground engineering [19]. Polymers have shown excellent performance in improving soil liquefaction issues [20,21,22]. Permeable polymer slurry is a water-soluble polymer material. Many scholars have conducted research on the polymer diffusion mechanism and reinforcement effect of sandy soil. Wang et al. [23,24] conducted a series of experiments to investigate the diffusion behavior of permeable polymers in fine sand. Guo et al. [25,26] carried out a series of tests to investigate the diffusion characteristics of polymer grouting materials in sand, gravel, and sand-gravel mixtures. Wang et al. [27] designed a visual constant pressure grouting test device to investigate the polymer’s diffusion and reinforcement mechanisms in silt under different pressures. Permeable polymers have been widely used in the reinforcement of dam and subgrade engineering [28,29]. The strength and impermeability of permeable polymers have been investigated. The information on the use of permeable polymers to improve fine sand layers is limited. The strength performance of fine sand reinforced by permeable polymers needs to be investigated.

The grouting effect is greatly influenced by the layer conditions and grouting parameters. Moisture content and porosity are important characteristics of the fine sand layer. Grouting pressure is the most important grouting parameter. This paper considers grouting pressure, moisture content, porosity of fine sand, curing days, and the effects of different factors on the compressive strength of grouting stone. The research on the strength performance of grouting stones was based on the orthogonal experimental design method. The theoretical calculation model was established between strength, grouting, and sand layer parameters. The research results provide new ideas for reinforcing fine sand layers and theoretical support for treating engineering disasters.

2. Test Content and Scheme

2.1. Test Materials

Permeable polymer is a two-component grouting material. The ratio of A and B can be adjusted to satisfy the requirements of different engineering. The ratio of component A to component B of permeable polymers is 1:1 in the test. The main active ingredients include composite polyether polyols, plasticizers, surfactants, isocyanate, auxiliary catalysts, and other blended materials. The A and B components of the permeable polymers are shown in Figure 1. The reactant is shown in Figure 2.

The physical parameters of the fine sand layer used in the experiment are shown in

Table 1. The physical parameters of the fine sand.

Parameters	Gs	ρdmax (g/cm3)	ρdmin (g/cm3)	emax	emin	Cu	Cc
Fine sand	2.65	1.61	1.41	0.88	0.64	2.77	1.0

. The particle grading of fine sand is shown in Figure 3. The permeable polymer was selected as the grouting material in the experiment. The main components of permeable polymers include composite polyether polyols, plasticizers, surfactants, isocyanates, co-catalysts, and other blend materials.

2.2. Test Equipment

The size of the compressive test block is Φ 50 mm × 100 mm. The grouting equipment is shown in Figure 4. The grouting model is shown in Figure 5. The grouting equipment injects grout into a mold filled with sand and waits for six hours before demolding. WHY-2000 universal testing machine was selected as uniaxial compression testing equipment. The maximum loading force of the testing machine is 100 kN, and the loading speed range is 0.05 mm/min~500 mm/min.

2.3. Test Scheme

An orthogonal experiment was established to investigate the effects of grouting pressure, fine sand’s moisture content, and fine sand’s porosity on the strength of grouting stones. The orthogonal test method is adopted, with grouting pressure (0.1 MPa, 0.15 MPa, 0.2 MPa, 0.25 MPa), moisture content (0.08%, 0.1%, 0.12%, 0.15%), and porosity of fine sand (39.62%, 41.5%, 43.39%, 45.28%) as the control factors for the experiment. In this experiment, the loading rate is 0.5 mm/min. According to the pre-test, the curing period at the peak strength of the grouting stone is twenty-eight days. Each group of experiments is tested six times, and the average of the remaining values after excluding outliers is the experimental result.

3. Interfacial Strength of Polymers and Concrete under Different Conditions

The results of compressive strength tests under different conditions are shown in

Table 2. Results of compressive strength test.

Test Code	Grouting Pressure p (MPa)	Moisture Content (ω/%)	Porosity (n/%)	7 d σ/(MPa)	14 d σ/(MPa)	21 d σ/(MPa)	28 d σ/(MPa)
1	0.1	45.28	8.0	3.21	3.88	3.98	4.04
2	0.2	45.28	8.0	3.35	4.03	4.11	4.18
3	0.3	45.28	8.0	3.59	4.24	4.36	4.41
4	0.1	43.39	8.0	3.32	3.97	4.09	4.13
5	0.2	43.39	8.0	3.47	4.15	4.25	4.32
6	0.3	43.39	8.0	3.72	4.38	4.48	4.53
7	0.1	39.62	8.0	3.47	4.12	4.23	4.29
8	0.2	39.62	8.0	3.62	4.29	4.39	4.46
9	0.3	39.62	8.0	3.84	4.50	4.61	4.66
10	0.1	45.28	12.0	2.97	3.62	3.72	3.78
11	0.2	45.28	12.0	3.12	3.79	3.89	3.93
12	0.3	45.28	12.0	3.27	3.93	4.04	4.08
13	0.1	43.39	12.0	3.11	3.77	3.87	3.92
14	0.2	43.39	12.0	3.22	3.88	3.98	4.04
15	0.3	43.39	12.0	3.39	4.04	4.15	4.20
16	0.1	39.62	12.0	3.22	3.89	3.98	4.03
17	0.2	39.62	12.0	3.30	3.96	4.05	4.11
18	0.3	39.62	12.0	3.47	4.13	4.23	4.29
19	0.1	45.28	15.0	2.77	3.42	3.54	3.59
20	0.2	45.28	15.0	2.91	3.58	3.67	3.73
21	0.3	45.28	15.0	3.06	3.72	3.84	3.89
22	0.1	43.39	15.0	2.86	3.53	3.62	3.68
23	0.2	43.39	15.0	3.03	3.69	3.81	3.86
24	0.3	43.39	15.0	3.23	3.90	4.01	4.06
25	0.1	39.62	15.0	2.98	3.66	3.76	3.81
26	0.2	39.62	15.0	3.12	3.77	3.89	3.95
27	0.3	39.62	15.0	3.30	3.95	4.09	4.13

.

3.1. Failure Forms of Grouting Stones

(1)

The failure mode of grouting stones

The integrity of the grout and permeability uniformity of the grouting slurry was reflected by the failure mode of grouting stones. According to the compression strength test results, the failure mode of the grouting stones was found to be holistic. After the test starts, fine cracks appear in the grouting stone during the initial compression stage. Fine cracks gradually develop into single cracks as compression continues and then run through the entire solid. The above failure modes can be described as three stages: crack occurrence, crack development, and crack penetration failure. The failure mode of the grouting stone is shown in Figure 6.

(2)

Stress–strain curve

It can be seen from Figure 7 that the stress–strain curve of the grouting stone is divided into four stages. 1. Compression stage (A–B): according to the stress–strain curve characteristics of the grouting stone in this stage, as the axial strain increases, the axial stress slowly increases upwards. During the initial stage of uniaxial compression, pores appear inside the grouting stone and gradually compact under pressure. 2. Elastic stage (B–C). At this stage, the stress–strain curve of the grouting stone is almost in a straight state. The fine sand reinforced by permeable polymers undergoes continuous compression deformation. 3. Plastic stage (C–D). With the increase of pressure, small cracks gradually appear in the grouting stone and develop into larger cracks. The grouting stone is about to be destroyed. 4. Destruction stage (D–E). The grouting stone reaches destructive strength, and cracks develop throughout the entire structure. The grouting stone has been destroyed.

3.2. Influence of Grouting Pressures and Curing Days on Compressive Strength

The relationship curve between the compressive strength and the grouting pressure when the moisture content is 8% is shown in Figure 8. It can be seen from Figure 8a–c that the compressive strength of the grouting stones shows an increasing trend with the increase of grouting pressure. Under the same grouting pressure, the compressive strength of the grouting stones shows an increasing trend with the increase of curing days.

It can be seen from Figure 8a that the compressive strength of the grouting stone shows a rapid growth trend from 7 to 14 days, with an average increase in compressive strength reaching about 96% of the 28 days’ strength. There is a slow growth trend from 14 to 28 days. On the 28th day, the compressive strength of the grouting stone basically reached its maximum.

The fine sand reinforced by permeable polymers shows an increasing trend with the increase of grouting pressure. As the pressure increases, the pore structure of the fine sand is filled with slurry, making the fine sand denser before it is fully solidified. Therefore, the compressive strength of the grouting stone is affected by the grouting pressure. As the curing time increases, the bonding effect between permeable polymer slurry and fine sand particles becomes stronger. As the grouting pressure increases, the amount of grouting per unit time increases. The bonding effect no longer increases and remains stable after reaching a certain index.

3.3. Influence of Porosity on Compressive Strength

The relationship curve between the compressive strength and porosity of grouting stone when the grouting pressure is 0.1 MPa and the moisture content is 8.0%, 12.0%, and 15.0% are shown in Figure 9. It can be seen from Figure 7 that the strength of the grouting stones decreases with the increase of the porosity of the fine sand. As the porosity of fine sand increases, the diffusion and viscosity resistance of permeable polymers is smaller during the grouting process, making it easier to penetrate more slurry. With the evacuation of the pore structure, the slurry cannot bond well with the pore framework of fine sand during solidification, forming a dense overall structure. Therefore, with the increase of porosity, the compressive strength of the grouting stone of permeable high polymer solidified fine sand shows a decreasing trend.

3.4. Influence of Moisture Content on Compressive Strength

The relationship curve between the strength and moisture content of grouting stones when the grouting pressure is 0.1 MPa and the porosity is 45.28%, 43.39%, and 39.62% is shown in Figure 8. It can be seen from Figure 10 that the compressive strength shows a decreasing trend with the increase of the moisture content of the fine sand. The same permeable polymer slurry was injected, and as the moisture content of the fine sand increases, more water will react with the permeable polymer slurry. As the moisture content of fine sand increases, injecting the same permeable high polymer slurry will result in more water reacting with permeable high polymer. The participation of water will decrease the strength of the permeable high polymer itself and is not conducive to bonding with fine sand particles. Therefore, as the moisture content of the fine sand increases, the compressive strength of the permeable polymer solidified fine sand grouting stone shows a decreasing trend.

4. Linear Regression Analysis of Compressive Strength of Grouting Stones

4.1. Sensitivity of Strength to Different Factors

After 6 h of the grouting reinforcement test, the corer removes the test blocks from the mold. The test blocks are placed separately in the curing box and cured. The schematic diagram of coring is shown in Figure 11. The schematic diagram of grout maintenance is shown in Figure 12. The results of sample coring prove that the grouting of each group of stones is uniform. After 28 days of curing according to the test plan, the grouting stones were subjected to uniaxial compressive strength testing. The test results are shown in

Table 3. Compressive strength test results.

Test Code	Grouting Pressure (p/MPa)	Moisture Content (ω/%)	Porosity (n/%)	Curing Time (d)	Compressive Strength (σ/MPa)
1	0.1	0.08	39.62	28	3.48
2	0.1	0.1	41.5	28	3.22
3	0.1	0.12	43.39	28	3.04
4	0.1	0.15	45.28	28	2.78
5	0.15	0.08	41.5	28	3.85
6	0.15	0.1	39.62	28	3.72
7	0.15	0.12	45.28	28	3.51
8	0.15	0.15	43.39	28	3.45
9	0.2	0.08	43.39	28	3.91
10	0.2	0.1	45.28	28	3.98
11	0.2	0.12	39.62	28	4.05
12	0.2	0.15	41.5	28	3.88
13	0.25	0.08	45.28	28	4.36
14	0.25	0.1	43.39	28	4.31
15	0.25	0.12	41.5	28	3.98
16	0.25	0.15	39.62	28	3.89

. The range analysis results of the compressive strength of the grouting stones are shown in

Table 4. Range analysis of compressive strength.

	Grouting Pressure (p/MPa)	Porosity (n/%)	Moisture Content (ω/%)
K1	3.13	3.785	3.9
K2	3.6325	3.7325	3.8075
K3	3.955	3.6775	3.645
K4	4.135	3.6575	3.5
R	1.005	0.1275	0.4
Order of influencing factors	Grouting pressure > Moisture content > Porosity

.

It can be seen from Table 4 that the influence of grouting pressure on compressive strength is greater than that of moisture content and porosity, with porosity having the smallest impact compared to both. The effect curve of each factor is shown in Figure 13. The compressive strength of grouting stone shows an increasing trend with the increase of grouting pressure. As the grouting pressure increases, the initial pressure of the slurry increases, making it easier to penetrate the pores of the fine sand. After the slurry solidifies, it binds more tightly to the fine sand, thereby increasing its strength. The compressive strength of the grouting stone shows a decreasing trend with the increase of the moisture content of the fine sand. The permeable polymer slurry will react with more water as the water content increases. The decrease in compressive strength also hinders the bonding effect between permeable polymer slurry and fine sand, resulting in a decrease in strength. The compressive strength of grouting stone shows a decreasing trend with the increase of sand layer porosity. The increase in porosity of the sand layer reduces the viscous resistance of the permeable polymer slurry during the diffusion process. The increase in pore structure prevents the formation of good bonding between the slurry and fine sand particles, leading to a decrease in the overall strength of the grouting stone.

4.2. Establishment and Verification of Theoretical Model

The multiple regression equation was established based on the compressive strength test data of grouting stones in Table 3. The quantitative relationship between the compressive strength of grouting stones and various influencing factors was determined.

The model between the compressive strength of the grouting stones and various factors is:

$$\sigma =a{X}_1{}^{b_1}{X}_2^{c_1}{X}_3^{d_1}$$

(1)

In the formula, a is constant. b₁, c₁, d₁ are regression coefficient values. X₁ is grouting pressure (p). X₂ is porosity (n). X₃ is moisture content (ω).

Fit the experimental data. The multiple linear regression coefficients are shown in

Table 5. Multiple regression coefficients.

Regression Coefficient	a	b1	c1	d1
Value	3.246	0.3021	−0.3216	−0.1792

.

The calculation model for compressive strength of grouting stones is:

$${\sigma }_{28d}=3.246p^{0.3021}{n}^{-0.3216}{\omega }^{-0.1792}$$

(2)

The p-value (0.136) in residual analysis is greater than 0.05. The regression process shows that the fitted model has significant statistical significance at the α level of 0.05. The numerical fitting residual normal probability distribution diagram between the compressive strength of the grouting stones and the grouting parameters is shown in Figure 14. The residual follows normal distribution, and the correctness of Formula (2) is verified.

5. Microscopic Characteristics of Grouting Stones

5.1. Microscopic Morphology of Grouting Stones

The grouting pressure of the specimen is 0.1 MPa, the porosity is 45.28%, the moisture content is 8%, and the curing period is 14 days. Select a small portion of each for microscopic electron microscopy scanning. The SEM electron microscope uses 30 µm, 50 µm, and 100 µm level scanning samples, and the scanning images are shown in Figure 15.

Figure 15 shows the SEM microscopic scanning images of permeable polymers and fine sand grouting stones under immersion conditions. Figure 15a–c shows the scanning images of 30 µm, 100 µm, and 50 µm levels of grouting stone bodies before immersion. The figure shows that the permeable polymer gel binds the fine sand particles together, and the slurry penetrates between the fine sand particles to fill the soil, effectively reducing the porosity. Permeable polymers increase the bonding force between soil particles through chemical bonding and physical filling, changing the porous and loose structure of fine sand and its poor cementation characteristics and improving fine sand’s compactness and compressive strength. After immersion, the bonding force between permeable polymers and fine sand particles decreases, and some sand particles or aggregates undergo dispersion after immersion. Overall, the soil sample still maintains good integrity, and the permeable polymer binder can still maintain its original appearance. The solidified body after soaking has more pores and lower strength compared to the one before soaking. Due to the strong bonding ability and viscoelasticity of permeable polymer binders after immersion, most fine sand particles still exist in agglomeration. The stone body injected with soaked fine sand still has strong compressive strength.

5.2. X-ray Powder Diffractometer

After scanning with an X-ray diffractometer, the XRD diffraction pattern of the permeable polymer grouting stone sample can be automatically generated by computer software fitted and quantitatively analyzed through relevant software. The XRD test data was analyzed using MDI Jade 6.0 software, which can be used to search for wave peaks and determine mineral composition. Qualitative analysis of phases is based on the unique characteristics of diffraction patterns, with each phase having its unique diffraction pattern. The diffraction patterns of the two different phases are not the same, and there will always be some differences from other phase patterns. Samples containing multiple phases simultaneously exhibit superimposed diffraction peaks of each single phase. All phases in the sample can be identified by comparing the chromatogram of the test sample with the “standard card” in the PDF card library.

The experimental results are plotted based on the sample’s diffraction angle and peak intensity, as shown in Figure 16. Different diffraction peaks represent different substances; a larger peak indicates higher substance content. According to the results in Figure 16, there were no new diffraction peaks in the sample before and after the addition of permeable polymers, and the diffraction intensity fluctuated slightly. The main material before and after grouting is still quartz. After injecting permeable polymers into fine sand, the mineral composition of the soil remained unchanged, and no new substances were produced. When considering changes in diffraction peak intensity, it is important to note that two samples are being tested, and their positions are random. Due to the different distribution of substances in fine sand, the intensity of diffraction peaks varies. There is no chemical reaction between fine sand and permeable polymers to produce new compounds.

6. Conclusions

This paper investigated the reinforcement effect of permeable polymers on dam engineering with fine sand. The influence of different factors on the compressive strength of fine sand reinforced by permeable polymers was analyzed by considering grouting pressure, sand layer moisture content, sand layer porosity, and curing days. The function calculation model between the strength of grouting stones and influencing factors was established based on the orthogonal experimental method. The main factors influencing the deterioration pattern of grouting stones were studied. The main conclusions are as follows:

(1)

With the increasing grouting pressure and curing time, the compressive strength of the fine sand reinforced by permeable polymer increases. The pore structure of fine sand is filled with slurry as the grouting pressure increases. The grouting stone becomes denser before it fully solidifies. The compressive strength of grouting stones shows a rapid growth trend from 7 to 14 days, reaching about 96% of the maximum strength;

(2)

With the increasing grouting pressure and curing time, the compressive strength of the fine sand reinforced by permeable polymer decreases. The slurry becomes more permeable as the porosity increases. The poor bonding effect of the pore framework between the slurry and fine sand is caused by the dispersion of the pore structure;

(3)

The compressive strength of fine sand reinforced by permeable polymers is inversely proportional to the moisture content of fine sand. As the moisture content of fine sand increases, more water will react with permeable polymers. The reduction in the strength of the permeable polymer and the weakening of the bonding effect of the fine sand particles are both caused by the participation of excess water;

(4)

The influence of grouting pressure on compressive strength is greater than that of moisture content and porosity. The influence of porosity on compressive strength is minimal;

(5)

Permeable polymers effectively reduce the porosity of fine sand and increase the bonding force between soil particles through chemical bonding and physical filling. No new material was produced after grouting.