Physical and mechanical characteristics of fine-grained soils impregnated with acrylate-based hydrostructural resin

. The information is given on the use of impregnation of fine-grained soils with acrylate-based hydrostructural resin for the creation of impervious elements of soil dams both as an independent impervious device and when repairing already existing ones. The results of specially set and performed complex laboratory tests of hydrogel on permeability in sandy loam, very fine, fine, and medium sands are analyzed; tests of obtained soil polymers on uniaxial compression at different impregnation temperatures are analyzed and discussed; single-plane shear and virgin compression; triaxial compression; injection of acrylate-based hydrostructural resin with viscosity from 7 to 16.2 mPa*s into sands of different particle size distribution (13 stands), through submerged injection tools. Some gaps in the study of the characteristics of formed soil polymers for use in hydraulic engineering construction have been revealed: there is insufficient data on their long-term filtration strength, the values of critical head gradients, the stability of characteristics in actual working conditions, and erosion resistance to surface cutting. Recommendations are given on the perspective application of acrylate-based hydrogel in hydraulic engineering construction, both for the creation of main impervious elements in the form of vertical or inclined diaphragms in the dam body, and additional repair of local impervious elements (impervious tampons), for example, in the joint area of adjacent columns of clay-cement-concrete diaphragms performed by the method of bored-section piles, or in the areas of adjoining impervious elements of other structures to concrete or slope piles.


Introduction
In soil dams, one of the most critical elements of the structure is the impervious surface.In the most traditional variant, it is a central core or a sloping screen on the side of the upper slope made of poorly permeable soils (clay, loam) SP 39.13330.2012[1].
In the practice of hydraulic engineering construction, impervious devices made of nonsoil materialswood, metal, concrete/iron concrete, asphalt concrete, plastic, clay-cement concrete, soil polymers, etc., which can be made according to the "wall in the ground" technology and erected by various methods: sheet piling, trench technologies, bored piles, jet grouting and impregnation of the ground with polymer compositions, have also become widespread.These materials are used as screens on the upstream side of the slope and diaphragms along the dam axis.
We have considered in detail one of the types of primer-polymer PFCs based on the method of impregnation of soils with acrylate-based hydrostructural resin (hydrogel).
Due to the lack of a full-fledged experience of its application for the creation of IFD SD considered ways of work, equipment, and some options for its auxiliary application in hydraulic engineering construction.
As an example, the cross-section (Figure 1) shows the construction zone of the impervious flow device (IFD) of a real soil dam with identified problems of progressive loss of filtration strength.In this soil dam, it is assumed to form a wall in the ground by forming mating columns of soil polymer.
To form soil-polymer columns in sands, small-diameter-driven injection tools with loose tips are used in conjunction with injection pumps, hammering, and extraction machines.All equipment is small and mobile.
For the formation of soil-polymer columns in the rocky base, the drilling of boreholes with the diameter from 112 mm to the specified depth with the step of drilling of impregnation boreholes 1050 mm, the scheme of arrangement of which is presented in (Figure 2).The injection line consists of paired 2-component pumps connected to the high-pressure hoses of a 2-component packer.Impregnation of the fractured soil is carried out by upward flow with the interval of installation of the inflating packer in the borehole every 2 m with subsequent lifting (Figures 1 and 2).
Also, the method of impregnating soils with an acrylate-based hydrostructural resin (hydrogel) may be recommended as additional boreholes for bored-secant piles (BSP).This recommendation is due to the fact that prohibitive pile divergence may occur during the construction of CCCD (clay-cement-concrete dams) using the BSP method, which does not provide a reliable interface for split piles.The method of elimination of this defect for today is the accepted method of additional drilling of bored piles in this area; the device of an additional small column of soil polymer will noticeably reduce the cost of defect elimination.
Also, unprojected tensile stresses leading to cracking may occur during the operation of the BSP-performed CCCD.To increase the filtration strength of the BSP interface area in both of these cases, it is recommended to make additional boreholes of 132 mm diameter with the spacing between them equal to the BSP spacing.An injection tool is plunged into the drilled hole, and the hydrogel is injected, subsequently forming a soil-polymer column.Additional wells are located on the upstream side (Figure 3).The layout of the additional wells is shown below (Figure 4)   The given examples of possible use of the method of impregnation of soils with acrylate-based hydrostructural resin (hydrogel) for the formation of impervious elements in soil dams during their construction and repair show broad prospects of its use in hydraulic engineering construction both in the form of independent and in the form of additional IFDs.The available equipment allows the forming of not only vertical but also arbitrarily oriented in-space IFDs.
Prospects for the application of this material are confirmed by the example of technical and economic comparison of specific volumes of some methods of IFD construction in the form of "wall in the ground" made of metal sheeting, trench filled with clay-cement concrete, bored piles filled with clay-cement concrete, jet grouting and impregnation of soil with polymer compositions, the results of which are presented in (Figure 5 and 6).In this comparison, the impregnation method has a clear advantage in terms of speed of execution and total cost of construction.
As can be seen from (Figure 5) the cost of erection of IFD by the variant Pr 1-400 (soil impregnation of column diameter 400 mm) is noticeable, 40-45% less, than the cost of execution of works by the method TR (trench method), and by the method BSP (boredsection piles), slightly inferior at the same time to the methods Sh (sheet pile) and Jg (jet grouting).
According to the data shown in Figure 6, the advantage of using the method of hydrogel impregnation according to the variant Pr 1-400 (impregnation of the ground with the diameter of columns 400 mm) is apparent in terms of the speed of erection of IFD of the soil dam in comparison with the works by the method of BSP (bored-section piles)in 7 times, by the method of TR (trench method)in 3 times, by the method of Jg (jet grouting) in 2 times, and practically comparable with the method of Sh (sheet pile).

Materials and methods
We have carried out a series of special studies to prove the guarantees of the formation of reliable, weakly water-permeable soil polymers by impregnation of soils of different granulometric composition with acrylate-based hydrostructural resin (hydrogel): -laboratory tests of hydrogel permeability in sandy loam, very fine, fine, and medium sands were performed according to GOST 25584-2016 [2]; -laboratory tests of soil polymer based on very fine sand for uniaxial compression at different impregnation temperatures were performed; -laboratory tests of sand and ground polymer at hydrogel viscosity of 7 mPa*s were performed for single-plane shearing and compression compression according to GOST 12248.1-2020[3], GOST 12248.4-2020[4]; -laboratory tests of sand and soil polymer on triaxial compression according to GOST 12248.3-2020[5] were performed; -experimental tests of injection of acrylate-based hydrostructural resin with viscosity from 7 to 16.2 mPa*s into sands of different granulometric composition (13 stands) through submerged injection tools were carried out.

Results
The results of laboratory permeability tests of the hydrogel in sandy loam, very fine sand, fine sand, and medium sand are shown in Figures 7 and 8. Figure 7 appearance of the soil prepared for testingsandy loam Figure 8 shows the studies of the filtration coefficient of 4 varieties of soilssandy loam, very fine sand, fine and medium hydrogel, and water sand.For the tested sands, the hydrogel filtration coefficient produced values ranging from 3.78 m/day to 2.07 m/day.At the same time, both hydrogel and water values for fine sand were the highest -3.78 m/day and 7.66 m/day, respectively, values of filtration coefficient of fine sand were respectively 10-20% lower.In sand, the average difference in filtration coefficients for hydrogel and water differed very slightly, and their values were 2.07 m/day and 2.33 m/day, respectively.The values of filtration coefficients of sandy loam by hydrogel and by water obtained in the experiments practically did not differ and amounted to about 0.1 m/day.-Laboratory test results of very fine sand-based soil polymer on uniaxial compression at different impregnation temperatures are presented in Table 1.The effect of soil temperature during its impregnation with hydrogel in uniaxial compression testing of soil polymer was determined for 2 temperatures: +24.0 °C and +16.2 °C.Five specimens of very fine sand impregnated with hydrogel with a viscosity of 5-7 mPa*s were tested.The strength of the samples impregnated at 16.2 °C, which amounted to 0.375 MPa, was 36.6% higher than the strength of the samples impregnated at 24 °C (0.275 MPa).
-laboratory test results of sand and soil polymer at hydrogel viscosity of 7 mPa*s for single plane shear and compression compression are presented in Table 2. -laboratory test results of sand and soil polymer in triaxial compression are given in Table 3.         -experimental test results of injection on stands of acrylate-based hydrostructural resin with viscosity from 7 to 16.2 mPa*s into sands of different granulometric composition (13 stands) through submerged injection tools are illustrated in (Figure 9) and presented in Table 4.The dependence of hydrogel viscosity and soil particle diameter on its permeability in fine sands was experimentally established.It was found that to form a 400-450 mm diameter column from fine sand, the value of hydrogel viscosity should be within 3-7 mPa*s.At the viscosity of hydrogel with a value higher than 8 mPa*s, the permeability of hydrogel in fine sands does not occur (Table 4).

Discussion
It should be noted that earlier applications of the method of impregnation of soils with acrylate-based hydrostructural resin (hydrogel) for the formation of reliable, weakly waterpermeable soil polymers having a given shape (configuration) for the purpose of creating impervious elements of hydraulic structures was not practically considered.The reason for this was the insufficient study of this method and the lack of experience in impregnating soils using mobile equipment and injection tools with loose tips.
We have analyzed the experience of using this method to create relatively waterresistant elements during the repair of gaskets at various HPPs (Svetogorskaya HPP, Sayano-Shushenskaya HPP, Mainskaya HPP, Nizhne-Bureyskaya HPP, Votkinskaya HPP, Krivoprozhskaya HPP, Ondskaya HPP, Matkozhnenskaya HPP, Verkhne-Svir stop-gate, Ust-Kamenogorsk stop-gate (Kazakhstan), Tishrin (Syria), Iffezheim HPP (Germany) [6] and the experience of creating impervious elements in transportation construction at the construction site of the underground passage between metro stations between Vokzalnaya metro station and Ploshchad Lenina metro station in Minsk [6] and taking into account the nomenclature and fleet of available equipment, machinery, and mechanisms; the conclusion was made about the possibility of construction on an industrial scale of impervious devices on hydraulic structures.
Preliminary evaluations have shown acceptable technical and economic indicators of the walls in the ground formed by this technology and their competitiveness in comparison with the majority of currently used methods of their construction both in terms of cost and construction time.
Taking into account the insufficient study of both the processes of interaction of acrylate-based hydrostructural resin (hydrogel) with various soils and physical and mechanical characteristics resulting from impregnation and further polymerization of soil polymers, we planned and conducted a series of special laboratory studies, which allowed us to establish: -high permeability of hydrogel in sandy loam, very fine, fine, and medium sands, quite commensurate with their permeability by water, so the data of permeability of hydrogel in sandy loam with the value of 0.09 m/day were obtained.
-values of strength of soil polymer based on the impregnation of very fine sand on uniaxial compression at different temperatures were obtained in the range of 0.275-0.375MPa -in the study of single-plane shearing for the soil polymer, the angle of internal friction 45 ○ -47 ○ and adhesion 0.033-0.036MPa exceeding these values for fine sand by 30-45% and 7 times respectively were obtained.
-in triaxial compression tests, the strength value of the soil polymer was obtained as 1.933 MPa at an omnidirectional pressure of 0.3 MPa.
-tests of injection on stands of acrylate-based hydrostructural resin with viscosity from 7 to 16.2 mPa*s into sands of different granulometric composition (13 stands), through submerged injection tools showed that the main criterion of impregnation of fine sands is the viscosity of the hydrogel, it was experimentally established that the confident formation of columns with a diameter of 400 mm occurs at a viscosity of hydrogel for fine sands with a value of 3-7 mPa*s.
Thus, the results of our special studies provide evidence of guarantees of the formation of reliable, slightly water-permeable soil polymers by impregnation of soils of different granulometric composition with acrylate-based hydrostructural resin (hydrogel) and confirm the possibility of the formation of impervious elements of hydraulic structures from this material.
Further research into the performance characteristics of these soil polymers is currently ongoing.
Performed complex studies of physical and mechanical characteristics of acrylatebased hydrogel on permeability in sandy loam, very fine, fine and medium sands; obtained ground polymers on uniaxial compression at different impregnation temperatures; on single-plane shear and virgin compression; on triaxial compression; injection of acrylatebased hydrogel resin with viscosity from 7 to 16.2 mPa*s into sands of different granulometric composition (13 stands) through submerged injection tools showed the suitability of acrylate-based hydrogel to form stable water-retaining structures in soils capable of performing the functions of impervious elements of pressure hydraulic structures (soil dams).
2. Some gaps in the study of the characteristics of the formed soil polymers were revealed: insufficient data on their long-term filtration strength, the values of critical head gradients, the stability of characteristics under real working conditions, and erosion resistance to surface cutting.
3. Recommendations are given on the prospective application of acrylate-based hydrogel in hydraulic engineering construction both for the creation of basic impervious elements in the form of vertical or inclined diaphragms in the dam body and additional repair local impervious elements (impervious tampons) in the joint area of adjacent columns of clay-cement-concrete diaphragms carried out by the method of bored piles, in the area of adjoining impervious elements of other structures to concrete or rock.

Fig. 1 .
Fig. 1.Section of the soil dam with IFD in sandy and rocky soils.

Fig. 5 .
Fig. 5. Comparison of the cost of some methods of erecting IDF SD (TR -trench method, Sh -sheet pile, Jg -jet grouting, PSP -bored-section piles, PR 1-400 impregnation of soil diameter of columns 400 mm, PR1-533 impregnation with a diameter of 533 mm, PR 2-800 impregnation in 2 rows width of the wall 800 mm).

Fig. 6 .
Fig. 6.Comparison of the speed of erection of the IFD SD by different methods.

Fig. 8 .
Fig. 8. Graph of values of filtration coefficients of tested soils by hydrogel and water.

Table 3 .
Laboratory test results of sand and soil polymer in triaxial compression.