Modern approaches to the construction of anti-filtration elements at the base of hydrotechnical construction

. The technologies of the device of anti-filtration elements in ground dams of hydrotechnical construction are considered. Currently, the following problem is relevant, which of the methods of the device of anti-filtration elements is more expedient, profitable and less time-consuming. Promising areas of innovative developments for the construction of underground dams are: the method of jet cementation; the method of injection curtains. A detailed analysis of technologies for the construction of anti-filtration elements at the Sangtudinskaya hydroelectric power station (HPP)-1 is carried out on the example of the jet cementation method. A wide range of non-ground construction materials is used to create anti-filtration elements in ground dams and non-rock foundations of dams. These are stone building materials based on binders, polymer materials, wood and even steel. However, the materials used for anti-filtration devices differ from those used in industrial and civil construction. They may differ in structure, composition, and application technology.


Introduction
Anti-filtration elements play a key role in the construction of underground dams, as they perform the main function of the dam -water retention.They can be different in their design and in the type of material that ensures their water permeability [1,2].Water filtration occurs both in the body and at the base of dams, anti-filtration elements can be located in the dam itself and at its base.
One of the types of anti-filtration elements are anti-filtration curtains.Curtains can be performed by injection methods or by methods such as "wall in the ground".
Injection curtains are implemented in fractured and filtering bedrock.The advantage of this method is that it can be performed at very large depths (several hundred meters).The method used to create injection curtains is to inject pressure and inject a special solution into cracks and holes in the bedrock, which reduces the permeability of the bedrock.Clay cement, cement, bitumen solutions can be injected into cracks.Injection curtains can also be performed in non-rock soils.The first injection curtain in a non-rock base was made at the Serre-Ponson dam (France) in 1954-1956 "figure 1" [3,4].

Fig. 1. Images of Serre-Ponson ground dam, France
The process of injection of non-rock soils is technologically complicated due to the high permeability of large-block soils.Another disadvantage of injection curtains in non-rock soils is a large amount of work and material consumption.The thickness of the injection curtain in large-block soils in accordance with building regulations should be approximately 20 percent of the pressure.
Therefore, in non-rock soils, it is more rational to install anti-filtration walls or "walls in the ground".Their main advantages are that they allow you to reliably cut through the permeable layer, but at the same time do not require a large consumption of sealing material.The thickness of the anti-filtration walls is approximately 0.6 to 1.2 meters.
Recently, a new method of creating anti-filtration walls has been used -the method of jet cementation.In this method, excavation is not performed, it is part of the artificial stone of the anti-filtration wall.The jet cementation method uses high-pressure jets of cement and water to erode the soil mass and create a space for filling a special cement mortar.This method was used in the construction of the curtain wall of the channel part of the Sangtudinskaya HPP-1 dam (Tajikistan) "figure 2" [5], and in the Xiaoloongdi dam (People's Republic of China) "figure 3" -the anti-filtration element of the riding bridge [6].

Methods
Previous work and experience in designing the creation of curtains (walls) in these soils with popular technologies indicated that the technologies used to create injection curtains, chemical compaction using silicatization, or polymer or other solutions caused high material and economic costs [7].Proceeding from the fact that, first of all, it is necessary to implement a multi-row curtain to ensure the appropriate density of the curtain, taking into account the minimum radius of distribution of injection compositions up to 0.5 meters, an impressive amount of drilling work was required at the construction site using specific cuff columns.The use of the provided technology also led to high material and economic costs, as well as the use of bulky and heavy equipment.
In the "Institute Gidroproekt" (Russian Federation), with representatives from the design organization and the management of the construction headquarters, taking into account the design competence and experience in designing anti-filtration elements by foreign states, it was decided to use, under the circumstances, a little-known but promising method (technology) of jet cementation, which was supposed to guarantee a high density anti-filtration curtain (wall) and higher minimization of construction and installation work.However, before the main stage of construction, it was decided to conduct pilot work in order to work out the main little-known components of modern technology and improve the skills of specialists in production conditions.
The basis of the technology of jet cementation of soils in relation to the construction of the Sangtudinskaya HPP-1 was the use of a high-pressure jet of cement mortar to destroy an array of alluvial soils in the riverbed under the base of the dam, in particular under the core, with synchronous mixing of soil with cement mortar, as a result, after hardening of the cement mortar, ground concrete is formed, having higher strength, deformation and necessary filtration characteristics [8].
The logical completion of the construction and installation works carried out in accordance with the design decisions is the creation of an anti-filtration curtain (wall) at the base of the hydraulic structure, the Sangtudinskaya HPP-1 dam by the "jet cementation" method, instead of the existing well-known injection curtain method.Based on the experience of construction and design of foreign companies, it was decided to use a solution based on class B30 cement with the addition of bentonite clay in an amount of up to 5 percent of the weight of cement to produce an impenetrable curtain using jet cementation technology [9,10].
Given the lack of experience in the production of impenetrable curtains by jet cementation, the work was divided into two stages: primary work and pilot work.The above technologies and industrial works are necessary for the production of curtains along the entire width of the Vakhsh riverbed.
Taking into account the need to start work on the construction of an anti-filtration curtain in the riverbed at the beginning of the calendar year and the fact that experimental work was planned to be completed in the middle of the calendar year, it was decided to carry out experimental and industrial work simultaneously.
At the site of the production of experimental work in the riverbed on the axis of the hydraulic structure (pilot site №2, right bank), it turned out that there were serious complications.Based on the fact that cementation is not applicable in aquiferous soils in which the actual flow velocity of the filtration flow in the voids exceeds 2400 (m 3 /day).Under these circumstances, it is necessary to take measures in advance to minimize the flow velocity by means of temporary jumpers, pumping or diverting the filtration flow, using setting accelerators.But, before the construction work on the right bank of the Vakhsh River, these conditions were not met.The horse bridge was an absolutely filtering dam, located in the riverbed, through the body of which significant water flows emanated.In addition, there was a significant collateral flow of water through the cracks of the rocky banks of the riverbed.At the same time, partial engineering and hydrological data included in the project for the production of works characterizing the filtration characteristics of alluvial deposits were not confirmed by field observations made at the experimental site in the riverbed.
The jet cementation method was to be followed by barrier cementation to prevent nonproduction losses of the injection material in areas of high filtration flow, since the filtration path could be carried beyond the anti-filtration curtain formed by the injection solution.
Barrier cementation (reduction of porosity of gravelly soil and elimination of intensive filtration) was proposed to be carried out in two rows of wells.The distance between the rows was 5.0 meters (the rows were located at a distance of 2.5 meters from each other on both sides of the axis of the dam), and the distance between the wells in the rows was 3.0 meters.
To determine the absorption and injection of the cementing (grouting) solution (hydraulic seal) at the experimental site №2, an experimental test was conducted based on the technical and design solutions of experimental works in which heavy cement mortar was fed into the casing at certain time intervals [11,12].Soil-concrete columns were created in six wells at the experimental site №2 "figure 6".
Based on the results obtained when creating concrete columns in alluvial soils, their minimum diameters and the minimum distance between piles for creating an anti-filtration curtain were determined.The minimum diameter of the ground concrete columns was about 1.1 meters.Experiments have shown that in boulder-pebble deposits of the Vakhsh riverbed (50-80 percent of the soil has a particle size of 10-60 (cm)) the diameter of the piles practically does not depend on the speed of the equipment (the lifting speed of the projectile is 10-30 (cm/min)) and basically this fact is also determined by the presence of pores in the soil.The optimal lifting speed was 15-20 (cm/min).
Thus, the device of a two-row curtain made by the method of jet cementation was provided: the width of the curtain is 1.2 meters; the distance between the axes of wells (columns) in a row is 1.0 meters; the distance between the axes of rows of wells (columns) is from 0.5 to 0.7 meters.The evaluation of the strength characteristics of the ground concrete performed at the experimental site №2 after 7 days "figure 7" by the method of non-destructive testing showed that its strength is 10-20 (MPa) [13].After the completion of the experimental work, barrier cementation was carried out near the right bank.Industrial work on the creation of an anti-filtration curtain was carried out by the method of jet cementation in the channel part, in order to accelerate the construction of the curtain, a conclusion was made on the creation of ground concrete piles after drilling lead wells, which were produced at specified points and up to the design mark in a rotational manner with bentonite clay washing, however, at the stage of experimental work, it was assumed that piles of considerable depth (17 meters and more) will be performed by grabs in a descending way [14].At the construction stage, due to the presence of a manmade layer formed during the overlap of the Vakhsh riverbed by an explosion, a significant number of columns (piles) had to be captured within the boundaries of heterogeneous soils, which led to an extension of the construction time of the anti-filtration curtain.
The curtain "figure 8" has a length of 55 meters with an estimated width of 1.2 meters.
The height (depth) of the anti-filtration curtain made by jet cementation in the direction from the sides of the river canyon to its center varies from 7 to 39 meters.The curtain consists of 2 rows of interfacing concrete columns [15].The total number of columns is 106.

Discussions
Describing the process of construction of the curtain, we emphasize that the construction work was carried out in extremely unfavorable engineering-geological and hydrogeological conditions -a plume of oversized and large stone, collapsed into the dam as a result of blocking the riverbed by an explosion in December 2006; dumping of gravel-pebble soils from the surface of the river bottom; the strongest filtration, the speed is 3000 (m 3 /day).It should also be noted that neither the granulation of alluvial deposits in the riverbed, nor the contours of the rocky sides of the canyon in alluvial deposits are known.The complicating processes were the extremely tight deadlines for the production of hydraulic engineering works [16].
A control partial test of the strength of the material of the cemented curtain (wall) produced by the jet method, as well as its hydro-testing, implemented after the completion of construction by the contractor, showed that the strength of the ground concrete is from 10 to 20 (MPa), and in 6 out of 9 wells water absorption was minimized multiple times and almost equals zero values.In the remaining wells, water occurrences had a local character, in particular at the boundary of the deposited boulder-pebble massif with an alluvial base, as well as at the contact of the cliff slope with the ruslov alluvium.In general, the results of hydraulic testing are satisfactory, given the particularly difficult conditions of construction and geological structure of the massif [17][18][19].

Conclusions
Modern technologies make it possible to create high anti-filtration protection of hydraulic structures in any soil due to the use of special materials.During the construction of the Sangtudinskaya HPP-1 on the basis of the technical project of the "Institute Gidroproekt", developed jointly with foreign countries (the Russian Federation, the People's Republic of China, the European Union), carrying out both scientific research and support of industrial work at the base of the Sangtudinskaya HPP-1 dam (in the alluvial deposits of the Vakhsh riverbed), in extremely difficult and complex engineering-geological and hydrogeological conditions, work was carried out on the installation of an anti-filtration curtain by jet cementation to a depth of up to 40 meters.The quality of construction work on the device of the curtain is generally recognized as very satisfactory.

Fig. 4 .
Fig. 4. Experimental site №1 (left bank, bridge of the hydroelectric power station under construction).When performing experimental work on the creation of an anti-filtration curtain by the method of jet cementation, the equipment of the Casagrande group company was used.The equipment is safe and complies with strict European Union safety standards and ISO 9001 quality requirements.The extension mast of the machine HBR-205 GT "figure5" of the company "Casagrande group" was used for drilling operations, the height of which is 19 meters.