Dependence of the wetting rate of the loess base on the moisture conditions

The increasing demand for improved irrigation systems in the world requires a more comprehensive and widespread study of the reliability and safety of hydraulic structures. In this regard, increasing the reliability during the operation of hydraulic structures of irrigation systems, especially on collapsible soils, is very important because loess subsidence soils are widespread globally, a significant part of loess rocks are also found in Central Asia, including the Republic of Uzbekistan. This article discusses the study of the features of the joint work of full-scale irrigation structures with their loess subsidence bases and the intense deformative state of subsidence foundations during their joint work with hydraulic structures on irrigation canals of the Kashkadarya region.


Introduction
The practice of irrigation development of loess lands shows that the disturbances in the operation of hydraulic structures that have occurred so far are due to insufficient knowledge and neglect in designing the features of joint work and interaction of structures with their subsidence bases. The design of hydraulic structures also does not fully take into account these features.
To ensure long-term, reliable, and economical operation of hydraulic structures on collapsible soils, it is necessary to improve their design and construction methods, taking into account the peculiarities of interaction with loess bases.
The experiments were preceded by the development of a methodology, their thorough preparation, the choice of methods for processing the results obtained following the recommendations of N.N. Frolov [3], S.V. Zasov [11].

Methods
In the course of the stamping tests of loess subsidence soils carried out in the zone of irrigation systems, simulating the operation of hydraulic structures, the dependence of the nature of the process of moistening on the conditions of soaking the soil mass was studied.
In the course of the experiments, it was found that if a stamp serving as a model of the GTS flatbed transmits to the ground a pressure greater than the initial subsidence pressure, then the rate of advance of the wetting front into the soil mass and the outlines of the moisture zone are different than at a pressure lower than the initial subsidence pressure. Immediately under the stamp, the wetting rate is, in this case, less than slightly away from it. This can be explained by the deformations occurring in the core under the stamp, which result in a decrease in porosity and a decrease in the filtration capacity of the soil. In addition, the load transmitted by the stamp to the soil affects the horizontal dimensions of the wetted part of the soil massif, which largely determine the nature of the deformation of the subsidence of loess soils.

Results and Discussion
If the area of the pit is large enough, then when the stamp transfers pressure P> P, the filtration flows passing outside the deformable zone on the sides of the stamp, as it were, flow around it over time, which contributes to a more intensive increase in the depth of moisture outside the core. As can be seen from Figure 1, when moistening the bases of round dies with an area of 1 m 2 located in the center of the pit D = 5 m, 24 hours after the start of soaking, the depth of the moistened zone under the center of the punch, which transfers a load of 0.2 MPa to the ground, is 60 cm less than under an unloaded stamp. However, after 6 days of soaking, this difference will stop insignificant. In this case, the front of soil moistening is at a depth of more than 6 meters.
A different picture is observed when the bases of the stamps are soaked through small pits. In such experiments, the most dramatic slowdown in the process of movement of the wetting front into the depth of the massif was observed with an increase in the pressure transmitted by the stamp to the ground. Figure 2 shows the graphs of the dependence of the depth of the moistening zone of subsidence foundations with a round stamp with an area of 1m² transmitting various pressures to the bases when they are moistened through pits with an area of 1, 23 m² on the time of soaking. As can be seen from the figure, the pressure on the depth of the wetted zone becomes more significant with time. The soil cushion formed due to loading, which has a low filtration capacity, causes the accelerated formation of a stabilized suspended contour of the humidification zone.
Thus, the deformed zone in the soaked loess base of the loaded punch has a reduced filtration capacity and affects the nature of the soil moistening process.
Studies have shown that the rate of advance of the front moistened in the loess subsidence foundation is the greater, the greater the volume of water supplied to the pit and the area of the pit through which soaking is carried out, and the less, the larger the area of the foundation and the pressure transferred by this foundation to the ground.
Often, in cases where the calculation of relationships between any parameters causes certain difficulties, researchers also resort to using diagrams, which are one of the simple and visual methods for graphically describing (modeling) the process under study. They make it possible, without complex calculations, by simple geometric constructions to reveal a complex relationship between several parameters.
For any three parameters under consideration, with a constant fourth, a flat diagram can be constructed. Such a diagram does not allow identifying the relationship between the four variables. However, this relationship can be clearly shown in a 3-D chart. So, based on experimental data, it is possible to construct a volumetric diagram for the soaking depth of the subsidence base of the structure, depending on the size of the water source and foundation, the time of soaking, and the magnitude of the pressure transmitted by the foundation to the ground. Figure 3 shows a diagram showing the dependence of the depth of the wetting zone of loess foundations on time at pressure from the foundation on the ground, P = 0.05 MPa for various ratios F K /F f , where F K is the area of the pit through which the base of the stamp is soaked, F f is area of the stamp (foundation) Using this diagram, it is possible, given the ratio F K /F f , for which there is a curve, and the soaking time, to determine the depth of soil wetting.
If the value F K /F f differs from the values for which the experimental data are available, then it can be determined by the interpolation method. So for point b, the parameters were obtained, where t = 144 hours, F K /F f = 15, H 3 = 6 m.
A similar diagram can be constructed for any three of the considered parameters (F K /F f ; t, P, H3) with a constant fourth. A flat plot does not allow establishing a relationship between four variables. This relationship can be clearly shown in a 3-D chart. The presence of several planes in the diagram in Fig. 4 makes it possible to determine any of the four considered quantities not only at the values of P, for which these planes are plotted according to the experimental data but also at its intermediate values, using the interpolation method.
For example, let us determine the depth of the moistened zone of the base of the stamp transmitting pressure to the ground P = 0.12 MPa 72 hours after the start of soaking at F K /F f = 6. To solve this problem, it is necessary to determine the position of the point on an imaginary plane corresponding to P = 0.12 MPa at given values, and F K /F f .
To do this, postponing the values of P and F K /F f along the x and y axes, respectively, we determine the position of point A', which is a secondary projection of the desired point A. To find the point itself, you need to transfer the line of the secondary projection (a space of the diagram taking into account the given time t = 72 hours. In this case, with an error insignificant for practical purposes, the line ab can be considered a segment of a straight line. The segment AA′ expresses on the scale of the drawing the value of the depth of the wetted soil zone H3. In our case, it is obtained equal to 3.1 m.
Suppose the specified value of t does not coincide with the values for which the corresponding curves are drawn within the displayed planes. In that case, it can also be found by interpolation between adjacent curves. The volumetric diagram makes it possible to clearly demonstrate the influence of such factors as the ratio of the areas of the pit and the foundation, as well as the pressure transmitted to the ground by the foundation, on the rate of advance of the front moistened into the depth of the soil mass.
However, the forecast obtained using the proposed diagram will be valid only when the soil conditions of the object under consideration are similar to those of the experimental site. By comparing the diagram, one can make sure that the rate of advance of the moistening front of the soil mass in the area of the 2-X distributor is 10-16% lower than in the area of the 4-X distributor. This indicator is valid for cases with similar experimental conditions, regardless of the size of the pit and the pressure transmitted by the stamp to the ground. The foregoing makes it possible to use the proposed diagrams in predicting the time of soaking of loess collapsible soils with other physical and mechanical characteristics. To do this, it is necessary to find out the ratio of the rates of soaking of the investigated and "reference" massifs by setting up one or several experiments in the area under consideration. A soil massif for which there is already a diagram of a similar type should be taken as a reference.
At the same time, it must be admitted that carrying out stamp tests, even in limited quantities, requires considerable effort and time.
In the course of experimental work in the zone of the distributor 4-X massif "Turkmenistan", we investigated the process of wetting the massif of soil both through the above-mentioned round and through rectangular pits, which simulate canal sections and have dimensions (in meters) of 0.5x0.5 ; 0.5 x 2.5; 0.5 x 5; 1, 1x1, 1; 1, 1x2, 2; 1, 1x5, 5; 1, 1 x 11; 2.0x2.0; 2, x 6, 0; 2.0 x 12.0. The depth of filling the pit with water was 0.4-0.5 m. The obtained data were compared with the results of a study of the process of wetting the foundations of canals built on the same soil area and having a width of 0.5; 12; 2.0 meters.
From the graphs in Fig. 6, constructed according to experimental data, it can be seen that with an increase in the length l of the pit, the rate of advancement of the moistening front into the depth of the soil massif also increases. However, there is no direct proportionality between these values. The rate of advance of the humidification front at В = сonst also tends to its limit if l → ∞. Figure 7 shows graphs characterizing the growth of the depth of the soaked part of the soil massif Н3 with an increase in the area of the pit through which the soaking Н′3/Н3 = f (F2 /F1) for time t = 10 days is given after the start of soaking. Curve 1 was obtained by comparing the values of H3 when the soil was moistened through compact (round or square in plan) pits of different areas, and the second curve was obtained by comparing the results of soaking rectangular pits having the same width B but different lengths l and compact pits with area F = B². In the region Н′3/Н3, covered by the experiments, the positions of the points are described by the expression y = ае bx + c, where the coefficients found by the least squares method, with the natural passage of the curves through the point, are а = 8.65*10; b = 4.79; c = 10 with a correlation coefficient of 0.825 (for the first curve) and a = 2.28*10 -3 ; h = 26.81; c = 0,9 (for the second curve). The standard deviation for these curves is, respectively, ± 0.1 and ± 0.035.   From the graphs shown in Figure 7, it can be seen that an increase in the area of a square pit due to the length of the parallel sides in the plan by a factor of 8, 2 gives the same increase in the rate of wetting the massif of loess soil, as an increase in the area of a compact pit by 2, 2 times, provided that its shape. Using such a comparison, the graph is shown in Figure 8.
Based on the above graph, we can conclude that the rate of advancement of the moistening front in the loess base of a rectangular pit with dimensions B x x is equal to the velocity in the same base of a compact pit with an area where K f is the coefficient of reduction of the pit shape is taken from the graph in Figure 8 and takes into account the change in the length l of the water source (takes into account the equivalence of the soaking rate)

Conclusions
The device used in this work, proposed by the authors of [13] for studying the physical and mechanical properties of soil, creates additional possibilities for studying the process of moistening loess soils.
An experimental-graphical method for determining the rate of soaking of an array of loess foundations of canals and structures on them has been developed, making it possible to determine the optimal sizes of pits intended for preliminary soaking of the foundations of hydraulic structures.
The volumetric diagrams of the loess soil moistening process proposed in work clearly demonstrate that the rate of advancement of the moistening front into the depth of the base of the stamp (foundation) decreases with an increase in the soaking time, the pressure transmitted by the stamp (foundation) to the soil and the area of this stamp, and increases with an increase in the intensity of moistening and the area of the water mirror in the pit through which the soaking is carried out.