HYDROPHYSICAL PROPERTIES OF SOILS, Water permeability of soils - Ground science

HYDROPHYSICAL PROPERTIES OF SOILS

The hydrophysical properties of soils are manifested as a result of the action of hydrodynamic fields on the soils (water resistance, water permeability) or characterize the changes occurring in them, caused by an increase or decrease in the moisture content (shrinkage, swelling).

Water permeability of soils

Water permeability is the ability of water-saturated soils to pass through the water through the pressure gradient (the ratio of the difference between the hydrostatic head of water and the length of the filtration path). Water permeability is associated with one of the most important processes of mass transfer in soils - filtration of water or other liquids), the study of which is of great importance in engineering geology.

The degree of water permeability is quantitatively expressed in filtration coefficients. The filtration coefficient Kf, m/day, is the speed of water filtration in the soil with a pressure gradient equal to unity. The filtration coefficients are used to calculate the precipitation over time, inflows into the production, losses for filtration in canals and reservoirs, and calculations of drainage systems.

Water permeability is also characterized by coefficient of permeability Kf cm. The ratio between the coefficients of filtration and permeability follows from the expression

where is the density of water, g/cm; ц - dynamic viscosity of water, cP.

The more common unit of permeability is Darcy, with 1 d approximately equal to 10 cm ". For water with a kinematic viscosity of v = 0.010 cm/s at 20 ° C, from the formula Kf = gfv -A , we obtain 1 d = 10 980/0.01 cm/s = 10 cm/s = 0.9 m/day, i.e. for water, the permeability coefficient, expressed in darcy, is close to the filtration coefficient expressed in m/day. At the present time, it is used by its dolphin part - a square micrometer, i.e. 10 "m.

The filtration of water through the ground can only be carried out through the communicating voids, in the rocky ground it is through and communicating caverns, cracks and open pores, in dispersed open pores. Any non-rocky soil consisting of solid mineral particles can be considered, depending on the degree of its water saturation, as a two- or three-phase system with effective bulk density (density). equal to the sum of the masses of particles of water and air per unit volume of soil. In practical calculations, the notion of dry soil density is more often used. With known density of particles p s , the porosity and porosity coefficient of the soil can be determined.

The porosity of various soils in natural occurrence usually varies from 0.2 (all-grained alluvial soils) to 0.6 (soft moraine clay). To assess the filtration properties of non-scorching soils, the most important is the characteristic expressing the change in the relative content in the soil of particles as a function of their fineness (characteristic of the weight distribution of particles by fractions). This characteristic is represented in the form of a graph of the grain composition (Figure 5.1), constructed on a semilogarithmic scale, in order to more accurately express the fraction of small particles whose presence in the soil strongly affects its filtration properties.

The heterogeneity of the grain composition of the soil is usually characterized by the coefficient of heterogeneity, or heterogeneity, - C and . This characterization of soils was first proposed by A. Khazen. The grain composition of the soil should be considered heterogeneous if the coefficient and is greater than 10. Hazen also showed that the water permeability of unbound earth in the first approximation depends on the value of this coefficient and the size of y/called the effective particle size. Later, to assess the water permeability of soils II. Sauerbey suggested using the particle size as the correlations between the filtration coefficient and the structural parameters of the soil are more stable (95)

Fig. 5.1. The distribution curves of the granulometric composition of soils [95]

The permeability of granular soils (excluding clay soils) also depends on the shape of their particles. The effect of the shape of the particles is taken into account by introducing into the calculated dependences the coefficient of the shape of the cross sections of the pre-wave channels F:

where kf0 is the soil filtration coefficient with well-rounded particles, in shape approaching the ball; F = 0,45 (1 + 0,3Б), where Б - a score of roundness on the scale А.В. Khabakov, according to which the particles of angular form, characteristic for detrital soil, have a zero point, particles with a uniformly well rounded surface - four points.

As can be seen, the influence of the shape of the particles on the water permeability of the soil is relatively small: the soil filtration coefficients with well-rounded and angular particles (for identical dimensions and packing density) differ approximately by two times.

Because the particles of different dispersity can form a differently shaped framework, porosity or porosity coefficient (the definitions of which are given above) give only an initial understanding of the structure of the soil. If the particles were identical balls, the stable equilibrium structure of the soil consisting of such particles would have a porosity n = 0.395 and is close to the average of its boundary values ​​of 0.26 and 0.48, corresponding to the maximum dense and loose packings of equal size balls. Since the porosity of the granular soil in the volume is equal to the porosity in any of its cross sections, the average diameter of the soil pores is:

(5.1)

where d c - is the average size of the ground particles.

The porosity of the soil decreases noticeably as its grain size increases. The dependence n (ц х ), obtained experimentally, can be represented as a formula

where n and - the porosity of the monofractive soil.

Accordingly, the porosity coefficient is calculated by the formula

Thus, with normal compaction of an uneven-grained ground with particles of average roundness,/≤ m = 0.43 and a = 0.135.

Influence of grain heterogeneity and density on the size of the widths of its pores. Kondratiev proposed to take into account, introducing in the form of a factor in the expression 5.1 the inhomogeneity parameter:

where d n and d1-n are the particle sizes determined from the auxiliary graph (line 2 in Figure 5.1) of the grain composition of the fictitious soil with a lognormal distribution of particles. Presenting the filtration model of granular soil in the form of a bundle of meandering capillaries (Jacen capillaries), it is not difficult to understand that the water permeability of the soil should be determined not by the average diameter, but by the size do of the widths of the pore channels at the sites of their narrowing. Therefore, the size do is called the hydraulically equivalent diameter of the pore channel of the soil, which is about three times smaller than the mean d c . Hydraulically equivalent pore diameter in granular soil it is recommended to determine by the formulas:

For monocrystalline soils (with particles of the same particle size) with the most probable porosity, close to 0.4, the diameter is: do = 0.2d50 [95].

Fig. 5.2. Setting for determining the filtration rate

When justifying the structure of the basic filtration law, it should be noted that, because of the low rates of the filtration flow, the magnitude of the velocity head can be neglected and the main laminar filtration regime can be considered. A linear relationship is established between the flow velocity and the drop in pressure, which was first discovered by Darcy on the basis of filtration experiments in a sand column of constant cross section. Darcy, who investigated the filtration of water through a layer of sand, used in his experiments the setup shown in Fig. 5.2. Through a vertical vessel of constant cross section, filled with sand. water was passed through at a constant pressure difference.

The thickness of the sand layer, the fractional composition and the difference in pressure were different in different experiments. Based on the research Darcy derived dependence:

where Q is the flowrate of the filtration flow with a cross section S with the pressure gradient /; kf is the coefficient of proportionality, called the coefficient of filtration.

At the present time, the following expression of Darcy law:

where v - rate of laminar fluid filtration; kf is the coefficient of laminar filtration.

Coefficients of laminar filtration of granular soils should be determined for uncertified sandy soil using the formula

for coarse-clastic soil). whose pore diameter do is greater than 0.2 cm.

where

So, at the temperature of water filtered in the ground t and . = 4 ° C (respectively, for viscosity v = 0.016 cm/s), the numerical value of the function and at t ". = 20 ° C

(v = 0.010 cm/s). , where do - in centimeters.

The Darcy law has a very wide scope and is rightfully considered the main law of filtration. At the same time, there are conditions under which the Darcy law is violated, and the upper and lower limits of its use take place. The upper limit of the Darcy law application is manifested in high permeability rocks at high filtration rates. Its nature is associated with a significant manifestation of inertial and pulsating forces, which are proportional to the square of the rate of filtration. Proceeding from the principle of independence of the action of viscous friction and pulsational forces, it can be assumed that the most reliable form of the basic filtration law in this case is the two-term dependence of Proni-Forchheimer proposed as a general filtration law and substantiated by a number of theoretical and experimental studies:

(5.2)

The advantage of a two-term dependence is its universality, since it covers the limiting conditions: the onset of a laminar regime at low speeds

filtering, when the term becomes negligibly small compared to and turbulence at very high filtering rates, when one can neglect the linear member in comparison with a square one.

With the transition of the laminar flow to turbulent (which is often observed in coarse-grained soils), the law of resistance changes sharply and becomes quadratic, that is, the pressure drops increase in proportion to the square of the velocity, and therefore, determining the water permeability of the soil in a specific area of ​​the base of the pressure head , it should be checked whether the laminar filtration regime in the given region will persist i under the predicted gradiite. Laminar mode is preserved under the condition:

where is the critical pressure gradient calculated for granular soils by the formula

Accordingly, the critical velocity v K (at the beginning of the filtration deviation from Darcy's law) is determined by the formula:

the critical Reynolds number:

where is the criterial ratio of the force of gravity to the force of the elasticity of water <( = 2.06 · 10 MPa) .

If, according to preliminary estimates, the average speed of water flow in the pores of the ground in the considered base area will exceed 5 cm/s, then the filtration calculations for this region should be performed in accordance with the following:

where k'f is the coefficient of turbulent filtration, determined by the formula

Hydraulic resistance to the steady motion of water in finely dispersed sacred material (clay soil). For cohesive (clay) soils that do not have a macro-aggregate (lumpy) structure

(5.3)

where k'o is a coefficient having a velocity dimension whose average value at a temperature of 20 ° C is 4-10 cm/s; e - the coefficient of porosity (in units of) of the fine-grained component of the soil with particles d < 0.1 cm; (I is an indicator characterizing the intensity of the decrease in soil permeability when it is densified. [95] It is recommended that the value of β be calculated using the formula

where еї - coefficient of soil porosity at the boundary of its fluidity, defined in fractions of unity for a soil test having a moisture content equal to w L .

In the absence of these data, the value of ei is determined from the experimentally established relationship:

(5.4)

where p s is the density of soil particles;/& gt; ", is the density of water.

The dependence (5.3) can be represented in the form of graphs, which are shown in Fig. 5.3. It follows from this dependence that the low-plastic sandy loam soils (e <0.7) have the highest rate of reduction in water permeability during compaction, and the heaviest clays and decomposed peat (e /> 1.5) have the lowest intensity [95].

Fig. 5.3. Graphs of the relationships between the coefficients of porosity and filtration of clay soils [95]

Hydraulic resistance to the steady movement of water in cracks . Based on the gradient-velocity characteristics obtained in studying the process of water filtration in cracks with various openings 6 and the degree of roughness of their walls, it is established that the average velocities of water movement in the cracks:

with laminar mode:

(5.5)

for turbulent:

(5.6)

The effect of the roughness of the walls of cracks on the resistance to movement of water in them is taken into account by introducing the coefficients and in the calculated dependences (5.5) and (5.6).

The most important feature of the process of water filtration in cracks is that when the pressure gradient increases beyond the critical value, the laminar regime comparatively quickly changes to turbulent. The critical pressure gradient and the rate of flow of water in the fracture are calculated [95]:

(5.7)

It follows from (5.7) that when the opening of very thin cracks increases, for which A and B 6, the critical gradient rapidly decreases inversely proportional to the fourth degree of crack opening. Accordingly, the critical Reynolds number [95]

The hydraulic parameters A and B of the roughness of the crack walls with its known opening 6 can be determined from two points on the gradient-velocity characteristic [95] :

in the laminar mode:

in the turbulent regime:

Fig. 5.4. Dependence of the filtration rate on the hydraulic gradient

Of great practical interest is the analysis of the anomalies of the basic filtration law arising at low filtration rates characteristic of weakly permeable rocks. In highly dispersed soils with low permeability (clay, peat), deviation from the linear law is observed, and filtration in such soils begins only when a certain pressure gradient, called initial filter gradients (Figure 5.4).

The nature of these anomalies is associated with the influence of the forces of molecular interaction between particles of water and rock. In the works of IF. Bondarenko and S.V. Nerpine [6] explanation of such anomalies is based on the notion of the viscoplastic character of the water flow in the smiling norch channels. Considering for analysis the regularities of the viscous-plastic filtration regime, the simplest model of a porous medium,

consisting of the same capillary tubes with a radius r r , it can be shown that in this case the viscous flow begins with a pressure gradient < 0 , determined by the formula

where w is the initial resistance to shear in the liquid, which, according to NF. Bondarenko, is of the order of 10 MPa; p " is the density of water; g is the acceleration of gravity.

For /o, the basic filtering law is described by the equation:

For large gradients, when /> gt;/o, this graph has a linear asymptote:

Initial gradients have been studied for a long time, but there is still no consensus about their nature and even the very fact of their existence. The initial gradient is allocated conditionally, the motion of the fluid is not excluded, but it is almost not fixed. After exceeding I0, the filtration rate increases nonlinearly, gradually leaving a straight section. The pressure gradient, at which the filtration rate becomes linear, is called the limiting gradient.

The reason for the existence of the initial filtration gradient in clays is the presence in the pores of bound water with anomalous rheological properties (increased viscosity). And in order to move layers of bound water in thin pores of clay, we must overcome their shear resistance. In the opinion of V.M. Goldberg [13], the influence of the pressure gradient on the permeability of clays is due to the viscoplastic properties of bound water, the heterogeneity of its connection with the hard surface of clay particles. Viscoplastic properties appear in the initial gradient of filtration, and the inhomogeneity of the energy bond is that with the increase in the pressure gradient, most of the bound water is involved in the motion. These two features - the initial gradient and the heterogeneity of the relationship - are interdependent. The values ​​of the initial gradients in the sands are of the order of to = 10, in peats to 15, the Na-forms of montmorillonite clays, the smallest kaolin clays, have the highest initial filtration gradient (up to 60-70). Such values ​​undoubtedly have a real significance, so that under natural conditions the manifestations of viscoplastic flow seem to require careful analysis. It is important to take into account that the viscoplastic flow has a relaxational character, which causes the possibility of flow in the plastic region I & lt;/o, but the effective permeability of the rock will be much smaller here.

All the factors that lead to the destruction of the structure of the layers of bound water around the particles (temperature increase, increase of the concentration of the burrow solution, etc.) influence the reduction of the initial filtration gradient. As the pressure gradient increases, the diameter of the pores increases due to the involvement of an increasing part of the bound water and the reduction in the thickness of the remaining fixed water. Under the influence of the applied pressure difference, the bound water is pressed and releases the hole space. With increasing pressure drop, effective porosity should increase. and in the field of large head - to remain constant. At the same time, there are data [53], which indicate the damping of the head and the manifestation of deformation of the head under the action of the filtration flow. When pressure is applied, the pores are compressed. which leads to a decrease in filtration properties. The magnitude of the destructive gradient is from 11 to 51.

Determination of the filtration coefficient. There are four widely used methods for determining the coefficient of soil permeability (moisture permeability):

• Field test methods, such as determining permeability by pumping, pouring, reservoir testing or flow measurement;

• method of empirical connection with the particle size distribution of particles;

• method of obtaining the estimated indicators in a test using a compression device (odometer);

• method for determining soil permeability on test specimens under laboratory conditions [126].

The choice of the method for determining the filtration coefficient depends on the geological (hydrogeological) conditions of the section of the proposed construction and the tasks assigned to the surveys. So. in conditions close to the groundwater table and the relatively high permeability of soils, the method of experimental evacuation should be applied. At very high permeability of soils, for example, boulder-pebble deposits, methods of injecting or pouring water into wells are used. If the groundwater is at a depth of more than 4 ... 6 m, then the filtration coefficient of the aeration zone is better determined by the method of pouring water into pits. Determination of the soil filtration coefficient for the design of the drainage from excavations should be carried out only by the method of experimental evacuation. A general assessment of the filtration properties of the soils of the site of the proposed construction can be performed for sandy soils by the method of calculation by the granulometric composition or by monitoring the restoration of the water level in the wells. If it is necessary to assess the forecast of flooding of the site, folded with soils, the best results of determining the filtration coefficient can be obtained by conducting experiments in compression-filtration devices. It is possible to optimize the estimation of the permeability coefficient if several of the above methods are used in combination with each other. In Table. 5.1 lists the main laboratory and field methods for determining the filtration coefficient and indicates the limits of their application for various soils.

Table 5.1

Laboratory and field methods for determining the filtration coefficient

Laboratory methods

Field methods

Direct

Indirect

Types of soil

Direct

Types of soil

Device

G.N. Kamensky

-

Sand and clay disturbed and undisturbed addition

Experienced pumping from wells and pits

All species except clays and loams

The device G. Time

-

Sandy disturbed addition

Experienced weights and pits

Sandy and clayey, except for clays with a deep groundwater table (4 ... 6 m)

Handset

G.N. Kamensky

-

Sandy disturbed addition

KFZ device

(Handset

Spetsgeo )

Sand and clay disturbed and undisturbed addition

Experienced water injection into the well

Rocky, semi-craggy, fractured and dry grainy-pebble

The PV device

-

Claying impaired and undisturbed addition

-

11 on the results of granulometric analysis

Sandy homogeneous

Observation of water level restoration in wells

All Views

Based on the results of compression tests

Claying impaired and undisturbed addition

The sandy soil filtration coefficient is determined with a constant preset pressure gradient with a water flow from the top to the bottom or from the bottom up, when the soil sample is pre-saturated with water from the bottom up.

The filtration coefficient of silty and clay soils is determined for given ground pressure and variable head gradient with water flow from top to bottom or from bottom to top, when the soil sample is pre-saturated with water from the bottom up without swelling.

For saturation of soil samples and filtration, groundwater is used from the site of soil selection or drinking-quality water. In cases established by the research program, it is allowed to use distilled water.

The results of the determination of the filtration coefficient should be accompanied by data on the particle size distribution, particle density, dry bulk density, yield and rolling limit, moisture degree and porosity coefficient. The number of particular definitions of the filtration coefficient for each engineering-geological element (soil layer) should be at least six, they are established by the method of statistical processing of the results of partial determinations [19]. The calculated values ​​of the filtration coefficients should be taken equal to the normative ones.

Determination of the sand filtration coefficient with a constant head gradient [35]. The coefficient of filtration is determined on samples of undisturbed addition or disturbed addition of a given density. The maximum particle size of sandy soils should not exceed 1/5 of the internal diameter of the device to determine the filtration coefficient.

The set of equipment for determining the filtration coefficient should include: a device KF-OOM, KFZ (Figure 5.5) or KF-01, PKF-3, FV-3 devices, the SPETSGEO tube is improved.

Fig. 5.5. The device for determining the filtration coefficient of sandy soils (the filtration tube KF-1 is part of the device) [35, 143]

The filling of the cylinder with the tested soil of undisturbed addition is carried out in the following order. The pre-weighed cylinder is put with a pointed edge on the leveled ground surface and slightly pressed into the ground, indicating the boundaries of the future sample for testing; the soil at the sharpened edge of the cylinder (from its outer side) is cut with a sharp knife in the form of a column 0.5 to 1.0 mm in diameter larger than the diameter of the cylinder and about 10 mm in height. At the same time, as the cutting process is progressed, the cylinder is gradually pushed onto the ground, without skewing, until the cylinder is completely filled. In a ground from which it is impossible to cut out a column, the cylinder is pressed; the upper end of the sample of the ground is stripped with a knife level with the edges of the cylinder and covered with a pre-weighed plate; pick up the cylinder with the ground from below with a shovel, turn it over, clean the bottom end of the sample of the ground level with the edges of the cylinder and also cover with a pre-weighed plate; weigh the cylinder with the sample of the soil and the plates covering it; determine the density of the soil, then put on the bottom of the cylinder with a sample of soil with a brass net covered with circles of gauze.

The soil filtration coefficient is determined in the following order: by rotating the lifting screw, a cylinder with soil is set up to coincide with the mark of the necessary pressure gradient on the bar with the upper edge of the housing cover and add water to the housing up to its upper edge. Tests are carried out with a gradual increase in the values ​​of the pressure gradient; measure the temperature of the water; fill the measuring glass cylinder with water and close the hole, tilt it with the hole down, bring it as close as possible to the cylinder with the ground and quickly insert it into the coupling of the filtration tube so that its neck contacts the brass mesh, and small bubbles of air gradually rise in the balloon. If large bubbles of air break through the measuring cylinder, then it must be lowered, producing small bubbles. When the water level reaches the scale of the measuring cylinder 10 (or 20) cm, fix the time, taking it as the beginning of the water filtration. In the future, the time is recorded when the water level reaches, respectively, divisions of 20, 30, 40, 50 (or 20, 40, 60.80) cm or other multiple values. Four samples are produced.

The filtration coefficient? p, m/day, reduced to the conditions of filtration at a temperature of 10 ° C, is calculated by the formula:

where V w is the volume of filtered water at one measurement, cm; t "- the average duration of the filtration (according to the measurements at the same water discharge), s; S - cross-sectional area of ​​the cylinder of the filtration tube, cm;/- pressure gradient; Т = (0,7 + 0,03 7F) - correction for bringing the value of the filtration coefficient to the water filtration conditions at a temperature of 10 ° С, where 7F is the actual water temperature at the test, ° С; 864 - conversion factor from cm/s in m/d.

To calculate the filtration factor, a calculation table should be compiled for a constant flow of water from the cylinder of a certain cross-sectional area under different pressure gradients and temperature. The filtration factor is calculated to the second significant digit.

Determination of the clay soil filtration coefficient with variable pressure gradient [35]. The technique of laboratory tests for the water permeability of cohesive soils has a number of peculiarities caused by:

• very low filtration coefficients, the values ​​of which usually do not exceed 10 cm/s and rapidly (exponentially) decrease in the process of soil compaction;

• the ability of cohesive soils to acquire an aggregate (lumpy) structure that significantly affects the water permeability and the nature of the changes in the compaction of the soil;

• The ability of many clay soils to swell when soaked or, on the contrary, to become denser (which is characteristic of the so-called subsidence loess-like loams);

• relatively large potentials for capillary absorption of water;

• the dependence of soil compaction (at a given compaction energy) on its initial humidity.

Compaction of cohesive soil proceeds in two stages: first, the destruction of individual lumps and the disappearance of the largest secondary pores as a result, the second - the decrease in the volume of primary porosity. If the initial soil moisture is less than optimal, the compaction energy is expended to destroy the clods.

The set of equipment for determining the coefficient of filtration of clayey soils should include: a compression-filtration device (Figure 5.6), which allows testing under load with a variable pressure gradient; Libra; thermometer; stopwatch.

Fig. 5.6. Device for determining the coefficient of filtration of clay soils Znamensky - V.I. Hauston [76]

A sample of soil is saturated with water from the bottom upwards through a piezometer. Water should be produced for at least 2 days for sandy loam, at least 5 days for loam; the duration of water saturation of clays is set by the task. With a moisture content of more than 0.98, water saturation can be avoided. Fill the soil sample with water (to the edges of the nozzle or top of the cover), and transfer the preset pressure to the sample in steps. The values ​​of the pressure stages and the time of their exposure are prescribed in accordance with GOST 12248 [17]. If the specified pressure is p s " (corresponding to the structural strength), the sample is loaded with pressure steps of 0.0025 MPa before compression begins.

In instruments having two piezometers connected to a lid and a pallet, the initial head is equal to the level difference in the piezometers. When examining downstream filtering, the piezometer attached to the top of the instrument must be filled to the top mark, and the piezometer attached to the bottom part to the bottom mark, and vice versa. Open it! a tap connecting the piezometer to the instrument, and mark the start time of the water filtration. Measure the number of divisions on which the water level in the piezometer has risen (risen), at identical intervals of time and the water temperature with an accuracy of 0.5 ° C. The piezometer readings are made depending on the filtration rate. Sample time intervals can be 5, 10, 15, 30 minutes, 1 hour, with slow filtering - twice, at the beginning and at the end of the working day. Produce at least six samples. If the water level in the piezometer is reduced by one division in a time exceeding 40 s, then the piezometers must be replaced by thinner tubes.

The soil filtration coefficient (? p), m/day, reduced to filtration conditions at a temperature of 10 ° C, is calculated by the formula

where S is the observed drop in the water level in the piezometer, measured from the original level, cm; But - initial head, cm; - dimensionless coefficient, determined depending on ;/- time of falling water level, s; S n is the cross-sectional area of ​​the piezometer, cm; S K is the area of ​​the ring, cm; h is the height of the sample of the ground, equal to the height of the ring, cm; Т = (0,7 + 0,03 7F) - correction for bringing the value of the filtration coefficient to the water filtration conditions at a temperature of 10 ° С, where ГF is the actual water temperature at the experiment, ° С; 864 - conversion factor from cm/s in m/d.

The filtration factor is calculated for each sample by the piezometer, the result is the arithmetic mean of the individual calculated values ​​and is expressed to within a second significant digit [35].

The structural and texture characteristics of the soil are most significantly affected by the structural and textural features of the soil: the granulometric composition, its homogeneity (heterogeneity), shape, tortuosity, pore size and channels, crack opening width, etc. Depending on these factors, the filtration coefficient of various soils varies in wide limits. In Table. 5.2 provides the approximate values ​​of the filtration coefficients of dispersed soils. The classification of rock soils according to the degree of water permeability is given in Table. 5.3 [84].

Table 5.2

Values ​​of filtration coefficients of dispersed soils

Primer

Coefficient of filtration to $, m/day

Pebble

200

Gravel

100 ... 200

Coarse-grained with sandy aggregate

100 ... 150

Gravel sand

50 ... 100

Coarse sand

25 ... 75

The average size of sand

10 ... 25

Fine sand

2 ... 10

Dusty sand

0.1 ... 2

Sandy loam

0.1 ... 0.7

Loam

0.005 ... 0.4

Clay

0.005

Peat

weakly decomposed

1 ... 4

medium degraded

0.15 ... 1.0

strongly decomposed

0.01 ... 0.15

Table 5.3

Classification of rock soils by water permeability

Characteristics of rockfalls

Coefficient of filtration, m/day

Specific water absorption. l/min

Practically waterproof, non-perishable

& lt; 0.01

<0.005

Very slightly water-permeable and slightly cracked

0.01 ... 0.1

0.005 ... 0.05

Weak waterproofness and weakly cracks

0.1 ... 10

0.05 ... 5.0

Water permeable and weakly fractured

10 ... 30

5 ... 15

Highly water-permeable, strongly-fractured

30 ... 100

15 ... 50

Very strongly water-permeable and highly fractured

& gt; 100

& gt; 50

The mineral composition affects the value of the soil filtration coefficient through the associated dispersion and porosity. In dispersed soils, including sands, an admixture of clay minerals leads to a decrease in the filtration coefficient. Adding only 10% of clay particles to sand reduces water permeability by more than 50 ... 60%.

The degree of saturation of some soil types can affect the filtration coefficient up to three orders of magnitude. The chemical composition of the penetrant can change the filtration coefficient by several orders of magnitude.

Among external factors, the most significant effect on water permeability of soils is caused by temperature. Research LI. Kulchitsky, I.A. Briling, V.A. The queen and others showed that with an increase in the positive temperature in the range 0 ... 90 ° C there is a significant increase in the filtration coefficient in clay soils. The reasons for this are: a decrease in the viscosity of the pore liquid upon heating, including layers of bound water with increased viscosity, which leads to a drop in A'ph; partial destruction of the structure of layers of bound water around particles with anomalous rheological properties due to heating, transferring part of the water from bound to free; increase due to heating of the degree of aggregation of particles and an increase in the size of inter-aggregate pores, which leads to an increase in kf. The largest structural changes in bound water occur in the temperature range 65 ... 75 ° C, which is reflected in a sharper growth of the clay soil filtration coefficient at this temperature [50].

Numerous experimental studies show that with long-term water filtration, soil kp remains constant only in the case of its rigid structure, stable in time, which is characteristic for rocky soils. In most dispersed soils, studies have shown that as a result of prolonged filtration, their structure and texture are disturbed due to salt dissolution and removal, loss of cohesion between individual particles, removal of particles, pore colmatation, and further compaction of the soil. Hydrostatic head acts as a sealing load. As a result, the filter coefficient in time gradually decreases and reaches a constant value. In this process, the reduction process can be smooth, which is characteristic of plastic clays, or spasmodic.

A sharp decrease in the rate of filtration is also revealed in organic soils. Many authors explain the change in the pH and gel formation during the experiment [8, 74], swelling [53], pore collation during the experiment [59], viscosity change [77], and a combination of all the factors listed above [65]. F.G.I. Vinokurov notes [8], "that none of the listed reasons for slowing the filtration rate in peat soils over time is not reliable, because each of them is rather the result of subjective conclusions." According to N.F. Bondarenko and N.P. Kovalenko [6], in the period of long-term determinations of the filtration coefficients in peat, anaerobic processes pass, which leads to the formation of compounds such as methane, methyl compounds and a complex of aldehydes, which reduce the amount of flow in time. Stabilization of flow rates reflects the equilibrium of anaerobic processes, dissolution, colmatation, suffusion for a given pressure gradient.

Even if the soil layer is homogeneous in composition, its filtration factor can vary considerably due to small changes in the specific load, void ratio, structure, particle size and stratification. Therefore, the soil filtration factor must be described with the values ​​of the minimum and maximum limit levels. The determination of the filtration coefficient of homogeneous sand can be made quite accurately, using the relationship with the particle size distribution. When using odometric test results to calculate the filter coefficient of clay soils, only an approximate estimate is obtained. Odometer testing at a constant speed provides a more accurate measure of permeability. The most reliable method for obtaining the value of the coefficient of filtration is the field test method.

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