Determination of the characteristics of deformation...

Determination of the characteristics of deformation of saline soils

The bases formed by saline soils should be designed taking into account their specific features, which cause the formation of long-term water filtration and leaching of salts of suffosive precipitation.

The subsoil deposit of the base s s j, folded by saline soils, is determined by the formula [92]

where e s /j - relative suffosion compression of the soil of the ith layer at a pressure p, equal to the total vertical stress at the considered depth from external load a : p and the own weight of the soil a zg ; n - thickness of the/th layer of the saline soil; n is the number of layers on which the zone of suffusion of saline soils is divided.

The deformation properties of saline soils characterize! such indicators as:

• absolute suffosive compression Δ h sf , mm;

• relative suffosion compression esfy

• initial pressure of suffusion compression p s /, MPa.

These characteristics are determined by the results of tests of soil samples in compression-filtration devices, which exclude the possibility of lateral expansion of the soil sample when loaded with a vertical load.

The composition of the installation for testing the soil for suffosive compression should include: a compression-filtration device with a working ring; mechanism for the vertical loading of the soil sample; Devices for measuring vertical deformations of a soil sample. The design of the compression-filtration device should provide: water supply to the soil sample from below (upflow scheme) or from above (downflow scheme); drainage of water filtered through a sample of soil and its accumulation in a measuring vessel; continuous filtration of water throughout the test; tightness of the main parts of the device; the initial load on the sample, created by the weight of the stamp and fixed on it by measuring instruments.

Testing by suffusion compression as a result of dissolution and removal of salts during water filtration Compression and filtration tests of soils are carried out according to the scheme of the descending or ascending flow of the filtering liquid , as well as for any values ​​of the pressure gradient (/), provided there is no mechanical suffusion. Tests of saline soils should be carried out according to the schemes [30]:

• single curve (when testing one soil sample) - to determine the compressibility characteristics of the soil of natural moisture at different relative subsidence pressures and relative suffosive compression at a given pressure value

• three curves (when testing three soil samples) - to determine the compressibility characteristics of the soil of natural humidity at different pressures, relative subsidence and relative suffosion compression.

In the "one curve" test , the load on the soil sample of natural moisture is produced by loading steps up to a given pressure p similarly to the compression test. After the conditioned stabilization of the deformation of the soil sample at a pressure of p , it must be soaked with water, continuing soaking until the conditional stabilization of subsidence is carried out [30]. After the end of the subsidence deformations or in the absence of subsidence, a continuous filtration of water through the sample should be initiated (at a given pressure p) until the conditional stabilization of the suffosive compression.

The "three curves" test is carried out on three undisturbed soil samples taken from one monolith and differing in density of dry soil by no more than 0.05 g/cm, or on three samples of disturbed addition having the same (predetermined) degree of salinity and density of dry soil.

One sample is tested at natural humidity, while the load on the soil sample is produced by loading steps up to a given pressure p.

The second sample must be soaked before full water saturation and then apply the load to the sample with load stages up to a given pressure. After the conditional stabilization of the compression of the water-saturated soil, it is necessary to begin continuous filtration of water through the sample (at a given pressure p) until the conditional stabilization of the suffosive compression.

The third sample should be soaked to full water saturation (without transferring the load to the soil sample and without the use of an arrestor) and then to leach the salts (soil desalinization) by continuously filtering water or liquid of the specified composition through the sample. After conditioned desalinization of the soil, it is necessary to apply the load to the sample in steps up to a given pressure p , maintaining the sample in the water-saturated state.

For the criterion for conditional desalinization of the soil (J $) during the leaching of salts without load (when testing under the scheme of "three curves"), the values ​​given in Table 1 should be determined. 8.26.

For the criterion of conditional stabilization of the suffosive compression of the soil at a given pressure p , the increment of the relative strain Δ% should not be taken, not exceeding 0.001 with an increment of the leaching degree of salts φ, = 0 , 1, and also provided that the degree of leaching of salts β, is not less than 0.6.

Table 8.26

The value of the conditional desalinization criterion

Content

water-soluble salts,%

The value of the conditional desalinization criterion В х . д. сд.

Sand

Sandy loam

Loam

& lt; 5

0.80

0.65

0.60

5 ... I0

0.85

0.70

0.65

11 ... 20

0.90

0.80

0.75

21 ... 30

0.95

0.90

0.85

& gt; 30

1.00

0.95

0.90

After the filtration of water through the soil sample begins, the readings of the strain indicators should be recorded in the first hour after 15 minutes, then an hour later (but not less than five times) during the working day, then once a day when testing soils containing readily soluble salts, and once every two to three days when testing soils containing medium-insoluble salts.

During the leaching of salts, it is necessary to take out the filtrates to determine the amount of salts in them by the mass of the dense residue: the first filtrate - after collecting 50 ml of liquid, the second - after 2 ... 3 days, then - once a week when testing soils, containing readily soluble salts, and once every two weeks - when testing soils containing soluble salts.

After the test, the working ring with wet ground is weighed, a reference measurement of the height of the soil sample in the ring is made, the soil moisture and the mass of the dry soil are determined, and the composition of the salts.

To determine the characteristics of mo, E and p m , the test results for each loading stage are calculated:

the absolute vertical stabilized deformation of the soil sample (Δh), mm, as the arithmetic mean of the readings of the measuring devices minus the correction for the deformation of the compression device;

relative vertical deformation of the soil sample (t) using the formula

where h " g is the height of the soil sample at a pressure equal to the voltage from the self-weight of the soil at the sampling depth minus the absolute deformation at this pressure.

For the calculated values, a graph of the dependence c =/(o) is plotted (Fig. 8.22, a). In the case of a swollen pear swelling when testing the "sin curve" scheme ; it is necessary to determine the free relative swelling and to include the point corresponding to rjm in the graph e. f (σ), laying it on the y-axis upwards from the abscissa.

Fig. 8.22. Registration of the results of testing saline soil during compression in a compression-filtration device: a graph of f. (Σ), (according to the "one-curve" scheme); b the graph of e. f (β); c is the graph of e. f (σ), (according to the scheme of "three curves") [30]

Relative suffosive compression of the soil s S f when tested under the single curve and in the presence of subsidence deformations should be defined as an additional relative compression of the soil sample at a given pressure and continuous water filtration using the formula:

і de Д/ix/suffosion compression of soil at constant filtration and preset pressure (absolute suffosion compression), mm; And Ш . Р is the height of the sample of the soaked soil at a given pressure, mm; h a / p is the height of the soil sample after compression under conditions of constant fluid filtration at a given pressure, mm.

Relative suffusion compression of the soil E s f when tested in the "one curve" scheme and in the absence of subsidence deformations should be determined by the formula

where h , is the height of the soil sample of natural moisture at a given pressure, mm.

Relative subsidence e s f for different pressures when testing the "three curves" should be defined as the difference between the values ​​of the relative compression of the samples in the water-saturated state sat , p and the natural moisture content e and . or the difference between the ordinates of the corresponding curves of the graphs c. -Yes).

Relative suffosion compression E s f for different pressures when tested under the "three curves" should be determined as the difference between the values ​​of the relative compression of the samples of the leached ground and in the water-saturated state or the difference between the ordinates of the corresponding curves of the graph c = Da).

The values ​​of є */should be used to plot the relative suffosion pressure versus pressure.

The initial pressure of suffosive compression p s /, MPa, should be determined from the plot of relative suffosion compression versus pressure e. f (σ), assuming a pressure at which the relative suffosive compression is 0.01 (Fig. 8.22, c). The relative suffosive compression is calculated with an accuracy of 0.001, the initial pressure of suffosive compression is with an accuracy of 0.02 MPa.

In the process of compression-filtration testing of soil with continuous filtration of water and a specified pressure p , a plot of the relative suffusion compression% on degree of salt leaching β, д. units, (figure 8.22, b), defined by the formula

where k - the number of definitions of the dense filter remainder during the test; V H - the volume of water filtered through the soil sample during the period between two subsequent determinations of the dense residue, l; X - the arithmetic mean of the two subsequent determinations of a dense residue of filtrates minus a dense filter fluid residue, g/l; D sa i ~ the degree of salinity of the soil (taken from the results of determinations of the average degree of salinity of the monolith, from which the soil sample is cut), d. then is the mass of the dry soil sample before the test,

After completing the compression-filtration test, as well as after leaching of the salts without loading and subsequent loading of the sample, we must correct the graph & pound; S ( ~/(D), substituting in the formula for calculating β, instead of the value D M i , the value of the initial degree of salinity of the soil sample Do, sal

where m - is the mass of the dry soil sample after the test, r; D - the degree of salinity of the soil sample after the end of the test, d. [30].

For the sake of preliminary calculations of the suffusion of precipitation the bases of the structures of the I and II levels of responsibility and for the final calculations of the structures of the III level of liability, it is allowed to determine the relative suffosion compression ratio of%, clayey gypsum soils, if they are represented by: a) loams with w = 0.02 ... 0.04, h = 0.08 ... 0.12, pd = 1.2 ... 1.6 g/cm, e = 0.75 ... 1.1; b) with sandwiches with w = 0.01 ... 0.03, //. = 0,03 ... .0,07, pd = 1,4 ... 1,45 g/cm; e = 0.9 ... 1.0, using the formula

where к is a coefficient that depends on the type of soil, the content of gypsum and pressure, is taken from Table. 8.27; do - the initial content of gypsum in the soil, d. pd is the initial density of dry soil, g/cm; p g is the density of gypsum particles, g/cm; (i is the degree of leaching, i.e. n is the coefficient taken for loams equal to 1, for sandy loam - 1/3 [92].

For preliminary calculations, the normative values ​​of the suffosive compression of gypsum loam and sandy loam are also allowed to be determined from Table. 8.28.

Table 8.27

The values ​​of the coefficient k for different types of soils

Type of soil

The content of gypsum, etc. units

Coefficient A] under pressure. MPa

0.1

0.2

0.3

0.4

0.1

0.86

0.70

0.52

0.43

Sandy loam

0.2

0.95

0.90

0.83

0.76

0.3

0.97

0.95

0.90

0.85

0.1

0.08

0.15

0.30

0.46

0.2

0.15

0.27

0.50

0.84

Loam

0.3

0.45

0.60

0.80

1.10

0.4

0.85

0.96

1.07

1.30

0.5

1.08

1.15

1.22

1.38

Table 8.28

The normative rotations of the density of dry soil, the filtration coefficient and the relative suffosive compression of gypsum loam and sandy loam [III]

Primer

Degree of salinity

The content of gypsum. %

The density of dry soil pj, g/cm '

Coefficient of filtration A *, cm/s

Relative suffusion compression, with #

Loam

Unsunged

& lt; 5

1.60 ... 1.65

10

<0.005

Weakly-gypsum

5 ... 10

1.55 ... 1.60

10 ... 10

0.005 ... 0.01

Medium-plated

10 ... 20

1.45 ... 1.55

10 ... 10

0.10 ... 0.05

Heavily plated

20 ... 35

1.40 ... 1.50

10 ... 10

0.05 ... 0.18

Excessively plastered

& gt; 35

1.25 ... 1.40

10 ... 10

& gt; 0.18

Sandy loam

Mesa plastered

& lt; 5

1.45 ... 1.55

10

<0.01

Weakly-gypsum

5 ... 10

1.40 ... 1.50

10 ... 10

0.01 ... 0.03

Medium-plated

10 ... 20

1.35 ... 1.45

10 ... 10

0.04 ... 0.10

Closed-grained

20 ... 30

1.35 ... 1.40

10 ... 10

0.10 ... 0.17

Excessively plastered

& gt; 30

1.30 ... 1.40

10 ... 10

& gt; 0.17

According to [111], the bases formed by saline soils should be designed taking into account their peculiarities, which cause:

• formation of a suffusion precipitation s S f with long-term water filtration and leaching of salts;

• a change in the leaching of salts of physical and mechanical properties of the soil, accompanied, as a rule, by a decrease in its strength characteristics;

• Increased aggressiveness of groundwater to the materials of underground structures due to dissolution of salts contained in the ground.

It should also be borne in mind that subsidence or swelling may appear in saline soils when soaked.

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