Soil density - Ground science

Soil density

Density - the physical property of soils quantitatively estimated by the ratio of their mass to the volume occupied. The physical properties that characterize the relationship between mass and volumes of rocks or minerals are called density. Density is used as a direct calculation factor in calculating household pressure, pressure on the retaining wall, in calculating the stability of landslide slopes and slopes, , the distribution of stresses in the foundation soils under the foundations, in determining the volume of excavation work, etc.

The following characteristics are used in engineering-geological studies: density of solid ground particles, soil density, dry ground density, soil density under water, density of the skeleton of the dried soil, etc. The first indicators of the indicator are most commonly used.

Soil density p , g/cm, kg/m, or wet ground density is the mass per unit volume of soil with natural moisture and undisturbed by addition:

To determine the density of soils, apply direct and indirect methods. Direct methods include methods based on direct measurement of the mass and volume of the soil, usually small samples. Methods for determining the density in laboratory conditions, according to the current regulatory documents [41], are given in Table. 4.5. Their disadvantage is the small volume of soil in the measured samples (obtaining "point" values) and the need to extract them from the array. Indirect methods are based on determining the density of soil without direct measurements of mass and volume of soils. First of all, they include penetration and nuclear (gamma-ray) methods, which make it possible to determine the density of soils directly in the massif. They are very efficient, have enough accuracy for practical purposes and can be used for single and multiple determinations, which is important for stationary observations.

Table 4.5

Methods for determining the characteristics of soil density [41]

Characteristic

ft

Method of determination

Soils (area of ​​applicability of the method)

Density

Soil

Cutting ring

Easy to be cut or not retaining its shape without a ring, loose and with a massive cryogenic texture

Weighing in wax of paraffined samples

Dusty-clayey nemerzlye, prone to crumbling or difficult to cut

Weighing in a neutral liquid

Frozen

3D methods

Frozen, rocky and coarse-grained soils

Gamma-ray methods

All soils

Density of dry soil

Estimated

All soils

Density of soil particles

Pycnometric with water

All soils, except saline and swelling

The same. with a neutral liquid

Saline and swelling

The method of two pycnometers

Saline

Maximum

Density

Layered soil tamping

Sands, clay soils, coarse-grained (only gravel) soils

Determination of density by the cutting ring method [44]. When using the cutting ring method, select the cutting ring-sampler, which is lubricated on the inside by a thin layer of vaseline or grease. The upper peeled plane of the soil sample is leveled by cutting off the excess with a knife, placing the cutting edge of the ring on it and screw press or manually pushing the ring into the ground through the nozzle, fixing the boundary of the test piece. Then the ground outside the ring is cut to a depth of 5 ... 10 mm below the cutting edge of the ring, forming a column with a diameter of 1 ... 2 mm larger than the outer diameter of the ring. Periodically, but as far as the cutting of the soil is concerned, by pressing lightly with the press or nozzles, the ring is placed on the column of the soil, avoiding any distortions. After filling the ring, the ground is cut 8 ... 10 mm below the cutting edge of the ring and separated. The ground protruding beyond the edges of the ring is cut with a knife, the ground surface is peeled with the edges of the ring and the ends are closed with plates. The ring with the ground and the plates is weighed and the density calculated with an accuracy of 0.01 g/cm.

The method for determining the density of soil by weighing in water paraffin samples [44] is used to determine the volume of small monoliths in the laboratory. The soil sample is cut by a volume not less than 50 cm, it is given a rounded shape, after which it is tied with a thin strong thread with a free end of 15 ... 20 cm in length, which has a loop for hanging to the earring of the scales.

The stranded sample of the soil is weighed and covered with a paraffin coat, immersing it for 2 ... 3 seconds in paraffin heated to a temperature of 57 ... 60 ° C. In this case, the air bubbles found in the frozen paraffin shell are removed, piercing them and smoothing the puncture points with a heated needle. This operation is repeated until a dense paraffin shell is formed.

To avoid cracking the paraffin shell, paraffin must be applied as soon as it melts. The waxing of the sample must be carried out very carefully. Deepening in the surface, including cavities from the fallen stones, must be covered with molten paraffin with a brush.

When the sample is placed in water, care must be taken to ensure that the bubbles do not linger under them. The cooled paraffin sample is weighed before immersion in water, and then in a vessel with water. To do this, above the pan of the scale, place a stand for the vessel with water so as to exclude its touch to the scale pan (or remove the suspension by balancing the balance with an additional load). The sample is suspended from the rocker arm and lowered into a vessel with water. The volume of the vessel and the length of the thread should ensure that the sample is completely immersed in water. In this case, the sample should not touch the bottom and walls of the vessel. When the sample is placed in water, care should be taken so that air bubbles do not linger under the sample.

It is allowed to apply the inverse weighing method : a bowl with water is placed on the bowl of the dial scale and weighed. Then a sample immersed in a liquid suspended from a tripod, and the vessel is again weighed with water and a sample immersed in it. The balance must be supported by a stand or platform above the container so that there is sufficient free space between the stand and the top of the container (Figure 4.8). Densitometers can also be used to determine the density. The container should be filled with water almost to the top, and the test specimen should be completely immersed in water, so that the suspension is in the water, without touching either the bottom or the walls of the container.

Fig. 4.8. Method for determining the density by weighing in water [180]

The weighed sample is taken out of the water, soaked in filter paper and weighed to check the tightness of the sheath. If the sample mass increased by more than 0.02 g compared to the original sample, the sample should be rejected and the test repeated with another sample.

The density of the soil p , g/cm, is calculated from the formula

where m is the mass of the soil sample before the waxing, g; m is the mass of the paraffined soil sample, g; m2 is the result of the sample weighing in water (the difference between the masses of the paraffined sample and the water displaced by it), g; p p is the density of the paraffin, taken to be 0.900 g/cm, p w - the density of water at the test temperature, g/cm.

When using the inverse weighting method, the soil density is calculated by the formula

where m - is the mass of the soil sample before the waxing, r, p p is the paraffin density taken to be 0.900 g/cm; p w is the density of water at the test temperature, g/cm, tu - mass of the vessel with water, g; pi is the mass of a vessel with water and a paraffined sample immersed in it, g.

For dense rock and semiccate soils, whose porosity is a fraction of a percent or 1 ... 2%, the bulk density can be determined without waxing [76].

Fluid displacement method . The metal container must be installed on the base and filled with water to a level higher than the level maintained by the siphon. The receiver for the displaced water is installed below the outlet end of the siphon.

The soil sample and the receiver must be weighed to within 0.1 g. All surface voids must be filled with an insoluble material in the liquid. Depressions from fallen stones should not be filled. If necessary, the sample can be completely covered by re-immersion in molten paraffin. The paraffinized sample should be cooled and weighed to within 0.1 g.

Fig. 4. 9. The method for determining the density by fluid displacement (130)

The sample of the soil must be completely immersed in the container, the tap on the siphon must be opened to allow the expelled liquid to flow into the receiver, then the receiver with the liquid should be weighed accurately to 0.1 g.

A representative part of the sample, free of paraffin, plasticine or putty, is taken to determine moisture.

The method of weighing a sample in a neutral liquid [44] is used to determine the density of frozen fine-grained soils with thin-bedded and fine-mesh cryogenic textures with a thickness of mineral layers not more than 0.5 cm. The sample is weighed in a vessel of 1000 cm capacity, two-thirds filled with a neutral liquid. In the process of working, the temperature of the liquid and its density are measured, from the beam of technical scales remove the left handle with the cup and balance the scales with a bag with a shot suspended on the hook of the left arch. A sample of frozen soil with a volume of less than 50 cm is bandaged with a kapron thread, suspended from the left earring and weighed. On the support of the balance on the left side, a vessel with a neutral liquid is placed, the sample of frozen ground is loaded into the liquid to a depth of at least 5 ... 7 cm and weighed again. The sample of frozen soil should be in contact with the bottom and walls of the vessel during weighing. After weighing the frozen monolith in air and then in a neutral liquid, the total density of the frozen soil is determined. The density measurement accuracy is 0.02 g/cm.

The neutral liquid used for determining the volume of the soil should have a freezing point below the freezing point of this soil, do not react with the soil and dissolve the ice. Typically, as a neutral liquid, kerosene, glycerin, toluene and naphtha are used. The density of these liquids is determined by the hydrometer.

The method for measuring regular geometric shapes [86] (volumetric method) is used to determine the density of rock and frozen soils. When selecting a monolith, it is given a definite shape, which makes it possible to determine the volume of the soil in an undisturbed addition. The sampled soil sample is weighed and the total soil density is set, and after it is dried to a constant weight - the density of the soil skeleton. Normally, when determining the soil density, the monoliths are shaped like a cube or parallelepiped. To determine the approximate value p for monoliths (volume not less than 50 cm) extracted from boreholes, their diameter, height (to within 0.01 cm) and mass are measured.

Fig. 4.10. Determination of the density of soils by the method of displacement of the volume: a - using polyethylene lined in the hole: б-е using a sand-loading machine: в - with a device with a rubber cylinder

The well method (volumetric method) [38] is used to determine the total density of frozen dispersed rocks with massive and schlieren cryogenic textures and for coarse clastic rocks (Figure 4.10). The method is used when working in open mine workings. The bottom of the working is leveled and cleaned. In the bottom of the hole make a hole - a hole no less than 30 30 30 cm. The soil selected from the hole is weighed on the scale with an accuracy of 1.0 g. After the selection of the bottom, the bottom of the hole is lined with a synthetic film (Figure 4.10, a), then the hole is filled with water or covered with dry sand with a grain size of 0.5 to 3.0 mm. Measured sand should be homogeneous and clean. Measure the volume of sand or the volume of water needed to fill the well, and thus establish the amount of ground extracted from the well. Having determined the mass of the soil and its volume, calculate the total density of the soil.

Radioisotope methods are mainly used to measure the density of soils under natural conditions. There are two methods for measuring the density using gamma radiation: the gamma-ray method and the method of scattered gamma radiation. As sources of gamma radiation, mainly the isozones cesium-137 and kobala-60 are used.

The gammascopic method is based on the weakening of the intensity of the gamma-ray beam as a function of the density of matter through which the beam passes. In practice, three versions of the gamma-ray method are used: a - the gamma radiation source and detector are placed in parallel wells in the ground; b - the radiation detector is on the surface, and the source is in the ground; in - the source and the radiation detector are on either side of the object under study (sample, monolith, etc.) [86]. The gammascopic method is applicable for measuring soil density to a depth of 1.5 ... 2.0 m.

The scattered gamma-ray method [86] is used to measure soil density in wells. If a gamma-ray source is placed in the borehole and a detector is located at some distance from it, then a part of the gamma-quanta coming from the well into the ground due to scattering by the electrons of the soil atoms will be returned to the well and recorded by the detector. To measure the density by radioisotope methods, the domestic industry produced a radioisotope moisture density meter UR-70 and a surface-depth density meter PPGR-1, designed for downhole measurements to a depth of 30 m. A density meter of the IOMP-2 type is used to measure the density of the upper layer of the soil to a depth of 0.3 m. The accuracy of density measurement varies within ± (0.02 ... 0.04) g/cm depending on the type of instrument. The measurement time at one point does not exceed 3 minutes.

In general, the density of dispersed soils varies from 1.30 to 2.20 g/cm. Soils, characterized by the presence of rigid crystallization bonds between the particles, have a high density, the value of which, with a small porosity, approaches values ​​for solid particles. Thus, the density of igneous rocks varies within the range 2.50 ... 3.40 g/cm (increases from acid rocks to basic and ultrabasic); argillites and siltstones - 2.20-2.55; limestones - 2.40-2.65; marls - 2,10 ... 2,60; sandstones - 2.10-2.40 g/cm. The density of watered peat due to the low density of the skeleton varies from 1.02 to 1.10 g/cm.

The density of the soil depends on the mineral composition, humidity and the nature of the addition (porosity): with increasing heavy mineral content, soil density increases, and with increasing content of organic substances - decreases; With increasing humidity, the density of the soil increases: the maximum for a given porosity it will be in the case of complete filling of the pores with water; With increasing porosity, soil density decreases.

The density of a significant part of the sedimentary rocks depends more on their porosity and humidity and, to a much lesser extent, on the mineral composition, which is explained by the wide limits of variation in porosity (humidity and gas saturation) of these rocks, by a sharp difference in the density of solid, liquid and gaseous components and a relatively constant density of the most common rock-forming minerals. The magnitude of the soil density of igneous, metamorphic and in a significant part of the chemogenic rocks is mainly determined by their mineral composition, as the porosity of these rocks is usually insignificant [50].

The density of the solid particles of the soil p s , g/cm or kg/m, is the mass of the solid component (represented by a mineral or organic component) per unit volume of soil represented by only a solid component:

The value of the density of solid particles of the soil is determined by the mineral composition, the presence of organic and organic mineral substances and is the weighted average density of these soil components in the absence of voids and moisture.

Determination of the density of soil solids by the pycnometric method [44]. A sample of soil in the air-dry state is crushed in a porcelain mortar, an average sample weighing 100-200 g is selected by quarting and sieved through a No. 2 mesh sieve, the remainder on the sieve is ground in a mortar and sieved through the same sieve. From the mixed average sample take a sample of soil from the calculation of 15 g for every 100 ml of the pycnometer capacity and dry to constant weight. A sample of ground ground or peat should be taken from an average sample at a rate of 5 g of dry soil for every 100 ml of a pycnometer tank, which in this case must be at least 200 ml. It is allowed to use the ground in an air-dry state, determining its hygroscopic humidity.

A pycnometer filled with 1/3 distilled water is weighed. Then, through the funnel, a dried sample of soil is poured into it, again weighed, shaken and put to boil on a sand bath. The duration of a calm boiling (from the moment of boiling) should be: for sand and sandy loam - 0.5 h, for loams and clays - 1 h. After boiling, the pycnometer should be cooled to room temperature and add distilled water to the dimensional risks on the neck to the bottom the meniscus coincided with it. The pycnometer is wiped from the outside and weighed. Then pour out the contents of the pycnometer, pour distilled water into it, stand in a bath of water at the same temperature and weigh.

Density of soil particles /> "g/cm is calculated by the formula

where mo is the mass of dry ground, g; m1 is the mass of a pycnometer with water and soil after boiling at the test temperature, g; m2 is the mass of a pycnometer with water at the same temperature, r; p n , is the density of water at the same temperature, g/cm.

In the case of using ground in the air-dry state, w 0 is calculated by the formula

where m is the mass of the sample of air-dry ground, g; p - hygroscopic soil moisture,%.

In determining p, the soil should be taken into account: the possibility of dissolving simple salts during the determination, resulting in underestimated values ​​ p s to avoid this in determining the specific gravity of saline soils water is replaced by neutral liquids (kerosene, gasoline, toluene, etc.); the possibility of a strong compression of the water layer around the colloidal particles of clay caused by molecular forces of attraction, resulting in exaggerated values; To prevent this, use liquids with a small surface tension (toluene, xylene, etc.); the possibility of incomplete removal of air adsorbed on the surface of the particles, resulting in underestimated values.

In accordance with the density of the most common rock-forming minerals, the density of solid particles of most soils varies from 2.50 to 2.80 g/cm. It increases with increasing content of heavy minerals in soils, therefore the density of basic and ultrabasic rocks is significantly higher (3.00 ... 3.74 g/cm) than in acidic ones (for example, in granites of 2.63 ... 2.75 g/cm, more often 2,65 ... 2,67 g/cm). In Table. 4.6 are given the approximate values ​​of the density of dispersed soil particles, which do not contain water-soluble salts and organic substances. These average values ​​are usually adopted in the absence of direct determinations of solid particle density to calculate a series of soil properties, in particular porosity and porosity.

Table 4.6

Density particle density of dispersed soils

Soil type

The average value of the particle density g/cm

Sands

2.65

Sandy loam

2.70

Loams

2.71

Clay

2.74

Peat

1.50

The presence of organic substances sharply reduces the density of solid ground particles, since their density is small compared to the mineral component. That is why the density of the solid component of peat, ground soils and soils is significantly lower than in mineral soils [50].

In peat p s varies from 1.20 to 1.89 g/cm, in normal-ash peat - up to 1.84 g/cm2, in grounded soils - up to 2.08 g/cm. The values ​​of p 3 occur more frequently in the interval from 1.4 to 1.6 g/cm ', in calculations it is assumed to be 1.5 g/cm'. The minimum values ​​of the index at close values ​​of ash content are noted for the peat of peat group and peat. containing wood residues, the maximum - in moss peat [70].

In connection with the laboriousness of the determination, the density of peat particles can be calculated from the formula [69]

Given that the density of organic particles p s or G = 1.5 g/cm, the average density of mineral particles p in * w = 2.65 g/cm, then the formula is simplified:

With increasing salt content in dispersed saline soils, the density of solids becomes smaller. The normative values ​​of the density of soil particles, depending on the nature of salinity, are given in Table. 4.7.

Table 4.7

Normative particle density changes of saline soils [111]

Content

Density of soil particles p s , g/cm3, with type of desalination

salts .%

NaCl

Na-SO.1

Na 2 CO,

MgCI 2

MgS0 4

CaCl;

NaCl + MgSO4

0

2.67

3

2.66

2.67

2.65

2.65

2.67

2.67

2.67

5

2.66

2.67

2.64

2.64

2.65

2.65

2.65

7

2.64

2.64

2.62

2.62

2.64

2.63

2.64

10

2.64

2.61

2.59

2.59

2.62

2.61

2.62

The density of the skeleton of the soil p d , g/cm or kg/m, is the mass of the solid component in unit volume of soil, dried at a temperature of 105 ° C, with a natural (undisturbed) structure:

The density of the soil skeleton is used to calculate the porosity, porosity coefficient, and also to characterize the degree of compactness of clay soils in bulk structures.

The density of the skeleton of the soil is determined experimentally, or the bowl is calculated from the soil density (p) and humidity (u-) according to the formula:

For the density of the skeleton p d all soils are subdivided into varieties (Table 2.2) [34]

Fig. 4.11. Ideal models for stacking loose and dense sandy soils

Density of soil Id - When building embankments, embankment dams, earth dams and other bulk earthworks, it is necessary to know the density of soils under loose and dense addition. Sandy soils can vary significantly in the degree of density or nature of the addition. For example, depending on the nature of stacking balls of the same size, the porosity of the system can vary from 47.64% for the most loose cubic packing to 25.95% for the most dense tetrahedral packing (Figure 4.11). In real sandy-dusty soils, due to the difference in the particle size of their particles, the porosity varies within a wider range - from 8 ... 10 to 80%.

For sandy soils, for which it is not always possible to determine practically the density of the skeleton with a natural structure, it is often performed on air-dry samples with broken addition at two states: extremely loose and dense.

In order to quantify the sand density, the relative density or density ratio (Id),

where e is the porosity coefficient for natural or artificial addition; emax - coefficient of porosity in the extremely dense addition; e min - coefficient of porosity in the extremely loose build.

For the calculation of I D , it is necessary to have the results of the field definitions of the value e and for this soil. Under laboratory conditions, determine emax and e min. To find e min, loose soil is usually used in a measuring vessel, and to determine emax, dynamic methods of compacting the soil in a measuring vessel are used.

But the degree of density Id sands are subdivided according to Table. 2.3 [34]. When // & gt; = 0 the soil is in the friest state, and for Id = 1 the soil has the densest addition.

Different in the grain composition of soils have significantly different values ​​of emax and e min, and with an increase in their size they decrease. The limiting shape of the porosity coefficients is less affected by the shape of the particles. With increasing pelletness and sphericity, they decrease, so using as the characteristic of the addition density of the relative density Id, taking into account both the grain composition and the shape of the particles, gives the most objective criterion of the addition density.

To determine the characteristics of the compacted soil, use the method of determining the maximum density, which consists in establishing the dependence of the density of the skeleton of the soil on its moisture when tampering the samples with a constant expenditure of work on their compaction and in determining from this dependence the maximum value the density of the skeleton of the soil (rham). The moisture at which the maximum density of the skeleton of the soil is reached is optimum moisture wptt

The method of laboratory determination of maximum density [28] (standard sealing method) is to establish the dependence of the density of dry soil on its moisture content when compaction of soil samples with constant compaction performance and sequential increase in soil moisture.

The composition of the installation (Figure 4.12) for testing the soil with the standard seal method should include: a device for mechanized or manual compaction of soil falling from a constant height of the load; form for a sample of soil. The design of the soil compacting device shall ensure that the weight of the load (2500 ± 25) g is dropped from the guide bar from a constant height (300 ± 3) mm to the anvil (99.8 0.2) mm. The ratio of the weight of the load to the weight of the guide rod with the anvil should not be more than 1.5. With the mechanized sealing method, the device should include a mechanism for lifting the load to a constant height and a counter for the number of strokes. Installation should be placed on a rigid horizontal slab (concrete or metal) with a mass of at least 50 kg. The deviation of the surface from the horizontal should not be more than 2 mm/m.

The form for the soil sample should consist of a cylindrical part, a pallet, a clamping ring and a nozzle. The cylindrical part of the mold should have a height (127.4 ± 0.2) mm and an internal diameter (100.0 + 0.3) mm. The temporary resistance of the metal of the cylindrical part of the mold must be at least 400 MPa. The cylindrical part of the mold can be integral or consisting of two detachable sections.

To test the soil by the method of standard compaction, the samples of the ground of disturbed addition, selected from the mine workings (pits, excavations, boreholes, etc.), outcrops or stored arrays are used.

The mass of the soil sample for disturbed addition when natural moisture is required for soil sample preparation must be at least 10 kg if there are particles larger than 10 mm in the ground and not less than 6 kg in the absence of particles larger than 10 mm. The sample of the broken-up soil sample submitted for testing is dried at room temperature or in an oven to an air-dry state. Drying in the drying cabinet of non-bonded mineral soils is allowed to be carried out at a temperature of no more than 100 ° C, and bonded - no more than 60 ° С. During the drying process, the soil is periodically mixed. Grind aggregates of soil (without crushing large particles) in a grinder or in a porcelain mortar.

Fig. 4.12. Instruments for standard soil compaction: a - device Ltd. "NGO" Geotech " (140)); b - the device Soyuzdornii (with two glasses); c - diagram of the device Soyuzdornii f28f: I - pallet; 2 - a detachable cylinder with a capacity of 1000 cm *:

3 ring; 4 nozzle; 5 anvil: 6 weight of 2.5 kg; 7 guide rod; 8 - the restrictive ring; 9 - clamping screws

The ground is weighed and sieved through a sieve with holes of 20 mm in diameter and 10 mm in diameter. In this case, the entire mass of the soil must pass through a sieve with holes of 20 mm in diameter. Then, the screened large particles are weighed. If the mass of soil particles larger than 10 mm is 5% or more, a further test is carried out with a sample of soil passing through a 10 mm sieve. If the mass of ground particles is larger than 10 mm is less than 5%, further sifting of the soil through a sieve with holes of 5 mm in diameter is made and the content of particles larger than 5 mm is determined. In this case, a further test is carried out with a sample of soil passing through a 5 mm sieve.

Samples are taken from the screened large particles to determine their moisture content and average density of solid particles. From the soil passing through the sieve, samples are taken to determine its hygroscopic humidity. Calculate the content in the ground of large particles K ,%, with an accuracy of 0.1% by the formula

(4.1)

where is the mass of the screened large particles, r; w g - the moisture content of the sifted soil in the air-dry state,%; t p - mass of the sample of soil in air-dry state, g; it. - humidity of the screened large particles,%.

A sample of soil for the test (/ Ir ') with a mass of 2500 g is selected from the sifted soil. The entire test cycle using one sample is allowed. The sampled sample is placed in a metal test cup.

The amount of water Q , g, to dampen the selected sample to the humidity of the first test is calculated by the formula

(4.2)

where m p ' is the mass of the selected sample, g; w - soil moisture for the first test, assigned by gabl. 4.8, %; w g - humidity of the sieved soil in air-dry state,%.

Table 4.8

Soil moisture values ​​for the first test

Primers

Soil moisture for the first test w1,%

Sand gravel, coarse and medium size

4

Sand is fine and dusty

6

Sandy loam, loam light

6 ... 8

Heavy loam, clay

10. .12

A calculated amount of water is introduced into the selected soil sample in several steps, mixing the soil with a metal spatula, then transferring the soil sample from the cup to a desiccator or a tightly closed vessel and holding it at room temperature for at least 2 hours for unbound earths and at least 12 hours for cohesive soils.

The cylindrical part of the mold (pre-weighed) is installed on the pallet without clamping it with screws, the clamping ring is mounted on the upper edge of the cylindrical part of the mold, the cylindrical part of the mold is clamped alternately with the screws of the pallet and the ring, wiping the inner surface with technical petroleum jelly. The assembled form is placed on the base plate and the bearability of the guide rod and the cylindrical part of the mold and the free running of the load along the guide rod are checked.

The test is carried out consistently increasing the soil moisture of the test sample. At the first test, the soil moisture content should correspond to the value set in Table. 4.11. For each subsequent test, the soil moisture should be increased by 1 ... 2% for non-cohesive soils, 2 ... 3% for cohesive soils.

The amount of water for moistening the test sample is determined by the formula (4.2), taking in it for w g and w respectively humidity at the previous and the next tests.

The soil sample test is carried out in the following order: the sample is transferred from the desiccator into a metal cup and mixed thoroughly; layer of soil thickness

5 ... .6 cm is loaded into the assembled form from the sample and the surface is lightly compacted by hand. The seal is produced by 40 shocks of the load from a height of 30 cm but an anvil fixed on the guide rod. A similar operation is performed with each of the three soil layers successively loaded into the mold. Before loading the second and third layers, the surface of the previous compacted layer is bled with a knife to a depth of 1 ... .2 mm. Before laying the third layer, a nozzle is placed on the mold; after compaction of the third layer, the nozzle is removed and the projecting part of the ground is cut flush with the end face of the mold. The thickness of the projecting layer of cut soil should be more than 10 mm. If the protruding part of the ground exceeds 10 mm, it is necessary to perform an additional number of strokes at the rate of one blow per 2 mm of the excess.

The depressions formed after stripping the surface of the sample, due to the precipitation of coarse particles, are filled manually with soil from the remaining part of the sampled sample and leveled with a knife.

Weigh the cylindrical part of the mold with the compacted soil (mі) and calculate the soil density p {, g/cm, using the formula

і de m, - mass of the cylindrical part of the mold with compacted soil, g; m, - mass of the cylindrical part of the mold without soil, r; V - the capacity of the form, cm.

The compacted soil sample is extracted from the cylindrical part of the mold, samples from the upper, middle and lower parts of the sample are taken to determine the soil moisture. The soil removed from the mold is attached to the part of the sample remaining in the cup, ground and mixed. The size of aggregates must not exceed the largest particle size of the test soil.

After the addition of water, the soil is thoroughly mixed, covered with a damp cloth and aged for at least 15 minutes for non-cohesive soils and at least 30 minutes for cohesive soils. The second and subsequent soil tests should be carried out in accordance with the procedure described earlier.

The test should be considered complete when, with increasing humidity of the sample, the subsequent two tests consistently reduce the mass and density values ​​of the compaction soil sample, and also when the water is squeezed or the liquefied soil is extracted through impacts during formulations. The consolidation of homogeneous granulometric composition and draining soils is terminated after the appearance of water in the form compounds, regardless of the number of impacts during compaction of the sample.

Based on the density and soil moisture values ​​obtained as a result of successive tests, the dry soil density g/cm is calculated with an accuracy of 0.01 g/cm using the formula

where pi - is the density of the soil, g/cm '; wi - soil moisture at the next test,%.

The results of the tests are presented in the form of graphs of the dry soil density versus humidity (Figure 4.13). At the highest point of the graph, for cohesive soils, the maximum density value and the corresponding optimum moisture value are found.

Fig. 4.13. Graphs for determining the maximum density and optimum humidity: a) cohesive soils: b) non-cohesive soils

For non-cohesive soils, the standard seal schedule may not have a marked maximum. In this case, the value of the optimum humidity is taken at 1.0 ... 1.5% less than humidity and at which water is squeezed out. The maximum density is taken from the ordinate corresponding to it. At the same time, 1.0% takes gravel, large and medium size for sand; 1,5% - for fine and silty sands.

If the soil contains large particles that were removed from the sample prior to testing, then, to account for the effect of their composition, the set value of the maximum density of dry soil is corrected using the formula

Where p * is the density of large particles, g/cm; K - the content of large particles in the soil,%.

The value of the optimum soil moisture w opl , %, is determined by the formula

To check the correctness of the test of cohesive soils, a "zero air content line", is plotted showing the change in the density of dry soil from moisture with complete saturation of its pores with water. The pairs of numbers p and w, for the construction of the zero air content line at the density of the ground particles p 5 sub> are determined by setting the moisture values ​​using the formula

Where p, is the density of soil particles, g/cm ', p and is the density of water equal to 1 g/cm'.

The descending part of the standard seal schedule should not intersect the zero-air line.

The number of successive soil tests with increasing humidity should be at least five and sufficient to detect the maximum density of dry soil according to the standard seal schedule. Allowable discrepancy between the results of parallel definitions . obtained under repeatability conditions, should not exceed 1.5% for a maximum density of dry soil, for an optimum moisture content of -10% . [28]

To determine the maximum density and optimum soil moisture (according to BS, ASTM and other foreign standards), the Proctor method and the Proctor modified method are used. The procedure for testing by the Proctor method and their processing are similar to the above procedure, the requirements for soils and equipment are also close: the diameter of the particles is not more than 20 mm; The hammer's weight, according to the BS, is 2.5 kg (or 4.5 kg); drop height 300 mm (or 450 mm); according to ASTM, the weight of the hammer is 2.5 kg (or 4.5 kg); the height of the fall is 305 mm (or 457 mm). The difference between the United States standard and foreign ones is that the diameter of the hammer in foreign devices is 50 mm, and in domestic devices the diameter of the hammer corresponds to the internal diameter of the glass 99.8 mm. Hammer for manual and automatic soil compaction by ELE, as well as a graph for determining maximum density and optimum soil moisture, according to BS. are shown in Fig. 4.14 [136].

Adjustment of the maximum density and optimum moisture values ​​for the main soil types determined by the standard compaction method to the values ​​obtained by the Proctor methods is carried out by multiplying by the transition coefficients given in Table. 4.9.

Fig. 4.14. Proctor's method: a - Proctor's prao for manual compaction of the soil;

6 - mechanism for automatic compaction of soil; in the graph for determining the maximum density and optimum soil moisture (136)

Table 4.9

The coefficient of reduction of the values ​​of the maximum density and the optimum soil moisture to the values ​​obtained by Proctor's methods

A kind of soil

Soil Test Method

Sand

Sandy loam

Loam and clay

Pd max

'opt

Pmax

Prx

W 0 pi Pitmax

W "pt

Proctor's standard method

1.0

1.0

0.99

1.02

0.96

1.03 T 0.97

1.02

Proctor modified method

1.02

0.87

1.05

0.84

1.06

0.85 1.06

0.88

The test results are also presented in the form of graphs of the dry soil density versus humidity (Figure 4.14). For optimum humidity, the humidity corresponding to the maximum density is taken.

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