(GOST 22733-77).
Goal of the work:
Establishing the dependence of the density of dry soil on its moisture content when compacting samples.
Equipment:
1. Soyuzdornia device for standard soil compaction; 2. Scales for measuring the mass of device parts with a limit value of 10 kg and an error of 1 g; 3. Laboratory scales for determining soil moisture with an error of 0.01g; 4. Porcelain mortar with rubber pestle; 5. Drying cabinet; 6. Sieve with 10mm holes; 7. Desiccator; 8. Measuring cylinders 100 and 500 ml; 9. Vernier caliper; 10. Laboratory knife; 11. Weighing bottles.
Preparing the device for operation:
Preparing the device for testing must be carried out in the following sequence:
Install the cylinder into the pan without clamping it with screws;
Install the ring on the side of the cylinder;
The cylinder is clamped alternately with the screws of the pan and the ring;
Check the dimensions of the cylinder with a caliper; in this case, the internal diameter and depth should be equal to 100 and 127 mm, respectively.
Determine the mass m 4 of the assembled container (cylinder with tray and ring) with an error of up to 1 g and record the data in a log;
Place the assembled container of the device on a rigid, fixed base weighing at least 50 kg.
Progress.
1. A soil sample weighing m 3 = 2.5 kg, previously dried to an air-dry state and ground in a mortar and pestle with a rubber tip, is sifted through a 10 mm sieve. From the grains that have passed through the sieve, 30 g samples are taken to determine the moisture content W 1 .
2. Re-moisten the samples to the initial moisture content (W 3), taken equal to 4% for sandy and gravel soils and 8% for clayey soils. The amount of water Q required to rehydrate the soil sample is determined by the formula.
where W 1 is the moisture content of the soil sample before additional moisture.
3. Add the required amount of water and mix the soil thoroughly.
4. The prepared soil sample is loaded in layers into the device cylinder, pressing the soil with a tamper. Each layer should be 5-6 cm high and compacted with 40 blows of the load, while the tamper rod is held in a vertical position. Before laying the third layer, put a nozzle on the cylinder. After compaction, the nozzle is removed and the soil is cut flush with the end of the cylinder. The thickness of the layer of cut soil should not be more than 10mm. For larger thicknesses, the test must be repeated.
5. Determine the mass of the container with soil (m 5) with an error of up to 1 g and calculate the density of the wet soil sample γ with an error of up to 0.01 g/cm 3 using the formula
(19)
where V is the capacity of the cylinder, equal to 1000 cm3.
6. Remove the pan and ring, open the cylinder and remove the compacted soil sample; one 30 g sample is taken from the upper, middle and lower parts to determine moisture content.
7. The soil removed from the cylinder is added to the soil remaining in the cup, ground, mixed and the humidity of the sample is increased, then it is also placed in the device. The test is considered complete if the soil stops compacting and begins to be squeezed out of the device when the load hits.
8. The test results are recorded in table 11. Based on the values of density and moisture content of compacted samples obtained as a result of testing, the density of the soil skeleton γ sk is determined with an error of up to 0.01 g/cm 3 according to the formula:
(20)
9. Based on the data obtained, a graph is drawn in Fig. 5 of the dependence of skeleton density on soil moisture. Find the maximum of the obtained dependence and the corresponding values of the maximum density of the soil skeleton γ max and optimal humidity. W opt.
Fig. 7 Diagram of the Soyuzdornia device for standard soil compaction: 1. pallet; 2. split cylinder with a capacity of 1000 cm 3; 3. . ring; 4. nozzle; 5. anvil; 6. load weighing 2.5 kg; 7. guide rod; 8. restrictive ring; 9. clamping screws.
Table 11.
Determination of dry soil density
ROAD CONSTRUCTION TECHNOLOGY SECTION
Laboratory work No. 9
METHOD FOR ACCELERATED DETERMINATION OF THE MAXIMUM STANDARD DENSITY AND OPTIMUM HUMIDITY OF SOILS STRENGTHENED WITH INORGANIC BINDING MATERIALS.
At a certain optimal moisture content of the soil mixed with the binder, fast-hardening binders such as cement introduce certain distortions into the test results. The time at which cement and soil begin to harden depends on the dispersion of the soil, its mineralogical and chemical composition. The composition of cement and its quantity also have a certain influence. The influence of soil on the nature of hardening of cement-soil mixtures can be assessed by the number of clay particles. Thus, when treating clay with cement, the hardening process of the mixture begins earlier than when processing sand.
Slow-setting fly ash-type binders also affect standard test results, although to a lesser extent.
In connection with the established phenomenon, a special accelerated method has been developed for determining the maximum standard density and optimal moisture content of soil mixtures with binders. According to this method, it is proposed to determine the maximum density of reinforced soil either by a single compaction in a standard compaction device at its optimal moisture content after a certain period of time (for example, 2 hours) after wetting, or by calculation. The optimal moisture content of the mixture is determined by calculation based on the value of the optimal moisture content of the original soil. The calculation of the maximum density and optimal moisture content of reinforced soil is based on the parameters of the standard compaction of the original soil (see laboratory work No. 9).
Necessary devices:
1. Technical scales; 2. Aluminum bottles; 3, Drying cabinet with thermometer and thermostat; 4. Desiccator with dehydrated calcium chloride (no moisture absorption); 5. Spatula.
Technique for performing the work:
1. Methodology for accelerated determination of the optimal moisture content and maximum density of a mixture of soil with mineral binders (cement, shale fly ash).
In accordance with the methodology outlined in laboratory work No. 9, W opt and γ s.max of the original soil or material are determined.
The optimal moisture content of the mixture of soil and binders W opt cm is determined by the formula:
W opt cm = W opt +a, (21)
where a is the correction factor adopted according to Table 13 (depending on the type of binder material).
W opt – optimal moisture content of the initial soil. The optimal humidity can also be determined by calculation based on the moisture content of the yield boundary:
W opt = α W t (22)
where α – 0.75-0.7 – (sand and light sandy loam)
0.6-0.55 – (heavy sandy loam, light loam)
0.5-0.45 – (heavy loams, clays);
or according to the humidity of the rolling boundary (W р, %)
W opt = W р –в, (23)
where in – 1-2 (heavy sandy loam, light loam), 2-3 (heavy loam, clay).
The maximum density of a mixture of soil with mineral binders γ sc.max can be calculated using the formula:
γ s.max cm = γ s.max k g (24)
where k g is the correction factor adopted according to Table 14;
γ sk.max is the maximum density of the original soil.
To determine γ sk.max cm experimentally, take a sample of soil in the amount of 2 kg and add the required amount of binder to the soil. After mixing the soil with the binder, water is added to the mixture in an amount determined by formula (21) taking into account the hygroscopic moisture of the original soil. The mixture is thoroughly mixed again and kept in a humid environment for the following time:
For a mixture of soil and cement – 1.5 hours;
For a mixture of cohesive soil with shale drift soil – 5-6 hours;
For a mixture of non-cohesive soil and ash – 24 hours.
After the specified time, the mixture is compacted once in a large standard compaction device (120 blows per 3 layers of the mixture). The obtained values of the average density of the skeleton are taken as the maximum density of the reinforced soil γ sk.max cm
Table 12
Coefficient value α
Table 13
K coefficient value
Laboratory work No. 10
Knowing the quantities ρ ,ρs And W it is possible to calculate a number of derived soil characteristics:
Dry soil density ρ d – the ratio of the mass of the soil skeleton (excluding pore water) m s to the volume of this soil V o:
, t/m 3; where: ρ – soil density, g/cm 3 ; w – soil moisture, %.
Soil porosity n
– ratio of pore volume V pores to the volume of the entire soil V 0: 
;
where: ρ – soil density, g/cm 3 ; ρ d – density of dry soil, g/cm 3 ; ρ s – density of soil particles, g/cm 3 ; w – soil moisture, %.
Porosity coefficient e – ratio of pore volume V pores to the volume of soil particles V 0:
Sandy soils according to their density are divided, depending on the porosity coefficient, into: Strong (dense) Medium strength (medium density); Low strength (loose).
Humidity levelSr – the proportion of soil pores filled with water - the ratio of humidity W to the total moisture capacity of soil W sat:
where: ρ w – density of water, g/cm 3. According to the degree of moisture, soils are: a) low-moisture (0
Optimal soil parameters are determined in a soil pre-compaction device. Soil is placed into the device in layers and each layer is compacted with 30-40 blows of a load falling from the same height.
Humidity at which max. The possible compaction effect is called optimum moisture content.
The density of the soil skeleton achieved during the swing. Humidity is called the optimal soil density.
5. Deformability of soils. Compression dependence and its analysis.
Soil compressibility– their ability to decrease in volume (give sediment) under the influence of external pressure. The degree of soil compressibility depends on the structure of the soil and is an important characteristic of the mechanical properties of the soil, which is used to calculate settlements of buildings and various structures. The compressibility of soils is caused by a change in their porosity when a load is applied and occurs due to the occurrence of mutual displacements of particles. Reducing the thickness of water-colloidal films by squeezing out water in water-saturated soils and due to the destruction of crystallization bonds in highly structured soils. Due to the fact that soil compressibility is associated with a decrease in their porosity, in soil mechanics it is customary to characterize soil compressibility by the dependence of the porosity coefficient on compaction pressure. This dependency is called compression and is determined experimentally in laboratory conditions using two types of devices:
-odometer(uniaxial compression device with rigid side walls of the cage in which the soil sample is enclosed) also called a compression device;
- stabilometer(a triaxial compression device with elastic side walls that enclose the soil).
Compaction of the soil from which the roadway is constructed is one of the most important technical processes, as a result of which the design strength, stability and stability of the road structure as a whole is achieved during future operation. The construction of embankments without layer-by-layer compaction is allowed only in certain cases:
1) in embankments in swamps;
2) in embankments with a spillway;
3) when constructing an embankment using the hydraulic alluvium method from one-dimensional fine dune sands;
Density of compacted soil into the design is estimated by the compaction coefficient cat. It is the ratio of the actual density of the embankment soil to the maximum standard density at optimal humidity (standard compaction method)
K y= ρ d /ρ max, ρ d = ρ/(1+0.01W)
The required density of soils in the embankment can be achieved at optimal humidity. The highest soil density can be achieved by using machines and mechanisms that provide the maximum permissible contact pressure under the strength conditions for a given soil. To determine the optimal thickness of the compacted layer and the number of passes along one track, test compaction should be performed or empirical dependencies should be used, guided by theoretical premises:
1) the soil mass in the embankment is a 3-phase system;
2) the degree of convergence of the elements of the solid phase for a given soil can be judged only by the density of dry soil; when soil is compacted, the density can increase only due to the removal of the gaseous phase and partially due to the squeezed out liquid phase;
3) an important question is the required density (it must provide the necessary resistance of the soil to stress from loads and weather-climatic factors);
4) the most widely used empirical method is assigning the required density
ρ sk tr =K y ρ sk max
To determine the maximum density, the soil is tested in a standard compaction device (the soil is compacted in a cylinder layer by layer by thrombosis using a falling weight). As a result of the tests, a standard compaction curve is obtained (the dependence of the density of dry soil on humidity)
The required minimum density of dry soil d, g/cm 3, t/m 3, must be such that the embankment soil, when exposed to temporary train loads, operates in an almost elastic stage.
The dry soil density d required in the subgrade for sandy and clayey soils is determined by the formula:
where k is the minimum compaction coefficient, for the upper and lower parts, see Table 5.4 p. 297;
Maximum density of dry soil, t/m3.
Thus:
The density of the embankment soil taking into account humidity is determined by the formula:
where: - optimal humidity.
The specific gravity of the embankment soil is determined by the formula:
A protective layer is a layer of drainage soil that must have an appropriate compaction coefficient and a thickness such that plastic deformations do not occur underneath it. A protective layer is placed under the main platform to prevent heaving.
According to STN Ts-01-95, the thickness of the protective top layer h protect for an embankment filled with sandy loam is 0.5-0.7 m. The value h protect = 0.5 m was taken into account for the calculation. The protective layer is poured from a sand-gravel mixture with the parameters : s = 1 kPa; φ=33º.
Maximum Density (Standard Density)- the highest density of dry soil, which is achieved when testing the soil using the standard compaction method.
Optimal humidity- soil moisture value corresponding to the maximum density of dry soil.
Since when the structural connections of the soil are disrupted, its properties change, it is necessary to study the state of the soil with an undisturbed structure. To do this, in the process of engineering-geological surveys, monoliths are selected from pits and wells - large samples of soil with an undisturbed structure. Smaller samples are taken from these monoliths in the laboratory and three main characteristics are experimentally determined:
· density(volumetric mass) soilρ natural (undisturbed) structure, equal to the ratio of the mass of the soil sample to its volume;
· density(volumetric mass) solid particles of soilρ s equal to the ratio of the mass of solid particles to their volume;
· natural soil moisture contentω, equal to the ratio of the mass of water contained in it to the mass of solid particles.
Rice. 1.3. Diagram of the constituent parts (components) of a soil sample
Let us select a sample of volume V = 1 cm3 from the soil and mentally divide it into two parts: one occupied by solid particles, volume V1, and the other, occupied by pores located between these particles, volume V2 (Fig. 1.3). The space occupied by pores can generally be divided into two parts, one of which is occupied by water, the other by air. Let the mass of solid particles in volume V be g 1, and the mass of water - g 2 (the mass of air does not affect the calculation results).
According to definitions
The density of the soil is determined by weighing, most often using a sample taken in a cutting ring, sometimes waxed or other methods, including gamma ray logging. The density of solid particles is found using a pycnometer. Soil moisture is determined by weighing a sample of natural moisture before and after drying (to constant weight) at a temperature of 105°C.
The purpose of artificial soil compaction is to increase their strength, reduce water permeability and the height of capillary rise, as well as reduce unevenness and accelerate settlement. Compaction of bulk soils containing water and air in their pores occurs mainly not due to the displacement of water, but due to the displacement of air when particles approach each other, therefore the compaction process is greatly influenced by soil moisture. When humidity increases to a certain limit, the density of the soil increases with the same expenditure of compacting energy. With a further increase in humidity, the density decreases with the same amount of work expended (see Fig. 5).
The dry density of the soil is usually taken as an indicator of the degree of soil compaction. ρ d.
Rice. 6. Density dependence ρ d on the number of blows nat constant humidity
In laboratory conditions, the determination of optimal humidity and the corresponding maximum density is carried out using a standard compaction device (Fig. 7). This standard compaction corresponds to the moisture content and density obtained when compacting soils with medium-weight rollers in industrial conditions.
The essence of the standard compaction method is to determine the optimal soil moisture w opt, at which the greatest compaction is achieved (the maximum value of soil density in dry form ρ d). In the SoyuzdorNII device, a series of separate tests are carried out on layer-by-layer (in three layers) compaction of soil with a consistent increase in its moisture content w, but with a constant number of blows (120 blows, i.e. 40 blows for each of the three layers) of loads weighing 2. 5 kg, freely falling from a height of 300mm. For sandy and gravel soils, the first test is carried out at an initial moisture content of 4%, and in subsequent tests the moisture content is successively increased by 1-2%. Similarly, for clay soils, tests are carried out at an initial moisture content of 8% with a subsequent increase in it by 2-3%.
Rice. 7. Standard compaction device SoyuzdorNII
Soil testing is carried out in the following order:
– a prepared soil sample weighing 2.5 kg is loaded in layers into the device cylinder, and each layer is compacted with 40 blows of the load;
In this case, the tamper rod is held in a vertical position (before laying the third layer, a nozzle is put on the cylinder);
– after compacting the third layer, the nozzle is removed and the protruding part of the sample is cut flush with the end of the cylinder;
– the density of a wet soil sample is determined by the formula:
Where m 0– mass of the assembled container (cylinder with tray and ring) g;
m 1– mass of the container with soil, g;
V– cylinder capacity, cm 3;
– the cylinder is opened and one sample (weighing at least 30 g) is taken from the upper, middle and lower parts of the sample to determine soil moisture (see work 2).
Then, by adding a certain amount of water (see Appendix 2), the soil moisture is increased and subsequent tests are carried out. The tests should be considered completed when, with an increase in the sample moisture content of the next two or three compaction tests, a consistent decrease in the density values of the compacted soil samples occurs.
Based on the values of density and moisture content of compacted samples obtained as a result of testing, the density of the soil in a dry state is determined:
A graph of the dependence of the density of dry soil on humidity is constructed (see Fig. 5), the maximum of the obtained dependence and the corresponding values of the maximum density of dry soil ( ρ d max) with an accuracy of 0.01 g/cm 3 and optimal humidity ( w opt) with an accuracy of 0.1%.
The maximum density obtained with standard compaction is taken as the initial value when assessing the density of artificial soil compaction.
Ratio of dry soil density to maximum dry soil density ρ d max called the standard compaction coefficient:
The required minimum embankment density is determined by multiplying by the coefficient K Tab (K Tab = K s), adopted according to SNiP 2.05.02-85, depending on the location of the soil layer along the height of the embankment, the type of coating, the road climatic zone and the conditions of the embankment.
Determination of optimal humidity and maximum density is mandatory when working: on the impact of embankments; final finishing of the subgrade; installation of road pavements and soil cushions in the foundations of structures.
In the laboratory, the teacher conducts a demonstration experiment on soil compaction at one humidity value. To build a dependency ρ d =f(w) data from Table 13 is used.
1. As directed by the teacher, according to direct determination data using the method described above (see Appendix 2) or according to those specified in the table. Using 13 values of the mass of the container with soil m 1 and humidity w for a series of six experiments, determine the values of soil density in a dry state (formula 23); record the results in the journal (form 13).
2. Construct a standard compaction curve (form 14).
3. Determine the values of the maximum density of dry soil and optimal humidity w opt; record the results in the journal (form 15).
Table 13
Note:
Weight of assembled container m 0=3600 g; cylinder capacity V=1000cm 3.
