Compaction coefficient of sand and gravel table. Technological map for the planning and compaction of the ASG. Sealing on site

The compaction coefficient of any bulk material shows how much its volume can be reduced with the same mass due to tamping or natural shrinkage. This indicator is used to determine the amount of aggregate both during purchase and during the construction process itself. Since the bulk weight of crushed stone of any fraction after compaction will increase, it is necessary to immediately lay a stock of material. And in order not to buy too much, a correction factor will come in handy.

The compaction factor (K y) is an important indicator that is needed not only for the correct formation of the order of materials. Knowing this parameter for the selected fraction, it is possible to predict further shrinkage of the gravel layer after loading it with building structures, as well as the stability of the objects themselves.

Since the ramming ratio is the rate of volume reduction, it changes under the influence of several factors:

1. The method and parameters of loading (for example, from what height the filling is performed).

2. The peculiarities of transport and the duration of the journey - after all, even in a stationary mass, a gradual compaction occurs when it sags under its own weight.

3. Fractions of crushed stone and grain content smaller than the lower limit of a particular class.

4. Flakiness - needle stones do not settle as much as cuboid stones.

The strength depends on how accurately the degree of compaction was determined. concrete structures, foundations of buildings and road surfaces.

However, do not forget that ramming on the site is sometimes performed only on the upper layer, and in this case the calculated coefficient does not fully correspond to the actual shrinkage of the pillow. This is especially true for home craftsmen and semi-professional construction crews from the near abroad. Although, according to the technology requirements, each layer of backfill must be rolled and checked separately.

Another nuance - the degree of ramming is calculated for a mass that is compressed without lateral expansion, that is, it is limited by walls and cannot creep. At the site, such conditions for filling any fraction of crushed stone are not always created, so that a small error will remain. Take this into account when calculating the settlement of large structures.

Sealing during transportation

Finding some standard value of compressibility is not so easy - too many factors affect it, as we talked about above. The crushed stone compaction factor can be indicated by the supplier in the accompanying documents, although GOST 8267-93 does not directly require this. But transporting gravel, especially in large batches, reveals a significant difference in volume at loading and at the final point of delivery of the material. Therefore, the correction factor, taking into account its compaction, must be entered into the contract and monitored at the reception point.

The only mention from the outside current GOST- the declared indicator, regardless of the fraction, should not exceed 1.1. Suppliers, of course, are aware of this and try to keep a small stock so that there are no returns.

The measurement method is often used during acceptance, when crushed stone is brought to the site for construction, because it is ordered not in tons, but in cubic meters. With the arrival of transport, the loaded body must be measured from the inside with a tape measure in order to calculate the volume of gravel delivered, and then multiply it by a factor of 1.1. This will allow you to roughly determine how many cubes were poured into the machine prior to shipment. If the figure obtained taking into account the seal is less than that indicated in the accompanying documents, it means that the car was underloaded. Equal or more - you can command unloading.

Sealing on site

The above figure is taken into account only for transportation. In the conditions of a construction site, where the compaction of crushed stone is carried out artificially and using heavy machines (vibrating plate, roller), this coefficient can increase to 1.52. And performers need to know the shrinkage of the gravel backfill for sure.

Usually the required parameter is set in the project documentation. But when exact value not necessary, use the averaged indicators from SNiP 3.06.03-85:

• For strong crushed stone of fraction 40-70, a compaction of 1.25-1.3 is given (if its grade is not lower than M800).
• For rocks with a strength up to M600 - from 1.3 to 1.5.

For small and medium size classes 5-20 and 20-40 mm, these indicators have not been established, since they are more often used only when the upper bearing layer is split from 40-70 grains.

Laboratory research

The compaction ratio is calculated on the basis of laboratory test data, where the mass is compacted and tested on various devices. There are methods here:

1. Substitution of volumes (GOST 28514-90).

2. Standard layer-by-layer compaction of crushed stone (GOST 22733-2002).

3. Express methods using one of three types of densitometers: static, water-cylinder or dynamic.

Results can be obtained immediately or after 1-4 days, depending on the selected study. One sample for a standard test will cost 2,500 rubles, in total they will need at least five. If data is needed during the day, express methods are used based on the selection of at least 10 points (850 rubles for each). Plus, you will have to pay for the visit of the laboratory assistant - about 3 thousand more. But in the construction of large facilities, one cannot do without accurate data, and even more so without official documents confirming the contractor's compliance with the project requirements.

How to find out the degree of ramming yourself?

In the field and for the needs of private construction, it will also be possible to determine the desired coefficient for each size: 5-20, 20-40, 40-70. But for this, you first need to know their bulk density. It varies depending on the mineralogical composition, albeit insignificantly. The crushed stone fractions have a much greater influence on the bulk density. For the calculation, you can use the averaged data:

More accurate density data for a specific fraction is determined in a laboratory way. Or by weighing a known volume of construction crushed stone, followed by a simple calculation:

• Bulk density = weight / volume.

After that, the mixture is rolled up to the state in which it will be used on the site and measured with a tape measure. The calculation is performed again using the above formula, and as a result, two different densities are obtained - before and after tamping. By dividing both numbers, we find out the compaction coefficient specifically for this material. With the same sample weight, you can simply find the ratio of the two volumes - the result will be the same.

Please note: if the indicator after ramming is divided by the original density, the answer will be more than one - in fact, this is the safety factor of the material for compaction. In construction, it is used if the final parameters of the gravel pad are known and it is necessary to determine how much crushed stone of the selected fraction to order. The reverse calculation results in a value less than one. But these figures are equivalent and when calculating it is important not to get confused, which one to take.

Preparing for development, special studies and tests are carried out to determine the suitability of the site for work ahead: take soil samples, calculate the level of occurrence groundwater and investigate other soil characteristics that help determine the possibility (or lack thereof) of construction.

Carrying out such measures contributes to an increase in technical indicators, as a result of which a number of problems arising during the construction process are solved, for example, subsidence of the soil under the weight of a structure with all the ensuing consequences. Its first external manifestation looks like the appearance of cracks on the walls, and in combination with other factors, to the partial or complete destruction of the object.

Compaction factor: what is it?

The coefficient of soil compaction is a dimensionless indicator, which, in fact, is a calculation from the ratio of soil density / soil density max. The soil compaction factor is calculated taking into account geological indicators. Any of them, regardless of the breed, are porous. It is permeated with microscopic voids that are filled with moisture or air. With the development of the soil, the volume of these voids increases significantly, which leads to an increase in the looseness of the rock.

Important! The density index of the bulk rock is much less than the same characteristics of the compacted soil.

It is the coefficient of soil compaction that determines the need to prepare the site for construction. Based on these indicators, they prepare sand cushions for the foundation and its base, additionally compacting the soil. If this detail is missed, it can cake and begin to sag under the weight of the structure.

Soil compaction indicators

Soil compaction factor indicates the level of soil compaction. Its value ranges from 0 to 1. For a concrete base strip foundation an indicator of> 0.98 points is considered the norm.

Specificity of determining the compaction coefficient

The density of the skeleton of the soil, when the subgrade is amenable to standard compaction, is calculated in laboratory conditions. Schematic diagram the study consists in placing a soil sample in a steel cylinder, which is compressed under the influence of an external brute mechanical force - impacts of a falling weight.

Important! The highest indices of soil density are observed in rocks with a moisture content slightly above normal. This relationship is shown in the graph below.

Each subgrade has its own optimum moisture content, at which the maximum compaction level is achieved. This indicator is also studied in laboratory conditions, giving the rock different moisture content and comparing the compaction rates.

Real data is final result research, measured at the end of all laboratory work.

Compaction Methods and Factor Calculation

The geographical location determines the qualitative composition of soils, each of which has its own characteristics: density, moisture, the ability to subsidence. Therefore, it is so important to develop a set of measures aimed at qualitative improvement characteristics for each type of soil.

You already know the concept of the compaction coefficient, the subject of which is studied strictly in laboratory conditions. This work is carried out by the relevant services. The index of soil compaction determines the method of impact on the soil, as a result of which it will receive new strength characteristics. When carrying out such actions, it is important to take into account the percentage of amplification applied to obtain the desired result. Based on this, the soil compaction factor is deducted (table below).

Typology of soil compaction methods

There is a conventional system of subdivision of compaction methods, the groups of which are formed based on the method of achieving the goal - the process of removing oxygen from the soil layers at a certain depth. So, distinguish between superficial and in-depth research. Based on the type of study, specialists select the equipment system and determine the method of its application. Soil research methods are:

• static;
• vibrating;
• drums;
• combined.

Each type of equipment represents a method of applying force, such as a pneumatic roller.

Partially, such methods are used in small private construction, others only when building large-scale objects, the construction of which is coordinated with the local authorities, since some of these buildings can affect not only a given site, but also the surrounding objects.

Compaction factors and norms SNiP

All operations related to construction are clearly regulated by law, therefore they are strictly controlled by the relevant organizations.

Soil compaction factors are determined by SNiP clause 3.02.01-87 and SP 45.13330.2012. The actions described in regulatory documents, were updated and updated in 2013-2014. They describe seals for different kinds soil and ground cushions used in the construction of foundations and structures of various kinds, including underground.

How is the compaction ratio determined?

The easiest way is to determine the soil compaction coefficient by the method of cutting rings: a metal ring of a selected diameter and a certain length is driven into the soil, during which the rock is tightly fixed inside the steel cylinder. After that, the mass of the device is measured on a balance, and at the end of weighing, the weight of the ring is subtracted to obtain the net mass of the soil. This number is divided by the volume of the cylinder to give the final density of the soil. After that, it is divided by the maximum possible density and a calculated compaction coefficient for a given section is obtained.

Compaction Factor Calculation Examples

Consider the definition of the soil compaction coefficient using an example:

• the value of the maximum density of the soil - 1.95 g / cm 3;
• cutting ring diameter - 5 cm;
• cutting ring height - 3 cm.

It is necessary to determine the coefficient of soil compaction.

Such a practical task is much easier to cope with than it might seem.

To begin with, the cylinder is completely driven into the soil, after which it is removed from the soil so that inner space remained filled with earth, but no accumulation of soil was noted outside.

Using a knife, the soil is removed from the steel ring and weighed.

For example, the mass of the soil is 450 grams, the volume of the cylinder is 235.5 cm 3. Calculating using the formula, we get the number 1.91 g / cm 3 - the density of the soil, whence the coefficient of soil compaction is 1.91 / 1.95 = 0.979.

The erection of any building or structure is a responsible process, which is preceded by an even more crucial moment of preparing the site to be built, designing the proposed buildings, calculating the total load on the ground. This applies to all, without exception, buildings that are designed for long-term operation, the life of which is measured in tens or even hundreds of years.

Obligatory compaction of soil, crushed stone and asphalt concrete in the road industry is not only an integral part of the technological process of the subgrade, foundation and pavement, but also serves as actually the main operation to ensure their strength, stability and durability.

Earlier (until the 30s of the last century), the implementation of these indicators of soil embankments was also carried out by compaction, but not mechanically or artificially, but due to the natural self-settling of the soil under the influence, mainly, of its own weight and, in part, traffic. The erected embankment was left, as a rule, for one or two, and in some cases for three years, and only after that the foundation and covering of the road were arranged.

However, the rapid motorization of Europe and America, which began in those years, required the accelerated construction of an extensive network of roads and a revision of the methods of their construction. The then existing technology for erecting the roadbed did not correspond to the new tasks that arose and became a brake in their solution. Therefore, there is a need for the development of scientific and practical foundations of the theory of mechanical compaction of earth structures, taking into account the achievements of soil mechanics, in the creation of new effective soil compaction means.

It was in those years that they began to study and take into account the physical and mechanical properties of soils, evaluate their compaction taking into account the granulometric and moisture state (Proctor's method, in Russia - the standard compaction method), the first classifications of soils and standards for the quality of their compaction were developed, methods were introduced field and laboratory control of this quality.

Until the indicated period, the main soil compactor was a trailed or self-propelled smooth drum static roller, suitable only for rolling and leveling the near-surface zone (up to 15 cm) of the dumped soil layer, and even manual ramming, which was used mainly for compaction of coatings, for repairing potholes and for compaction shoulders and slopes.

These simple and ineffective (in terms of quality, thickness of the worked layer and productivity) compaction means began to be replaced by such new means as plate, ribbed and cam (remember the invention of 1905 by the American engineer Fitzgerald) rollers, ramming plates on excavators, multi-hammer ramming machines on a caterpillar tractor and a smooth roller roller, manual blast rammers ("jumping frogs") are light (50–70 kg), medium (100–200 kg) and heavy (500 and 1000 kg).

At the same time, the first soil-compacting vibrating plates appeared, one of which from Lozenhausen (later Vibromax) was quite large and heavy (24–25 tons together with a basic caterpillar tractor). Its vibrating plate with an area of ​​7.5 m 2 was located between the tracks, and the engine with a power of 100 hp. made it possible to rotate the vibration exciter with a frequency of 1500 count / min (25 Hz) and move the machine at a speed of about 0.6-0.8 m / min (no more than 50 m / h), providing a productivity of about 80-90 m 2 / h or not more than 50 m 3 / h with a thickness of the compacted layer of about 0.5 m.

More versatile, i.e. The compaction method has proven to be capable of compacting various types of soils, including cohesive, incoherent and mixed.

In addition, during ramming, it was easy and simple to adjust the force compaction effect on the soil by changing the height of the fall of the ramming plate or ramming hammer. Due to these two advantages, the impact compaction method became the most popular and popular in those years. Therefore, the number of ramming machines and devices multiplied.

It is pertinent to note that in Russia (then the USSR) they also understood the importance and necessity of the transition to mechanical (artificial) compaction. road materials and the establishment of the production of sealing technology. In May 1931, the first domestic self-propelled road roller was produced in the workshops of Rybinsk (today ZAO Raskat).

After the end of the Second World War, the improvement of the technique and technology of compaction of soil objects proceeded with no less enthusiasm and efficiency than in the pre-war period. Trailed, semi-trailed and self-propelled pneumatic rollers appeared, which for a certain period of time became the main soil compactor in many countries of the world. Their weight, including individual copies, varied within a fairly wide range - from 10 to 50-100 tons, but most of the produced models of pneumatic rollers had a tire load of 3-5 tons (weight 15-25 tons) and the thickness of the compacted layer, depending from the required compaction coefficient, from 20–25 cm (cohesive soil) to 35–40 cm (disconnected and poorly connected) after 8–10 passes along the track.

Simultaneously with pneumatic rollers, vibratory soil compactors - vibratory plates, smooth drum and cam vibratory rollers, developed, improved and gained more and more popularity, especially in the 50s. Moreover, over time, trailed models of vibratory rollers were replaced by self-propelled articulated models, which are more convenient and technologically advanced for performing linear earthworks, or, as the Germans called them, "Walzen-zug" (pull-push).

Smooth drum vibratory roller CA 402
by DYNAPAC

Each modern model The compactor vibratory roller, as a rule, has two versions - with a smooth drum and a cam drum. At the same time, some companies manufacture two separate interchangeable drums for the same uniaxial pneumatic wheeled tractor, while others offer the buyer of a roller instead of a whole cam drum, just a "shell attachment" with cams, easily and quickly fixed on top of a smooth drum. There are also firms that have developed similar smooth roller "shell attachments" for mounting on top of a cam drum.

It should be especially noted that the cams themselves on vibratory rollers, especially after the beginning of their practical operation in 1960, underwent significant changes in their geometry and dimensions, which had a beneficial effect on the quality and thickness of the compacted layer and reduced the depth of loosening of the near-surface zone of the soil.

If earlier the “tenon” cams were thin (support area 40-50 cm 2) and long (up to 180-200 mm and more), then their modern counterparts “padfoot” became shorter (the height is mainly 100 mm, sometimes 120-150 mm) and thick (support area about 135-140 cm 2 with a side of a square or rectangle about 110-130 mm).

According to the laws and dependencies of soil mechanics, an increase in the size and area of ​​the contact surface of the cam contributes to an increase in the depth of effective soil deformation (for cohesive soil it is 1.6–1.8 times the size of the side of the cam support area). Therefore, the compaction layer of loam and clay with a vibratory roller with “padfoot” cams, when creating proper dynamic pressures and taking into account 5–7 cm of the depth of immersion of the cam into the ground, began to be 25–28 cm, which is confirmed by practical measurements. This thickness of the compaction layer is commensurate with the compaction capacity of pneumatic rollers weighing at least 25–30 tons.

If we add to this the significantly greater thickness of the compacted layer of non-cohesive soils by vibratory rollers and their higher operational performance, it becomes clear why trailed and semi-trailed pneumatic rollers for soil compaction began to gradually disappear and are now practically not produced or are produced rarely and in small quantities.

Thus, in modern conditions, a self-propelled single-drum vibratory roller articulated with a uniaxial pneumatic tractor and having a smooth working body as a working body (for disconnected and loosely connected fine-grained and coarse-grained soils, including rocky coarse) or cam drum (cohesive soils).

Today, there are more than 20 companies in the world that produce about 200 models of such soil compacting rollers of various sizes, differing from each other in total weight (from 3.3-3.5 to 25.5-25.8 tons), the weight of a vibratory drum module (from 1 , 6–2 to 17–18 t) and their dimensions. There is also some difference in the device of the vibration exciter, in the vibration parameters (amplitude, frequency, centrifugal force) and in the principles of their regulation. And of course, at least two questions may arise before the road builder - how to choose the right model of such a roller and how to use it most effectively to carry out high-quality soil compaction at a specific practical facility and at the lowest cost.

When solving such issues, it is necessary to establish in advance, but rather accurately, those prevailing types of soils and their condition (particle size distribution and moisture content), for the compaction of which a vibratory roller is selected. Especially, or first of all, attention should be paid to the presence of dusty (0.05–0.005 mm) and clay (less than 0.005 mm) particles in the soil, as well as its relative humidity (in fractions of its optimal value). These data will give the first ideas about the compaction of the soil, the possible method of its compaction (purely vibrational or power vibroimpact) and will allow you to opt for a vibratory roller with a smooth or cam drum. The moisture content of the soil and the amount of dusty and clay particles significantly affect its strength and deformation properties, and, consequently, the required compaction capacity of the selected roller, i.e. its ability to provide the required compaction coefficient (0.95 or 0.98) in the soil dumping layer, set by the technology of the subgrade device.

Most modern vibratory rollers operate in a certain vibration shock mode, expressed to a greater or lesser extent depending on their static pressure and vibration parameters. Therefore, soil compaction, as a rule, occurs under the influence of two factors:

• vibrations (vibrations, shocks, movements), causing a decrease or even destruction of the forces of internal friction and small adhesion and engagement between soil particles and creating favorable conditions for effective displacement and denser repackaging of these particles under the influence of their own weight and external forces;
• dynamic compressive and shear forces and stresses created in the soil by short-term but often impact loads.

In the compaction of loose loose soils, the main role belongs to the first factor, the second serves only as a positive addition to it. In cohesive soils, in which the forces of internal friction are insignificant, and the physicomechanical, electrochemical and water-colloidal cohesion between small particles are much higher and are predominant, the main acting factor is the pressure force or the compressive and shear stresses, and the role of the first factor becomes secondary.

Studies by Russian specialists in soil mechanics and dynamics at one time (1962–64) showed that compaction of dry or almost dry sands in the absence of external loading begins, as a rule, at any weak vibrations with vibration accelerations of at least 0.2g (g is the earth's acceleration) and ends with their almost complete compaction at accelerations of about 1.2–1.5 g.

For the same optimally wet and water-saturated sands, the effective acceleration range is slightly higher - from 0.5g to 2g. In the presence of external surcharge from the surface or when the sand is in a clamped state inside the soil massif, its compaction begins only with a certain critical acceleration equal to 0.3–0.4 g, above which the compaction process develops more intensively.

At about the same time and almost exactly the same results on sands and gravel were obtained in the experiments of the Dynapac company, in which it was also shown using a paddle impeller that the shear resistance of these materials at the moment of their vibration can be reduced by 80–98% ...

On the basis of such data, it is possible to construct two curves - changes in critical accelerations and attenuation of the accelerations of soil particles acting from a vibrating plate or vibratory drum with distance from the surface where the source of vibrations is located. The intersection of these curves will give the depth of interest for effective compaction of sand or gravel.

Rice. 1. Curves of decay of vibration acceleration
sand particles when compacted with a DU-14 roller

In fig. 1 shows two curves of the decay of the vibration acceleration of sand particles, recorded by special sensors, when it is compacted with a trailed vibratory roller. DU-14(D-480) at two working speeds. If we take a critical acceleration of 0.4–0.5 g for sand inside the soil massif, then it follows from the graph that the thickness of the worked layer with such a light vibratory roller is 35–45 cm, which has been repeatedly confirmed by field density control.

Insufficiently or poorly compacted loose loose fine-grained (sandy, sandy-gravel) and even coarse-grained (rocky-coarse-gravel, gravel-pebble) soils, laid in the subgrade of transport structures, rather quickly reveal their low strength and stability in conditions of various kinds of shocks, impacts , vibrations that can occur during the movement of heavy freight road and rail transport, during the operation of all kinds of percussion and vibration machines for driving, for example, piles or vibration compaction of layers of road pavements, etc.

The frequency of vertical vibrations of the elements of the road structure when a truck travels at a speed of 40–80 km / h is 7–17 Hz, and a single blow of a tamper plate weighing 1–2 tons on the surface of an earth embankment excites it as vertical vibrations with a frequency of 7–10 to 20-23 Hz, and horizontal vibrations with a frequency of about 60% of the vertical.

In soils that are not sufficiently stable and sensitive to vibrations and shocks, such vibrations can cause deformations and noticeable settlements. Therefore, it is not only advisable, but also necessary to compact them with vibrational or any other dynamic influences, creating vibrations, shaking and stirring of particles in them. And it is completely pointless to compact such soils by static rolling, which can often be observed on serious and large road, railway and even hydraulic facilities.

Numerous attempts to compact low-moisture uniform sands in the embankments of railways and highways and airfields in the oil and gas regions of Western Siberia, on the Belarusian section of the Brest-Minsk-Moscow highway and at other sites, in the Baltic States, the Volga region, the Komi Republic and the Leningrad Region with pneumatic rollers. did not give the required density results. Only the appearance of trailed vibratory rollers at these construction sites A-4, A-8 and A-12 helped to cope with this acute problem at one time.

The situation with the compaction of loose coarse-grained rocky-large-block and gravel-pebble soils may turn out to be even clearer and sharper in its unpleasant consequences. The construction of embankments, including heights of 3–5 m and even more, from such solid and resistant to any weather and climatic manifestations of soils with their conscientious rolling with heavy pneumatic rollers (25 tons), it would seem, did not give serious reasons for concern to the builders, for example, one of the Karelian sections of the Kola federal highway (St. Petersburg – Murmansk) or the “famous” in the USSR railway Baikal-Amur Mainline (BAM).

However, immediately after they were put into operation, uneven local subsidence of incorrectly compacted embankments began to develop, amounting to 30–40 cm in some places of the road and distorting the general longitudinal profile of the BAM railway to a "sawtooth" with a high accident rate.

Despite the similarity general properties and behavior of fine-grained and coarse-grained loose soils in embankments, their dynamic compaction should be performed by vibration rollers of different weight, dimensions and vibration intensity.

One-size sands without dust and clay are very easily and quickly repackaged even with minor shocks and vibrations, but they have low shear resistance and very low traffic on them by wheeled or roller machines. Therefore, they should be compacted with lightweight and large-sized vibratory rollers and vibratory plates with low contact static pressure and medium-intensity vibration impact, so that the thickness of the compacted layer does not decrease.

The use of trailed vibratory rollers on one-size sands of medium A-8 (weight 8 t) and heavy A-12 (11.8 t) led to excessive immersion of the drum in the embankment and squeezing out the sand from under the roller, with the formation of not only a shaft of soil in front of it, but and moving due to the "bulldozer effect" of a shear wave, visible to the eye at a distance of 0.5–1.0 m. As a result, the near-surface zone of the embankment to a depth of 15–20 cm turned out to be loosened, although the density of the underlying layers had a compaction coefficient of 0.95 and even higher. In light vibratory rollers, the loosened near-surface zone can decrease to 5–10 cm.

Obviously, it is possible, and in some cases advisable, to use medium and heavy vibratory rollers on such one-size sands, but with an intermittent drum surface (cam or lattice), which will improve the passability of the roller, reduce sand shift and reduce the loosened zone to 7-10 cm. This is evidenced by the author's successful experience in compaction of embankments from such sands in winter and summer in Latvia and the Leningrad region. even with a static trailed roller with a lattice drum (weight 25 t), which ensured the thickness of the embankment compacted to 0.95 up to 50–55 cm, as well as the positive results of compaction with the same roller of one-size dune (fine and completely dry) sands in Central Asia.

Coarse-grained rocky-coarse-grained and gravel-pebble soils, as practical experience shows, are also successfully compacted with vibratory rollers. But due to the fact that in their composition there are, and sometimes predominate, large pieces and blocks up to 1.0-1.5 m in size and more, it is not so something easy and simple.

Therefore, on such soils, large, heavy, durable smooth-drum vibratory rollers with a sufficient intensity of vibration impact should be used with a weight of a trailed model or a vibro-drum module for an articulated version of at least 12-13 tons.

The thickness of the worked layer of such soils with such rollers can reach 1–2 m. This kind of dumping is practiced mainly on large hydraulic engineering and airfield construction sites. They are rare in the road industry, and therefore there is no particular need and expediency for road workers to purchase smooth drum rollers with a working vibratory drum module weighing more than 12-13 tons.

Much more important and serious for the Russian road industry is the task of compacting fine-grained mixed (sand with one or another amount of dust and clay impurities), just silty and cohesive soils, which are more common in everyday practice than rocky-coarse-grained and their varieties.

Especially a lot of troubles and troubles arise for contractors with dusty sands and purely dusty soils, which are quite widespread in many places in Russia.

The specificity of these non-plastic, low-cohesive soils is that with their high moisture content, and such waterlogging is primarily the North-West region, under the influence of traffic or the compacting effect of vibratory rollers, they turn into a "liquefied" state due to their low filtration capacity and the resulting increase in pore pressure with excess moisture.

With a decrease in moisture content to the optimum, such soils are relatively easily and well compacted by medium and heavy smooth-drum vibratory rollers with a vibratory drum module weighing 8-13 tons, for which the backfill layers compacted to the required standards can be 50-80 cm (in a waterlogged state, the layer thicknesses decrease to 30- 60 cm).

If a noticeable amount of clay impurities (at least 8-10%) appears in sandy and silty soils, they begin to show significant cohesion and plasticity and, in terms of their compaction capacity, approach clay soils, which are very poorly or not at all amenable to deformation by a purely vibration method.

The research of Professor N.Ya. Kharkhuta showed that when compacting practically clean sands in this way (impurities of dust and clay less than 1%) optimal thickness the layer compacted to a coefficient of 0.95 can reach 180-200% of the minimum size of the contact area of ​​the working body of the vibrating machine (vibrating plate, vibratory drum with sufficient contact static pressure). With an increase in the content of these particles in the sand to 4–6%, the optimal thickness of the worked layer is reduced by 2.5–3 times, and at 8–10% and more, it is impossible to achieve a compaction coefficient of 0.95.

Obviously, in such cases, it is advisable or even necessary to switch to the power method of compaction, i.e. on the use of modern heavy vibratory rollers operating in vibro-shock mode and capable of creating 2-3 times more high pressures than, for example, static pneumatic rollers with a ground pressure of 6-8 kgf / cm 2.

For the expected force deformation and the corresponding soil compaction to occur, the static or dynamic pressures created by the working body of the compacting machine should be as close as possible to the compressive and shear strengths of the soil (about 90–95%), but not exceed it. Otherwise, shear cracks, gaps and other traces of soil destruction will appear on the contact surface, which, moreover, will worsen the conditions for transferring the pressures necessary for compaction to the underlying layers of the embankment.

The strength of cohesive soils depends on four factors, three of which relate directly to the soils themselves (particle size distribution, moisture and density), and the fourth (the nature or dynamics of the applied load and assessed by the rate of change in the stress state of the soil or, with some inaccuracy, the duration of this load ) refers to the effect of the compacting machine and the rheological properties of the soil.

Cam vibratory roller
by BOMAG

With an increase in the content of clay particles, the strength of the soil increases up to 1.5-2 times compared to sandy soils... The actual moisture content of cohesive soils is a very important indicator that affects not only the strength, but also their compaction. In the best way, such soils are compacted at the so-called optimal moisture content. When the actual moisture content exceeds this optimum, the strength of the soil decreases (up to 2 times) and the limit and degree of its possible compaction are significantly reduced. On the contrary, with decreasing humidity below optimal level tensile strength increases sharply (at 85% of the optimal - 1.5 times, and at 75% - up to 2 times). This is why it is so difficult to compact low-moisture cohesive soils.

As the soil compresses, so does its strength. In particular, when the compaction factor in the embankment reaches 0.95, the strength of the cohesive soil increases by 1.5–1.6 times, and at 1.0, by 2.2–2.3 times compared to the strength at the initial moment of compaction ( compaction ratio 0.80–0.85).

In clayey soils with pronounced rheological properties due to their viscosity, the dynamic compressive strength can increase by 1.5–2 times at a loading time of 20 ms (0.020 s), which corresponds to the frequency of application of a vibro-shock load of 25–30 Hz, and for shear - even up to 2.5 times the static strength. In this case, the dynamic modulus of deformation of such soils increases up to 3–5 times or more.

This indicates the need to apply to cohesive soils higher compaction pressures of a dynamic nature than a static one in order to obtain the same deformation and compaction result. Obviously, therefore, some cohesive soils could be effectively compacted with static pressures of 6–7 kgf / cm 2 (pneumatic rollers), and when switching to their compaction, dynamic pressures of the order of 15–20 kgf / cm 2 were required.

This difference is due to the different rate of change in the stress state of cohesive soil, with a growth of 10 times its strength increases 1.5–1.6 times, and 100 times - up to 2.5 times. For a pneumatic-wheeled roller, the rate of change in contact pressure over time is 30–50 kgf / cm 2 * sec, for rammers and vibratory rollers - about 3000–3500 kgf / cm 2 * sec, i.e. the increase is 70-100 times.

For the correct assignment of the functional parameters of vibratory rollers at the time of their creation and for control technological process performing by these vibratory rollers the very operation of compacting coherent and other types of soils is extremely important and it is necessary to know not only the qualitative influence and tendencies of changes in the tensile strength and deformation moduli of these soils, depending on their grain size composition, moisture, density and dynamic load, but also have specific values ​​of these indicators ...

Such approximate data on the ultimate strength of soils with a density coefficient of 0.95 under their static and dynamic loading were established by Professor N. Ya. Kharkhuta (Table 1).

Table 1
Strength limits (kgf / cm 2) of soils with a compaction coefficient of 0.95
and optimal humidity

It is pertinent to note that with an increase in density to 1.0 (100%), the dynamic compressive strength of some highly cohesive clays of optimum moisture content will increase to 35–38 kgf / cm 2. With a decrease in humidity to 80% of the optimum, which can be in warm, hot or arid places in a number of countries, their strength can reach even greater values ​​- 35–45 kgf / cm 2 (density 95%) and even 60–70 kgf / cm 2 (100%).

Of course, such high-strength soils can only be compacted with heavy vibration-impact cam rollers. The contact pressures of smooth-drum vibratory rollers, even for ordinary loams, the optimum moisture content will obviously not be enough to obtain the compaction result required by the standards.

Until recently, the assessment or calculation of contact pressures under a smooth or cam-shaped drum of a static and vibration roller was made in a very simplified and approximate way according to indirect and not very substantiated indicators and criteria.

Based on the theory of vibrations, the theory of elasticity, theoretical mechanics, mechanics and dynamics of soils, the theory of dimensions and similarity, the theory of permeability of wheeled vehicles and the study of the interaction of the roll die with the surface of the compacted linearly deformable layer asphalt mix, crushed stone base and subgrade soil, a universal and rather simple analytical dependence was obtained to determine the contact pressures under any working body of a wheel or drum type roller (pneumatic tire wheel, smooth hard, rubberized, cam, lattice or ribbed drum):

σ o - maximum static or dynamic pressure of the drum;
Q in - the weight load of the roller module;
R o - the total force of the impact of the roller with its vibrodynamic loading;
R o = Q in K d
E o - static or dynamic modulus of deformation of the compacted material;
h is the thickness of the material layer being compacted;
B, D - width and diameter of the drum;
σ p - ultimate strength (fracture) of the compacted material;
K d - dynamic factor

More detailed methodology and explanations to it are set out in a similar collection-catalog "Road equipment and technology" for 2003. Here it is only appropriate to point out that, in contrast to smooth drum rollers, when determining the total material surface settlement δ 0, maximum dynamic force R 0 and contact pressure σ 0 for cam, trellis and ribbed rollers, the equivalent smooth-roller width of their rollers is used, and for pneumatic and rubber-lined rollers - an equivalent diameter.

Table 2 shows the results of calculations according to the specified method and analytical dependencies of the main indicators of the dynamic effect, including contact pressures, of smooth-drum and cam vibratory rollers of a number of companies in order to analyze their compacting ability when filling one of the possible types of fine-grained soils with a layer of 60 cm into the subgrade (in loose and in a dense state, the compaction coefficient is 0.85–0.87 and 0.95–0.96, respectively, the deformation modulus is E 0 = 60 and 240 kgf / cm 2, and the value of the real vibration amplitude of the drum is also, respectively, a = A 0 / A ∞ = 1.1 and 2.0), i.e. all rollers have the same conditions for the manifestation of their compacting abilities, which gives the results of calculation and comparison the necessary correctness.

CJSC "VAD" has in its park a whole range of properly and efficiently working soil compacting smooth-drum vibratory rollers of the "Dynapac" company, starting from the lightest ( CA152D) and ending with the most difficult ( CA602D). Therefore, it was useful to obtain the calculated data for one of these rollers ( CA302D) and compare with the data of three models of the Hamm company, similar and close in weight, created according to a peculiar principle (by increasing the weight of the oscillating drum without changing its weight and other vibration indicators).

Table 2 also shows some of the largest vibratory rollers from two companies ( Bomag, Orenstein and Koppel), including cam analogs, and models of trailed vibratory rollers (A-8, A-12, PVK-70EA).

table 2

Data analysis table. 2 allows us to draw some conclusions and conclusions, including a practical plan:

• created by vibrating rollers, including medium weight (CA302D, Hamm 3412 and 3414 ), dynamic contact pressures significantly exceed (on compacted soils by 2 times) the pressures of heavy static rollers (pneumatic type, weighing 25 tons and more), therefore they are able to compact disjoint, poorly connected and light cohesive soils quite effectively and with a layer thickness acceptable for road workers;
• cam vibratory rollers, including the largest and heaviest, in comparison with their smooth roller counterparts, can create 3 times higher contact pressures (up to 45-55 kgf / cm 2), and therefore they are suitable for successful compaction of highly bonded and sufficiently strong heavy loams and clays, including their varieties with low humidity; analysis of the capabilities of these vibratory rollers in terms of contact pressures shows that there are certain prerequisites to slightly increase these pressures and increase the thickness of layers of cohesive soils compacted by their large and heavy models, up to 35–40 cm instead of the current 25–30 cm;
• The experience of Hamm in creating three different vibratory rollers (3412, 3414 and 3516) with the same vibration parameters (vibrating drum mass, amplitude, frequency, centrifugal force) and different total mass of the vibratory drum module due to the frame weight should be considered interesting and useful, but not 100%, and above all from the point of view of the insignificant difference in the dynamic pressures created by the rollers, for example, in 3412 and 3516; but on the other hand, in 3516, the pause time between loading pulses is reduced by 25–30%, increasing the contact time of the drum with the soil and increasing the efficiency of energy transfer to the latter, which contributes to the penetration into the depth of the soil of a higher density;
• on the basis of comparing vibratory rollers by their parameters or even according to the results of practical tests, it is incorrect, and hardly fair, to assert that this roller is generally better, and the other is bad; each model may be worse or, conversely, good and suitable for its specific conditions of use (type and condition of the soil, thickness of the compacted layer); we only have to regret that so far no samples of vibratory rollers have appeared with more universal and adjustable compaction parameters for use in a wider range of types and conditions of soils and thicknesses of the dumped layers, which could save the road builder from the need to purchase a set of soil compactors different types by weight, dimensions and sealing capacity.

Some of the conclusions drawn may seem not so new and even already known from practical experience. Including the uselessness of using smooth-drum vibratory rollers for compacting cohesive soils, especially low-moisture ones.

At one time, the author worked out at a special landfill in Tajikistan the technology of compaction of Langar loam, laid in the body of one of the highest dams (300 m) of the now operating Nurek hydroelectric power station. The composition of loam included from 1 to 11% sandy, 77–85% silty and 12–14% clay particles, the plasticity number was 10–14, the optimal moisture content was about 15.3–15.5%, the natural moisture content was only 7– 9%, i.e. did not exceed 0.6 of the optimal value.

Compaction of loam was carried out by various rollers, including a very large trailed vibratory roller specially designed for this construction site. PVK-70EA(22 t, see Table 2), which had rather high vibration parameters (amplitude 2.6 and 3.2 mm, frequency 17 and 25 Hz, centrifugal force 53 and 75 tf). However, due to the low soil moisture, the required compaction of 0.95 was obtained with this heavy roller only in a layer of no more than 19 cm.

More efficiently and successfully this roller, as well as A-8 and A-12, compacted loose gravel and pebble materials, laid in layers up to 1.0–1.5 m.

Based on the measured stresses by special sensors placed in the embankment at different depths, a curve of the damping of these dynamic pressures along the depth of the soil compacted by the three indicated vibratory rollers was plotted (Fig. 2).

Rice. 2. Damping curve of experimental dynamic pressures

Despite rather significant differences in total weight, dimensions, vibration parameters and contact pressures (the difference reached 2–2.5 times), the values ​​of the experimental pressures in the soil (in relative units) turned out to be close and obey the same regularity (the dashed curve in the graph in Fig. 2) and the analytical dependence shown in the same graph.

It is interesting that exactly the same dependence is inherent in the experimental stress decay curves under purely shock loading of the soil massif (ramming plate with a diameter of 1 m and a weight of 0.5–2.0 tons). In both cases, the exponent α remains unchanged and equal to or close to 3/2. Only the coefficient K changes in accordance with the nature or "severity" (aggressiveness) of the dynamic load from 3.5 to 10. With more "acute" loading of the soil, it is more, with "sluggish" - less.

This coefficient K serves as a kind of "regulator" of the degree of stress damping along the depth of the soil. With its high value, the stresses decrease faster, with distance from the loading surface, and the thickness of the soil layer being worked out decreases. With decreasing K, the character of the decay becomes smoother and approaches the curve of decay of static pressures (in Fig. 2 for Boussinet α = 3/2 and K = 2.5). In this case, higher pressures "penetrate" into the depths of the soil, and the thickness of the compaction layer increases.

The nature of the impulse effects of vibratory rollers does not vary very much, and it can be assumed that the values ​​of K will be in the range of 5–6. And with a known and close to stable attenuation of the relative dynamic pressures under vibratory rollers and certain values ​​of the required relative stresses (in fractions of the ultimate strength of the soil) inside the soil embankment, it is possible, with a sufficient degree of probability, to establish the thickness of the layer in which the pressure acting there will ensure the implementation of the coefficient seals, for example 0.95 or 0.98.

Practice, test compaction and numerous studies, the approximate values ​​of such soil pressures are established and are presented in table. 3.

Table 3

There is also a simplified technique for determining the thickness of the compacted layer with a smooth-roller vibratory roller, according to which each ton of the weight of the vibratory roller module is capable of providing approximately the following layer thickness (with optimal soil moisture and the required vibratory roller parameters):

• sands are large, medium, ASG - 9–10 cm;
• fine sands, including sands with dust - 6–7 cm;
• light and medium sandy loam - 4–5 cm;
• light loams - 2-3 cm.

Conclusion. Modern smooth drum and cam vibratory rollers are effective soil compactors capable of providing the required quality of the erected subgrade. The task of the road builder is to competently comprehend the possibilities and features of these means for the correct orientation in their choice and practical application.

The need for knowledge of the exact density of bulk building materials arises during their transportation, ramming, filling containers and pits and the selection of proportions when preparing mortars. One of the indicators taken into account is the compaction coefficient, which characterizes the compliance of the stacked interlayers with the requirements of standards or the degree of reduction in the volume of sand during transportation. The recommended value is indicated in the design documentation and depends on the type of structure being built or the type of work.

The compaction ratio is a standard number that takes into account the degree of reduction in the external volume during delivery and laying with subsequent compaction (you can find information on compaction of crushed stone). In a simplified version, it is found as the ratio of the mass of a certain volume taken during sampling to the reference parameter obtained under laboratory conditions. Its value depends on the type and size of filler fractions and varies from 1.05 to 1.52. In the case of sand for construction works it is 1.15, they are repelled from it when calculating building materials.

As a result, the real volume of supplied sand is determined by multiplying the measurement results by the compaction rate during transportation. The maximum allowable value must be specified in the purchase agreement. The opposite situations are also possible - to check the conscientiousness of the suppliers, the volume is found at the end of the delivery, its quantity in m 3 is divided by the coefficient of sand compaction and is checked against the delivered one. For example, when transporting 50 m 3 after ramming in the body of a car or wagons, no more than 43.5 will be brought to the facility.

Factors influencing the coefficient

The given number is the average, in practice it depends on many different criteria. These include:

• Sand grain size, purity and other physical and chemical properties determined by the location and method of extraction. The characteristics of the source of production can change over time, as the quarries are excavated, the looseness of the remaining layers increases; to eliminate errors, the bulk density and related parameters are periodically checked in laboratory conditions.
• Transportation conditions (distance to the object, climatic and seasonal factors, type of transport used). The stronger and longer the vibration affects the material, the more efficiently the compaction of sand is carried out, the maximum compaction is achieved when it is moved by road transport, a little less - during rail transport, and the minimum - during sea transport. At the right conditions transport exposure to moisture and sub-zero temperatures reduced to a minimum.

These factors should be checked immediately, the values ​​of the indicators of permissible natural humidity and bulk density are prescribed in the passport. Additional volumes of bulk solids caused by losses during transportation depend on the delivery distance and are taken equal to 0.5% within 1 km, 1% - over this parameter.

Using the coefficient in the preparation of sand cushions and road construction

A characteristic feature of any bulk building materials is the change in volume when unloading in a free area or its ramming. In the first case, the sand or soil becomes loose, during storage, the particles settle and adhere to each other with practically no voids, but still do not comply with the normative ones. At the last stage - the laying and distribution of the compositions at the bottom of the pit, the coefficient of relative compaction of sand is taken into account. It is a criterion for the quality of work carried out in the preparation of trenches and construction sites and varies from 0.95 to 1, the exact value depends on the intended purpose of the layer and the method of filling and tamping. It is determined by calculation and must be indicated in the project documentation.

The compaction of the backfilled soil is considered the same compulsory action, as well as when laying a sand cushion under the foundations of buildings or when arranging a roadbed. For achievement desired effect special equipment is used - rollers, vibrating plates and vibrating stamps, in the absence of it, ramming is carried out with a hand tool or with feet. The maximum permissible thickness of the processed layer and the required number of passes refer to the tabular values, the same applies to the recommended minimum filling over pipes or utilities.

In the process of compaction of sand or soil, their bulk density increases, and the volumetric area inevitably decreases. This must be taken into account when calculating the amount of purchased material, along with the total losses due to weathering or the amount of stock. When choosing a compaction method, it is important to remember that any external mechanical influences affect only the upper layers; vibration equipment is required to obtain a coating with the desired quality.

Crushed stone is a common building material that is obtained by crushing hard rock. Raw materials are extracted by blasting during quarrying. The breed is crushed into appropriate fractions. In this case, the special coefficient of compaction of crushed stone is important.

Granite is the most common, since its frost resistance is high, and its water absorption is low, which is so important for any building structure. Abrasion and strength of crushed granite meets the standards. Among the main fractions of crushed stone: 5-15 mm, 5-20 mm, 5-40 mm, 20-40 mm, 40-70 mm. The most popular is crushed stone with a fraction of 5-20 mm, it can be used for various works:

• construction of foundations;
• production of ballast layers for tracks and railways;
• additive to building mixtures.

Compaction of crushed stone depends on many parameters, including its characteristics. Should be considered:

1. The average density is 1.4-3 g / cm³ (when compaction is calculated, this parameter is taken as one of the main ones).
2. Flakiness determines the level of the plane of the material.
3. All material is sorted into fractions.
4. Resistant to frost.
5. Radioactivity level. For all work, you can use crushed stone of the 1st class, but the 2nd class can only be used for road.

Based on these characteristics, a decision is made which material is suitable for a particular type of work.

Types of crushed stone and technical characteristics

Crushed stone for construction can be used in various ways. Manufacturers offer different types of it, the properties of which differ from each other. Today, according to the type of raw material, it is customary to divide crushed stone into 4 large groups:

• gravel;
• granite;
• dolomite, i.e. limestone;
• secondary.

For the manufacture of granite material, the appropriate rock is used. It is a non-metallic material that is obtained from hard rock. Granite is solidified magma with great hardness, its processing is difficult. Crushed stone of this type is manufactured in accordance with GOST 8267-93. The most popular is crushed stone, which has a fraction of 5/20 mm, since it can be used for a variety of works, including the manufacture of foundations, roads, platforms and others.

Crushed gravel is a building bulk material that is obtained by crushing rocky rock or rock in quarries. The strength of the material is not as high as that of crushed granite, but its cost is lower, as is the radiation background. Today it is customary to distinguish between two types of gravel:

• crushed type of crushed stone;
• gravel of river and sea origin.

By fraction, gravel is classified into 4 large groups: 3/10, 5/40, 5/20, 20/40 mm. The material is used for the preparation of various building mixtures as a filler, it is considered indispensable for mixing concrete, building foundations, paths.

Limestone crushed stone is made from rock sedimentary rock. As the name implies, the raw material is limestone. The main component is calcium carbonate, the cost of the material is one of the lowest.

The fractions of this crushed stone are divided into 3 large groups: 20/40, 5/20, 40/70 mm.

It is applicable for the glass industry, in the manufacture of small reinforced concrete structures, in the preparation of cement.

Secondary crushed stone has the lowest cost. Make it from construction waste, for example, asphalt, concrete, brick.

The advantage of crushed stone is its low cost, but in terms of its main characteristics it is much inferior to the other three types, therefore it is rarely used and only in cases where strength of great importance does not have.

Back to the table of contents

Compaction Ratio: Purpose

The compaction factor is a special regulatory number determined by SNiP and GOST. This value shows how many times the crushed stone can be compacted, i.e. reduce its outer volume when tamping or transporting. The value is usually 1.05-1.52. According to existing regulations, the compaction factor can be as follows:

• sand and gravel mixture - 1.2;
• construction sand - 1.15;
• expanded clay - 1.15;
• gravel crushed stone - 1.1;
• soil - 1.1 (1.4).

An example of determining the compaction coefficient of crushed stone or gravel can be given as follows:

1. It can be assumed that the density of the mass is 1.95 g / cm³, after the compaction was carried out, the value became equal to 1.88 g / cm³.
2. To determine the value, it is necessary to divide the actual density level by the maximum, which will give the crushed stone compaction coefficient 1.88 / 1.95 = 0.96.

It should be borne in mind that the design data usually indicates not the degree of compaction, but the so-called density of the skeleton, i.e. during the calculations, it is necessary to take into account the level of humidity, other parameters of the building mixture.