Mandatory compaction of soil, crushed stone and asphalt concrete in the road industry is not only integral part technological process of constructing the subgrade, base and coating, but also serves as the main operation to ensure their strength, stability and durability.
Previously (until the 30s of the last century), the implementation of the indicated indicators of soil embankments was also carried out by compaction, but not by mechanical or artificial means, but due to the natural self-settlement of the soil under the influence, mainly, of its own weight and, partly, traffic. The constructed embankment was left, as a rule, for one or two, and in some cases for three years, and only after that the base and surface of the road were built.
However, the rapid motorization of Europe and America that began in those years required the accelerated construction of an extensive network of roads and a revision of the methods of their construction. The technology of roadbed construction that existed at that time did not meet the new challenges that arose and became a hindrance in solving them. Therefore, there is a need to develop the scientific and practical foundations of the theory of mechanical compaction of earthen structures, taking into account the achievements of soil mechanics, and to create new effective soil compaction means.
It was in those years that the physical and mechanical properties of soils began to be studied and taken into account, their compactability was assessed taking into account the granulometric and moisture conditions (the Proctor method, in Russia - the standard compaction method), the first classifications of soils and standards for the quality of their compaction were developed, and methods began to be introduced field and laboratory control of this quality.
Before this period, the main soil-compacting means was a smooth-roller static roller of a trailed or self-propelled type, suitable only for rolling and leveling the near-surface zone (up to 15 cm) of the poured soil layer, and also a manual tamper, which was used mainly for compacting coatings, when repairing potholes and for compaction curbs and slopes.
These simplest and ineffective (in terms of quality, thickness of the layer being worked and productivity) compacting 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, tamping slabs on excavators, multi-hammer tamping machines on a caterpillar tractor and smooth roller, manual explosion-rammers (“jumping frogs”) 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 including the base crawler tractor). Its vibrating plate with an area of 7.5 m2 was located between the tracks, and its engine had a power of 100 hp. allowed the vibration exciter to rotate at a frequency of 1500 kol/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 approximately 80–90 m2/h or not more than 50 m 3 / h with a thickness of the compacted layer of about 0.5 m.
More universal, i.e. capable of compacting various types soils, including cohesive, non-cohesive and mixed, the compaction method has proven itself.
In addition, during compaction, it was easy and simple to regulate the force compacting effect on the soil by changing the height of the fall of the tamping plate or the tamping hammer. Due to these two advantages, the impact compaction method became the most popular and widespread in those years. Therefore, the number of tamping machines and devices multiplied.
It is appropriate to note that in Russia (then the USSR) they also understood the importance and necessity of the transition to mechanical (artificial) compaction of road materials and the establishment of production of compaction equipment. 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 equipment and technology for compacting soil objects proceeded with no less enthusiasm and effectiveness than in pre-war times. Trailed, semi-trailer and self-propelled pneumatic rollers appeared, which for a certain period of time became the main soil-compacting means in many countries of the world. Their weight, including single copies, varied quite widely - from 10 to 50–100 tons, but most of the produced pneumatic roller models 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 (loose and poorly cohesive) after 8–10 passes along the track.
Simultaneously with pneumatic rollers, vibratory soil compactors - vibratory plates, smooth roller and cam vibratory rollers - developed, improved and became increasingly popular, especially in the 50s. Moreover, over time, trailed models of vibratory rollers were replaced by more convenient and technologically advanced ones for performing linear earthworks self-propelled articulated models or, as the Germans called them, “Walzen-Zug” (push-pull).
Smooth vibratory roller CA 402
from DYNAPAC
Each modern model The soil compacting vibratory roller, as a rule, has two versions - with a smooth and a cam drum. At the same time, some companies make two separate interchangeable rollers for the same single-axle pneumatic wheel tractor, while others offer the buyer of the roller, instead of a whole cam roller, only a “shell attachment” with cams, which is easily and quickly fixed on top of a smooth roller. There are also companies that have developed similar smooth roller “shell attachments” for mounting on top of a padded roller.
It should be especially noted that the cams themselves on vibratory rollers, especially after the start of their practical operation in 1960, have undergone significant changes in its geometry and size, which had a beneficial effect on the quality and thickness of the compacted layer and reduced the depth of loosening of the near-surface soil zone.
If earlier “shipfoot” cams were thin (supporting area 40–50 cm2) and long (up to 180–200 mm or more), then their modern analogs “padfoot” have become shorter (height is mainly 100 mm, sometimes 120–150 mm) and thick (supporting area about 135–140 cm 2 with a side size 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 deformation of the soil (for cohesive soil it is 1.6–1.8 times the size of the side of the cam support pad). Therefore, the layer of compaction of loam and clay with a vibrating roller with padfoot cams, when creating the appropriate dynamic pressures and taking into account the 5–7 cm depth of immersion of the cam into the soil, began to be 25–28 cm, which is confirmed by practical measurements. This thickness of the compaction layer is comparable to the compacting ability 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 using vibratory rollers and their higher operational productivity, it becomes clear why trailed and semi-trailed pneumatic wheel rollers for soil compaction began to gradually disappear and are now practically not produced or are rarely and rarely produced.
Thus, in modern conditions The main soil-compacting means in the road industry of the vast majority of countries in the world has become a self-propelled single-drum vibratory roller, articulated with a single-axle pneumatic-wheeled tractor and having a smooth (for non-cohesive and poorly cohesive fine-grained and coarse-grained soils, including rocky-coarse-grained soils) or a cam roller ( cohesive soils).
Today in the world there are more than 20 companies producing about 200 models of such soil compaction rollers of various sizes, differing from each other in total weight (from 3.3–3.5 to 25.5–25.8 tons), weight of the vibrating drum module (from 1 ,6–2 to 17–18 t) and its dimensions. There are also some differences in the design 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 for a road worker: how to choose the right model of such a roller and how to most effectively use it to carry out high-quality soil compaction at a specific practical site and at the lowest cost.
When resolving such issues, it is necessary to first, but quite accurately, establish those predominant 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, you should pay attention to the presence of dusty (0.05–0.005 mm) and clayey (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 soil compaction, possible way its seals (pure vibration or power vibration-impact) will allow you to choose a vibratory roller with a smooth or padded drum. Soil moisture and the amount of dust and clay particles significantly affect its strength and deformation properties, and, consequently, the necessary compacting ability of the selected roller, i.e. its ability to provide the required compaction coefficient (0.95 or 0.98) in the soil backfill layer specified by the roadbed construction technology.
Most modern vibratory rollers operate in a certain vibration-impact 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 (oscillations, 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 repacking 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 frequent impact loads.
In the compaction of loose, non-cohesive 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 physical-mechanical, electrochemical and water-colloidal adhesion between small particles is significantly higher and predominant, the main acting factor is the force of pressure or compressive and shear stress, and the role of the first factor becomes secondary.
Research by Russian specialists in soil mechanics and dynamics at one time (1962–64) showed that compaction of dry or almost dry sand in the absence of external loading begins, as a rule, with any weak vibrations with vibration accelerations of at least 0.2g (g – earth acceleration) and ends with almost complete compaction at accelerations of about 1.2–1.5 g.
For the same optimally wet and water-saturated sands, the range of effective accelerations is slightly higher - from 0.5g to 2g. In the presence of an external load from the surface or when the sand is in a clamped state inside the soil mass, 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 sand and gravel were obtained in experiments by the Dynapac company, in which, using a bladed impeller, it was also shown that the shear resistance of these materials when vibrating can be reduced by 80–98% .
Based on such data, two curves can be constructed - changes in critical accelerations and attenuation of soil particle accelerations acting from a vibrating plate or vibrating drum with distance from the surface where the source of vibrations is located. The intersection point of these curves will give the effective compaction depth of interest for the sand or gravel.
Rice. 1. Damping curves of vibration acceleration
sand particles during compaction with a DU-14 roller
In Fig. Figure 1 shows two decay curves of the acceleration of oscillations of sand particles, recorded by special sensors, during its compaction with a trailed vibratory roller DU-14(D-480) at two operating speeds. If we accept a critical acceleration of 0.4–0.5 g for sand inside a soil mass, then it follows from the graph that the thickness of the layer being processed with such a light vibratory roller is 35–45 cm, which has been repeatedly confirmed by field density monitoring.
Insufficiently or poorly compacted loose non-cohesive fine-grained (sand, sand-gravel) and even coarse-grained (rock-coarse-clastic, gravel-pebble) soils laid in the roadbed of transport structures quite quickly reveal their low strength and stability under conditions of various types of shocks and impacts , vibrations that can occur when driving a heavy truck and railway transport, when working with all kinds of drums and vibration machines for driving, for example, piles or vibration compaction of layers of road pavements, etc.
The frequency of vertical vibrations of road structure elements when a truck passes at a speed of 40–80 km/h is 7–17 Hz, and a single impact of a tamping slab weighing 1–2 tons on the surface of a soil embankment excites vertical vibrations in it with a frequency of 7–10 to 20–23 Hz, and horizontal vibrations with a frequency of about 60% of vertical ones.
In soils that are not sufficiently stable and sensitive to vibrations and shaking, such vibrations can cause deformations and noticeable precipitation. Therefore, it is not only advisable, but also necessary to compact them by vibration or any other dynamic influences, creating vibrations, shaking and movement of particles in them. And it is completely pointless to compact such soils by static rolling, which could often be observed at serious and large road, railway and even hydraulic facilities.
Numerous attempts to compact low-moisture, one-dimensional sands with pneumatic rollers in embankments of railways, highways and airfields in oil and gas regions 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. 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 the time.
The situation with the compaction of loose coarse-grained rock-coarse-block and gravel-pebble soils may be even more obvious and more acute in its unpleasant consequences. The construction of embankments, including those with a height of 3–5 m or even more, from such soils that are strong and resistant to any weather and climatic conditions 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 federal highway “Kola” (St. Petersburg–Murmansk) or the “famous” Baikal-Amur Mainline (BAM) railway in the USSR.
However, immediately after they were put into operation, uneven local subsidence of improperly 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 track to a “sawtooth” with a high accident rate.
Despite the similarity of the general properties and behavior of fine-grained and coarse-grained loose soils in embankments, their dynamic compaction should be carried out using vibrating rollers of different weights, dimensions and intensity of vibration effects.
Single-sized sands without dust and clay impurities are very easily and quickly repacked even with minor shocks and vibrations, but they have insignificant shear resistance and very low permeability of wheeled or roller machines. Therefore, they should be compacted using light-weight and large-sized vibratory rollers and vibrating 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 single-size sands of medium A-8 (weight 8 tons) and heavy A-12 (11.8 tons) led to excessive immersion of the drum into the embankment and squeezing out sand from under the roller with the formation in front of it of not only a bank of soil, but and a shear wave moving due to the “bulldozer effect”, visible to the eye at a distance of up to 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. With light vibratory rollers, the loosened 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 same-sized sands, but with an intermittent roller surface (cam or lattice), which will improve the roller's permeability, reduce sand shear and reduce the loosening zone to 7–10 cm. This is evidenced by the author’s successful experience in compacting embankments of such sands in winter and summer in Latvia and the Leningrad region. even with a static trailed roller with a lattice drum (weight 25 tons), which ensured the thickness of the embankment layer compacted to 0.95 was up to 50–55 cm, as well as positive results of compaction with the same roller of one-size dune (fine and completely dry) sands in Central Asia.
Coarse-grained rock-coarse-clastic 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 measuring up to 1.0–1.5 m or more, it is not possible to move, stir and move them, thereby ensuring the required density and stability of the entire embankment. -easy and simple.
Therefore, on such soils, large, heavy, durable smooth roller vibratory rollers with sufficient intensity of vibration impact should be used, weighing a trailed model or a vibrating roller module for an articulated version of at least 12–13 tons.
The thickness of the layer of such soils processed by such rollers can reach 1–2 m. This kind of filling is practiced mainly at large hydraulic engineering and airfield construction sites. They are rare in the road industry, and therefore there is no particular need or advisability for road workers to purchase smooth rollers with a working vibratory roller 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 varying amounts of dust and clay), simply silty and cohesive soils, which are more often encountered in everyday practice than rocky-coarse-clastic soils and their varieties.
Particularly a lot of trouble and trouble arises for contractors with silty sands and purely silty soils, which are quite widespread in many places in Russia.
The specificity of these non-plastic, low-cohesion soils is that when their humidity is high, and the North-Western region is primarily “sinned” by such waterlogging, under the influence of vehicle traffic or the compacting effect of vibratory rollers, they pass into a “liquefied” state due to their low filtration capacity and the resulting increase in pore pressure with excess moisture.
With a decrease in humidity to the optimum, such soils are relatively easily and well compacted by medium and heavy smooth-roller vibratory rollers with a vibratory-roller module weight of 8–13 tons, for which the layers of filling compacted to the required standards can be 50–80 cm (in a waterlogged state, the thickness of the layers is reduced to 30– 60 cm).
If a noticeable amount of clay impurities (at least 8–10%) appears in sandy and silty soils, they begin to exhibit significant cohesion and plasticity and, in their ability to compact, approach clayey soils, which are very poorly or not at all susceptible to deformation by purely vibrational methods.
Research by Professor N. Ya. Kharkhuta has shown that when almost pure sands are compacted in this way (impurities of dust and clay less than 1%), the optimal thickness of the layer compacted to a coefficient of 0.95 can reach 180–200% of minimum size contact area of the working body of the vibrating machine (vibrating plate, vibrating drum with sufficient contact static pressures). With an increase in the content of these particles in the sand to 4–6%, the optimal thickness of the layer being worked is reduced by 2.5–3 times, and at 8–10% or more it is generally impossible to achieve a compaction coefficient of 0.95.
Obviously, in such cases it is advisable or even necessary to switch to a force compaction method, i.e. for the use of modern heavy vibratory rollers operating in vibro-impact mode and capable of creating 2–3 times more high pressure than, for example, static pneumatic rollers with a ground pressure of 6–8 kgf/cm 2.
In order for the expected force deformation and corresponding compaction of the soil to occur, the static or dynamic pressures created by the working body of the compaction machine must be as close as possible to the compressive and shear strength limits of the soil (about 90–95%), but not exceed it. Otherwise, shear cracks, bulges and other traces of soil destruction will appear on the contact surface, which will also worsen the conditions for transmitting 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 (grain size distribution, moisture and density), and the fourth (the nature or dynamism of the applied load and estimated by the rate of change in the stressed state of the soil or, with some inaccuracy, the time of action of this load ) refers to the effect of the compaction machine and the rheological properties of the soil.
Cam vibratory roller
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 their strength, but also their compactability. In the best possible way Such soils are compacted at the so-called optimal moisture content. As the actual humidity exceeds this optimum, the strength of the soil decreases (up to 2 times) and the limit and degree of its possible compaction significantly decreases. On the contrary, with a decrease in humidity below the optimal level, the tensile strength increases sharply (at 85% of the optimum - 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 compacts, its strength also increases. In particular, when the compaction coefficient in the embankment reaches 0.95, the strength of 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 coefficient 0.80–0.85).
U clay soils, which have pronounced rheological properties due to their viscosity, the dynamic compressive strength can increase by 1.5–2 times with a loading time of 20 ms (0.020 sec), which corresponds to a frequency of application of a vibration-impact load of 25–30 Hz, and shear strength - even up to 2.5 times compared to 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 higher dynamic compaction pressures to cohesive soils than static ones 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 an increase of 10 times its strength increases by 1.5–1.6 times, and by 100 times – up to 2.5 times. For a pneumatic 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 correct purpose functional parameters of vibratory rollers at the time of their creation and for control technological process When these vibratory rollers perform the very operation of compacting cohesive and other types of soils, it is extremely important and it is necessary to know not only the qualitative influence and trends in changes in the strength limits and deformation moduli of these soils depending on their granular composition, humidity, density and dynamic load, but also to have specific values of these indicators .
Such indicative data on the strength limits of soils with a density coefficient of 0.95 under static and dynamic loading were established by Professor N. Ya. Kharkhuta (Table 1).
Table 1
Strength limits (kgf/cm2) of soils with a compaction coefficient of 0.95
and optimal humidity
It is appropriate to note that with an increase in density to 1.0 (100%), the dynamic compressive strength of some highly cohesive clays of optimal moisture will increase to 35–38 kgf/cm2. When the humidity decreases to 80% of the optimum, which can happen in warm, hot or dry 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 cm 2 (100%).
Of course, such high-strength soils can only be compacted with heavy vibro-impact pad rollers. The contact pressures of smooth drum vibratory rollers, even for ordinary loams of optimal moisture content, will be clearly insufficient to obtain the compaction result required by the standards.
Until recently, the assessment or calculation of contact pressures under a smooth or padded roller of a static and vibrating roller was carried out very simply and approximately using 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 cross-country ability of wheeled vehicles and the study of the interaction of a roller die with the surface of a compacted linearly deformable layer of asphalt concrete mixture, crushed stone base and subgrade soil, a universal and quite a simple analytical relationship for determining the contact pressures under any working part of a wheeled or roller-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 – weight load of the roller module;
R o is the total impact force of the roller under vibrodynamic loading;
R o = Q in K d
E o – static or dynamic modulus of deformation of the compacted material;
h – thickness of the compacted layer of material;
B, D – width and diameter of the roller;
σ p – ultimate strength (fracture) of the compacted material;
K d – dynamic coefficient
A more detailed methodology and explanations for it are presented 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 settlement of the material surface δ 0, the maximum dynamic force R 0 and the contact pressure σ 0 for cam, lattice and ribbed rollers, the width of their rollers equivalent to the smooth drum roller is used , and for pneumatic and rubber rollers - the equivalent diameter.
In table Figure 2 presents the results of calculations using the specified method and analytical dependencies of the main indicators of dynamic impact, including contact pressures, smooth drum and cam vibratory rollers from a number of companies in order to analyze their compaction ability when pouring into the roadbed one of the possible types of fine-grained soils with a layer of 60 cm (in loose and in a dense state, the compaction coefficient is equal to 0.85–0.87 and 0.95–0.96, respectively, the deformation modulus E 0 = 60 and 240 kgf/cm 2, and the value of the real amplitude of vibration of the roller 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 calculation results and their comparison the necessary correctness.
JSC "VAD" has in its fleet a whole range of properly and efficiently working soil-compacting smooth drum vibratory rollers from Dynapac, starting from the lightest ( CA152D) and ending with the heaviest ( CA602D). Therefore, it was useful to obtain calculated data for one of these skating rinks ( CA302D) and compare with data from three Hamm models similar and similar in weight, created according to a unique principle (by increasing the load of the oscillating roller without changing its weight and other vibration indicators).
In table 2 also shows some of the largest vibratory rollers from two companies ( Bomag, Orenstein and Koppel), including their cam analogues, and models of trailed vibratory rollers (A-8, A-12, PVK-70EA).
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Table 2
Data analysis table. 2 allows us to draw some conclusions and conclusions, including practical ones:
- created by Glakoval vibratory rollers, including medium weight (CA302D, Hamm 3412 And 3414 ), dynamic contact pressures significantly exceed (on sub-compacted soils by 2 times) the pressures of heavy static rollers (pneumatic wheel type weighing 25 tons or more), therefore they are capable of compacting non-cohesive, poorly cohesive and light cohesive soils quite effectively and with a layer thickness acceptable for road workers;
- Cam vibratory rollers, including the largest and heaviest ones, compared to their smooth drum counterparts, can create 3 times higher contact pressures (up to 45–55 kgf/cm2), and therefore they are suitable for the successful compaction of highly cohesive and fairly strong heavy loams and clays, including their varieties with low humidity; an analysis of the capabilities of these vibratory rollers in terms of contact pressures shows that there are certain prerequisites for slightly increasing these pressures and increasing the thickness of the layers of cohesive soils compacted by large and heavy models of them to 35–40 cm instead of today’s 25–30 cm;
- The experience of the Hamm company in creating three different vibratory rollers (3412, 3414 and 3516) with the same vibration parameters (mass of the oscillating roller, amplitude, frequency, centrifugal force) and different total mass of the vibratory roller module due to the weight of the frame should be considered interesting and useful, but not 100% and primarily from the point of view of the slight difference in the dynamic pressures created by the rollers of the rollers, for example, in 3412 and 3516; but 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 facilitates the penetration of higher density soil into the depths;
- based on a comparison of vibratory rollers according to their parameters or even based on the results of practical tests, it is incorrect, and hardly fair, to say 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); one can only regret that samples of vibratory rollers with more universal and adjustable compaction parameters have not yet appeared for use in a wider range of types and conditions of soils and thicknesses of backfilled layers, which could save the road builder from the need to purchase a set of soil compactors of different types by weight, dimensions and sealing ability.
Some of the conclusions drawn may not seem so new and may even be already known from practical experience. Including the uselessness of using smooth vibratory rollers to compact cohesive soils, especially low-moisture ones.
The author at one time tested at a special testing ground in Tajikistan the technology of compacting Langar loam, placed in the body of one of the highest dams (300 m) of the now operating Nurek hydroelectric power station. The composition of the loam included from 1 to 11% sandy, 77–85% silty and 12–14% clay particles, the plasticity number was 10–14, the optimal humidity was about 15.3–15.5%, the natural humidity was only 7– 9%, i.e. did not exceed 0.6 from the optimal value.
The compaction of the loam was carried out using various rollers, including a very large trailed vibratory roller specially created for this construction. PVK-70EA(22t, see Table 2), which had fairly 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 with this heavy roller was only achieved in a layer of no more than 19 cm.
More efficiently and successfully, this roller, as well as the A-8 and A-12, compacted loose gravel and pebble materials laid in layers up to 1.0–1.5 m.
According to the voltages measured by special sensors placed in the embankment on different depths, a curve of attenuation of these dynamic pressures along the depth of the soil compacted by the three indicated vibratory rollers was constructed (Fig. 2).
Rice. 2. Decay curve of experimental dynamic pressures
Despite quite significant differences in total weight, dimensions, vibration parameters and contact pressures (the difference reached 2–2.5 times), the values of experimental pressures in the soil (in relative units) turned out to be close and obey one pattern (dashed curve in the graph of Fig. 2) and the analytical dependence shown in the same schedule.
It is interesting that exactly the same dependence is inherent in the experimental stress decay curves under purely shock loading of a soil mass (tamping slab with a diameter of 1 m and a weight of 0.5–2.0 t). In both cases, the exponent α remains unchanged and is 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 “sharp” soil loading it is greater, with “sluggish” loading it is less.
This coefficient K serves as a “regulator” for the degree of stress attenuation along the depth of the soil. When its value is high, the stresses decrease faster, and with distance from the loading surface, the thickness of the soil layer being worked decreases. With decreasing K, the nature of the attenuation becomes smoother and approaches the attenuation curve of static pressures (in Fig. 2, Boussinet has α = 3/2 and K = 2.5). In this case, higher pressures seem to “penetrate” deep into the soil and the thickness of the compaction layer increases.
The nature of the pulse effects of vibratory rollers does not vary very much, and it can be assumed that the K values will be in the range of 5–6. And with a known and close to stable nature of the attenuation of relative dynamic pressures under vibratory rollers and certain values of the required relative stresses (in fractions of the soil strength limit) inside the soil embankment, it is possible, with a reasonable degree of probability, to establish the thickness of the layer in which the pressures acting there will ensure the implementation of the coefficient seals, for example 0.95 or 0.98.
Through practice, trial compactions and numerous studies, the approximate values of such intrasoil pressures have been established and presented in Table. 3.
Table 3
There is also a simplified method for determining the thickness of the compacted layer using a smooth roller vibratory roller, according to which each ton of weight of the vibratory roller module is capable of providing approximately the following layer thickness (with optimal soil moisture and the required parameters of the vibratory roller):
- sands are large, medium, AGS – 9–10 cm;
- fine sands, including those with dust – 6–7 cm;
- light and medium sandy loam – 4–5 cm;
- light loams – 2–3 cm.
Conclusion. Modern smooth drum and pad vibratory rollers are effective soil compactors that can ensure the required quality of the constructed subgrade. The task of the road engineer is to competently comprehend the capabilities and features of these means for proper orientation in their selection and practical application.
The compaction coefficient must be determined and taken into account not only in narrowly focused areas of construction. Professionals and ordinary workers performing standard procedures for using sand are constantly faced with the need to determine the coefficient.
The compaction coefficient is actively used to determine the volume of bulk materials, in particular sand,
but also applies to gravel and soil. Most exact method Determining compaction is a weight method.
Wide practical application was not found due to the inaccessibility of equipment for weighing large volumes of material or the lack of sufficiently accurate indicators. Alternative option coefficient output – volumetric accounting.
Its only drawback is the need to determine compaction at different stages. This is how the coefficient is calculated immediately after production, during warehousing, during transportation (relevant for road deliveries) and directly at the end consumer.
Factors and properties of construction sand
The compaction coefficient is the dependence of the density, that is, the mass of a certain volume, of a controlled sample to the reference standard.
It is worth considering that all types of mechanical, external seals can only affect top layer material.
The main types and methods of compaction and their effect on the upper layers of soil are presented in the table.
To determine the volume of backfill material, the relative compaction coefficient must be taken into account. This is due to changes in the physical properties of the pit after sand is pulled out.
When pouring a foundation you need to know correct proportions sand and cement. By going through, familiarize yourself with the proportions of cement and sand for the foundation.
Cement is a special bulk material, which in its composition is a mineral powder. O various brands cement and their use.
With the help of plaster, the thickness of the walls is increased, which increases their strength. find out how long it takes for the plaster to dry.
P = ((m – m1)*Pв) / m-m1+m2-m3, Where:
- m – mass of the pycnometer when filled with sand, g;
- m1 – weight of an empty pycnometer, g;
- m2 – mass with distilled water, g;
- m3 – weight of the pycnometer with the addition of distilled water and sand, after getting rid of air bubbles
- Pv – water density
In this case, several measurements are taken based on the number of samples provided for testing. The results should not differ by more than 0.02 g/cm3. If the received data is large, the arithmetic average is displayed.
Estimates and calculations of materials and their coefficients are the main component of the construction of any objects, as it helps to understand the quantity required material, and accordingly costs.
To correctly draw up an estimate, you need to know the density of the sand; for this, information provided by the manufacturer is used, based on surveys and the relative compaction coefficient upon delivery.
What causes the level of the bulk mixture and the degree of compaction to change?
The sand passes through a tamper, not necessarily a special one, perhaps during the moving process. It is quite difficult to calculate the amount of material obtained at the output, taking into account all the variable indicators. For an accurate calculation it is necessary to know all the effects and manipulations carried out with sand.
The final coefficient and degree of compaction depends on various factors:
- method of transportation, the more mechanical contact with irregularities, the stronger the compaction;
- duration of the route, information is available to the consumer;
- presence of damage from mechanical influences;
- amount of impurities. In any case, foreign components in the sand give it more or less weight. The purer the sand, the closer the density value is to the reference value;
- the amount of moisture that has entered.
Immediately after purchasing a batch of sand, it should be checked.
What samples are taken to determine the bulk density of sand for construction?
You need to take samples:
- for a batch of less than 350 tons - 10 samples;
- for a batch of 350-700 tons – 10-15 samples;
- when ordering above 700 tons - 20 samples.
Take the resulting samples to a research institution for examination and comparison of quality with regulatory documents.
Conclusion
The required density depends greatly on the type of work. Compaction is mainly necessary to form the foundation, backfill trenches, creating a cushion under road surface etc. It is necessary to take into account the quality of compaction; each type of work has different requirements to compaction.
In the construction of highways, a roller is often used; in places difficult to reach for transport, a vibrating plate of various capacities is used.
So, to determine the final amount of material, you need to set the compaction coefficient on the surface during compaction, this attitude specified by the manufacturer of the compacting equipment.
Always the relative density coefficient is taken into account, since soil and sand tend to change their indicators based on the level of humidity, type of sand, fraction and other indicators.
The compaction coefficient of crushed stone is a dimensionless indicator that characterizes the degree of change in the volume of the material during compaction, shrinkage and transportation. It is taken into account when calculating the required amount of filler, checking the weight of products delivered to order and when preparing the bases for load-bearing structures along with bulk density and other characteristics. The standard number for a specific brand is determined in laboratory conditions; the real one is not a static value and also depends on a number of inherent properties and external conditions.
The compaction coefficient is used when working with bulk building materials. Their standard number varies from 1.05 to 1.52. The average value for gravel and granite crushed stone is 1.1, expanded clay - 1.15, sand-gravel mixtures - 1.2 (read about the degree of sand compaction). The actual figure depends on the following factors:
- Size: the smaller the grain, the more efficient the compaction.
- Flakiness: needle-shaped crushed stone and irregular shape compacts less well than cube-shaped filler.
- Duration of transportation and type of transport used. The maximum value is achieved when gravel and granite stone is delivered in dump truck bodies and railway cars, the minimum value is achieved in sea containers.
- Conditions for filling into a car.
- Method: manually achieving the desired parameter is more difficult than using vibration equipment.
In the construction industry, the compaction coefficient is taken into account primarily when checking the mass of purchased bulk material and backfilling foundations. The design data indicates the density of the structure skeleton. The indicator is taken into account in conjunction with other parameters of building mixtures; humidity plays an important role. The degree of compaction is calculated for crushed stone with a limited volume of walls; in reality, such conditions are not always created. A striking example is a backfilled foundation or drainage cushion (fractions extend beyond the boundaries of the layer), an error in the calculation is inevitable. To neutralize it, crushed stone is purchased with a reserve.
Ignoring this coefficient when drawing up a project and carrying out construction work leads to the purchase of incomplete volumes and deterioration performance characteristics constructed structures. With the correct degree of compaction selected and implemented, concrete monoliths, building and road foundations can withstand the expected loads.
Degree of compaction on site and during transportation
The deviation in the volume of crushed stone loaded and delivered to the final point is a known fact; the stronger the vibration during transportation and the further the distance, the higher its degree of compaction. To check the compliance of the amount of material brought, a regular tape measure is most often used. After measuring the body, the resulting volume is divided by a coefficient and checked with the value indicated in the accompanying documentation. Regardless of the size of the fractions, this indicator cannot be less than 1.1, with high requirements the accuracy of delivery is discussed and specified in the contract separately.
If this point is ignored, claims against the supplier are unfounded; according to GOST 8267-93, the parameter does not apply to mandatory characteristics. The default value for crushed stone is 1.1; the delivered volume is checked at the receiving point; after unloading, the material takes up a little more space, but over time it shrinks.
The required degree of compaction when preparing the foundations of buildings and roads is indicated in project documentation and depends on the expected weight loads. In practice, it can reach 1.52, the deviation should be minimal (no more than 10%). Tamping is carried out in layers with a thickness limit of 15-20 cm and the use of different fractions.
The road surface or foundation pads are poured onto prepared sites, namely with leveled and compacted soil, without significant level deviations. The first layer is formed from coarse gravel or granite crushed stone; the use of dolomite rocks must be permitted by the project. After preliminary compaction, the pieces are separated into smaller fractions, if necessary, even to the point of filling with sand or sand-gravel mixtures. The quality of work is checked separately on each layer.
The compliance of the obtained tamping result with the design one is assessed using special equipment - a density meter. The measurement is carried out provided that there is no more than 15% grains with a size of up to 10 mm. The tool is immersed 150 mm strictly vertically, maintaining the required pressure, the level is calculated by the deflection of the arrow on the device. To eliminate errors, measurements are taken at 3-5 points in different places.
Bulk density of crushed stone of different fractions
In addition to the compaction coefficient for determining exact quantity required material, you need to know the dimensions of the structure being filled and specific gravity filler. The latter is the ratio of the mass of crushed stone or gravel to the volume it occupies and depends primarily on the strength of the original rock and size.
| Type | Bulk density (kg/m3) with fraction sizes: | ||||
| 0-5 | 5-10 | 5-20 | 20-40 | 40-70 | |
| Granite | 1500 | 1430 | 1400 | 1380 | 1350 |
| Gravel | 1410 | 1390 | 1370 | 1340 | |
| 1320 | 1280 | 1120 | |||
The specific gravity must be indicated in the product certificate; in the absence of accurate data, you can find it yourself empirically. To do this, you will need a cylindrical container and a scale; the material is poured without compaction and weighed before and after filling. The quantity is found by multiplying the volume of the structure or base by the obtained value and by the degree of compaction specified in the design documentation.
For example, to fill 1 m2 of a 15 cm thick cushion of gravel with a fraction size ranging from 20-40 cm, you will need 1370 × 0.15 × 1.1 = 226 kg. Knowing the area of the base being formed, it is easy to find the total volume of filler.
Density indicators are also relevant when selecting proportions for cooking concrete mixtures. For foundation structures, it is recommended to use granite crushed stone with a fraction size in the range of 20-40 mm and a specific gravity of at least 1400 kg/m3. Seal in in this case is not carried out, but attention is paid to flakiness - for the manufacture of reinforced concrete products, a cube-shaped filler with low content grains of irregular shape. Bulk density is used when converting volumetric proportions to mass proportions and vice versa.
Crushed stone is a common building material, which is obtained by crushing rock hard rock. Raw materials are extracted by blasting during quarrying. The rock is divided into appropriate fractions. In this case, the special compaction coefficient of crushed stone is important.
Granite is the most common, as its frost resistance is high and water absorption is low, which is so important for any building structure. The abrasion and strength of granite crushed stone meets the standards. Among the main crushed stone fractions we can note: 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 highways and railway tracks;
- additive to construction mixtures.
The compaction of crushed stone depends on many indicators, including its characteristics. Things to consider:
- The average density is 1.4-3 g/cm³ (when compaction is calculated, this parameter is taken as one of the main ones).
- Flakiness determines the level of plane of the material.
- All material is sorted into fractions.
- Frost resistance.
- Radioactivity level. For all work, crushed stone of the 1st class can be used, but the 2nd class can only be used for road work.
Based on such characteristics, a decision is made which material is suitable for a particular type of work.
Types of crushed stone and technical characteristics
Various crushed stones can be used for construction. Manufacturers offer different types of it, the properties of which differ from each other. Today, based on the type of raw material, crushed stone is usually divided into 4 large groups:
- gravel;
- granite;
- dolomite, i.e. limestone;
- secondary.
To make granite material, the appropriate rock is used. This is a non-metallic material that is obtained from hard rock. Granite is solidified magma that is very hard and difficult to process. Crushed stone of this type is manufactured in accordance with GOST 8267-93. The most popular is crushed stone having a fraction of 5/20 mm, as it can be used for a variety of works, including the manufacture of foundations, roads, platforms and other things.
Crushed gravel is a bulk construction material that is obtained by crushing stony rock or rock in quarries. The strength of the material is not as high as that of granite crushed stone, but its cost is lower, as is the background radiation. Today it is common to distinguish between two types of gravel:
- crushed type of crushed stone;
- gravel of river and sea origin.
According to the fraction, gravel is classified into 4 large groups: 3/10, 5/40, 5/20, 20/40 mm. The material is used for preparing various building mixtures as a filler; it is considered indispensable for mixing concrete, building foundations, and paths.
Crushed limestone is made from 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 to the glass industry, in the manufacture of small reinforced concrete structures, in the preparation of cement.
Recycled crushed stone has the lowest cost. It is made 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 the strength of great importance does not have.
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Compaction factor: purpose
The compaction coefficient is a special standard number determined by SNiP and GOST. This value shows how many times crushed stone can be compacted, i.e. reduce its external volume during compaction or transportation. The value is usually 1.05-1.52. According to existing standards, the compaction coefficient can be as follows:
- sand and gravel mixture - 1.2;
- construction sand - 1.15;
- expanded clay - 1.15;
- crushed gravel - 1.1;
- soil - 1.1 (1.4).
An example of determining the compaction coefficient of crushed stone or gravel can be given as follows:
- It can be assumed that the mass density is 1.95 g/cm³; after compaction was carried out, the value became 1.88 g/cm³.
- To determine the value, you need to divide the actual density level by the maximum, which will give a crushed stone compaction coefficient of 1.88/1.95=0.96.
It is necessary to take into account that the design data usually does not indicate the degree of compaction, but the so-called skeleton density, i.e. During calculations, it is necessary to take into account the level of humidity and other parameters of the building mixture.
The compaction coefficient of any bulk material shows how much its volume can be reduced with the same mass due to compaction or natural shrinkage. This indicator is used to determine the amount of filler both during purchase and during the construction process itself. Since the bulk weight of crushed stone of any fraction will increase after compaction, it is necessary to immediately lay down a supply of material. And in order not to purchase too much, a correction factor will come in handy.
The compaction coefficient (K y) is an important indicator that is needed not only for correct formation ordering materials. Knowing this parameter for the selected fraction, it is possible to predict further shrinkage of the gravel layer after loading it building structures, as well as the stability of the objects themselves.
Since the compaction coefficient represents the degree of volume reduction, it varies under the influence of several factors:
1. Loading method and parameters (for example, from what height is backfilling performed).
2. Features of transport and the duration of the journey - after all, even in a stationary mass, gradual compaction occurs when it sags under its own weight.
3. Fractions of crushed stone and grain contents of smaller size than the lower limit of a specific class.
4. Flakiness - needle-shaped stones do not give as much sediment as cuboid ones.
The strength of concrete structures, building foundations and road surfaces subsequently depends on how accurately the degree of compaction was determined.
However, do not forget that compaction on the site is sometimes carried out only on the top layer, and in this case calculated coefficient does not quite correspond to the actual shrinkage of the pillow. Home craftsmen and semi-professional construction teams from neighboring countries are especially guilty of this. Although, according to technology requirements, each layer of backfill must be rolled and checked separately.
Another nuance - the degree of compaction is calculated for a mass that is compressed without lateral expansion, that is, it is limited by the walls and cannot spread out. At the site, such conditions for backfilling any fraction of crushed stone are not always created, so a small error will remain. Take this into account when calculating the settlement of large structures.
Sealing during transport
Finding some standard compressibility value is not so easy - too many factors influence it, as we discussed above. The compaction coefficient of crushed stone can be indicated by the supplier in accompanying documents, although GOST 8267-93 does not directly require this. But transporting gravel, especially large batches, reveals a significant difference in volumes during loading and at the final point of delivery of the material. Therefore, an adjustment factor that takes into account its compaction must be included in the contract and monitored at the collection point.
The only mention from the current GOST is that the declared indicator, regardless of the fraction, should not exceed 1.1. Suppliers, of course, know this and try to keep a small supply so that there are no returns.
The measurement method is often used during acceptance, when crushed stone for construction is brought to the site, because it is ordered not in tons, but in cubic meters. When the transport arrives, the loaded body must be measured from the inside with a tape measure 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 put into the machine before shipping. If the figure obtained taking into account the compaction is less than that indicated in the accompanying documents, it means that the car was underloaded. Equal or greater - you can command unloading.
Compaction on site
The above figure is taken into account only for transportation. Under construction site conditions, where crushed stone is compacted artificially and using heavy machines (vibrating plate, roller), this coefficient can increase to 1.52. And the performers need to know the shrinkage of the gravel backfill for sure.
Usually the required parameter is specified in the design documentation. But when exact value no need, use average indicators from SNiP 3.06.03-85:
- For durable 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 of up to M600 - from 1.3 to 1.5.
For small and medium size classes of 5-20 and 20-40 mm, these indicators have not been established, since they are more often used only when decluttering the upper load-bearing layer of grains 40-70.
Laboratory research
The compaction factor is calculated based on laboratory test data, where the mass is compacted and tested for 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 density meters: static, water balloon or dynamic.
Results can be obtained immediately or after 1-4 days, depending on the study chosen. One sample for a standard test will cost 2,500 rubles, and at least five of them will be needed in total. If data is needed during the day, express methods are used based on the results of selecting at least 10 points (850 rubles for each). Plus you will have to pay for the departure of a laboratory assistant - about 3 thousand more. But during the construction of large projects it is impossible to 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 compaction yourself?
IN field conditions and for the needs of private construction, it will also be possible to determine the required coefficient for each size: 5-20, 20-40, 40-70. But to do this, you first need to know their bulk density. It varies depending on the mineralogical composition, although slightly. Crushed stone fractions have a much greater influence on the volumetric weight. For calculations, you can use averaged data:
| Fractions, mm | Bulk density, kg/m3 | |
| Granite | Gravel | |
| 0-5 | 1500 | — |
| 5-10 | 1430 | 1410 |
| 5-20 | 1400 | 1390 |
| 20-40 | 1380 | 1370 |
| 40-70 | 1350 | 1340 |
More accurate density data for a specific fraction is determined in the laboratory. Or by weighing a known volume of building rubble, followed by a simple calculation:
- Bulk weight = mass/volume.
After this, the mixture is rolled to the state in which it will be used on site and measured with a tape measure. The calculation is made again using the above formula, and as a result, two different densities are obtained - before and after compaction. By dividing both numbers, we find out the compaction coefficient specifically for this material. If the sample weights are the same, you can simply find the ratio of the two volumes - the result will be the same.
Please note: if the indicator after compaction is divided by the initial density, the answer will be greater than one - in fact, this is the material reserve factor for compaction. It is used in construction if the final parameters of the gravel bed are known and it is necessary to determine how much crushed stone of the selected fraction to order. When calculated back, the result is a value less than one. But these numbers are equivalent and when making calculations it is only important not to get confused which one to take.
