Scientists from Perm Polytechnic University and St. Petersburg Mining University have developed a method for calculating the optimal density and humidity of road building materials — sandstones and clays. The technique makes it possible to effectively use local soils when laying highways and save imported materials.
Modern infrastructure construction — roads, dams, etc. — faces a difficult dilemma: on the one hand, it is necessary to reduce the cost and time of work, and on the other — to guarantee the durability and stability of structures. One of the main cost factors is logistics: it is often necessary to transport high-quality crushed stone or sand-gravel mixture (SGS) for hundreds of kilometers, which dramatically increases the cost of any project. Therefore, more and more attention is being paid to the use of local soils as artificial foundations and for filling out the roadbed.
However, a serious problem becomes the situation when highly weathered sedimentary deposits, sandstones and mudstones, which are prone to frost heaving, swelling and having low strength, act as such a local soil for filling. These negative properties of local rocks force designers and builders to make compromises, each of which has serious drawbacks.
Most often, the most radical way is chosen — the complete replacement of soil with imported material (crushed stone, sand, PGS), which entails enormous logistical costs. An alternative is to try to "improve" sandstones and mudstones directly on the construction site by adding cement, lime or other chemicals. However, this process requires special equipment and precise control, turning a simple filling into a complex high-tech operation.
To solve the problem, the authors of the study created and tested a series of samples in which sandstone and mudstone were mixed in different proportions. Each mixture was compacted according to a standard procedure that simulates the process of soil tamping in real conditions. During the tests, the key indicators were measured — optimal humidity and maximum density. This experiment allowed us to determine exactly how the composition of the material affects its building properties.
"The results showed that there is an optimal proportion at which maximum density will be observed for the mixture. This effect is achieved with a content of 40 to 50% argillite. This is explained by the fact that small clay particles fill the voids between the larger sandstone particles. It turns out to be a dense, homogeneous mass," explained Evgenia Akbulyakova, one of the authors of the development, Associate Professor of the Department of Construction Production and Geotechnics at PNRPU, Candidate of Technical Sciences.
According to scientists, with this composition, the soil not only becomes denser, but also its optimal humidity for compaction (that is, the "golden mean" at which the soil is best rammed) approaches natural humidity. This means that during construction it will not be necessary to spend additional energy and resources on artificial humidification or dehumidification of huge masses of this material.
Moreover, the researchers managed to describe the found dependence mathematically. They derived two calculation formulas, which gives engineers a simple method for designing the optimal composition of a mixture of mudstones and sandstones.
"It is enough to substitute the mudstone content into one formula to calculate the maximum density of the mixture. The second calculation allows us to determine the optimal humidity for compaction of the mixture, depending on the content of argillite," added Evgeniya Akbulyakova.
The developers note that the accuracy of the density forecast exceeds 99%. Thanks to the new method, designers will be able to determine in advance and accurately the physical characteristics of the resulting soil and the mode of its laying, without wasting time and money on expensive laboratory samples for each new site. And using local raw materials will reduce the time and cost of construction.
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