Eluvial soils: construction features and classification indicators

Eluvial deposits are called massifs of debris formed as a result of physical and chemical destruction of rocks. Such layers in Russia are found almost everywhere. Construction on eluvial soils of various kinds of buildings and structures has, of course, some of its own characteristics.

What are

In geology and construction, soils of this type are generally classified as low-strength. Only some of them having a special structure can be considered as medium-strength strata or strong. In our country, even private traders, not to mention large companies, often have to erect various structures on eluvial soils. What are these layers and how do they look?

Such soils are formed due to decompaction, cracking, grinding and breaking of rocks. Geological processes of this type in time usually last a very long time. In this case, in fact, the eluvial layer itself during the weathering form, of course, only fragments remaining in place above the parent rock. That is, arrays of this type are formed by fragments, which over time were not carried away by water or winds. Roughly speaking, this type of soil can be called weathering crust.

Power eluvial layers can have from one to several tens of meters. Most often, soils of this type occur:

• on gentle slopes;
• flat and low watersheds;
• in the valleys of the rivers.

The structure of such deposits is complex and consists mainly of unbound clay and loose, for example, sand, gravel, wood, and rocks. In the photo on the page you can see how eluvial soil can look. There are many examples of such sites in our country. In Russia, soils of this variety are most often found in Siberia, the Urals, and Karelia.

Features

Construction on such soils is a rather complicated procedure and requires the right approach. The result of a violation of the technologies for the construction of buildings and structures on the bases of this type can be distortions, cracking of the building envelopes or even their collapse.

Features of eluvial soils that complicate the construction on them are:

• heterogeneity in depth;
• sharp differences in strength and deformation characteristics in different places;
• the possibility of reducing strength and even transitioning to a quick condition in the area of ​​ditches and trenches dug under the foundation;
• tendency to swelling and heaving;
• the presence of sites with high acidity.

How to evaluate

Before starting the construction of a building or structure on such strata, of course, geological surveys are mandatory. In the first place, experts identify the petrographic composition of the mother breed and its genetic appearance. When conducting research, geologists also determine in such areas:

• profile and structure of weathering crust;
• fracturing, degree of layering and slatiness of the layer;
• the presence of pockets and weathering languages;
• the amount, size and shape of large debris;
• the presence and location of the elements of strike and dip;
• change the properties and composition vertically.

Which may have signs

Eluvial soils are strata, in assessing the condition and degree of suitability of which usually pay attention to the construction:

• weathering coefficient (Kwr);
• weathering rate coefficient (Kcb);
• uniaxial compression resistance (Rc);
• softening coefficient in water (Ksop).

The first indicator is then defined as the ratio of the density of eluvium to the density of the parent rock. When determining Kcb, the volume indicator of weathered rocks is divided by the area of ​​the layer. Ksop is defined as the ratio of the ultimate strength of the soil to uniaxial compression of samples in an air-dry and water-saturated state. In this regard, in turn, distinguish soils:

• softened with Ksop less than 0.75;
• non-softening with Ksop greater than 0.75.

Also, when assessing the condition of such soils, geologists identify zones with different properties and composition in them, and also make forecasts of the intensity and speed of weathering processes when digging pits and excavations.

Soil zones

Depending on the characteristics of the parent rock, mineralogical composition and geochemical processes, the eluvial layer from top to bottom can be represented by the following zones:

• dispersed clay, sand or silty clay;
• clastic with woody, wood-gravel or coarse-grained formations with dusty clay or sand filler;
• block, lying in the form of an array with unsystematically located cracks and sometimes with small-fragment aggregate;
• fractured, which is a solid rock mass in the initial weathering stage.

In many cases, eluvial soils are classified as low-strength. However, in some areas, for example, in the Urals, in their engineering-geological section, layers may be present that, according to their formal characteristics, can be classified as semi-rock or even rocky, but with noticeable compressibility.

Species by degree of weathering

According to this indicator, eluvial soils are distinguished:

• non-weathering;
• weakly weathered;
• weathered;
• strongly weathered, or loosers.

The classification of eluvium by this indicator corresponds to the division of rocky soils in terms of uniaxial compression in a water-saturated state according to GOST 25100-82:

• non-weathering eluvia can be attributed to strong and very durable soils (500 kgf / cm ² );
• weakly weathered - to the bases of medium strength (150 kgf / cm ² );
• weathered - to low-strength (50 kgf / cm ² );
• loosening - to soils with reduced and low strength (10 kgf / cm ² ).

Of course, eluvial soils, depending on the degree of weathering, have different physical characteristics. You can find them in the table.

How soil behaves in the pit

Any buildings, including on clay or gravelly eluvial soils, are, of course, built on foundations. Several types of such supports under the building envelope can be used:

• tape
• plate;
• columnar
• pile.

Most often, on such soils, pile foundations are built up, stitching through the unstable layer. Also, buildings in such areas can be erected on a continuous slab. In this case, the structure is subsequently deformed as a whole, and therefore, cracks do not occur in its enclosing structures.

Foundations on eluvial soils can be laid in some cases, either tape or column with a grillage. Such support bases, when erected in areas of this type, are carefully reinforced with all the required technologies.

In any case, under foundations, including on eluvium, pits or trenches are pre-excavated. Further, in the formwork, in fact, the supporting structure itself is poured.

The mechanical properties of eluvium, as already mentioned, in the open pit during construction can vary significantly. When performing construction work on soils of this type:

• dispersion and deformability increase;
• strength decreases to a depth of 1 m.

The stabilization of eluvium usually occurs only about 1-2 months after digging the foundation pit and filling the base of the building.

Most of all, when digging holes and trenches, structural hard clay and coarse fragments are weakened. In particular, petrified clays and dusty soils change their properties greatly. Under the influence of water and temperature fluctuations from stable, such massifs pass into a fluid state, bypassing the plastic one.

Assessment of durability in pits

The way in which the strength of the eluvial soil in the trench or pit will decrease, geologists determine by changing its characteristics during the weathering period, as well as by changing the values ​​of density, decay intensity, water absorption coefficient of samples taken at the site of the future construction (in the form of fragments and blocks).

For the expected period, an assessment of the resistance of eluvium at the construction site to additional atmospheric weathering during opening is done by determining:

• the rate of decline of the desired parameter of the degree of weathering A for a period of time t: (A1 - A2) / t;
• the degree of decrease in parameter A: (A1 - A2) / A1;
• the total quantitative decrease in parameter A for the entire period t: (A1 - A2).

Quantitative values ​​of the parameter A are determined at given time intervals t established taking into account the construction time, as well as the specific features of the area. The same factors also affect the choice of the maximum allowable residence time of the eluvial soil in the open state.

Destruction measures during digging pits

So that the characteristics of eluvium do not deteriorate, of course, certain measures should be taken at the beginning of the construction of a building or structure. According to the rules, for example, when arranging foundations in this case, the standards do not allow interruptions. Also, waterproofing measures should be taken at the site before digging the pit.

The thickness of the shortfalls in eluvies, according to the rules of GOST and SNiP, should not be less:

• 0.3 m - in dusty and clay layers;
• 0.1-0.2 m - in others.

Sometimes in soils of this type there are quite large sections of carbonaceous or compressed layers that go to the level of the base of the foundation. In this case, the value of the shortfall should be equal to at least 0.8 m. The protective layer during the development of the pit to the design depth in the future, in accordance with existing standards, can be performed with soil with a disturbed structure by compaction with tampers or rollers.

What measures can be taken in the construction of buildings

Construction on eluvial soils of various kinds of structures should be carried out in compliance with certain rules. In order for the constructed structure to subsequently be safe in operation and have a long service life, the measures in this case are usually carried out as follows:

• The device under the foundations of distributing and cushioning pillows made of sand, gravel, gravel and other similar rocks.
• Fixing the eluvial soils themselves, for example, by cementing, bituminizing or claying them.
• Replacing pockets and weathering nests on the site with coarse or sandy soil.
• Deep foundation laying with cutting eluvial soil to the entire depth.

Additional measures

Also, to improve the bearing capacity of such layers, the construction site is protected by all possible means from atmospheric waters. A feature of the construction of buildings and structures on eluvial soils is also that in foundation pits, in this case, a large number of waterproofing materials are usually used. Laying the walls and bottom of pits and trenches in such areas allows you to protect the underground part of the supports of buildings from exposure to soil acidic environment.

The blind areas of structures during construction on soils of this type are usually made as wide as possible. At the same time, when pouring such protective tapes, waterproofing materials are also required to be used, laying them either with a thick layer (clay) or in several sheets (roofing material).