Eluvial soils: construction features and classification indicators

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

What are

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

Eluvial soils

Such soils are formed due to decompaction, cracking, grinding and breaking of rocks. Geological processes of this type are usually very long in time. At the same time, in fact, the eluvial layer itself during weatheringform, of course, only fragments left in place, above the parent rock. That is, massifs of this type are formed by fragments that have not been carried away by water or winds over time. Roughly speaking, soils of this type can be called a weathering crust.

Thickness 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 river valleys.

The structure of such deposits are complex and consist mainly of unbound clayey and loose, for example sand, crushed stone, gruss, rocks. In the photo on the page you can see what the eluvial soil might look like. 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.

Rock weathering

Features

Construction on such soils is a rather complicated procedure and requires the right approach. A violation of the technologies for erecting buildings and structures on foundations of this type can result in distortions, cracking of enclosing structures or even their collapse.

Features of eluvial soils that complicate construction on them are:

  • heterogeneity in depth;
  • sharp differences in strength and deformation characteristics in different places;
  • possibility of strength reduction and even transition to a floating state in the area of ​​foundation pits and trenches dug under the foundation;
  • tendency toswelling and swelling;
  • presence of areas with high acidity.

How an evaluation is done

Before the construction of a building or structure on such layers, of course, geological surveys are mandatory. First of all, specialists identify the petrographic composition of the parent rock and its genetic appearance. Also, when conducting research, geologists determine in such areas:

  • profile and structure of the weathering crust;
  • fracturing, layering and schistosity of the layer;
  • presence of pockets and weathering tongues;
  • number, size and shape of large debris;
  • presence and location of strike and fall elements;
  • changing properties and composition vertically.
Geological surveys

What signs may have

Eluvial soils are layers, in assessing the condition and degree of suitability for construction, they usually pay attention:

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

The first indicator is defined as the ratio of the density of eluvium to the density of the parent rock. When determining Kcb, the volume of weathered rock is divided by the area of ​​the layer. Ksop is defined as the ratio of soil tensile strength for uniaxial compression of specimens in air-dry and water-saturated states. In this regard, in turn, soils are distinguished:

  • softened with Ksop less than 0.75;
  • unsoftened with Ksop greater than 0.75.

Also, when assessing the state of such soils, geologists identify zones with different properties and composition in them, and also make predictions 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 clayey, sandy or silty clay;
  • clastic with grusly, gruss-crushed stone or large-clastic formations with silt-clay or sandy filler;
  • blocky, occurring in the form of an array with randomly located cracks and sometimes with fine-grained aggregate;
  • fissured, which is a solid rock mass in the stage of initial weathering.

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

Eluvium structure

Types by degree of weathering

Eluvial soils differ in this indicator:

  • unweathered;
  • slightly weathered;
  • weathered;
  • very weathered, or friable.

Classification of eluvium according to thisindicator corresponds to the division of rocky soils in terms of uniaxial compression in a water-saturated state according to GOST 25100-82:

  • unweathered eluvium can be classified as strong and very strong soils (500 kgf/cm2);
  • slightly weathered - to bases of medium strength (150 kgf/cm2);
  • weathered - to low-strength (50 kgf/cm2);
  • loosers - for soils with reduced and low strength (10 kgf/cm2).

Of course, eluvial soils, depending on the degree of weathering, have different physical characteristics. They can be found in the table.

Characteristics of eluvial soils of varying degrees of weathering

Variety

Physical properties

Density at occurrence (y) (g/cm3)

Porosity factor (e)

Ultimate strength in water-saturated state MPa (kgf/cm2)

Features of interaction with water

Light wind (0.9≦Sun<1)

More than 2, 7

Less than 0, 1

Over 15 (150)

Unsoftened

Weathered (0.8≦Qus<0.9)

2, 5≦γ≦2, 7

0, 1≦e≦0, 2

50≦Rc≦150

Virtually unsoftened

Heavy weathered (Qus<0, 8)

2,2≦γ≦2, 5

Over 0, 2

Under 50 (50)

Soften

How the soil behaves in the pit

Any buildings, including those on clayey or gravelly eluvial soils, are erected, of course, on foundations. Several types of such supports for building envelopes can be used:

  • tape;
  • slab;
  • columnar;
  • pile.

Most often, pile foundations are built on such soils, piercing through an unstable layer. Also, buildings in such areas can be erected on a solid slab. In this case, the structure subsequently deforms as a whole, and, consequently, no cracks appear in its enclosing structures.

Foundations on eluvial soils can be laid in some cases and tape or columnar with grillage. Such supporting foundations, when erected on sites of this type, are carefully reinforced, in compliance with all required technologies.

In any case, foundation pits or trenches are dug out beforehand for foundations, including those on eluvium. Further, in the formwork, in fact, the supporting structure itself is poured.

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

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

Stabilization of eluvium is comingusually only about 1-2 months after digging the foundation pit and pouring the base of the building.

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

Evaluation of durability in pits

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

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

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

The quantitative values ​​of the parameter A are determined at specified time intervals t, established taking into account the construction time, as well as the specific features of the area. The same factors also influence the choice of the maximum allowable time for the eluvial soil to remain in the open state.

Laboratory research

Measures to prevent destruction during diggingpits

In order for the characteristics of eluvium not to deteriorate, certain measures must, of course, 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 for breaks. Also, water protection measures should be carried out on the site before digging the pit.

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

  • 0, 3 m - in dusty and clayey formations;
  • 0, 1-0, 2 m - in others.

Sometimes in soils of this type there are quite large areas of carbonaceous or compressed interlayers that extend to the level of the base of the foundation. In this case, the amount of shortfall should be at least 0.8 m. The protective layer during the development of the pit to the design depth in the future, according to existing standards, can be carried out with soil with a disturbed structure by compacting it with rammers or rollers.

Instability in the pits

What measures can be taken when building buildings

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

  • Device under the foundations of distribution and damping pads made of sand, gravel, crushed stone and other similar rocks.
  • Fixation of the eluvial soils themselves, for example, bycementing, bituminization or claying.
  • Replacement of pockets and weathering nests at the site with coarse or sandy soil.
  • Deep laying of foundations with cutting through the eluvial soil to the full depth.

Additional measures

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

Samples for laboratory research

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, it is also mandatory to use waterproofing materials, laying them either in a thick layer (clay) or in several sheets (roofing material).

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