Physical properties and classification of soil, mechanical properties of soil

Soil refers to a layer of loose material on the earth's surface, composed of various granular minerals, organic matter, water, air, microorganisms, etc., which can grow plants. The soil is composed of minerals weathered from rocks, animals and plants, organic matter produced by the decomposition of microbial residues, soil organisms (solid substances), water (liquid phase substances), air (gas phase substances), and oxidized humus. Solid matter includes soil minerals, organic matter and nutrients obtained by microorganisms through light sterilization.

Liquid matter mainly refers to soil moisture. Gases are the air present in the pores of the soil. These three types of substances in the soil constitute a contradictory unity.

They are interconnected, mutually restrictive, provide the necessary living conditions for crops, and are the material basis of soil fertility.

The substances in the soil can be summarized into three parts: the solid part, the liquid part, and the gas part.

Soil minerals are mineral particles of different sizes (sand, soil and gum) formed by the weathering of rocks. There are many types of soil minerals, and the chemical composition is complex, which directly affects the physical and chemical properties of the soil, and is one of the important nutrients of crops. Solid soil particles composed of minerals and humus are the main body of the soil, accounting for about 50% of the soil volume, and the pores between the solid particles are occupied by gas and water.

The vast majority of soil gas is oxygen and nitrogen entering the atmosphere, and a small part is carbon dioxide and water vapor produced by life activities in the soil. The moisture in the soil enters the soil mainly from the surface and includes many dissolved substances. There are also various animals, plants, and microorganisms in the soil.

The amount of organic matter is an important indicator of soil fertility, and it is closely combined with minerals. In the general cultivated layer, the organic matter content only accounts for the dry weight of the soil, and the cultivated layer is less, but its role is great, and the masses often call the soil containing more organic matter "oil soil". Soil organic matter is divided into fresh organic matter, semi-decomposed organic matter and humus according to the degree of its decomposition.

Humus refers to the gray and black earth-colored colloidal substances formed by the transformation of fresh organic matter through enzymes, and the soil that retains its nutrients after the harmful bacteria and viruses and parasites that are killed by sunlight generally accounts for more than 85-90% of the total soil organic matter.

Soil can be divided into: soil can be divided into sandy soil, clayey soil, loam three types of sandy soil properties: more sand content, rough particles, fast water seepage, poor water retention performance, good aeration properties of clay soil:

The nature of loam with low sand content, fine particles, slow water seepage rate, good water retention performance, poor aeration performance: general sand content, general particles, general water seepage speed, general water retention performance, and average aeration performance.

Soil is a multiphase medium composed of particles, water and air, and its mechanical properties are one of the main objects of soil mechanics research. The lower buried surface is several important mechanical properties of soil:

1.Compressibility: Compressive deformation occurs when the soil is subjected to load. The compressive properties of soil mainly include compressive modulus, compressive coefficient, etc.

2.Shearability: Shear deformation occurs when the soil is subjected to shear stress. The shear properties of soil mainly include shear strength, shear modulus, cohesion and internal friction angle.

3.Porosity: There are many pores in the soil, and they affect the water movement and permeability of the soil. The pore properties of soil mainly include porosity ratio, porosity, permeability coefficient and so on.

4.Plasticity: Plastic deformation occurs while the soil is subjected to load. The plastic properties of soil mainly include state positive plastic index, compression index, etc.

5.Liquefaction: When some special soil layers are subjected to vibration or dynamic loads, liquefaction will occur and lose strength and stability.

The mechanical properties of these soils are very important for engineering design and soil mechanics analysis, and the research and application of soil mechanics can help people better understand and improve the behavior and properties of soils, so as to ensure the safety and reliability of soils in various engineering applications.

(1) Physical and mechanical properties of soil

Physical properties of soil.

The physical properties of soil are some properties that indicate the physical state; It reflects the lightness, weight, dryness, wetness, and density of the soil.

2.Mechanical properties of soil.

The properties of soil under the action of external forces are called the mechanical properties of soil. It mainly includes the permeability, compressibility and shear resistance of the soil under stable load (static load), the dynamic compaction and rheology of the cohesive soil.

1) Permeability of soil.

The permeability of the soil is the ability of water to pass through the soil under the action of the hydraulic slope (water pressure difference). The seepage of water in the soil can sometimes deform or destroy the soil, which is called seepage deformation, which includes two basic types: flowing soil and pipe gushing.

Basic physical indicators of soil.

(2) Compressibility of soil.

The compressibility of the soil soil under the action of pressure, the porosity ratio will be reduced due to the removal of gas or water in the pores, resulting in the reduction of the volume of the soil, this property is called the compressibility of the soil, and there are many indicators to measure the compressibility of the soil.

3) The shear strength of the soil.

The ultimate stress of deformation and failure of the soil per unit area is called the shear strength of the soil under the action of the shear plane parallel to the soil, and the unit is Pa or MPa.

The shear strength of soil can be divided into: shear strength of non-cohesive soil, shear strength of unsaturated cohesive soil, and shear strength of saturated cohesive soil.

1) Engineering classification of soil.

There are many types of soil, and the soil used as the foundation of buildings is divided into rock, gravel soil, sand, silt, cohesive soil and special soil (such as silt, peat, artificial fill, etc.). Rocks can be divided into hard and soft as well as slightly weathered, moderately weathered, strongly weathered, fully weathered and residual soils; Gravel soil is divided into boulder, block stone, soft stone, gravel, round gravel and breccia gravel; Sand is divided into grave sand, coarse sand, medium sand, fine sand and silt sand, as well as compact, medium-dense, slightly dense and loose sand; Cohesive soils can be divided into clay, silty clay, and hard, hard, hard, plastic, soft plastic, and flow plastic.

In the construction of earthworks, according to the difficulty of soil excavation, the soil is divided into eight categories: soft loose soil, ordinary soil, solid soil, gravel solid soil, soft stone, sub-hard stone, solid stone and extra hard stone. The first four types belong to general soil, and the last four types belong to rocks.

1.The origin of soil.

Residue: the product that has not been transported in situ after the weathering and denudation of the original rock is called the residue;

Slope deposits: Rock weathering products are eroded and moved down by rain and snow flows, and deposited on the gentle slope waist or slope foot, which is called slope deposits;

Flood deposits: flash torrents caused by heavy rains or large amounts of snowmelt wash the surface of the land, carrying a large amount of sediment to accumulate at the outlet of a ravine or on the sloping plain in front of the mountain, forming an alluvial deposit;

Alluvium: On both sides of rivers, sediments of one order are formed, called alluvium;

Sediments: sediments from oceans and lakes, mostly pebbles, gravel and sand in coastal and lakeshore areas.

2.Three-phase and engineering classification of soil.

1) The three-phase soil of the soil is composed of a three-phase system, including: solid phase - solid particles (soil particles); Liquid phase – water in the voids between particles; Gas phase – a gas in the void between particles.

2) The engineering classification of soil is divided into: gravel soil, sand (gravel sand, coarse sand, medium sand, fine sand, silt sand), cohesive soil, silt and silty soil, red clay, silt and artificial fill.

Classification of soil (engineering) 1, gravel soil gravel soil is a typical coarse-grained soil, if the particle size of the soil is greater than 2mm content is higher than the weight of the whole soil 50, the soil belongs to crushed.

Stony soil. According to the grain coarse and particle shape, the gravel soil can be further subdivided, as shown in the following table. Classification of gravel soil Soil name Particle shape description.

Particle group content.

Bleoms are rounded.

The particle content of the particle size greater than 200mm exceeds 50 stones, edges, corners, pebbles and rounding of the whole soil.

The particle content of more than 20mm particle size exceeds 50 of the whole soil.

The particle content of more than 2mm particle size exceeds 50 breccia edges and corners of the whole soil.

2. Sand Sand is fine-to-medium-grained soil, non-plastic, composed of fine rocks and mineral fragments. The diameter of the sand grains varies between 0 75 and 2 mm, and the soil content of more than 0 075 mm is more than 50According to the content of the grain group, sand can be further divided into gravel sand, coarse sand, medium sand and fine sand.

and five types of silt, as shown in the table below. Classification of sand.

Soil name Particle group content Gravel sand Particle size greater than 2mm particle size accounts for 25 a 50 coarse sand Particle size greater than 0 5mm fine particle content exceeds 50 of total weight Medium sand Particle size greater than 0 25mm particle size content exceeds 50 of total weight Fine sand Particle size greater than 0 075mm particle size content exceeds 85 of total weight Silt More than 0 075mm particle size particle content exceeds 50 of total weight 3, silt Silt Silt is fine-grained soil, particle size changes between 0 002 075mm, and soil particles are larger The content shall not be exceeded.

50, plasticity index i p 10In conclusion, silt is intermediate in nature from sandy and clay. Inorganic silt is also known as "rock powder".

4. Cohesive soil Cohesive soil is a typical fine-grained soil, with a particle size of less than and an irregular shape. Cohesive soils can be subdivided into two categories: silty clay and clay.

Earth. It is classified on the basis of the plasticity index ip, as shown in the table below. Classification of cohesive soils Plasticity index i p Soil name i p > 17 Clay 10< i p 17 Silty clay.

5. Artificial filling Artificial filling is the soil formed by human action. Common artificial fills include plain fill, compacted fill, miscellaneous fill and flush fill. Plain fill soil can be.

Contains all kinds of soil. Miscellaneous fill is the artificial soil formed by the mixing of various garbage, which may be industrial waste or municipal waste. Flushing.

Soil is formed by hydraulic action, such as the soil formed by sand and dredging of river embankments and river embankments. In addition, there are many special properties of soil distributed in nature, including silt, silty soil, expansive soil, collapsible loess, red clay, etc.

These soils are distributed in different parts of the country. They all have their own specifications, and the corresponding specifications can be selected and used in the actual project.

The structure of soil is divided into single grain structure, honeycomb structure, and flocculent structure.

The single-grain structure is formed due to the sinking of coarse soil particles in water or air, and the salient feature is that there is no connection between the soil particles, or the connection is very weak and negligible. The mutual position of the soil particles in space and the particle size and shape of the soil particles determine the soil particle density of the single grain structure.

The honeycomb structure is formed by the continuous deposition of clay mineral sheets, which are mainly formed by the contact of one grain at the edge, and many chains are surrounded by polygonal rings. Honeycomb soils have large pores, but with the help of their intergranular connections, they can bear general static loads.

The flocculent structure is that when the clay particles with very fine particle size are suspended in the water for a long time and move in the water, they form small chain-shaped soil aggregates and sink, and when a small chain-link touches another small chain-link, they attract each other and continue to expand to form a large chain-ring.

The interarrangement and connection of soil particles is called the structure of soil. The structure of soil is gradually formed in the process of soil formation, which reflects the influence of soil composition, genesis and age on the engineering properties of soil. The structure of soil can be divided into three types: single grain structure, honeycomb structure and flocculent structure according to the arrangement and connection of its particles, as shown in the figure.

1。Single-grain structure.

The monogranular structure is a structural characteristic of gravel and sandy soils. It is characterized by the presence of no connection between the soil particles, or the connection is very weak. The tightness of the individual grain structure depends on the mineral composition, particle shape, particle size composition, and uniformity of the gradation.

2。Honeycomb structure.

The honeycomb structure is a structural feature dominated by silt particles, when the soil particles with a particle size of 0002 002mm are deposited in water, they are basically a single particle sinking, and when they encounter the deposited soil particles in the sinking process, the gravitational force of the soil particles is relatively large enough for their own weight, so the particles stay in the initial contact position and no longer sink, forming a honeycomb structure with large voids.

3。Floccular structure.

The flocculent structure is a unique structure of cohesive soil particles, and when the clay particles suspended in water change, the soil particles polymerize with each other to form a flocculent structure that sinks and is deposited into a large-pore flocculent structure. Among the above three structures, the dense single-grain structure has the best engineering properties, and the honeycomb structure and flocculent structure have low strength and high compressibility if the natural structure is damaged by disturbance, and cannot be used as a natural foundation.

Soil physical properties include soil structure and porosity, soil moisture, soil air, soil heat and soil tillage.

Among them, soil moisture, air and heat, as the constituent elements of soil fertility, directly affect the fertility of soil, and the other physical properties restrict the activities of soil microorganisms and the transformation, existence form and supply of mineral nutrients by affecting soil moisture, air and heat, and then have an indirect impact on soil fertility.

It is of great significance to learn and master the basic theories of soil physical properties and their regulation measures for continuous soil fertilization, soil productivity, and sustainable utilization of soil resources.

Soil physics includes the color, texture, porosity, structure, moisture, heat and air conditions of the soil, and the mechanical and physical properties and electromagnetic properties of the soil. Various properties and processes are interrelated and restricted, among which soil texture, soil structure and soil moisture are dominant, and their changes often lead to changes in other physical properties and processes of soil.