Collapsible loess foundation

I. Characteristics and distribution of loess

Loess is a quaternary sediment with a series of internal material composition and external characteristics, which is different from other sediments of the same period.

Loess has all the following characteristics:

1. The color is mainly yellow, brown yellow, sometimes gray yellow;

2. The particle composition is mainly powder (0.05-0.005mm), the content is generally more than 60%, almost no particle size greater than 0.25mm;

3. The porosity is relatively large, generally around 1.0;

4. Rich in calcium carbonate salts;

5. Vertical joint development;

6. There are generally large pores visible to the naked eye.

When one or more of these features are missing, the loess soil is called loess.

Classification of loess

Loess can be divided into primary (or typical) loess and secondary loess according to its origin. Aeolian loess without stratification is generally considered to be primary loess. The loess with more sand and even fine grains in the bedding is called secondary loess after the primary loess is washed and redeposited by running water.

Distribution of loess

Loess is widely distributed, covering an area of 13 million square kilometers, accounting for about 9.3% of the total land area. The proportion of loess coverage in the total area of the world's continents is: 7% in Europe, 5% in North America, 10% in South America, 3% in Asia, in addition, there are sporadic distributions in Australia and North Africa.

China's loess distribution area of 635280 square kilometers, accounting for about 4.9% of the world's total loess distribution area, mainly distributed in 33-47 degrees north latitude, the most developed between 34-45 degrees, belongs to arid and semi-arid climate type.

The collapsive-loess distribution area in China accounts for about 60% of the total loess distribution area in China, which is 270,000 square kilometers, most of which is in the middle loess area, starting from near the Great Wall in the north, reaching the Qinling Mountains in the south, Wushao Mountains in the west, and Taihang Mountains in the east, that is, between 34-41 degrees north latitude and 102-114 degrees east longitude.

Collapsible loess generally covers the non-collapsible loess layer lying below, the thickness is larger in the west of Liupan Mountain, up to 30 meters, the thickness is slightly thinner in the east of Liupan Mountain, such as the Fen-Wei River Valley is mostly several meters to ten meters, and then the east to the west of Henan is even less, and non-collapsible loess is located between the collapsible loess layers.

Ii. Causes and influencing Factors of Loess collapsibility

Various arguments on the causes and mechanisms of loess collapsibility can be summarized into two aspects: internal causes and external causes. The internal cause is mainly due to the material composition of the soil itself (particle composition, mineral composition and chemical composition) and its structure, while the external cause is the effect of water and pressure.

1. Capillary hypothesis

Terzaghi pointed out that when the discontinuous water in the wet sandy soil accumulates at the particle contact point, the surface tension at the junction of water and air in the adjacent particle pores makes the soil particles pull together. When water is immersed in the soil, the surface tension disappears, and the sand collapses. Some scholars have used this point of view to explain the collapsibility of loess, but later it was opposed. J.G. udley believes that capillary pressure is an important factor for the formation of fine silt and flocculated clay adhesion in loess. Capillarity exists in loess, but it is questionable to regard it as the main cause of collapsibility. Chang Baoqi once made samples with air-dried disturbed soil. Although the capillary channel was destroyed and the liquid surface bending was eliminated, it still had great collapsibility.

2. Dissolved Salt hypothesis

There is a large amount of soluble salt in loess. When the water content of loess is small, the soluble salt is in the state of microcrystal, attached to the surface of particles, and plays a certain cementation role. This cementation is part of the cohesive force of loess, which is lost when water is soaked and soluble salt dissolves, resulting in collapsibility.

The content of soluble salt in collapsible loess in China is small, which is not the main part of the consolidation cohesion. Although the content of insoluble salt is high, its dissolution is very slow. Therefore, more views believe that the dissolution of soluble salt is not the main cause of collapsibility.

3. Insufficient colloid

The collapsibility of loess is an inherent property of soil containing less than 0.05mm particles and less than 10%, which lacks colloidal part. If there is a significant amount of colloid, the expansion prevents the occurrence of collapse. Zhu Haizhi believed that loess does not have collapsibility when the clay content is greater than 15-20%, but found that loess with clay content greater than 30% on the second-order terrace on the north bank of Lanzhou West Basin has strong collapsibility.

4. Water film wedging theory

Low water content loess on the surface of fine particles (mainly clay) wrapped in the bound water film is generally very thin, dissolved in the negative, cation electrostatic attraction is strong, the surface of the negatively charged clay connected to form a certain condensation strength. When water enters the soil, the bound water film thickens and separates the firmly connected particles like a wedge, causing the surface of the soil particles to expand, the volume to increase, the gravity to weaken, and the cohesion strength to decrease, resulting in collapsibility. The water-film wedging theory can explain the phenomenon that loess will collapse immediately when water enters. However, it is not enough to explain the occurrence of various complex collapsibility phenomena (such as the strength of collapsibility, self-weight collapsibility and non-self-weight collapsibility).  

5. Undercompaction theory

Loess is formed in arid or semi-arid climate. The surface of weathered loess is affected by atmospheric precipitation during the deposition process. Under the condition of dry and low rainfall, the thickness of the wet zone of atmospheric precipitation is often less than that of the zone affected by evaporation. During the precipitation period, the soil above the A-A line has a high water content in the soil and is in the optimal compaction condition. However, due to the thin soil layer and low dead weight pressure, it cannot be effectively compacted. With the continuous accumulation of loess, the a-a line increased, and in the soil layer between the new a-a line and the b-b line, the atmospheric precipitation did not affect it, but the evaporation process continued. As the water is reduced, the salts are precipitated, and the colloids coagulate to create a cohesive force.

Although the overlying soil pressure increases, it is not enough to overcome the reinforcement cohesion formed in the soil, so it becomes an undercompaction state. This cycle makes the accumulated undercompacted soil layer thicker and thicker, and once the water is immersed deeper, the strengthening cohesion is lost, resulting in collapsibility. When the precipitation is low and the dry period is long, the undercompaction degree is large, and the undercompaction soil layer is thicker. On the contrary, the degree of loess undercompaction is weak, and the undercompaction soil layer formed is also thin. The theory of undercompaction is easy to explain why the collapsibility of loess is strong in the northwest and weak in the southeast of China.

Treatment of collapsible loess foundation

There are many treatment methods for collapsible loess foundation. In different areas, different treatment methods should be selected according to different soil quality and different structures. In the investigation stage, after field sampling and analysis of test data, it is determined whether the loess is self-gravity collapsible or non-self-gravity collapsible, as well as the thickness, collapsible grade and category of collapsible loess layer, and through economic analysis and comparison, many factors such as process environment and construction period are comprehensively considered. Finally, a most suitable foundation treatment method is selected and optimized to ensure that the treated foundation has sufficient bearing capacity and deformation conditions.

The methods used are:

* Heavy weight surface compaction and dynamic compaction

* Soil cushion

* Soil compaction pile

* Pile foundation

* Chemical reinforcement method

* Other reinforcement methods (pre-immersion method, thermal reinforcement method, underwater blasting method, electric spark reinforcement method)

Expansive soil foundation

Expansive soil is also a very important regional special soil type, according to China's "expansive soil area building Technical Code" (GBJ112-87) (hereinafter referred to as the "expansive soil code") definition, expansive soil should be clay clay in the soil is mainly composed of hydrophilic minerals, and has significant water absorption expansion and water loss shrinkage of two deformation characteristics. It is well known that generally cohesive soil also has the characteristics of expansion and contraction, but its amount is not large, and it is not of great practical significance for engineering. And the expansion of expansive soil. Shrink. The periodic deformation of reexpansion is very significant and often brings harm to engineering, so it is distinguished from the general cohesive soil in engineering and treated as special soil.

Expansive soil is widely distributed in China. According to the available data, expansive soil exists in more than 20 provinces, autonomous regions and cities such as Guangxi, Yunnan, Hubei, Anhui, Sichuan, Henan and Shandong. The same is true abroad, such as the United States, 40 of the 50 states contain expansive soil, in addition to the vast areas of India, Australia, South America, Africa and the Middle East, also have varying degrees of distribution. At present, the engineering problem of expansive soil has become a worldwide research topic.

I. Identification of expansive soil and expansion and contraction grade of expansive soil foundation

a, the main factors affecting the expansion and shrinkage characteristics of expansive soil

The internal mechanism affecting the swelling and shrinking properties of expansive soil mainly refers to the two aspects of mineral composition and microstructure. Experiments show that expansive soil contains a large number of active clay minerals, such as montmorillonite and illite, especially montmorillonite, which has a huge suction force on water at low water content. The content of montmorillonite in soil directly determines the size of the swelling and shrinking properties of soil. In addition to the mineral composition factors, the spatial connection of these mineral components also affects the expansion and contraction properties. Scanning electron microscope analysis of expansive soil in a large number of different locations showed that the surface of expansive soil was... Surface-connected polymers are a common structural form of expansive soils, which have greater capacity of absorbing and expandable water and shrinking than aggregate structures.

The biggest external factor affecting the swelling and shrinking properties of expansive soil is the action of water on expansive soil, or more precisely, the migration of water is the key external factor controlling the swelling and shrinking properties of expansive soil. Because only if there is a gradient and a way of water migration in the soil, it is possible to cause soil expansion or contraction. Although a clay has a potentially high expansion potential, if its water content remains constant, no volume change will occur; On the contrary, a slight change in water content, even as little as 1-2%, has been shown to be sufficient to cause harmful expansion. Therefore, the indexes of swelling and shrinking of expansive soil reflect the swelling and shrinking amount and swelling force of expansive soil when the water content changes.

b, expansion and shrinkage index of expansive soil

1. Free expansion rate def

The ground dried soil sample prepared manually is injected into a measuring cup through a neck-less funnel, and its volume is measured, and then poured into a measuring cylinder with water. After it is fully absorbed and expanded and stabilized, its volume is measured again. The ratio of the increased volume to the original volume def is called the free expansion rate.

2. Expansion rate dep and expansion force Pe

Expansion rate represents the ratio of the increased height of the soil sample to the original height after it is immersed in water and expanded to stabilize under a certain pressure in the confined compression instrument.

The expansion rate dep under all levels of pressure is taken as the ordinate and the pressure p as the horizontal coordinate, and the test results are plotted as the P-DEP relationship curve. The intersection point of the curve and the horizontal coordinate is called the expansion force of the sample, and the expansion force represents the maximum internal stress caused by the expansion of the original soil sample when the volume remains unchanged. Dilatancy is a useful indicator when selecting foundation type and base pressure. In the design, if the expansion deformation is to be reduced, the base pressure should be close to the expansion force.

2. Identification of expansive soil

According to the practical experience of more than ten years in China, the main basis for judging expansive soil is engineering geological characteristics and free expansion rate. Therefore, the "expansive soil Code" stipulates that any site with the following engineering geological characteristics, and the free expansion rate def≥ 40% of the soil should be considered expansive.

1. Cracks develop, often smooth surface and scratches, and some cracks are filled with gray and gray green clay. In a hard or hard plastic state under natural conditions;

2. Most of them appear in the terrace of second or above second grade, the hill front and the hilly area of the basin edge, the terrain is gentle, and there is no obvious natural steep;

3. Common shallow plastic landslide, ground crack, newly dug pit (trough) wall collapse easily;

4. Cracks in buildings open and close with climate change.

Iii. Measures to be taken for the design and construction of bridge and Culvert foundation engineering on Expanded soil foundation

1. Soil exchange pad

2. Reasonable choice of foundation embedment depth

3. Lime grouting reinforcement

4. Reasonable selection of base types

5. Reasonable selection of construction methods

Frozen soil

Definition of Frozen soil

Frozen soil is soil with a temperature below zero degrees Celsius and containing ice. Frozen soil is a kind of temperature sensitive and unstable soil. Ice in frozen soil can exist in the form of ice crystals or ice sheets, ice crystals can be as small as microns or even nanometers, and ice sheets can be as thick as meters or hundreds of meters, thus forming a variety of cold structures in frozen soil.

Distribution of frozen soil

The distribution area of permafrost on the Earth accounts for about 23% of the land area, mainly distributed in Russia, Canada, China and Alaska in the United States. The permafrost is mainly in the alpine permafrost zone and the discontinuous permafrost zone, and the permafrost area is about 206.8&times. 104 km2 is second only to Russia (1000× 104 sq. km) and Canada (390× 104 square kilometers) is about 1.5 times the size of the United States. China is the third largest country of frozen soil in the world, accounting for about 10% of the world's permafrost distribution area, accounting for about 21.5% of China's land area, and China's permafrost is mainly distributed in the middle and low latitudes. The Roof of the world ” On the Qinghai-Tibet Plateau, others are distributed in the Pamir, the western high mountains (Qilian Mountains, Alshan Mountains, Tianshan Mountains, Sijunggar Mountains and Altai Mountains, etc.), the northeast and the Greater Hingan Mountains, and the east.

Three, anti-frost heave measures

At present, frost heave is usually prevented by reducing frost heave force and improving the frost heave property of surrounding frozen soil:

1. To replace the soil on four sides of the foundation, the coarse grain soil such as pure sand and gravel is used to replace the frozen soil around the foundation, and the filling earth is compacted.

2. To improve the smoothness of the foundation side surface, the foundation must be cast tightly and have a smooth surface. The sides of the foundation can also be coated with industrial vaseline and residual oil in the range of frozen soil to reduce tangential frost heaving force. The tangential frost heave force can also be reduced by concrete casing for pile foundation.

3. The anti-frost heave foundation is selected to change the section shape of the foundation, and the self-anchoring action of frost heave reaction is used to increase the uplift resistance of the foundation.