Comprehensive Analysis of Collapsible Loess

—Loess is a kind of special sediment with pore structure and rich carbonate in its composition. Due to the characteristics of structure and composition, most loess has collapsibility. There are rich loess resources distributed in the northwest area of China. With the improvement of the traffic network in the west area, the use of loess as roadbed is gradually increasing. Therefore, during the construction process, the engineering problems encountered by loess also increased, which seriously hindered the construction process of the traffic network. Most of loess in our country is collapsible loess. So the common problem in engineering is how to dispose the collapsibility of collapsible loess.In this paper, the characteristics and evaluation indexes of loess are summarized and studied, the mechanism of loess collapsibility is further explored, and the three methods of loess collapsibility treatment are summarized, which will provide guidance for future engineering problems in collapsible loess areas.


1
Characteristics of loess

Definition and causes of loess
Loess is a kind of Quaternary sediments with a series of characteristics of internal material composition and external morphology. Throughout the ages, the human understanding of loess has been gradually developed. In the beginning, people called the "pale yellow calcareous silt" loess. Later, B.A. Obluchev [1], Pecsi [2], Smili [3], K. Pai [4] and others have defined loess. In China, scholars led by Liu Dongsheng have explored China's loess and combined with the central idea of Obruchev's point of view, the yellow powdery with large pores, no secondary disturbances, no layered yellow powder, and wind transport accumulation of soil-like sediments with large pores and no secondary disturbances have been called loess, and sediments of other causes that have similar characteristics in some respects are called secondary loess. [5] The genesis of loess is controversial. Researchers have different basic views on loess, so their conclusions are also different. In recent years, three widely accepted theories have been formed: the theory of water formation, the theory of wind formation and the theory of weathering residue.

Classification and distribution of loess
Due to the differences in the era and environment of loess formation, loess has different development laws and engineering properties. According to the era of loess formation, it can be divided into Wucheng loess, Lishi loess, Malan loess, Newly stacked loess and Loess-like soil,as shown in the table1; According to whether the loess has collapsibility, it can be divided into self-weight collapsible loess, non -weight collapsible loess and noncollapsible loess, as shown in the table2. After being soaked in water, it will produce collapsibility due to its own gravity or external force is called collapsible loess. Under the influence of water and no external force, the soil layer will also be collapsible due to its own gravity.

Nongravity collapsible loess
After being soaked by water, the loess will not produce collapsibility without external force.

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Non-collapsible loess After being soaked by water, the loess will not produce collapsibility either under the influence of self-weight or external force. Loess covers about one-tenth of the world's land area, and is intermittently distributed in the forest-steppe, steppe and desert-steppe areas in the mid-latitudes of the northern and southern hemispheres in an east-west band. China is the country with the widest distribution and thickness of loess in the world, covering an area of about 630,000 square kilometers. It is distributed in Inner Mongolia, Hebei and Shanxi in North China, Xinjiang, Qinghai, Ningxia, Shaanxi and Gansu in Northwest China, Heilongjiang and Liaoning in Northeast China, Shandong in East China, and Henan in Central South China [6]. The distribution of loess varies from 200m to 2400m, and the distribution location of individual loess has reached an altitude of more than 3000m. The Loess Plateau in northwest China is the largest Loess Plateau in the world, and the Loess Plain in North China is the largest Loess Plain in the world.

The formation of the Loess Plateau
In China, there are two main types of air currents that form the Loess Plateau: one is the winter wind, which starts from the strong storms in the Gobi and Inner Mongolia Deserts of Mongolia, carries the dust to the southeast, and forms sedimentary dust on the Loess Plateau with dry landing and wet landing. The other is the rapids, which carry dust wrapped in the desert from high above and lifted up by the updraft, across the Loess Plateau to the Pacific Ocean. [7] Figure 1 is the picture of Loess Plateau.

Figure 1 Loess Plateau
As shown in the figure 2 .In the process of dust transport by wind, the separation of loess material is also completed. The loess contains coarse silt, fine silt and clay particles, and the clay particles are mostly bonded with fine silt to form aggregate size. Similar to wind dust, the large amount of coarse silt in loess makes it suitable for suspension in the air and long-distance transport. When the wind blows, the coarse sand rolls, forming a close distance transport; Medium and fine sand can be transported over long distances in the form of jumping. Silt, especially fine silt and clay, is suspended in the air, floating in the wind. This sorting process migrates the dust from the northwest Gobi to the desert, where the dust falls and deposits due to the weakening wind speed or the terrain obstacles and rough ground, and finally accumulates into the Loess Plateau. [8]  Index classification of loess evaluation

Index of physical properties
The main physical properties of loess include density, gravity, pore ratio, etc. They are the basic properties of loess. The physical properties of loess are shown in the table 3. Annotation: m-the quality of the soil; V-the volume of the soil; m s -the quality of soil particles; m v -the quality of water in the soil; V s -the volume of soil particles; V wthe volume of water in the soil; V a -the volume of pores in the soil;G s -the specific gravity of soil particles; ρ w -the density of water; n-poriness.

Index of hydrological properties
Hydrology refers to the study of the properties of loess with water variation due to quantity and quality, it mainly includes the plasticity index (I P ) and liquidity index (I L ) of loess.Its classification is shown in the table 4 and table 5.

Index of mechanical properties
The mechanical properties index mainly includes compression coefficient a and compression modulus E S , which are directly measured by indoor tests, and the calculation formula and compressibility classification of soil are shown in the table 6 and table 7.

Index of loess collapsibility
The collapsibility indexes include the collapse coefficient (δ S ), the self-weight collapse coefficient (δ ZS ), etc, which are generally measured by indoor tests. [9] The classification of loess collapsibility classification is shown in the table 8.

Analysis of loess collapsibility
The root cause of loess collapsibility is the extremely unstable granular overhead pores in the loess, as shown in the figure 3. After wetted by water, under the dead weight or external load, the overhead pores will collapse and damage, the large pores will become medium pores, and the medium pores will become small pores, which will reduce the structural strength of the loess, break the balance of the original soil particles, and then collapse and other situations will occur. Under the domino effect, the loess will collapse. By collecting a large number of literature and the research achievements of predecessors ,we can find that stress, water content, porosity, depth and severe, clay content, the soluble salt, cause of the loess, effect of the loess are the influencing factors of loess collapsibility.The initial water content, loess depth, loess severe, dry density, clay content and pedogenesis time are negatively correlated with the coefficient of collapsibility, while the immersed water content and pore ratio are positively correlated with the coefficient of collapsibility. [10] [11] Loess is characterized by its rich pores, and it is precisely because of so many pores that it provides space for the wetting of loess. The basic consensus is that the more developed the pores between the skeleton particles, the greater the amount of collapsibility, which is basically medium pores or intergranular pores, and macroscopic pores such as root holes and wormholes have no effect on collapsibility.
Since the formation of loess requires a long period of geological evolution, the efficiency of wind-blown accumulation or flood deposition is relatively low, and the speed of solidification and connection of loess particles is faster than the increase of the load pressure in the upper layer. Therefore, the loess will not be in a state of undercompression due to the action of the upper load, which is one of the factors affecting the extremely collapsible structure of the loess. The collapsing nature of loess and its compressibility, from top to bottom, will be weakened to a certain extent, physical indicators also show regular changes In addition, the load state of loess, to a certain extent, will have a greater impact on the initial pressure of loess collapse, and also affect the category of loess collapse, loess with excessive initial pressure will appear non-self-weight collapse or no collapse during the period of high humidity, and vice versa will appear selfweight collapse .

Treatment of loess collapsibility
In engineering, there are many ways to eliminate the collapsibility of loess, such as destroying the porous structure of soil, improving the engineering properties of soil, eliminating or reducing the collapse deformation of foundation, preventing water from immerging in building foundation, and improving the stiffness of building structure. The main methods are divided into three kinds: foundation treatment, waterproof measures, structural measures. Foundation measures are mainly used to improve the physical and mechanical properties of soil.
Waterproof measures (such as waterproof ground, drainage ditch) are mainly used to prevent or reduce the foundation by water immersion; The structural measures are mainly used to reduce and adjust the uneven settlement and make the superstructure adapt to the deformation of collapsible loess foundation. Among them, the foundation measures mainly include cushion method, compaction method, dynamic compaction method, presoaking method, pile foundation method and chemical consolidation method. [12] [13]

Conclusions
The collapsibility of loess has been a long-standing problem. Researches on loess collapsibility mainly include characteristics of loess, evaluation indexes of loess, analysis of loess collapsibility and treatment methods and so on.The following conclusions are drawn: 1. There are many different views on the origin and classification of loess,and the cause of loess has mainly formed three widely accepted theories: the theory of water formation, the theory of wind formation and the theory of weathering residue; The classification of loess is mainly based on engineering geology and collapsibility.
2. Evaluation indexes mainly include physical properties, hydraulic properties, mechanical properties and collapsibility indexes. In addition, the physical and mechanical indexes of loess have a certain relationship with the collapsibility. In the future, the correlation between the representative indexes of soil and the collapsibility coefficient can be determined by comparing the relationship between the indexes and the collapsibility at different depths, so as to better serve the actual engineering.
3. Loess collapsibility is a very complicated physical, hydraulic and mechanical process, which is the result of the interaction of the special factors inherent in the soil and the appropriate external conditions. The basic principle of eliminating loess collapsibility is to improve soil quality and block water. The commonly used methods include cushion method, compaction method, dynamic compaction method, presoaking method, pile foundation method and chemical consolidation method. In actual construction, the project can comprehensively consider the scheme to eliminate the collapsibility according to the thickness of collapsible loess layer, the form of superstructure, the depth to be treated and the cost.