The collapsibility of loess is very harmful to human engineering activities, which often causes damage to buildings, channels, reservoir banks and road slope protection. The following is my research paper on collapsible loess foundation treatment, to share with you.

By combing the causes of collapsible loess and the application scope, advantages and disadvantages of various foundation treatment methods, combined with the engineering example of collapsible loess foundation treatment scheme of Hou railway house, through technical and economic analysis, the general methods of railway house foundation treatment in collapsible loess area are obtained, which has a good reference for railway house construction in collapsible loess area.

Key words: collapsible loess; Railway houses; Foundation treatment cushion method; Compaction method

Collapsible loess is a kind of soil with special properties, which has the property that under the action of self-weight or external load or both, the structure is quickly destroyed and suddenly sinks after being soaked in water. It is widely distributed in China, mainly in most areas of Shanxi, Shaanxi, Gansu and western Henan. In addition, Xinjiang, Shandong, Liaoning, Ningxia, Qinghai, Hebei and Inner Mongolia are also distributed, but not continuously.

Railway house is the core of railway production and operation, which plays the role of command, dispatch, monitoring and service, plays a very important role in the safe operation of the whole railway and is an indispensable part of the whole railway system. In view of the harmfulness of collapsible loess and the importance of railway station building, treatment measures must be taken to ensure the structural safety of railway station building in collapsible loess area.

Because there are few special studies and examples on the collapsible treatment measures of railway houses in China, this paper discusses this problem from the perspective of combining theory with practice.

Collapsibility mechanism and prevention measures of 1 loess

1. 1 Collapsibility mechanism of loess

The structural characteristics of loess and the water solubility of cementing materials determine the mechanism of loess collapse [1]. Collapsible loess is a kind of unsaturated undervoltage dense soil with large pores and developed vertical joints. Under natural humidity, it has low compressibility and high strength. However, when it is immersed in water, its strength is obviously reduced. Collapsible deformation caused by additional pressure or under the action of additional pressure and soil weight pressure is a kind of large settlement and fast settlement.

(1) The mechanical properties of loess have changed from the inside. Under the factors of water immersion and external load, the shear stress exceeds the shear strength, leading to collapse.

(2) The loess is wetted by water, which reduces the friction of the soil itself and causes collapse due to external disturbance.

(3) The internal structure of loess collapses, which weakens the bond strength between loess particles and makes the particles migrate relatively, accompanied by small particles entering the big gaps. At the same time, because the bond between particles is dissolved by water, the strength is out of balance under external disturbance, which leads to the destruction of soil structure.

1.2 preventive measures for collapsible loess

The collapsibility of loess is very harmful to human engineering activities, which often destroys buildings, channels, reservoir banks and road slope protection. The author thinks that changing the structural characteristics of collapsible loess is the core to prevent loess from collapsing, and preventing or reducing the collapse of building foundation is the key to prevent loess from collapsing.

Comprehensive measures to prevent the collapse of collapsible loess foundation mainly include foundation treatment, waterproof measures and structural measures, among which foundation treatment measures are mainly used to improve the physical and mechanical properties of soil and reduce or eliminate the collapse deformation of foundation; Waterproof measures are mainly used to prevent or reduce the foundation from being soaked by water; Structural measures are mainly used to reduce or adjust the uneven settlement of buildings, or to adapt the superstructure to the deformation of foundation [2]. The functions, functions and emphases of the three measures are different. In practice, comprehensive measures should be taken to treat both the symptoms and root causes, highlight key points and eliminate hidden dangers.

The foundation treatment measures to prevent loess from collapsing mainly include the following:

(1) cushion method: including soil cushion and lime-soil cushion. It is to remove part or all of the collapsible soil layer below the basement, replace it with 2:8 or 3:7 lime soil, and tamp it by layers. It is suitable for removing collapsible loess below 1 ~ 3m basement. It has the advantages of simple and fast construction and low cost, but the thickness of loess treated by this measure is limited and it is not applicable.

(2) Dynamic compaction method: the rammer (10 ~ 60t) is repeatedly lifted to a certain height (10 ~ 40m) to make it fall freely, giving the foundation impact and vibration energy, thus improving the bearing capacity of the foundation, reducing its compressibility and improving the foundation performance. This method has the advantages of simple equipment, short construction period, labor and material saving and low cost. The treatment depth is generally 3. The effect on non-self-weight collapsible loess is obvious. The site is required to have enough space, high soil water content and large vibration caused by construction, which will have an impact on the surrounding buildings. It is not suitable for urban use, and the test before construction and the inspection after completion take a long time (30 days).

(3) Compaction method: the piles compacted with lime soil or soil layer form the reinforcement, and together with the compacted soil between piles form the composite foundation to bear the upper load of the foundation. Hole-forming compaction mainly includes pipe sinking, impact, tamping expansion and blasting expansion, which is suitable for collapsible loess with a thickness of 3 ~ 15m. Due to the double action of piles and soil between piles, soil compaction is caused.

(4) Pre-soaking method: it is suitable for treating sites with a thickness greater than 10m and a calculated weight collapse of not less than 500mm. After soaking, the ground subsidence below 6m can be completely eliminated, and the ground subsidence within 6m can also be greatly reduced. However, this method takes too long, often affects the construction period and consumes a lot of water, so it is not suitable for water-deficient areas. Floods lead to ground subsidence and cracking around the site, which easily affects the safety of nearby buildings. Therefore, it is stipulated that the distance from the existing building should not be less than 50m, and it is generally necessary to carry out supplementary investigation after flooding, re-evaluate the collapsibility of the foundation soil, and treat the upper collapsible loess layer with cushion or other methods, which invisibly increases the cost and prolongs the construction period.

When the above foundation treatment measures cannot meet the design requirements, pile foundation can be used to eliminate the collapsibility of loess. This method is safe and reliable, but the investment cost is high.

Two engineering examples

2. 1 Railway Station Building Complex

The newly-built Baishui Station, Chengcheng Station and Heyang North Station of Hou Railway are located in Weinan City, Shaanxi Province in Longdong. Northern Shaanxi? In the collapsible loess belt in the west of Shanxi Province, the self-weight loess is widely distributed in this area, the thickness is generally greater than 10m, and the foundation collapsibility is generally 3 ~ 4, so the collapsibility is sensitive. Through field drilling, it is found that all three stations are grade 4 self-weight collapsible loess, which conforms to the judgment of Building Code for Collapsible Loess Area (GB 50025-2004). Taking the construction site of Chengcheng Station as an example,

(1) Artificial fill: about 1. 1m thick.

(2) Cohesive loess: about 23.8m thick, yellow-brown, mainly composed of clay particles, with average viscosity, uniform soil, loose structure, needle-like pores and wormholes, and a small amount of calcium filaments and calcium nodules. The core is loose and short columnar, in which:15.10 ~15.70 m contains a lot of calcium wires. The self-weight collapsibility grade is Grade IV. fak= 150kPa。 (3) Sandy loess: about 1.3m thick, brown, mainly composed of clay particles, with average viscosity, uniform soil texture, compact structure, a small amount of needle-like pores, hard plastic, short columnar core and self-weight collapsibility of Grade IV. FAK = 65438。

Through on-site drilling detection, Baishui Station and Heyang North Station, like Chengcheng Station, are all Class IV self-weight collapsible loess with large thickness. The station building is a small railway station building with 300 people. The building structure adopts fully cast-in-place reinforced concrete structure, with main body 1 floor and local 2 floors, building height 12m, building area of 2500m2, design service life of 50 years, and building foundation.

According to the design requirements, the characteristic value of bearing capacity of composite foundation after treatment is not less than 180kpa, and the average compaction coefficient of soil between piles is not less than 0.93.

According to the comprehensive analysis and calculation of the site situation, building structure and the requirements of relevant national codes, the foundation of the third station building is treated with lime-soil compaction piles. The lime-soil compaction pile material is 2: 8, the pile diameter is 400mm, the pile length is 10m, and the pile spacing is 900 mm. After construction, the characteristic value of the bearing capacity of the composite foundation of the three-station building is 225kPa, and the soil between piles is evenly compacted.

2.2 Single-story small masonry houses

The waiting room and purification room of Hancheng Station of Hou Railway are located in Hancheng Railway Station in hancheng city, Shaanxi Province. Geological data of the proposed house are as follows:

(1) Miscellaneous fill: It is composed of silty clay and a large number of brick and tile fragments, with disordered structure and uneven soil quality, and the thickness is about 1m, which is Grade II ordinary soil.

(2) Cohesive loess: distributed below the bottom surface of the fill, yellow-brown, hard plastic? Plastic, wormholes and macropores developed, collapsible, distributed in the construction site, moderately compressible soil, with a thickness of about 10m, and a brownish red paleosol layer at the bottom. It is Grade II ordinary soil, fak= 120kpa, and its self-weight collapsibility is Grade III.

The building area of the waiting room and clean room is 68.64m2, 1 floor and 3.6m floor. The structure is brick-concrete structure with a design service life of 50 years. The design level of foundation is Grade C, and the foundation adopts strip foundation under column and wall. The characteristic value of the treated foundation bearing capacity shall not be less than 180 kPa.

During foundation treatment, a 3:7 lime-soil cushion is replaced under the foundation, with a thickness of 3m and a width of not less than 1.5m on each side. The cushion is compacted by layers, and the compaction coefficient is not less than 0.97. After foundation treatment by replacement method, the compaction coefficient is not less than 0.97, and the characteristic value of foundation bearing capacity is 195kpa, which meets the design requirements.

2.3 Multi-storey frame houses

(3) Zhiyang Station of Hou Railway is a comprehensive building with a building area of 639.52m2 and a two-story frame structure with a height of 8.9m The geological data of the proposed building are: cohesive loess; Light brown? Yellow-brown, with a thickness of more than 20m, uniform soil, containing calcareous reticulate and sporadic ginger stone, hard plastic, general soil of Class II, fak= 150kpa, self-weight collapsible loess, with a collapsibility grade of Grade IV and a collapsibility soil layer thickness of about 25-35m. Surface water is underdeveloped. No groundwater was found in the drilling depth, and the freezing depth of soil was 37 cm. It is required that the characteristic value of bearing capacity of composite foundation after foundation treatment is not less than 65438.

3:7 lime-soil compaction pile is proposed for foundation treatment. The pile length is 6m, the pile diameter is 450mm, and the pile spacing is 1000mm. However, due to the fact that the site construction site can't meet the requirements of the width of lime-soil compaction pile, the foundation treatment scheme is modified to 3: 7 lime-soil replacement treatment, and the treatment thickness is 3m below the basement and 2m outside the foundation edge, so the compaction coefficient of lime-soil is not high.

After field construction, the characteristic value of foundation bearing capacity is greater than 180kpa, which meets the design requirements.

2.4 Project cost analysis

As can be seen from the above table, lime-soil compaction piles should be used for foundation treatment of important buildings of railway houses, such as railway station houses, to ensure structural safety, and lime-soil cushion should be used for foundation treatment of other houses. Compared with the compaction method, it can not only effectively save the construction period, but also significantly reduce the cost, so it can be used as the first choice for the treatment of collapsible loess in railway houses.

3 Conclusion

(1) Taking the collapsible loess foundation treatment of the railway building in Shaanxi Province as an example, according to the site and geological conditions where the building is located, the foundation treatment schemes of cushion method and compaction method are discussed, and the economic and technical analysis is made. It is concluded that the lime-soil compaction method is suitable for railway station buildings, and the cushion method is suitable for geological treatment of collapsible loess for general buildings.

(2) At present, the research results of collapsible loess mainly focus on local civil buildings, while railways mainly focus on subgrade treatment [3,4], and the research on railway houses is few and single [5]. Because of the particularity of railway houses, the next step should be to strengthen the exploration and research on the foundation treatment methods of railway houses in collapsible loess areas, especially other technologies (such as DDC piles, bored piles and bored piles) that are less used in local civil construction.

(3) In the treatment of collapsible loess foundation, according to the requirements of Building Code for Collapsible Loess Area (GB50025) and Technical Code for Building Foundation Treatment (JGJ79), technical and economic analysis should be made in combination with the characteristics of buildings, geological conditions, required foundation bearing capacity, construction difficulty and investment saving, so as to select the most suitable building foundation treatment scheme and avoid waste of resources.

(4) The lime-soil compaction pile construction machinery belongs to large-scale construction machinery. Due to the cost, the machinery used now is relatively old. Therefore, it is necessary to study a set of feasible safety control methods and safety operation procedures to ensure construction safety.

(5) After foundation treatment, collapsible loess building foundation can eliminate or reduce collapsibility and meet the requirements of building load. However, in actual operation, structural measures should also be taken (such as considering frame structure and setting deformation joints inside and outside the structure, etc.). ), waterproof (such as expanding the apron width outside the station building, increasing waterproof coiled material, etc. ), continue to research and explore the structure, waterproofing, maintenance and other aspects, and constantly explore ways to prevent collapsible loess from collapsing in practice.

References:

(1) Zhang Xiaoyu. Foundation Treatment Measures for Buildings in Collapsible Loess Area [J]. Shanxi Architecture, 20 14 (3): 76.

(2)GB 50025-2004, Code for Building in Collapsible Loess Area [S].79.

(3) Yao Yuchun, Li Anhong, Luo, Sun Ying. Study on treatment technology of collapsible loess foundation in Zhengxike College [J]. Journal of Railway Engineering Society, 20 13 (9): 15? 19.

(4) Zhao Ruyi. Research and Design of Treatment Measures for Collapsible Loess Foundation of Hou Railway [J]. Science and Technology Innovation Guide, 2013 (5):113.

(5) Yuan Erli. Treatment scheme and cost analysis of building foundation in collapsible loess area [J]. Railway engineering cost management, 20 13 (3): 8- 10.

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