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Summary of soft soil foundation treatment methods

Du Yanhua (Jingmi Project Department, No.5 Engineering Co., Ltd., China Communications No.1 Bureau)

This paper introduces the definition and characteristics of soft soil and soft soil foundation, discusses the harm caused by soft soil foundation in highway engineering, and introduces several treatment measures of soft soil foundation, which has certain guiding significance for soft soil foundation construction.

Key words: dry jet grouting pile method for soft soil foundation, geogrid soil replacement cushion method

Since the reform and opening up, China's highway transportation industry has experienced unprecedented development opportunities and made brilliant achievements. With the development of national economy, the influence of highway on economic development is increasing, and it is also paid more and more attention by the state. Although the level of expressway construction in the southeast coastal areas ranks among the top in China, highway diseases on soft soil subgrade also occur from time to time. In particular, the phenomenon of bump at the bridge head is serious, which affects the use function of expressway. Because the settlement difference between the bridge head and the embankment is too large, it causes traffic accidents and has to be treated repeatedly, which not only costs money, but also causes serious social impact. In order to ensure the safe operation of roads, the treatment of soft soil subgrade is particularly important.

1 Soft soil and soft soil foundation

1. 1 soft soil

Soft soil refers to fine-grained soil with high natural water content, large void ratio, high compressibility and low shear strength deposited in coastal areas, lakes, swamps, valleys and flood areas. It has the characteristics of high natural water content, large natural void ratio, high compressibility, low shear strength, small consolidation coefficient, long consolidation time, high sensitivity, large disturbance, poor water permeability, complex layered distribution of soil layers and great difference in physical and mechanical properties between layers.

1.2 Soft soil foundation

There is no definition of soft soil foundation in China's highway industry specification. Japan's high-grade highway design code defines it as: mainly composed of soft soil with high fine particle content such as clay and silt, organic soil with large pores, peat and loose sand. Foundation and settlement with high groundwater level and poor stability of filler and structure. Japanese code also classifies soft soil foundation, and puts forward the general judgment standard of this type. When the definition of soft soil foundation is given, it is pointed out that soft soil foundation can not be determined simply by foundation conditions, but changes with the shape of fill and construction conditions. It is necessary to judge whether soft soil foundation should be treated on the basis of fully studying the types, forms, scales and foundation characteristics of fill and structures.

2 the harm of soft soil foundation in highway engineering

(1) The survey and design are not detailed and accurate, which leads to the failure to design the road section that should be treated with soft foundation.

(2) It is known as a soft soil foundation, but the soft soil foundation is not properly treated, which leads to the instability of the embankment or endangers the buildings outside the line.

(3) Although the soft soil foundation has been treated, the measures are not effective and improper construction causes the embankment to be unstable.

(4) Improper stacking, failure to fill in layers according to regulations, too fast filling and improper rolling, resulting in instability of the embankment.

(5) Disturbing the "hard shell" or improper filling will destroy the "hard shell" and lead to the instability of the embankment.

3 Soft soil foundation treatment methods

There are many methods of foundation treatment. Compared with other foundation treatment such as building construction, soft foundation treatment of expressway has its own characteristics. Generally, the geological stability of subgrade is considered from the following aspects:

(1) Improved shear characteristics

The shear failure and stability of subgrade under earth pressure depend on the shear strength of subgrade soil. Because in order to prevent shear failure and reduce earth pressure, certain measures need to be taken to increase the shear resistance of subgrade soil.

(2) improve the compression characteristics

Measures should be taken to improve the compressive modulus of foundation soil to reduce the settlement of foundation soil.

(3) improve water permeability

Because it is a problem in groundwater movement, it is necessary to take measures to make the foundation soil impervious or reduce its water pressure.

(4) improving dynamic characteristics

Saturated loose silt (including some silt) will liquefy during earthquake, so measures should be taken to avoid liquefaction of foundation soil and improve its vibration characteristics in order to improve the seismic performance of foundation.

(5) Using special soil to improve the characteristics of poor foundation.

It mainly refers to eliminating or reducing the bad foundation characteristics of special soil such as collapsibility of loess and expansion and contraction of expansive soil.

The methods of foundation treatment can be classified from different angles. Generally speaking, according to the principle of foundation treatment, they can be roughly divided into the following methods.

3. 1 soil replacement cushion method

When the bearing capacity or deformation of the soft soil foundation can't meet the design requirements, and the thickness of the soft soil layer is not very large, part or all of the soft soil layer within the treatment range under the foundation will be excavated, and then sand with high strength or other materials with stable performance and no corrosiveness will be replaced in layers, and compacted to the required density. This foundation treatment method is called soil replacement cushion method, which is called replacement filling method for short. It is suitable for treating silt, muddy soil, collapsible loess, plain fill and miscellaneous fill foundation.

The reinforcement mechanism of replacement method is to remove the soft soil layer by manual, mechanical or other methods, and replace it with sand, gravel, plain soil, lime soil and other materials with stable performance and no erosion, and compact (or tamp) it to the required density. When the thickness of soft soil is less than 3 meters, the method of full excavation and replacement can generally be adopted. When the thickness is more than 3 meters, usually only local excavation is adopted for filling. All excavation and replacement are fundamental measures to improve the foundation, leaving no future troubles, with the best effect and the most thorough measures. When the soft soil layer that the expressway route passes through is located on the surface, the thickness is thin (less than 3 meters), and there are soft soil or swamps in some areas, it is often appropriate to adopt the method of full excavation and replacement.

This method can be divided into: mechanical soil replacement method, blasting silt squeezing method, riprap silt squeezing method and sand cushion method.

3.2 Dynamic compaction method

Dynamic compaction was first developed in France in the late 1960s and early 1970s. It was called dynamic compaction abroad to distinguish it from static consolidation. Generally, a 50-ton rammer is used to lift the large-tonnage rammer (100 ~ 400 kn) to a height of 6 ~ 40 meters, which exerts strong impact energy on the foundation soil, forms shock waves and dynamic stresses in the foundation soil, and makes the foundation soil vibrate compactly, thus strengthening the foundation soil, improving the strength, reducing the compressibility, improving the anti-liquefaction conditions of sand soil and eliminating collapsible loess.

Dynamic compaction method is mainly suitable for strengthening sandy pebble soil, low saturation silt and cohesive soil, collapsible loess, miscellaneous fill and plain fill. Because of its remarkable reinforcement effect, simple equipment, convenient and fast construction, economy, material saving and environmental protection, it is quickly popularized around the world.

3.3 Constraint method

Driving wooden stakes, reinforced concrete piles or setting flaky tooth walls near the slope toe on both sides of the embankment can limit the extrusion of soft soil in the basement, thus ensuring the stability of the basement. After the lateral restraint of foundation is implemented, the filling speed of embankment can be uncontrolled. Compared with the back-pressure berm, it saves earthwork and occupies less cultivated land, but it needs a certain amount of three materials, and the cost is higher. This method is suitable for the case of thin soft soil layer, hard soil layer at the bottom and urgent construction period, especially the case of cross slope in the underlying layer.

3.4 geotextile reinforcement method

This kind of artificial composite soil can withstand stretching, compression, shearing or bending by burying solid geopolymer, tension bar and stress bar in the soil layer, thus improving the bearing capacity of foundation, reducing settlement and increasing the stability of foundation. Suitable for all kinds of soft foundation.

The basic principle of the reinforcement method is to transfer the tensile stress in the soil to the reinforcement through the friction between the soil and the reinforcement, and the reinforcement bears the tensile stress, and the soil between the reinforcement bears the compressive stress and shear stress, so that the reinforcement and the soil in the reinforced soil can better play their respective roles.

Common geotextiles include geogrids, geotextiles and geogrids, among which geogrids, like geotextiles and geogrids, can delay or cut off the sliding surface of foundation damage, thus improving the bearing capacity of foundation. Moreover, the geocell can give three-dimensional constraints to the soil particles in the geocell, so that the soil particles and geocell become a whole with much greater stiffness than the foundation, which can better distribute the load imposed on it and make the foundation bear more evenly, thus improving the bearing capacity of the foundation.

3.5 DJM pile method

DJM pile method is to use special equipment and machinery to forcibly mix reinforcing agent powder (cement or lime) with foundation soil through the transportation of compressed air, so that it can produce sufficient physical and chemical reaction and form a pile with certain strength (hereinafter referred to as DJM pile). This is a soft soil foundation reinforcement method to improve soil quality and improve foundation strength, and can be widely used for foundation reinforcement of muddy soil, miscellaneous fill and soft clay.

DJM pile is a method to treat soft foundation by deep mixing method. It uses compressed air to transport powdery reinforcement materials such as lime and cement to soft soil layer, so that it can be mixed with in-situ soft soil for compaction. Through a series of physical and chemical actions between reinforced materials and soft soil, soft soil is hardened into columnar reinforced soil with integrity, water stability and certain strength, and forms a composite foundation with in-situ soft soil layer, which improves the bearing capacity of soft soil foundation and reduces the cost.

3.6 High pressure jet grouting method

In China, it is referred to as high-pressure injection method or rotary injection method. In this method, a grouting pipe with a nozzle is drilled into the soil layer with a drilling rig to a designed depth, and slurry or water is ejected from the nozzle at high pressure to form a jet to impact and destroy the soil layer. When the dynamic pressure of high-energy and high-speed pulsating jet is greater than the structural strength of soil layer, soil particles will be stripped from the soil layer, and some fine particles will come out with mud or water. The remaining soil particles will be stirred and mixed with the slurry under the impact force, centrifugal force and gravity of the jet, and will be rearranged regularly according to a certain proportion and quality of the slurry. After the slurry solidifies, it will form a consolidated body in the soil layer, which can improve the bearing capacity of the foundation and reduce the settlement. Suitable for silt, muddy soil, cohesive soil, loess, sandy soil, artificial fill and gravel soil.

3.7 light subgrade fly ash treatment method

Fly ash is a light, porous, uniform particle and water-stable non-stick material. Because fly ash contains a certain amount of CaO, SiO2 _ 2, MgO and other components, their volume expands during the hydration of fly ash, and this expansion rate can be used to improve the reinforcement effect of soft foundation. Its road performance meets the technical requirements of highway.

Fly ash is light, and its maximum dry bulk density is about 1. 19/ resistance, which is about 40% lighter than ordinary soil. Filling fly ash on soft soil subgrade can effectively reduce the self-weight of embankment, reduce subgrade settlement and post-construction settlement, thus affecting subgrade treatment scheme and reducing foundation treatment cost.

Fly ash has high strength and high friction coefficient. In pavement design, fly ash improves the recovery of soft soil.

Elastic modulus value, reduce the design thickness of pavement accordingly.

The compaction test shows that the mixture of fly ash and soft soil has good dry density, water content and maximum dry weight.

The curve of density relationship is gentle, which is more conducive to field construction.

3.8 Cement-soil mixing method

Mixing cement, lime and other materials with soft foundation soil into piles with a blender. This pile becomes a cement-soil pile, a lime-soil pile or a cement-soil pile, which has certain strength and water stability. The composite foundation composed of mixing piles and surrounding soft soil can improve the bearing capacity, strength and deformation modulus of the foundation. Therefore, the mixing method can improve the bearing capacity of the foundation, reduce the settlement of the foundation, prevent the flow of water, enhance the stability of the foundation and prevent the infiltration of groundwater. Cement-soil mixing method is divided into deep mixing method (wet method) and powder spraying mixing method.

When dealing with soft soil foundations such as normal consolidated silt, muddy soil, cohesive soil with high water content, silt, etc., when it is used to deal with erosive peat soil or groundwater, its applicability should be determined through experiments.

When roads and bridges are built on soft soil foundation, problems do not always appear. As long as the design and construction measures are appropriate, the stability and use effect of embankments and bridges can be guaranteed. The design and construction scheme of embankment on soft soil foundation should be fully demonstrated in combination with local engineering geological conditions, material supply, investment environment, time limit for a project, environmental protection and other factors, in accordance with the principles of adjusting measures to local conditions, using local materials, constructing by stages and comprehensive treatment, so as to make the design and construction scheme technically advanced and economically reasonable.

There are many treatment methods for soft soil foundation. In a word, the purpose of soft soil foundation treatment is to increase foundation stability and reduce uneven settlement after construction. Therefore, construction technicians must be aware of the harmfulness of soft soil foundation, resolutely speak with data, carefully calculate the bearing capacity of foundation, and adopt practical treatment schemes according to different geological conditions, different investments and different construction periods. At the same time, they must collect post-construction settlement data of bridges and culverts to accumulate experience and lay a solid foundation for future construction.

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