Research on Soft Soil Subgrade Treatment Technology

Soft soil subgrade cannot simply be determined based on subgrade conditions. Due to the fill shape and construction conditions, it is necessary to fully study the fill and structures. Based on the type, form, scale and subgrade characteristics, judge whether it should be treated as soft soil subgrade. The following is a research paper on soft soil roadbed treatment technology that I compiled for you. I hope it will be helpful to you.

Abstract: In recent years, the problem of soft foundation treatment in road engineering construction has increasingly become the main factor affecting the project cost and road construction quality, and has attracted more and more people's attention. Therefore, the study of soft soil roadbed treatment technology is also of great significance. This article first explains the concept and characteristics of soft soil roadbed, and discusses several common soft soil roadbed treatment technologies in detail.

Keywords: highway; soft soil subgrade; reinforced soil; cement mixing pile; drainage consolidation

1. The concept and characteristics of soft soil subgrade

(1) The concept of soft soil roadbed

It is mainly composed of soft soil with a large content of fine particles such as clay and silt, organic soil with large voids, peat soil and loose sand. Their members Although the structures and shapes are different, they all have the characteristics of large water content, high compressibility, low strength and poor water permeability. Soft soil subgrade cannot simply be determined based on the subgrade conditions. Due to the fill shape and construction conditions, it is necessary to fully study the type, form, scale, and subgrade characteristics of the fill and structures to determine whether it should be determined according to the soft soil subgrade. deal with.

During the road construction process, some soft soil foundations were deformed during the filling process, and the pavement could not be paved in a final shape; some roads were barely paved, but the soft foundation was deformed and the work was not completed until acceptance. The road surface begins to become unstable; sometimes it deforms during operation, which not only requires annual repairs, consuming a lot of manpower, material and financial resources, but also affects driving safety and interrupts traffic. When building roads on soft soil foundations, reinforcement treatment must be carried out first; therefore, strengthening research on the effects of soft foundation treatment is of great significance to ensuring the quality of road projects.

(2) Characteristics of soft soil roadbed

Soft soils of different origins in various parts of my country have nearly the same corrosion resistance. The main manifestations are: 1. High natural moisture content , the void ratio is large. The water content is between 34% and 72%, the void ratio is between 1.0 and 1.9, the saturation is generally greater than 95%, and the liquid limit is generally 35% to 62%. 2. Poor water permeability. The permeability coefficient of most soft soils is 10-8 ~ 10-7㎝/s. 3. High compressibility. The compression coefficient is 0.005~0.02, which is a highly compressible soil. 4. Low shear strength. my country's soft soil is naturally undrained, with a shear strength of less than 20kpa and an effective internal friction angle of 20 to 350. Under the action of load, if the soft soil roadbed can be drained and consolidated, the shear strength of the soft soil will change significantly. The faster the drainage and consolidation speed of soft soil, the more obvious its strength improvement effect will be. 5. It has the ability to change gender. Once disturbed, the strength of the soil decreases significantly and even becomes a fluid state. 6. Significant rheology. Under the action of shear stress, the soft soil is subjected to shear stress, which slows down the shear deformation and may lead to the attenuation of the shear strength. After the consolidation settlement is completed, the soft soil may also produce considerable secondary settlement.

2. Key technical points for the treatment of soft soil subgrade in highway engineering

(1) Treatment methods for shallow layers of soft soil subgrade Shallow layer treatment of soft soil subgrade refers to the treatment depth of the road bed. If it exceeds 5m, the treatment methods mainly include reinforced soil method, dynamic compaction method, filling method and riprap siltation method, etc., which will be explained in detail below.

1. Reinforced soil method.

The reinforced soil method is to embed geotextiles or geogrids into the foundation soil. The two form a whole and increase the pressure diffusion angle, thus improving the bearing capacity of the foundation and reducing its settlement. The reinforced soil method is generally suitable for embankments formed by backfill soil, and is suitable for soft soil, sandy soil and clay soil. Geogrid can usually be used as the same layer as the sand cushion layer. Through this layer, the load of the embankment body is transferred to the soft soil foundation. This layer has a different stiffness from the embankment itself and the soft soil foundation. It is both The flexible foundation of the embankment is also the drainage surface when soft soil is consolidated. After the treatment of this cushion layer, the foundation becomes uniform, the construction speed is fast, the final settlement of the center of the roadbed is smaller than that of materials without geosynthetic materials, and the lateral deformation of the embankment will also be reduced due to the installation of geogrids. It can quickly achieve the purpose of improving the bearing capacity and stability of the foundation.

2. Strong compaction method.

The dynamic compaction method uses lifting equipment to lift a rammer with a large weight (80-300KN) and a certain shape and structural specifications to a certain height (usually 6-30m), and then drops it freely to give it a The foundation soil is rammed with powerful impact energy, which causes strong vibration and high dynamic stress in the foundation soil. The soil layer is tamped to a certain depth below the ramming surface to increase the bearing capacity and stability of the foundation and reduce the compressibility. A soft soil foundation treatment method. Due to its large ramming capacity and large reinforcement depth, it has a good effect on general soft soil foundation reinforcement.

It is a method of quickly reinforcing soft foundations. The construction equipment is simple, the construction process is simple, no reinforcement materials are needed, the cost is low, the cycle is short, it is applicable to a wide range of soil types, the reinforcement effect is significant, and it can achieve higher bearing capacity. Generally, foundations The strength can be increased by 2 to 5 times; the deformation and settlement is small, and the compressibility can be reduced by 2 to 10 times; the depth of reinforcement influence can reach 6 to 10m. The soil particles are closely combined and have high structural strength. It is a commonly used soft soil Foundation treatment methods.

3. Dredging and filling method.

This is the most commonly used method. This method is to dig out all or part of the weak soil and replace the soft soil and silt soil with good soil. When it is convenient to obtain materials, try to replace the filling with permeable soil in order to ensure the stability of the filling and reduce settlement. It is suitable for surface silt, peat soil, no hard shell, thickness no more than 3m and regional drainage construction. When excavation, the depth is within 2m, and it is directly removed manually or mechanically to the area outside the roadbed and stacked or transported to the borrow pit. Return to the field; when the depth exceeds 2m, it must be excavated in layers from the ends to the center, and temporary access roads must be built to be carried out of the pit by vehicles; the depth of replacement and filling must be determined according to the bearing capacity.

4. Dumping method.

In lakes, rivers or water-logged depressions, where water accumulates all year round and is difficult to drain, and the thickness of soft soil is thin, throw-in gravel should be used. The gravel should not be less than 30cm. When throwing and filling, start from the center line to both sides. When the cross slope is steeper than 1:10, start throwing and filling from high to low, so that the silt is squeezed out to both sides. After the debris is thrown out of the water, it should be filled with small stones and leveled with heavy-duty materials. The road roller will crush it, lay a filter layer on it, and then fill it with soil.

(2) Treatment technology for deep soft soil subgrade

1. Cement mixing pile reinforcement.

(1) Reinforcement principle. The basic principle of cement mixing pile reinforcement is based on the physical and chemical reaction process of cement-reinforced soil. Mechanical equipment is used to spray cement into the soft soil subgrade of the road to be treated, and continuously stir it up and down evenly to promote hydrolysis and hydration reactions between the cement and soil. And form a gel, eventually forming a stable structural whole, thereby improving the overall strength of the land and meeting the requirements for the bearing capacity of the roadbed.

(2) Reinforcement method. Cement mixing piles can be divided into two types according to the construction method: wet method and dry method. Wet mixing piles use cement as a curing agent and use machinery to perform continuous deep mixing. The soft soil and curing agent are vigorously stirred deep in the roadbed to form a composite foundation with sufficient strength. Cement mixing pile reinforcement is divided into slurry spraying method and powder spraying method. When the natural moisture content of the soil is greater than 30% and the plasticity index is greater than 10, the powder spraying method is generally used. Because under normal circumstances, within the same mixing time, the soft foundation treated by the powder spraying method is stronger than the slurry spraying method, but the slurry spraying method is convenient for construction and easy to control the construction quality.

(3) Issues that should be paid attention to during construction. Cement mixing piles must be constructed according to the technical parameters determined by the test. The drilling depth of the drilling rig, the powder spraying elevation and the ash stop surface should be controlled to ensure that the mixing piles have sufficient length. When spraying powder to connect piles, it is necessary to ensure that the overlap length of powder spraying is greater than 1m. During the construction of mixing piles, the cement pumping process must be continuous, and the dosage error of the curing agent should be controlled within 1%. After completing the mixing construction, lift the drill bit off the ground, turn on the air compressor to remove residual powder and attached soil in the pipes and nozzles, and then move the pile driver to the next pile position.

2. Drainage and consolidation method

The method of pressurizing the soft soil foundation and cooperating with internal drainage to accelerate the drainage of the soft soil foundation and accelerate the consolidation of the soft soil foundation is called Drainage consolidation method. It is suitable for treating various types of saturated cohesive soil foundations such as silt, silty clay and filling. There are mainly the following reinforcement methods:

(1) Loading preloading method and overload preloading method. This method is 1. The treatment of soft clay, silt, organic sediment and miscellaneous fill soil is simple, feasible and effective. There is a mature theory and the soil quality is relatively uniform; however, it takes a long time and requires the transportation of a large amount of earth and stone.

(2) Sand well (various plastic drainage boards): This method is suitable for inorganic soft clay. This method has mature design and construction experience and calculation theory. It is often combined with loading and preloading to achieve the best effect. It's possible, but construction and preloading will take months.

(3) Vacuum pre-pressure method: It is suitable for processing soft clay and can avoid the trouble of moving earth and stone. However, the pre-pressure is limited and the processing depth is not large.

3. High-pressure jet grouting method.

The high-pressure jet grouting method is a method in which a grouting pipe with a special nozzle is placed at a predetermined depth in the soil layer, and the solidified slurry is mixed with the soil using high-pressure jet flow to solidify, harden and strengthen the foundation soil. It is suitable for foundations such as silt, silty soil, clay soil, silt, loess, sandy soil, artificial fill soil and gravel soil. The high-pressure jet grouting method has developed rapidly in Italy, Japan and the Federal Republic of Germany. Italy and Radio have also developed machinery that can simultaneously detect the soil quality of the formation during drilling, control the machine, automatically adjust the designed grout volume, and collect feedback information. The country also attaches great importance to it and has conducted some exploratory experiments.

References

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