The method of reinforcing the soft soil subgrade of highway bridges and culverts is very important. Only by understanding the original intention of the reinforcement can we understand the details of the method, and understand all the contents to achieve the expected results during implementation. Zhongda Consulting will explain to you the methods of reinforcing soft soil subgrade for highway bridges and culverts.
The roadbed is an important part of the road or railway. It is a strip-shaped structure built according to the route location and certain technical requirements. It should have sufficient strength, stability and durability to withstand the load from the road. Soft soil roadbed is often encountered in highway projects.
The so-called soft soil generally refers to cohesive soil in a soft or flowing state. It is characterized by large natural water content, large void ratio, high compression coefficient, low strength and creep and thixotropy. Special engineering geological properties such as engineering geological conditions are poor. If the roadbed is built on soft soil foundation without treatment or improper treatment, the roadbed will often become unstable or excessively subsidence, causing the road to be damaged or unable to be used normally. It is customary to refer to silt, silty soil, and soft clay soil as soft soil.
The main characteristics are high natural moisture content and large porosity ratio. The water content is between 34-72. The void ratio is between 1.0-1.9. The saturation is generally greater than 95. The liquid limit is generally 35-60. The plasticity index is 13-30. The commonly used reinforcement methods for soft soil roadbeds are when the embankment cannot be verified by stability calculation or settlement calculation. Soft soil foundations must be reinforced to meet design requirements.
1. Common reinforcement methods
1. Bagged sand well (plastic drainage board) drainage consolidation method
It is set in soft soil roadbed A series of vertical drainage bodies (bagged sand wells, plastic drainage boards), on which sand cushions or sand trenches are laid to artificially increase soil consolidation drainage channels, shorten drainage distances, and cooperate with load preloading and vacuum preloading. Pressure or vacuum stacking combined with preloading can accelerate the consolidation of soft soil and accelerate the growth of strength. The effect of drainage consolidation method on eliminating the consolidation settlement of soft foundation is not obvious.
2. Compacted sand piles
Sand piles are formed when steam or diesel pile drivers or vibration pile drivers impact or vibrate into loose sandy soil or artificial fill soil and form holes. A pile formed after filling with sand. During the pile formation process, the surrounding sandy soil produces a squeezing effect, or a squeezing or vibrating effect occurs at the same time, thereby increasing the density of the surrounding soil, improving the load-bearing performance and overall stability of the foundation, and reducing the settlement of the foundation. Compacted sand piles were initially mainly used for compacted sand foundations. With the emergence of high-efficiency special machines, they were gradually used for the reinforcement of liquefied silt foundations. In recent years, through combined use with preloading method, good results have been achieved on weak and sticky soil foundations, and it has become an extremely versatile foundation treatment method.
3. Gravel pile (vibration displacement method)
It uses one-way or two-way lifting of rocks to hit its own vibrating head, and sprays high-pressure water while sinking into holes. , and then fill in the gravel while vibrating it to form a gravel pile; the pile body and the original clay soil form a composite foundation to improve the bearing capacity of the foundation and reduce settlement. However, according to the "Technical Specifications for the Design and Construction of Highway Soft Soil Subgrade Embankments", if wet construction (water vibration) is used, the shear strength of the cross plate of the foundation should be greater than 15KPa. For dry construction (immersed tube method, etc.), the cross plate of the foundation The shear strength should be greater than 10KPa. For soil quality that fails to meet the requirements, caution must be used when using gravel piles, and its applicability should be determined through tests.
4. Cement spray pile (mixing pile)
It uses cement as the main agent of the curing agent. Through a special deep mixing machine, the soft pile is mixed deep in the foundation. The soil and curing agent are forcedly mixed to form a hard mixing body, which forms a composite foundation with the original stratum. It is divided into two types: 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 should be used, and the powder spraying method is better than the slurry spraying method in the same mixing time. The strength should be high and the dispersion of strength should be small; however, the powder spraying method is not as simple to construct as the slurry spraying method, and its construction quality is not as easy to control as the slurry spraying method. Due to its high project unit price, it has a greater impact on the overall project cost.
5. Dynamic drainage consolidation
It sets up a vertical drainage system (bagged sand well or plastic drainage board) and a horizontal drainage system (medium-coarse sand cushion and blind ditch), combined with static load (filling load) and dynamic load (strong ramming energy), so that the foundation completes most of the consolidation settlement in a short period of time, becoming super-consolidated soil, greatly reducing post-construction settlement and rapidly Improve the bearing capacity; in addition, it can also achieve pre-seismic effect on the foundation and effectively eliminate sand liquefaction; this method is also beneficial to the excavation and burial of underground pipelines. Using this method, the physical and mechanical properties of the foundation soil can be fundamentally improved in a relatively short period of time, which has comprehensive advantages in terms of effect, construction period, and economy. However, the treatment depth of this method usually does not exceed 10 meters and is limited by site conditions.
2. Commonly used methods for handling soft soil subgrade and spring soil at construction sites
Most of the situations often encountered during construction are not soft soil foundations, because if there is a soft soil foundation, it is generally not the case in the design. Processing methods should be proposed based on geological data. In most cases, the geological conditions in some areas are different from the original design, and the bearing capacity of the local foundation cannot meet the design requirements, or the water content in the local area is too high, the original drainage system is not smooth, and the original soil soil has poor water permeability, causing the foundation to be soft and elastic, muddy and spring soil. part. According to these situations, the commonly used methods are:
Replacement: This is the most commonly used method.
For example, if the soft base surface area is small and the soil layer is thin, such as some silty soil, all the soft soil under the embankment can be excavated manually or mechanically and replaced with high-strength cohesive soil or permeable materials such as sand, gravel, pebbles, and rubble. To improve the bearing capacity of the base, the depth of filling should be determined based on the bearing capacity. The maximum effective processing depth of this method is 3 meters.
Ripstone filling: It is to fill the road sections with soft soil or spring soil and water accumulation with stones to a height that exposes the original soil layer or water accumulation height of the road section to be treated. During the stone filling process, be sure to use a bulldozer to compact the stones so that they do not appear soft and elastic. Then fill in the earth. Blind ditch: It is to dig a blind ditch horizontally or vertically in the road section to be treated according to the length of the road section to be treated. Blind ditch is usually made of permeable large-pore filler or rubble masonry. Stones of different grades can also be filled in to provide drainage. Note that the outlet of the blind ditch must be connected to the drainage ditch to drain the water from the roadbed.
Drainage sand cushion: Drainage sand cushion is a layer of sand laid on the ground at the bottom of the embankment. Its function is to add a drainage surface on the top surface of the soft soil. During the filling process, the load gradually increases to promote the soft soil. Water that seeps out after foundation drainage consolidation can be drained away from the sand cushion. To ensure smooth drainage of the sand cushion, materials with good water permeability should be used. It is advisable to use medium-coarse sand with a mud content of less than 0.074mm and not more than 15. The general thickness of sand cushion is 0.6-1.0 meters. In order to ensure the water seepage of the sand cushion, a layer of sticky soil should be filled on the sand cushion to seal the water and prevent it from returning to the roadbed. Drainage channels should be repaired on both sides of the roadbed to ensure the stability of the roadbed.
Lime shallow pit method: Due to the influence of the moisture content of the clay soil, soft soil often occurs during construction. Generally, the lighter ones can be excavated, sun-dried, crushed and backfilled: 34 The lime shallow pit method 34 can be used on various road sections of different areas, that is to say, it can be used in large areas and small areas. The specific method is: Dig a 40cm-50cm square or circular pit with a depth of generally about 1m. Remove the seepage water in the pit. It is best to remove the seepage water the next day after digging the pit. Add quicklime to a depth of 1/3 of the pit depth and then backfill and roll it. The row spacing and pit spacing of pits are 5-6m in mild spring sections and 3-4m in severe spring sections.
3. Soft foundation treatment plan design
The effects of various soft foundation treatment measures adopted in newly built highways, bridges and culverts in recent years are analyzed and compared, and combined with the soft foundation treatment measures in surrounding areas. Based on the experience of foundation treatment engineering, consider the comprehensive use of foundation treatment methods such as shallow cushion replacement, bagged sand wells, and stack preloading. The characteristics and adaptability of various methods are described as follows:
1 , treatment characteristics of thicker hard shell layer
In the deep soft soil layer of the road section, due to the small fill height of the roadbed, the total settlement of the roadbed is not large (the post-construction settlement must be ensured to be less than 30cm), the main consideration is to improve the surface ( To improve the bearing capacity of road bed or embankment and reduce the problem of uneven settlement after construction, the method of replacing the cushion layer with multi-layer transverse reinforcement can be used, which can not only achieve better results, but also reduce the cost and shorten the construction period.
2. Treatment of thin hard-shell layers
For sections with thicker soft soil layers, if the construction period is relatively relaxed, bagged sand wells can be used to stack the soft foundation or overload preloading to treat the soft foundation. The pre-pressure period is 4-6 months. However, judging from the geological longitudinal profile, the length of this section is not large, about 200-300 meters. It can also be treated with cement mixing piles, and the treatment depth is controlled within 15 meters, which will have little impact on the total cost.
3. For sections with higher post-construction settlement requirements at both ends of the structure, cement mixing piles should be used.
The composite foundation method can effectively eliminate secondary consolidation settlement of roadbed. The distance between piles is determined based on the settlement calculation of the composite foundation. It is about 1.5m at the culvert and 1.2m-1.5m near the abutment. In order to make a better transition between the settlement of the roadbed and the bridge abutment, 30m in the direction of the roadbed is a gradual pile spacing section. The pile spacing is arranged according to the principle of gradually changing from 1.2m to 1.5m in two sections from the bridge abutment to the roadbed. The spacing is 10m, and a layer of gravel cushion is laid on the top of the pile, and a layer of bidirectional tensile geogrid reinforcement is placed on top of it. No geogrid is laid within the culvert foundation range. For structures such as pipelines and culverts, the design elevation of the base is the elevation of the graded gravel cushion.
4. For sections located under high-voltage lines where mixing piles or bagged sand well racks cannot be erected, high-pressure jet grouting piles can be used to treat soft foundations.
3. Summary
The soft foundation treatment construction technology is difficult and the quality requirements are high. If it is not mastered well, deviations can easily occur and cause hidden dangers to the project quality. The above-mentioned methods for handling soft foundation and spring soil commonly used on construction sites require different treatment plans based on the construction environment and geological conditions, project level and construction period requirements, cost control and other factors. Sometimes several methods can be alternated or used together. use. At the same time, construction should be carried out in strict accordance with the relevant provisions of "Design and Construction Technology of Highway Soft Soil Embankment" JTJ017-96 and the specific instructions in each design drawing. It is recommended that thin-layer wheel addition construction be used for roadbed filling based on careful, meticulous, accurate and timely dynamic observation. During construction, observation work should be highly valued, observation points should be arranged according to design requirements for observation, observation data should be organized in a timely manner, and abnormalities should be discovered in a timely manner Measures are taken to ensure project quality and construction deadlines.
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