The development direction of China highway is that the design standards and construction quality of heavy-haul high-speed lines put forward better requirements for subgrade strength and settlement. To achieve this requirement, we must attach importance to its reinforcement measures from the preparatory stage of subgrade construction. All construction procedures must be scientifically organized, reasonably arranged, strictly managed and meticulously constructed according to the owner's design, specifications and requirements, and pay close attention to the construction quality from the source to ensure the effective realization of the three major objectives of subgrade engineering cost, progress and quality.
1 technical measures for shallow treatment of subgrade soil in highway engineering
When filling subgrade, the soil moisture content is high, and the compaction degree can't meet the requirements. The newly-built subgrade passes through paddy fields and depressions, or the water level on both sides of subgrade is high, so the subgrade is soft in the compaction process, or some subgrade can't be rolled at all, the groundwater level is high, the soil moisture content in the lower layer of subgrade is high, the compaction degree of subgrade can't meet the quality standard, and the soil structure in the lower layer of subgrade has soft substratum or newly deposited loose soil, with large pores. The bearing capacity is low, and it is easy to cause large settlement after filling subgrade to build pavement, and it will move and collapse in serious cases, resulting in pavement cracking, especially for high filling subgrade. When filling embankment in soft soil area, the settlement and stability should be monitored and the embankment filling speed should be strictly controlled. When the settlement and lateral displacement are greater than the design requirements, measures should be taken to prevent collapse.
1. 1 soil drying and shallow replacement method
If the weather is hot, the climate is dry and the soil water content is high, you can loosen the soil with a plow or bulldozer to evaporate the water. When the optimum water content is reached, paving and compaction can be carried out. When the soil moisture content of the excavated subgrade is high, the top 40 cm soil can be pushed out by bulldozer, and the treatment depth is usually 60 cm. When the water content is close to the optimum water content, that is, 20cm at the bottom, and 40cm at the compaction surface, and then paving and compacting twice. If the elastic soft water content of subgrade soil is usually large and the depth is changed to 60cm, the subgrade soft soil layer can be excavated. If the moisture content of the bottom layer is still very high or the groundwater level is very high, the measure to reduce the moisture content of the bottom layer is to reduce the groundwater level to ensure the compactness of the backfill.
1.2 particle reinforcement method
When the fill in ditches, depressions and ponds can't be rolled up after drainage and dredging, or the compactness of the fill can't meet the compaction requirements because of the high water content of the lower soil, the particle reinforcement method is used for reinforcement. The material of granular reinforcement must be a material that is not easily soaked in water or decomposed by dry-wet cycle, and has good water stability, such as slag, gravel, stone or gravel, concrete block, etc. , the maximum particle size shall not exceed 30cm. Paving stones and concrete blocks should not be empty and should be packed neatly. The thickness of each layer shall not exceed 30cm, and the gaps shall be filled and compacted with gravel. When using stones or concrete blocks for reinforcement, it is best to spread a layer of small gravel, crushed stone or crushed stone with a particle size not exceeding 25 microns. The thickness shall be not less than 5cm to prevent the stones from squeezing into the mud after filling, and the pavement will be damaged and deformed after completion.
1.3 lime soil treatment method
After adding lime to soft soil, the optimum water content increases with the increase of lime content, and the plasticity index decreases obviously. The peak section of the compaction curve has a wider range of optimum water content than before adding lime. The increase of the maximum dry density ether lime content can reduce the water content that meets the design compaction requirements during backfill construction, which is easy to control.
In practical engineering, the mixing proportion of ash should be controlled at 5% ~ 8%. Theoretical research lags far behind practical engineering application. In soft soil area, lime is often used as subgrade filler to improve soft soil, but the research on mechanical properties of lime-improved soil under traffic load is still immature. The design adopts lime-soil reinforcement, and the treatment depth of large-area elastic soft subgrade is determined to be 60cm. Usually 8% lime soil is used for treatment, and the treatment depth can also be determined according to the actual situation. Therefore, it is of great theoretical significance and practical value to study the dynamic characteristics and deformation characteristics of lime-improved soil under traffic load.
In order to make the content of clods meet the requirements of lime-soil base, lime with 8% soil weight is used to push the top 40 cm soil out of the bottom 20 cm, and then the broken clods are leveled and compacted with a plow. After that, the top 40cm is divided into two layers, each layer is 20cm, and then the mixture is evenly mixed into the ash for compaction. For the compaction of soft soil subgrade, the first layer can be used to make the compaction degree reach more than 90%. When rolled with 8t flat roller, the compaction degree of the second layer is more than 95%. The compaction degree of the third layer is required to be above 98%, and it can be rolled by a large roller above 12t. For lime soil whose compactness can't meet the quality requirements, the sewage ditch is deep. In order to prevent large settlement and cracking, lime soil treatment must be carried out after precipitation with water or compaction with filler.
1.4 mixed reinforcement method: according to the construction conditions and material sources, the above methods can be comprehensively adopted for subgrade reinforcement in the same section.
2 Technical measures for deep treatment of subgrade soil in highway engineering
2. 1 plastic sheet pile drainage consolidation method
Its equipment is basically the same as that of bagged sand drain. Gantry driving equipment is installed on two tracks covered with sand cushion, and the pile driver can move longitudinally on the tracks. The circular sleeve steel sleeve attached to the steel guide frame is a circular steel pipe with a rectangular pile head, which is guided by the steel guide frame with a sufficient height. Because the gantry rail-type driving equipment can move freely in the longitudinal and transverse directions, it can also move horizontally by using its own rails on the gantry, so the time consumed by equipment movement and movement can be reduced. The plastic plate belt is erected on the roller on the ground, and the plastic plate enters the steel casing through the pulley attached to the upper end of the guide frame, and protrudes from the rectangular pile tip to be clamped with the pile shoe. The pile shoe is a steel plate welded by portal reinforcement, and the plastic plate extends from the pile head, passes through the gap of the pile shoe and is inserted into the pipe, so that the pile shoe steel plate is closely attached to the pile head.
The procedure of driving plastic sheet piles: firstly, horizontally drive plastic sheet piles between tracks, then move the portal pile frame longitudinally with the pile spacing as the moving distance, and continue to drive the second row of plastic sheet piles. The construction method of plastic sheet pile is the same as that of bagged sand drain. First, move the steel frame so that the pile tip of the pile casing is aligned with the pile position, and then drive it into the pile casing. Finally, pull out the casing, cut the plastic plate with tools, then pass the plastic plate through the pile shoe, move the pile frame to the pile position, and drive the second plastic plate pile in this order for construction control.
2.2 bagged sand drain reinforcement method
The woven bag with bagged sand drain should have good water permeability, certain aging resistance and corrosion resistance, and the bag material should have sufficient tensile strength, so that the sand in the bag is not easy to leak. At present, polypropylene woven bags should be protected from aging and long-term sun exposure. Generally, the diameter of bagged manholes is 7cm, the spacing is 1-2m, and the depth depends on the design requirements.
The dry sand used should have a silt content of less than 3% and good water permeability. Special construction equipment for bagged sand drain piling includes track gantry, crawler crane, etc. Its top end is fixed on the chuck of the vibrating hammer. The circular casing at the top of the casing is guided by a steel guide frame and has a funnel for loading sandbags. Pulleys should be installed at the population funnel of sandbags to prevent sandbags from breaking, and detachable precast concrete pile tips should be installed at the lower end of the casing. The pile tip should have sufficient strength and be closely spaced from the top surface and the contact of the steel casing to prevent soft soil from squeezing into the pipe and affecting the sinking of the sand bag. First set out the center line of the road, and then set out the pile line from the starting point of the sand drain with standard brazing according to the spacing of the sand drain.
Spread a layer of sand cushion with a thickness of 3050cm on the leveled soil foundation, and sprinkle water for rolling to achieve the required degree of compaction; Move the piling equipment, align the pile head with the pile position, and install and insert the precast concrete pile head tightly; When the sand bag sinks to the bottom of the pipe and the casing is pulled out, the sand bag can be put into the casing funnel, or water can be slowly injected into the pipe with the sand bag to reduce the friction resistance between the sand bag and the pipe wall, and the vibrating hammer is started to sink the casing to the required depth. The sand bag should be long enough to extend into the drainage sand cushion layer, so that the bagged sand and the drainage cushion layer can be well connected. Mobile drilling equipment continues to drill bagged sand wells according to the above procedures.
2.3 Back pressure protection method
When the bearing capacity of subgrade foundation is very low or the embankment is filled to a certain height, the slope toe on both sides is prone to uplift and settlement acceleration, and in severe cases, landslide or collapse will occur, so the back pressure berm method should be adopted. The back-pressure berm method can prevent the shear slip of soft foundation and ensure the stability of subgrade. Back-pressure dike refers to the construction of dikes on both sides of embankment, which are constructed together with embankment. The cross section of the back pressure berm is determined by the design. The filling speed of the back pressure berm shall not be lower than that of the main embankment, so as to avoid over-filling. The main dike should be fully compacted, leveled by layers, and have a certain cross slope to facilitate drainage. First fill sand cushion, including back pressure berm and embankment, and then fill the main embankment. During or after the construction of the main dike, the part above the design height of the back pressure dike can be excavated. These materials can be used to fill the main embankment if it can be determined that the foundation strength under the back pressure berm has increased to the required value.
2.4 Light packaging method
When light materials are used to fill embankment, the settlement value can also be reduced due to the reduction of load. This method is suitable for bridge head filling. Lightweight filler requires light weight, low compressibility, good water stability, no pollution, no decay and deterioration. Such as cinder, shells, fly ash, pumice, etc. Lightweight materials shall be filled and compacted in layers, and the compaction thickness of each layer shall not be greater than 20cm.
In the width range above 100cm on both sides of the subgrade filled with light materials, the cohesive soil should be cultivated and compacted to prevent the loss of light materials. The top surface should be covered with 20 cm filler.
2.5 Overload preloading drainage consolidation method
This method is usually combined with vertical drainage. Overload preloading is to increase the fill height at the top of the embankment after the embankment reaches the design elevation, so that the overloaded earthwork will become a part of the embankment after settlement. When embankment is filled, it shall be compacted as required, and settlement observation plates shall be buried. After embankment filling, the settlement curve should be observed regularly and drawn. After being stationary, the excess overloaded earthwork on the embankment can be removed for the filling of other sections.
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