1 Importance of subgrade construction in expressway Soil subgrade includes embankment and road. The basic operations are excavation, transportation and filling. The process is relatively simple, but the conditions are relatively complicated. Highway construction is field work, with poor natural conditions, inconvenient transportation, difficult supply of materials, equipment and construction team and high speed. Subgrade construction sites are scattered, and the working face is narrow, so it is easy to encounter special geology. In the face of this complicated situation, in order to ensure the engineering quality and realize fast, efficient and safe construction. Attention must be paid to construction technology and management. As far as the current situation is concerned, it is necessary to have a stable professional construction team, equipped with corresponding technical backbones and machines and equipment, establish and improve the inspection and acceptance system of construction technical operation rules and systems, and adopt modern construction management methods, which is the need of the rapid development of highway industry and the only way to realize refined construction. 2. Construction requirements of pavement cushion and subgrade 2. 1 Technical requirements of graded macadam cushion Graded macadam is the main material of pavement cushion, but the quality of macadam itself directly affects the function of cushion and the overall engineering quality of pavement, so the supervision engineer should master the technical requirements of macadam and control the engineering quality of graded macadam cushion. These technical requirements include: the crushing value of gravel in graded gravel should be less than 30%. The content of 0.5~5cm particles in gravel should not be less than 50%, and the largest particle should not be greater than 6cm. The content of 0.074mm powder in graded crushed stone should not be greater than 7%, and the plasticity index should not be greater than 6. In view of the fact that the strength of graded macadam cushion is mainly related to particle grading and compaction, the grading of graded macadam should be required. 2.2 Graded macadam cushion construction technology Graded macadam cushion construction can be divided into five processes: road groove rolling, transportation leveling, plastic water spraying rolling and initial maintenance. Graded gravel is a loose material. In order to prevent it from being damaged by traffic, the supervision engineer should remind the construction unit that half of the road is open to traffic, so as to avoid loose surface, poor flatness and prolonged construction period of graded macadam cushion due to traffic chaos. 2.3 Cement stabilized gravel base can be divided into two forms according to the construction method: factory mixing method and road mixing method. Subbase can use road mixing method. Plant mixing method can be used at the grass-roots level. As the base of high-grade highway pavement, cement stabilized macadam is an important part of pavement structure, and it is a key part related to the overall strength of pavement and whether it can guarantee the normal use of pavement in the design cycle. 2.4 Cement stabilized gravel base construction technology There are two kinds of cement stabilized gravel base construction technology. Cement-stabilized gravel base is constructed by road mixing method, and the working procedures are divided into: initial gravel leveling, sprinkling water blocks, spreading cement, sprinkling cement, mechanical stirring, shaping and leveling, sprinkling water and rolling maintenance, and constructing cement-stabilized gravel base by plant mixing method. The working procedures are divided into gravel preparation, cement dosage, water mixing, material transportation on the road, paving and rolling, water spraying and maintenance, etc. 3. Graded macadam cushion 3. 1 Graded macadam cushion shall be rechecked and established by the engineer before construction. The completed subgrade must be rechecked and accepted, and the handover procedures shall be handled by the pavement construction unit. The handover procedure shall be organized by the resident supervision engineer, and attended by the leaders of subgrade and pavement construction units and technical leaders. After the subgrade is completed, the subgrade construction unit shall conduct self-inspection on the subgrade self-inspection report, which includes compactness, flatness, width, slope, deflection value of subgrade top surface, elevation of subgrade top surface and small and medium-sized bridges and culverts, list of straight curve angles and level. 3.2 Material Test The pavement construction unit must submit the following graded macadam material test to the supervision engineer. In the screening test of gravel particles, the supervision engineer takes samples from the construction section every half kilometer on average and submits them to the supervision engineer for review. Graded macadam in this section must meet the technical requirements of graded macadam before it can be used. For graded broken stones that do not meet the technical requirements, measures should be taken to ensure the quality of graded broken stones, such as mixing broken stones, removing broken stones larger than 6cm or discarding them all and replacing the yard. Gravel capacity test and heavy standard laboratory should be carried out to obtain the best water content and maximum compaction. 4. Quality control measures in construction 4. 1 subgrade quality control. 4. 1. 1 subgrade soil control. Subgrade is generally built with natural soil. Natural soil should be tested and analyzed before subgrade filling to determine its physical properties. Determine the optimum water content and maximum dry capacity to guide subgrade construction and detection of subgrade filler products. From the analysis of related test results, the finer the soil particles, the lower the corresponding resilience modulus, while the higher the resilience modulus of sand. This is what is usually called sand, which is a good road building material. When choosing soil field in construction, we choose soil with small plasticity index to fill subgrade. 4. 1.2 degree of compaction control. Ensure the best water content of soil. Only when the soil is compacted at the optimum water content can the maximum density be reached. Therefore, the water content of soil must be controlled at any time during the compaction of subgrade fill. When the water content is too large, it should be dried in the air until it is crushed to the optimal water content. The construction process should be carried out continuously, reducing rain and exposure to prevent the soil moisture from changing greatly. Reasonable compaction machinery shall be selected, and the thickness of filled soil shall not exceed 30cm, and compaction shall be carried out by layers. In the construction, try to use heavy roller. For the same kind of soil, the maximum dry density obtained by light compaction is less than that obtained by heavy compaction, and the optimal water content is greater than that obtained by heavy compaction. 4. 1.3 strength control subgrade engineering. Compactness reflects the compactness of each layer of subgrade, and deflection reflects the overall strength of the upper part of subgrade, both of which meet the qualified requirements. The overall strength, stability and durability of subgrade can meet the requirements, and the technical requirements for subgrade construction are not complicated. As long as the regulations are strictly implemented and the construction is serious and responsible, high-quality roads will be built. 4.2 Control of water damage. Water has great influence on the performance of highway subgrade, which reduces the strength of subgrade. Because expressway generally designs high embankments and has many hard shoulders. The water damage in the subgrade is not serious, mainly to prevent the underwater leakage of the integral ballast bed, resulting in the structural damage of the integral ballast bed. In our country, a certain amount of pits and troughs often appear after the rain in the built Qin-Shen-expressway, which is caused by water damage, especially in the high temperature weather in summer, and rainwater seeps into the whole track bed. Forming high-temperature water. Under the driving load, the gravel peeled off and separated. Under the driving load, potholes are formed. Surface construction is treated as a waterproof layer, so that water can enter the interior of the structural layer, thus avoiding this damage. 4.3 expressway usually adopts monolithic ballast bed to prevent cracks. The types of cracks in monolithic ballast bed can be basically divided into two categories, namely, reflective cracks formed by cracks at the grass-roots level and temperature shrinkage cracks generated by the surface layer itself. This is the first category and belongs to non-load cracks; On the other hand, the cracks caused by repeated traffic loads are irregular network cracks caused by the tensile stress of the whole ballast base exceeding its bending strength, which should be fully considered in the design. Good quality control in the construction process is to solve the first crack. 4.3. 1 Crack control of monolithic ballast bed basement. When the cement stabilized structure with small shrinkage is selected as the base, the crack mechanism of cement stabilized material should be considered in the construction process. There are two main reasons for its shrinkage, namely temperature shrinkage and dry shrinkage, which are related to the water content and plasticity index of the material. When selecting materials, the plasticity index of materials should be tested. The plasticity index of materials can only be purchased within the scope permitted by the specification. In the construction, retarding water reducer and other methods can be used to make the cement stabilized material reach the best water content as much as possible. Make sure there are few or no cracks. 4.3.2 Prevention and treatment of surface cracks of monolithic ballast bed. The non-load crack of asphalt monolithic ballast bed is the sum of low temperature and fatigue crack, which is related to the quality of asphalt, mainly the temperature sensitivity and penetration of asphalt. Many tests at home and abroad show that the higher the penetration index, the lower the temperature sensitivity, and the lower the temperature sensitivity of high viscosity asphalt. These factors should be fully considered when selecting the material of monolithic ballast bed, because after cracks appear, rainwater will penetrate down along the cracks, erode the bottom structure layer, reduce its strength, and cause serious monolithic ballast bed. Conclusion: The quality of subgrade in expressway has a great influence on the performance of expressway. Therefore, subgrade construction should be carried out in strict accordance with the requirements of the code, and different concrete measures should be taken for different subgrade projects. There are many ways to control the quality of subgrade construction in expressway. The above are just some experiences of the author, which are intended to inspire everyone to discuss with you.
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