Influencing Factors and Countermeasures of Submarine Vacuum in Soft Foundation of Expressway

Western Exploration Project 1: Influence Factors and Countermeasures of Subgrade Vacuum in Soft Foundation of B Expressway Nie Yuegao, Zhao Weibing, Shi Jianyong, Hong Baoning (Institute of Geotechnical Engineering, Hohai University, Nanjing, Jiangsu 210098, China) Settlement and other characteristics, but due to this technology At present, there are no comprehensive design and construction specifications, which limits the application of this technology. Based on the actual construction conditions of the Xintai Expressway, the effects of sand layers, depth of packed sand wells, arrangement of drainage filter tubes, and subgrade filling on the vacuum degree under the membrane were analyzed in detail. Improvement has achieved satisfactory results.

The vacuum preloading method was first proposed by Prof. W.kellman of the Swedish Royal Institute of Geology after a series of field tests was conducted in 1952. Afterwards, the relevant countries have conducted exploration and research. However, due to the sealing problem, they have not been well resolved. No suitable vacuum device has been researched, so it is not easy to obtain and maintain the required degree of vacuum, and thus it has not been well used in practical engineering. In the early 1960s, China started to research vacuum preloading and reinforcement of soft ground. Due to limitations of theory and test methods at that time, indoor model tests and qualitative research were mainly used, and field tests were also limited to smaller scales. . In the 1980s, with the continuous deepening of the mortar and the improvement of the on-site treatment methods, the problem of vacuum sealing has been well solved. The degree of vacuum under the membrane can reach 80 kPa or more, and it is widely used in port terminals. Based on the characteristics of the soft soil foundation in the Pearl River Delta region, Guangdong Province has boldly tried to apply the vacuum-loaded combined preloading technology to the highway soft ground treatment, and achieved many useful results. However, due to the long line of the expressway and the complex topography and geomorphology along the line, the distribution of the soil layer is very different. At present, there is no perfect design and construction specification, which brings great difficulties to the design and construction, thus limiting the promotion of the technology. And applications. This paper introduces the effect of vacuum bed-load combined preloading on the softness of Xintai Highway Project. The effect of sand layer in sand, length of sand-packed wells, layout of horizontal drainage filter, subgrade filling, etc. Therefore, some improvement measures are proposed for reference by similar projects.

1 Project Overview A soft base section of the Xintai Expressway project is reinforced with a vacuum-loaded combined preloading method. The total length of the reinforcement zone is about 1,450m and the width is about 45m. It is divided into 10 sections for implementation. The thickness of the sludge layer in this section is between 9. 0~19.5m, of which the 6th section is the thickest, and the thickness of the thinnest 1st to 4th sections of the 5th section is between 10~11m, and the difference is not significant; 6~10. The thickness of the silt layer is between 19.5 and 13.2 m, showing a decreasing trend. The main construction procedures include: site leveling, laying sand cushion (70 cm), setting up sand wells for bags, laying drainage filter tubes, sealing film laying, and vacuum System installation, vacuum preloading, surcharge and vacuum combined preloading. For expressways, vacuum-loaded combined preloading is used to reinforce the soft ground, and the roadbed filling can be used for the surcharge. The consolidation of the soil under vacuum load and the combined action of the surcharge can increase the strength by 2~3. Times. At the same time, due to the negative pressure generated by the vacuum, the soil shrinks inwards, which can counteract the outward extrusion deformation caused by the heap loading. The foundation will not be stable due to the fast filling rate. Drainage filter tube; Sand preloading is safer and more reliable than surcharge preloading. Prior to the vacuum preloading of the soft section, bagged sand wells were set up, with a 7-m spacing of 7 mm and a length of more than 12 m. In the process of vacuum preloading, the vacuum under the vacuum of 4-7d reaches the designed value of 65kPa and is maintained at 65-90kPa for a long time. However, the degree of vacuum under the membrane at different times varies greatly. The following specific analysis is performed.

2 Reasons for the difference in the degree of vacuum under the membrane The causes of the difference in the degree of vacuum under the membrane are various. However, the regularity of the entire construction process and the analysis of the geological conditions of the entire section can be found. The main factors that cause this difference are: 2.1 The sand layer and the length of the sand well in the foundation. During the construction, it is found that the same construction technology is used in the reinforcement section of the entire section, and the vacuum degree varies greatly. Among them, the degree of vacuum in the 1~6 section reinforced area is 65~79kPa, the 8~10 section is 87~90kPa, and 1~6 sections occasionally stop vacuum. After about 1~2hi, the vacuum degree quickly drops to 0kPa and 8~10. Section vacuum pumping vacuum 1d only reduced by about 20kPa. From the perspective of the geological condition of the whole section, the 1~6-segment foundation contains a thick sand layer with a depth of 8~10m. The length of the sand-shaft is long and penetrates the silt layer into the sand layer. In this way, during the evacuation process, the water outside the reinforcement zone passes through the sand layer—bag sand wells* sand cushion—drainage filter—vacuum system. This causes the water pumped by the partial evacuation system to be outside water, and has nothing to do with the pore water of the soil in the consolidation zone, which reduces the evacuation effect of the evacuation system, resulting in a low degree of vacuum under the membrane. At the same time, when the vacuum is occasionally stopped, the external water source continues to flow along the sand layer in the foundation, and the sand well enters the reinforcement zone and flows into the negative pressure zone, causing the vacuum degree under the membrane to drop quickly to 0kPa2.2. In pressure construction, the drainage filter tube adopts three different arrangements, as shown. In the initial stage of construction, the arrangement shown in (a) is adopted. From the observation of the vacuum table buried under the membrane, the vacuum degree under the membrane is inhomogenously different by 3~5 kPa, and the vacuum degree far from the side of the drain main filter tube is relatively low, and the side near the main filter tube is relatively high. From this kind of arrangement form, it is mainly that the distance from the end of the drainage filter tube to the main pipe is too long, the vacuum pressure loss is large, and it is much smaller than that near to the main pipe when passing to the end of the filtering pipe, so that the vacuum degree under the membrane has 3 ~5kPa difference. In addition, this kind of arrangement is unevenly accommodating electro-elastic foundations (planned layout of drained filter tubes in the Western Exploration Project, which is used in the monitoring of roads and roads. The basic difference is eliminated, and the vacuum degree under the membrane is uniform. 0-arrangement The branch filter tube is disconnected near the centerline of the roadbed, and it can better adapt to the uneven settlement of the foundation.

2. With the increase of vacuum pumping time, the under vacuum between the membrane and the vacuum in the drainage pipe increases gradually, increasing to 10kPa, or even 20kPa. This is mainly due to the vacuum time. The lengthened, the sand cushion is continuously compacted, and the drainage pipe filter is continuously pressed closely, so that the vacuum pressure in the sand cushion transmission resistance increases, the permeability of the filter membrane decreases, resulting in increased negative pressure difference The degree of vacuum under the membrane is relatively reduced.

2. Subgrade filling In the subgrade filling construction, the sealing membrane has various degrees of damage. If the filling of the first layer is too thin during the filling process, the walking on the construction machinery such as the bulldozer will be more serious. As a result, serious damage to the sealing membrane occurred in the early stage of subgrade filling, and a lot of air leakage occurred, resulting in greatly reduced vacuum.

3 Conclusions and recommendations Through the comprehensive analysis of the whole process of joint preloading and the geological conditions of soil distribution in Xintai Expressway, the following conclusions can be drawn: (1) The effect of sand layer on the vacuum under the membrane in the foundation is very significant. Big. Deeply buried in the sand layer, the water outside the reinforcement zone continuously flows into the reinforcement zone, resulting in a relatively low degree of vacuum under the membrane. If the sand layer is buried in shallow depth, it will cause leakage of air leakage, so it must be blocked before vacuum preloading, otherwise it will lead to failure of vacuum preloading. Into the sand layer, so as to avoid the formation of continuous seepage channels in the sand and sand wells, the effect of vacuum preloading will be better.

The layout of the drainage filter tube is better with the 0) 0) type configuration, especially for the larger width of the site, so that the vacuum distribution under the membrane is uniform.

When the vacuum preloading time is very long, it is necessary to pay attention to the problem of filter failure, and take necessary measures during design and construction to ensure the reinforcement effect of vacuum preloading.

In the subgrade filling construction, it is necessary to pay attention to the protection of the sealing film, the use of non-woven geotextile is better, and the thickness of the first layer of filling should be at least 60cm, otherwise it will damage the sealing film.

4 Conclusion As the application of the vacuum one-load combined preloading method to soften expressway soft foundation technology is not common in highway soft foundation treatment, it will inevitably encounter some problems. In this paper, combined with Xintai Expressway adopting a vacuum-loaded preloaded soft foundation reinforcement project, some major factors affecting the vacuum degree under the membrane are discussed and analyzed, and some useful work is done. However, because different highways have their own characteristics, their application to other projects will also encounter new problems. A large number of tests, researches, and other work will be needed to facilitate the promotion and application of this technology in highway projects.

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