Study on Vacuum-filling Self-supporting Combined Preloading Soft Foundation Reinforcement Technology and Its Application

KeyWord : Reinforcement technology ; vacuum consolidation ; self - loading combined precompression soft foundation and application ; Li Changning ; Technology Development Center of China Railway First Group Group Co., Ltd. , Xi'an 710054 ; Affect control technology issues. This paper combines the actual situation of soft soil foundation treatment in a high-speed railway in Shanghai, through comprehensive analysis and field test tests, the design method, construction technology and control measures for the surrounding environment of the deep soft foundation reinforced by vacuum-filled self-loading combined preloading method. Etc., conducted a pilot study. The prediction analysis of the post-construction settlement shows that the requirement of 10cm and 5cm of settlement after work can be met respectively after one month and 11 months after vacuum unloading. The method is a high-efficiency and low-cost soft foundation treatment method. It has the advantages of high filling speed, short construction period and preloading time, large treatment depth, and high consolidation speed, and can effectively reduce post-construction settlement to achieve preloading and overloading. the goal of.

1 Overview As the contradictions between the rapid economic growth in eastern China and the demand for transportation are becoming increasingly prominent, it is imperative to realize the leap-forward development of the railway by building the Beijing-Shanghai high-speed railway and the intercity high-speed railway in the east. The high-speed railway demands very high deformation control of the roadbed. It increases the requirements for the base conditions of the road and emphasizes the strict requirements for post-construction settlement. This requires an economic and effective treatment of the foundation. Soft soil foundations along the Beijing-Shanghai and eastern intercity high-speed railways are not only widely distributed, but also have complex types of genesis and large thickness. Generally, the foundation treatment methods are costly and difficult. The vacuum-filled self-loading combined preloading technology has good economics and unique advantages. It is characterized by that the roadbed's packing can be used as a combined preloading of the pile load and the vacuum, and there is no problem in the stability of the subgrade. The rate is also not very limited; after the subgrade is completed, it is not necessary to continue to carry the overloaded earthwork; the vacuum is equivalent to overloading to accelerate the consolidation speed of the soil, and no overloaded earthworks are piled and excavated to shorten the construction period; Compared with other foundation treatment methods, the construction depth is simple and the cost is low, so it is widely used in the soft soil foundation reinforcement project. For deep soft foundations, especially for foundations with a hard interlayer and sand permeability with good water permeability, the most important concerns in engineering construction are the vacuum preload reinforcement effect, post-construction settlement, and its impact on the surrounding environment. At present, there are not many large-scale engineering practices and theoretical studies in this area. In this paper, combined with the actual construction of the work site in the test section of settlement control of a soft ground in a high-speed railway in Shanghai, the application of the vacuum combined preloading technology, existing problems, solutions, and problems to be further studied are analyzed.

2 Overview of the construction site of the test section The construction site of the test section is an alluvial plain in the Yangtze River Delta. The elevation of the ground is 3.8-4.2m, the terrain is open, the east is high and west is low, and the river ditch is vertical and horizontal. The stratum is a Quaternary Holocene Chonghai formation, with a thick soft soil layer and a deep soft soil layer. The main soil distribution and lithology from top to bottom are as follows: The soft soil foundation of the test section is treated with a plastic drain plate + vacuum-filled soil self-loading combined preloading scheme (see below). The specific parameters are: the reinforced area is about 1200m2, The insert depth is 27m, the plastic drain spacing is 1.2m, the equilateral triangle is arranged, the sand cushion thickness is 40cm, and the design seal groove depth is 1.5~2m. A layer of composite geotextile is laid under the vacuum membrane and a layer of geotextile is first laid on the membrane. A 20cm-thick reinforced one-way reinforced geogrid sand cushion was laid, and the embankment height was 4.5m. 3 Vacuum-filled self-loading combined preloading technology 3.1 Development of vacuum-filled soil self-loading combined preloading technology Characteristics The theory of vacuum preloading was first proposed by the Swedish Royal Geological Institute (W.Kjellman) in 1952. In 1958, the United States' Philadelphia Airport used the method of combining vacuum well point precipitation and sand wells for the first time to deal with the foundation of the runway expansion project. The problem was later reported by Japan, Finland, the Soviet Union, France, the United States, and Sweden. However, due to the fact that key problems such as vacuum equipment efficiency, gas-water separation technology, and sealing technology were not available at the time. Good solution, for a long time the construction method is not widely used. In the 1950s, China began to study the engineering method, but at the time it did not make it successful for the reinforcement of on-site foundations. Until the 1980s, the Ministry of Communications, the Ministry of Communications, and other agencies did a lot of work on vacuum preloading and reinforcement of construction techniques and design methods for soft soil foundations. Only in this way can they be successful in engineering applications. The major developments include a series of construction methods such as the development of a single vacuum preloading method for the consolidation of soft soils to vacuum combined surcharge, vacuum combined electroosmosis, vacuum combined precipitation, and vacuum combined with gravel piles to reinforce soft ground.

The vacuum-filling self-loading combined preloading method is developed on the basis of vacuum preloading method. It is combined with the actual conditions of highway and high-speed railway subgrade filling. The subgrade filling is taken as a ready-made load and combined with vacuum preloading. Soft ground treatment. In China, a vacuum combined preloading method was developed in 1983. Through indoor centrifugal simulation tests and field large-scale tests, it was shown that the negative pressure produced by vacuum and the positive pressure generated by the earth loading can be superimposed, thus forming a vacuum- Filling and self-loading combined preloading method.

The construction process of vacuum-filled self-loading combined precompression reinforcement of soft ground includes the following steps: laying a 30-50 cm thick sand cushion as a horizontal drainage channel on the surface of the soft foundation to be reinforced, and then setting a m-distance plastic drainage plate as Longitudinal drainage channels, then airtight membranes are laid on the sand cushion as sealing measures, and if necessary, silt mixing piles shall be set up around the reinforcement area as vertical sealing curtains. The vacuum pump extracts the air and water in the soil below the membrane through the main and branch filter tubes embedded in the sand mat. The subgrade filling the membrane is used as the load and the vacuum load together to create a pressure difference in the reinforced area. Promote the formation of a seepage field in the soil body, and make the soil drainage consolidation, strength increase. The plastic drain plate acts as a vertical drainage channel, reduces the drainage distance, and accelerates consolidation of the soil.

In subgrade filling, the construction period of subgrade filling depends on the height of filling and the speed of filling. High-speed railways require stricter post-construction settlement of roadbeds. If general preloading methods are used, it is often necessary to increase the overload height. (or extend the pre-pressing time) to reduce the post-construction settlement, because the soft soil layer of Beijing-Shanghai section has a low shear strength and a small permeability coefficient, which limits the filling rate; in addition, it needs to be dug after the pre-pressing is completed. In addition to overfilling part of the earth, it will take a long time to fill the roadbed with the surcharge preloading method. The vacuum-filling self-loading combined preloading method can make up for the above-mentioned insufficiency. Its characteristics are: According to the principle of effective stress of Taisaji, vacuum preloading is to increase the effective stress of the earth by reducing the pore water pressure. Therefore, vacuum preloading Only change the ball stress of the soil, but does not produce shear stress, therefore, the soil will only produce inward contraction deformation. Even if the vacuum load is applied instantaneously at one time, the soil will not occur as if the preload of the preload is too large. Destructive situation. If the vacuum degree under the membrane is more than 80kPa, it is equivalent to a one-time plus 4~5m height filling load, while the soft clay layer of Beijing-Shanghai section generally has a filling height of about 2.0m, and the vacuum degree is increased from zero to 80kPa only. It takes about 7 days. Due to the inward contraction deformation of the soil due to the evacuation, it can offset the lateral extrusion deformation caused by the embankment filling load, and in addition, the strength of the soil in the vacuum preloading stage increases, so the embankment filling can achieve continuous filling. The purpose is basically that there is no stability problem. When the roadbed is filled to the design elevation, the vacuum load is equivalent to overload. Under the dual effects of vacuum and filling load, the consolidation speed of foundation soil is fast. After the combined preloading is completed, the vacuum load can be removed from the pump, which reduces the workload of the unloading earthwork with overloaded preloading and saves time.

In terms of the consolidation speed: In the vacuum preloading stage, the negative pressure is gradually transmitted to the inside of the soil through the filter tube, sand cushion, and plastic drainage plate. The pore water in the soil is continuously discharged outward under the effect of pressure difference, so that The soil is drained and consolidated. The combined preloading stage is based on vacuum preloading, and then the roadbed filling is used for self-loading preloading. On the one hand, due to the effect of self-loading, the pore water pressure in the soil increases; on the other hand, due to the effect of vacuum preloading The pressure of the pores in the channels of the sand cushion and the plastic drainage channel is in a state of negative pressure, so that the pressure difference between the pore water and the drainage body in the foundation is greater, which accelerates the discharge of water in the soil, accelerates the consolidation speed, and strengthens the consolidation effect. obvious. In depth of reinforcement: The depth of reinforcement depends mainly on the depth of the pore water pressure. The measured data show that the attenuation along the depth of the pore water pressure difference of the combined preloading with vacuum-filled soil is significantly lower than the attenuation along the depth of the total stress during the loading. Therefore, the depth of reinforcement of this method is greater than that of ordinary surcharge preload, which is more suitable for reinforcing foundations with deep soft soil layers. In terms of strength increase: In the vacuum-filled soil self-loading combined precompression reinforcement soft foundation method, when the vacuum degree under the membrane reaches 80kPa, the vacuum load is equivalent to a 4~5m height filling load, which makes the method in the preloading process. The high equivalent load in the high-speed, combined with the high speed of consolidation and deep depth of influence, all make the strength of the foundation grow fast and increase greatly. In terms of stability, the vacuum-filled soil self-loading combined preloading method is based on vacuum preloading and applying external loading. Due to the negative pressure generated by the vacuum, the inward contraction and deformation of the soil can be offset, and the out-squeezing deformation caused by the surcharge can be cancelled out. Therefore, the foundation does not have a stable foundation due to the high filling rate.

Analyze from another angle, the inward contraction deformation of the foundation is equivalent to the application of back pressure protection pathways on both sides of the roadbed. Therefore, the vacuum generated by the vacuum is conducive to the stability of the roadbed.

3.2 The main construction process of vacuum-filled earth-filled self-loading combined precompression reinforcement soft-base includes construction site cleaning, laying of sand cushion, plastic drainage board, sealing trench excavation, drainage pipe installation, sealing film laying, and vacuum pump installation. , vacuum pumping, embankment filling, site monitoring and other procedures, construction methods such as pipe connection, all suction pipes need to be buried in sand mats, and through the membrane 3.3.1 settlement with load, the law of time 3.2.1 The preparations for construction mainly include: acceptance of the previous process, construction of a temporary warehouse on the construction site and arrangement of the power supply network. The measurement and pay-off line shall be measured and released by a professional measurement engineer according to the design drawings, and the boundary of the vacuum preloading zone shall be marked clearly and vacuum pre-arranged. Pressure equipment and materials entered the site, a 0.4 m thick sand mat was laid, and then plastic drainage boards were set up.

3.2.2 Excavation of Sealed Ditch Considering that there is a layer of silty sand with strong permeability in the strata (see stratum case 3), the actual excavation depth is 3~4m, and all excavation is performed along the perimeter, and the clay is replaced and compacted. After the sealing film is laid, the sealing groove is filled with silt or clay.

3.2.3 The suction pipe is divided into the main (dry) pipe and branch filter pipe, the main pipe is 090mm PVC pipe, and the branch pipe is “75mm PVC pipe. The three-way and four-way connection between the main pipe and the branch filter pipe are adopted. The pipe joint is connected to the vacuum pump through the wire suction glue and suction pipe. While digging the sealing groove, it is possible to connect, install, and bury the main pipe and the branch filter pipe, and remove the sunken shells and other impurities from the surface of the sand pad, and bury the plastic drainage plate head that leaks out of the sand pad surface. in.

3.2.4 Laying the sealing film Before laying the film, the pebbles, shells, and other debris on the surface of the sand mat should be removed, and the surface of the mat be leveled. After the vacuum gauge probe and other observation instruments are to be buried, two layers of polyvinyl chloride film are placed to cover the entire pre-pressing area, and the film is buried in a sealed groove around it, and backfilled with clay or clay.

Membrane connection and installation of the vacuum pump system: The vacuum master is connected to the vacuum pump through the ejector. The connector of the filmer must be firm and reliable.

After the above work is completed, vacuum can be started. After the vacuum is applied, the vacuum level in the reinforcement zone will continue to rise. When the vacuum degree under the membrane reaches and stabilizes at above 80kPa, it enters the normal stage of vacuum preloading.

3.2.6 Subgrade filling First lay a layer of geotextile on the vacuum membrane, then lay a protective layer of geogrid reinforced sand cushion with a thickness of 0.2m, and then fill the filler. The thickness of the first layer should be greater than 0.5m, and use less than 16t roller compactor static pressure or vibration compaction.

3.2.7 Unloading Acceptance The roadbed is filled to the design elevation. Under the condition that the vacuum degree under the membrane in the reinforcement zone reaches 80kPa or more, when the foundation consolidation degree meets the design requirements, or when the settlement rate of the foundation is less than 1mrn/d, the vacuum unloading can be stopped. Holland, acceptance inspection.

3.3 Vacuum-filled soil self-loading Combined pre-compression field test and analysis It is a vacuum preloading stage during the start of vacuuming and during the construction of the official embankment. At this stage, the settlement is closely related to the degree of vacuum under the membrane. At the initial stage of vacuum pumping, the sedimentation rate is 1~12 days, and the average sedimentation rate is 1.3~1.7cm/d. After that, the sedimentation rate begins to decrease and averages at 10 days. 6 ~ 0.75cm / d. When the vacuum is stopped, the settlement basically stops, even a small amount of rebound deformation. After the vacuum under the membrane reached 80kPa again, the sedimentation rate became larger again. Before the embankment was formally started on August 12, 2003, the sedimentation rate has been reduced to 3~5mm/d, indicating that the main consolidation of soil in the vacuum preloading stage is also a gradual convergence process with preloading The laws are basically the same. When the total settlement of the K0+010 and K0+020 sections is formally filled, the settlement rate will increase again. During the loading period, the sedimentation rate is mostly 4~6mm/d. After completion of the filling, the sedimentation rate decreases with time. Before the vacuum unloading, the sedimentation rate has decreased to 2~3mm/d. The total settlement of the two sections is respectively After 100.3 cm and 104.3 cm. vacuum unloading, the settling rate becomes significantly smaller. From the vacuum unloading on December 10, 2003 to the last seven months of July 14, 2004, the settlement of the two sections was only 3.2~3.9cm, the settlement was 11.7mm in the first month after vacuum unloading, and the settlement was 1.8mm in the seventh month. 3.3.2 Variation of the settlement along the depth The results of the layered settlement test show that at the end of the vacuum preloading (before embankment filling), the settlements of the two sections below the foundation surface are 3.2cm and 4.8cm, and the settlement at 27m is 0.5m. It can be said that the vacuum preload reinforcement depth can reach 27m, that is to the bottom of the plastic drainage board (see).

3.3.3 Pore water pressure change law After vacuum, negative super pore pressure appears in the soil. The initial 5d pore pressure decreases rapidly. With the increase of time, the pore pressure decreases gradually. During the vacuuming period from July 26 to August 4, 2003, shallow pore pressures increased rapidly. After re-evacuation and maintaining the vacuum below the membrane to be greater than 80kPa, the pore pressure begins to decrease again. The negative pore pressure at the surface of 2~3m shallow soil can reach 78~83kPa. From October 9, 2003 (filling height 1.5m) The pore pressure in the foundation soil began to rise, but the increase was not significant, with a maximum of 24 kPa, indicating that the positive pore pressure generated by the heaping was greater than the negative pore pressure generated by the vacuum and appeared 2~3 days after the completion of the filling. Maximum pore pressure, the original negative pore pressure rises quickly after vacuum unloading. From the perspective of pore pressure and depth, the effect of vacuum preloading also reaches the bottom of the insert plate (see 3.3.4 post-construction settlement, mA-36mK0+010 section cross-hole pressure-load-time curve due to the post-production of the foundation The settlement is composed of three parts: one is the main consolidation settlement that has not been completed until the start of track laying under the action of the roadbed load, and it is related to the time after the completion of construction to the track laying; the second is the post-construction settlement caused by the track and train load. The size of the subsidence is directly related to whether the soft soil treatment method can meet the control standards for high-speed railway subsidence. Third, the subgrade consolidation of soft soils, due to the use of vacuum combined with preloading, the load in the joint phase is greater than the load. Large 70~80kPa, this part of the secondary consolidation settlement will not be too large.Under the consideration of sub-consolidation settlement, the calculated settlement after work is shown in the following table 1. It can be seen that one month after vacuum unloading and 11 Months, post-construction settlement can meet the standards of 10cm and 5cm, respectively.

Table 1 Post-construction Settlement and Pre-compression Period\Post-construction Settlement and Post-construction Settlement (cm) Pressure Period 1 month after vacuum unloading Residual settlement 11 months after vacuum unloading Post-construction settlement caused by train load on the remaining subsidence track 4 Pair of surrounding environment Impact analysis and main control measures In order to study the influence of the vacuum-filled self-loading combined preloading method on the surrounding environment, inclinometers, side piles and water level pipes were buried to monitor the deep displacement, surface displacement and the water level in the vacuum process. Changes.

The test data show that the horizontal displacement of the surface layer is consistent with the trend, the maximum is 40cm (2m away from the boundary of the reinforced area), there is also 6cm displacement at the boundary of 10m away from the reinforced area, the scope of impact is greater than 10m. The trend of deep horizontal displacement In the initial stage of vacuuming (before embankment filling), the soil moves to the consolidation zone, and the displacement to 10m below the surface before embankment filling reaches 7.2mm (K0+010), and it is filled horizontally with the embankment. The change in displacement gradually decreases inwards and starts to move outwards, and it still moves with time. When the vacuum unloading has a large amount of change, the deformation is basically stable after 8 months. For the crack group formed around the reinforcement zone during a vacuum test (before embankment filling) in a test section, it can be seen that the vacuum pump's water tank base has serious tilt, deformation and cracking. This aspect shows that the effect of vacuum preloading is obvious. On the other hand, it also shows that the inward horizontal displacement caused by vacuum load can not be ignored. The groundwater level descends in the reinforced area, and has the characteristics of being in the first reinforcement zone outside the edge of the reinforcement zone due to the horizontal displacement of the soil, and then moving to the outside of the reinforcement zone. If there are underground pipelines or structures on the ground within the affected area, this reciprocal deformation of the soil may cause pipeline damage and cracking or even collapse of structures. The influence of the soil shall be monitored and effective control measures shall be taken during construction to reduce the impact. degree.

The main measures adopted in actual construction are: vacuum load classification loading; embankment load is added after the vacuum degree under the membrane is stabilized; and the size of vacuum load is matched with the size of embankment load; isolated piles are used for isolation; real-time monitoring , dynamic construction.

5 Conclusions and prospects Using vacuum-filled self-loading combined preloading method to soften soft soil, the actual total settlement of the pilot site is 107.7~108.9cm; the effective depth of reinforcement can reach 27m, that is, the bottom of the drainage plate; the prediction analysis of post-construction settlement shows that After 1 month and 11 months of vacuum unloading, the requirements for post-construction settlement of 10 cm and 5 cm can be met respectively. The method is a high-efficiency and low-cost soft foundation treatment method. It has the advantages of high filling speed, short construction period and preloading time, large treatment depth, and high consolidation speed, and can effectively reduce post-construction settlement to achieve preloading and overloading. The purpose is therefore to have broad application prospects in the soft ground treatment of high-speed railways (roads), especially deep soft grounds.

Of course, there are also problems such as differential settlement control in reinforced areas, testing methods and instrument performance problems in vacuum environment, excessive problems between vacuum preload soft foundation treatment section and CFG pile-net composite structure treatment sections, etc. , need further study and solution.

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