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Discussion on Vacuum Preloading in the Soft Foundation Construction of Port Flyover Abutment Xia Xiaoming S Mao Jie1, Lü Wei2 (1. Guangdong Guanlu Bridge Co., Ltd., Guangzhou 510630, China; 2. Guangdong Daxiong Economic and Technical Consulting Company, Guangzhou 510623, China) , Reduce the disturbance to the soil and complete the construction of the sand wells in bags. To reduce the difference in settlement between adjacent soil layers and accelerate the settling and consolidation effect of the accident section, change the original surcharge preloading to vacuum combined surcharge. Pre-compression, but in the vacuum preload construction due to lack of understanding of the geological conditions caused by the vacuum can not meet the design requirements, and cause damage to adjacent buildings. Combined with this case, the experience and suggestions for the consolidation of soft ground with vacuum preloading method are presented.
1 Overview The Port Interchange is located in the port town of Zhongshan City and is the connecting line between the Guangzhou-Zhuhai expressway (Zhuzhou)-Zhuhai Section and the Zhongshan-Jiangxi Expressway. Its engineering geology is dominated by silt and silt soil. The maximum thickness of sludge is 36.8m, and the water content is 45.8%~61.0%. The compression coefficient is 0.788 ~ 1.422. The basic bearing capacity of the foundation is 54~104kPa, which is typical of the Pearl River Delta. Land base. In the design, it adopts bag sand wells and sand cushions for drainage consolidation. Vertical drainage body bags are designed with a length of 18~20m, a diameter of 7cm, a spacing of 1. 3m, a plum blossom arrangement, and a sand cushion of 0.8. m. The height of the design of the 0 main bridge of the port interchange is 7.93m. The construction began in April 2001.
2 Problems encountered during construction and its treatment The construction of the main line base of the port started at the end of 2000. In April 2001, the construction of sand-packed sand wells began. During the construction of the right section of K1+440~K1+540, a sudden slip occurred. The slip range was approximately 100m (length of the roadbed) X50m (direction of vertical roadbed). The construction unit immediately stopped the construction of this section.
The original landform of this section is the slope of the old river. There is a large amount of humus sludge on the surface and there is a natural sliding surface. No special treatment is required in the design drawings. The flotation of the surface is cleaned only according to the design requirements. Although the filling height of the work mat and sand cushion before slumping is only about 2.5m, the ultimate filling height calculated by geological data has not yet reached 3.5m. However, due to the construction of the vibrating immersed tube process for bagged sand wells, the original soil The layers formed disturbances and the bearing capacity of the foundation was reduced (according to the relevant experience and achievements of the adjacent section of the Beijing-Zhuhai Expressway Guangzhu section, its bearing capacity decreased by about 20 to 30%). In addition to the fact that there was a natural sliding surface, the embankment that was built was moved along the river bed surface of the old river.
In view of the disturbance and destruction of the soil structure after the embankment sliding, the strength was greatly reduced, and the problem of the demolition of the neighboring K1+540~K1+638 house was not completely resolved, the decision was made to deal with it as follows: The soil strength was waiting for some time. After recovery, continue construction according to the original design. The original vibratory immersed pipe was changed to the hydrostatic immersed pipe process, and only one sand-shaft machine was allowed to operate at the same time to reduce the disturbance of the sand-packed well to the soil. The original line-by-line construction was changed to interval construction, ie, the remaining sand wells were to be constructed after the completion of all construction of one sand well, and the construction time interval of adjacent sand wells was used to make full use of the wells completed. Drain consolidation, improve the bearing capacity of soil.
Practice has proved that this scheme is successful. After a lapse of 12 months, the construction of sand-filled sand wells for the remaining K1+440~K1+620 segments began. The roadbed was basically stable, there was no recurrence of slippage, no treatment fees were imposed, and costs were effectively controlled.
At this time, the adjacent subgrade has been constructed for more than one year. It has been basically filled to the design elevation, and most of the subsidence has been completed. The adjacent K1+400 has settled by 2.4m, accounting for 88% of the total design subsidence. The settlement of K1+440 640 section will reduce the settlement difference at the subsequent junctions. After consultation with the design agency, it is decided to implement vacuum combined surcharge preloading for K1+ 440~K1+640. For the integrity of the strong joint embankment, the right roadbed of K1+440~K1+550, which has been built, is first unloaded to a height of about 2.5m. When the surrounding roadbed is filled to the same height, the roadbed will be lifted. The joints shall be appropriately covered with several layers of two-way geogrid, and the geogrid shall extend into the filled embankment 7m (see and).
On the vacuum prefabricated facade, vacuum preloading began on the 10th of the month, the initial progress was relatively smooth, and the vacuum level rose steadily. After 3 days, it reached 80% of the design requirements, and the settlement at each observation point was obvious. Observations on July 1 showed that the maximum settlement was 48.7 cm (K1 + 575) and the minimum settlement was 7 cm (K1 + 480 right). After reaching the design requirement of about 80% vacuum and stable for 7 days, it began on July 2. The design requirements are to be filled. In the afternoon of the same day, the vacuum level began to decline, and the vacuum level dropped to about 40% to 50% on the following day. The initial suspicion was that improper filling had caused damage to the vacuum film or a problem with the sealing film. A thorough inspection of the vacuum film and the sealing groove was conducted. No damage or leakage was found. During the inspection of the vacuum pump, it was found that the amount of water was significantly larger and the water quality was also changed. The original salt water turned into fresh water, and there was a negative pressure at the K1 +600 inclinometer pipe nozzles on both sides. Sealed, vacuum level has not yet risen. On a certain day, when the rain dropped, the vacuum level rose slowly. However, when the rainfall stopped for about 4 to 6 hours, the vacuum dropped back to 40% to 50%. According to the above analysis, there may be sand traps in the shallow section of the site. In order to verify the hypothesis for geological survey, nine static contact points were placed around the vacuum preload (see). Static penetration results are shown in Table 1. Geological borehole No. Position Sand depth/m Right The results of the left-up survey are basically consistent with the hypothesis. The geology of the site is relatively complex. There is a sand layer between 4m and 10m below the ground, and there is a muddy soil in the middle of the sand layer. The author believes that the sand layers in this lot are interconnected. This is the main reason why the vacuum preloading cannot meet the design requirements due to the lack of watertightness due to vacuum preloading. In order to solve the problem of vacuum pre-press sealing, after discussions with design agencies and experts, it was decided to lay underground continuous anti-seepage curtains at the periphery of the vacuum preloading zone. The impervious curtain is constructed by a deep-spraying shotcreting machine with a diameter of 70cm, overlapped by 10cm, and a depth of 12m. The sprayed rice is prepared with cohesive soil. The designed amount is 40~50kg/m. The actual construction time is 10kg/m for safety reasons. Cement dosage.
Impervious curtain structure see. At the end of the construction of the anti-seepage curtain, the degree of vacuum has risen significantly. In order to prevent the impervious curtain that has just been completed from being damaged, only one vacuum pump was scheduled to operate in the initial stage, and then gradually added to four, and the vacuum pump was arranged to work in rotation. One week after the vacuum reached again and stayed around 80%, the re-starting of the building was resumed. Since then, work has progressed normally.
3 Other conditions A three-story residential building 35m to the right of the foot of the roadbed sloped after vacuum preloading. The house floor and exterior walls cracked. The courtyard doorpost was sheared and the maximum crack of the 9cm wall was displaced up and down 2.2. Cm, the entire house is basically destroyed, see.
Cracked door frame 4 re-understanding of vacuum preloading Vacuum preloading as a new technology applied on the highway in recent years has reflected its unique advantages: fast consolidation of soil, eliminating embankment slippage within a certain range The danger of stability can effectively shorten the construction period, and the price is relatively cheap in several rapid reinforcements of soft foundations, which has been rapidly promoted.
However, vacuum preloading causes the destruction of surrounding buildings and facilities to occur in other highways in the Pearl River Delta region of Guangdong. For this reason, the author believes that vacuum preloading and its applicability must be discussed.
4.1 Mechanism of Vacuum Preloading to Reinforce Soft Soil Foundation The mechanism of vacuum preloading to reinforce soft soil foundation is to reduce the void water pressure in the soil by vacuuming so that the effective stress of the soil can be increased. The degree of vacuum (pressure difference) formed in the sealing film during the vacuum process is diffused in the soil. The water and gas in the soil are permeated to the drainage channel under the effect of the vacuum degree. The seepage flow is from near to far, until the seepage resistance. Equal to vacuum (see). This near and far seepage range determines the scope of the vacuum preload, within the scope of the vacuum preload pressure, the surrounding soil will produce settlement and lateral displacement to the direction of the preload center, when the vacuum preloading construction area When there are buildings or other facilities nearby, vacuum preloading will have a certain effect on neighboring buildings or facilities.
Vacuum Pre-Pressure Settlement Affected Range Diagram Given that the vacuum preloading method has a large impact on the surrounding environment and may even cause serious consequences in some cases, it is necessary to consider this effect in design and construction. According to surveys and studies of multiple projects, the effect of vacuum preloading method for foundation reinforcement construction can reach 20 (with the boundary less than 5cm), and its impact degree gradually weakens from near and far. The scope of influence is related to the permeability coefficient of the soil, the permeability coefficient has a large influence range, the permeability coefficient is small, and the impact scope is small. If there are building facilities in the range of 15 ~ 35m outside the vacuum preloading zone, protective measures should be considered to block the horizontal diffusion of vacuum.
4.2 Mechanism of impervious curtains Impervious curtains can be in the form of cement mixing and mud (sludge) agitating continuous walls, with slightly different mechanisms of action. Cement mixing is achieved through the cement and the sand of the water permeable layer as a whole to reduce its permeability coefficient and achieve the anti-seepage effect. The mud (sludge) agitation is to plug the pores in the permeable layer through the mud (sludge), reduce the permeation channels, and reduce the permeability coefficient to achieve the anti-seepage effect. The anti-seepage effect of cement stirring is obviously better than mud agitation. For this reason, the author believes that for buildings with close distances, high permeability, and high requirements for prevention, cement-mixed underground continuous curtain walls should be adopted; for buildings without surrounding impact, only anti-seepage treatment can be used. Mud (sludge) Stir continuous curtain wall.
In order to ensure the effectiveness of the anti-seepage curtain, it is recommended that the anti-seepage curtain be carried out before the vacuum preloading and be spaced at a certain time to facilitate the consolidation of the cement and the binding of the mud. It is recommended that the cement mixing process should be about 5 days apart and the mud mixing process should be about 10 days apart.
5 Some Experiences and Recognitions in Other Aspects 5.1 Understanding of Engineering Geological Exploration The mastery of engineering geology sometimes directly relates to the success or failure of the project. The current "Technical Code for Design and Construction of Highway Soft Ground Embankment" stipulates that the drilling distance between complex sites in the detailed survey stage is 300 ~ 500m, and there is no requirement for surveying each embankment horizontally. According to this regulation, it is difficult to reveal a small range of stratigraphic differences, and miss many necessary stratigraphic information, which will adversely affect the subsequent design and construction. Therefore, it is necessary to carry out additional surveys on key sites before construction.
5.2 Understanding of geomorphology The initial cause of embankment slippage in this segment is that part of the area from K1 +440 ~ K1 + 540 is in the slope of the old river. For such a lot, there are inherently unstable factors. The design and construction units did not pay enough attention to this. For this reason, the author believes that the construction unit must pay attention to the landscape before construction, and find that unreasonable or vulnerable areas should communicate with the owner, supervision, and design unit in time to prevent it.
5.3 Recognition of the applicable conditions of vacuum preloading Before the development of the program for vacuum combined surcharge preloading of soft ground, the engineering geological conditions, hydrogeological conditions and surrounding buildings and facilities must be ascertained. Through the analysis of the site engineering geological conditions, the physical properties of the soft soil layers can be ascertained, the types of groundwater (pressured water, submersible), and the recharge relationship between groundwater and surface water can be ascertained to determine the tightness of the soft ground ( Air tightness and watertightness) Vacuum preloading solutions can only be developed if the tightness meets the requirements.
When analysing the hydrogeological conditions in the field, it is necessary to ascertain the development of the sand interlayer and to find out whether the sand interlayer has water conservancy links with the surface water (river, pond, ditch, and ravine). Generally, a strong permeable layer containing more than 1m in 15m depth can be isolated by deep mixing silt (or cement) to form the underground sealing wall.
There must be careful consideration of buildings and facilities in the immediate vicinity. If vacuum preloading must be used, it is recommended that appropriate protective measures be taken to ensure that nearby buildings and facilities are not damaged.
5.4 Understanding the Subgrade Slump Treatment Restoration The intensity of the restoration continues to follow the original design and construction method. Although successful, there are also shortcomings: First, it takes a long period of time to control the overall duration. Second, the sand-packed well To improve the overall anti-slip effect is not obvious, the sliding surface has not been effectively dealt with, and there are still hidden dangers in the subgrade filling period.
Great care must be taken in the construction of the roadbed in the future. In addition, if the interval time is not well mastered, slipping again will waste a lot of valuable time.
Therefore, we believe that as far as the conditions permit, it is recommended to treat such issues as possible in the form of FCB. Through cement or other colloidal materials to improve the physical and chemical properties of silt and silty soil, improve soil strength and bearing capacity, and cement soil as a root anchor to connect the upper and lower soil layers as a whole can make it anti-slip The effect is greatly improved.
6 Conclusion After fully learning the lessons learned from the port interchange, we carried out a geological supplement survey on the soft base section of the Zhongjiang Expressway that was started later, and conducted a detailed survey of the key predicated sections such as the vacuum preloading section in the design. The section that is not suitable for vacuum preloading is to be removed, and a mixing impervious curtain is added to ensure the airtightness and watertightness of the section, and the original design remains unchanged. After the implementation of vacuum preloading, no port-like interchange occurred.
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