Vacuum preloading method used in highway construction

Because vacuum preloading has obvious advantages over preloading, it was widely used in the soft foundation treatment of port engineering in the late 1980s. However, the load provided by vacuum preloading is often limited (about 80kPa, equivalent to 45m fill Loads), when it can not meet the requirements of bearing capacity and settlement, etc., need to be supplemented with pile load, theoretical research and engineering practice have shown that the two types of loads can be combined. The vacuum-surcharge combined preloading method was developed based on the vacuum preloading method and the surcharge preloading method. At present, this method is widely used in the soft foundation reinforcement of hydraulic structures such as high-grade highways, large tank storage yards and port culverts. However, in practical applications, some reinforcement effects are unsatisfactory, even failed engineering examples. Except for the reasons such as design and construction quality, the application conditions of the law are largely not considered enough, so it is necessary to have a vacuum-reactor. The conditions for applying the combined preloading method are further studied. 1 vacuum-surcharge combined preloading mechanism mechanism Introduction First, through the vacuum in the reinforcement zone in the soil caused by negative pressure, so that the boundary of the pore pressure is reduced, for the vacuum preloading phase, R is a constant, so dRc =-du, that is, the pore The reduction in water pressure is the increase in effective stress. Therefore, the mechanism of vacuum preloading is essentially the mechanism of the pore water pressure change in the soil. The effective stress Rc before the vacuum is equal to the pressure of the earth's own weight. With the passage of time, the pore pressure in the soil gradually decreases and the hole is reduced. The pressure is converted into effective soil stress. After the vacuum consolidation process is completed, the vacuum pressure is converted into effective stress. The higher the degree of vacuum, the smaller the attenuation along the depth, the greater the increased effective stress, the better the reinforcement effect. For natural foundations, the foundation soil is generally in the K0 consolidation state, ie, Rc30 = K0Rc10. Since the pore water pressure is a ball stress, the reduced pore pressure (increased effective stress) during vacuum preloading is equal to that of the foundation. The moire circle size of the soil unit does not change, but only shifts to the right. When the load is removed, the consolidated soil is changed from the normal consolidation state to the over consolidation state, and the strength is increased compared with that before reinforcement.

Brief Introduction of Construction Technology for Consolidation of Soft Foundation by Vacuum-Surface Combined Preloading Method

(1) Reconciliation: In order to find out whether there is a relatively strong water-permeable soil layer (silt, silt, etc.) in the reinforced area, a detailed engineering geology supplementary survey should be conducted. The purpose is to find out the depth, extent of the pervious layer, and whether it is necessary to use curtains or other blocking measures.

(2) Drainage board construction: Drainage board shall be inserted vertically and horizontally into the design elevation, and the deviation of verticality shall not be greater than 1.5%. The plastic drainage board construction shall strictly comply with the provisions of the 5 plastic drainage board construction regulations.

(3) Sand cushion construction: Firstly, the site of the reinforcement area should be leveled, and the thickness of the laid sand cushion should be about 50cm. The surface elevation should be within 10cm.

(4) Vacuum preloading: After the preliminary work is completed, the sealing trench can be excavated, the drainage pipe can be buried, the front geotextile can be laid, the sealing membrane can be buried, the vacuum pump can be installed, and the vacuum gauge probe can be installed. Then vacuum is started.

(5) Vacuum-surcharge combined pre-compression construction: When the vacuum degree under the membrane is stable at about 80kPa, the upper loading construction can be performed after vacuuming for 10 days.

The applicable conditions of vacuum-surcharge combined preloading method for soft foundation reinforcement The purpose of soft soil foundation treatment is to improve the properties of foundation soil, increase the shear strength and bearing capacity of foundation soil, and reduce settlement or uneven settlement. Although there are many methods for foundation treatment, there is no universal method. It is necessary to make technical and economic comparisons based on factors such as the soil conditions in the reinforcement site and the actual reinforcement effect required. The reason why the reinforcement effect is unsatisfactory and failed is that, besides the reasons for design, construction quality, etc., the application conditions of the law are not considered sufficiently. From the reinforcement mechanism, it is known that maintaining the vacuum degree under the membrane and ensuring that the vacuum degree is transmitted smoothly to the deep soil are the key to the success of the vacuum-surcharge combined preloading method, and the vacuum degree under the membrane is closely related to the sealing of the membrane seal and the deep soil. . Applying the vacuum-surcharge combined preloading method to reinforce the soft foundation, the applicable conditions of the method must be studied.

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