3. Key technologies in the construction of vacuum combined load method

3.1 Reinforcement effect Vacuum combined heap load preloading method is a soft base treatment method developed on the basis of vacuum preloading and heap load preloading. It has the dual effects of vacuum preloading and heap load preloading, and is suitable for projects with large dry load and high bearing capacity requirements. The reinforcement effect of this method has the following three characteristics:

(1) Consolidation velocity In vacuum preloading stage, the vacuum degree is gradually transferred to the upper body through the filter pipe, sand cushion and vertical drainage channel, and the pore water in the soil is continuously discharged outward under the action of such pressure difference, so that the soil has drainage consolidation. On the basis of vacuum preloading, the embankment is filled to carry out the preloading. On the one hand, the role of heap loading increases the pore water pressure in the soil. On the other hand, the effect of vacuum preloading makes the pore pressure of sand cushion and vertical drainage channel in the state of negative pressure, so that the pore water pressure difference in the foundation is greater, which accelerates the discharge of water in the soil, the consolidation speed is natural fast, and the reinforcement effect is more obvious.

The depth of reinforcement mainly depends on the depth that causes the change of pore water pressure. The vacuum degree is one of the main factors that cause the change of pore water pressure in the method of strengthening soft foundation by vacuum combined with pile-load preloading. A large number of field measured data show that the attenuation of vacuum degree along the depth is obviously lower than the attenuation of total stress along the depth under load. Therefore, the reinforcement depth of vacuum combined preloading is greater than that of normal preloading.

(3) strength increasing soft foundation in a vacuum combined pile load preloading method, when 80 kpa vacuum, vacuum load equal to 4 m ~ 5 m height of fill in the load, which makes the method in the process of preloading equivalent height at the overload, combined with the consolidation speed, deep influence depth and other factors, makes the foundation strength increase fast, and improve.

3.2 Foundation stability Vacuum combined heap load preloading method applies external load on the basis of vacuum preloading. The negative pressure generated by vacuum causes the soil to shrink and deform inwards, which can offset the extruding deformation caused by heap load. Therefore, the foundation has good stability in the process of filling. From another point of view, the foundation produces inward shrinkage and deformation, which is equivalent to applying backpressure protection on both sides of the subgrade. It can be seen that the anti-slip distance is increased, so the vacuum generated negative pressure is conducive to the subgrade stability.

4. Construction technology of vacuum combined preloading method

4.1 Drainage channels shall be first set up on the wet and soft foundation, that is, sand cushion shall be laid first according to the conventional method, and then sand Wells shall be bagged. The length of sand Wells shall be perforated through the silt layer or not less than 20m, and the spacing shall generally not be greater than 1.5m.

4.2 Embedding monitoring instrument. Based on the homosexual love situation, we arranged two monitoring sections in this test section, and buried instruments such as void water pressure meter, inclinometer, surface settlement plate and vacuum degree meter to monitor the stress and strain changes of soft foundation before and after reinforcement and control the filling. Due to vacuum preloading lateral displacement towards the center of the subgrade. Due to the continuous decrease of pore pressure, the settlement is not strictly required and the lateral movement and pore pressure coefficient are the main control indexes.

4.3 Clear the site and install drainage (gas) pipes. The surface of the sand cushion should be cleared of debris. Do not have gravel, bamboo stick and other sharp objects to prevent puncture surface of the sealing film. Then vacuum drainage (gas) pipe (PVC pipe) is arranged. Highway embankment is long strip, according to this characteristic, there are two kinds of main pipe and filter pipe. The diameter of the main pipe is 6cm. Two filter pipes with a diameter of 6cm are arranged along the longitudinal interval of 30m. Two filter pipes with a diameter of 6cm are set on both sides of the main pipe at an interval of 6m.

4.4 The vacuum pipeline is connected from the main pipe and connected with the jet pump through the sealing film. Each jet pump controls the reinforcement area of 600 ~ 800 square meters. When connecting pipes, the joints should be firm, sealed and flexible.

4.5 After the pipeline is laid, the bedding sealing film shall be followed. The sealing film should be good anti-aging, high toughness, strong puncture resistance, good integrity and sealing, with a thickness of 0.14 ~ 0.16mm. Then, a sealing trench should be dug around the reinforcement area with a depth of no less than 1.5m and 50cm should be entered into the silt. At the same time, the sealing film should be turned over and anchored in the trench, backfilled with cohesive soil, and a cofferdam should be constructed to seal with water injection.

4.6 After the sealing film is covered, start vacuuming and check for air leakage at any time. Air leakage usually occurs in foundation, edge ditch, film, joint and other places. Under normal conditions, the vacuum degree within the membrane can reach more than 70kPa for about 7-10d. After being stabilized for 2-3d, stacking is started. Before stacking, a layer of non-woven fabric and other fabrics must be added to the sealing membrane to prevent the filling material from bursting the sealing membrane.