Project content

The key point of site vacuum preloading is to strengthen the area along the route of the road and the pipeline of the drainage box in the plant area by vacuum preloading, and to enlarge the width of the edge line of each block outward by about 2 meters (to meet the requirement of embedding sealing membrane groove), so as to form a flat site. The treated foundation can meet the requirements of natural excavation of foundation pit; Before vacuum preloading, the sand cushion and bagged sand Wells laid in the previous stage should be inspected. The thickness of the sand cushion should be 0.5 meters, and the bagged sand Wells should be buried within 20 ~ 30 cm of the sand cushion. After the above procedure meets the requirements, the vacuum tube is installed, the upper sand cushion is laid, the upper sand cushion is flattened, and the second layer of 0.13m thick PVC sealing film is laid.

1. Materials: PVC sealing film about square meters, medium sand cubic meters, 4 inch galvanized iron pipe meters, 4 points galvanized iron pipe meters, 4 inch galvanized iron pipe joints, 4 inch -4 points galvanized iron pipe tee, four, 4 inch galvanized iron pipe two-way opening and closing, PVC glue and other materials.

2. Equipment: JS45 type water jet pump station is to be used for construction on site, and JS45 type water jet pump station is to be replaced as a spare equipment. It is proposed to adopt 3 dongfeng 15-ton trucks and 1 8-ton truck crane for equipment entry transportation.

3. Personnel: there are 5 monitoring personnel, a total of contractors and lifting workers in the construction team.

4. Water and electricity for construction: the construction power is 150KW, and the construction water is mainly domestic water, both of which are provided by the construction unit.

5. Acceptance criteria: the vacuum degree shall be stable at least 80KPa for three months, and the soil consolidation degree shall be ≥80%.

6. Schedule: It will take about 12 days to prepare for the entry, installation of vacuum tubes and vacuum machines, digging sealing trenches, and laying sealing films; It takes 12 days to vacuum up to 80KPa, 90 days to preload and 30 days to unload. The whole process requires about 142 days.

Construction organization design

Construction procedure of vacuum prepressing soft foundation: Before vacuum prepressing, the construction preparation shall be fully done and the construction technical measures shall be formulated. Check whether the sandbag, PVC film and medium and coarse sand meet the technical standards and design requirements, and submit the inspection report to the supervision engineer for approval.

After obtaining the approval of the supervision engineer, the next step of vacuum preloading construction shall be started. Main construction steps: measuring unreeling - laying main filter drain - laying upper sand cushion and sand surface leveling sealing groove - > Settings - laying polyvinyl choride (PVC) - construction surveying markers to install the vacuum pump, vacuum preloading consolidation soil this vacuum preloading engineering unit piece of vacuum preloading, plans a total of 7.5 kW vacuum pump units, design is a total capacity of 120 kW. In order to ensure the power supply of vacuum preloading engineering, generators and power supply network layout, distribution way for cable shunt feed, namely, those of the generator, set the multiloop distribution box, with a soft rubber copper core cable sectional area for five core along the cofferdam power to the power points, and all the control box of waterproof and leakage protection device to consider the rainy season.

1) measuring and setting out :(P14) plane layout, measuring and setting out 12 unit blocks with theodolite and level, releasing the exact position of the edge line of each reinforcement unit with wooden stake (1 for every 20 meters) or bamboo pole (interpolation) and white lime, and marking the elevation of the top surface of sand cushion with red paint; Red nylon rope can also be used to connect the wooden stake with the small bamboo pole.

2) Laying the main branch filter drainpipe: the main branch filter drainpipe can be divided into main (dry) pipe and branch filter pipe. The main pipe is 4-inch galvanized iron pipe, and the supporting pipe is 2-inch galvanized iron pipe. The main pipe and the branch filter pipe are connected with three or four ways of reducing diameter, and the butt joint with the pipe diameter is connected with screw thread. All suction pipes shall be buried in sand layer and connected with vacuum pump through membrane extractor and suction pipe. At the same time of digging the sealing ditch, the main (dry) pipe and branch filter pipe can be processed, connected and installed and buried. At the same time of this process, the plastic drain head exposed to the sand cushion surface should be buried in the sand cushion.

3) According to the design requirements, sand cushion shall be laid within the vacuum preloading range. A total of one cubic meter of sand cushion shall be laid. Medium and coarse sand with silt content less than 5% and 0.25 meters thick shall be laid at a time. The paving method adopts artificial block paving.

4) In order to ensure the reinforcement effect of vacuum preloading, vacuum preloading sealing film trench shall be excavated between two adjacent unit blocks, and a 2-meter interval shall be reserved within the unit block without sand cushion. The sealing groove is arranged around each unit block, and it plays the role of peripheral sealing in the vacuum preloading construction. According to the position of sealing groove, it can be divided into sealing groove outside the reinforcement block and sealing groove between two unit blocks, which have different sections. The construction of sealing ditch adopts hydraulic backhoe combined with manual excavation. After laying sealing film, the sealing ditch should be backfilled with silt or clay.

5) After the completion of the above two tasks, the sharp pebbles, sharp shells and other debris on the surface of the sand cushion shall be picked up first, and the artificial fine flat sand cushion surface shall be added. After embedding the probe of the vacuum gauge and other observation instruments, the whole vacuum preloading unit block is covered by manually layering two layers of PVC salt with a thin film. Then the surrounding of the membrane body is buried into the sealing trench, and the sealing trench is backfilled with silt and clay and compacted, and then the sealing water cofferdam is built upward.

6) Membrane outlet connection and vacuum pump system installation: The vacuum main pipeline is connected to the vacuum pump through the membrane outlet and suction hose. The connection of the ejector must be firm and the sealing must be reliable and safe.

7) After the above work is completed, vacuum pumping and filling sealing water film can be started. After the vacuum is pumped, the subintimal vacuum of the reinforcement unit block will continue to rise. When the vacuum degree under the membrane reaches and stabilizes above 80KPa, it enters the normal preloading stage of vacuum preloading engineering. According to our construction experience, we can enter the normal vacuum preloading stage after about 25 days of vacuum test.

8) Vacuum preloading unloading acceptance When the vacuum preloading reinforcement unit block is vacuumized continuously for three months with a vacuum degree of 80KPa under the film, or when the foundation consolidation degree Ut≥80%, it can be stopped for unloading and handed over for acceptance.

　　Construction technical guarantee measures

1. Construction quality assurance measure 2. Before the sand laid on the upper layer enters the site, samples shall be sent to the quality engineer for approval and relevant quality inspection and appraisal units for inspection.

The vacuum time is closely related to the soil quality conditions and the distance between vertical drainage water bodies, so as to achieve the same degree of consolidation. The smaller the distance is, the shorter the time will be. In the case of tight construction period, the smaller the distance may be, and in the case of loose construction period, the larger the distance may be, so as to reduce the cost.

The average degree of consolidation required in the reinforcement area shall not be less than 80% in general. If time limit permits, a larger degree of consolidation shall be adopted as the degree of consolidation required by design as far as possible.

The effect of vacuum preloading is greatly related to the vacuum degree in the sealing film. According to the experience of some domestic projects, the vacuum degree in the membrane can be generally maintained at about 600mmHg column height, equivalent to the heap pressure of 80KPa.

The total area of vacuum preloading shall not be less than the area surrounded by the outer edge of the building foundation and shall exceed the outer edge of the building foundation by 2 ~ 3m. The area covered by each thin film should be as large as possible. If preloading of each block is needed, the space between each block should not exceed 2-4m, and at least two vacuum pumps should be set in each preloading area.

Vacuum prepressure sealing film should be made of impermeable material with good anti-aging property, good toughness and strong puncture resistance. When the sealing film is hot, use the flat lap of two hot joints, the lap length should be more than 15mm. 3 layers of sealing film should be laid. Trench and tile can be used around the covering film.

When laying polyvinyl chloride sealing film, attention should be paid to the vacuum reinforcement around the block, the sealing film four sides should retain enough 3-5m margin.

After the beginning of the vacuum pumping process, if the vacuum degree under the membrane is less than 80KPa as expected, corresponding technical measures shall be taken to deal with it. If can use the spare film extractor to add pump to increase the vacuum capacity, or carefully check, repair seal groove, so as to ensure that the vacuum preloading can achieve the design effect.

2. Construction safety guarantee measures

1) Conduct safety education for workers before operation and make safety disclosure. Workers must strictly abide by the rules of mechanical operation and operation. It is strictly prohibited to violate the rules of operation and operate blindly.

2) The electrician shall be responsible for checking the insulation of the wires, especially whether the insulation of the power line and the leading-line of the distribution box and container is reliable.

3) Strictly implement the company's management regulations on construction safety. Workers entering the construction site must wear hard hats and are not allowed to wear slippers or go to work after drinking.

4) Team members should take care of each other, clarify post responsibilities and improve safety concept.

5) Implement the system of working with certificates for special types of work, and prohibit those without certificates to work.