General construction technology and supervision points of vacuum preloading method

General construction technology and supervision points of vacuum preloading method

1). Sand cushion and bagged sand well construction

For sand cushion and bagged sand well construction, the ground can be leveled, and according to the specific conditions of the soil foundation, a certain thickness of sand cushion is laid and stabilized by machine. Position the pile tube in a vertical position, place the casing at the designed depth, place the sandbag filled with sand into the casing, and then pour a small amount of water into the casing. The most important thing in this process is to immediately fill the head of the sand well.

2). Laying vacuum filter tube

The sand bag head is buried in the sand cushion, placed flat, and then the vacuum filter tube is laid. The vacuum filter tube is taken out from the main filter tube, sealed from the membrane discharger, and finally connected to the membrane discharger and the jet vacuum pump [3].

3). Filming and lamination

A layer of non-woven fabric is applied to the sand cushion to avoid damage to the sealing film. Then, in the windless climate, a sealed polyethylene film will be set up. It should be laid layer by layer. After the first layer is checked, there is no problem. Then lay the next layer. If there is any problem during the inspection, it should be repaired in time. After the film is laid, the film setting is set, and the jet vacuum pump and the observation instrument are intermittently installed along the roadbed slope. The sealing film should be completely buried in the laminating groove, and the lamination groove should be trapezoidal and filled with clay.

4). Road filling

After the jet pump and the settlement table are placed, the readings of various side instruments can be set, and then the test pumping test can be used to check whether there are any loopholes on the membrane and take corresponding measures. After confirming the correctness, press it to the designed preloading height according to the general embankment filling construction method.

The roadbed is piled up to handle the soft soil foundation, and the original foundation will produce lateral deformation. However, due to the effect of vacuum preloading, the lateral deformation caused by the vacuum preloading and the lateral deformation caused by the stacking are mutually eliminated. Moreover, the consolidation of the foundation soil caused by the vacuum preloading makes the effect of the loading treatment better.

5). Preload control

The output power of the vacuum pump is greater than or equal to 7.5 kW, and the seal must be ensured before installing the vacuum pump. A PVC drainage filter is arranged in the middle of the gravel drainage layer, and the spacing is generally about 4 meters, and the maximum is not more than 6.5 meters. In the area where the vacuum preloading is strengthened, when the vacuum is more than 85 kPa and the vacuum is continuously pumped for 4-5 months, or the foundation consolidation degree meets or exceeds the design requirements, the unloading should be stopped and handed over to the relevant parties. acceptance.

6). Instrumentation

Before the completion of the first layer of sof soil treatment and  backfilling, the measuring point should be set up with the settlement board, the horizontal settlement tube, the earth pressure box, the single hole water meter, the inclined tube, the groundwater level well and the side pile. The pore water pressure gauge is placed in the road and shoulder position of the cross section of the subgrade, and each position is placed 48 in the depth direction. 

Twelve deep subsidence marks were placed in the road. The inclinometer is placed on the outer side of the subgrade slope and on the outer side of the side ditch. The pile is placed 80m at the foot of the slope, and one of the two observation sections is set. A settling plate is provided on the road in the cross section of the roadbed, on the left shoulder and on the right shoulder. Horizontal sections are placed in each section. A pressure box is placed at the center of the carriageway, at the center of the central divider, at the shoulders and at the foot of the slope. A single-hole water discharge machine is designed in the middle of the road and at the shoulder [4]. The groundwater level well is generally designed 2m outside the subgrade trench, and an earth pressure box is placed between each two observation sections. After all the meters are set, they should be returned to zero in time. Look at the effect every two days or so. At least 3 consecutive readings are stable.