The Vacuum Drainage Preloading Method

The vacuum drainage preloading method is a relatively new reinforced soft soil technology. It is a kind of drainage consolidation method. It is laid by laying a horizontal drainage sand cushion and a vertical drainage body arranged in a soft foundation, and then in the sand pad. A gas-tight membrane sealing device is laid on the layer, and a vacuum is formed by means of a pipe embedded in the sand cushion layer to form a negative pressure in the soil body, thereby extracting pore water in the pores of the soil body, thereby reducing pore water pressure. Increase the effective stress, make the soil consolidation, reduce the settlement at the later stage, and improve the bearing capacity of the foundation.

Principle of vacuum preloading

When vacuuming, negative pressure is gradually formed in the surface sand cushion and the vertical drainage channel, so that a pressure difference is formed between the interior of the soil and the drainage channel and the cushion. Under this pressure difference, the pore water in the soil is continuously discharged by the drainage channel, thereby consolidating the soil. Mainly reflected in the following aspects:

1) The film is subjected to a load equal to the pressure difference between the inside and outside of the film.

2) The groundwater level is reduced and the additional stress is increased accordingly.

3) The closed bubble is discharged, and the permeability of the soil is increased.

The vacuum preloading is to draw a vacuum under the sealing film covering the ground to form a pressure difference between the inside and the outside of the film, so that the clay layer generates a consolidation pressure. That is, a method of increasing the effective stress by reducing the pore water pressure while the total stress is constant. Vacuum preloading and precipitation preloading are drainage consolidation under negative superhydrostatic pressure, known as negative pressure consolidation.

Before the vacuum preloading construction, the construction preparation work should be fully prepared and the construction technical measures should be formulated. Check whether the sandbag, PVC film and medium coarse sand meet the technical standards and design requirements, and submit the inspection report to the supervision engineer for review and approval. After obtaining the approval of the supervision engineer, the next step of vacuum preloading is started.

Vacuum preloading construction process

Measure the discharge line → lay the main branch filter drain pipe → lay the upper sand cushion layer → sand surface leveling → lay the PVC film → construct the sealing groove → set the measurement mark → install the vacuum pump → vacuum preload the consolidated soil layer.

The vacuum drainage preloading method is mainly composed of four parts:

(1) Construction of a vertical and horizontal drainage channel, ie construction of plastic vertical drains（PVDS） and sand cushions;

(2) Construction of a sealing layer that ensures the insulation of the foundation from the atmosphere is ensured;

(3) To set up a set of high-efficiency vacuuming devices. That is, the main pipe and the filter pipe network are laid in the sand cushion layer, and a vacuum pump or the like is installed outside the sealing system;

(4) It is necessary to set up a set of inspection systems that can be constructed according to the design requirements.