Principle of vacuum preloading

The vacuum preloading is to first lay a sand cushion layer of about 30~50cm thick on the soft soil foundation, arrange the main filter tube in the sand cushion layer, and then apply a film on the sand cushion layer to penetrate the main pipe from the film. The vacuum pump is connected, and the vacuum pump is turned on for pumping and pumping to achieve the purpose of reinforcement. 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 sandbags, polyvinyl chloride film and medium coarse sand meet the technical standards and design requirements.

Vacuum Preloading Method Combined with Reinforcement Soft foundation Technology

Vacuum preloading method combined with reinforcement soft foundation technology

Technical content

(1) The vacuum preloading method is to set up a sand well or a plastic drainage board in the soft clay foundation to be reinforced, and then lay a sand cushion on the ground, which is covered with a gas-tight sealing film to isolate the soft soil from the atmosphere, and then pass through the embedding. The water filter pipe in the sand cushion is evacuated by a vacuum device to discharge the air inside the film, thereby generating a gas pressure difference inside and outside the film, and this part of the air pressure difference becomes a load acting on the foundation. The foundation is consolidated as the isotropic stress increases.

(2) The vacuum preloading combined preloading method is based on the vacuum preloading, after the vacuum degree of the membrane reaches the design requirement and is stabilized, the staged loading is carried out, and the stacking rate is controlled according to the deformation of the foundation and the change of the pore water pressure. . Before the preloading construction, the sealing film must be covered with a protective layer such as non-woven geotextile and clay (fly ash) for protection, then layered backfilled and compacted. Compared to pure preloading, the loading rate is relatively fast. After the end of the loading, enter the joint preloading phase until the rate of deformation of the foundation meets the design requirements, then stop the vacuuming and end the vacuum combined with the preloading.

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What are the adaptation areas of the preloading method?

The pre-pressing method is suitable for the treatment of saturated clay foundations such as silt soil, silt and flushing soil. Generally, when the thickness of the soft soil layer is less than 4m, the natural foundation preloading method can be used. When the thickness of the soft soil layer exceeds 4m, in order to accelerate the preloading process, vertical drainage preloading such as plastic drainage belts and sand wells should be adopted. The law handles the foundation. 

For vacuum preloading, vertical drains must be placed in the foundation.

Main research directions of geotechnical engineering

Main research directions of geotechnical engineering

1, Urban underground space and underground engineering: 

taking urban underground space as the main body, studying various environmental geotechnical engineering problems in the development and utilization of underground space, rational utilization strategy of underground space resources, and design and calculation methods of various underground structures and Construction techniques of underground engineering (such as shallow buried excavation, shield method, freezing method, precipitation drainage method, sinking method, TBM method, etc.) and its optimization measures.

2, Slope and foundation pit engineering: 

focus on the impact of foundation pit excavation (including foundation pit dewatering) on ​​adjacent existing buildings and environment, design calculation theory and method of foundation pit supporting structure, and optimal design of foundation pit supporting structure And reliability analysis techniques, slope stability analysis theory and the development and application of new support technologies.

3, Foundation and foundation engineering: 

focus on foundation model and its calculation method, parameter research, research on new technology, new method and detection technology of foundation treatment, building foundation (such as column foundation, cross foundation, raft foundation, box) Research on the mechanism and law of interaction between the basic foundation and the pile foundation and the superstructure.

Introduction to soft base processing

Introduction to soft base processing

Common methods and characteristics of soft foundation treatment：

1. Composite foundation method: 

cement soil mixing pile, powder spray pile, gravel pile, etc.; Disadvantages: high cost；

2, Drainage consolidation method：

(1) Combined drainage of prefabricated vertical drains: long construction period, unsatisfactory effect；
(2) Plastic drainage board combined with vacuum preloading: after 90 days of construction, the effect is easy to control and the cost is low；

3, Dynamic consolidation:

 shortcomings: quality is uncontrollable, easy to form "spring soil"；

4, No drainage sand cushion vacuum preloading: 

new construction method, short construction period, low cost, cost savings compared with plastic drainage board vacuum preloading one-third, the effect is reliable；

5, Explosion squeezed drainage method.

What is Prefabricated Vertical Drains？

Prefabricated Vertical Drains

Prefabricated Vertical Drain is a plastic drainage belt with various shapes such as wave type and harmonica type. In the middle is an extruded plastic core plate, which is the skeleton and passage of the drainage belt. The cross section is parallel cross, and the two sides are wrapped with a non-woven geotextile as a filter layer. The core belt supports the water and the water infiltrated by the filter layer is discharged upward. We also call it PVDs or wick drains。

PVD wick drain is a good vertical channel for soft foundation treatment of saturated viscous and mixed fillings such as silt, silt soil and flushing soil, which greatly shortens the consolidation time of soft soil.

Vacuum preloading reinforcement mechanism

Vacuum preloading reinforcement mechanism

The vacuum preloading method uses atmospheric pressure as a preloading load. Firstly, a layer of water-permeable sand cushion is laid on the surface of the soft soil to be reinforced, and then several layers of air-tight plastic film or rubber cloth are covered thereon, which are sealed around the atmosphere and isolated from the atmosphere.

A drainage pipe is buried in the sand cushion layer, and then communicated with the vacuum pump to perform air pumping, so that the water permeable material maintains a high degree of vacuum, and a negative pore water stress is generated in the pore water of the soil, and the pore water and air in the soil are gradually sucked out. Thereby consolidating the soil.

Brief Introduction To Vacuum Preloading

Brief Introduction To Vacuum Preloading

Vacuum preloading

The vacuum preloading method is to set up prefabricated vertical drains or sand well in the soft clay, and lay a sand layer on the upper layer, and then cover the film to seal, and pump the gas to make the drainage band and the sand layer in the membrane partially under vacuum, and remove the moisture in the soil. A ground treatment method that pre-consolidates soil to reduce sedimentation in the later stages of the foundation.

The principle of vacuum preloading is 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 performed by vacuuming under a sealing film covering the ground, and a pressure difference is formed inside and outside the film to cause a consolidation pressure of the clay layer. 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.

Mechanism and advantages of vacuum preloading

Mechanism and Advantages of  Vacuum Preloading

vacuum preloading

1. Reinforcement mechanism

 Taking the atmospheric pressure as the preloading load, a layer of permeable sand cushion is laid on the soft soil to be reinforced as a lateral drainage channel, then a vertical drainage channel is set, and a water seepage pipe is buried in the sand cushion layer, and then on it. Cover one layer of geotextile and two to three layers of non-breathable plastic film, seal nearby and block the atmosphere, and then connect the water seepage pipe with the vacuum pump to pump air, so that the water permeability data adheres to a higher vacuum and the negative water in the pore water of the soil The pore water pressure, a soft land reinforcement method that gradually sucks out the pore water and air in the soil to consolidate the soil.

 Before pumping, the film is exposed to atmospheric pressure. The gas in the pores of the soil and the groundwater surface are at atmospheric pressure. After pumping, the gas in the sand cushion in the film is first extracted, and the pressure is gradually lowered. A pressure difference is formed in the film surface, so that the film is closely attached to the sand cushion. This pressure difference is called "vacuum degree". 

The vacuum formed in the sand cushion is gradually transmitted downward along the depth through the vertical drainage body, and the vacuum degree is extended by the vertical drainage channel to the nearby soil, which causes the pore water pressure in the soil to decrease, forming a negative ultra-permanent pore water. pressure. The so-called negative means that the pore water pressure is smaller than the pore water pressure in the original atmospheric condition, and the increment is a negative value. 

Under the premise of no external load, the vacuum preloading reduces the pore water pressure in the longitudinal drainage body, so that it is smaller than the porous pressure in the Tuhuaxia, forming a hydraulic gradient, so that the water in the soil empty space may be from the soil to the soil. The vertical drainage channel seeps and finally is pumped out by the vertical drainage channel to the surface sand cushion to consolidate the soft soil.

 2, The advantages of the vacuum preloading method

 Compared to the preloading method or the forced handling on soft soil improvement, the vacuum preloading has the following significant advantages:

1), Save preload data and reduce the cost of the project.

2), Fast consolidation and short construction period. When the vacuum preloading operation is carried out, the deformation of the clamp will not occur, and as long as the deformation is tightened, the ground will not be unstable. Therefore, it is not necessary to control the loading rate during construction, and the load can be applied at a rapid rate, and the speed of reinforcement is fast and the construction period is short.

 3), construction is simple. Vacuum preloading does not require the use of a lot of construction machinery during loading, and the construction disturbance is small, and it is not easy to cause construction problems. The construction tools and equipment are simple, the construction is convenient, and the work efficiency is high, which is suitable for large-scale ground reinforcement. The effect of reinforcement is also quite good.

 4), reinforcement depth is deep. Soft soils also use well point precipitation to enhance concrete construction. The effect of vacuum preloading on deep consolidation is more significant than the use of well point precipitation to enhance enthalpy.

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Introduction of Drainage Consolidation

Drainage consolidation

Drainage consolidation method, also known as preloading method, includes preloading method, overload preloading method, vacuum preloading method, combined vacuum and stacking method, groundwater level reduction method and electroosmosis method.

Under the preloading force, the pore water in the soft clay soil is discharged, the soil is consolidated, the pore volume in the soil is reduced, and the soil strength is increased, which reduces the settlement after foundation construction and improves the bearing capacity of the foundation.

Ground Improvement Preloading Method

Preloading Method

It is suitable for the treatment of saturated clay foundations such as silt, silty soil and flushing soil. According to the pre-pressing method, it is divided into a preloading method and a vacuum preloading method. Preloading pre-compressed plastic drainage belts or sand well foundation preloading and natural foundation preloading. 

When the thickness of the soft soil layer is less than 4m, the natural foundation preloading method can be used. When the thickness of the soft soil layer exceeds 4m, it should be treated by vertical drainage preloading method such as plastic drainage belt and sand well. For vacuum preloading, a drainage shaft must be placed in the foundation. 

The preloading method is mainly used to solve the problem of settlement and stability of the foundation.

Ground Treatment Method

The foundation treatment method is to carry out the necessary reinforcement or improvement of the foundation according to the requirements of the superstructure on the foundation, improve the bearing capacity of the foundation soil, ensure the stability of the foundation, reduce the settlement or uneven settlement of the superstructure, and eliminate the collapsibility of the collapsible loess and methods to improve resistance to liquefaction.