Technical Points for Vacuum Preloading in Large Land Reclamation Projects

The characteristics of vacuum preloading make it very suitable for large-scale dredging and dredging soil-building projects. To make full play of its advantages, it is necessary to closely combine its technical key points with the landfilling and dredging land reclamation project.

1.1 Vacuum preloading block

For large-area vacuum preloading, first of all, scientific and reasonable plane segmentation should be carried out. Whether the block is reasonable or not directly affects the construction period and cost, the larger the single block processing area, the shorter the construction period and the lower the cost, but it is subject to the current vacuum preloading. 

With the limitations of sealing technology, it is impossible to limit the processing area of ​​a single block. At present, the area of ​​a single block can reach 5*10000 square meters, but according to past experience, it is generally suitable to divide the block by 3*100000 square meters per block. A plurality of single blocks can be combined into one zone, and each can be constructed synchronously. Flowing operations are performed between the zones to reuse the vacuuming equipment and reduce the engineering input. The division of the blocks should be combined with the design load and reinforcement requirements of the site.

1.2 sand cushion

Sand pad material selection, coarse sand, mud content should be less than 5%, the thickness of the sand cushion layer depends on the construction process, if the backfilling method by artificial road, the thickness should not be less than 50cm, if it is filling, sand cushion Should be thicker.

The physical and mechanical properties of the flushed silt produced by landfilling and landing are extremely poor, and the surface of the dredged silt must be treated to ensure the smooth follow-up construction. Before the construction of the sand cushion layer, a layer of geogrid and geotextile can be laid in the area with large sludge content. If the treatment of the geotextile fabric is still carried out, the mulled mud phenomenon may occur, and the 801cm-1m thick medium may be laid first. Fine sand cushion, re-laying, coarse sand cushion, which can guarantee the quality of medium and coarse sand cushion, so that it has good horizontal drainage effect.

For the dredging process, in order to ensure the flatness of the sand cushion layer, an energy-eliminating head should be provided at the outlet of the blow-fill pipeline to reduce the flushing of the pipe head to avoid a large punching hole at the position of the pipe head.

1.3 Insert Plastic Drainage Vertical Drains（PVD Wick Drains）

As a vertical drainage channel for vacuum preloading, the plastic drainage board not only plays the role of vertical drainage, but also accelerates the consolidation of the soil, and plays a role in transmitting the vacuum. It must be fully functional, except that the material itself needs to have a good In addition to the water permeability, the spacing and depth of the installation are also very important. The drainage plate spacing is determined by the Taishaji and Balun consolidation theory according to the consolidation index of the foundation and the treatment period. The depth of the setting is determined according to the depth of the soft soil. In principle, it is required to penetrate the soft soil layer.

If the drainage board needs to penetrate the sandy soil that communicates with the outside world, the clay and the content of the clay should be analyzed. If most of the soil samples contain more than 15% of the powder and clay content, and there is cementation, it will not affect the vacuum preloading. Otherwise, the drainage board should be prevented from penetrating the soil layer or other special areas. Reinforcement plan.

1.4 Setting Sealing Wall

To ensure the reinforcement effect of vacuum preloading, effective sealing of the reinforced soil is a key part. The sealing form around the reinforcement zone is sealed groove and sealing wall. Due to the excessive water content of the surface dredger fill or the mud, or the thickness of the sand layer is large, it is not suitable to adopt the form of the seal groove. Therefore, the seal wall is generally used to seal the periphery of the reinforcement zone, due to the mud resources of the reclamation land. It is very rich, so it can be used as a sealing wall with mud mixing piles.

Since the construction of each block is carried out, a common sealing wall will appear between the adjacent two reinforcing blocks. During the vacuuming process, the mud particles in the wall will be affected by the soil shrinkage in the reinforcement area. The vacuum pressure is taken away, and the long time will cause the mud mixing pile to lose the sealing effect. For this reason, for the adjacent area where the vacuuming time interval is more than 15d, the common sealing wall should adopt a double-row mud mixing pile with a combination of length and shortness. This problem is solved by increasing the upper width of the wall. The length of the short pile is generally 3-4 meters, and the length of the long pile is determined according to the geological conditions.

1.5 Vacuum Preloading Soft Soil Processing

The vacuum system includes a filter tube, a filter tube outlet device, and a vacuum pump.
The filter tube is made of pvc plastic tube, the spacing of the filter tube should not be more than 6.5 meters, the depth of the filter tube is 0.3cm, and the vacuum gauge under the membrane is arranged between the two filter tubes.

The sealing film is made of 2 layers of polyethylene film, the size of which is predetermined according to the plane size of the reinforcement zone. For the adjacent two reinforcement zones, a narrow strip of vacuum film can be pre-buried in the common sealing wall in the later construction area. The remaining membrane joints are then bonded during subsequent construction. The second sealing membrane of the former zone is covered with the upper part of the common sealing wall and bonded to the sealing film of the latter zone to ensure that the sealing wall is sealed during vacuuming. There will be no air leaks.

Vacuum pump-7.5kw is arranged in a set of about 800 square meters, according to the order of motor, water pump, water tank, gate valve, check valve, and film outlet. The joints are sealed with rubber mat, glass glue, etc. The mouth and tank inlets are kept at the same level.

1.6 Water Cover on The Sealing Membrane

After the vacuum is started and the vacuum under the film reaches 60 kPa, the film can be covered with water. The effect of water covering is: a, to avoid the aging of the sealing film under the long-term exposure of sunlight, so that cracking is easy, b, the tiny pores in the reinforcing zone are filled by the infiltration of water, and the sealing property of the reinforcing zone can be further improved. Strengthen, c, the weight of the water increases the preloading load, which can speed up the construction progress.

Under normal circumstances, the depth of overburden does not exceed 0.5m. It is pointed out in particular that seawater is very cheap and convenient for reclamation land. If the depth of overburden is properly deepened, the construction period can be exchanged at a very low cost. Overburden preloading is gradually being promoted.

Vacuum Preloading Soft Soil Improvement Technique Application Prospects In Land Reclamation

With the rapid development of our social economy, the land resources available for economic construction in the coastal areas are becoming more and more tense. Therefore, the pace of reclamation and land reclamation is accelerating with the needs of economic development, and its engineering technology is also constantly developing and maturing.
In recent years, there are mainly two ways to reclaim land for land reclamation. One way is to open a mountain for reclamation, and the other is to make land by dredging the dredged soil. The former method is more and more damaging to the environment due to high cost, and its use is increasingly restricted by government departments. The latter method is not only low in cost, relatively environmentally friendly, but also has a short construction period. More importantly, with the increase of port construction, the landfilling of dredged soil can minimize the external throwing of dredged soil in port engineering. While greatly reducing the cost of dredging projects, the use of dredged soil as a reclamation resource, so that the dredged soil that should be discarded has a value, can be described as a multi-pronged.

Because landfilling by dredging and filling land has great advantages in both technical and economic benefits, this land reclamation method is more and more accepted by people and has been widely used in reclamation projects in coastal areas. . This land reclamation method also brings greater application prospects to vacuum pre-loading foundation treatment technology.

The application prospect of vacuum pre-loading in land reclamation

The purpose of reclamation is to provide land areas that can be used for economic construction. If the site is to meet the requirements of use, ground treatment must be carried out. Vacuum preloading is widely used in large reclamation projects because the technology has the following characteristics:

1) Save pre-compressed materials and reduce project cost. Vacuum preloading uses atmospheric pressure as the motive force for preloading. On the one hand, it does not need to use a large amount of piled materials, such as sand and stone, as preloading load, which solves the trouble of supplying preloaded materials, and on the other hand, the atmosphere Pressure is a free resource, which reduces engineering costs, especially for large-area soft-base treatments, which can greatly save engineering investment.

2) Fast consolidation and short construction period. Since the foundation does not undergo shear deformation under the action of vacuum pressure, no matter the degree of vacuum during the loading process, the foundation will not be unstable when applied once. With the maturity of vacuum preloading reinforcement technology in China, the vacuum degree under the film can reach about 85 kPa. Although the speed of consolidation does not depend on the magnitude of the preloading load, the overload is greater when the load is equal. The speed of residual settling must be faster. Since the load of the vacuum preload is large and the loading process is fast, the reinforcement time is short.

3), The construction is simple, the vacuum preloading is achieved by pumping air, then the unloading can be stopped as long as the pumping is stopped, that is, there is no problem that a large amount of unloading sand needs to be treated, which is in areas with poor external traffic conditions, It looks very good. In addition, vacuum preloading does not require the use of a large number of construction machines during loading, construction disturbance is small, construction accidents are not easy to occur, and after the required vacuum is stably maintained, the remaining work is only daily inspection and maintenance.

4), The reinforcement effect is good, the degree of consolidation of the vacuum preload can reach more than 90%, which can eliminate most of the settlement of the foundation. Moreover, since the atmospheric pressure is evenly distributed throughout the reinforced zone, the vertical deformation of the soil is also uniform throughout the reinforced zone (constrained by the surrounding soil, and the settlement at the boundary of the reinforced zone is slightly smaller), so The vacuum preloading strengthens the soft foundation, and the uneven settlement of the site will be smaller.

For sites with large ground design loads, such as container yards, if the preloading scheme is used, when the treatment area is large, the amount of earthwork used for the loading will be very amazing. Due to the impact of the stacking process, the construction period will be It has become very amazing. Due to the impact of the stacking process, the construction period will become very long. For example, in a project in Nansha, Guangzhou, the disposable treatment area is 1.50*10~5 square meters. If the preloading scheme is used, the purchased heap is required. Load sand 4*40~5 cubic meters, then divide the treatment area into three large blocks and use the piled sand in stages. The required construction period is not less than 2 years, and the construction cost reaches 400 million yuan. In the case of increasingly tight supply of sand and gravel resources, It is very difficult to obtain such a large amount of sand in a short period of time. The handling of a large amount of unloading sand is also extremely difficult. What is more important is that even if the engineering cost and construction difficulty are aside, such a long period of time cannot be used for investors or Accepted by the owner.

Because the vacuum preloading soft soil improvement has the advantages of low cost, short construction period, saving earthwork and ensuring the reinforcing effect, the treatment of large-area deep soft soil, especially the port engineering with relatively high ground design load, has the advantage that the preloading can not be compared. As mentioned above, the Guangzhou Nansha foundation project uses the vacuum preloading scheme to not only greatly reduce the project cost, but also the total processing time is only one year. Obviously, the application prospect of vacuum preloading in the landfilling project of dredging and filling soil is Very bright.

Latest Ground Improvement Techniques and Vacuum Consolidation

Latest Ground Improvement Techniques & Vacuum Consolidation of Soft Soil:

1. Vibro Compaction;
2. Vacuum Consolidation;
3. Preloading of soil;
4. Mechanically stabilized earth structures;
5. Soil nailing;
6. Micro-piles;
7. Grouting;
8. Soil stabilization by heating or vitrification;
9. Ground freezing;
10. Vibro-replacement stone columns.

Following are the recent methods of ground improvement Techniques used for stabilization of the soft soil improvement:

Applications of Vacuum Consolidation of Soft Soil:

• Replace standard preloading techniques eliminating the risk of failure.

• Combine with a water preloading in scare fill area. The method is used to build large developments on thick compressible soil.

• Combine with embankment pre-load using the increased stability

Preloading or Pre-Compression of Soil for Ground Improvement:
Applications of Preloading of Soil

• Reduce post-construction

• Settlement

• Reduce secondary compression.

• Densification

• Improve bearing capacity

Vaccum reloading has been used for many years without change in the method or application to improve soft soil properties. Preloading or pre-compression is the process of placing additional vertical stress on a compressible soil to remove pore water over time. 

The pore water dissipation reduces the total volume causing settlement. Surcharging preloading is an economical method for ground improvement. However, the consolidation of the soils is time dependent, delaying construction projects making it a non-feasible alternative.

The soft soils treated are Organic silt, Varved silts and clays, soft clay, Dredged material The design considerations which should be made are bearing capacity, Slope stability, Degree of consolidation.

Soft Base Processing Specification

Soft Base Processing Specification

Layout the base:

1 Try to use the way of cross-laying, so that the width of each compartment corresponds to the width of the embankment as much as possible, because the two-way strength of the compartment is consistent of the soft base improvement;
The 2 cells are pulled vertically in the direction of the bank axis, and no slack is allowed. It should be manually tightened. The geocell can be sewn as needed;
3 by weighting (referring to the filling in the grid of the marginal compartment) or inserting the nail to position the compartment on processing of the soft ground, without moving due to filling;
4 Check the cell for damage before paving the soft soil. If found, it should be disposed of immediately;
5 After paving the high-strength geocell, it should be filled on the soft base within 48 hours. 

Filling the soil:

A. There should be no slack in the laying of high-strength geotechnical cells, and try to tighten them manually before filling.
B. Fill the soil symmetrically from the center to the sides, and it is inverted U-shaped. The filling is used to keep the high-strength geocells under tension. C. The height of the earth unloading shall not exceed 1m to prevent local subsidence.
D. Pay attention to the size and weight of the PVD construction machinery to avoid the occurrence of ruts larger than 7~8cm. 
E. The filled soil is 50cm after being crushed.

More tips or information about vacuum preloading soft soil improvement, please contact our sales engineer or visit our official webiste at http://www.sunzo99.com.

Foundation Treatment Method Selection Principle

Foundation Treatment Method Selection Principle

 The soft foundation treatment project must ensure the quality of the project, economic rationality and advanced technology. China has a vast territory, and the engineering geological conditions are ever-changing. The construction machinery conditions, technical level, experience accumulation, and the variety and price of construction materials vary greatly. When selecting the ground treatment method, it must be adapted to local conditions. Specific project analysis should give full play to local advantages. Use local resources. There are many ground treatment methods, each of which has a certain scope of application, limitations, advantages and disadvantages. No ground treatment method is omnipotent. According to the specific project conditions, the appropriate ground treatment method should be determined according to local conditions. Blindness should be overcome when referring to a method of a foreign or external unit, paying attention to regional characteristics. Adapting to local conditions is an important selection principle for the selection of ground treatment methods such as pvd wick drains.

 First, it is determined whether the soft foundation improvement needs to be treated according to the various requirements of the foundation (structure) and the natural foundation conditions. If the natural foundation can meet the requirements of the foundation of the building (structure), natural foundation should be used as much as possible. If the natural foundation does not meet the requirements of the building (structure) to the soft clay foundation, then the scope of the natural formation to be treated and the requirements for foundation treatment need to be determined by vacuum preloading.

       When the natural foundation cannot meet the requirements of the foundation of the building (structure), the superstructure, soft foundation should be considered together. When considering a ground treatment plan, attention should be paid to the interaction of the superstructure, foundation and foundation. It is not only possible to consider the reinforcement of the soft foundation, but also consider whether the upper structure is reasonable and whether the overall rigidity is sufficient. When determining the ground treatment plan, the scheme of treating only the foundation should be considered at the same time, or the combination of strengthening the rigidity of the superstructure and the treatment of the foundation should be selected, otherwise it will not only cause unnecessary waste but may bring about adverse consequences.

       Before the specific soft ground treatment plan is determined, the feasibility study of the ground treatment plan should be carried out according to the conditions of the natural formation, the principle of the foundation treatment method, the experience of the past application, the equipment and the material conditions, and various technically feasible solutions are proposed; Then, the technical, economic, and progress aspects of the proposed multiple schemes are compared and analyzed, and the environmental protection requirements are considered. One or several ground treatment methods are determined. This is also the optimization process of the ground-based treatment plan. Finally, based on the pre-determined ground treatment plan, decide whether to conduct a small field test or conduct a supplementary survey as needed. Then carry out the construction design and then carry out the foundation treatment construction. During the construction process of soft ground improvement, monitoring and testing should be carried out. If necessary, reverse analysis should be carried out. The design can be modified and supplemented according to the situation. The detailed understanding of the natural geological conditions of natural foundations in actual projects is often neglected. Many engineering accidents caused by foundation problems or soft ground treatments fail to achieve the intended purpose, often due to insufficient understanding of the engineering geological conditions. Detailed engineering geological survey is one of the important basis for judging whether the natural foundation can meet the requirements of the foundation of the building (structure). Detailed engineering geological survey data is one of the main basic data for determining a reasonable foundation treatment method if ground treatment is required.

       Through engineering geological survey, investigate the topography and geomorphology of the building site, and identify the geological conditions, including the nature of the rock and soil, the genetic type, geological generation, thickness and distribution range. It is necessary to understand whether there are alums, dark scorpions, ancient rivers, ancient wells and ancient tombs in the foundation. 

For the layer, the degree of weathering and the contact relationship of the stratum should also be ascertained, the geological structure of the natural stratum should be investigated, and the hydrological and engineering geological conditions should be ascertained to determine whether there are any unfavorable geological phenomena: such as landslides, collapses, karst, soil caves, gullies, Debris flow, shore scouring and earthquakes. The physical and mechanical properties of the foundation soil are determined, including: natural gravity, relative density, particle analysis, plasticity index, permeability coefficient, compression coefficient, compression modulus, shear strength, and the like. 

Finally, according to the requirements, the stability and suitability of the site, the uniformity of the foundation, the bearing capacity and the deformation characteristics are evaluated. In addition, it is necessary to emphasize the comparison of multiple schemes for ground treatment. For a specific ground improvement project, there are often several technically feasible ground treatment schemes. The comprehensive analysis of the technology, economy, and schedule, as well as the impact on the environment, should be carried out to optimize the ground treatment plan to obtain a better ground treatment plan.

vacuum preloading

vacuum preloading soft soil treatment work site

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.

The basic introduction to surcharge preloading techiniques in China

The basic introduction to surcharge preloading techniques in China

The foundation is drained and consolidated under the preloading load, the foundation is deformed, the strength of the foundation is improved, and the prefabricated load is removed, and then the building (structure) is constructed, which not only improves the bearing capacity of the foundation, but also has small settlement after construction.
China's soft soil areas are widely distributed. Its characteristics are: large water content, low strength and high compressibility, which is difficult to meet the needs of engineering construction. However, with the development of the national economy, many engineering projects have to be constructed on soft soil foundations. Therefore, it must be reinforced, otherwise it will cause large settlement or even damage.

In engineering practice, the traditional and effective soft soil foundation reinforcement method is the preloading method. However, due to the insufficient source of materials, transportation shortage, and difficulty in turnover, the construction materials have a direct impact on the construction schedule, resulting in long construction period and high project cost. To this end, the vacuum preloading method at home and abroad solves the problem of insufficient material for stacking and long construction period. However, with the improvement of reinforcement requirements, the vacuum preloading method cannot meet the requirements of engineering practice. Therefore, a new method of vacuum combined with preloading preloading soft soil foundation has been created.

The surcharge preloading method has been widely promoted and used in domestic port engineering, industrial and civil construction, airport runway construction and other foundation treatment projects. It has achieved good reinforcement effect and objective economic benefits, and accumulated a large amount of practical experience. At present, the method has matured after more than ten years of engineering application.