Prefabricated vertical drains are commonly used materials in soft soil foundation treatment

Plastic drainage boards are commonly used materials in soft soil foundation treatment. 

In vacuum preloading and load preloading treatment of soft foundations, the vertical drainage body has the dual functions of drainage and vacuum transmission, so the performance and construction of the vertical drainage body Quality will directly affect the reinforcement effect. Plastic drainage board soft foundation has the advantages of good reinforcement effect, fast construction speed and low cost, and is widely used in soft foundation reinforcement projects.

Vacuum consolidation is a method of soft soil foundation treatment

 Vacuum consolidation is a method of soft soil foundation treatment.

The basic principle is to lay a sand cushion layer on the surface of the soft soil foundation that needs to be reinforced, then lay a vertical drainage body such as a plastic drainage board, and then lay an airtight sealing film on the sand cushion layer to isolate it from the atmosphere. The device extracts the air from the soil under the membrane, thereby forming an air pressure difference inside and outside the membrane. This air pressure difference is equivalent to applying a preload load to the foundation, prompting the water in the soil to be discharged, consolidating the foundation, and improving the strength. To achieve the purpose of strengthening the foundation.

Vacuum consolidation usually has the following significant effects:

advantage:

Accelerated consolidation: It can effectively accelerate the consolidation speed of soft soil foundation, allowing the foundation to achieve higher strength and stability in a shorter period of time.

Improve the bearing capacity: It can greatly improve the bearing capacity of the foundation to meet the requirements of subsequent project construction.

Reduce settlement: It can significantly reduce the later settlement of the foundation and ensure the long-term stability of the project.

Wide scope of application: It can be applied to many types of soft soil foundations, such as silt soil, silt, etc.

Vacuum consolidation is mainly suitable for the following ranges:

Silt, silty soil and other weak foundations: Such soft soil layers are widely found in coastal areas, around lakes and other places.

Large-area site reinforcement: such as large-area soft soil foundation treatment in port terminal yards, airport runways, etc.

Projects with higher requirements for deformation control: because it can effectively reduce settlement and uneven settlement.

Foundation treatment in the early stage of the filling project: Reinforce the soft foundation before carrying out the filling construction.

Foundations with higher groundwater levels: can effectively reduce groundwater levels and improve foundation conditions.

Ground improvement vacuum consolidation method

 The vacuum consolidation method is to lay a sand cushion on the ground within the soft clay foundation that needs to be reinforced, and then set up a plastic drainage board. The horizontal water filter pipe and vacuum equipment are connected through the plastic drainage board, and an airtight sealing film is covered on it to make the soft soil Isolated from the atmosphere, use a vacuum device to evacuate the air inside the membrane, thus creating an air pressure difference inside and outside the membrane. This part of the air pressure difference becomes a load acting on the foundation. The foundation solidifies as the isotropic stress increases. Knot.

Vacuum consolidation technology is suitable for the reinforcement of soft soil foundations. There are widespread soft clay layers deposited in marine, lake and river phases. This kind of soil is characterized by high water content, high compressibility and low strength. , poor water permeability. Weak clay foundations can undergo considerable deformation or differential deformation under building loads. For weak clay foundations, especially when large-area treatment is required, such as building docks, airports, etc. on soft clay foundations, vacuum preloading method and vacuum stacking combined preloading method are one of the more effective methods for treating such weak clay foundations.

THE VERTICAL WICK DRAIN PRINCIPLE AND ITS APPLICATIONS

THE VERTICAL WICK DRAIN PRINCIPLE AND ITS APPLICATIONS

When construction work such as road and airfield embankments, bridge approached, dykes, land reclamation or buildings on soft compressible soils, significant settlements may occur due to on solidation of these soils under the superimposed loads. To avoid serious and potentially expensive problems due to such settlements, it is desirable to cause this consolidation to occur at the outset of project, and in the shortest possible time – during the construction period.

Consolidation of compressible soils involves removal of pore water from the soil. This is traditionally done by applying a surcharge of pre-load to the construction area to “squeeze” the water out. Unfortunately, compressible soils are also often low-permeability soils (peats, silts, clays), and as such the water is not easily squeezed out.To facilitate the de-watering process, it is necessary to install vertical drains into the soil, to provide a conduit for the water flow. 

Traditionally, these drains took the form of sand columns – holes drilled into the low-permeability soil and filled with higher permeability sand. But these were relatively expensive, and inconvenient to place at close spacing. PVD's are a very economical replacement for sand drains. They are relatively inexpensive, provide higher conductivity (up to 30 times more effective than a 300 mm diameter sand drain) and can easy be installed at close spacing, thus shortening the path of the pore water in the impermeable soil and expediting the consolidation process.

Principles of soft foundation treatment for water dike project

The soft foundation of the water dike project means that the geology of the project construction site is made of soft soil. Since the soft soil has the characteristics of large water content and large gap, low bearing capacity, poor water permeability and easy to be disturbed, it requires high technology in soft soil improvement and pvd construction. Guarantee to ensure that the stability of the soft soil foundation meets the needs of the dike project.

 If improperly handled, the shear stress of the soft soil foundation exceeds its own shear strength, which will cause the foundation to lose stability and cause the sliding damage of the embankment.

1 Principles of soft foundation treatment for water dike project

1.1 Control the ground settlement index;

Avoiding problems with settlement differences and cause problems in the water dike project, it is necessary to control the settlement index of the soft foundation. Generally speaking, the settlement period of the foundation after completion is required to be above 15 years, and the remaining settlement of the foundation after the completion of the general section project should be less than 30 cm, the foundation settlement of the bridge and culvert and its adjacent road sections should be low to 10 cm after completion, and the transition section of the bridge head foundation and the general foundation should exceed 50 meters.

1.2 Strictly control PVD construction time;

Make ensure the integrity of the soft foundation settlement control of the water dike project, to reach prescribed standards, it is need to strictly control the construction time of pvd construction and wick drains installation. 

Generally, the construction of the entire water conservancy project needs to be completed within at least 1-2 years, and the roadbed construction needs to be done within one year.

1.3 To be done in bridge foundation treatment and the soft soil improvement;

Bridgehead safety is an important guarantee for the safety of the entire dike project soft soil improvement. Therefore, it is necessary to do a good job in the foundation treatment of the bridgehead. 

Specifically, the following two aspects can be used: 1st,  the length of the bridgehead needs to be processed is about 5-7 times of the fill height, and the treatment should start from the center line of the abutment; 2nd,  the foundation of the abutment slope protection needs to be properly treated, and the bridge head small structure The length of the transition section should be around 10 meters.

1.4 Use appropriate packing.

Soft Soil and Soft Based Ground Requires Ground Improvement Before Construction

Soft Soil & Soft Based Ground Requires Ground Improvement Before Construction

ANY development on soft soil requires ground improvement work prior to construction. Otherwise, the structure, such as roads and railways, would fail due to unstable soil foundation. If you have ever driven on roads riddled with potholes, chances are it might be due to poor soil foundation. Or when a house has cracks, it means the structure is unstable, probably due to the poor soil foundation of the house. Poor or soft soil condition will cause a movement in soil foundation, which will lead to the movement of the structure or anything on the soil.

Therefore, ground improvement work is crucial to ensure the soil foundation is solid.

There are many types of ground improvement work that can be carried out to ensure the soil foundation resists load from structure or infrastructure. The cheapest type is excavation and replacement. But nowdays the pvd（prefabricated vertical drains）and vacuum preloading method is more popular in the practice when in ground engineering. This requires the excavation and removal of soft soil and replaced with the good backfill materials.

The limitation of this method is the depth of excavation is only up to 3m due to the safety and stability of the cut soil.The other low-cost method is surcharge load.

Surcharge load is used to expedite the consolidation of the soft soil where the backfill soil is placed on top of soft soil for a certain period of time. The consolidation is the process to drain out water from the soft soil. The backfill represents the load of the structure to be constructed, so that when the structural load is put on the soil foundation later on, the movement of the soil foundation is minimised.

If the depth of the soft soil is deeper, prefabricated vertical drain (PVD) is preferred. PVD is installed in a soft soil area to expedite the vacuum consolidation process and it is combined with surcharge to effectively consolidate the soft soil. If the depth of soft soil is critical and the structural load is large, the pile embankment is the best option, but it is costly.

This is the most effective method for deep soft-soil areas. The current technology and research proposed the use of lightweight material as a ground improvement for soft soil. For example, the soft soil is removed at shallow depth and replaced with expanded poly-styrene foam or polyurethane foam to reduce the load imposed on the underlying soil. Apart from the cost, implementation of the ground improvement takes into consideration the time allocation and safety of the surroundings.

What is Soil Improvement and Its Importance in Dredging Industry

WHAT IS SOIL IMPROVEMENT?

In the dredging industry soil improvement is typically implemented to:

- prevent excessive settlement of reclaimed land when it is being used for construction purposes (roads, airports, bridge and other foundations);

- enhance the soil stiffness and density to prevent liquefaction

which can lead to damage to structures in earthquakesensitive regions;

- improve the shear strength of the soil to prevent slip failure

and increase the bearing capacity of the soil;

- immobilise or stabilise contaminants in dredged soil in order to mitigate and preferably eliminate environmental impacts.

Soil improvement techniques vary depending on the characteristics of the soils and subsoils. Some techniques are applied to consolidate existing loose subsoils and some are specifically for compaction of newly reclaimed soil.

WHY IS SOIL IMPROVEMENT IMPORTANT?

Typically, potential reclamation sites are shallow coastal zones or marshy lowlands. The soil in these areas often consists of thick layers of soft clay or silts. Reclamation work increases the load on these soft layers, causing widespread settling.

Waiting for the natural settlement of this land is time consuming and thus costly and may put a project in jeopardy if it cannot be accomplished in a designated period of time.

To accelerate the settlement process, dredging contractors have developed specialized solutions to consolidate these soft soils. In addition, newly reclaimed soil is often in a loose state and may not meet the demands of the planned infrastructure

and therefore needs to be improved.

WHAT IS THE DIFFERENCE BETWEEN CONSOLIDATION AND COMPACTION?

A distinction is made between consolidation techniques for cohesive soils (clay, silt) and compaction techniques used to solidify granular soils. The consolidation phase has become an essential part of soil improvement and several techniques have been developed to stabilise new ground. 

Amongst these are preloading or surcharging the area that is subject to consolidation with a temporary load of sand, using Prefabricated Vertical Drains (PVDs), sand drains and a system known variously as BeauDrain, Intensive Forced Compaction (IFCO) or Press-To-Drain (PTD) as well as vacuum consolidation. These are all methods in which soil improvement takes place over a longer period of time.

Compaction is done by methods such as smooth rollers, High Energy Impact Compaction (HEIC), or Dynamic Compaction (DC) – also known as heavy tamping – or Rapid Impact Compactors (RIC) and vibroflotation.

What are vertical or wick drains, and vacuum consolidation

WHAT ARE VERTICAL OR WICK DRAINS?

Soil stabilization using Prefabricated Vertical Drains (PVDs) or wick drains are applied in areas with loose, compressible and water-saturated soils such as clay and silty clay. These soils are characterized by a very weak soil skeleton and a large pore space, usually filled with water (pore water). 

When a load such as a road embankment, a hydraulic fill or a dike, is placed on soft compressible soils, significant settlements may occur. These settlements can create serious problems. Any increase in load can also result in an increase of pore water pressure. In soils with low permeability, this water dissipates very slowly, gradually flowing from the stressed zone. Increased pore pressure may also cause soil instability and

slip plane failures may result. 

A vertical drainage system – drains are generally placed in a square or triangular pattern, spaced at about 1 to 3 meters – allows for faster removal of excess pore water, thus decreasing the risk of slip plane failure. The consolidation of soft cohesive soils using vertical drains can reduce settlement time from years to months ensuring that bearing capacity is adequate and construction can commence rapidly.

WHAT IS VACUUM CONSOLIDATION?

Vacuum consolidation is a process whereby vacuum pressure is applied to an area where PVDs have already been installed. The purpose is to potentially increase the drains’ effectiveness. 

Generally, this technique requires the application of a surcharge loading to squeeze water out of the soft clay soils. Such loading must be equal to or in excess of the service loading to which the developed land will be subjected. In vacuum consolidation, the vacuum pressure applied contributes to the surcharge loading, and therefore actual surcharge heights are reduced. 

An additional important advantage of the vacuum is the isotropic nature of the vacuum pressure and the correlated improvement of the stability under Pre-loading, reducing considerably the risk of slope failure resulting from the surcharge.

Note:this articles are excerpted from the iadc dredging, facts about soil improvement, and here we thank the authors give us the knowledge about dredging and soil improvement.

Cement mixing pile and soft foundation treatment

Cement mixing pile is an effective form of soft foundation treatment，just like vacuum consolidation and vibration. 

It is a kind of main agent that uses cement as a curing agent. The cement is sprayed into the soil by a mixing pile machine and fully stirred to make a series of physical and chemical reactions between cement and soil. To harden the soft soil and increase the strength of the foundation. Cement mixing piles are divided into single-axis, double-shaft and three-axis mixing piles according to the main construction practices. 

In the 1970s, the use of cement mixing piles to reinforce soft soil foundations has been around for more than 40 years. It uses a special deep mixer to force the soft soil and cement (curing agent) to stir, and uses a series of physical and chemical reactions between cement and soft soil to consolidate the soil to form a whole body, water. Stability and strength of cement soil piles. This process is mainly used for the treatment for soft soil foundations enhancement.

For more method and technologies about soft soil improvement, please take a tour at our official website at www.sunzo99.com .

pvd installation vedios

Introduction To Non-cushion Sand Layer Vacuum Preloading Method

The method relates to a large area soft ground treatment method, in particular to a vacuum preloading method without a drainage sand layer. 

Non-cushion sand layer it includes the following main steps: 

1) laying drainage plates on the treatment site according to the plum pile type; 

2) arranging the horizontal drainage system to replace the medium coarse sand cushion used in the previous vacuum preloading construction, two kinds of laying can be used One of the ways; 

3) Connect the main pipe of the horizontal drainage system to the vacuuming device, and then vacuum the construction after filming. 

The vacuum preloading method for the non-drained sand cushion of the invention is suitable for the reinforcement of large-area soft soil foundation, especially for the dredger fill and the newly deposited ultra-soft soil, and the utility model saves a large investment of laying the sand cushion layer. The vacuum transmission effect is good, and the horizontal drainage system of the wire spring tube + tube plate connector + drainage plate can realize large-scale finished product and normalized construction, and the laying time is short, and can be recycled and reused. The construction period is short and the unique advantage of economic saving.

The main purpose of the dredging projects

The main purpose of the dredging project is to excavate the harbor basin, the inbound waterway, etc., to fill the land to build the dock, the port area and the port industrial zone, the coastal city land and recreational land and land reclamation, the shore beach maintenance, the water conservancy and flood control and the reservoir area. Improvement of water environment such as dredging, rivers, lakes and seas, ecological restoration, construction and landfill of various underwater pipelines. The role of dredging projects in human social progress, environmental improvement and economic development is very important.

Use dredging methods to dig shallow sections of deep rivers or bays to improve navigation or flood discharge capacity; an economically viable primary method of landfilling dredged soils in excavation channels or harbor basins into nearby low-lying areas for land reclamation . 

Dredging has traditionally been treated in a manner that is abandoned or dumped in the waters adjacent to the project. The dredging project has created a new way of turning waste into treasure and comprehensively processing it, and then developed it into the use of dredged soil as building materials and materials for building buildings. The dredging project is also extended to excavate the bottom of the river or the bottom of the sea to bury the river or cross-sea pipeline (water pipes, oil pipes, transmission cables, communication cables, etc.); remove the sand and stone for hydraulic engineering with strong underwater displacement capacity. Material foundation and soft ground improvement; blowing sand to protect the beach and so on.

In the past few decades, human beings have increasingly demanded environmental standards. Preventing and reducing the pollution of waters and land by dredging activities has become an important issue that must be considered in dredging projects. Dredging methods are used to excavate underwater contaminated soils and carry out engineering. Processing has also become an important part of dredging projects.

What is Dredging and Its Application

Dredging is to dredge, widen or dig deep waters such as deep rivers and lakes, and use manmade power or machinery to carry out underwater earthwork or excavation works.

The generalized dredging includes the reef and the fried beach, which are carried out by the blasting method under water. A small river that is constructed by artificially breaking the flow. Various dredgers are widely used in mechanical construction, and onshore construction machinery such as rope shovel is sometimes used.

About the dreging history, the mechanical dredging began in 1600, and the prototype of the chain bucket dredger appeared in the construction of the port of Rotterdam, the Netherlands. China began construction of dredgers in the Huangpu River in 1889. In 1929, the Jiangnan Shipyard caused a chain bucket dredger. In the early 1980s, China had an annual capacity of about 300 million cubic meters of mechanical dredging.

Dredging works are widely used: 1, to excavate new waterways, ports and canals. 2, Deep, widen and clean up existing waterways and ports. 3, dredge the rivers, channels, and dredging of the reservoir. 4, Excavation of foundation pits for hydraulic structures such as docks, docks and ship locks. 5, Combine dredging to carry out landfilling, reclamation and other projects. 6, Clear underwater obstacles.

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.

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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.