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.