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.