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 .