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Vibro Stone Column or Vibro compaction method is a developed method of vibro-flotation. This method consists of three stages, including drilling, compaction and reinforcement and stone pillow execution. In this method, loose grain soils, such as clean stone and sandy soils, lose their previous arrangement due to vibrations and are in the most compaction state. In Vibro Stone Column or Vibro compaction method, the vibrator rod with a jet of pressurized water penetrates into the soil. The soil particles separate in the vicinity of the vibrator rod and the effective stress between them reaches zero and they float in the water. These particles are in the most compaction state due to the movements of the vibrator rod and their weight. Since the grains are placed next to each other without applying special stress, the obtained compaction will be stable.

Vibro Stone Column - Istasazeh Engineering Construction Co
Vibro Stone Column – Istasazeh Engineering Construction Co

In this method of ground improvement, columns with stone and sandy materials with appropriate granulation are executed in a regular mesh. During the execution of this method, loose and problematic soil is replaced with suitable soil and with partial and relative density of soil mass, the ground improvement is completed.

Vibro Stone Column or Vibro compaction method - Istasazeh Co
Vibro Stone Column or Vibro compaction method – Istasazeh Co

Vibrator rod – Istasazeh Co
Vibrator rod – Istasazeh Co

History of Vibro Stone Column or Vibro compaction method

The history of the vibro stone column method Vibro Stone Column or Vibro compaction method dates back to France in the 1830s. This method has been popular since 1930 in Europe (albeit in the form of floating vibration) and in 1970 in the United States and other parts of the world. This method has a physical-mechanical nature and is used for deep modification of granular, sticky and mixed soils. Vibro stone column method is one of the most efficient methods of soil improvement. The American Federal Highway Administration (FWHA) also published a regulation entitled design and construction of stone columns on the theory, design methods and execution of this method in 1983.

Design basics of Vibro Stone Column or Vibro compaction method
Design basics of Vibro Stone Column or Vibro compaction method

Depending on the problem of loose soil, the design of the stone column will be different:

  • If the goal is to increase the bearing capacity, the compressive strength of the stone column is considered as a pile in the calculations.
  • If the goal is to reduce subsidence, calculations should be performed by calculating the modulus of elasticity of the soil-stone column system.
  • If the goal is to solve the liquefaction problem, the reduction of the pore water pressure created by the stone columns should be considered in the calculations.

Vibro stone column execution method (vibro-compaction)

This method consists of three stages, including drilling, compaction and reinforcement and stone pillow execution.

1) Drilling: The vibrator rod hangs from the crane and penetrates into the soil under the influence of its weight with the help of water jet or air jet. In the vicinity of the vibrator rod, the granular soil becomes fluid.

2) Compaction and reinforcement: With the penetration of the vibrator to the depth of improvement and as it rises step by step, the real compaction of the soil begins. In this way, the effect of compaction on the ground is plotted and a large conical cavity is formed around the point of penetration of the vibrator. After about 2 minutes of placing the vibrator at the lowest point, the vibrator rises 30 to 50 cm and then the compaction operation is repeated, after which the appropriate granular material is poured from the ground into the compaction hole. Thus, with the slow rise of the vibrator rod, a uniform and continuous compaction is created in the soil mass and the decrease in soil volume is compensated with suitable granular materials. The addition of granular materials increases the specific dry weight of the soil by approximately 10 to 15%. At the ground level, a subsidence will be created equal to 5-15% of the compaction depth. The amount of subsidence depends on the soil compaction before and after improvement. The maximum operating depth for vibro-compaction is about 30 m and it is not more economical.

3) Stone pillow: At the end of the ground improvement operation, to smooth the final surface, a stone pillow is applied on the stone columns.

The main equipments for the execution of stone columns are:

  • A vibrator rod with a diameter of 30 to 40 cm
  • Length 2 to 4 m
  • Weight 2 to 3 tons

The rod (which has several names: Probe, Vibrator, Vibroflot, Pocker, etc.) sinks into the ground. The frequency of horizontal vibration and in some cases the vertical vibration frequency of this vibrator rod is about 30 to 50 Hz and its free vibration amplitude is between 7 to 25 mm. In this case, the resulting horizontal centrifugal force will be about 10 to 20 tons.

Execution stages of stone column (vibro compaction)
Execution stages of stone column (vibro compaction)

Formation of a conical cavity around the vibrator during vibro stone column operation
Formation of a conical cavity around the vibrator during vibro stone column operation

Execution operation of stone column method (vibro compaction method)
Execution operation of stone column method (vibro compaction method)

Different methods of vibro stone column

The vibro stone column method is used in 4 ways:

1) Vibro Compaction method: In Vibro Compaction method, the existing soil mass is compacted by vibration without adding coarse-grained materials. The vibration and compaction resulting from this vibration is not useful for fine and coarse soils. Therefore, the application of vibro compaction method is limited to coarse-grained soils. Vibro Compaction method is suitable for dry and wet soils.

2) Vibro Replacement method: Vibro Replacement method is one of the most common methods of stone columns, which is usually used in normal consolidated clay soils, soils with organic matter, saturated silt and alluvial soils. In Vibro Replacement method, the weak soil mass is replaced with suitable granular materials and because the diameter of the drilled borehole is larger than the size of the vibrator rod, there is no significant compaction in the surrounding soil. On the other hand, due to the instability of the drilling hole in weak soils, water jets should be used to push the drilled soil out and the mud from the fine material to cover the entire wall and ensure its stability. Hence this method is also called as wet method of stone columns.

Vibro replacement method has been successfully implemented in clay soils with un-drained strength of 7 kPa, but should not normally be used in soils with no drained shear strength less than 14 kPa; because in such conditions, there will be no lateral resistance necessary for the proper operation of stone columns. Vibro Replacement method is not suitable as an improvement method for deep clay deposits with high organic matter. The construction of stone columns takes place in several places inside the clay. The diameter of each column is normally equal to 0.9 m, but this diameter varies from 0.5 to 1.5 m. The maximum diameter depends on the characteristics of the clay and each column can withstand a load of about 30 tons.

In soft soils, the vibrator rod penetrates the soil with the help of a water jet, resulting in a hole larger than the size of a vibrator. Hanging the vibrator from the crane ensures that the drilled hole is vertical. The vibrator rod penetrates to the desired depth and is then slowly pulled out. With the exit of the vibrator rod, coarse-grained materials are poured into it from the top of the hole and pass through the empty space between the vibrator and the excavation wall and are compressed by the vibrator at the end of the excavation. Therefore, the common method of constructing replacement stone columns is feeding from above. Of course, this does not mean that the construction of these columns is not done by the method of feeding from below. As compaction occurs, the surrounding soil shifts laterally. This process takes place in successive steps of 1 meter and finally a stone column is formed. In general, the dimensions of the grains in embankment materials vary from 5 to 100 mm and the amount of particles smaller than 5 mm should not be more than 15%. In some references, the dimensions of the grains are mentioned from 30 to 80 mm.

The process of compaction of embankment materials inside the excavation together with the exit of water jet from the body of the vibrator rod, may cause local failures in the surrounding fine-grained soil, especially when the surrounding soil is sensitive. Failed materials come out of the borehole with the flow of water and the diameter of the hole increases and more embankment materials will be needed.

Stages of constructing replacement stone columns - wet method of feeding from above
Stages of constructing replacement stone columns – wet method of feeding from above

The concept of compaction phenomenon in granular soils
The concept of compaction phenomenon in granular soils

3) Vibro-displacement method: Vibro displacement is a dry method of constructing stone columns in insensitive clay soils. Dry method means that water jet is not used during the construction process. In this type of soil, the minimum amount of un-drained shear strength is equal to 20 kPa. The dry method is used in conditions where the hole remains stable and there is no risk of groundwater penetrating into it. Vibro displacement method is very effective in unsaturated clay soils. In Vibro displacement method, the displacement of the surrounding soil due to compaction will increase the soil strength between the stone columns. There is no such thing as a vibration replacement method or its effect is very small. For example, if the distance between the columns is equal to 1.5 m, the resistance increase will be about 1.5 times.

During the stone column construction process, the vibrator rod penetrates into it by cutting and moving the surrounding soil, and because it is completely in contact with the hole, there is no circular space between the vibrator rod and the borehole for pouring embankment materials. Therefore, it is necessary that the vibrator rod is pulled out and then the embankment material is poured into the excavation as feeding from above. Vibro-displacement stone columns are also constructed as feeding from below. In this method, the type of vibrator rod is slightly different and the stone material is fed through a tube that is installed in its body. In this type of vibrators, special equipment is used to feed and fill the hole, which is called Vibrocat.

Stages of constructing stone columns - dry method of feeding from below
Stages of constructing stone columns – dry method of feeding from below

4) Driven stone columns:

Driven stone columns method is used in insensitive clay soils. Execution of these columns is done in 3 stages:

  1. Drilling a stone column hole
  2. Construction of lower resistant bubble
  3. Construction of a stone column

After drilling the stone column borehole, granular stone material is poured into 30 cm layers in the hole and hammered with a special hammer. This hammer can be in the following 2 ways:

  • Manual winch and rope system
  • Hydraulic

A hydraulic hammer can deliver an energy equivalent to 1 to 2 million pounds feet per step, with around 400 beats per minute. As a result of the compaction caused by the hammer in the first stage, a low resistant bubble is formed, which causes the fine-grained soil to move laterally and downwards. After that, the stone column is constructed step by step and deforms laterally due to the applied compaction of the surrounding soil. If the surrounding soil does not have the necessary strength to stabilize the borehole wall after drilling, the use of casing will be necessary. In this case, when pouring stone material into the hole, the casing is pulled out slowly and step by step.

An example of hydraulic hammers for constructing driven stone columns
An example of hydraulic hammers for constructing driven stone columns

An example of hydraulic hammers for constructing driven stone columns

Applications of vibro stone column method

The stone column method is currently used in a wide range of soil types, although its main application is in fine-grained soils.

  • Stone columns increase the bearing capacity of loose non-adhesive soils and their function as drainage in this type of soil is not very important.
  • In conditions where the soil mass is liquefied, in addition to eliminating the liquefaction potential, the drainage performance of stone columns is also considered.
  • In adhesive soils, in addition to increasing the bearing capacity, the consolidation subsidence is significantly reduced due to the incoming loads and its drainage is faster.
Scope of application of each method of vibro-replacement and vibro-compaction depending on the soil granulation curve
Scope of application of each method of vibro-replacement and vibro-compaction depending on the soil granulation curve

Advantages of vibro stone column method

  • High execution speed
  • Easy to execute
  • Suitable price to execute
  • Increasing the bearing capacity of foundations
  • Reducing the settlement of foundations
  • Reducing the liquefaction potential
  • Increasing the speed of consolidation of fine-grained soils
  • Stabilization of natural or artificial slopes (embankments)

Advantages of vibro stone column method
Advantages of vibro stone column method

Related contents:

Federal Highway Administration (FHWA)
Keller , Vibro Stone Columns

Menard , STONE COLUMNS

Milano , STONE COLUMNS

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