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Dynamic Compaction (DC) is one of the ground improvement methods that increases the soil compaction by throwing a steel or concrete weight. The weights usually weigh 5 to 40 tons and are released from a height of 10 to 40 m. These weights are thrown on the ground with a special pattern according to the executive plans.

Dynamic Compaction method is used to improve problematic soils by increasing bearing capacity, reducing settlement of the foundation and reducing liquefaction potential for residential and office buildings, reservoirs, highways, railways, airports and ports.

Dynamic Compaction - Istasazeh Co.
Dynamic Compaction method- Istasazeh Co.

History of Dynamic Compaction method

Throwing weights on the ground is a very old method, to the extent that some attribute the concept of DC to the time of the Roman Empire. However, studies of Dynamic Compaction (DC) in the literal sense were first conducted in 1957 by the British Road Research Laboratory, and in 1970 the French engineer L. Menard introduced this method with a patent. An article published by Menard and Brevis (1975) provided the theoretical basis for the consolidation of fine-grained soils by heavy compaction. Dynamic Compaction method was developed in 1973 in England and in 1975 in the United States.

The first application of Dynamic Compaction method in Iran was reported in late 1973 for the purpose of ground improvement in the Shahriar region in the southwest of Tehran. The use of Dynamic Compaction ground improvement method has been considered especially in the seventies and simultaneously with the expansion of large development projects in the south of the country.

DC method
Dynamic Compaction (DC) method

Design basics of Dynamic Compaction method

The applied energy of each impact (W.H) is the most important factor in the depth of improvement. Various studies have been performed on the relationship between energy and depth of improvement and relations have been presented. Among the proposed relations, the Menard empirical relation is more common among engineers:

D=n (W.h)0.5

Where, D, W, h and n are the depth of improvement (m), the weight of the weight (tons), the height of the compacting (m) and a constant coefficient between 0.3 to 0.8, which is a function of the composition, grain size, etc. of the soil.

Dynamic Compaction ground improvement
Dynamic Compaction ground improvement

Dynamic Compaction execution method

  1. Preparing the site by removing additional objects (trees, etc.), leveling the ground, drainage and filling holes. If there is groundwater level about 2 m above the ground, either drainage or embankment should be done. If the surface soil is too weak to withstand the load of machinery and tools, an operation platform must be constructed first.
  2. If there are structures or vital lines near the project site, a separation trench is required to minimize vibrations and lateral displacements. The trench should be at least 2 to 3 m deep and 1 m wide.
  3. Placing the nail in the center of the landing points of the weights and check the height of the ground.
  4. Deploying equipment and lifting 5 to 40 tons of weight to a height of 10 to 40 m on the desired point.
  5. Reading the height of the weight above the desired point.
  6. The weight rises to the desired height and is released freely on the ground. When the weight is still inside the hole, the height above the weight is read. If the weight is removed from the perpendicular position when it hits the ground, the floor of the hole will be leveled after lifting the weight.
  7. Until the number of collisions on the collision points reaches the required value, stage 6 is repeated and then it is the turn of the next compacting point.
  8. Stages 4 to 7 are repeated until the compacting points are completed for the first time.
  9. A bulldozer is used to level the ground and the ground level is measured. The difference between the current level and the previous level indicates the amount of settlement created.
  10.  After the passage of time, depending on the soil and groundwater level conditions, if necessary, stages 3 to 8 are repeated until all the points of compacting are completed for the second time.
  11.  Ironing stage is done for the whole ground area of the project.
Change of soil particle arrangement after different stages of Dynamic Compaction (DC)
Change of soil particle arrangement after different stages of Dynamic Compaction (DC)

Applications of Dynamic Compaction method

The Dynamic Compaction method can be used for all types of granular soils. The result of improving this method is suitable in the following soils:

  1. Inorganic soils
  2. Loose and semi-saturated embankments
  3. Saturated soils with free drainage
  4. Pads with a plasticity index less than 8
  5. Low saturation clay soils (percentage of moisture less than the plastic limit)
  6. Heterogeneous fillers
  7. Improved ground and soil (areas built through the rehabilitation of wetlands, lakes or shores)
  8. Regenerated areas with variable specifications
  9. Where there are underground networks or a piece buried underground
  10. Soils that have large air cavities (such as buried garbage and ground filled with poor soil)

In general, Dynamic Compaction ground improvement is not recommended for clay soils with a high plasticity index (greater than 8) and a high degree of saturation. Of course, Dynamic Compaction method has also been used to improve fine-grained clay soils in some countries. Drainage is often required to reduce pore water pressure due to deep Dynamic Compaction (DC), and a certain waiting time is required to eliminate excess pore water pressure. High groundwater level (about 2 m above ground level) minimizes the effect of Dynamic Compaction (DC), which drainage is necessary for such conditions. In general, deep DC is cost-effective when the project area is more than 5000 m2.

Comparison of DC method and other methods of soil improvement in different soil types
Comparison of Dynamic Compaction (DC) method and other methods of soil improvement in different soil types

Quality control of DC method

Compacting operation
Compacting operation
  • The height and point of fall must be checked before each compacting.
  • It is important to observe in person during the compacting operation.
  • Corrections may be made based on controls and observations.
  • For example, if one of the holes is deeper than the other, it indicates that the soil is weaker. As a result, special measures such as drilling and replacement should be considered to improve these areas.
  • If the extra compacting creates large bumps around the hole, it means that more impact will be useless and the compacting operation in this area should be suspended or interrupted.
  • Typical field controls include the following:
  • Piezometer inside fine-grained saturated soils
  • Slope gauge to detect horizontal displacements
  • Accelerometer for ground vibrations
  • After completing the compacting operation, field tests should be performed to evaluate the degree and depth of improvement.
  • Depending on the soil type and groundwater level, for coarse-grained soils, field evaluation should be done at least 1 to 2 weeks and for fine-grained soils at least 3 to 4 weeks after the completion of the compacting operation.
  • Field surveys include sampling for SPT, CPT or PMT laboratory tests.
  • The depth of the test should also be lower than the design depth for improvement.
  • Static loading tests may also be performed on large projects.
  • Since PMT and plate loading tests are more sensitive to changes in soil stiffness than SPT and CPT, they are good ways to meet this goal.

Advantages of Dynamic Compaction method

•	Deep Dynamic Compaction (DC)
Deep Dynamic Compaction (DC)
  • Deep Dynamic Compaction (DC) can improve a relatively large area in a short period of time at a low cost.
  • Using this method, loose semi-saturated materials with a fineness of less than 15% are compacted.
  • This method can often identify very weak or loose areas during the operation, to be well improved using methods such as replacement and so on.
  • Dynamic Compaction (DC) can make a heterogeneous soil into a denser, more uniform and more resistant material.
  • The main equipment required for this method is a crane and a weight, which many contractors have

Disadvantages of Dynamic Compactio method

Disadvantages of DC Dynamic Compactio method
Disadvantages of DC Dynamic Compactio method
  • Deep Dynamic Compaction (DC) generally has no effect on saturated clay soils.
  • Special measures, such as providing drainage and long waiting times, should be taken to dissipate additional pore water pressure for this method.
  • Impacts to the soil by this method cause sound, vibration, and lateral displacement that may cause problems for surrounding buildings, infrastructure, and vital lines.
  • This method usually requires a tool to control vibration, sound level and ground movements.
  • When this method is used in saturated clay soils, piezometers are needed to control the formation and dissipation of pore water pressure.
  • Falling weights may cause dust which is dangerous for workers inside the site.

Related contents:

Federal Highway Administration (FHWA)
Keller , Dynamic compaction

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