Dynamic Compaction is used in loose coarse grained soils deposits, to increase in-situ density and improve stiffness characteristics, resulting in improved settlement performance.
Dynamic Replacement is an alternative to the more commonly used Dynamic Compaction technique, utilising dynamic impact techniques to improve soils at the boundary between fine- and coarse-grained materials.
Dynamic methods are often used to improve deep deposits of made ground in-situ. This can help to avoid any requirement for the use of more expensive traditional piling methods, allowing the adoption of lightly reinforced shallow foundations, and ground bearing slabs.
Both Dynamic Compaction and Dynamic Replacement utilise tracked crawler cranes, dropping compaction weights from heights of up to 20.0m. The depth of influence achieved is in the typical range of 5.0m – 8.0m, potentially increasing to as much as 15.0m, dependent on drop heights, tamper weight and soil conditions.
With prior approval Dynamic Replacement can utilise recycled aggregates, offering environmental sustainability benefits.
The Dynamic Replacement Process
Ahead of commencement on site GeMech will complete a detailed ground improvement design and prepare a compaction layout based on a set of construction issue foundation drawings – supplied to GeMech in DWG format by the Client.
Step 1 – Prior to the crane arriving to site the client surveys the area for services and prepares a suitable working platform at either top of foundation level or underside of slab level and drives a numbered pin into the working platform at each compaction point location.
Step 2 – A health and safety briefing will take place, the rig will be prepared to work, and moved to the first pile position.
Step 3 – A compaction trial is carried out, to determine depth of drop weight penetration and saturation energy, through measurement of the ground heave profile. This will inform the compaction process across the working areas.
Step 4 – Stone is delivered to site and tipped on the working platform, in a stockpile positioned close to the piling rig. GeMech will move stone from the stockpile to the individual columns using a 360 excavator.
Step 5 – The shaped drop weight is positioned over the compaction point and lifted to the predetermined drop height. When at the required height, the weight is dropped in free fall to impact the surface of the working platform. This process is continued until the require depth of penetration, or saturation energy is achieved.
Step 6 – Stone is placed in the resultant void, with each layer successively compacted by the drop weight during the compaction process. This process is repeated until all primary compaction points have been completed.
Step 7 – A lighter, flat bottomed, drop weight is attached to the crane, and the entire surface of the working area. This process recompacts the near surface soils which are prone to disturbance during the initial higher energy compaction pass.
The Dynamic Compaction Process
Dynamic Compaction deploys a very similar construction process to Dynamic Replacement.
However, there is no requirement to compact imported stone in the formed compaction craters. Instead, the working platform is graded into the compaction craters, resulting in a global site level reduction.
The required increase in in-situ density can be related to an overall volume reduction, therefore, with Dynamic Compaction, achievement of the compaction requirements can be related to a target surface level reduction.
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John Reed, Capstan
Clear, professional & practical communication during the pre-construction phase allowed for a smooth operation on site. Correct methods of piling for the presenting ground conditions were discussed and employed, resulting on hitting programme targets aswell as budget constraints.
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