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FAQ Compaction

Design and engineering

This section will provide specific answers to design and engineering questions related to compaction

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How is a compaction programme designed?

A compaction programme is designed by matching the selected compaction technique to the soil conditions, project requirements and performance criteria. The objective is to introduce sufficient energy into the ground to achieve the required increase in density, bearing capacity, stiffness or liquefaction resistance. While geotechnical design methods and empirical correlations provide the basis for the design, the optimisation of a compaction programme often relies on practical experience and engineering judgement.

The most efficient compaction programme is not necessarily the one that applies the most energy. In many cases, the most economical solution is achieved by optimising the treatment depth, grid spacing and energy level to achieve the required ground improvement without unnecessary treatment.

The design process typically starts with a review of the available geotechnical information, including CPTs, boreholes, laboratory test results and groundwater conditions. These data are used to identify the soil types present, the thickness of the layers to be treated and the target treatment depth.

Based on the ground conditions and project requirements, the most suitable compaction technique is selected. Depending on the project, this may include Cofra Dynamic Compaction (CDC), Vibro Compaction (CVC) or Rapid Compaction (CRC).

The compaction programme is then developed by determining:

• The required treatment depth
• The target level of improvement
• The applied energy level
• The grid spacing of the treatment points
• The number of treatment locations
• The required number of passes or blows
• The anticipated production rates

Where appropriate, a trial section may be performed to verify the design assumptions and optimise the treatment parameters before full-scale production begins. The effectiveness of the programme is typically verified through field testing before and after treatment. Depending on the project objectives, this may include CPTs, SPTs, density measurements, plate load tests or shear wave velocity measurements.

Because every project is different, the final compaction programme is generally developed through a combination of geotechnical analysis, practical experience and field verification. The optimum design is often a balance between the required performance, construction schedule and overall project economics.

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How is compaction performance verified?

The performance of a compaction programme is verified by comparing the condition of the soil before and after treatment. The objective is to confirm that the required improvement in density, bearing capacity, stiffness or liquefaction resistance has been achieved.

The verification method depends on the project requirements, the soil conditions and the selected compaction technique. In most projects, a combination of field testing and construction monitoring is used.

Common verification methods include:

• Cone Penetration Tests (CPTs)
• Standard Penetration Tests (SPTs)
• Plate load tests
• Density measurements
• Shear wave velocity measurements
• Topographic settlement monitoring
• Machine data obtained during compaction

For densification projects, CPT testing is often the preferred verification method because it provides a direct indication of the achieved increase in soil resistance with depth. Pre-treatment and post-treatment CPTs are commonly compared to quantify the improvement achieved by the compaction works.

For liquefaction mitigation projects, additional assessment methods may be used. Improvements in CPT resistance, SPT values or shear wave velocity can be incorporated into liquefaction analyses to demonstrate that the required reduction in liquefaction risk has been achieved.

Where appropriate, a trial section may be performed prior to full-scale production. The trial section allows the effectiveness of the selected compaction technique, grid spacing and energy level to be verified and optimised before the main works commence.

The verification programme should always be tailored to the project objectives. While one project may focus on bearing capacity improvement, another may be driven by settlement reduction or liquefaction mitigation. The selected verification method should therefore demonstrate that the required design performance has been achieved.

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How deep can compaction techniques improve the ground?

The achievable treatment depth depends on the selected compaction technique, the soil conditions and the project requirements. Different compaction methods transfer energy into the ground in different ways and therefore have different effective depths of influence.

In general, treatment depth is influenced by:

• The compaction technique used
• The applied energy level
• The soil density and grading
• The groundwater conditions
• The presence of weak or intermediate layers
• The required degree of improvement

Surface compaction techniques such as CRC and CDC are typically most effective in the shallow, up to 2-3m, to intermediate depth range of 7-8 meters. The highest improvement levels are generally achieved in the upper part of the treated zone.

Vibro Compaction (CVC) is capable of treating significantly deeper granular deposits because the energy is introduced directly into the ground through a vibrating probe. Depending on the project conditions, treatment depths in excess of 30 m can be achieved.

The achievable treatment depth should therefore not be viewed as a fixed value. Instead, it is determined by the interaction between the equipment, soil conditions and performance requirements of the project. Different contractors may also apply different equipment configurations and treatment approaches to achieve the required level of improvement.

For critical projects, a trial section is often performed to verify the achievable improvement depth and optimise the treatment programme before full-scale production begins.

The required treatment depth is usually determined during the geotechnical design phase and is based on the depth of the soils contributing to settlements, bearing capacity limitations or liquefaction susceptibility. In many projects, the objective is not to improve the entire soil profile, but only the layers that govern the performance of the development.

In practice, the most economical ground improvement solution is often achieved by treating only the critical depth range that influences the performance of the project. A proper geotechnical assessment can therefore significantly reduce treatment quantities and project costs.

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What challenges can affect compaction performance?

The performance of a compaction programme depends on the interaction between the selected compaction technique, the soil conditions and the project requirements. Successful compaction is not only determined by the amount of energy applied, but also by the ability of the soil to respond to that energy.

One of the most important factors is the soil composition. Compaction techniques generally perform best in loose granular soils such as sands and reclamation fills. As the fines content increases, the effectiveness of densification often decreases because the soil particles become less free to rearrange into a denser configuration.

Groundwater conditions can also influence performance. Most densification techniques rely on particle movement and temporary pore pressure generation to achieve improvement. Depending on the soil type and compaction method, groundwater conditions may therefore affect both the achievable improvement and the production rates.

The presence of weak, compressible or heterogeneous intermediate layers may also influence the results. Variations in soil stratigraphy can cause differences in the response to the applied energy, resulting in non-uniform improvement across the treatment area.

Construction control is another important aspect. Verification testing should be carried out during the works to confirm that the required bulk improvement is being achieved. If the measured improvement differs from the design assumptions, adjustments to the treatment programme may be required. It should be noted that the selected verification method should focus on the overall bulk behaviour of the treated ground rather than solely on isolated weaker locations, as the performance of the soil mass is governed by its bulk properties.

For this reason, trial sections and performance verification are often used to optimise the compaction programme before and during full-scale production. This helps ensure that the required bearing capacity, settlement performance or liquefaction resistance is achieved in the most economical manner.

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How is compaction monitored during construction?

During construction, the compaction process is continuously monitored to verify that the treatment programme is being executed as designed and to identify any deviations that may require adjustment of the works.

For CDC, CRC and Vibro Compaction projects, the installed energy is typically recorded through dataloggers and site observations. Depending on the technique, the recorded parameters may include:

• Treatment locations
• Applied energy levels
• Number of blows or passes
• Depth
• Settlement rate
• Production rates
• Equipment operating parameters

The collected production data is reviewed throughout the project to confirm that the required treatment programme is being achieved and that the equipment is performing as expected.

In addition to production monitoring, occasional verification testing is commonly carried out during construction. Depending on the project requirements, this may include CPTs, SPTs or other field tests to confirm that the achieved improvement is consistent with the design assumptions.

This combination of continuous production monitoring and periodic field verification allows the treatment programme to be adjusted when necessary and helps ensure that the required ground improvement performance is achieved.

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Technical Sales Cofra

For design questions or technical information.

Csm Limelights Cofra Jeroen 04 126Bf92f29
Jeroen Dijkstra
+31 (0)20 69 34 596 Jeroen.Dijkstra@cofra.com

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