The construction industry is one of the largest contributors to carbon emissions across the globe and is under growing pressure to find solutions to more sustainable development. A large proportion of the carbon cost is created by the vast quantities of construction waste being produced, with it making up one third of all global waste. In addition to the excavation, transportation, and disposal of waste to landfill having a high carbon cost, the disposal of waste soils also produces a need for replacement material. This results in unnecessary pressure on the supply of finite natural resources and can have substantial financial implications to developers and contractors.

Finding methods of reusing on-site material has therefore become a significant issue over recent years both in the UK and globally. Ground improvement techniques are increasingly being used to cut the amount of site-generated waste, ultimately reducing both the carbon and financial costs of construction projects. Continuous advancements of these methods in recent years are further helping the industry work towards a ‘Net Carbon Zero’ future.

One such ground improvement technique is Rolling Dynamic Compaction (RDC), also known as High Energy Impact Compaction. RDC works similarly to conventional roller compaction where a load is applied to the surface using weighted drums. The weight of the drum compacts the underlying soil increasing the strength and bearing capacity of the ground. Where RDC differs from more conventional methods is the use of noncircular drums (typically with 3, 4 or 5 sides); as the drum passes over the ground and reaches the pinnacle of the drum it then ‘falls’. The periodic impact of the drum on the ground surface dynamically loads the near surface soils but also compacts the deeper underlying soils by inducing pressure waves deep into the ground.

Benefits of RDC

Dynamic compaction using non-circular drums provides several advantages over traditional circular drums:
• The dynamic loading compacts the ground to much greater depths. Where circular drums would be expected to achieve compaction to depths of around 0.5m, RDC can achieve compaction to depths in excess of 1m and, in favourable conditions, can reach up to 4m below the surface.
• The use of RDC in large earthwork projects allows the material to be placed in thicker layers, reducing the number of layers required and thus saving time and reducing vehicle movements. Thicker lifts also allows the use of larger maximum particle sizes, reducing the need for processing site-won material.
• In addition to achieving compaction to greater depths, RDC drums can often be towed at higher speeds (10-12km/h) comparable to conventional rollers (4-5km/h). This allows an area to be compacted significantly quicker when applying RDC.

Typically, the ground being treated is split into strips and passed over a set number of times at a set speed to achieve a required strength. Advancements in the plant now enables the response of the ground to the compactive effort from the drum to be recorded with each rotation. Via a digital display in the cab, the plant operator is provided with the Surface Stiffness Modulus in real time along with their position using GPS. The energy of compaction can then be automatically adjusted by altering the position of the two internal counter-rotating weights to vary the drum vibrations.

This continuous feed of information to the operator allows them to confirm if the specification has been achieved or whether further passes of the RDC roller are required. These technological advancements have led to more efficient coverage of sites and can produce more consistent results across large areas

Applications for RDC

RDC is most suited for large open sites that require significant areas of ground improvement. In recent years it has been utilised on a range of projects such as land reclamation, compaction of non-engineered fill (i.e. historical landfill sites or quarry restoration with poorly engineered backfill), improving mining haul roads and tailing dams and in agriculture to help reduce water loss.

A significant advantage of this ground improvement technique is that it can be carried out without the need to disturb or handle the ground, which, in the case of a former landfill site, could contain significant contamination or poorly graded backfill soils. The use of RDC (often in conjunction with other ground improvement methods) allows the material to remain in-situ, reducing the risk of exposing contaminants to the environment, construction workers and future site users. Methods such as RDC (that remove or significantly reduce the need to handle or disturb waste) offer a practical solution to designers and clients as it provides a potential way to eliminate, so far as is reasonably practicable, foreseeable risks to the health and safety of site users as is required with the CDM 2015 regulations. Of course, the potential impact from potentially contaminated soils on controlled waters and the wider environment would need to be assessed and mitigated, as required.

RDC can also be applied to site where a thin mantle of weak soil with poor bearing characteristics overlies more competent ground. The use of RDC on these sites may allow the foundations to be placed at shallower depths, reducing the amount of excavation undertaken and decreasing the quantity of concrete required. However, the long-term consolidation and creep settlement of the compacted soils need to be understood and included in the design of building foundations along with external areas such as pavements and landscaping. The potential for differential settlement between buildings founded on deeper, competent soils (e.g., piled structures) and external areas also needs to be carefully considered in the design.

The figure below shows the improvement achieved across a former landfill using RDC to compact the top 3m to 4m of ground. The plant outputs clearly show the increase in the Surface Stiffness Modulus (Evib) across the treated ground. The outputs, which are presented on a digital display in real-time in the cab to the machine operator, clearly highlight areas that may require additional passes. These outputs can then be verified with in-situ testing and may be used in reporting to confirm that the works specification requirements have been met.

Limitations of RDC

As with all ground improvement techniques, the effectiveness of RDC treatment is dependent on the ground conditions. RDC is best suited to granular soils, where excess pore water pressure is rapidly dissipated, although can still be applied to less permeable clays and silts in certain conditions. RDC is generally not considered to be suitable where the ground comprises soft clays or high organic content material, such as peat, or where high groundwater is present.

Predicting the depth of compaction achieved can also be problematic, especially where there is significant variability within the compacted soils. The depth of improvement can be variable and the factors affecting this are not fully understood. Methods for assessing the depth of influence often include comparing in-situ test results undertaken pre- and post- compaction. However, the reliability of the data can be dependent on the quality of the testing and the interpretation of the results. It is advisable to carry out initial trials on a section of the proposed works to understand the performance of the chosen compaction method and to finalise the compaction methodology, plant-type, etc. for the wider works.

The RDC plant measures ‘refusal compaction’ at the moisture content of the in-situ material at the time of compaction; should a material be significantly drier than the optimum moisture content during the compaction it may reach refusal, however, if then later inundated with water, collapse settlement may occur. To prevent this, conventional compaction compliance testing is often required to show that the material being compacted is near to optimum moisture content.

Conclusions

RDC can be an effective method of ground improvement and has been successfully applied to numerous sites in recent years. It can reduce the amount of plant movement and off-site soil disposal required for a project and can improve the bearing characteristics of the ground so that a more economical foundation solutions may be considered. Advancements in technology are working to address some of the limitations of the technique. Overall, the application of ground improvement techniques such as RDC, can be used across the industry to help work towards a ‘Net Carbon Zero’ future.

References

Avsar, S., Bakker, M., Bartholomeeusen, G. and Vanmechelen, J. (2006). Six Sigma Quality Improvement of Compaction at the New Doha International Airport Project. Terra et Aqua no. 106.

Scott, B., Jaksa, M. and Mitchell, P. (2019). Depth of influence of rolling dynamic compaction. Proceedings of the Institution of Civil Engineers – Ground Improvement, pp.1–10.

https://www.forbes.com/sites/heatherfarmbrough/2023/10/04/why-and-how-construction-companies-can-move-towards-net-zero/?sh=64a714b325b3

https://www.bbc.com/future/article/20211215-the-buildings-made-from-rubbish#:~:text=Roughly%20half%20of%20the%20raw,the%20world’s%20carbon%20dioxide%20emissions.

Article provided by Rose Ashmore, Senior Geotechnical Engineer at CampbellReith