Soil Modification (fill drying)

Soil modification is the process of creating a controlled fill drying method to ensure that soil is at the correct moisture content to achieve maximum compaction.

Structural earthworks have been carried out successfully in the UK for many years. However, to maximise land values or realise land investments, many sites are re-profiling to meet the required design levels for new development. Therefore, it makes sense to make use of the site-won fill materials, and place them in a controlled manner which avoids the need to transfer foundation loadings back down through the upfill materials and on to the firm natural strata.

In the UK, most soils found in the upper weathered layers tend to be naturally above their optimum moisture content, and therefore, it can be very difficult to meet the very stringent requirements of a high performance density-driven specification as required for structural earthworks under, say, building foundations. This, coupled with the frequent risk of inclement weather, means there is the likelihood that the soils will already be in-excess of their natural optimum moisture content, or will become in-excess of their optimum once disturbed during the course of earthworks operations.

Considering the UK climate and its soil conditions and given there is no accurate method of controlling the site-won soils moisture content clients face a real challenge to the meeting the requirements of a high performance density driven specification, especially when the industry-required standard is minimum 95% compaction less than 5% air voids for many high end performance situations such as under building foundations.

We are successfully using soil modification techniques as an effective means of gaining control of the soil moisture content during the course of undertaking structural earthworks operations. This fill drying technique has been proven to achieve the required density driven specification, and it has met the requirements of engineers, clients and insurers on various projects throughout the UK.

Advantages of Soil Modification

  • It conditions non-engineered material to an appropriate engineered solution;
  • High strength fill material with minimal settlement or swell;
  • Engineered back-fill ready for pad / strip foundation and ground bearing floor slabs;
  • Reduces the need for deep foundation works,imported stone, mass concrete and piling;
  • No requirement for stone capping layers on roads, car parking areas etc;
  • Creates a capability to recycle on-site material, thereby reducing material going off-site and decreasing aggregates coming back on-site; and
  • Reduces the environmental impact through better use of existing site materials and reduced haulage.


Virtually any earthworks project where there is a desire to maximise the reuse of site won soils and minimise the cost of the foundation solution for both buildings and road pavements.

GDL are always pleased to provide contractors and specifiers with practical technical advice on materials suitability for any soil stabilisation process.

The process

The fill formation is first prepared ready to accept upfill, commonly by means of proof rolling using a heavy soil compactor. Dependent on the requirements of the specification, frequently soft compressible materials which are identified during proof rolling are excavated and replaced with re-engineered material to the same performance requirements as the subsequent upfill.

Soils intended for modification may be treated either in – situ in the cut area prior to excavation and deposition as fill elsewhere on-site or alternatively in-situ once placed in a control layer thickness in the fill situation. There are merits to each approach and this would be determined on a project specific basis. However, it is critical that adequate grading takes place to ready the materials for stabilisation because we must make sure that materials are treated in controlled layer thickness, and this allows us to check that all materials are treated to the required degree without thin laminations of untreated materials between layers.

The required dose of proprietary dry binder is then spread over the materials which are undergoing treatment using a computer-controlled binder spreader. We operates a fleet of modern self-propelled spreaders which offer a higher degree of accuracy and minimise the production of any airborne dust during the process. The type and quantity of binder that is required for each project is determined through an initial design testing that is normally carried out prior to site start, and then on an ongoing basis to allow for variations in the ground. The most commonly used binders are hydrated quicklime and cements, but others such as pulverised fuel ash (PFA) and ground granulated furnace bottom slag GGBS may also be used depending on the requirements for each project.

Mixing takes place to incorporate the binder into the soil. Our preference is to utilise one of our highly-efficient Wirtgen self-propelled stabilising machines, although our fleet also includes tractor mounted variants for use on small sites and in lighter soil conditions to ensure that we can deliver exactly what our clients need.

In certain circumstances, such as when working in heavy clay types soils, secondary mixing may also be required to achieve the correct degree of pulverisation.

In these cases, materials are graded to ensure the correct layer thickness is maintained for compaction and then compacted with the specified number of passes by a heavy soil compactor. The process is repeated for subsequent layers, until the required design level is attained.

Throughout the process our dozers and excavators utilise GPS technology to ensure that we accurately control the layer thicknesses and ensure that no area of the fill is over-cut or overfilled.

Testing is carried out during the works in accordance with the specified requirements which commonly includes density testing and strength testing by means of CBR or Plate Load testing.

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