Soil stabilisation enables engineers to build on soils that would otherwise be too weak, too compressible, or too variable. Soil stabilisation, or ground stabilisation, involves the improvement of the engineering properties of a soil by mechanical or chemical means.
In road engineering, the layer of soil immediately below the road construction is referred to as the subgrade. If this layer is weak or variable, soil stabilisation, or subgrade stabilisation as it is commonly called by road engineers, provides an alternative to excavation and replacement of the weak soil.
Tensar is a global leader in the design and manufacture of innovative soil stabilisation products for use in a wide range of applications, including, roads, working platforms, foundations and more. Read on to learn about the process of soil stabilisation, the methods and products that can be used, and the benefits unlocked by ground stabilisation techniques.
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- What is soil stabilisation?
- Why might we need to stabilise soil?
- Soil stabilisation or ground stabilisation – what is the difference?
- The impact of ground stabilisation on the properties of soil
- Which layers of soil are typically stabilised?
- Ground stabilisation techniques
- Soil stabilisation products
- Projects that may require soil stabilisation
- What benefits can ground stabilisation unlock?
- Next steps
What is soil stabilisation?
Soil stabilisation, or ground stabilisation, refers to improvement of the physical properties of a soil by the addition of a stabilising component. This may be chemical – lime, cement, or other chemical agents combined and mixed in-situ, or mechanical – by the inclusion of one or more layers of a stabilisation geogrid.
In the context of road engineering, soil stabilisation, or subgrade stabilisation as it is commonly called by road engineers, can also describe the process whereby a mechanically stabilised layer is placed over the weak soil to distribute load to the subgrade. The layer typically comprises a compacted fill material, typically an aggregate, incorporating a stabilisation geogrid. In this case, the subgrade soil properties remain unchanged, but the mechanically stabilised fill layer provides protection to the subgrade and increases load-carrying capacity.
Why might we need to stabilise soil?
Everything we build above ground must transfer load into the ground. The capacity of the ground to bear that load, and the settlement associated with it, will depend on the physical characteristics of the soils.
Inadequate soil strength
The bearing capacity of a soil is dependent upon its physical characteristics – mineral composition; particle size, shape, and distribution; water content; and the confinement pressure. Inadequate soil strength will require soil stabilisation, or ground stabilisation.
Excessive soil settlement
Soil will deform and compress under load. At the surface, the effect of this is settlement. The soil type, arrangement of particles and the degree of saturation will determine the amount of settlement. As these factors may vary across a site, the potential for settlement varies, leading to differential settlement. Potential for excessive settlement can be overcome with soil stabilisation, or ground stabilisation.
Soil stabilisation or ground stabilisation – what is the difference?
Soil stabilisation and ground stabilisation are used interchangeably. Some ground improvement treatments with a deeper influence, such as chemical injection and dynamic compaction, are more likely to be described as ground stabilisation, rather than soil stabilisation. But to most engineers the two stabilisation terms mean the same thing – the improvement of the physical properties of a soil by the addition of a stabilising component to increase the bearing capacity of the ground and limit settlement.
The impact of ground stabilisation on the properties of soil
The most common problems with natural soils are low soil strength and high compressibility. Other problems may include low permeability, susceptibility to creep deformation, freeze/thaw and shrink/swell behaviour. The objective of ground stabilisation is to improve the short- and long-term properties of the soil by preventing or restricting soil particle movement.
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Which layers of soil are typically stabilised?
When weak or compressible ground is encountered in road, rail, and other construction, soil stabilisation can improve bearing capacity and reduce settlements. Soil stabilisation can be introduced at several levels.
Subgrade stabilisation
The subgrade, which is the naturally occurring soil layer immediately below formation level, may be stabilised using, compaction, or chemical treatments, such as cement or lime. As an alternative the weak subgrade can be protected, and the bearing capacity improved, by subgrade stabilisation using a mechanically stabilised layer.
Capping layer stabilisation
The capping layer is an improvement layer placed over weaker subgrade to protect the subgrade and improve the formation stiffness. Usually constructed using a lower quality fill material compared to the subbase. The performance of this layer can be enhanced by stabilisation, often enabling a reduction in thickness. Although chemical stabilisation can be used, mechanical stabilisation is usually adopted, where one or more layers of geogrid are incorporated into the granular fill material, forming a mechanically stabilised layer.
Subbase stabilisation
The subbase is a platform on which the main structure of the pavement is constructed. This layer is typically an unbound granular material - a well graded crushed rock or recycled aggregate. The performance of the subbase can be improved by incorporating one or more layers of geogrid to form a mechanically stabilised layer.
Base stabilisation
The road base is the primary structural layer in a flexible pavement that supports the asphalt surfacing. It can be a bound layer – usually asphalt. Or it can be unbound – using a high quality well graded crushed aggregate. Where adequate quality materials are not available, cement may be used to create a cement treated base. The performance of an unbound base layer can be improved by mechanical stabilisation with a geogrid. The result is an increase in pavement life, or pavement optimisation.
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Mechanical stabilisation of capping layer, subbase, and base layer in a road pavement using Tensar geogrids Ground stabilisation techniques
The most common ground stabilisation techniques include:
- Lime stabilisation
- Cement stabilisation
- Other chemical soil modifiers
- Mechanical stabilisation with geogrids
- Tensar mechanically stabilised layer (MSL)
Lime stabilisation
Mixing lime into a soil to alter its characteristics has been known for centuries. More correctly termed soil modification, this technique is still used in many countries to stabilise weak soils.
The lime is mixed in place, often using specialist equipment. Most of the improvement is expected to occur within 72 hours, however the strength of the mix will continue to increase for up to a year after construction.
The addition of lime reduces the moisture content and plasticity of certain clay soils, improving workability. Lime treatment is therefore often seen as a useful temporary construction expedient by road engineers.
This method is not suitable for stabilisation of non-cohesive soils, or for clays containing high sulphate levels. Laboratory classification and reactivity of the soil and appropriate mix design are critical to ensure a suitable site-specific solution.
Lime is commonly obtained from chalk or limestone. This process has extremely high energy and CO2 costs for production. During construction, dust from in-situ mixing might affect adjacent properties and local stakeholders.
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Cement Stabilisation
Portland cement is mixed with the soil, to a pre-defined depth (typically up to 300mm). Water must be added in a second process to activate the cement. The hydrated cement combines chemically with the soil to bind the particles together, creating a stronger material. Water content must be tightly controlled to achieve the required properties.
The method is not suitable for cohesive soils and soils with more than 2% organic content, however with help of other binders some issues with these more difficult soils can be overcome.
The energy and CO2 manufacturing costs for Portland cement are high, as production entails quarrying, crushing limestone and the use of kilns operating at 1500°C.
In both temporary and permanent road applications, deformation of cement stabilised soils must be limited, as deformation can lead to cracking of the stabilised soil. In some countries it is assumed that cement improvement of soil is temporary and with time soil strength will deteriorate.
Specialised in-situ mixing equipment is available, but even with this, cement dust clouds arising during in-situ mixing can be an issue affecting adjacent properties and local stakeholders.
Other chemical modifiers
Synthetic polymers and biopolymer additives are offered for soil stabilisation. These are often proprietary agents with limited independent research to verify performance. Consequently, they are less commonly used for stabilisation than cement or lime. They are typically more suited to use with granular soils with higher fines content. Whilst they may offer a more environmentally friendly solution than lime or cement, they are often higher-cost alternatives.
Mechanical stabilisation with geogrids
When an aggregate is compacted over a suitable geogrid, the aggregate particles interlock with the geogrid. Provided the geogrid has adequate in-plane stiffness at low strain, combined with strong integral junctions and a high rib profile, the aggregate particles are confined within the geogrid apertures. This particle confinement restricts the movement and rotation of the aggregate particles in contact with the geogrid. The aggregate particles immediately above and below the confined layer interlock with the fully confined particles and are then also confined. Thus, a zone of confinement is formed around the geogrid layer. This mechanically stabilised layer has increased strength and stiffness compared to the non-stabilised aggregate.
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Tensar mechanically stabilised layer (MSL)
The increased strength and stiffness of a Tensar mechanically stabilised layer allows for a thinner aggregate layer to be used over weak soil, reducing imported materials cost and associated CO2 emissions.
The properties of mechanically stabilised layers for a range of aggregate types and Tensar geogrid grades have been characterised by extensive testing. Design parameters have been defined and these have been validated by full-scale load testing and trafficking tests.
With suitable installation techniques, this technology can be used on extremely weak soils. Tensar can provide specific advice on the use of Tensar geogrids for mechanical stabilisation over weak and variable subgrade soils.
Soil stabilisation products
Lime
Available in most regions, lime is supplied in bags and bulk carrier. It has been used for decades to modify soil. A significant amount of research has been done to describe the mechanism and benefit of lime to soils. The primary application is subgrade stabilisation of wet clay soils. It is also used in lime columns and to modify clay fill for use in embankment construction. Typically mixed in-situ using specialised equipment. Lime stabilisation is a two-stage process. The initial stage takes a few hours or days in which the plasticity and workability of clay is improved. A second stage, over a much longer term sees an increase in soil strength.
Portland cement
Widely available globally in bags and in bulk deliveries. Cement is used for subgrade stabilisation of granular soils and for stabilisation of aggregate road base material, referred to as cement treated base (CTB). Subgrade stabilisation is typically conducted in-situ using specialised equipment, while the production of CTB usually takes place in a batching plant. The hydrated cement forms a matrix between particles binding the soil particles together.
Geogrids
Soil stabilisation using geogrids has been widely adopted since the early 1980s for soft ground problems across the globe. Geogrids are supplied in roll format, typically 4m wide and up to 100 m in length. There are several manufacturing types of geogrid available. The most common type used for soil stabilisation uses the punched and drawn manufacturing method. Made from polypropylene, these have stiff ribs and integral junctions. The grid structure can be biaxial (with rectangular apertures), triaxial (with triangular apertures) or multi-axial (with triangular, hexagonal, and trapezoidal apertures).
Tensar InterAx – is the best performing stabilisation geogrid that Tensar has developed. Visit our Tensar InterAx page to learn more about its proven performance.
Projects that may require soil stabilisation
Weak and variable ground can hamper progress on any construction project. Very often the first problem to be solved is how to get access and operate over extremely weak soils. The next issue may be how to protect the weak subgrade from excessive deformation and remoulding that would further reduce strength. The final problem may be how to improve bearing capacity to adequately support the construction above with acceptable settlements, during construction and in the longer term. These issues, in various forms, arise on the full range of construction projects.
Tensar mechanically stabilised layers have been used to solve these problems and more, across the globe for over 40 years.
Soil stabilisation in roads and highways
As an alternative to excavating and replacement of weak subgrades soils, a Tensar mechanically stabilised capping layer or subbase can be used to protect the subgrade and increase bearing capacity. This enables a reduction in the volume of imported capping/subbase materials.
Tensar geogrids are also used to stabilise the unbound pavement layers to increase strength and stiffness, improving support to the surfacing layers and increasing overall pavement life.
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Stabilisation in railways
Mechanical stabilisation of the sub-ballast (blanket layer) increases the strength and stiffness of the track foundation. Tensar stabilisation geogrids are used in track refurbishment and new track works to increase bearing capacity.
The migration and deterioration of track ballast can be improved by mechanical stabilisation of the ballast layer. Tensar stabilisation geogrids incorporated into, or below, the ballast layer can extend the period between regular maintenance.
The transition in stiffness between rigid foundations, such as culverts and bridgeworks, and the adjacent soil subgrade can result in differential settlements. Multiple layers of Tensar stabilisation geogrids are used to create stiffness transition zones to reduce differential settlement in these areas.
Soil stabilisation for housing & residential developments
In a typical residential development, roads are often the first and last construction activity. The road layout is put in place and constructed up to subbase level to enable quick and clean access for construction of the properties. The final road surfacing may not be put in place until all properties are completed. Where weak soils are present, the cost of excavation and replacement can be eliminated by adopting Tensar mechanically stabilised capping and/or subbase layers. In some cases, this may avoid the need to expose contaminated soils, multiplying the cost savings.
The overall pavement construction thickness of residential roads can be reduced by incorporating Tensar stabilisation geogrid in the unbound pavement layers. This may enable a reduction in excavation to the formation level as well as reduced imported aggregate costs. In some cases, the asphalt layers may be reduced in thickness without reducing pavement life.
Similar weak ground problems arise in commercial and industrial development sites. Tensar mechanically stabilised layers are often adopted as the solution.
Soil stabilisation for renewable energy projects
The development of onshore wind farms involves the construction of roadways to access the turbine locations. These are used during construction but are also needed for future turbine maintenance. These unsurfaced roads need to carry very high vehicle loads and tracked plant. Very often wind farm sites are located in areas of weak soil. Many in the UK are on moorland peat bogs. On these organic soils Tensar mechanically stabilised layers are used to create ‘floating roads,’ minimising the disruption to the peat bog and significantly reducing the volume of imported materials.
Crane access pads adjacent to the turbines need to carry high load with limited settlement. Tensar mechanically stabilised layers and in some cases the Tensartech® Stratum® system have been used for the foundation of these crane pads.
Stabilisation of airport pavements
Deformation of the base layer and subgrade below pavements leads to accelerated deterioration and reduction of pavement life. Tensar mechanically stabilised layers have been used in airports to strengthen the foundation of new runways and taxiways and reduce differential settlement.
What benefits can ground stabilisation unlock?
Ground stabilisation, or soil stabilisation, turns weak variable soils into workable engineering materials, enabling construction to proceed. By changing the characteristics of the soil or by capping weak soil layers, the bearing capacity is increased to a sufficient extent to carry construction and service loads without excessive deformation.
Reducing project costs
By avoiding the need to excavate and replace weak soils with imported fill, the cost of construction can be reduced. Soil stabilisation provides an improved and more consistent bearing capacity across the site. This allows the designer to adopt more economical designs for the work above. Pavement design need not be overly conservative to allow for subgrade variation. Rail trackbed design can reflect the improved bearing capacity leading to more economical design.
Increasing the profitability of projects
Contractors faced with gaining access over weak soils have traditionally placed thick fill layers to create an acceptable working surface for plant movements and storage areas. Project profitability relies on the construction team to get in, get the job done, and get out, as quickly as possible. Soil stabilisation can rapidly provide a strong working surface without the cost and time associated with importing large volumes of fill. The time saved can have a positive impact on project planning and overall project profitability.
Tensar mechanical stabilisation of unbound layers in the road pavement offers the opportunity to reduce overall pavement thickness, reducing both bound and unbound layers, without reducing service life. As a value engineered alternative to a non-stabilised pavement design this can offer major construction cost and time savings, increasing project profitability.
Meeting sustainability targets
By reducing the volume of imported fill materials and in some cases the export and disposal of unsuitable, sometimes contaminated soils, construction related CO2 emissions are reduced. This comes as a direct result of the reduced transportation, plus quarrying and compaction related emissions.
Climate change resilience
Soil stabilisation solutions are being employed where problems related to expansive soils, freeze/thaw cycles and loss of permafrost, are becoming increasingly prevalent due to climate change. Incorporating mechanical stabilisation into new construction provides greater resilience for the future where potential for these problems exists.
Next Steps
This guide has offered a comprehensive look at ground stabilisation, exploring why we need to stabilise soils and the methods and products that make it possible. Tensar offers a range of industry-leading soil stabilisation products suitable for various industries and markets, including:
Tensar’s design team can also produce a free of charge “Application Suggestion” to illustrate what Tensar can achieve and how much value can be added to your project. To do this, simply visit our design support page.