Down to content

What is a retaining wall?

A retaining wall is a wall that holds back earth or water on one side of it.

Above ground, retaining walls are vertical, or near-vertical, structures designed to create level areas on sloping sites to maximise development space; to create terraces for infrastructure to run along slopes; and to provide additional support to natural slopes at risk of failure. They are also used to form the walls of basements, underground car parks and metro stations, in towns and cities.

How does a retaining wall work?

Retaining walls are designed to restrain soil, or engineering fill, at an angle steeper than the material’s angle of repose – the steepest angle it can hold naturally, without failing. To do this, they need to be able to withstand the horizontal – or lateral – earth pressure, exerted by the material being retained.

Lateral earth pressure is dependent on the vertical stress imposed by the material behind the wall, which is a function of the height of backfill and its density. It follows then, that the greatest lateral earth pressure is exerted at the base of the wall, because the deeper the backfill, the higher the vertical stress.

There are three types of lateral earth pressure:

  • Earth pressure at rest: When the wall is ‘at rest’ and the backfill has not experienced significant post-construction movements
  • Active earth pressure: If the wall moves away from the backfill, the lateral earth pressure falls until it reaches a minimum. This is the active earth pressure – beyond that point, failure will occur
  • Passive earth pressure: If the wall moves into the backfill, pressure increases until it reaches a maximum value, equal to the maximum resistance of the soil – this is the passive earth pressure. In embedded walls, typically used for basements, passive earth pressure is an important factor in stability (but more of that later).

Types of retaining wall.

Retaining walls come in all types, shapes and sizes – from simple gravity walls to bored pile walls for basements and reinforced soil walls using geogrids – to suit a wide range of project needs, and site conditions.

Gravity walls

Gravity walls are the simplest, and earliest recorded, type of retaining wall. Built of masonry, brick, concrete blocks or mass cast-in-situ concrete, these hard-wearing structures rely on their large weight to resist toppling and sliding caused by the lateral earth pressure from the soil behind them.

Gravity walls are typically wider at their base, with sloped faces, enabling them to resist the higher lateral earth pressures at depth. This means that, while they are easy to build and suitable for retained heights of up to about 3m, any higher and they tend to take up too much space and can end up being too heavy for the ground below, leading to bearing capacity failure. This can lead to failure of the soil being retained by the wall.

Cantilever retaining walls

Cantilever walls are built using reinforced concrete, with an L-shaped, or inverted T-shaped, foundation. The vertical stress behind the wall is transferred onto the foundation, preventing toppling due to lateral earth pressure from the same soil mass.

Additionally, a T-shaped foundation benefits from the weight of soil (and therefore vertical stress) in front of the wall, providing further stability. Foundations sometimes include a ‘key’ in their base, that sticks into the ground to prevent sliding failure.

A big advantage of cantilever walls is that they take up little space once built, and are suitable for retained heights of up to 5m. However, construction does require space behind the wall, so they are not particularly suited to retaining existing slopes, unless temporary support is provided during construction.

Embedded retaining walls

Embedded retaining walls are used to form near-surface underground structures, such as basements, car parks and metro stations. Walls can be huge – those for Westminster underground station on the Jubilee Line in London, next to the Houses of Parliament, are 40m deep, for example.

They are built using a number of different methods, depending on ground conditions, how watertight the excavation has to be, constructability (ie time, cost and excavation method) and the retained depth required.

For deep excavations, methods include diaphragm walls and panels, as well as bored concrete piles, where piles are either interlocking (secant) or installed next to one another (contiguous). For shallow and temporary excavations, sheet piles and king post walls are commonly used, as shown in the image above.

Embedded retaining walls act like cantilever walls, extending deeper than the excavation to take advantage of passive earth pressure of the ground below to, at least partly, counteract the active earth pressure being exerted on the wall above. Additional support is provided by internal propping – usually from the base slab, ground slab and any intermediate floor slabs – or by ground anchors installed through the wall.

Reinforced soil, or mechanically stabilised earth, retaining walls

Reinforced soil walls, sometimes referred to as mechanically-stabilised earth walls, use layers of geogrid to reinforce soil, which are mechanically connected to a range of facings, including precast concrete blocks and panels, gabions and crib walls, depending on the aesthetic requirements of the project.

The benefits of geogrid reinforced soil walls

Load-bearing reinforced soil walls can cut construction costs by up to 75% and halve build times compared with traditional solutions, while being robust and low-maintenance.

Reinforced (or mechanically-stabilised) soil is becoming a standard way of forming cost-effective walls and bridge abutments on roads and railways, instead of the more traditional options that frequently involve piling and reinforced concrete.

The approach uses layers of geogrid to reinforce soil, increasing bearing capacity and increasing resistance to differential settlement. Reinforced soil structures have lower bearing pressures, which can eliminate the need for expensive foundations.

A big advantage of using geogrids, particularly the HDPE geogrid Tensar manufactures – is that they work with a huge range of materials, opening up the possibility of using marginal fill (including selected site-won fill) and waste products such as pulverised fuel ash (PFA). This is a great way to reduce the environmental impact of a retaining wall; plus it can save time and money.

Design guidance for retaining walls

Retaining walls are designed to withstand the active earth pressure, with a factor of safety, and the start point is to analyse how the wall might fail. The main types of failure are:

  • Toppling, or overturning, caused by lateral earth pressure increasing to a point where it is a higher than the wall can resist
  • Bearing capacity failure of the ground beneath the wall
  • Sliding, when there is insufficient friction (or sliding resistance) at the base of the wall
  • Internal failure, where lateral earth pressure increases to the point where structural elements (blocks, panels, brickwork etc) of the wall fail.

A retaining wall can have design life of 100 years, so it is important to consider any future plans (as far as possible) for the site: Is something going to be built on ground above the wall? Will construction materials be stored temporarily there? If so, the design needs to account for a higher vertical stress, and therefore higher lateral earth pressures, to ensure the wall remains stable

Designing reinforced soil structures for the long-term

Assessing the long term performance of geogrid used in reinforced soil walls and slopes is crucial, with some structures having a design life of up to 120 years.

The polymers used in geogrids are viscoelastic, which means their strength and stiffness are affected by temperature and how frequently or how long they bear a load. Therefore, Tensar test the creep strength of their geogrids by subjecting them to long-term loading. Creep strength is used to calculate the long term design strength (LTDS), which is the predicted strength of the geogrid at the end of its serviceable life (for example, 120 years). Note that partial reduction factors - including the effect of chemical degradation, biological degradation, soil temperature and installation damage are applied to the creep strength in LTDS calculations.

Drainage for Retaining walls

Drainage is a key consideration when designing earthworks, including reinforced soil structures, using compacted clay fill.

It is very important to consider negative pore pressure when designing earthworks, including reinforced soil structures, particularly when using well-compacted clay fill.

Clay fill compacted to a normal earthworks specification is likely to have negative pore pressure up to considerable heights. Pore pressures are only likely to become positive at the base of very high structures (more than 10m to 15m high), or at lower heights, if the clay was on the wet side and therefore soft during placement.

Suction is typically ignored in design and so provides an additional margin against failure or poor performance of the structure. As a result, it is a good idea to maintain suction in the long term.

Internal slope drainage must let water drain out and must not let water in and be designed to intercept groundwater flow and be free-draining, allowing water to drain out easily and staying ‘dry’ most of the time. It should also not ‘daylight’ at the upper surface of the structure, preventing run-off from flowing into the fill (run-off should be handled by surface water drains, to avoid ponding).

Any movement of a soil structure has the potential to disrupt, or even reverse, the fall of a drain, which could result in water flowing back in – something worth considering when specifying maintenance regimes.

Choosing the right retaining wall

There are many factors that can influence a decision when selecting the most suitable retaining wall solution for a project. Here we look at the five most important. Understanding and defining each of these at the early stages of a project will lead to the most economic and appropriate choices.

  • Aesthetics
  • Design life (Durability)
  • Footprint - available space for construction (face angle)
  • Geotechnical considerations (soil types and foundation)
  • Budgetary constraints

Aesthetics

What is appropriate for the location and environment? Are there adjacent structures or features that need to be considered? Is the new structure going to be prominent and highly visible to the public? Would a green faced (vegetated) structure enhance the local environment or be out of place? Would a high vertical structure be intimidating to pedestrians as opposed to a stepped face or steeply sloping one.

It will be possible to select a type of structure that will be sympathetic to the location and enhance the local environment where appropriate.

Design Life

How long must the structure last? Are all components sufficiently durable? What level of inspection and maintenance is appropriate and likely to be provided throughout the life of the structure? How will the structure eventually be dismantled and disposed with minimum disruption and cost? If the structure is temporary, has the design fully recognised this to deliver minimum costs – both construction and removal. Is the structure exposed to vandalism and graffiti?

Designing the structure to meet the required service life, considering the durability of all components is of course crucial. Certification of systems by recognised bodies such as BBA can provide valuable confirmation of the suitability of methods and materials.

Footprint– Available space for construction

What are the space constraints – during construction and for the completed structure? Is there an economic or practical benefit to be gained by minimising the footprint or area taken by the structure? Is there sufficient space to enable use of a steep slope at lower cost to a vertical wall? Is there space for a crane and larger plant during construction or is access an issue – favouring solutions that require only smaller plant and equipment? Is the structure supporting an embankment or an excavation – is over-excavation possible to facilitate construction of certain structure types?

By carefully examining site specific constraints and the best use of space, a solution can be adopted that best meets the needs of all stakeholders.

Geotechnical considerations

What are the foundation soil conditions? Piled foundations may be needed for certain structure types – is this cost effective - can it be avoided? Is groundwater going to be an issue - how will this be dealt with? Are there suitable structural fill materials available within the site or locally from recycled materials – can these be utilised?

Clearly, adequate geotechnical information is crucial to developing the most appropriate and cost-efficient design.

Budgetary constraints

Generally, steep slope structures can be built at lower cost than vertical walls. Structures requiring piled or rigid foundations are more costly that those requiring no foundation. Hard faced structures using concrete or masonry are more expensive to build than ‘green’ faced or flexible faced structures. Depending on type and location, some level of maintenance may be required for ‘green’ faced structures.

The construction cost of different retaining wall solutions varies widely, it is important to examine the alternatives and determine the most appropriate lifetime cost that meets all the above constraints.

TensarTech wall and slope systems

With 30 years of construction knowledge, design experience and innovative geogrid products, Tensar’sTensarTech permanent and temporary retaining wall and slope systems provide a number of facing types and construction options to suit the structure’s end use, location and required design life.

Our retaining wall solutions include precast concrete, dry-laid modular block systems (with the option of adding architectural, masonry or brick finishes); precast concrete panel systems; gabion and crib walls; and robust units suitable for aggressive marine environments. Our reinforced soil slope solutions can create vegetated slopes with angles of up to 70˚.

Tensar Case study - Enabling safe construction of load-bearing bridge abutments on the A21

Our TensarTech TW3 modular block reinforced soil system was the ideal solution for building the load bearing bridge abutments and wing walls for overbridges at two new grade separated junctions between Tonbridge and Pembury on the A21 in Kent.

We worked with WSP to design the retaining structures for main contractor Balfour Beatty. The abutments were built using Department of Transport Type 6I/J aggregate, designed to meet bank seat loads of up to 566kN/m and to resist horizontal loads of up to 54kN. TW3 was used to build a total of 194 linear metres of abutments and 80 linear metres of wing walls up to 7.6m high. It was also used to build a 60m long, 3.6m high retaining wall on the route.

Aside from technical performance, Balfour Beatty chose TW3 because traffic flow had to be maintained throughout the works. TW3’s modularity meant it could be built in the limited space available, without heavy lifting equipment or propping, and with minimal disruption to road users.

Read the full case study here

Tensar Case study - Providing robust support to the A14 improvements

Tensar’s reinforced soil systems were used extensively on Highways England’s £1.5bn A14 Cambridge toHuntingdonimprovement scheme, delivering permanent structures for junction improvements and load bearing abutments for temporary bridges.

As well as widening and improvements to 34km of the A14, the improvement scheme included the new 20km Huntingdon Southern Bypass, to take traffic off the A14, plus improvements to the A1(M), Huntingdon town centre and local roads, with better connections for horse riders, cyclists, and pedestrians.

Tensar was involved from the very start, working with the A14 Integrated Delivery Team, a joint venture of contractors Balfour Beatty, Skanska and Costain, plus consultants Atkins and Jacobs.

Read more about our work on the A14 improvement scheme here

TensarSoil Design Software

Our TensarSoil design program allows design engineers to produce designs for reinforced soil walls, slopes and bridge abutments.

By using the design programs offered by Tensar, design engineers can save valuable time by generating designs in-house. To assist the design engineer with this process, the Tensar Design and Technical Support team is available to give advice and offer a design checking service.

TensarSoil Software allows you to Design Temporary and Permanent works including:

  • Reinforced Soil Walls
  • Slopes
  • Bridge Abutments

Request our TensarSoil Software here