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