Tuesday, July 17, 2018

Retaining Walls - Part 2 - Construction Types



A retaining wall is not randomly constructed, but should be selected and designed to suit individual ground and site conditions

In my last article, I provided some rather dramatic images of retaining wall failures and demonstrated that design and consequently selection of an appropriate construction method is crucial. This is because there are many considerations that will affect the type, size and construction of a retaining wall which in most cases require the advice of a professional such as a Civil/Structural Engineer. Finally I provided a graphic showing the many pressures/forces that occur behind a retaining wall which can significantly influence the stability of a retaining wall. You can read my previous article from the following (Link).

Once thorough ground/site investigations have been completed there are a number of different types of retaining wall that may be selected. Basically, a retaining wall is not randomly constructed, but should be selected and designed to suit individual ground and site conditions. There are various types of retaining solutions such as embedded retaining walls (e.g sheet piling) and reinforced soil walls, however, for the purposes of this article I plan to focus on gravity retaining walls (also known as mass retaining walls) which rely solely on their own weight to stand up and withstand all of the loads and pressures that they will be exposed to. There a wide selection of gravity retaining wall solutions including Masonry (unreinforced), Concrete, (sometimes with masonry facing), Gabions and Crib retaining walls:

Masonry (unreinforced) Retaining Walls

These types of retaining wall are some of the earliest know retaining structures and are formed from masonry which has been mortared together. The lateral forces from backfill are resisted by the weight of wall itself, and due to their robust nature, they develop little or no tension. Masonry retaining walls are not usually reinforced and are generally economical for heights of up to approximately three metres. 

You will note from the image below that a masonry retaining wall will need to incorporate weepholes positioned at approximately two metre centres in order to help relieve hydrostatic pressure. Construction of a masonry  retaining wall is generally labour intensive, often requiring deep excavation, temporary support, construction and backfilling. Clearly this can provide significant safety issues, can be time consuming and often prove to be very expensive.

Source: Chudley R. & Greeno R (2005), Building Construction Handbook. Elsevier Butterworth Heinemann

Masonry Retaining Wall - Source: http://www.archiexpo.com/

Concrete Retaining Walls

Concrete retaining walls provide some flexibility in design as they allow wet concrete poured in-situ with the use of temporary timber formwork, or pre-cast concrete units to be used. Mass concrete retaining walls will be designed using similar principles described above for masonry retaining walls and will have the same safety, labour and cost issues.  Concrete retaining walls are typically designed with a horizontal footing and a vertical stem wall.These types of retaining wall are referred to as Concrete Cantilever retaining walls. These walls allow a much thinner stem and therefore less construction material because the cross section of the wall is much thinner. Concrete Cantilever walls utilize the weight of the backfill to provide most of the resistance to sliding and overturning, and because of this are economical up to heights of ten metres, much higher than the possibilities for unreinforced masonry retaining walls. The image below details a section through a Concrete Cantilever wall where you will note how the backfill load applied to the horizontal footing will help to stabilise the wall.  The further image below also demonstrates how concrete pre-cast units can be used in addition to in-situ for Concrete Cantilever retaining walls.

Source: Chudley R. & Greeno R (2005), Building Construction Handbook. Elsevier Butterworth Heinemann
Pre-cast Concrete Retaining Wall - Source: http://groundtechgeo.com.au/

A slight variation to the Concrete Cantilever wall is the use/installation of Monolithic Counterforts. Counterfort  walls are cantilever walls strengthened with counterforts monolithic (formed as a single unit) with the back of the wall slab and base slab. The counterforts act as tension stiffeners and connect the wall slab and the base to reduce the bending and shearing stresses. Counterforts are used for high walls with heights greater than 8 to 12 m. They are also used for situations where high lateral pressures occur, e.g. where the backfill is heavily surcharged.

Pre-Cast Concrete Counterfort Retaining Wall - Source: http://designerpropertiestexas.com/

Gabions

Gabions are welded wire or rectangular wire mesh boxes, which are then filled with rock, and used for construction of erosion control structures and to stabilize steep slopes. Gabion design is based upon mass gravity design as detailed above and therefore relies on the mass and self weight of the Gabions to resist any imposed loads or forces. Stability is achieved through a combination of the self weight of the rocks and its interlocking and frictional strength. Gabions are often constructed with stepped face and because of the voids between the rocks are able to more easily absorb forces by retained earth and flowing water. Generally Gabions are more economical than other retaining structures as they require little maintenance, no skilled labour, minimum foundation preparation and no costly drainage systems.

Gabion Retaining Wall - Source: http://terraqua-es.co.uk/erosion-control/woven-gabions/

Crib Retaining Walls

Crib walls are made up of interlocking individual ‘boxes’ which are usually consist of timber or pre-cast concrete (see image below). Crushed stone or other coarse granular materials to create a free draining structure are used to fill the individual boxes. Crib walls are popular because they are versatile, strong, light weight, cost effective, fast and easy to install. Another advantage of crib walls, due to their form, is that they can be further enhanced within built geo-textile planters, allowing a faster ‘green up’ time than with other retaining solutions. The advantage of using concrete instead of timber is that concrete components provide for long term durability and will not rot or warp, however the component cost is likely to be higher.

Gabion Retaining Wall - Source: http://terraqua-es.co.uk/erosion-control/woven-gabions/
Concrete Crib Retaining Wall - Source: http://www.retainingsolutions.com.au/
I have provided an overview of typical types of retaining wall solutions above however there are numerous other engineering solutions than may be used depending upon ground conditions, loadings, location and the like.  In order to select an appropriate retaining wall, in most cases it will be necessary to seek professional advice, as the consequences of poor or inappropriate design can prove to be disastrous.

Author: Gary O’Neill

Please feel free to share this article and other articles on this site with colleagues, friends and family who you think would be interested


Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog. 

Monday, July 9, 2018

Retaining Walls – Spectacular Collapses!



Sudden retaining wall collapse will be a result of the effect of forces/influences behind the wall and often inappropriate design. Not all damage to a retaining wall is as significant as the examples shown in this article and in fact much more subtle movement can occur over a long period of time before a retaining wall will eventually fail

Retaining walls are an important feature in many construction projects, often a necessity, as they can provide a solution where land is uneven or at a gradient. Very few construction sites are completely flat and where cut and fill site levelling is not possible it may be necessary to consider retaining walls as a way of maximising development space, or solving access issues within or around a construction site.

Wordnetweb define a retaining wall as ‘a wall that is built to resist lateral pressure (especially a wall built to prevent the advance of a mass of earth)’. There are many considerations that will affect the type, size and construction of a retaining wall which in most cases require the advice of a professional such as a Structural Engineer. This is because there are lots of pressures/forces occurring behind a retaining wall that can significantly influence the stability of a retaining wall. The photographs below show the dramatic impact of what can happen when retaining walls fail:

Source: wivinah-hefner.blogspot.co.uk

Source: James Lockyer Associates

Source: http://wqad.com/

Source: http://www.thestar.com.my/

Source: http://blogs.agu.org/

Source: http://www.nibiru-planetx.com/
Sudden retaining wall collapse will be a result of the effect of forces/influences behind the wall and often inappropriate design. Not all damage to a retaining wall is as significant as the examples above and in fact much more subtle movement can occur over a long period of time before a retaining wall will eventually fail. It is therefore worth understanding the function of a retaining wall as well as the forces/influences that require consideration during design.

I published an article a few weeks ago on basement construction where I considered the influence of ground water and particularly hydrostatic pressure on the basement structure (link). This is also a significant factor when designing retaining walls, which if not controlled can effectively push against the internal face of the wall and cause it to fail. So in addition to all of the forces that will potentially affect a retaining wall, other factors for consideration will include the type and nature of the soil, the types of wall construction to be used, materials to be used in the wall construction, ease and speed of construction, the provision of expansion joints, health & safety issues and obviously cost! As you can imagine there is an awful lot to think about and it is no surprise that professional advice may be needed. The diagram below shows a range of different forces that require consideration during the design of a retaining wall:

Source:Own

In my next article, I will discuss retaining wall construction and consider different types of retaining wall.


Author: Gary O’Neill

Please feel free to share this article and other articles on this site with colleagues, friends and family who you think would be interested

Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog.