) We hope this section provides some useful background information to prospective clients. Not all the information here may be applicable, or, all encompassing for your particular situation. (I think that is called a Disclaimer). The details of any particular wall are normally worked out by your contractor (the “Stonemason”), the City or Shire Council and a suitably qualified Engineer. Sometimes Land Surveyors are also involved. In short, we have tried to keep the information here Generic. It is largely based on our experience and reading of just about every book we can find on the craft of building retaining walls.
Sometimes the information available to our clients is filled with terminology and language that is not easily understood by the layman. I have attempted to reduce the technical aspects to more easily understood terms. However; some “stoney jargon” and explanation of applicable building terms is often helpful. At the appropriate point a Definition may be inserted, explanation by way of example or a Question and Answer format is provided. The authors penchant for a bit of Humour and Sarcasm may also be accommodated in these pages. 
Stoneworkers are generally requested to build walls for three purposes:
1.) Retaining Walls - to hold back earth where a change in level is required
2.) Free Standing Walls – most commonly as a fence, division screen or garden structure. You will also find examples of free standing walls in parks, public places and as part of monuments.
3.) Load Bearing Walls – to support a vertical load such as walls for houses, sheds, garages or as footings for the construction of houses or other buildings.
It is common for the purpose of a wall to be two-fold i.e. a retaining wall may also be accommodating a vertical load. The important thing to remember is that a wall is designed and constructed for a specific purpose and adding a load without proper investigation may exceed the capability of the wall.
We often further catergorise walls based on aesthetics. There are those walls, and those clients, for which appearance is of no great significance. These walls do their fundamental task and are walked past without a second glance. The other type is where the wall must also serve to enhance or compliment its surrounds. Even the most basic of retaining walls can, with care, take on a pleasing appearance. Our section on “Stone” discusses appearance in more detail.
Surcharge: Term commonly used when referring to retaining walls. The Surcharge for a retaining wall is any external load applied to the wall, other than the basic retaining task (holding back the soil). Most commonly, this is loads applied by fencing mounted directly on top of the wall, vehicle/driveway loads and building loads. If a building is in close proximity to a retaining wall an Engineer will determine the surcharge and design the wall accordingly. Never underestimate the Surcharge to a retaining wall by wind acting on a fence mounted to the wall. Just ask a yachtsman how much power can be generated from a few square metres of sail and then consider the windage area of a backyard fence.
There are numerous different types of retaining walls and even more methods and materials of construction. It is not the purpose of this article to provide the ultimate definitions but to simply examine some of those types available and gain a clearer understanding of the physical aspects of retaining wall. We will look at the following wall types:
- Mass Retaining Wall (MRW)
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Cantilever Wall
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Soil Stabilised and Segmental Retaining Walls (SRW)
- Concrete Post & Panel Type Construction
Mass Retaining Wall (MRW): In Western Australia these are the most common system of block retaining walls seen in Residential and Commercial applications. Usually of a limestone product, I generally refer to this as an “all balls no brains” approach to retaining. This is somewhat besmirching the designers and builders of these structures but it certainly worked for the Ancient Egyptians. The Mass Wall can be of Mortared or “Dry Stack” construction (No mortar) with the former being far and away the most common here in the West. Mass Walls have traditionally been constructed with whatever stone was available locally. The principle remains the same the world over although the techniques and materials will vary.
If we look at the design in its most basic form the name (and my crude description) becomes clearer. The wall is of a mass equal to or greater than the sum of all loads/surcharges applied. The higher the wall, the more mass is applied. A good description of the fundamental design might be a pyramid cut vertically in half through its apex.
In practice MRW constructed of cut stone or fabricated blocks will have “backers” (backing blocks) installed behind the wall. See Figure 1 below. The amount of backers is proportionate to the height and surcharge and simply increases the mass of the wall. Engineers will then apply various physical methods to further enhance the structural capability of the wall. Mass walls will have the lowest course(s) of blocks in the ground such that the bottom of the wall effectively forms its own footing. Engineers may specify various blocks be laid at right angles to the line of the wall effectively “cross-bonding” the blocks. These cross-bonding blocks are often called “headers” or “soldiers”.
Q. How come you didn’t pour a concrete footing for the retaining wall?
A. Limestone Block MRW have the first course(s) imbedded in the ground to a depth determined by the height of the wall. The burying of these blocks in the ground effectively forms the footing for the wall. (see also our section on Compaction under the page Stonework Stuff). It would seem that there is some sort of perception that a poured concrete footing somehow glues itself to the core of the earth. Unfortunately this is not the case and over reliance on the stability of a poured footing can have disastrous results. Free standing feature walls, such as those used for water features, or stone wall fences will usually have a poured concrete footing. Where possible, natural limestone free standing walls should have the footing constructed from the same stone as which the wall is going to be constructed.
The main advantage of the MRW is that designers can easily “beef up” the wall to accommodate extra surcharge by adding more mass. Here in Western Australia the various types of limestone block are readily available and, on a “pound for pound” basis, it is a relatively cheap building product. That said, due to the large mass required at greater wall heights, the mass wall can, in some cases, become uneconomical when compared to segmental walls. More about that later.
Another advantage of the Mass Wall is it will generally allow for fencing to be core drilled and mounted directly to the top of the wall. See Imagine X.
A properly constructed Mass Retaining Wall should well and truly outlive the poor decrepit stoneworkers who took the time to build it properly. A reconstituted limestone block retaining wall of 1.8 metres height will weigh in excess of 2.1 tonnes per lineal metre. That’s a solid wall in anybody’s terms. Imagine the surprised look on a client’s face when I told him that his “little” 500mm high garden wall stretching around his backyard amounted to 13 tonnes of stone and that didn’t include the mortar.
The sheer weight of the MRW has its advantage in its unrivalled durability. A Mass Wall can suffer many fractures and all sorts of damage and still carry on doing its job. I once had a Front End Loader backfilling a large commercial wall we had only completed the week before. The operator managed to reverse his 14 tonne machine into the wall with enough force for me to feel the ground shake some 100 metres away. The wall survived unscathed save some chipping damage on face stonework. Not one crack was to be seen. Mass Stone walls have survived centuries.
The disadvantages of the Mass Wall System become clear when you see this type of wall being constructed. The blocks are heavy, up ¼ tonne for some 1 metre blocks. These require machinery to transport them into site and then again to lay each block. You wouldn’t believe the number of home owners who have told me how they built a small wall themselves by hand. The phrase “never again” inevitably follows such stories. Even the common 500mm long blocks will weigh from 42kg to 76kg. In some cases, where access is restricted for machinery, manual labour to get the blocks to where the wall is being constructed can be as costly and time consuming as constructing the wall itself.
Another characteristic of the Mass Wall that requires consideration is the space that it occupies when built and size of the excavation required in order to build the wall. Figure X below shows a typical excavation and site prep for a 4 course wall which gives a net retaining of about 1200mm.
Nett Retaining: The actual above ground retaining height, not including that portion of the wall below the mean ground level (buried in the ground).
Cantilever Walls: The principle of the cantilever design can be seen in most types of retaining walls, perhaps with the exception of the post or piling type construction. Figure 2 below shows the basic “L” type construction. The mass of the backfill material (material immediately behind the wall) acts to prevent the wall “racking” or rolling over.
Most commonly used in commercial applications the cantilever wall is usually constructed of pre-cast steel re-enforced concrete. However, figure 3 shows how the cantilever system can be used in Mass Brick construction. I often hear Mass Brick referred to as a type of wall. It is not!. Mass Brick is a method of construction only effective when the brickwork is tied by steel to the bottom leg of the “L” effectively forming a Cantilever. In other words it’s really a Cantilever wall.
Brick laid on a basic concrete strip footing (See Figure 3a) is not an effective retaining wall at anything over about 500mm in height. This can be evidence by all the fractured and leaning brick retaining in almost any West Australian suburb that has a few hills. It lacks both the Mass of the Mass Retaining System and the mechanical capability of the Cantilever Wall. I make a good living pulling out failed brick retaining.
This is not to say that house bricks are a poor product rather that it is the application and method of construction that is incorrect. Typical brick construction is designed for vertical loads and has a very limited capacity to accept horizontal loads. If you are the owner of a basic brick constructed retaining wall that is standing the test of time….good luck to you, but it is probably more testament to the skill of the bricklayer than selection of a suitable wall design.
Soil Stabilised and Segmental Retaining Walls (SRW): A system that has been very popular in many countries and in Australia’s Eastern States for some decades. The Segmental Wall is rarely seen in Western Australia. Consisting of interlocking cavity blocks with a region of stabilised soil behind, the system might be considered a mechanical interlocking method in so far as the Segmental Blocks are concerned but this is only part of the system. For the SRW to achieve heights above 600-800mm, soil stabilisation is required. Soil stabilisation might be described as complicated backfill. Figure X below shows a typical example of a soil stabilised Segmental Walls. Imagine X shows a typical Segmental Wall constructed for commercial property development.
One of the main advantages of the SRW is that is available in different colours. The irony of the situation here in Western Australia is the wall is not so popular partly due to the lack of colour choices by comparison to the Eastern States. Sometimes only two or three colours are available in WA (which is more than limestone). However, the reason why the manufacturers don’t have more colour choices available is because the wall is not so popular….you’ve heard of the chicken and egg story haven’t you???
A specific advantage of the SRW is that once the blocks are on-site heavy lifting is avoided. Most of the blocks weigh around the 8-12kg range meaning they can easily be laid by hand. However, some machine work is often still required to bring in aggregate and backfill material in order to achieve the stabilisation. Careful compaction in stages is also required. Often the engineer will specify “Geo-Grid” in the construction as shown in Image X.
In very general terms the Soil Stabilised SRW is more expensive than a Mass Wall below about 1.8 metres in height but above this level it can achieve considerable cost savings. A significant cost is involved in the Soil Stabilisation and my personal thoughts are this is because the product suffers from “Easternstateitis”. My reasons for this can be found in the specifications for the stabilisation. It is very heavily biased by construction in poor draining clay type soils where water (hydrostatic pressure) often travels horizontally rather than vertically as in our free draining sand country of the West.
This is probably a result of the products North American origins. It is difficult to get an Engineer to sign off on a SRW wall design that is less onerous than the manufacturer recommends. I think this is a shame as the product offers greater flexibility and speed of construction. Some types of SRW available in Western Australia are the Boral Keystone and the Allan Block. Some fantastic SRW can be seen around the country on road embankments and freeway overpasses.
Concrete Post & Panel Retaining: Often referred to by the brand name “Twinside” post & panel consists of pre-cast slotted posts which are concreted into the ground. Concrete panels are then slid into the slots between the posts to provide the retaining mechanism. In my experience post & panel is a good system that has its place when properly installed. It is at its most useful in restricted access sites. It generally requires less earthworks to prepare than other walls and it is relatively easy to achieve retaining up to 900mm. Above this height the system requires some engineering and it has been our experience that it often lacks the capacity for any significant surcharge.
The most common cause of failures in post & panel construction is in the application and workmanship. A post & panel wall installed with half a bag of rapid set (concrete) on each post is doomed to failure, as it is when installed on sloping sites with very loose sand. In my opinion Post & Panel is a very economical method of achieving retaining heights of 900mm and less….if you can stand the appearance.
There are numerous other types of retaining walls, methods of construction, and variations of above systems. Back anchored walls, stone pitching and sheet piling are just a few. We have briefly covered the types we most commonly encounter however, modern technology is always coming with something new……….but most often it seems the basic principles never change. A good stoneworker will be capable and fluent with two or three types of retaining wall construction.