Why do Retaining Walls Fracture or Fail?
From time to time we are consulted on retaining wall failures and are often asked to conduct repairs.
The purpose of this section is to examine some of the basics of stone wall construction such that structural problems can be reduced or averted. It is biased toward Mass Limestone Retaining Walls and free-standing stone walls but can be equally applicable to most wall types.
When choosing a contractor, designer or simply planning a retaining (or other) wall a good general knowledge of wall failures can help you sort the “wheat from the chaff” so to speak.
Advice given here is generic in nature and based on our own research and experience. You should always consult an engineer for expert advice on your own particular situation. Information contained herein is compiled by Victor Taylor, Director of Cockburn Stoneworks Pty Ltd and Founder of costone.com.au A list of recommended technical references and resources is listed on the Further Information Page.
In Western Australia's sandy terrain it has been our experience that the following factors commonly figure in wall failures:
- Uneven or Inadequate Compaction:
- Mortar Failure (Poor workmanship)
- Incorrect Design or Overloading
- External Damage or Other Factors
Illustration: This original wall fractured at the corner and rotated forward to leave the large gap shown (Thats my size 13). The wall had effectively been raised in two sections under various owners. This additional surcharge was clearly beyond the original design intent with resultant catastrophic failure. The wall was in a dangerous state and had to be completely demolished. A new wall was designed and built to cope with the additional height.
In addition to the above; Factors that arise in Clay soils are:
- Hydrostatic Pressure - the build up of, or persistent, water behind the wall
- The "heaving" that occurs between very wet and very dry clays in our climate
Footing Preparation, Compaction & The Stonewall Builder
Footings
One of the first steps in any significant building or construction is a soil analysis (normally arranged by the builder). The resultant report allows engineers to stipulate the necessary preparations required for retaining and building footings. In the main, it is the more unusual situations that bring special requirements for footings. This may be removal of resident soils, special drainage or high degrees of compaction.
For most residential situations in Western Australian coastal sands the stone wall builder can follow these rules of thumbs: The wall footing area should be over excavated and raked clean to ensure:
- It is free of any organic matter which can decompose and thus destabilise the wall. (A good rule is not to have anything thicker than a pencil). This includes matted grass, roots, timber etc. The scourge of retaining wall builders is the palm tree. Palm roots are numerous and small making a fibrous mat that is impossible to compact properly without removing the roots.
- It is free of voids. Old septic tanks, leach drains and rubbish holes are common traps for the unwary.
- It is free of significant clay and suitably above the water table. For clay soils special preparations and drainage are required. This matter is discussed further below.
- It is free of rocks, bricks, metal objects and loose stones which will unsettle the bottom course of stone or otherwise provide hard nodes on which stone is laid. (nothing bigger than an apple).
- The footing does not bear on or contact any pipe, conduit or service duct nor should it attach to any other independent structure such as a building or neighbours wall.
Illustration : This site had it all. A failed brick retaining wall and fence with "some" crude concrete footings, palm stumps, old sleepers, resident rock and plastic soakwells that caved in during the job. All in a days work for the stone wall builder.
Tips for Footing Preparations
- Underground Services: Always Dial-Before-You Dig and determine the location of underground services. Never assume the area is free of underground services....if in doubt, there is a new invention designed to help locate services, its called a shovel. Service piping and conduits (phone, sewer, water, gas, electricity etc.) may well dictate modifications be made to the wall or that the service be relocated. You should discuss this eventuality with your wall builder prior to commencement of work.
Remember, The Dial-B4-U-Dig plans rarely provide detailed information about the actual physical location of services inside private property. If in doubt, use a shovel!. Commercial service location contractors can also assist with specialist location equipment but this is not often viable nor available for residential work.
- The wall builder cannot quote on what can't be seen. Your quotation/contract should always have an explanation or clause covering “hard digging”. Hard digging includes extra digging effort required when ground is harder or more persistent than the resident ground. This includes rock, hard clay, “coffee rock” stumps or buried debris such as concrete. Also bear in mind that such material if removed from the footings area may have to be transported away and disposed which would incur additional costs. We once had the misfortune to dig up a Volkswagen Kombi Van.....it was checked for backpackers and hippies before being sent to the recyclers.
- Planning: Footings for some walls may be as simple as a hand dug trench, or they may involve the use of machinery to shift hundreds of cubic metres of soil. Good planning is essential to maintaining the flow of work and ensuring the site remains safe. Stone and mortar mixing should be as close to their end location as possible.
- Property Boundary: If the wall concerned is a boundary wall you will need (at minimum) to do two things:
- Accurately locate the boundary line by finding the survey pegs (should they still exist) or obtaining a new survey. The stone wall builder is not an expert surveyor. Location of walls in relation to boundaries is a whole can of worms to open, suffice to say here that; Recent changes to laws regarding duty of disclosure at the time of a property sale mean that you don't want to get your boundary line wrong. Councils may insist on a new survey before processing an application for development.
- Consult your neighbour as to how the works will affect their property and formulate a plan to deal with any of their specific needs or concerns. An angry neighbour is no help to building a good wall.
Compaction (2nd Favourite Topic)
What is compaction? There is a lot of technical information, reference material and definitions on compaction available, well beyond the scope of this website. For the layman (and in plain speak) compaction can be described of the fitting of soil particles together so as to remove the air. Thus, soil which has acheived 95% compaction comprises only 5% of voids. Generally this will require the application of mechanical energy and H2O or more succintly, a suitable compactor and some water!. Water alone will not acheive deep compaction. Water is required to lubricate the particles such as they slide together and fill the voids during compaction.
Illustration: A Vertical Rammer (Foreground) and a Plate Compactor in use preparing a footing for a boundary retaining wall, fence and house footings within 1 metre of the wall. Note the sewer pipes at the right. The ground here required many hours extra compacting and water to acheive the specified number of blows on the Penetrometer.
Generally, two types of compactors are used for preparing building and wall footings; These being:
- Vibratory plate compactors (medium to high frequency - around 60Hz), and
- Vertical Rammers, (Low frequency - around 12-15 Hz) sometimes called Jumpin Jacks...Ours is called "Dolores"....she has a mind of her own.
Now, so far as compacting sands, your typical 70 - 80 kg small plate compactor, much favoured by brick paving and landscape contractors, is just not going to do the job. Whilst vibrating plate compactors are often referred to by their mass (kg) it is the Kilonewtons (kN) of force which should be the determining factor. For example, your typical 75kg small plate compactor will lay down about 15kN which, in sand, will barely provide much compaction past about 150mm depth. For most small residential walls we need to be acheiving compaction in the zone of 150mm to 450mm deep. My smallest vibrating plate compactor is rated at 35kN and it will acheive this compaction when moisture and soil conditions are favourable. If the soil contains lots of clay or organic matter then less depth will be acheived. As a wall height increases engineers will specify deeper levels of compaction be tested (450 - 750mm and deeper). For this larger compactors (50kN +) and compaction in lifts (layers) will often be required.
Engineers will specify a level of compaction for a given site. The compaction may be expressed as a percentage (98% typically). More typically for WA Sands the compaction requirement will be stated in "blows" for a Perth Sand Penetrometer........sounds complicated but really its an age old test. A penetrometer is akin to a specifically weigthed and calibrated slide hammer which, when correctly operated, will provide a good measure of the compaction. Most commonly the specifications calls for around 6 to 8 blows per 300mm tested. The first 150mm is not tested with the Perth Sand Penetrometer. Remember, the feel underfoot is not a measure of compaction. I have had sites on which semi-trailers have been driving on so I could load them...but 300mm under the surface crust the sand would not hold 2 blows on the penetrometer. Not a good result for building a 3 storey house on the side of a sandhill.
Even if you have no specifiied compaction requirement your wall builder should still be testing and/or compacting your footings. A penetrometer is very cheap to hire and they can be purchased for less then $500. Good insurance if you ask me. Unfortunatley, current practice results in a lot of (residential) walls being built without any consideration for compaction whatsoever. The fact that many walls show no ill effects from lack of compaction is immaterial. This is more testament to the inherent strength of Mass Retaining Walls and the fact that the wall builder is generally not around 5 or 10 years later when the wall eventually starts to fail.
Additionally, the compaction specification itself is deficient.....In my Humble opinion!. So why would we (most humble poor broken down stoneworkers) claim the Engineer's have got it all wrong. Well, they haven't really its just we need to make practical sense of the specification and apply some common sense. To understand this we should get our heads around the fundamentals of a MRW.
For example; Consider a 30 metre long wall of 6 courses (Reconstituted Limestone). This wall will be about 1.8m high (nett retaining) and comprise of anywhere up to 15 x 1m blocks per lineal metre. Thats a total Mass exceeding 80 tonnes and ground pressure of 2.8 to 3.4 tonnes per m2 (or around 32 kPa) depending on the wall design and stone used. To keep this in context most residential walls are designed to withstand around 2 to 5 kPa additional surcharge loading (loads applied by vehicles, buildings, soil build up etc in the zone behind the wall as wide as the wall is high). Thus, the most significant load ever likely to be applied to the footing is the wall itself and the weight of the back-fill material beyond the angle of repose. Ok Ok..plain language, one of the biggest threats to the wall is the weight of the wall itself. This is the reason why (as in most cases) good preparations are the key to building good wall - walls that will last for centuries.
Now lets consider our 30 m long and 80 odd tonne mass wall as a beam. Hold a ruler or stick out in front of you in the horizontal plain. It obvious that if you place a hard node/fulcrum (a finger will do) anywhere under the ruler, the wall will tend to fall away in one direction or another. Hard compaction at any one point under a wall with softer areas elsewhere will place stress on the wall. Most important is to consider this in section (looking from the end of the wall). I have looked at literally hundreds of walls which are rotating forward (leaning) from the loads applied, and more often than not, poor compaction at the foot/front of the wall.
Thus what we really need to add to our compaction specification is a limit on scatter, or range. A footing that technically passes the basic compaction specification is not necassarily a good footing. To be honest, in preparing wall footings I am more interested in how even the compaction is, from one end to the other, rather than the actual number of blows. This is not rocket science. It stands to reason that if our 30m long wall has a compaction of 10 blows plus in the middle (which meets the specification) and 6 blows at either end, it will, over the years, tend to settle and sag about the ends and probably fail to some degree.
I never cease to be amazed at the number of stone wall builders that do not own or use compaction equipment. Whilst the machinery required is expensive, once purchased it provides years of service and more importantly a huge amount of confidence in soundness of construction. An engineer once said to me “why construct a $15,000 retaining wall and forsake $500 worth of compaction”. Of course it was a rhetorical question, but why would you omit such a process?. Well, outwardly it would appear that not all of history is on my side, although closer examination reveals a different story. The subject is well worth examining. (Just bear with us..)
When I talk with other “stoneys” about compaction invariably someone will point out that modern vibratory compaction equipment was not available to the stonemasons of old. “What about the old-timers!”. Walls built 200 odd years ago are still standing strong. Many Inca terrace walls still stand strong and have done for nearly 600 years. In Israel and Turkey walls can be found that are older than anybody can remember. On the shores of the North Sea in Scotland, and the North Atlantic in Ireland, centuries old stone structures stand testament to ancient practice......and those Stonemasons didn't have Vibratory Plate Compactors!. What they did have, is time, lots of knowledge and great skills. They new about the problems that could occur with poorly prepared footings.
So why be concerned about compacting footings. Well, its fairly simple. Almost without fail, all those old retaining wall in Europe, South America etc that still stand strong, are not built on coarse even grained sand with little capacity for binding. Also, if we study the building methods used on these ancient walls it becomes clear that the stonemasons were fully aware of problems with soft footings and took many steps in the preparation of the base of the wall to effectively provide a compacted footing. Documents and sketches from the United Kingdom show the stonemasons preparing footings with “tampers”, being heavy wooden blocks with a handle attached for pounding the footings. Most of the retaining walls constructed where pitched backwards ie the walls leaned back into the higher ground to increase the strength of the wall and reduce the surcharge.
Examination of old walls can reveal much about the stonemasons skills. In may areas, builders of old, such as the Romans, dug down to bedrock on which to build their footings. On the topic of Romans....and before we get all expansive about what they did for modern civilisation, probably their greatest unsung acheivement was drainage (and their development of lime mortars). Roman walls often show intricate knowledge of the problems that can occur in wet areas and where persistent hydrostatic pressure can occur. Very wide base course stonework to distribute the weight and careful routing of water was typical in their constructions.
Many ancient walls were dry stack (without mortar) such that the wall could accomodate settling and movement without catastrophic failure and allow water to pass through. In mortared walls lime mortar was used which allows a degree of flexibility and movement in the wall. Lime mortar was usually weaker than the stone, thus when significant movement occured the stonemason could easily chisel out the fibrous mortar and repair it. This brings us to another topic. The reason why some old walls survived the test of time is because they were maintained and cared for by the Stonemasons. These days, we expect to build a stone wall and never touch it again.
Another reason we need to be more rigorous with our compaction in modern climes is simply the speed of construction. With modern machinery and equipment we can literally slam hundreds of tonnes of stonework up in a matter of days. A crew working on development walls with good access can easily lay up a hundred tonnes in a day. In days of yaw the stonemason often had to carry every stone by hand, work it into shape with basic hand tools and then lay it in the wall. As the stone was progressively laid the wall settled under its own weight in a time honoured fashion. Lime mortars cured slowly and with a degree of flexibility. What often took weeks, months or even years is now done in the blink of an eye. Couple high speed modern construction techniques with hard modern cement based mortars, which are brittle and inflexible, and you can see why limiting settlement is an important factor.
Modern methods of developing new urban areas also raises another good point in regard to compaction (I've got more!). In days gone by the wall builder could have some confidence in "Virgin Ground". Current development practice leaves the wall builder with earth that is definetly not a Virgin!. Huge amounts of earthworks, quarrying and filling go into preparing new areas. Couple this with deep sewers, underground power and other services and you may find the existing compaction is somewhat lacking.
Last but not least; its nice to get all romantic and teary eyed about fabulous old stonework but the fact is, the majority of old walls have gone. Ravaged by time, pollution, wars, the elements and lack of maintenance. What remains gives us clues and knowledge for building modern walls. Whilst stonemasonry remains, at heart, an ancient skill, today we are blessed with modern machinery and equipment with which to do our work. If we could go back 300 years and offer a stonemason a Vertical Rammer I'm sure he would be pleased to have such a device (if not a little wary)...save for the unavailability of Unleaded Petrol!. Perhaps too, more of his walls would still be standing today due to well prepared and compacted footings.
If you are about to spend your hard earned cash on a stone wall you only have to ask youself the rhetorical question mentioned earlier.
Illustration: This boundary brick wall failed catastrophically. It had inadequate footings and poor workmanship. The Local Council placed a demolition order on the wall.