REACTIVE FOUNDATION MOVEMENT— CAUSES & RESPONSIBILITIES
Foundation movement is a common cause of building distress and disputation. Sometimes it is not clear what the causes are or who is responsible. This article attempts to set out common practice in design and construction with a view to party responsibility.
The foundation of a building is the ground upon which a building's slab or footings are supported. Although the term "foundation" is commonly used to describe footings this is both incorrect and misleading. There are a large number of footing systems in use from piles of many types to strip and pad footings and slabs. In this article I am restricting my assessment to buildings of domestic scale and type with the two most common footing systems: raft slabs and strip footings with separate subfloors.
Foundation movement and resultant building distress can occur for many reasons but I am discussing the most common, the reactive movement of clay soils, that is foundation swelling and shrinking due to foundation moisture content changes.
When a clay foundation shrinks or swells it is difficult to avoid building movement. Under the normal range of moisture conditions clays shrink when dry and swell when wet but they can also settle when saturated as the foundation becomes a quagmire. These situations do occur but are something of a special case that result from a gross source of moisture such as an unconnected downpipe.
Therefore we have to accept some building movement on clay foundation soils as moisture changes are inevitable. Stronger, stiffer footing systems can resist small local movements and reduce or spread larger scale actions. The reality of some building movement due to foundation reactivity is accepted in design standards.
Reactive soils are considered by engineers to potentially give rise to a certain degree of movement that can be thought of as a range although it has geometric properties that complicate this but in simple terms a "Class H" soil, that is a heavy clay is expected to move some 40 to 60mm at the surface under normal seasonal conditions - that is without any significant extraneous influences. This is reduced by the foundation depth and again by the stiffness of the footing system so by the time it reaches the above ground structure it is usually much less. An example of this that can readily be observed is the tendency for stumps to settle internally. Modern brick veneer, plaster lined walls are remarkably tolerant to movement. I recently examined a house in Berwick with 67mm of movement over six metres with no internal cracking.
Design and construction of footing and foundation systems sets reasonable expectations of performance, builds accordingly to certain standards and attempts to guide development and maintenance. Site works often influence foundation performance.
What follows is a potted, sequential synopsis of foundation performance from design to distress.
Geotechnical Investigation and Report
At design stage a geotechnical consultant's report is usually commissioned. There are scant criteria for such reports and scope and type of testing and interpretation vary considerably. The geotechnical report should contain recommendations for construction, design and site maintenance.
It is best practice for the design engineer to provides a written brief to the geotechnical consultant. This is rarely done, however; for domestic projects someone usually just orders a "soil report". These can be provided for as little as $200 with fieldwork for such investigations often being carried out by under qualified field hands with scant professional supervision.
Geotechnical report content relevant to reactivity include the soil profile and moisture condition and the many factors likely to influence the foundation moisture on subject and adjacent sites. Adjacent trees and the overall drainage condition are often important. If the geotechnical consultant considers the site to be subject to abnormal moisture conditions he should classify it as "Class P" to AS 2870 and provide relevant discussion and recommendations.
The report should include recommendations for construction and site maintenance and many cover the site maintenance by a cross reference to Appendix B of AS 2870. Abnormal moisture conditions caused by significant trees are particularly invidious to deal with. It is difficult to design for the ongoing presence of growing trees and the removal of trees may itself give rise to abnormal conditions as the site wets up and heaves. It can take years for foundation conditions to return to normal as moisture migration through clay is very slow.
Therefore it can be seen that a good geotechnical assessment of a reactive site, particularly with abnormal moisture conditions is most important. The geotechnical consultant's report constitutes part of the endorsed building permit documents and should form the basis for the engineering design and the builders works.
Slab and Footing Design
If an AS 2870 Site Classification other than "P" (for problem) has been made in the geotechnical report and several other criteria are met that effectively define a standard domestic type building the slab or footing system can be specified from AS 2870 without engineering involvement. However as we increasingly build on confined sites with service trenches, other obstructions and abnormal moisture conditions not to mention non standard houses engineering involvement is common.
In the design of a slab or footing system the design engineer should consider many factors including the recommendations of the geotechnical report, applied loads, building requirements and many other factors. The intent of the design should be to achieve building performance commensurate with relevant standards and reasonable expectations. In most cases of reactive foundations it is impractical to eliminate the possibility of building movement even if all the recommendations of the geotechnical report are employed.
Control joints in masonry buildings are usually a necessity in order to attract building movement to the joints where no damage will occur. Ideally the engineer should design control joints as this requires more than simple adherence to spacing criteria; joints should be placed where they are most likely to attract movement and this requires an understanding of where the structure and masonry is most likely to articulate, however this is not commonly done and joints are at best indicated on the architectural elevations or at worst left up to the builder or brickie to improvise perhaps inspired by the recommendations of the geotechnical report. When things get to this stage you might as well write to Father Christmas for correct joint locations!
The building surveyor has the invidious task of checking all documents for permit and ensuring their adequacy or (in the case of engineering certification) of satisfying himself that the certification is from a good source. A streetwise building surveyor will cast a wary eye over structural content however credible the certification.
In addition to adherence to the building specification the builder should also adhere to the construction recommendations of the geotechnical report, these commonly relate to issues such as trench stability and drainage. Wet trenches can cause short term heave and medium term shrinkage settlement. This can be most difficult to diagnose post construction particularly as records of construction condition are rarely available.
Variations to design documentation intent are common due to site conditions, material availability, errors found etc. If these variations are significant the building surveyor may require an "as built" set of drawings to be submitted for the file for future reference. This does not always happen. Records of inspections are sometimes available which may contain comments as to what variations were made during construction.
The building surveyor's approval and ultimately his certificate of final inspection is the formal endorsement of compliance of construction. This has little to do with quality control. To ensure best quality building other inspections by engineers, architects, interior designers, the builder, and the client are necessary. If the building is rushed together by the builder with only minimal compliance checks by the building surveyor the result is unlikely to be pristine.
Often site works are conducted that are relevant to foundation performance. These may include anything that affects site drainage; pavements, retaining works, trees and garden beds etc. Often such works are carried out after the main building contract by the owner on a progressive basis making their influence difficult to determine. Some site development recommendations are included in Appendix B of AS 2870.
This is a key area, particularly the need to maintain suitable site drainage and control vegetation. In almost all cases site maintenance reverts to the owner who is unlikely to be an expert in such things. Formally the expectation is that the owner has read the site maintenance recommendations in the geotechnical report and the documents to which it is cross referenced which usually include AS 2870 and the CSIRO Homeowners' Guide to Foundation Maintenance. The practical reality is more likely that he had never heard of such a thing.
Plumbing must also be maintained, a common problem is leaking stormwater. As this is usually not readily discernible and not visible it can easily be missed. The current fashion for camera investigations of pipes is no replacement for pressure testing to find invisible leaks.
The whole process and the input of many is imperfect. It is rare for this writer to inspect a new house and not to find obvious errors and omissions that may result in structural defects, it is however most unusual to find problems that may result in major failures or catastrophic consequences such as collapse.
Reactive foundation movement often results in minor building faults that may not be classified as defects. These commonly include gapping of control joints and settlement of stumps under the central part of the building where the site tends to a drier condition than externally and stump pads settle. AS 2870 and the Building Commission's Guide to Standards and Tolerances offer some guidance to assist definition of building defects. This does not necessarily mean that remedial works may not be required to satisfy owner preferences but it is a strong indication that nothing is seriously amiss structurally.
More serious defects are not uncommon although it is generally true that modern designs and particularly raft slabs are less likely to be problematic compared to old buildings. There have been several major steps in improvements in building performance historically, the use of reinforced concrete from about the 1930s, the adoption of deeper strip footings post UBR from 1974 and the move to deeper strips again via AS 2870 in the 1980s combined with the general trend away from strip footings and stumps to raft slabs that has been going on for decades.
Abnormal reactive foundation movement may manifest itself in several ways, the two most common are settlement and rotation of footings. Settlement most often occurs at corners or projections where the footing structure has to cantilever to support the building when the foundation settles. Footings potentially rotate both outward and inward as the foundation at their sides shrinks or swells differentially but in practice inward rotation is rarely observed as the walls are usually effectively restrained against this action.
Outward rotation can cause gapping of brickwork around openings in brick veneer construction as the masonry tends to rotate with the footing while the roof frame tends to keep the stud wall straight. In solid masonry construction, particularly of older buildings where walls are not well fixed at their top, cracking typically occurs at the cornice line as walls rotate outward.
Both settlement and outward rotation tend to be deteriorating mechanisms as cracks tend to fill with debris and resist closure when foundation moisture condition reverses. Heave may also occur but in this writer's experience is less common. There is a school of thought that if the current drought ever ends we may see much more heave. All these mechanisms occur from normal seasonal moisture cycles and if design, construction and site maintenance are in order should not produce defined building defects, these tend to occur when something is amiss.
I am often asked if the drought has caused building distress and perhaps it has but I am yet to see an example where the drought is the only influence at play, invariably there are other factors which are probably exacerbated by the drought.
Diagnosis of foundation movement distress begins with an examination of the building and site with particular attention to levels and the plumbness of walls. Careful measurement, even with simple hand tools, is usually sufficient to ascertain the mechanism with which a building has moved in response to footing and foundation movement.
In some instances this points to site features that are so obviously that further investigation may not be warranted. Otherwise geotechnical investigation is required to establish the foundation moisture profile in the affected area and elsewhere as a reference. This is difficult as different foundation moisture contents commonly occur in different soil layers at differing depths. So if the soil profile is not consistent between the affected and reference areas comparisons may be meaningless. In the case of a building dispute borelogs constitute hard evidence but they are unlikely to offer a refined diagnosis beyond the fact that the foundation is wet or dry.
It is notoriously difficult to apportion the influence of several detrimental site factors. For instance if we have a building surrounded by several trees with detrimental site drainage we cannot be sure how each tree is drying the foundation and the site drainage may be affecting it. We are reduced to speculation about below ground influences we cannot readily see. Tree roots are notoriously variable and while some investigation can be done by borelogs, pits, air guns or radar roots can such moisture through clays from a considerable distance so even this is not definitive. In cases of potential liability such as the influence of a council street tree upon a house I have often been asked to nominate a % blame. If there is one factor and the history is known this may be possible but if there are several factors it becomes impractical.
It is unusual for building movement to point to a gross error of construction adequacy such as deficient foundation depth and one would hope that with modern inspection procedures such things are rare. The more common construction issues of poor drainage is fearfully difficult to diagnose when investigating a building years later. We have on occasion had recourse to meteorological records when we suspected a wet site but unless the site is close to the met station and one can show it was not adequately drained that is imperfect evidence.
Therefore most building movement diagnosis points to site features and these may be on the subject site or adjacent sites and may be the responsibility of the builder, owner or others.
In many cases remedial works are simply a matter of resolving site maintenance or development issues and repairing the building. If the site factors are clearly the problem and the building distress has not damaged the structure significantly this is usually sufficient. The range of footing movement that leads to readily repairable building damage does not usually suggest structural damage to the footings.
Revisions to site works however are not simply a case of reverting to the best practice of the design guidelines. For instance best may be to pave the entire perimeter of the house but if this has not been done and the house has persisted for many years to do it later may cause drying settlement. Site works have to be approached with consideration to how the building has performed as this is the best guide of what is actually happening below ground.
Another common problem is influence from adjacent sites and problems include drainage and vegetation issues but by far the most common issue is adjacent trees. My legal colleagues tell me that trees fall under the law of nuisance and an affected party on an adjacent site has rights to mitigate that nuisance. This may be true but for a variety of practical reasons it is not always possible to simply remove the tree.
The next likely option is the installation of a "root barrier". This is something of a misnomer as these barriers are primarily to reduce the moisture absorption affects of trees not just the physical presents of roots. The technique is to excavate a narrow, deep trench, commonly 1200mm deep and 150mm side and install a geotextile or concrete barrier and chemical root inhibitors. One problem with root barriers is that moisture seeping affects often go much deeper than this and recent research is suggesting that at least 1800mm and perhaps up to 3m is required.