By
Introduction
Site classification is an imperative step before designing any footing for a building. The foundation, being the ground which supports the building, is made up of a variety of materials that are subject to varying properties such as reactivity, bearing and shear strength.
Varying soil types behave differently under geological and localised conditions, which is known as geomechanical characteristics.
In order to safely design the footing for any structure, it is crucial to understand the load that is being transmitted by the structure and the behaviour and deformability of soils that support the footings.
Site classification and soil classification, though related, are distinct processes. A site classification takes many factors into consideration, soil testing being one of these. In order to classify a site, one must first understand the soil characteristics and reactivity. This case study focuses on site classification based on soil reactivity, in line with AS2870:
In summary, to ensure that the structural design is suitable for the site, the site classification must be known.
In recent years, the practice of utilizing generalised and often limited information to assume site classifications has become increasingly prevalent, to the extent that it now borders on being an industry norm or widely accepted standard.
Background on Site Classification Requirements
As Building Surveyors/Certifiers considering Site Classification in assessing structural plans, we must refer to the National Construction Code (NCC), as well as the relevant Australian Standards that it references.
The NCC is Australia’s primary set of technical design and construction provisions for buildings. As a performance-based code, it sets the minimum required level for the safety, health, amenity, accessibility and sustainability of certain buildings.
Part H1 of the NCC focuses on safeguarding people from injury caused by structural failure, loss of amenity caused by structural behaviour (deflections, creep, vibration, settlement and the like), protection of other property from physical damage caused by structural failure and safeguarding people from injury that may be caused by failure of, or impact with, glazing.
With specific regard to footings and site classification, the Deemed-to-Satisfy Provision for Footings and Slabs H1D4 outlines:
(1) Performance RequirementH1P1 is satisfied for the design and construction of footings and slabs if they comply with either (a) or (b):
a) One of the following:
ii) AS 3600.
Section 4 of the ABCB Housing Provisions may only be used for sites that are classified as A,S or M and fit within certain design criteria. Referencing the requirements of the ABCB Housing Provisions Standard first, 4.2.2 Site Classification outlines:
"The foundations where footings and slabs are to be located must be classified in accordance with AS 2870."
AS2870, Section 2.1 outlines:
"Sites shall be classified in accordance with Clauses 2.1.2 and 2.1.3 using the techniques and principles specified in Clauses 2.2, 2.3, 2.4 and 2.5."
If you are designing a residential footing under either the Housing Provisions Standard or AS2870, the site must be classified.
It is important to note that whilst AS3600 does not contain any requirements for site classification, the existence of AS2870 triggers Clause 1.1.3 - Exclusions, which states:
The requirements of this Standard shall not take precedence over design requirements and material specifications set out in other Australian Standards that deal with specific types of structures.
Now that we understand Site Classification and its significance in the design and approval of a residential building, lets look at a specific example where assumed Site Classification had an impact on a project.
A Review of Structural Plans
We received an application for a Construction Certificate (CC) for a Secondary Dwelling in Sydney, NSW. Upon review of the structural engineering plans, it was found that the site classification had been assumed. The Structural Engineer included notations such as these:
- FOOTINGS HAVE BEEN DESIGNED FOR AN ALLOWABLE BEARING INTENSITY OF 150 kPa TO BE CONFIRMED BY GEOTECH INVESTIGATION.
- IF A GEOTECHNICAL INVESTIGATION HAS NOT BEEN MADE, THE FOUNDATION CONDITIONS AND REACTIVITY CLASS ARE AN ASSUMPTION AND MUST BE CONFIRMED BY TRIAL EXCAVATIONS BY THE BUILDER.
- FOUNDATION MATERIAL SHALL BE APPROVED FOR THIS BEARING PRESSURE BEFORE PLACING MEMBRANE, REINFORCEMENT OR CONCRETE.
- RESIDENTIAL SLABS AND FOOTINGS HAVE BEEN DESIGNED FOR A REACTIVITY CLASS H1 TO AS2870.
These notations state that the Structural Engineer at the time of design, did not have the appropriate geotechnical information to be able to accurately design a footings system for a building. Due to this lack of information, the Structural Engineer assumed the site classification, and as a result designed to H1 (refer back to the introduction for characteristics of a H1 site).
The Structural Engineer also noted that the foundation conditions be tested and approved by a Geotechnical Engineer prior to placing membrane, reinforcement or concrete.
A Building Surveyor must determine whether the proposed building, as depicted in the plans and described in the specifications, will comply with the relevant requirements of the Building Code of Australia.
In addition to this, the Engineer provides design certification to accompany their plans, that is a statement that their design complies with the relevant standards and codes. By having an assumed classification, it can not comply with the requirements of the NCC/BCA and/or Section 2 of AS2870, as discussed above.
Request For Site Investigation & Findings
Due to the ambiguity of the notations, the Building Surveyor/Certifier requested that the applicant engage a Geotechnical Engineer to provide a definitive site classification, and provide a report to both the Certifier and Structural Engineer.
The applicant engaged a Geotechnical Engineer, and the report came back with the following findings:
- The site classification was not H1, but P, known as a problem site.
- The site contained soft or collapsing soils.
- There was evidence of underground flowing water and/or seepage.
- There was significant indicators of movement in surrounding roads, kerbs, pavements, masonry fences and/or ground surfaces.
Required Alterations To Structural Plans
As a result of the findings of the Geotechnical Report, the Structural Engineer made significant changes to the design. The following images outline the design prior to, and following the Geotechnical Investigation:
Before:
After:
The most notable changes to the design are as follows:
- The strip footing and engaged brick piers must be supported by piers at a maximum of 1800mm centres, requiring an additional 20 piers.
- The pier depth for all piers was increased by 50%.
These changes were deemed necessary by the Structural Engineer to ensure that the foundation could adequately take the load imposed by the proposed Secondary Dwelling.
The Structural and Economic Implications of Assumed Site Classification
In the event that a site classification has been assumed incorrectly, there is a greater propensity for economic loss and loss of amenity through structural failure. Economically, if the Structural Engineer designs to a Class H1, but the site is actually a Class A, S or M, the design is over-engineered, and the cost of building materials will be increased significantly. As an example, the changes to the footings for going from a H1 to a P site required an additional 10 tonnes of concrete.
Conversely, if the builder engages a Geotechnical Engineer to verify the site classification once works have commenced, and the site classification contains characteristics of higher vertical surface movement, delays in construction due to redesign and requirement of additional materials all pose a risk of increased costs.
An inadequate footing design, not tailored to the actual soil conditions can result in uneven settling, leading in cracks in the foundation, walls and floors. Settlement issues caused by inadequate footing design due to assumed site classification include include uniform settlement, tipping settlement and differential settlement, as shown below:
source: https://www.tensar.co.uk/resources/articles/understanding-settlement-in-geotechnical-engineering
The Legal and Ethical Implications of Assumed Site Classification
A Certifier is required to always act in the public interest, and the consideration by a Certifier of the suitability of documentation needs to be viewed through this lens and not the interests of the applicant or for expediency. The Practice Standard for Registered Certifiers was released in October 2022. It is issued under Section 14 of the Building and Development Certifiers Act 2018 and sets out the expected conduct of Registered Certifiers carrying out building certification work.
The Practice Standard for Professional Engineers was released in March 2024. It sets out obligations, responsibilities and standards required of registered Professional Engineers to complete engineering work under the Design and Building Practitioners Act 2020 (DBP Act). The Act is currently only enforced for Class 2, 3 and 9c buildings, not yet Class 1 which is where this specific example lies, though it is anticipated that this will extend to all classes of buildings in the future.
The DBP Act imposes new obligations on design practitioners and builders throughout the life of the building to ensure better compliance with the Building Code of Australia underpinned by higher quality design documentation.
The code of practice outlines that Engineers must take all reasonable steps to protect the health and safety of the community when carrying out professional engineering work, including by doing the following:
- identifying hazards;
- assessing risks;
- implementing appropriate strategies to manage risk; and
- take all reasonable steps to manage and avoid foreseeable adverse impacts of professional engineering work on the natural environment
In this scenario, the Certifier/Building Surveyor has a duty of care to ensure that the site is accurately classified. Ethically, all persons involved in the design of a building should share this duty of care. Furthermore, it is highly probable that the duty of care incumbent upon the Engineer will soon be legislated for Class 1 buildings.
Conclusion
This case study highlights the critical importance of accurate site classification in the assessment of an application for a building approval. In our review, we encountered a project where site classification was assumed rather than confirmed, resulting in amendments to the structural plans following the findings of a Geotechnical Investigation.
The initial assumption of the site classification could have led to significant structural failures, economic losses due to over-engineering or delays, and potential legal liabilities for parties involved. By requesting a Geotechnical Investigation prior to approval, we were able to ensure the safety and stability of the building in line with the requirements of the National Construction Code and our responsibilities as independent regulators of building work.
Moreover, this case underscores the ethical responsibility of Certifiers and Engineers to act in the public interest, ensuring that all potential hazards are identified and mitigated. The collaboration between the Building Surveyor, Structural Engineer and Geotechnical Engineer in this scenario serves as a model for best practices in the industry, emphasising the need for accurate site classification to safeguard people from injury caused by structural failure and loss of amenity caused by structural behaviour.
