Building a swimming pool is a great way to add value to your home and having the best team to assist with the design, structural engineering and construction, will make the whole process a wonderful experience.
In Australia, in-ground swimming pools are most commonly constructed from reinforced concrete or fibreglass. Each material has its advantages and disadvantages. Choosing between them becomes a question of picking the best construction method for your budget and particular site requirements. In this article, we will share some of our knowledge from our experience with concrete swimming pools and their structural engineering aspects, pool design considerations, construction requirements with recent projects across Adelaide, Hobart, Melbourne, Perth and Sydney.
WHAT ARE SOME OF THE STRUCTURAL ENGINEERING DESIGN CONSIDERATIONS OF CONCRETE SWIMMING POOLS?
The structural engineering design process will consider a range of factors to ensure that the soil conditions and the applied loading on the swimming pool structure can withstand the expected worst case scenario. The majority of the concrete swimming pools will be constructed using “shotcrete” which is essentially a mortar sprayed using a pneumatic pumping system.
Swimming pools may be subjected to a combination of critical loading combinations similar to retaining walls such as overturning, sliding and various scenarios of surcharge loading. In areas with a high water table, the possibility of uplift (or flotation) needs to be addressed for in-ground swimming pools as the effect of hydraulic pressure when the swimming pools is empty can cause damage to the pool structure. In some cases, residential subdivisions can adversely affect an existing swimming in close proximity to property boundary. The additional loading from construction equipment and vehicular surcharge pose a significant risk that could lead to structural failure in an existing swimming pool.
A wide variety of soil types encountered in Australia, proximity to existing buildings or structures, hydro-static water conditions and a host of other site specific conditions may exist. If the proposed site is on a slope and contains retaining walls, services and other buildings, it is recommended that a structural engineer conducts a site inspection before finalising the engineering details. In certain regions of Australia, for example the Perth area, the structural engineer may need to consider the seismic zone proximity in documenting the engineering specification.
In other parts of Australia where soft clay soils are prevalent such as Adelaide, Melbourne and Sydney, the swimming pool may require the implementation of bored piles to minimise settlement over time. In Perth, Brisbane and Darwin, most of the metropolitan area is built on stable sandy soils and this reduces many of the additional considerations in relation to foundation preparation.
The certified structural engineering drawings should include a site plan to show the location of the pool on the site and relevant information such as:
Soil classification in accordance with AS2870 and the design bearing pressure of the supporting soil;
Fully dimensioned plan, pool wall sections and detail for seats, stairs, corner lap etc.;
The lining or internal finish required;
The characteristic compressive cylinder strength of concrete at 28 days;
The concrete curing procedure;
Details of reinforcement;
Methods used to prevent flotation of the pool.
The key areas of concern to ensure that the construction of the swimming pool is successful is related to eliminating the possibility of undermining existing structures and assessing the likelihood of affecting existing services. If the pool is to be constructed close to an existing boundary, a structural engineer will assess the loading conditions and may include light vehicle surcharge of 5kPa.
In rare circumstances, if the pool is to be constructed close to the boundary with a highway, it may be required for the pool wall to be designed to withstand heavy vehicle loading (in the event that a truck will breakdown). This loading can be significant and we have been involved with projects where a 20kPa loading was required. The structural design of the swimming pool is always conducted on a case by case basis and while standard engineering drawings can be applied to low complexity projects, the reality is that each project needs to be customised.
WHAT IS THE USUAL PROCESS FOR OBTAINING A BUILDING PERMIT?
The building permit application varies across Australia. However, one of the objectives of the permit authority is to ensure that the applicable building standards and local swimming pool regulations to the following are addressed:
Swimming pool safety barrier and relevant considerations;
Structural adequacy of the swimming pool via certified engineering drawings;
Water re-circulation system;
Energy efficiency requirements if swimming pool heating is installed;
Water efficiency via inclusion of a compliant pool blanket;
Impact in relation to proposed zoning changes, land use changes and interfacing developments;
Generally, the following documents will be required to obtain building permit approval:
Swimming pool architectural design drawings;
Site plan produced by a land surveyor showing the size, shape and position of the block, location and names of streets, boundaries and Australian Height Datum (AHD) contours, location and dimensions of existing structures, trees, etc, location of existing or proposed retaining walls, location of septic tanks and leach drains. Finally, the site plan should show the AHD datum point, proposed FFLs and FGLs;
Certified structural swimming pool engineering drawings, details and specification signed by a practising structural engineer;
Certificate of design compliance or similar certification produced by a registered building surveyor.
For specific information in relation to the building permit process and obtaining approval in the following cities, please visit the relevant authority:
Adelaide, South Australia - Getting approval to build is provided by the Department of Planning and Local Government.
Brisbane, Queensland - Queensland Building and Construction Commission.
Canberra, Australian Capital Territory - Environment, Planning and Sustainable Development Directorate.
Darwin, Northern Territory - Building Advisory Services
Hobart, Tasmania - Consumer, Building and Occupational Services.
Melbourne, Victoria - the Victorian Building Authority (VBA) regulates building and plumbing practitioners.
Perth, Western Australia - For projects located in Perth the Department of Mines, Industry Regulation and Safety, prior to a building permit being granted in Perth by a permit authority (usually the relevant local government), a registered building surveyor is engaged to confirm that the proposed swimming pool and its safety barrier comply with the National Construction Code.
Sydney, New South Wales - refer to the Building Professionals Board.
This requirement for a Certificate of Design Compliance is applicable in all parts of Australia.
After the building permit is granted and consideration is given to aspects such as health and safety, structural engineering, environmental and heritage requirements in line with the local government and Building Code of Australia, the swimming pool is ready to be constructed.
It is essential that existing services are identified and not damaged during construction. It is standard practice that the structural engineer must consider the location of the sewer line which is available by engaging the Dial Before You Dig service. However, it is always prudent that a separate enquiry is also undertaken by the builder.
The site preparation should be undertaken in accordance with the Australian Standard 3798 "Guidelines on Earthworks for Commercial and Residential Developments. This will involve the removal of any organic material, rubbish, tree stumps & deleterious material such as rock or clay from the foundation area. The builder would usually obtain a compaction test certificate prior to constructing the swimming pool.
The following are some of the steps involved in the construction of a concrete swimming pool:
Ordering of equipment and long lead items;
Site inspection to identify existing service and structures;
Installation of retaining walls (if needed);
Installation of formwork;
Steel reinforcement and bar chairs;
Plumbing and electrical;
Pool construction typically using shotcrete;
Landscaping, fencing, screen walls, water features etc.;
Testing and commissioning.
In Australia, domestic concrete swimming pools are constructed with a specially formulated reinforcing bar which is designed to be easily cut and bent into shape on site to speed up construction. The reinforcing bars are 12mm diameter grade 250N and enable a wide range of options in shaping & forming the pool.
TREES, SHRUBS AND LANDSCAPING
Trees or shrubs in close proximity to foundations can cause a variety of structural issues over time. Available literature such as the CSIRO Building Technology File 18-2011 Foundation Maintenance and Footing Performance: A Homeowner's Guide (BTF18) provides guidance in regards to long term performance (for more information, visit https://www.publish.csiro.au/book/7076/).
The following is quoted from the BTF18:
“Trees and shrubs that are allowed to grow in the vicinity of footings can cause foundations soil movement in two ways:
Roots that grow under footings may increase in cross-sectional size, exerting upward pressure on the swimming pool structure;
Roots in the vicinity of the swimming pool structure may absorb much of the moisture in the foundations soil causing shrinkage or subsidence. This is particularly applicable to projects on reactive clays (eg. Adelaide, Western Sydney & Melbourne)“
For projects in Brisbane, additional guidance from the Queensland Building and Construction Commission, A guide to preventing structural damage states that “trees should not be planted within a distance from a building (or structure) that equates to their mature height.”
It is often the case that trees are added to a property after construction of various structures for their many significant benefits but certain tree types such as palm trees can cause damage to a pool shell. The Australian Standard 2870 – Residential slabs and footings contains a procedure in Appendix H for the design of footings for trees so it is possible to design structures if the trees to be planted are known to the design engineer.
SWIMMING POOL INSPECTION REQUIREMENTS and SAFETY
When building a new concrete swimming pool, it is recommended to have a qualified structural engineer undertake a steel reinforcement inspection. Site specific constraints such as surcharge loading due to adjoining buildings, excavation consideration in relation to existing services and undermining should be undertaken using a risk assessment approach on case by case basis.
While underperformance issues such as tile cracking or the swimming pool losing water are rare, a structural swimming pool inspection may be required to ascertain the cause and recommend remediation advice.
The team at Engineering Plans Australia can assist with all types of concrete swimming pools regardless of it being above ground, in ground, residential or commercial pool, under construction or in need of renovation. Our team of structural engineers includes experts in concrete engineering, with a focus on swimming pools & retaining wall designs.
It is important to highlight that the responsibility is on the property owner to ensure that fencing around a pool is maintained and operating effectively at all times. It is critical that young children are always supervised when in the proximity of the swimming pool area.
If you have any questions regarding structural engineering or construction of a concrete swimming pools, send us an email or calls us today.
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