The importance of correct excavation, compaction and site preparation

Proper site preparation is one of the most critical—and most overlooked—stages of residential construction. Before any concrete is poured or structural elements are installed, the building platform must be correctly formed, cleared, and compacted. Failure to manage this phase effectively is a leading cause of slab movement, cracking, water ingress and long-term structural issues.

Under the National Construction Code (NCC) and AS 2870 – Residential Slabs and Footings, builders have a responsibility to ensure that the foundation system is suitable for the soil classification and that the site is prepared to standard. This includes:

1. Removal of organic and unsuitable material
   Topsoil, roots, vegetation, and any decomposable materials must be completely removed from the building platform and trench bases. Organic material breaks down over time and creates voids, leading to slab settlement and differential movement.
2. Imported fill must be controlled and compliant
   Where fill is introduced, it must be clean, free of organic content and contamination, placed in controlled layers and compacted to the engineer’s specification. Unverified fill can contain debris, clay lumps or moisture pockets, which compromise foundation performance.
3. Correct compaction protects the structure
   Compaction testing should be undertaken to confirm that the achieved density meets the project’s geotechnical requirements. Poor compaction is one of the most common causes of post-construction slab heave or movement.
4. Correct Pier Installation

In the ACT, piers are required on the majority of residential sites due to local soil profiles and site classifications. Piers must always be installed strictly in accordance with the engineering design, including the specified diameter, depth, spacing, and vertical alignment. While it may seem obvious, it is not uncommon to find piers installed on an angle or not drilled to the required depth.

All pier holes must be free of moisture and debris before concrete placement, as water or loose material can soften the bedding layer and compromise load transfer. On sites with highly reactive or unsuitable ground conditions, piers are often the most critical structural element supporting the slab system. Poor installation can result in differential settlement and long-term structural movement.

Correct pier installation is fundamental to slab stability — it is not a step that can be rushed or overlooked.

5. Correct battering of excavations for site safety
   Excavations must be battered or benched to the appropriate angle to prevent collapse. Incorrect or vertical cuts can result in trench wall failure, posing a significant risk to workers and damaging surrounding structures. Ensuring safe battering is not only best practice — it is a WHS requirement under WHS Regulations for managing excavation work and preventing ground collapse.

Why it matters

Rectifying slab movement or drainage issues after construction is significantly more expensive and disruptive than doing the job properly at the outset. Ensuring correct site preparation is not only a requirement of AS 2870, but it also demonstrates due diligence and reduces warranty risk for the builder and homeowner.

At MBA ACT, we often see preventable defects linked to inadequate excavation and site preparation. A well-prepared building platform sets the foundation — literally — for the success of the entire project.

If you need support with site inspections, compaction documentation, or ensuring your site preparation meets NCC and AS requirements, our Building Advisory Team can assist.

Build it right from the ground up.

If you have any questions or need further guidance, please don’t hesitate to contact our Advisory team:

📞 (02) 6175 5900
📧 workplace@mba.org.au