Earth retaining structures are an important aspect of geotechnical engineering that are used to support slopes, embankments, and excavations. The Australian Standard AS4678 provides guidance on the design, construction, and maintenance of earth retaining structures.
The design process begins with a site investigation to gather information on the soil and rock properties, as well as the slope geometry and hydrology. Depending on the retaining wall type, a global stability analysis may be undertaken using limit equilibrium or finite element methods, to determine the factor of safety of the slope. The results of the global stability analysis are used to identify potential global failure modes forming below the retaining wall. A suitable earth retaining type may be recommended by the geotechnical engineer to mitigate these potential failures. Internal stability of the proposed retaining solution is often undertaken separately by the civil/structural engineer engaged on a project.
A shoring system is often required to support excavations for building basements and other structures. A shoring system could include flexible piled secant/soldier piles or soil nails rock bolts and shotcreting. The design process of a shoring system includes review of site investigation information followed by geotechnical analysis in to provide recommendation for retaining wall design (active, passive and at rest earth pressures). For complex projects, soil structure interaction analysis using finite element software may be required to provide the structural engineer with estimated deformations, indicative structural loads for a specific construction sequence. A number of iterations (typically 2-3) are often required to achieved convergence between the structural and geotechnical analyses models. The benefit of soil structure interaction analysis is often an optimised solution, resulting in reduced pile lengths and lesser materials.
The AS4678 standard provides guidance on several types of earth retaining structures, such as reinforced earth retaining walls, cantilevered L-shape gravity walls, flexible piled retaining walls, recommendations for cuts stabilised by soil nails / rock bolts. It also includes minimum requirements for material selection, drainage requirements and construction methods, as well as for the maintenance and inspection of the structures. The standard also provides guidance on the loadings and the structural design of the retaining structures.
It’s important for engineers and construction teams to follow the guidance of the Australian Standard AS4678 when designing, constructing and maintaining earth retaining structures, as these structures play a critical role in ensuring the safety and stability of slopes, embankments, and excavations. This standard provides a framework for the design and construction of earth retaining structures that can withstand the loads and the environmental conditions of the site, providing reliable and sustainable solutions.