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Looking for clues – geotechnical investigation

Geotechnical models lead

The objective of any ground and groundwater investigation is to reduce risk to a project.

In theory, the more investigations that are undertaken, the lower the risk of geotechnical and/or hydrogeological issues affecting excavation and construction risks including safety, schedules and costs.

Antoinette Walshe, one of our Melbourne based geotechnical engineers, asks the very important question: how much geotech is enough?

Geotechnical excavations and tests must be considered and tailored in the context of answering specific questions. The sum of the ground volume that can be inspected by an intrusive excavation represents a small fraction of the total volume of the ground mass within or upon which a structure will interact. Ground conditions are inherently variable. With this in mind a thorough understanding of the potential geological and hydrogeological risks associated with a site is required, together with appropriate risk mitigation measures.

“From project inception, a risk register should be maintained which quantifies the likelihood and consequence of adverse effects of geotechnical hazards,” says Charlie Shackell, Geotechnical Services Leader, Aurecon.

“It’s what makes each project so unique. Geotechnical engineers and engineering geologists have to develop a picture of the geotechnical story of a site and relay this to designers and construction teams by providing meaningful data and advice on potential geological and hydrogeological hazards,” adds Charlie.

Working inside a tunnelBenchmark from the beginning

A desktop study is typically undertaken at concept design stage to establish the geological and hydrogeological context of a project.

“As designs progress beyond concept phase and more information becomes available, any assumptions need to be reconsidered and appropriate design modifications made. An robust  geotechnical model will also ensure new data is interpreted appropriately and interpolations or extrapolations, where required are sensible,” says Charlie.

Once a preliminary geotechnical model is developed from existing geological information and any existing excavation records, further investigations can be designed which are targeted and cost effective.

The objectives are decided in consultation with the design engineers and ideally, the construction team. The origin of each geotechnical unit and their geological history will directly influence the identification of appropriate geotechnical properties for each stratum and the ground mass as a whole.

A setting might comprise deltaic (ancient river and estuarine) deposits which are stratified and compressible. Sediment particle sizes can vary from silts and clays to cobbles and boulders depending on the energy and chemistry of the transport and deposition system. Great difficulty can be encountered in excavating such sediments due to high groundwater inflow into excavations and abrasive cobble zones, not to mention the effect of excavation induced ground deformations and groundwater table lowering on the condition of surface or adjacent structures.

A city in a mountainous region might attribute its topography to tectonic plate collisions or tectonic uplift. Here the in-situ stress regime may be very irregular due to topographic effects and stress relieving or stress concentrating features such as faults and shear zones or deeply incised valleys.

Building a data base

It is important to understand the geomorphological framework within which the site exists. This is where ongoing processes may influence the type of loading regimes to be analysed and taken in to account when designing infrastructure.

“The range of possible geological environments within which a metro or urban tunnel or rail site can be situated is broad and a fascinating aspect of geotechnical engineering,” says Charlie Shackell, Geotechnical Services Leader, Aurecon.

Unforeseen ground conditions are often a key project hazard. As such, careful consideration must be given to how this risk is minimised and allocated in construction contracts. Successful treatment of this risk requires collaboration between clients and their technical, commercial and legal advisors.

It is vital that geotechnical engineers and engineering geologists are methodical in assessing a project site. Every geotechnical model should be developed where assumptions are clearly documented throughout every step.

Working inside a tunnelWhole of project approach

The Observational Method was first documented by R.B Peck in 1969 and presented in the Rankine Lecture of that year. The Observational Method is a continuous and integrated process of geotechnical assessment, monitoring and surveillance and review applied through project appraisal, design and construction stages.

It can be impossible to foresee every geotechnical condition that may affect construction and performance of a surface or underground structure. It is prudent to make some provision to allow revision of geotechnical models and the adopted design to suit actual conditions, as new information is collated from construction excavation stage and construction monitoring.

Economy of design and construction can be achieved without compromising safety by applying this methodology, provided it is adopted within an agreed contractual framework.

“The Observational Method can be used to optimise designs that may otherwise be based on over-conservative parameters due to the lack of site specific information,” says Charlie.

For various planning, contractual and risk management reasons, such a design methodology may be the preferred approach. Geotechnical and hydrogeological monitoring and surveillance would still be required through construction stage but the objective would be to confirm the actual ground conditions are in accordance with the design assumptions.

No two projects are the same and often the area of greatest variability is the ground conditions on or through which we are constructing. A flexible risk based approach, tailored to the specific project needs is the basic principal Aurecon ground engineering team bring to bear on all our projects.

In the end, a successful project is one that draws upon highly accurate geotechnical information and integrates this to the project programme to deliver best design and optimum project deliver performance.

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