Avoiding Surprises: Managing Ground Risk When Planning And Delivering Your Next Tunnel

Stephen Barrett

Principal

If you are about to embark on planning or delivering an underground infrastructure project such as a metro system, road tunnel, hydroelectric development or water transfer scheme, what key actions can you take at each project stage to proactively manage your ground risk?

Ground risk is associated with encountering conditions beneath the ground surface that are different to what was assumed. When such risks eventuate, there can be technical, contractual and legal implications. The technical implications relate to the physical consequences, while the contractual and legal implications relate to how these consequences should be valued and how the costs should be allocated among the parties affected.

The impact of ground risk can be considerable from many perspectives, including health and safety, environment, third parties, existing facilities, reputation, schedule and cost. In some instances, the impacts and required mitigation measures can be ongoing and extend throughout the asset’s operational life.

Not surprisingly, given this context, contract insurers have recognised the importance of proactive risk management for any form of underground construction. They have documented their insurance eligibility and maintenance requirements in the International Tunnelling Insurance Group’s (ITIG) Code of Practice for Risk Management of Tunnel Works. Proactive management of risk by owners, designers and contractors through the project lifecycle is therefore essential for these types of projects.

We suggest the following key actions at each stage of your next tunnelling project to help you and your project team proactively manage ground risk – and reduce the potential to encounter unwelcome surprises and the associated consequences.

Project development

The project development stage typically involves project feasibility studies, developing the business case, preparing concept designs and assessing environmental impacts. In these early stages, many key decisions are made about the form of the project. This is also when the greatest flexibility exists around the proposed alignment corridor and the ideal time to evaluate opportunities to avoid areas that contain known ground risks (e.g., poor ground conditions, obstructions or contamination).

In these early stages it is important to develop an initial understanding of the project’s geological and hydrogeological setting, along with potential contamination, ground gas and acid sulfate soil and rock issues. This is also the time to determine the historical and current land uses, as well as the likely existing subsurface infrastructure, excavations and foundation types present along the project corridor. It’s advisable to avoid (as far as practical) areas of densely developed residential housing or commercial buildings where there is the alternative of running the alignment below undeveloped ground or road reserves. Appropriate investments made by the owner at this stage will allow well-planned and targeted ground investigations to be completed.

To maximise the risk-adjusted return on investment, it’s best if the ground investigation is phased. Initial phases should focus on the more significant risks identified in desk studies. Further ground investigation can be completed later to fill in the data gaps as the design is progressively detailed and refined. Geotechnical, hydrogeological and contamination risk registers are essential tools in this process, providing a structure to guide the investigation scope and a central point for documenting and continually updating the assessed risks as each investigation phase is completed.

To assist design development and environmental assessment, the investigation results should be documented in factual data reports, with interpretation of the results documented in separate geotechnical, hydrogeological and contamination interpretive and design reports, updated after completion of each investigation phase.

From a ground risk perspective, environment effects statements (EES) for tunnel projects should consider ground movement, noise, vibration, groundwater, spoil management and contamination as a minimum.

Procurement

The ground risks identified and assessed during the project development stage should inform the project procurement strategy. This involves selecting a delivery model and contract form that appropriately allocates the identified risks between the parties, as well as the ‘known unknowns’ and the ‘unknown unknowns’. The risk of changed conditions may lie with the owner, the contractor, or be split between the two parties – depending on their appetites for risk.

To minimise the final project cost, the most appropriate strategy is to allocate the identified ground risks to the party best able to manage them. If the ground risk is not appropriately allocated, the project cost may grow due to contingency allowances being added into the contract price, or due to costly legal disputes during construction or operation.

Organisations such as the International Federation of Consulting Engineers (FIDIC) have developed and published standard contract forms to help engineers manage these issues and minimise the potential for disputes. These standard contracts are useful starting points for drafting the tender and contract documents.

In these standard contracts, ground risk is typically allocated through a clause relating to differing site conditions or unexpected ground conditions. This can be supported by other contractual instruments such as geotechnical baseline reports (GBR) and dispute review or adjudication boards (DRB/DAB). FIDIC, in association with the International Tunnelling and Underground Space Association (ITA), has recently developed a new standard contract form specifically for underground construction (known as the ‘Emerald Book’), which aims to provide fairer mechanisms for allocating ground risk for tunnel work. It is hoped that once trialled and refined, the Emerald Book will be broadly adopted and become the new standard for the tunnelling industry.

In addition to these contractual considerations, it is also important that sufficient ground investigation has been completed to support the adopted contract strategy prior to tenders being called. As a rule of thumb, the more ground risk a contractor is asked to accept, the more ground investigation should be completed by the owner to allow the contractor to properly assess the risk they are being asked to take. For tenderers to be able to prepare their bids effectively, they should also be given the opportunity early in the tender period to nominate what additional investigations they require to inform their bids and have this information provided to them prior to the closing of the tender period.

When preparing the tender documents, it is also important to consider what tender submittals to request and the information that should be provided in each, so that tenderers’ approaches to managing ground risk can be adequately assessed prior to contracts being awarded.

Detailed design and construction

Sharing the risk registers developed during the project development stage with the parties responsible for project delivery is an essential step in managing risk. For design-build projects, it is best that this knowledge is transferred at the procurement stage, so the learnings can be incorporated into the contractor’s tender design.

Procurement investigation data should, as a minimum, include a preliminary ground model and a summary of key ground-related risks, including any perceived gaps or uncertainties in the completed investigations at the time of tender. The detailed design investigation can then be tailored to address any remaining critical information gaps to inform the final design solution.

Due to the natural variability of the ground, the potential to encounter unexpected ground conditions remains even after the detailed design investigation is complete. To address this ‘known unknown’ in the final design solution, it’s important that the ground support designs consider a range of potential conditions beyond those expected to be the most likely.

Regardless of the mode of excavation and support system chosen, each ground support design package should include a geotechnical instrumentation design which allows the project team to observe and compare actual ground movement and changes to groundwater conditions against what was predicted. Specifying appropriate alert levels and the actions to be taken if they are exceeded is also essential when preparing these designs, so all parties are clear on what needs to be done when the unexpected inevitably happens.

This article is the first of a series of insights to help you manage ground risk when planning, procuring, designing and constructing underground infrastructure. We’ll explore some of the aspects raised in this article in more detail over the series, giving you a broader picture of a best-practice approach to managing ground risk. Follow us on LinkedIn or subscribe to our emails to be notified about upcoming articles in this series and other valuable content.

Stephen Barrett

Principal

About the Author

Related Insights

Golder uses cookies to ensure that we give you the best experience on our website. By continuing to use this website, we assume that you consent to receive all cookies on our website.

OK Learn More