The assessment area for geology and groundwater is separated into the three sections of the T2D Project and is closely aligned with the site contamination assessment area.

The following map shows ground investigation sample locations along the project corridor:

Map:  Ground investigation sites
Map disclaimer

The T2D Project is located in relatively flat area, with a gradual rise in elevation towards the foothills in the south. The conditions just under ground level, where the entrance and exits to the tunnels and open motorway sections will be located, feature soils, coarse-grained sands and gravels and clay most commonly deposited by ancient rivers or waterways.

Deeper soils that the tunnels will move through are made up of layers of sands and gravels located within numerous clay layers (aquitards). The design for the Southern Tunnels extends downwards through these layers and variably encounters limestone and sandstone at depth. A series of shallow confined aquifers are located in these (quaternary) sands and gravels with a deeper aquifer located in the underlying (tertiary) sandstone as shown in the cross section of the Southern Tunnels, northern portal below.

Diagram: Cross section towards the northern end of the Southern Tunnels

The Northern Tunnels are mostly located in the clay layers, but the southern portion of these tunnels encounters an ancient, non-active fault zone, the Para Fault, which has pushed underlying sandstone upwards. Sandstone and limestone are common at this depth across Australia and are relatively easy for a tunnel boring machine (TBM) to operate in. The below cross section shows the general geology below the project and the wider Adelaide metropolitan area.

Diagram: Regional geological section showing general geology below the T2D Project and wider Adelaide metropolitan area

Groundwater across the T2D Project generally drains from the hills in the east towards Gulf St Vincent in the west. Two deep aquifers located in the underlying sandstone extend along the length of the alignment. Groundwater in the deeper aquifers is typically under pressure, which is also common.

The construction and operation of the project has the potential to impact on the geological and groundwater conditions which in turn could cause unwanted impacts for the community or to the environment if not properly managed.

The objective of the geological and groundwater impact assessment is to document and assess the social and environmental (including heritage) risks during construction and operation of the project and identify management and mitigation measures to reduce these risks.

Further, assessments have also sought to determine the probability of underground construction shifting or impacting the ground at the surface – known as ground settlement.

Ground settlement issues can include aesthetic damage such as fine cracks in buildings and infrastructure through to severe stability and structural damage. This can occur if the removal of soil and groundwater required for tunnelling and construction is not managed properly.

Photo: Excavation undertaken for the Torrens Road to River Torrens project (T2T).

Construction impacts

Construction of the T2D Project will include excavation for sections of open motorways and portal dive structures which will connect to the two twin tunnels. The tunnels will be excavated using tunnel boring machines (TBMs) at an underground depth which is mostly below the water table.

Dewatering of some underground water will be required to enable construction of the open motorway section. This will also reduce pressure in some aquifers to prevent future uplift of excavation and construction at the tunnels and their entrances and exits.

The key impacts from construction and their principal mitigation measures are described below. It is important to remember that contamination is addressed in site contamination. The groundwater impacts discussed below relate to non-contaminated groundwater impacts only.

Video: Ground investigations and monitoring groundwater.

Operation impacts

The key impacts from operations and associated mitigation measures are described below. Subsidence impacts often occur over a long duration, although the impact may be experienced in the operational phase, the event that initiated the impact often occurs in the construction phase.

How impacts will be minimised or avoided

The project design has incorporated the results from extensive and detailed site investigations including careful consideration of risks and mitigation measures to minimise ground settlement and other risks.

For example, specialised retaining wall construction techniques, such as diaphragm walling, will reduce water ingress and mitigate impacts from any soil movements in the cut and cover tunnel sections and the open motorway, while base slabs below the water table will be appropriately reinforced and anchored to prevent uplift. The TBM tunnels are progressively lined by the TBM as excavations advance to prevent groundwater ingress and minimise surface settlement.

The Department’s Master Specifications (MS) have specific requirements applicable to the mitigation of impacts while the EPA and SafeWork SA practices and guidelines will also be followed.

These and other requirements listed below will be complied with throughout will sufficiently mitigate the identified impacts:

• MS PC-ENV1 – Environmental Management
• MS PC-ENV2 – Environmental Protection Requirements
• MS PC-ENV3 – Environmental Design
• MS PC-SI2 – Site Investigations
• MS RD-EW-C1 – Earthworks
• SA EPA Guideline for stockpile management (October 2020)
• SA EPA Construction environmental management plan (CEMP) Guidelines
• SA EPA Environmental management of dewatering during construction activities
• SafeWork Australia Excavation work code of practice.

A summary of the Department's Environment and Heritage Technical Manual (EHTM) that will be applied to avoid and mitigate impacts is included below:

• EHTM Attachment 6A – Protecting waterways Guideline
• EHTM Attachment 6B – Water Affecting Activities Best Practice Operating Procedure
• EHTM Attachment 10A – Guideline for the Preparation of a Contractor's Environmental Management Plan.

Approval, permits and authorisations

The Project Assessment Report (PAR) community consultation period concluded in April 2024. We thank you for your feedback.

The valuable information and local insights received during the consultation phase of the PAR are being considered by the project team and will inform the ongoing refinement of the T2D Project.