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Metro Tunnel

Site investigations types

Site investigations have been carried out to gather further information about ground and environmental conditions ahead of construction

If you have any questions about our site investigation activities please contact us.

Arborist assessments

Why did we undertake arborist assessments?

Arborist assessments were carried out on a number of trees along the project alignment. Arborist assessments documented the size and health of trees, their proximity to the project alignment and assessed the potential impact of the project on nearby trees.

The information gathered from the tree assessments informed project planning, including identifying and minimising potential impacts to trees from construction and operation of the new rail line.

How and where were these assessments undertaken?

Assessments were undertaken in public areas along the project alignment, between Kensington and South Yarra, by a team of up to four arborists. The studies involved:

  • Visual inspections to determine tree health, including evidence of insect infestation, soil pathogens and fungus.
  • Measuring the height and circumference of trees.
  • Assessing tree structure, including tree roots and canopy.

Basement surveys

Why did we undertake basement surveys?

We identified a number of properties along the proposed project alignment that have basements. To understand how the project's underground tunnels and stations will integrate with below ground features such as basements, we needed to undertake a visual inspection and 3D laser scan to ensure we fully understood the underground landscape in which the project will be built.

The information gathered through these studies helped inform project designs and proposed construction methodologies.

What does the survey involve?

The investigations involved a surveyor and structural engineer undertaking a visual inspection and 3D laser scanning of basements within buildings located near the proposed alignment.

The visual inspection involved taking photos to help document the existing dimensions and condition of the basement.

The 3D laser scanning involved a surveyor using a handheld device to scan the inside of the basement from various locations to produce a 3D image of the basement and its dimensions.

Cultural heritage surveys

Why are we undertaking cultural heritage investigations?

We undertook cultural heritage investigations to identify any Aboriginal cultural heritage artefacts present in the vicinity of the project.

We worked together with the Office of Aboriginal Affairs Victoria and Traditional Owner Groups to identify areas that required investigation.

The findings from these investigations guided the development of a Cultural Heritage Management Plan, which will recommend measures for the management and protection of Aboriginal cultural heritage.

What did these investigations involve?

Work at each location involved:

  • Setting up temporary fencing around each site.
  • Checking the site for underground services using non-destructive digging techniques
  • Digging a pit and sifting through the soil to identify any items of Aboriginal cultural heritage
  • Backfilling the pit and moving to the next site.

All sites were restored to their original or similar condition once works were complete.

Geotechnical investigations

Just like the landscape above ground has features like hills, rivers and different types of soil, underground Melbourne has its own landscape. This landscape includes a variety of geological conditions such as hard rock, softer rocks and clays, silt deposited by rivers and groundwater.

Geotechnical investigations help us to gain a greater understanding of local geological conditions and assisted in planning how the Metro Tunnel will be built and involved drilling into the ground to obtain rock and soil samples.

We are undertaking a range of investigations across the project corridor to better understand local ground conditions and the location of underground services ahead of the start of major construction.

Where did we conduct drilling?

The geotechnical drilling program involved drilling approximately 200 boreholes across the project from Kensington to South Yarra.

What do geotechnical investigations involve?

Stage one: Site establishment

Site compounds of varying sizes are set up to enable the work to be carried out safely. These areas will be reinstated at the completion of each borehole.

Stage two: Service identification

Prior to the commencement of geotechnical drilling, we undertake non-destructive digging which involves using a vacuum truck to dig carefully into the ground to confirm an area is free of services.

Stage three: Geotechnical drilling

100-millimetre diameter boreholes will be drilled up to a depth of 50 metres to retrieve a core sample which will provide information on ground conditions in the area. Drilling takes around one week to complete at each borehole.

Stage four: Well development

Groundwater and other materials which may have entered the well during drilling works are removed following geotechnical drilling. Well development typically takes half a day but may extend to a full day depending on the amount of groundwater encountered.

Stage five: Groundwater sampling

Groundwater is pumped from the borehole at a steady, controlled rate and groundwater samples will be recovered. Sampling typically takes a few hours to half a day to complete. Inspections of the groundwater are will be carried out every 2-3 weeks for several months to sample groundwater and measure groundwater levels.

Ground surface surveys

The ground surface survey is a low-impact activity designed to help us understand the way electrical currents may travel through certain ground conditions. The survey involves laying a 50-100 metre line of metal probes at several locations listed below. The small metal probes will be inserted approximately 5cm into the soil at regular intervals, and will collect data by releasing a low electrical current into the soil. A meter box is set-up to measure the resistance of the electrical current through the ground.

This survey needs to be undertaken in a number of parkland areas across the alignment as the probes cannot be inserted into asphalt. This method is commonly used in public spaces to test soil profile and density.

What does the survey involve?

The ground surface survey will involve:

  • Checking the site for underground services using ground penetrating radar
  • Sectioning off the work area using bollards and tape
  • Setting up a 50-100 metre line of metal probes, and inserting these by hand to a depth of approximately 5cm
  • Removing the probes on completion of the test

Groundwater monitoring

Why did we undertake groundwater monitoring?

Groundwater monitoring inspections were conducted at a select number of pre-existing boreholes and at some of our newly established boreholes to determine how construction activities could potentially influence groundwater sources in and around Melbourne.

The groundwater wells were located on both public and private land within 1.5 kilometres of the project alignment between Kensington and South Yarra.

What does groundwater monitoring involve?

Groundwater monitoring is undertaken by a specialist consultant who performs a visual inspection by lifting the lid of the well and taking a photo. Once the lid is replaced, the consultant leaves the property.

Laser scanning

What is laser scanning?

3D laser scanning took place at various locations along the project alignment. Laser scanning technology allowed the project to catalogue existing site conditions using photographs taken from a number of viewpoints to build a 3D model of the current environment.

Why did we undertake laser scanning?

Laser scanning helps to determine potential impacts from construction and operation of the new rail line on the existing environment.

What does laser scanning involve?

3D laser scanning involves a team of two people:

  • Assessing the current urban environment in the vicinity of the rail tunnel entrances in Kensington and South Yarra and in the vicinity of the underground stations (Arden, Parkville, State Library, Town Hall and Anzac).
  • A scanner mounted on a tripod which takes a series of images over a 15 minute period in each location.
  • Taking written notes to observe findings.

Level settlement monitoring

Level settlement monitoring helps us to understand horizontal and vertical ground movements along the alignment during construction.

The data gathered forms a baseline, allowing us to identify if any changes to ground levels have occurred during construction. Level Settlement Monitoring is a standard activity undertaken in advance of any major construction project.

Undertaking this type of monitoring does not mean that we expect ground movement to occur as a result of the project, in fact, a range of mitigation measures are built into construction planning such as grouting, groundwater monitoring and, if necessary, groundwater recharge, to manage any potential ground settlement impacts of the project.

Three types of Level Settlement Monitoring devices are used to understand horizontal and vertical ground movements:

  • Array Point: Similar to a household nail, these devices are drilled into the ground using a hand-held drill. They are placed at 10-30 metre intervals along public footpaths and on the grounds of the University of Melbourne.
  • Settlement Monitoring Points: These devices are between one and five metres in length and are approximately 10cm in diameter. They are inserted into the ground using a truck-mounted drilling rig. A metal cap is placed on top of the device to protect it. These devices may be installed in roadways, footpaths or parkland.
  • Permanent Survey Bench Marks: These are similar to the settlement monitoring points but are between 10 and 20 metres in length. They are installed into the ground using a drilling rig. A metal cap is placed on top of the device to protect it. These devices must be installed in areas of solid rock to provide a reference point to measure ground movement in an area. Like the Settlement Monitoring Points, these devices are installed in roadways, footpaths or parkland.

Noise and vibration

Why do we measure noise and vibration?

The purpose of the studies is to understand existing levels of noise and vibration in the urban environment. This helps us to:

  • Assess current and future noise and vibration levels in these areas.
  • Inform project designs and strategies to minimise potential impacts from noise and vibration during construction and operation of the new rail line.

How are we measuring noise?

Noise studies are undertaken using two different methods:

  • Attended studies, where consultants are present during the shorter-term monitoring event (generally up to 30 minutes).
  • Unattended studies, for longer-term data capture where logging equipment is left for up to one week to log noise levels in the surrounding environment.

How are we measuring vibration?

Loggers are placed on surfaces, such as footpaths, to record vibration events enabling consultants to identify peak vibration levels and measure these against specific events, such as passing trams or traffic. These measurements are recorded at a number of locations for approximately 30 minutes. All assessments are undertaken in public areas with consultants accompanying the equipment at all times.

How did we measure noise and vibration in the Yarra River?

Noise and vibration studies were undertaken in the Yarra River to document current underwater noise and vibration levels, which will help assess the potential impact of construction activities on aquatic life within the river.

These studies involved acoustic consultants lowering aquatic loggers into the river for approximately 30 minutes to obtain this data.

What is geophysical testing?

This testing helps us to understand how vibration travels through the ground so we could plan for and manage future construction impacts during the excavation of major shafts, tunnels and stations; as well as understand potential operational impacts from trains running through the tunnels and stations.

We undertook two types of tests over two nights:

  • For the first test, we placed geophones (a device that records ground movement) down an existing borehole and generated energy by using a hammer striking a metal plate on the ground continuously over a number of hours.
  • For the second test, we put compressed air down an existing borehole. Geophones were placed on the ground and record the vibration Peak Particle Velocity (PPV).

This testing needed to take place at night or in the early hours of the morning (between 8pm and 8am) when there were fewer competing sources of vibration such as trams or trains.

What is Continuous Sound Wave Testing?

This testing generates vibration at the ground surface. This vibration is measured using six geophones (a device that records ground movement) set out along the ground surface (for approximately five metres).

This testing needed to take place at night or in the early hours of the morning (between 7pm and 8am) when there are fewer competing sources of vibration such as trains and vehicles.

Service Investigations

Investigations were undertaken to identify the location of underground services in areas along the project alignment. These investigations included two activities, preliminary service investigations and trenching activities.

Preliminary Service Investigations

Electronic detection and visual inspections

Preliminary service identification involves the use of electronic detection equipment to confirm the location of underground services in the area.

Electronic detection is a safe and reliable method for accurately determining the location of underground services. Crews walk along streets with Electromagnetic Field equipment to locate pipes and cables beneath the surface, and determine the alignment and depth of those services. These investigations will involve lifting the lids of service pits and performing visual inspections.

Trenching activities

Trenching activities helps confirm the location of underground services.

This activity involves the excavation of narrow trenches in roadways and footpaths in areas along the project alignment.

Visual inspections

What do these studies involve?

A team of assessors undertook a range of visual inspections to document the existing conditions of areas along the project alignment. The assessors will took photographs to document the current condition of the urban environment..