An independent blast impact assessment was undertaken for the proposed expansion of the Montrose quarry.

The purpose of this assessment is to determine the effects risks and impacts of blasting in the proposed extension area and identify control measures required to ensure blasting is undertaken safely, with minimal impact to offsite receptors, and in accordance with regulations, standards and guidelines that apply to blasting under the regulatory framework for Victorian quarries.

The primary blasting risks and impacts considered in this report are;

• Ground vibration and airblast overpressure levels from blasting
• Flyrock risk and control
• Impacts of blasting to local amenity
• Risk of blasting to domestic animals and native fauna.

The bulk of the resource underlying Montrose quarry is classified by Geosciences Victoria as Coldstream Rhyolite located in the north, west and central pit areas, and Mount Evelyn Rhyodacite in the south and east. The units have similar characteristics (hardness, density, etc.) and both exhibit structures from highly weathered to fresh. A lesser quantity of welded tuff/breccia can be found in the southwest area and all units separated by clearly defined contact zones and faults. The structural characteristics of the rock and its behaviour under blasting is well known to quarry management.

The extension area is immediately south and southeast of the current WA100 boundary and covers ~12.7 hectares of the proposed 30.5 hectare Work Authority variation area. Approximately 3.9 hectares would be subject to extraction by blasting. The remaining 3.1 hectares would consist of overburden batters formed by mechanical excavation.

Under current pit designs blasting would occur on benches with maximum face heights of 16m and minimum berm widths of 10m. Extraction would occur from elevations RL 21 (the current and terminal pit floor level) to maximum elevations of RL 160 (south wall) and RL 176 (east wall). Further extraction is also proposed within the current Work Authority with production blasts along the east wall and limited blasting along northern benches to form terminal batters for site rehabilitation. Conceptual rehabilitation plans show infilling of the pit to commence from the north and east during latter stages of the proposed extraction phase.

Like other metropolitan quarries, numerous residences are located within a few kilometres of Montrose quarry and blast vibration and associated effects are perceptible to a relatively large number of people. Blast vibration levels reduce rapidly over distance and ground vibration and airblast (overpressure) levels are typically low in areas more than 1 km from blast sites, becoming imperceptible beyond 1.5 km. Ground vibration and airblast (overpressure) levels at residences less than 500m of the proposed extraction area were the primary focus of the assessment as these areas are where the highest levels would occur. Maintaining compliance with regulatory blast vibration limits at the closest houses will effectively ensure compliance is achieved in more distant areas.

There is estimated to be 150-200 residences (occupied houses) within 500m of proposed future operations, the closest located at Ash Grove and Kirkwood Court to the immediate northeast. Eight residences within 500m are identified along Jeanette Maree Court to the southwest, and five residences on Sheffield and Glascow Roads to the south.

The closest houses and their minimum separation distance to the proposed Limit of Blasting are;

• 33 Ash Grove, 160m northeast

• 13 Jeanette Maree Court, 205m southwest

• 245 Sheffield Road, 385m south-southeast

A total of 240 blast vibration measurements were analysed from 106 blasts fired at the quarry between August 2020 and July 2023. Blasting during this period was conducted from benches 6 to 11 with MICs ranging from 27 to 143 kg/delay, three monitors are installed for every blast.

Monitoring results show that Boral have been successful maintaining blast vibration levels below 5 mm/s at the closest houses through careful attention to blast design and loading. This includes a limited number of blasts located between 200-300m from Ash Grove, demonstrating that compliance with ERR limits can also be achieved from proposed future operations.

The assessment found that the risk of blast vibration-induced damage to houses from blasting the proposed Montrose quarry extension is low and PPV and airblast levels can be maintained below the ERR limits for human comfort.

This assessment demonstrates that blasting operations within the proposed extension area of Boral Montrose quarry can be undertaken safely, comply with prescribed regulatory criteria for quarry blasting, and would not result in substantially increased impacts from previous operations.

Blast and Vibration Information

Blasting is carried out approximately 1-3 times a month at Montrose Quarry. The quarry maintains a blast register for nearby neighbours and notifies people when blasting activities occur. If you would like to join the register, please send an email to community@boral.com.au with your name, address and contact details (email and mobile) and specify it is for Montrose quarry.  

What is Blasting?   

There are a number of ways to extract ‘hard’ rock, but the most common and effective method is ‘controlled blasting’. As blasting is very precise practice, each blast is designed and carried out by an independent third-party blast expert. A blast design is generated and is a carefully planned operation that involves drilling into the rock in a specified pattern, then placing a very precise amount of explosive in the holes. The explosives are then detonated in a precise sequence, designed to maximise the efficiency of rock breakage while minimising noise, vibration and dust. One of the advantages of blasting is that it reduces the need to operate large heavy equipment to extract the rock, in turn reducing noise and greenhouse gas emissions.

Depending on how close you live to a quarry, you may notice some vibration or noise associated with blasting, however all blasts as mentioned are precisely designed to prevent and minimise impacts to surrounding neighbours.  

It is common for some people living near quarries to believe blast vibration is the cause of cracks and defects that develop in all houses over time, regardless of the blast vibration levels that occur. Numerous investigations of blast damage claims have shown such cracks and defects are cosmetic or minor in nature and can be attributed to normal mechanisms that affect all houses including seasonal movement of foundation soils (particularly clay soils), poor drainage around houses, lack of property maintenance, and the effects of ageing and weathering.

What safeguards are in place during blasting?   

Strict national safety regulations apply to protect neighbouring homes, buildings and public places from the potential effects of blasting. Blasting operations must adhere to prescribed limits stipulated by the relevant regulatory authority that are well below the vibration levels which could cause structural or cosmetic damage.

The key standards and guidelines are Australian Standard: AS 2187.2 with all blasts measured at the nearest sensitive receptor as outlined in the site management plans. 

​ ​Airblast Overpressure 

​·       Maximum limit (95% compliance): ≤ 115 dB (linear peak) 

​·       Absolute limit (never to be exceeded): ≤ 120 dB (linear peak) 

​Ground Vibration (Peak Particle Velocity – PPV) 

​·       Maximum limit (95% compliance): ≤ 5 mm/s 

​·       Absolute limit (never to be exceeded): ≤ 10 mm/s 

Blasts are monitored at different locations with sensitive ground and air vibration equipment to ensure they remain within regulations.