Prosecution Details

Being an employer, failed, so far as was reasonably practicable, to provide and maintain a working environment in which its employees were not exposed to a hazard, and by that contravention, caused serious harm to an employee contrary to sections 19(1) and 19A(2) of the Occupational Safety and Health Act 1984.

SUMMARY OF THE INCIDENT

On 21 June 2019, a worker (victim) was seriously harmed when he was working in an excavation that collapsed (Incident).  Three lumps of sand, which had been hardened with microfine cement, came away from a wall of the excavation and one lump struck the worker.  The three lumps together weighed more than 500 kilograms. The victim suffered pelvis fractures, bruising to the pelvis and a laceration to his left calf, and required two surgeries.

The Incident occurred in Dalkeith WA, on residential land with a house on it and was the premises jointly owned by the Offender and his wife. The Offender was carrying on renovations as an owner builder.

The renovations included construction of a basement and adjacent lift shaft. Constructing the basement and lift shaft involved excavating approximately 3.6 metres below ground for the basement and approximately a further 1.2 metres below the basement for the lift shaft.

The location where the lumps of hardened sand sheared away from the wall was the central wall in the lift shaft excavation, which ran directly below a wall of the residence. At the time of the Incident, this wall was approximately 4.85 metres deep.

The Offender engaged a number of workers to assist him with the renovations and directed the victim and another worker to carry out work on 21 June 2019 which required them to enter the lift shaft excavation.

PLANNING THE EXCAVATION

The Offender started planning the renovations, including the excavation, in 2014.  He engaged a few contractors who had purported expertise in excavation work, either directly or, through a project manager (up to 2016), a consulting company.  These third parties included:

• a geotechnical engineering company, which provided a geotechnical report in November 2015 and provided ongoing advice to The Offender about the excavation until approximately March 2016;
• a structural engineering company, which was engaged on 16 June 2015 and provided advice on site and structural engineering designs dated 5 November 2015; and
• a grout injection contractor which was engaged by The Offender to provide advice and carry out microfine cement injections and the insertion of steel rods to reinforce those injections in the period from 2016 until 17 May 2019.

In May 2015 during the early stages of planning the works on site, the geotechnical engineering company suggested several measures the Offender could use to stabilise and temporarily retain the excavation. One measure was injecting microfine cement to stabilise the excavation. By the end of 2015, the Offender had decided to use microfine cement as a method to stabilise the excavation.

Microfine cement is an ultrafine cement which is injected into sand with spears. It fills the spaces between sand particles and hardens to form a continuous grout block. It is designed to provide temporary retaining to the soil whilst excavation takes place and permanent retaining methods are installed.

The grout injection contractor recommended to the Offender that the grout treatment area always be 300mm lower than the proposed maximum excavation depth. The Offender engaged the contractor to inject microfine cement to stabilise the existing central excavation face and to allow the lift shaft excavation to continue. The Offender and contractor agreed that the necessary injections would continue in a staged process following the initial injection of microfines and steel rods in February 2016.

In May 2019, the grout injection contractor carried out microfine injections and inserted steel reinforcement bars into the exposed part of the central wall as well as into the depth of the excavation.

From on or around 29 May 2019, the Offender continued to excavate the lift shaft excavation. However, the Offender did not engage the grout injection contractor to carry out further microfine cement injections.

To excavate the lift shaft excavation, the Offender dug sand out with an excavator, which he operated personally.

The days leading up to the Incident – 18 to 20 June 2019

On 18 June 2019 (three days before the Incident), the Offender, the victim and other worker were working on the lift shaft excavation. They  continued the excavation works in increments of 1.2 metres which the Offender carried out personally by operating the excavator. When excavating the lift shaft, the Offender excavated to a depth of approximately 1.2m, and then checked that there were microfines to a depth of at least 300mm under the new excavation depth as recommended by the grout injection contractor. He did this by scraping back by hand under the excavation face in random spots.

The purpose of checking that there were microfines to a depth of at least 300mm under the new excavation depth was to ensure that the excavation did not go below the depth of the microfines and thereby expose loose or un-retained sand at the wall of the excavation. If the excavation continued below the microfines, then the section of the lift shaft wall retained solely by microfines would be sitting on loose sand and, as such, would be at risk of shearing away from the wall.

On 20 June 2019 (the day before the Incident) at around 2pm, the other worker told the Offender that he had concerns about the microfine cement in the lift shaft excavation. The victim told the Offender that there were soft sections of sand both underneath and around the solid microfines in the central wall of the lift shaft excavation.  The Offender said words to the effect of "no, the wall is solid".

The victim picked up a shovel and scraped at the loose sand with it. He said words to the effect of "come and see for yourself".  The Offender went into the lift shaft excavation and poked at the central wall with the shovel. He felt loose sand fall from the wall in several places including the area.

The other worker said words to the effect of "we should get the microfines guys back to inject all the soft sand to make it safe".  The Offender determined that a shutter would be built and filled with concrete to secure the soft sand in the area — being, the lower part of the lift shaft wall including the area marked "void".

A shutter is an industry reference to a retaining system which in this situation would be comprised of the following:

• installing reinforcing bars into the wall parallel to the floor at the base of the microfine cement and through the microfine cement;
• installing a plywood shutter against the central wall of the base of the lift shaft excavation and bracing the plywood shutter;
• pouring concrete behind the plywood shutter; and
• once the concrete has set and cured, removing the plywood shutter.

There were two factors that influenced The Offender's decision to build a shutter: cost and time.

The Offender told the other worker that the shutter needed to be completed by the close of business the next day. The Offender said this because significant rain was forecast in the coming days which he was concerned would make the sand in the lift shaft softer and potentially undermine the house. Once the shutter was complete, concrete set and cured, and the shutter removed, the next phase of construction could be completed. The next phase of construction was the installation of a dincel which would have resulted in the entire lift shaft excavation being permanently retained.

The morning of the Incident – 21 June 2019

On 21 June 2019, the victim, the other worker and additional labourer started work at 6.30am. The Offender had intended to be on site but was unexpectedly detained with a family commitment.  At 7am the workers commenced work on the installation of the shutter.

The victim and the other worker did some quick cutting to the microfine cement walls, jackhammered microfine cement away from the wall using a small jackhammer and drove reinforcing bars into the core holes with a hammer and jackhammer. The additional labourer assisted as required.

They then cleaned and levelled the lift shaft excavation of loose material.  The victim then noticed that some of the cut reinforcing bars protruding from the wall were not the correct length. The victim used an angle grinder to cut the reinforcing bars.

THE INCIDENT

At or around 8.40am, the victim was crouching in the lift shaft excavation, cutting a reinforcing bar.  Three large lumps of hardened sand fell away from the central wall, which he was working on. The victim was wearing ear plugs but heard a loud cracking noise. He tried to launch himself out of the way.  He was knocked down by one of the lumps and trapped from the waist down.

The other worker  was also working in the lift shaft excavation at the time of the Incident but was not hit by the lumps. The additional labourer was inside the house fetching a tool at the time of the Incident.  Immediately after the Incident, the labourer ran downstairs to help the victim. At this time, sand was continuing to crumble off the wall that had just collapsed. The labourer got into the lift shaft excavation to help the victim. He used bottle jacks to lift the lump off the victim.  Another worker present at the site called emergency services. An ambulance arrived. Attending paramedics stabilised the victim and took him to Royal Perth Hospital.

The Offender was not present on site prior to the Incident.

THE VICTIM’S INJURY

The victim suffered pelvis fractures, bruising to the pelvis and a laceration to his left calf from the Incident. He spent 18 days in Royal Perth Hospital and underwent two surgeries, one to install a pubic symphysis plate and one to install a sacroiliac joint screw. As a result of his injuries, the victim used a wheelchair for four months and was incapacitated for this period. He has 16% permanent physical impairment and will require rehabilitation injury management for life. The victim was unable to work in any capacity for two years and will never be able to work as a carpenter again.

HAZARD

The hazard was an excavation collapse hazard, specifically earth or sand forming the walls of the excavation collapsing and burying or crushing a person in the excavation.

The Offender had seen, by November 2017 at the latest, the geotechnical report, which stated that "excavations in sand are particularly prone to instability unless support is provided". Both the geotechnical engineering and the grout injection contractors recommended or proposed using measures which can be understood to be for the purpose of minimising the risk of excavation collapse.

The Offender adopted various measures for the purpose of minimising the risk of excavation collapse, including:

• engaging a grout injection contractor to inject microfine cement and insert steel bars to stabilise the sand forming the excavation;
• shotcreting the first 1.5 metres of the face of the lift shaft excavation;
• instructing the victim and the other worker to drill core holes in the excavation walls to check the depth and consistency of the microfine cement; and
• on 20 June 2019, suggesting that the other worker install the shutter retaining system at the deepest section of the lift shaft excavation to enable a dincel to be inserted which would provide permanent retaining of the excavation.

CONTRAVENTION CAUSED SERIOUS HARM

A number of factors contributed to the Incident, namely:

• the microfine had not penetrated to a sufficient depth below the level of the excavation in the area where the hardened lumps of sand came away from the wall of the excavation. Therefore, the previously injected microfine in that area sat on loose sand;
• activities undertaken by the offender’s workers within the vicinity of the microfine cement grout block, causing three lumps to shear away from the excavation wall;
• the victim and the other worker used jackhammers in the lift shaft excavation, which caused excess vibrations which contributed to the three lumps becoming dislodged; and
• the lower portion of the lift shaft excavation (being at least 3.5 metres) was unsupported by any physical barrier to prevent collapse if the microfine cement sheared off the wall.

The Offender could have ensured that the workers were instructed not to enter the lift shaft excavation, and ensured this instruction was maintained until the risk of excavation collapse was assessed by a competent person.

This would have prevented the victim’s injury because he would not have been in the lift shaft excavation at the time it collapsed.  It was reasonably practicable for The Offender to have implemented this measure.

KNOWLEDGE THAT CONTRAVENTION COULD CAUSE DEATH OR SERIOUS HARM

The Offender knew there were pockets of loose sand near the base of the lift shaft excavation and was in the process of implementing a permanent solution to retain the excavation and also knew that, if the excavation were to collapse, it could cause a person death or serious harm.