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Landslide management on the east wall of Dubbo Quarry

 

In 2018-19, rock extraction from Dubbo Quarry’s east wall posed a landslide risk for Alasdair Webb’s team. As Alasdair explains, after extensive preparations to mitigate the hazard, the landslide occurred without injury or equipment damage, and with no interruption to the business.

Extraction of quarry rock from the east wall of Dubbo Quarry posed a risk of producing a landslide along a known geological contact. The hazard and associated risks were identified and assessed. The zone was surveyed and a landslide predicted. Before the predicted failure, the area was abandoned. A landslide occurred without injury or equipment damage. There was no significant interruption to our business.

This methodology and practice were highly effective at solving the issue. The potential for a landslide was identified well in advance and when the landslide began to move, survey targets allowed the motion of the ground to be monitored and ground failure predicted. This allowed the quarry access road relocation and removal of all quarrying activities from below. A potential landslide is a dangerous hazard and by effective management a serious incident was alleviated. We could not stop the landslide, but we could monitor it and predict when to abandon the quarry area below.

A preliminary geotechnical investigation by Xstract in 2017 at the Holcim Dubbo Quarry identified the potential for a landslide if the lower bench of the east pit was extracted.
This geotechnical investigation was the result of a company-wide campaign to assess
slope stability issues in Holcim quarries.

Due to quarry planning issues, a production blast had to be fired in the lower bench of the east pit wall. This liberated 86,000 tonnes of raw feed in September 2018. On 23 October, 2018, as extraction from the 86,000-tonne muck pile continued, some extension cracks were noticed in the quarry access road adjacent to the eastern pit upper face directly above the blast by the quarry supervisor “doing his rounds”. The access road was diverted, and the original access road closed. The discovery of the cracks was immediately entered into our internal safety database. The cracks were monitored and it was decided to survey the zone that was moving to see the speed and magnitude of the upper face movement. Discussions were held with our regular contract surveyor and a plan of ongoing monitoring was developed. Survey targets were set up and base data was obtained. Survey results were collected twice per week (sometimes more often) from 4 December, 2018 and indicated that the face was moving toward the open pit.

In mid-January 2019, the upper face began to move faster and further than previously measured. Consultants from GHD were engaged to perform a geotechnical investigation. The recommendations in the GHD geotechnical report (ie leave some material packed against the lower face and establishment of a catch trench) were adopted and extraction from that area ceased in early February 2019. All personnel were notified of the issue and all equipment and people were removed from the extraction area. On the night of 7 February, 2019, the higher part of the face slumped down approximately three metres. The quarry manager noticed the slope failure upon arriving at work at 6.00am on 8 February, 2019. This slope failure was predicted due to the past three months of observations. No one was injured and no equipment damaged. Business was not interrupted as we had taken the road at the top of the landslide out of service, constructed a new access road and removed all equipment and personnel from below.

The New South Wales Resources Regulator was contacted immediately and was satisfied with our level of focus. The landslide event was added to our internal safety database.

INNOVATION/ORIGINALITY

Minor extension cracks trending north/south were discovered on 23 October, 2018 in the ground (roadway) directly above the east wall extraction zone in an area where a landslide was predicted to occur. Upon examination of the cracks, a survey was designed in conjunction with a local surveyor to monitor movement on the top of the east pit’s east wall above the geological contact between the basalt deposit (hanging wall) and the underlying sloping contact of the underlying sandstone (footwall).

A prompt meeting with our regular contract surveyor covered topics such as base station establishment for reference points, survey target construction and survey target location.

An arbitrary local chainage was established across the top of the hanging wall to give some spatial control to the affected area and the location of survey targets was established to best indicate lateral and vertical ground movement.

The survey targets were made out of cut off star pickets with a mounting pad welded on top to accept yellow plastic survey prisms. The quarry supervisor invented the design for the survey prism mounts, which were then positioned along the marked chainage at critical locations to alert us to ground movement, ie east or west of the north/south trending extension cracks on the ground.

Figure 1. The survey data monitoring the movement of the quarry’s eastern wall. Data and graph (Figure 2) courtesy of Jason Helmuth, GHD.

The survey targets were hammered into the ground using a special welded cap that was removable so as not to damage the prism mounting pad (also an invention from the quarry supervisor). “Back sight” reference points were established mounted and surveyed in. It took about four weeks to develop the survey, establish the datum points, fabricate the targets and perform the initial reference survey to form the baseline data. 

Traffic was diverted to a new access road constructed immediately when the cracks were first discovered to remove traffic from the affected area.

Once the prism targets were in place, regular surveying was performed, starting on 4 December, 2018. The survey was done at 5.30am as it was summer and the potential for refraction was too great as the day warmed up. We were aiming for an accuracy of around plus or minus 3mm.

The survey data was presented in an innovative format designed in-house to present relative movement and, importantly, acceleration of ground movement.

Every Tuesday and Thursday mornings (sometimes more often) the surveyor took readings. We compared the survey targets set up along the arbitrary chainage on the zone that was moving to the reference points that were not moving and calculate the amount of movement. The results are presented in Figure 1 – titled “Monitoring Survey Results” – and Figure 2 – “Monitoring Data Graph Calculations” – for each of the survey chainage prism monitoring points. 

The “Monitoring Survey Results” in Figure 1 are presented as Eastings (E’) and Northings (N’) and changes in Relative Level (dRL). The column with black numbers is the amount of movement (in mm) relative to the datum and the column with the red numbers is the difference between the current reading and the last reading. The difference column showed us any changes in the speed of the movement (ie amount over time). The red indicates movement greater than 10mm when compared to the datum. The “Monitoring Data Graph Calculations” in Figure 2 show the same information but in a graphed form showing movement trends downwards. The graphs show movement in millimetres against time (days since monitoring began).

Movements were slow and imperceptible at first until late January 2019 (about 50 days into monitoring) when movement increased significantly as shown by increased slope downwards on the graphs. Quarrying continued below the landslide zone and the recommendations as per the GHD Geotechnical Report were put in place.

Two trenches were dug at 90 degrees across the extension cracks at the top to get a better idea of the potential plane of weakness between the hanging wall and the footwall. 

Once the movement was identified to be accelerating after 60 days and the RL started to accelerate downwards, quarrying was stopped and all equipment and personnel removed from the area below the potential landslide. It is estimated that we were unable to extract the final 6000 to 8000 tonnes of raw feed at the base of the landslide zone as the survey results indicated that we needed to abandon the area and the remaining rock was able to be used to form the “catch berm”.

On the night of 7 February, 2019, the landslide occurred as predicted along the plane of weakness caused by the geological contact between the basalt hanging wall and the sandstone footwall. The landslide was not a violent slope failure but a gentle slumping as the hanging wall slid down the sloping footwall. The old (now redundant) roadway moved down on top of the hanging wall as the block of rock slowly moved downwards. This completely severed the old access road.

The landslide came to rest as predicted on the “catch berm” at the bottom of the face. The survey targets and method were designed specifically for this landside and the results presentation derived in discussion between the Dubbo quarry manager and the surveyor. Much information was taken from the survey results. The information was pivotal in decision-making about when to abandon quarrying in the east pit and predict the landslide. Without the data, we would only have been able to rely on visual observation and memory.

The cross-section after the quarry blast to the hanging wall with the landslide.

IMPACT ON OPERATIONS

Once the cracks in the roadway at ground surface were first discovered on 23 October, 2018, the immediate impact was to cease production for the rest of the day and to call all personnel to a meeting to explain to them the situation and the way forward. During this meeting a Job Safety Analysis (JSA) was prepared in conjunction with all personnel to construct an alternative access road and decommission the main access road that had served the quarry for nearly 40 years. The JSA showed the input from all personnel who normally would be involved in the day’s production but now would be involved in construction of the alternative access road.

That afternoon, everyone set about their tasks to safely construct a new access road. Due to their involvement in the JSA everyone knew what was required to build an alternative access road. By 11.00am the next day, a new road was constructed and the traffic was permanently diverted away from the zone containing the potential landslide. Due to the actions of quarry personnel and the large amount of data gathered about this landslide, a course of events was mapped and a decision process was made possible. There was no impact on sales and essentially the business as a whole. Had we not acted, our main access road would have been
lost completely and there was potential for a fatal injury and equipment damage due
to engulfment.

COST-EFFECTIVE SOLUTION

A preliminary geotechnical investigation conducted by Xstract in 2017 identified the potential for a landslide if the lower bench of the east pit was extracted. While initial geotechnical surveys of whole quarry sites can be costly, the identification of high risk areas is worth the money. The potential for equipment damage, injury or loss of life is too great. The focused geotechnical review conducted by GHD was not expensive and yielded cost-effective solutions and advice. It was good to have had an expert to site to advise on such a hazard.

Once ground movement was actually detected, ongoing surveying was very inexpensive (especially with survey target mountings being able to be fabricated at the quarry) and gave us some high value information about the ground movement and ultimately the acceleration of the ground towards a landslide. Once set up, the surveyor would only spend about one and a half hours on site two to three times per week to survey all marker prisms. Within another couple of hours, the surveyor had prepared the report for the day so that movement could be quantified and ultimately the decision to abandon the area was able to be made quickly.

The estimated cost for all of the geotechnical work and surveying was around $50,000. The road reconstruction was done in-house using overburden and downgraded materials. The cost of such work was not quantified. The lost production was insignificant.

Now faced with the landslide rubble pile, we can utilise the area in several positive ways. First, we intend to use a dozer and excavator to stabilise the area and separate the unusable rock. This will yield us useable rock to run through the crushing plant. Gravity has moved it for us so we intend to use the good rock as raw feed to make a saleable product. Second, we have been planning the construction of an access ramp into the east pit in the area of the landslide for some time. Therefore, a dozer and excavator will be able to stabilise the area for extraction and also construct the ramp simultaneously.

TRANSFERABILITY

This technique is transferrable to any operation that has ongoing ground stability issues that could result in a landslide into an active quarry pit.

Understanding the geology of your quarry site is paramount to predicting slope stability. The way that the slope stability issue at Dubbo Quarry was handled resulted in a controlled landslide event that was well known in advance.

It was established upon later discussion with the geotechnical consultant from GHD that the consultants often see situations similar to this where there is potential for slope failure. They make recommendations and although their advice is heeded, a landslide rarely happens. The consultant felt that this event was exceptional in the course of events that followed from the initial identification, survey development, application of survey techniques, ongoing monitoring with relative movement recorded, implementation of the consultant’s recommendations resulting in the abandoning of the area and ultimately ground failure at the geological contact.

The role of the work team is also something that can be transferred. As soon as the cracks were discovered, production ceased, as nothing is more important than the safety of people at our site. The work team actively got involved with the JSA process and went out and built an alternative access road with a full understanding of what they were doing and the knowledge that their safety was more important than a day’s production. This is a transferable initiative that can be used at any workplace.

Hazard identified, risk assessed, innovative data gathering, controls and alternatives put in place, danger to people and equipment eliminated. 

Alasdair Webb is the quarry manager at Holcim Dubbo Quarry.

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