Geotechnical
8
 min read

SH4 Te Ore Ore slip

SH4 Te Ore Ore slip

This was a large slip that occurred along SH4 that was part of an older, larger, landslip. This unfortunately reactivated due to an ongoing increase in pore water pressure within the landslip mass and on the basal shear surface. ​ 

Pressures rose to a point where the material in the slip became buoyant and began to slide along a shallow preferential shear plane up to 40m below ground level. ​ 

A temporary road had been implemented and Cirro were asked to assist with the Instrumentation and Monitoring requirements.

Problems and Solutions 

Delivering an instrumentation solution on this particular site meant that we had to overcome a number of challenges:

  • This was a remote site with no signal coverage, requiring a robust and reliable solution to prevent unnecessary site visits. 
  • We checked the surrounding area and signal coverage was found at a nearby hilltop where we installed a ‘gateway’, creating our own network coverage across the site. 

 This photo shows the gateway that we deployed on the hilltop.

  • We had to integrate existing sensors and telemetry on site.  

The client wanted all instrument data, both new and existing, to be hosted in one online location so everyone would have access to the information they needed. 

  • To facilitate this, we developed Cirro to accommodate data formats, from a variety of different sources and providers to allow the data to be forwarded into our platform. 
  • High frequency reporting of data was required to effectively administer a Trigger Action Response Plan. 
  • With Datapoints initially being uploaded every 15 minutes, which then reduced to 1 hour intervals once water pressures had stabilised 
SENSORS MONITORED: 
  • A large number of VWP’s across numerous boreholes 
  • Flow rates and volumes of Ground water Pumps 
  • Rainfall 
  • Extensometers monitoring for translational movements of the slip 
  • As well as Tilt Meters, Webcams and Inclinometers. 

The System 

A custom site overview was implemented, so that users could see the latest drone imagery of the site, with the location of each instrument showing it’s icon in green, orange or red to indicate the alarm status. 

The users had set up multiple alarms to be notified of any exceedances which was key for successful management of the TARP.

  1. This is how the data solution looks in the platform for the client. Users can plot data from multiple instruments on a single graph, allowing great insights to the data. 
  2. After the initial investigation, data was processed and a ground model was prepared. The stability of the landslip under initial groundwater levels was assessed to have a Factor of Safety of 1.05. After modelling rises in groundwater pressures, the engineers were able to assess what the groundwater level had likely been at the time of the landslip. 
  3. During the investigation stage, a series of deep pumping wells were installed to assess the effectiveness of vertical pumping and the impact it had on groundwater levels, and therefore the overall stability of the landslip.  
  4. After pumping water for about 9 months, they could see the water levels receding across much of the slip area.
    This graph illustrates the effect of that pumping - showing data from 4 VWPs in one of the many boreholes on site, noting the drawdown of water level over time, as well as the groundwater pressure response to rainfall. 

The graph also illustrates the alarm and alert thresholds that had been established (the horizontal lines at the top of the graph). When these are triggered, SMS and emails are issued to subscribed users. 

The Geotechnical Lead on this project would log in to check the data coming in every day, keeping an eye on the groundwater and pumping activity. As the water pressures continued to decline, the factor of safety gradually increased.

TESTIMONIAL:  
Nick Peters
– Senior Engineering Geologist:


“I still log into check the data coming in every other day, keeping an eye on the groundwater and pumping activity. As the water pressures continue to decline, the factor of safety gradually increases. There’s a masters thesis written all over this project, in particular around the relationship between groundwater level response to rainfall, the effectiveness of the pumping wells installed, or looking more closely at the hydrogeological and geological models."

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