Raumati Escarpment – Detailed Case Study
Site Owner – Kiwirail
Lead Engineer – WSP Opus
New Zealand’s main trunk railway line runs north out of Wellington past the Raumati Escarpment where recent slippages began occurring. The clay soil began slipping towards the trunk line leading to landslides. KiwiRail commissioned WSP Opus to monitor this site for the risk of a landslide affecting the track itself.
To help drain the water pooling, trenches with pipes were added to remove excess water. Stakes were added to the ground and the distance between them measured over time to assess the relative slippage. However, this method of data capture was infrequent and not very accurate.
In order to monitor the movement on the escarpment Senior Geotechnical Engineer David Stewart from WSP Opus identified 10 site that would benefit from daily precise measurements using AccuMM sensors. Seven of these sensors would be located on the site of a previous slip while three of the sensors were located at the head of the slip that and not had any movements. These positions were selected so they could be seen from across where manual surveying could also occur in parallel.
Figure 1. Raumati Escarpment Slip site
Figure 2. Location of the AccuMM sensors at Raumati Escarpment
On the 19th January 2018 the field team hammered in 10 metal stakes into the soil and bolted the 10 AccuMM sensors to the stakes. As the slope was North West facing with good view of the sky a series of solar powered sensors were used that would charge during the day and turn off at night. To help compare the data from AccuMM, a reflective survey sticker was attached to each sensor. This would allow manual survey measurements to occur in parallel and help validate the AccuMM data.
Figure 3. Sensors installed at Raumati Escarpment
Figure 4. Sensor 305 at Raumati Escarpment near main railway line
On the 20th January 2018 the base station was installed. This could have been across the railway line and road which would have given excellent line of site to the sensors. However, because the site was not secure with a risk of vandalism, the base station was installed 100m south along the escarpment on a stable patch of ground next to the railway line but not visible from the road. The site has good cellular coverage – this meant the base station could use cellular data to pass information to the cloud for processing. In addition, a Yagi antenna was used to penetrate the 100m of shrub and enable the base station to connect with the sensors using a 2.4GHz frequency.
The geographic restraints in the site meant that sensor 301 could not connect to the base station. In order to get the signal from sensor 301, an AccuMM 2.4GHz repeater was added next to sensor 307 that had line of sight to sensor 301. With the repeater in place all the sensors could communicate to the base station.
Figure 5. AccuMM base station at Raumati Escarpment
Once installed the raw GNSS data was collected by the sensor network and transmitted via the base station to the cloud. This raw data was analysed to produce the relative position (North, East and Up) to the base station of each of sensors.
In order to aggregate the data from AccuMM into the alerting and monitoring system, AccuMM provided a JSON interface that allowed the geotechnical team to retrieve the reading via an API from the sensor network as they were processed in the cloud. AccuMM provided three sets of (North, East and Up) data for each sensor.
Below is the AccuMM Data for the twelve, six and two step size relative position data for sensor 305 at Raumati Escarpment for March 2018. The Horizontal 2DRMS is 4.39mm, 4.80mm and 7.09mm for the twelve, six and two step sizes respectively.
At 11am on 27th February 2018 WSP Opus moved sensor 301 and 305 by 230mm and 660 mm respectively. The AccuMM data for sensor 305 can be seen below. The twelve step size data could not calculate a result at midnight after the movement as half the data from front the old location and half from the new location. The first twelve step size measurement was obtained at midday on the 28th February 2018.
WSP Opus has drilled some boreholes and installed inclinometers in the landslide to allow him to measure the depth of movement. Inclinometers are useful as they help determine the appropriate method of remediation. However, field staff have to return regularly to see if the slope has moved and distorted the bore hole. With AccuMM sensors installed on site, WSP Opus knows now when the movement has occurred and can then tell the field staff to make a site visit.