Feature: Consistent Data for Unstable Ground
Professional Surveyor Magazine - March 2012
After a massive earthquake hit Christchurch, New Zealand, damage assessment using the latest in surveying technologies—GNSS instruments, 3D scanning, and RTN—is laying the foundations for recovery.
By Frances Mortimer
The road to earthquake recovery in Christchurch, New Zealand, is a bumpy one. As city and government officials attempt to repair and rebuild, aftershocks from the initial September 4, 2010 earthquake continue to cause further damage. Some estimates put the total cost of rebuilding to insurers between $16 - 23 billion (NZ$20 - 30 billion), making it by far New Zealand’s costliest natural disaster and the third-costliest earthquake (nominally) worldwide.
Post-quake 3D Record Aids Cadastral Recovery
Immediately following the February 2011 earthquake, Christchurch’s central business district (CBD) was cordoned off behind army barricades as additional, strong aftershocks continued to bring down city buildings. As soon as the extent of the damage was understood, Trimble
offered its MX8 mobile spatial imaging system (with an operator) to the government agencies responsible for all spatial data. The offer was accepted immediately.
The system was installed on an SUV belonging to Martin Hewitt, business development manager at GeoSystems New Zealand, Ltd
. Inside a “pod” on the vehicle’s roof, two scanners were installed at 270 degrees to each other in order to gather a complete model. The pod also held four cameras: three forward and one rear facing. Where the passenger seat should have been, the monitors were positioned and connected to a rack of computers—running Trimble Trident data capture software—that had been loaded in the rear of the vehicle. Also, an Applanix POS LV system with two antennas was included as part of the system. The system combines inertial, GNSS, and distance measurement technologies. Should GNSS signals be lost, for example in a tunnel, the inertial navigation system would continue to provide positioning data information.
With the assistance of the Ministry of Civil Defense and Emergency Management
, a three-person spatial imaging team gained entry to the CBD following an intensive induction similar to that of a major construction site. The team was briefed on safety equipment and procedures, as well as potential hazards such as major aftershocks and sinkholes.
When the team entered the highly restricted, cordoned-off area, deserted except for construction workers, they were in for a shock. As Hewitt put it, “It was eerie and so surreal.” Even though Hewitt is a Christchurch local, with familiar buildings now crumbled it was often difficult for him to determine where he was without the GNSS data.
Capable of operating at normal driving speeds, the system was limited to speeds of only about 12 mi (20 km) per hour due to debris and other obstacles on the roads. Still, it took only two days to capture all the data. Once the scanning project was complete, the data was post-processed at Trimble’s office in Montreal, Canada, using control data from a Trimble NetR9 reference station in GeoSystems’ iBASE network.
GeoSystems added the reference station to the network near Christchurch specifically to provide control for the area during the early recovery process. Free access to the network was offered to any surveyor in the affected area during this period. The iBASE network, based on VRS technology, is an RTK and DGPS corrections service spanning New Zealand and is accessible to users via cellular technology.
To make the CBD safe for rebuilding, authorities have rapidly demolished and cleared numerous damaged buildings. This process has eliminated countless survey marks, making re-measuring property boundaries extremely difficult. Additionally, the physical boundary of many commercial properties came down to the occupation on the site as the primary definition of the property. When buildings were cleared away, those physical occupations were lost for good.
“Thanks to the scanning operation, we have a unique 3D model of the CBD accurate to around 5 cm [2 in],” says Hewitt. “And it’s available for use by anyone who requires it for the recovery process, future development, or for a historical point of view, potentially making the Trimble MX scanning data critically important in the cadastral recovery process.”
Measuring Land Sinkage in the Orange Zone
Damage to the Christchurch area extends far beyond the central city. Many thousands of residential and commercial buildings have been affected throughout the region.
For many of the damaged properties, the main problem was often land sinkage from liquefaction, where wet sediment has seeped to the surface with each sizeable quake. (Approximately 440,924 tons [400,000 tonnes] of grey silt has since been carted away from Christchurch city streets and properties and even from inside homes.) Because the land on which the city is located was once mainly swamp, estuaries, and lagoons (two rivers, the Avon and Heathcote, still meander through the city), the Christchurch area is normally quite wet.
Loss of sediment from the ground has reduced the thickness of the crust between the surface and the ground water table, causing the level of the land to sink lower—in some areas by up to 4.9 ft (1.5 m)—and making it less able to support construction. Engineers and decision makers required survey data to understand how to bring the land back above the flood level and to determine whether it was worthwhile economically to do so.
Soon after the June 2011 quake, engineers divided Christchurch into four zones: green, red, white, and orange. The green zone encompassed approximately 100,000 properties where repair and rebuilding would be safe and economically viable. In the red zone, about 5,000 properties would be abandoned due to the extent of land damage. Properties in the white zone were still being mapped.
In the orange zone, approximately 9,000 properties needed further examination to determine whether they would be labeled red or green. Residents and business owners in the orange zone were understandably anxious for answers.
Geotechnical engineers from Tonkin & Taylor, Ltd
., were brought on as consultants for the Canterbury Earthquake Recovery Authority
(CERA) to map the orange zone using airborne lidar technology. However, progress was slow when poor winter weather conditions repeatedly grounded the aircraft. So Tonkin & Taylor turned to GNSS surveying to provide its geotechnical reports, awarding the survey contract to Paterson Pitts
, providers of land survey and resource management services to the South Island’s Otago region. Paterson Pitts director and registered professional surveyor Mike Botting managed the survey and all personnel.
Due to the wide area to be surveyed and the number of surveyors from different companies, the project’s success hinged on all data being consistent. GNSS surveying expert Reece Gardner of 3D World in Christchurch was brought in to define all surveying procedures for the project—from GNSS system to survey style, geoid to control—to ensure that consistency. Gardner was also responsible for processing, quality checking, and ensuring data completeness. Botting put it simply: “We couldn’t afford to have any mistakes; people’s lives, properties, and livelihoods were at stake.”
Personnel and procedures were not the team’s only concerns: additional surveying equipment was needed. For this, Paterson Pitts turned to GeoSystems, which quickly mobilized nine additional systems comprising Trimble rovers and controllers running Trimble Access or Trimble Survey
Controller field software. In accordance with the defined procedure, each system used the latest versions of software and firmware to ensure maximum performance. GeoSystems also provided control to the surveying project via its iBASE network, which has reference stations located both inside and outside the quake zone.
Surveying began on schedule with surveyors focused on the vertical movement of each property. A measurement was taken every 32.8 ft (10 m), which meant 8 to 10 measurements per property. Measurements were also taken from the berm and, if possible, from the centerline of the street, which typically hadn’t moved as much as the surrounding land. Up to 20 surveying teams were on the ground each day, and at peak demand the iBASE network reached a record number of concurrent users.
“It was just basic survey work, but at such a large scale—over 860 ha [2,125 acres]—that the VRS network and speed at which we could operate made a significant difference,” said Botting. “Teams would go straight to a site and start surveying in a couple of minutes thanks to the VRS.” Most points gathered were GNSS measurements, with GLONASS satellites proving especially valuable in areas where canopy was a factor, for example in established suburbs with large trees. On systems employing the latest Trimble TSC3 controller, the internal modem made data capture easier because no external cell phone was required to access the network.
Although one person with a GNSS rover could achieve most measurements, it was critical that each surveyor work with a partner close by because of the unique challenges of the project. For instance, surveyors frequently encountered dogs, many significantly stressed by the on-going earthquake; one surveyor was bitten. Responsible for the teams’ health and safety, Botting ensured that all teams had the appropriate ID and followed CERA protocols for entering properties. Teams checked in at the start and finish of each day.
The teams had to be cognizant that they were entering properties owned by people traumatized by many months of earthquakes and facing the potential loss of their homes. “It was shocking to see the damage,” says Botting. “Sometimes, though, you didn’t appreciate it till you stopped and got out of the car. I walked onto one property and looked straight into the gutter of a house that had sunk down about a meter. The whole house had completely dropped.”
Although three weeks were scheduled to finish the orange-zone survey, it was completed in just two weeks due to the speed and efficiency of the surveying teams. This clearly demonstrates the value that using advanced surveying solutions will add to Christchurch’s efforts to rebuild.
The Shaking—and Recovery—
In just the first half of January 2012, New Zealand’s geological hazard monitoring system GeoNet
—powered by Trimble NetRS and NetR9 CORS—listed more than 80 aftershocks in the Christchurch/Canterbury region. Most have been in the magnitude 2.6 to 3.9 range; 18 have exceeded 4.0 and, of those, 5 have been from 5.0 to 5.5. But thanks to today’s surveying and scanning technology and the many dedicated people helping to provide accurate, essential data, the long, bumpy road to recovery is being made a bit smoother.
At 4:35 a.m. on September 4, 2010, the residents of New Zealand’s second-largest city, Christchurch, were jolted awake by the intense shaking of a magnitude 7.1 earthquake. Since then, the Christchurch area has experienced almost 10,000 aftershocks. In particular, a magnitude 6.3 quake on February 22, 2011 resulted in 185 fatalities and wreaked havoc on buildings, city infrastructure, and land already weakened by the September quake. Although smaller in magnitude on the Richter scale, the February event was shallower and harder, with one of the world’s highest recordings of peak ground acceleration at 2.2g. The next major quake, a magnitude 6.4 on June 13, significantly set back the city’s rebuilding efforts.
Frances Mortimer is a freelance writer specializing in high-tech positioning solutions. Frances writes out of the United States and New Zealand.
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