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A Slippery Slope of Opportunity: Bulson Surveying Puts Remote Monitoring to the Test in Telluride

By Rowland Chen

The Telluride–Mountain Village Gondola is a free public, wind-powered transportation system that stretches between the mountain communities of Telluride and Mountain Village, Colorado. Managed by the Telluride Mountain Village Owners Association (TMVOA), this impressive four-station transport system provides some thousands of visitors and residents access to mountain hiking and biking trails in the summer and ski slopes in the winter. 

Heavy 2023 spring rains caused a significant release of soils from the hillside around Gondola Station 4 in Village Center. An assessment by geotechnical engineers indicated the slope instability was due to subsurface soil movement atop bedrock, a consequence of warming temperatures, melting snow and rain. The area of concern is roughly 2,500 square feet with an average depth of 5 feet [Figure 1].

Figure 1: Soil movement following spring rains around Gondola Station 4

According to TMVOA, crews removed around 1,270 cubic yards of dirt from the hillside as a precautionary measure to prevent soil from sloughing off the bedrock layer underneath and potentially sliding into the gondola station or the path of the gondola operating line. Of note, the integrity of the gondola station and gondola towers were not threatened by the incident.

However, at the recommendation of geotechnical experts, TMVOA contracted Telluride-based firm Bulson Surveying to set up a monitoring regime to scientifically track soil movements in the area over the course of six months. For Dave Bulson, owner of the survey firm, this project was a unique opportunity to demonstrate the power of today’s latest monitoring technology.

Remote Reliance

The goal of the monitoring campaign around Gondola Station 4 was to track ground movement across the entire area, with particular emphasis on the hillside retaining wall and around the gondola towers. Per the geotechnical consultant’s requirements, Bulson would need to track and check slope changes daily.

“We already had the equipment to effectively monitor a site this size. Our second Trimble S7 total station along with prisms are purpose-built for this kind of activity. Further, I had seen the Trimble 4D Control software at Trimble Dimensions and thought it would be ideal for this monitoring effort.”

Figure 2: Total station and prism arrangement

He called his Trimble dealer, invested in Trimble 4D Control (T4D), and after some training from the Trimble team, was ready to set up the monitoring system at the gondola station. 

The Bulson team mounted the S7 total station on the gondola station’s steel platform about 10 feet off the ground. The total station was connected to a Settop M1 monitoring controller that relays measurement data to the office. Then, they set 23 prisms along the slope, the retaining wall and the gondola towers (Figure 2). The S7 was set to collect data in 10-minute intervals (6 minutes to scan the prisms, 4 minute rest), 24 hours a day.

Bulson said, “Back in the office, I monitor the data collected from the S7 within T4D. Once set up, this system takes very little oversight. It collects, analyzes, develops reports and sends automatic alerts if movement exceeds a set threshold. Even the monthly report to TMVOA comes out of the web interface for T4D. Fortunately, we’re showing no movement that exceeds our thresholds at this point.”

Evaluating Anomalies

For the analysis, the key challenge was to evaluate movement of each of the prisms and the total station with the least amount of impact from environmental factors. These included the heating of the steel frames from the sun and the flexing of the gondola structure during operation. 

Bulson confirmed, “I noticed in the first group of data from the total station, that the relative movement of the gondola towers was as much as an inch and a half a day, largely triggered by the loading of the gondola and the warming of the steel frame from the sun. The slope prism position variations were on the order of a quarter of an inch a day due to these environmental factors. 

These environmental factors made determining soil movement a bit more challenging. We had to make some decisions on how we compare one day to another. The optimal solution would be to compare a day-to-day measurement with as many environmental factors removed as possible.”

For Bulson, that meant the optimum time to assess ground movement was when gondola operation was shut down overnight and the sun was no longer a factor. “Ultimately, we opted to compare day-to-day measurements on the prisms at 3 a.m. That summary comes to me as an email every day. And that has been really successful because those day-to-day measurements were very consistent, down to one-sixteenth of an inch. Then at the end of each month, I send our client a report with a summary of data, noting any anomalies.”

Figure 3: Client report automatically generated from the T4D web interface, displaying the daily 3 a.m. measurements. The green band represents the acceptable movement tolerance initially set by the monitoring team.
Figure 4: Chart of the raw measurements collected every 10 minutes on the same prism, which is also noted in Figure 3. Of significance are the cyclical variations caused by the environmental factors and the average change over the week that is represented by the pink trend line.

Bulson and his team can also look at results over a month or a year and scroll through the 23 prisms to see if anything stands out.

Bulson confirmed, “There are two kinds of movements the geotechnical engineers are concerned about—the fast movement from the rain event that triggers a release of soil—and then there’s the very slow creep of the hillside down the slope. Neither condition occurred during the monitoring time frame.” 

The monitoring project is anticipated to end early November as the ground begins to freeze.  

The First of Many 

When asked about lessons learned from this first time using the T4D, Bulson noted that he’s very satisfied. 

Bulson concluded, “There’s a learning curve to using an advanced solution such as T4D for remote monitoring. The Trimble support team helped considerably. Overall, it’s been a really positive experience. Considering the ongoing development in this area, the need for this type of monitoring will be essential going forward.”

Rowland Chen is a monitoring sales manager at Trimble. He has been at Trimble for more than six years, supporting and installing automated monitoring systems around the world. Rowland received a Bachelor of Science in Geophysics from the Colorado School of Mines and is based in Denver, Colorado.