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Monitoring–an intermittent series of observations in time, carried out to assess compliance with a specified degree of deviation from an expected norm–is an increasingly critical component in today’s project environment, as analysis of monitoring data can contribute to a better understanding of motion-related events and long-term trends.
Several factors are driving this demand. The first is safety: A monitoring system can detect motion and alert a project team in real time, giving them tools to promote a safer and more secure work site. Monitoring also supports better decisions, because it enables the team to predict events and take remedial action to prevent them. Finally, monitoring is often tied to compliance with regulations and contract conditions.
Monitoring at De Vere Gardens
Safety, improved decision-making, and compliance were the initial drivers behind a monitoring project currently underway in one of London’s wealthiest areas, Kensington and Chelsea. Home to some of the most expensive real estate in the world, the De Vere Gardens development is located on the site of the recently demolished Kensington Park and Kensington Palace Thistle hotels. The project will provide two new seven-story buildings and one five-story block incorporating retained façades. In the middle of the site, encompassed on three sides by extensive building work (including the enlargement of the existing basement to two levels), is a block of 15 privately owned flats. Sir Robert McAlpine, Ltd.– a leading UK building and civil engineering company and the main contractor on the De Vere project–is responsible for monitoring the buildings and the retained façades for structural damage as part of the contract.
The first stab at monitoring was less than satisfactory. Following the hotel demolition, a subcontractor was asked to provide a monitoring scheme that would produce data on a weekly basis. This method raised two problems, however: First, it provided data but no analysis; and second, if movement was detected, a weekly result did not provide a discernible pattern to put the readings into context.
Moving Targets
These drawbacks were underlined when, early in the project, 2 of the 12 targets on the back wall of the retained flats indicated movement while the remaining 10 did not. Although not excessive, this movement might indicate either structural damage or the influence of external factors. With no pattern to measure against, Sir Robert McAlpine lacked the necessary data to pinpoint the problem; and because any element of doubt was unacceptable, the team determined that a different monitoring scheme would be necessary.
The new system had several requirements. It had to be managed in-house. It had to be automated, real-time, reliable and easy to use. Above all, it had to be in place before piling began–a tight timeframe.
"The tight time schedule was significant for us," explains Senior Land Surveyor Vince Bridle, the dedicated manager tasked with installing and overseeing the system. "Understandably, local residents were concerned about structural damage to their property; we felt that a comprehensive monitoring program would help alleviate their fears." The team looked at several systems, ultimately selecting a real-time solution of two Trimble S8 Robotic Total Stations with Trimble 4D Control PC software.
"We were already familiar with the hardware and software, which cut down the learning curve," says Bridle. "And having seen the system demonstrated on a small-scale project, we knew that we could manipulate it for our specific requirements. Strong vendor support to get us up and running was also a factor."
14,000 Readings a Day
A control network of 13 base targets was established on site, and 98 25mm glass prisms were fixed on the structures to be monitored. The total stations were positioned at opposite ends of the site, one on the Sir Robert McAlpine site building and the other on the highest point of a retained façade wall, where there was sufficient room to power it with three solar panels. A 2.4 GHz standard wireless radio link connects the total stations with an office PC running the Trimble 4D Control software, where email and text alerts are generated automatically.
Trimble 4D Control software post-processes the data collected by total stations on monitoring or similar applications. It reads rounds imported from Trimble’s field software as individual sessions and reveals any movement of targets over time. The terrestrial engine in Trimble 4D Control processes each session and validates the data. It then stores the point information together with the raw data. Other modules of the software use this data to create warnings and alarms, as well as graphs and reports.
The system works continuously. At the end of each 24-hour period, Trimble software merges the 14,000 readings to provide a daily average and loads the data into a spreadsheet for comparison against base readings. Bridle then analyzes the data and prepares it according to the client’s required format.
"One of the major drawbacks of the initial subcontractor monitoring scheme was that we had data but no analysis," explains Bridle. "With the in-house system, the Trimble 4D Web module allows us to provide access to selected `guests’–including our geotechnical advisors–so they can view the data in graph form, as well as zoom in on data snapshots to analyze spikes or other unusual readings. This is particularly useful for our own in-house validation system."
The data goes to the geotechnical staff, where it is checked and then sent back so it can be prepared for the client. "We know that the information we are producing–whether graphs showing movement with different values, or charts of our monitoring locations–is in a format that is clear, informative and exactly what the client requires."
Converging Factors
Several elements made it possible for Sir Robert McAlpine to take monitoring in-house, including technological developments and economic factors. "As industry attitudes change, and monitoring becomes mandatory and a contractual requirement, companies must find cost-effective monitoring schemes," says Bridle. "Technology has helped us achieve cost efficiency in many ways on this project."
For example, the glass prisms used as targets are now smaller and lighter in weight, reducing the need for major fixings; at the same time, they offer improved accuracy and have almost halved in price during the last few years. The Trimble S8’s FineLock™ technology has also enhanced its suitability for monitoring purposes by using a much narrower field when automatically aiming the instrument toward a target. This is especially useful in engineering applications such as monitoring and tunnelling, where passive targets with very tight spacing are used. The Trimble S8 offers a FineLock accuracy of <1mm at 300m and has a range of 2.5km.
"Our approach is to monitor hourly so we can predict how climatic patterns and different conditions will affect our readings," says Bridle. "The Trimble system allows us to do this with no additional labor or maintenance costs–it’s self-sufficient and reliable in any kind of weather."
Assurance…and reassurance
Sir Robert McAlpine’s dedication to honesty and transparency is evident across the project. Instrument calibration can be checked from readings on a database for QA assurance. No recorded data c
an be deleted, and all findings are clearly stated in the reporting system. This data provides assurance for Sir Robert McAlpine’s team of structural and design engineers that they have done their job correctly, while reassuring all external parties (including the council and local residents) that the job is being handled properly and professionally. It also makes it easier to prove that any incidents, such as road damage or burst water mains, are not the result of the building work.
"This is a built-up area, and it’s vital that we have the confidence and trust of those around us so that the project will run smoothly," says Bridle. "If an incident does occur, however minor, we are in a position to investigate it within 24 hours, rather than within a week as previously done." During the piling operation, for example, a small movement was detected on a Friday night. The immediate text alert meant that by Saturday, the team had temporary measures in place; by Monday, the problem was rectified permanently.
"By working with this system, for the first time we are equipped to analyze and evaluate the data we collect," says Bridle. "The system is also scalable, making it easy to incorporate the two additional solar-powered Trimble S8s that we need to continue our work as the new structure goes up." The experience gained on this project will provide an excellent blueprint for other monitoring schemes within Sir Robert McAlpine; the team is already looking at new possibilities, including the addition of cameras to the total stations.
Concludes Bridle: "It is clear that monitoring is one of survey’s fastest growing disciplines, not just for the accepted reasons of safety, planning and compliance, but also for the `softer’ benefits of trust, confidence and openness. These factors can’t be measured, but they remain key to a smooth-running project and good community relationships."
Chris Gill has worked for Sir Robert McAlpine since 2004 as the survey manager and has worked on all of the company’s major UK projects including the London Olympic Stadium, M1 Motorway widening and Sizewell Nuclear Plant.
A 1.765Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE