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GPS: Total roll control

Automating the compaction process using GPS technology could improve productivity and save money, as Margo Cole reports.

Global positioning systems have become fairly widely accepted in everyday life over the last few years, and we now take them for granted in satellite navigation systems and mobile phones. Increasingly, the use of global positioning system (GPS) technology is also playing an important role in the construction process - and not just by ensuring the engineer’s satnav gets them to the site without the aid of a map.

On major road and development projects, dozers and graders are often fitted with GPS-based control systems that are used to determine line and level for cut and fill. A display in the machine’s cab lets the operator know when they have achieved the required level at any location on the site.

But that is usually where the machine control element stops, with traditional manual testing and surveying techniques taking over once the formation level has been reached. Now, however, GPS is being introduced for the next phase of the construction process - compaction.

Cat dozer

A Cat dozer fitted with antenna, sensors, displays and rollers

Surveying equipment specialist Korec has launched a GPS-based machine control system in the UK that can be fitted to a wide range of compaction equipment manufactured by all the big suppliers, including Bomag, Caterpillar, Volvo and Liebherr.

Once fitted, the system can be used to either record the number of passes a roller has made on any section of ground or even to determine if the material has reached the specified level of compaction.

The system, known as CCS900, has been developed by positioning specialist Trimble, and is based on the company’s highly successful GCS900 grader control equipment.

A GPS antenna on the roof of the cab both transmits and receives information from satellites regarding its exact position, while an onboard computer is programmed with information about the compaction requirements for every bit of the ground, such as the number of passes the machine must make to achieve the required compaction.

Because the machine’s position is constantly recorded by the GPS, the computer can calculate how many times it has passed over every location and compare this with the pre-set requirement.

Technicalities of the system

A display in the cab shows the operator a coloured map indicating the number of times the roller has passed over each area. The map changes to green when the pre-set number has been achieved. This system makes it simple for an operator to know when they need to move on, without having to keep their own record of how many passes they have done.

Often a roller operator will have to meet different pass requirements on different sections of the same site, depending on the type of material or amount of compaction required.

Keeping a manual tally of exactly how many passes are required - and how many have been done - in each area can be difficult on a large site. Often it is left to a clerk of works to keep records, but a GPS-based system takes that concern away - the operator simply keeps compacting until the screen turns green.

The information can also be relayed in real-time back to the site office using a radio link. It can also be downloaded at the end of the day, giving an “as built” compaction record for the site.

GPS Australia

A GPS equipped plant builds a temporary air base in Australia

“You can view where compaction has taken place in realtime,” says Trimble application engineer Dwayne Rosie.

“Data is displayed in job site co-ordinates, so you can move a cursor over any location and see how many passes have been done. It reduces over- and undercompaction of the work area.”

Midlands-based civil engineering contractor JME has been using the CCS900 machine control system on three rollers on a major redevelopment project in Corby.

Gary Shuttleworth, survey manager for the company’s subsidiary JME 3D, says the system provides the evidence the contractor needs to demonstrate the specification has been met.

“It’s always very difficult to prove you’ve done the number of passes.”

Gary Shuttleworth

“It’s always very difficult as a contractor to prove you’ve done the number of passes. This will prove invaluable because we have a verifiable way to prove that we have met the engineer’s specification.”

On many sites, the earthworks contractor is given the compaction specification in the form of a pass count. Alternatively, they may be told exactly what degree of compaction needs to be achieved at each location, but this is then translated into a pass count by doing trial sections to see how many passes are needed to achieve the required compaction.

This is, inevitably, a non-exact science, and can lead to either doing more passes than necessary - which reduces productivity - or leaving some particularly tricky sections under-compacted.

Korec says the more efficient way to be sure of achieving accurate compaction is to use the new CCS900 machine control system in its more advanced mode, which includes an accelerometer in the drum. This can measure the resistance of the ground and therefore give a compaction value for every area as the operator drives over it.

GPS in use

GPS in use

A test section of each material is first compacted to the required specification, and then the roller is run over it to measure the resistance. This value is used to calibrate the on-board computer, and is set as the target value for that material.

“You do the calibration first, which gives you a target value for that material at that depth,” explains Rosie.

“When you achieve that as your compaction value, you’ll get a nice green screen. It means you can detect areas where target compaction levels have not been met, with soft spots showing up as blue and hard spots as black.”

“When you achieve target value you can detect areas where target compaction levels have not been met”

Dwayne Rosie

The system will also identify if the drum of the roller is bouncing - an indication that either the material or the machine is being damaged. Again, the operator can see in real-time when the required level of compaction has been achieved.

Engineers back in the office can identify which areas are over- or under-compacted, as well as have a complete “as-built” record once the job is finished.

“Each pass is recorded on each soil, so you can use this as an asbuilt file,” says Rosie.

“It’s also a good way of knowing when the machine is on the final pass on that material.

“The office can analyse the data to see if it meets the design, if it is on grade and compact enough and, if there are any soft spots, where they are and why they are there.”

The system has obvious benefits to the contractor, including more efficient compaction operations.

“If you’re using the compaction sensor the benefit for the operator is that if he achieves a given compaction in three passes rather than four, he doesn’t have to do any more,” says Korec machine control sales manager Peter Brooks.

GPS dozer

GPS dozer

Further savings can be made if the amount of testing and checking is reduced. At the moment, one of the only ways the client and contractor (or main and subcontractor) can be sure they agree that the right level of compaction has been achieved is by independent third party testing at different locations - something that can cost as much as £500 a day.

“Independent testing is a safeguard for both sides, but it costs a lot of money for someone to come on site and do tests,” says Brooks.

“There is also the safety aspect to consider, because with this system you don’t have guys having to work in the field trying to do tests with all these big machines around.”

“The benefit of this is that you get a compaction record everywhere you drive, not just spot checks,” adds Rosie.

“Testers would still need to come out and adequately verify that the compaction is being achieved, but they wouldn’t need to be there every day.”

Both companies accept that engineers will be reluctant to swap independent third party testing for the output of a machine, but they are confident about the accuracy of the system and suggest running it in parallel with traditional testing until all sides are convinced.

Extending the application

As yet, the CCS900-fitted compaction equipment is only being used to monitor pass counts in the UK. But overseas it has already proved its worth in meeting compaction specification - the most spectacular example being the construction of a temporary airfield during a massive military exercise in the Australian outback.

The Joint Rapid Airfield Construction (JRAC) exercise involved 110 Australian and 105 US army, marines, navy and air force personnel, whose first task was to construct a military standard airfield in less than 25 days.

During construction, the servicemen and women used GPS location software, deployable communication systems and remote-controlled plant, including bulldozers, which could be operated from a distance of up to 300m away.

Five Caterpillar CS563E vibratory soil compactors were fitted with GPS-based machine control set to measure and record the compaction values being achieved.

The airstrip itself was built using rapid compaction together with mixed fibre and rapid-setting cement.

Three layers of material were placed and compacted on two aprons and taxiways, then fibre-cement stabilisation material was applied, topped with a fine polymer surface coating.

The rollers were operated by a team of young operators - all service personnel with limited experience in soil stabilisation and compaction techniques.

The equipment fed real-time data directly to a GPS base station via radio, tracking each piece of equipment for positions (within 1mm to 2mm), the level of compaction achieved and ground speed.

Construction was completed within the allotted timescale, and the airfield was inaugurated with the arrival of a US Air Force C-17 aircraft.

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