3D Milling: a South African runway success story
3D Milling: a South African runway success story
With a sound background in construction and engineering, the Power Group - through the group companies - Power Construction and Power Developments - is a supplier of civil, building, turnkey housing, project engineering and property development services. The business is built on sound principles: “To improve the quality of life in Africa through infrastructure development.” The Power Group has a nationwide footprint, operating from three regional offices in Cape Town (its head office), Port Elizabeth and Centurion, with a branch in Namibia: Power-Oyeno Namibia, based in Swakopmund.
The Power Group’s reputation has earned them numerous large infrastructural contracts and in August 2012, it was awarded a contract for the rehabilitation of the main runway at Cape Town international airport. According to Gary Hirst Technical Executive - Power Construction South Africa, “One of the key construction activities was the strengthening and re-grade of the existing runway slopes, which included the milling-off, to a specific design, of 4500m³ of asphalt and placing 35000t of wearing course asphalt.”
The 11-month window for completion of the project was immovable and the end date was set at 28 June 2013. (This was owing to a commitment by Airports Company South Africa to international airlines that the runway would be fully operational by the date and their planning was in place.)
To meet the stringent deadline there were a number of challenges to work around existing considerations. For one, the runway was active during the day and early evening and only available for construction between 00h30 and 05h15, so as to not interrupt the departure times of the first flights leaving at 05h45.
Every night, a minimum of 200t of the 35000t of wearing course asphalt had to be placed. To meet that criteria, the milling operation was critical and had to be complete by 02h30 at the latest and additionally, there was a tight tolerance on final levels of +8mm -8mm. There was a rideability specification IRI of 1.26 with a bonus/penalty system in place and, to compound the pressure, there were massive penalties per day for late completion.
Gary explained, “The operation began in the conventional way with lanes 2.1m wide and measure-up points every 5m set out by the surveyors so that steel wires could be placed on stools on which the milling machine sensors would run, to cut to the correct levels.”
However, this system’s serious fallibilities soon became evident. “With only two hours every night allocated to the milling process, the set-up alone took 40 minutes, equating to 33% of the available milling time. The system also drastically reduced the area that the milling machine could mill and consequently impacted severely on the tonnage of asphalt that could be laid,” he added anxiously.
It soon became apparent that quotas were not being met and of the 200t of anticipated asphalt due to be laid each night, only an average of 120 tons was being achieved - no matter how streamlined the survey operation. In reality this was only 60% of the production outcome required to meet the hand over date.
To counteract the shortfall and avoid the associated penalties, the staff complement soared as more and more workers were employed to alleviate the shortfall in production. “We threw people at the problem. Stress levels were at an all-time high, resulting in numerous mistakes being made by even the most experienced surveyors. We were basically looking down the barrel of a gun,” he added ominously.
A month into the contract, the Power Group’s reputation was on the line, as it realised that it was facing a massive problem.
After a serious re-grouping and following a think tank, it was decided to investigate if there was any technology that could help reduce, or even solve, the milling problem.
Previously, the company had achieved success with Trimble 3D GCS900 Grade Control System, so it was decided that Gary would attend Trimble Dimensions 2012 held in Las Vegas, “So we could interact with other construction companies and users of Trimble technology and benefit from their knowledge.”
He enthused about the experience: “The brilliant session at Trimble Dimensions 2012 with the contractors that built the Austin Texas formula one raceway and the demo in the quarry at the off-site venue - coupled with brilliant advice from fellow contractors and Trimble/SITECH staff - proved that the technology really works.
“I knew we had a solution in the 3D milling machine control system. A plan was hatched at the breakfast table to get the equipment as well as installers and trainers to South Africa, to get everybody through security clearance, so we could get the 3D milling machine control up and running - as the clock was ticking and failure was not an option…”
Advice received at Trimble dimensions 2012 was informative and vital. Gary explained, “If rideability is a major factor on a rehabilitation project, it is vital to get an accurate picture of the existing carriageway. This way, the design can be optimised, especially for tie-ins with existing asphalt, but most importantly, areas that are in-fill and not cut can be identified and repaired, so that when the milling operation takes place, an even thickness is milled. The material that is then placed, is uniformly thick. If not, the low spots reflect through the new mat, owing to differential settlement and this can dramatically affect the IRI (International Roughness Index) results.”
He continued: “While the equipment was being shipped, a scan of the runway was run, which took one night - as opposed to the six months the conventional survey took - and it showed some big problems with the design and tie-ins on the side joint, which could now be fixed before implementation. The scan ended up being a major factor in the planning and success of the milling operation.”
“On the 27 Nov 2012, the Trimble 3D PCS900 Paving Control for milling machines was installed on our two-metre Wirtgen milling machine and on the night of the 28 Nov 2012, we went live on the runway.”
While he admits there were some minor technical problems which were quickly sorted out by the Trimble/SITECH installers and trainers, by the third night, the operation was achieving the 220 –tonne, nightly target, “with very little effort. We had a winner,” he beamed and added wryly, “3D milling technology helped us to tie a knot in the barrel of the gun...”
The trend continued to the end of contract; the deadline was achieved and, according to Gary, “quality expectations were exceeded.”
Summary and lessons learned
The 3D system is now used on rehabilitation work to ensure good rideability and an appropriate slope to allow an effective drainage of the water / rain.
“We learned some valuable lessons during this process. Running a survey scan of the surface is vitally important, as it identifies low points that need to be pre-filled and the scanned model can be used to optimise the design as well as give a correct calculation of volumes.
“So too, the number and accuracy of benchmarks is vital to ensure quick set-ups and UTS changes. We had a benchmark every 150m that was levelled with the UTS to within 2mm.
“Milling machine drum maintenance is of vital importance and the condition of the drum must be constantly monitored to check that the picks wear evenly end ensure a better ride surface).
“A second UTS behind the milling machine for level checks is necessary to quickly get confidence in the system and to also to make minor adjustments as the drum wears. Initially we tried to operate with only a single UTS owing to cost concerns, but quickly saw the error of our ways and purchased a second one..”
Gary’s confidence in the system is evident: “The Trimble SPS930 and SPS730 UTS are very useful survey tools and can be used to pick up very accurate as-built surveys once the milling operation has been completed.”
He advised: “The milling machine's hydraulic system must be well maintained with no leaks as this affects the drums response time to the Grade control signals and ultimately accuracy.”
In conclusion, he noted: “Everybody in the team must be committed to making 3D control work. This includes foreman, survey, machine operators and management. Time must be taken to introduce the team to the concept and explain the focus areas. The final averaged IRI value for the project was 1.16 versus a Target value of 1.26. This made us eligible for a rideability bonus. After such results the use of the system on other projects such as the East London Airport runway become obvious.”