When moving the earth to construct roads, building pads, civil works and other items, the traditional method of ensuring the design is followed is for a surveyor to place pegs. Bulk earthworks is the process where the majority of the soil and rock are moved. Topsoil is stripped and stockpiled, then the excavation or placement of material is undertaken to get the site ready for final trimming. It is during bulk earthworks that the survey pegs are most useful – and most removed!
When surveying for the construction of the Wellington Correctional Centre, Doherty Smith & Associates placed an estimated 2000 marks for construction, of which about half were steel fence pickets, painted and marked with levels. In some areas of the site, the cut and fill areas were up to 4 metres deep. Naturally, in these areas, the survey pegs placed for bulk earthworks were only useful until either excavation removed them, or they were buried and had to be replaced.
Since then, the adoption of machine control has allowed the placement of pegs for bulk earthworks to be minimised. Usually based on a GNSS (GPS) system, machine control uses a digital model of the finished surface of the site to guide the machine operator in their work. There are varying levels of automation with machine control, from simple guide lights indicating whether to raise or lower the surface, to full control units which control the blade.
The GNSS based machine control system is ideal for large sites and uses a reference station fixed in place, sending corrections by radio link in real time to receivers located on machines. The receivers on the machines are linked to the control unit. Typical accuracy of GNSS based machine control is stated at +/-20mm for level, although results are generally better than this.
The role of a surveyor in the adoption of machine control is to set up control marks on the site. The design is then converted to a digital model suitable for upload to the machine control system. The surveyor can then assist with setting up the system and ensuring that it is working correctly. Usually, once the system is set up and verified, the required input from the surveyor is minimal.
The advantages to the civil contractor are immediately obvious: less time for the surveyor on the site saves cost, there are little or no survey pegs on the site “getting in the way” and no surveyor getting cranky when they are removed and have to be replaced. Initial setup cost for the surveyor will be more expensive, however in the overall cost of the project the saving in field time makes the use of machine control worthwhile. To top it all off, the accuracy and efficiency of the work often increases, requiring less adjustment to get to finished levels.
Doherty Smith & Associates work in conjunction with Leica to provide machine control expertise, however have experience with other brands including Trimble and Topcon. Machine control is a smart way to streamline civil earthworks, save time and money.