International Dredging Review

International Dredging Review

CGC Dredging performed work at Port Geographe located in Western Australia with cutter suction dredge Cooper II. It dredged seaweed and sand accumulations and preparing the seafloor for rock placement for a new seawall which would diminish shoaling and alleviate maintenance dredging in the future.

The canal community and small boat harbor regularly dredges in order to cut back seagrass wrack and accumulated sediment and keep navigation open. The port redesigned its new seawall structure which was collecting seaweed and sand to instead be self-flushing.

Port Geographe is located in the town of Busselton in Western Australia. The harbor entrance has two seawalls which over the years have accumulated severe sand and seaweed build up. In order to keep the channel open and maintain an attractive and usable beachfront frequent dredging of the seafloor along the harbor entrance is required.

Faced with this challenge the local transport authority authorized a large-scale $28.15 million coastal reconfiguration project. WA Limestone in conjunction with Italia group were the project managers for this larger project providing stone and construction plant for constructing the new breakwaters and seawalls. A pre dredge hydrographic survey was performed in the area prior to dredging and placing material on the seafloor. In addition to modifying the shape of the western break wall and removing of the old seawall the team was also tasked with building a new seawall and realigning the shoreline.


The dredge used Trimble® HYDROpro™ construction software along with Trimble SPS461 GPS heading and positioning receivers.

“The basic rule of dredging is that you don’t want to over- or under-dredge” CGC Operations Manager Adam Whitcomb said. “Overdredging means you spent time and money like burning diesel that you didn’t need to; and if you under-dredge your floor may not meet design depth specification. You really need to be precise and efficient in your work and also have the ability to make adjustments according to design parameters.”

For the project CGC turned to its technology partner SITECH Western Australia (WA) and Trimble adopting the Trimble® HYDROpro™ construction software Trimble SPS461 GPS heading and positioning receivers; WA Limestone (WAL) used Trimble GCS900

Machine Control on their excavators and a SPS855 receiver with HYDROpro Navigation on a Trimble Site Tablet and a single beam echo sounder on their hydrographic boat.

CGC worked with Sitech WA to equip its Cooper II dredge with Trimble Site Positioning System GNSS receivers.

The company also installed the Trimble SPS461 GPS positioning receiver onto the dredge along with dual Zephyr antennas to position the dredge arm.


SPS855 receiver with HYDROpro Navigation on Trimble Site Tablet.

“A hydrographic survey was needed to provide an accurate baseline beforehand and to check our work throughout” Whitcomb said. “During the course of the project we also wanted as-built updates from the dredging. WA Limestone considered using multi-beam echosounder but the cost estimates were way over budget so Sitech WA recommended Trimble HYDROpro for collecting measurements and site positioning.”

For the hydrographic survey Trimble HYDROpro software and Trimble Site Tablet were used in combination with the receiver data and the echo sounder to determine the real-time position and elevation of the seabed surface. Since the GNSS receiver was so precise in the vertical plane a conventional tide gauge was not needed with their ongoing communication and maintenance issues.

Using RTK technology with a GPS base station on-shore at the site office at the harbor which sends GPS corrections to all the roving vessels and excavators that use the corrections. Those systems merge with their own GPS tracking signals to get centimeter accuracy in three dimensions.

Whitcomb said that with the Trimble marine positioning system his operator had precise dredge head positioning and real-time tracking capabilities. The receiver provided vessel positioning and heading. To position the cutter head underwater the dredge was fitted with a digital angle sensor on the dredge arm. With the length of the arm and angle the HYDROpro software displayed the precise position of the cutterhead underwater within an accuracy of 10 centimeters.

Whitcomb explains that cutterhead guidance is extremely important with this type of project because his operator needs the ability to re-start dredging at the precise location where the previous day’s or week’s work ended. The work requires precision and is somewhat tedious when accurate positioning is not available. This high level of accuracy was needed to locate and dredge the toe of the new structure. WA Limestone and Italia Joint Venture also used their equipment whenever location guidance was needed to ensure the successful removal of existing rock by the earthworks excavation team.

“If the dredge was taken to sheltered waters or in for maintenance the accurate cutter head guidance enabled our operator to return to the precise location the next day and continue” Whitcomb said. “With the depth guidance we were also able to precisely dredge to the design and keep up with changes which saved us more work and time and none of this would have been possible without Trimble technology or the support from Sitech WA.”

Sitech WA delivered remote support and service for the project. The CGC dredge was equipped with an Internet connection and a remote viewing software application was loaded on to the on-board computer system. With that support personnel could then remotely see exactly what the operator was viewing during the dredging process.

“The remote support experience from Sitech was absolutely essential to the success of the project” Whitcomb said. “Port Geographe is a three hour drive from Perth not to mention the logistics of getting that person on the dredge; it just wasn’t a cost-effective option. Despite having no experience with Trimble marine guidance with remote monitoring support our operator resolved questions extremely fast.”