REAL-TIME MONITORING OF GROUND IMPROVEMENT EFFECT UNDER ROLLING DYNAMIC COMPACTION (RDC)

OLAMBO TRYPHOSE

【Abstract】Rolling dynamic compaction （RDC）， which employs non-circular module towed behind a tractor， is an innovative soil compaction method that has proven to be successful in many ground improvement applications. RDC involves repeatedly delivering high-energy impact blows onto the ground surface， which improves soil density and thus soil strength and stiffness.

【Key words】ground improvement; rolling dynamic compaction （RDC）; real-time monitoring; earthwork; instrumentation

The quality of earth structures highly depends on the compaction state of fill layers， which can be made up of a wide range of various materials，e.g. non-cohesive and cohesive soils， granular material， artificial powders， fly ashes， grain mixtures， and stabilized materials. Thus， both compaction equipment and compaction procedure need to be selected carefully taking into account the used fill material since compaction mainly contributes to achieve sufficient bearing capacity and uniform settlement behavior of the earth structure.

Ground Improvement is a technique that improves the engineering properties of the treated soil mass. Usually， the properties modified are shear strength， stiffness and permeability. Ground improvement has developed into a sophisticated tool to support foundations for a wide variety of structures. Properly applied， i.e. after giving due consideration to the nature of the ground being improved and the type and sensitivity of the structures being built， ground improvement often reduces direct costs and saves time.

Common issues seen today in the field are：

€Y孴he operator loses track of his passes and the job becomes guesswork

€Y孴he supervisor cannot monitor pass count performance and cannot verify accurate completion of the compaction job

€Y孖nconsistent density

€Y孶nder-compaction or over-compaction

€Y孭enalties， missed bonuses， premature road failure and legal issues

Recent developments for measurement and analysis of machine power response caused by changes in physical soil properties have the potential to change completely the future of earthwork construction. The technologies which allow engineers to record and supervise the compaction process and see the changes in physical and mechanical properties of soils have been developed， among them：

Intelligent compaction is the use of real-time measurements of the response of a compaction machine to provide on-the-fly adjustments to the machine parameters that affect compaction， such as drum vibration， amplitude， frequency， and roller speed. The beginnings of intelligent compaction can be traced to work in Sweden， where vibratory compactors were instrumented to measure the accelerations of drum roller during the compaction process.

The basic principle of a CCC system is to detect the soil stiffness by evaluating the motion behaviour of the drum. The parameters influencing the motion behaviour of the drum influence the values of CCC systems as well.

The operator can focus primarily on maintaining proper vehicle speed and rolling pattern. Another significant development in intelligent compaction has been the incorporation of global positioning system （GPS） technology and compaction operations. There are a number of advantages in the incorporating the two systems. Won-Seok et al. document two major advantages. One is that it allows an operator to compact a section freely without being restricted to a predetermined area inputted in a computer system.

Historically， measuring soil compaction during earthwork construction operations has been a key element to ensure adequate performance of the fill. Current state-of-practice relies primarily on process control （lift thickness and number of passes） and/or end-result spot tests using nuclear moisture-density gauge or other devices to ensure adequate compaction and proper moisture control has been achieved. While providing relatively accurate information， these inspection approaches have several disadvantages：

1.require continuous observation for method/process control;

2.offer measurements only for a small percentage of the fill volume （typically 1：1，000，000） for spot tests;

3. require construction delays to allow time for testing;

4.result in downtime for data analysis; and

5. cause safety issues due to personnel in the vicinity of equipment.

The compaction monitoring system has a lot of advantages. A significant advantage of the compaction monitoring system is that measurements are often output to a computer screen in the cab of the roller in real time to allow the operator to identify areas of poor compaction and make necessary rolling pattern changes. By making the compaction machine a measuring device and insuring compaction requirements are met the first time， the compaction process should be better controlled to improve quality， reduce rework， maximize productivity， and minimize costs. Productivity should be improved and delays for post process inspections could be avoided. Improved safety is an additional benefit due to reduction of people on the ground for inspection measurements.

The CIR technology is a reliable control and verification tool which is fully integrated into to the quality control process. The process captures the specification requirements as established by the design/consulting engineers， develops correlations between measured data and the specific engineering properties being measured and requiring control， establishes control limits， captures data and plots the results for verification.

The soil’s response to the load delivered by the drum is monitor and thanks to the fully integrated GPS， the indication of soil’s strength/stiffness all over the site during the earthworks is accurately produced.

The principle of continuous impact response （CIR） is simple and similar to that of Froumentin et al. who conducted field trials on a prototype compaction aid that utilized the global positioning system （GPS） technology. The system was dual frequency and had an accuracy of +1 cm. The cab was fitted with a GPS receiver， on board processor， and a touch-sensitive color graphic screen （11.5 x 8 in）.

Like Froumentin et al. the CIR technology is used together with the continuous induced settlement （CIS） technology; the CIR measures and records the change in soil’s strength and stiffness and the number of passes while the CIS measures and records the continuous induced settlement， then a color-coded map of the two measurements can be generated. We have can distinguish two different frequencies， the first one comes from the CIR and the second one from the CIS.

A Global Positioning System （GPS） device allows the operator to quickly know the position of the roller equipment while operating. To display and save positions reported by a GPS device on the computer screen， the GPS device must have an input/output （I/O） interface. A GPS-compatible cable that connects to a computer is also needed.

The new generation GPS toolbar requires a communications （COM） port connection. The GPS Connection Setup dialog box will not recognize a GPS device connected via Universal Serial Bus （USB） port. Some GPS device manufacturers provide drivers to map the USB connection to a virtual COM port that then can be used. Bluetooth devices also can be configured to use a virtual COM port.

At the end of this analysis about real-time monitoring of improvement effect under rolling dynamic compaction （RDC）， we have noticed that sensors and the global positioning system （GPS） are of great importance in the recent advanced technologies used by civil engineers in geotechnical engineering especially for earthworks. In order to design a new monitoring device， one should have some electronic， electric and mechanic skills which are very useful for the purpose. The drum is instrumented with the accelerometer or impactometer. The engineer should be able to read/interpret the map generated by the sensors and the GPS. The real-time monitoring of improvement effect under RDC avoids the loss of time and reduced costs. There still have the desire to design a new monitoring system which will further the progress of science.