Clay-Based Soil Transformed into Solid Base
The A14 Motorway between Bologna and Taranto, Italy, has seen its traffic volume increase about 4% year on year. The most recent data suggests that 110,000 vehicles use the motorway daily. The trend looks set to continue which led the Italian government to plan an upgrade and expansion of the motorway. The multi-phased project took place along a 154.7 km section between Rimini North and a future junction at Porto Sant’Elpidio. The project widened the motorway from two to three lanes with the addition of an emergency lane. Ten existing junctions were upgraded and five new junctions built. They are located in Pesaro South, Fano North, Marina di Monte Marciano, Ancona West and Port Sant’Elpidio.
Experienced Contractor
Ghea Sistem, a firm involved in many high-profile Italian projects, handled most of the challenging soil stabilisation. Ghea Sistem recently completed the Northern part of Section 2 of the project, a 28 km stretch of the expansion from Cattolica to Pesaro. Because of the clay-based soil and of the high water table, the section required some major work to raise the level of the road.
“The main challenges include the proximity of the water table which is almost at ground level in places along the motorway,” said Alessio Cagnola, who owns Ghea Sistem with his brother, Paolo. “In addition, 95% of the works was carried out alongside the existing motorway route. This created issues with limited manoeuvring space, and sometimes large quantities of soil close to the traffic needed to be excavated to widen the road and to allow the construction of retaining walls. It could have interfered with the placement of concrete structures, such as large pipe and box culverts.”
Tasks included grading of slopes, and the saturated soil was unstable for the work crews at times because of heavy rain on Section 2. On the more even grades, sometimes the soil was too saturated to be moved or for equipment to operate on.
Stabilising The Road
Ghea used two approaches to stabilize the road. Wide, more even stretches of road were stabilised with the Cat® RM300 Rotary Mixer. In poor soil conditions, lime additives were sprayed on the soil and mixed by the RM300.
Ghea Sistems excavated clay-based soil to make room for culverts and other infrastructure such as two artificial tunnels used to divert traffic into the new tunnels to allow work to be carried out on the older ones. Pockets of unstable gumbo and saturated sub-soil were encountered. The challenge of establishing the load bearing capacity to keep thousands of kilograms of pavement in place was further compounded by the forces of nature and the instability of the remaining sub-surface soil.
Temporary soil stabilisation sites were established. Four-axle haul trucks transported thousands of cubic metres to each site. Non-usable soil was separated and blended into the surrounding terrain. The RM300 was deployed to blend lime additives into the soil that was deemed suitable for re-use. The stabilised soil was hauled back for use as fill material to build the new lane. The grade had to be raised from 2 to 5 m where the road crossed the high water table and areas of marginal soil consistency. The first step involved the placement of a base layer of graded aggregate hauled in from a nearby quarry. In addition to the reclaimed fill material, other suitable soil was excavated from high-ground pits. Soil topped with a lime additive was used to raise the grade to the required height.
Project specifications called for 20% of lime blended into each stabilised layer. Three Cat® RM300 passes were applied to each layer of the stratified soil-lime mix to consistently achieve the target composition. A motor grader insured each load was placed at the correct depth to consistently achieve the desired soil compaction. A soil compactor completed the third step to place each load. This three-step sequence was followed to establish each soil/lime layer and the Cat RM300 played a key role to help achieve a stable base. Depending on the time of year and on the soil moisture content, the soil was watered between the first and second passes of the Cat RM300. This insured that the lime was properly hydrated (up to 3% water) to achieve the best mix in preparation for the compacting finish.
With such a volume of material, productivity was a key element in all phases of the project.
“The RM300 has been giving us excellent results with regards to production, with an average of approximately 1,000m3 per day,” said Paolo Cagnola. “In addition, when fitted with a universal rotor, the RM300 makes the material suitable for placement with little processing.”
Operator Approved
The operator of the Cat RM300, Felice Catello, found the cab quiet and the heavy traffic only a few feet away, was barely audible. The air conditioning provided comfort, and the instrumentation was easy to read. The Electronic Monitoring System (ECM) clearly displayed the readings and showed the machine often worked at a depth of 450 mm, although capable of greater depths.
After making a pass, Catello reversed the travel direction with the help of the rear wheel steering, which enabled crab and coordinated steering. The RM300 moved forward along the trench cut during the previous pass. The rear tyres are wide with a traction-assisting tread pattern and a large ground contact area. The wheels are hydrostatically driven by two radial piston motors that can be activated by the operator when a true all-wheel drive is needed.
The machine worked extensively and Catello estimated it only needed to be refueled every 16 hours. He also stated that 5,000m3 of material could be stabilised in approximately 40 hours of weekly operation, using only two full fuel tanks.
The material left behind was consistent in size, with no large chunks or clumps of clay. Its colour was uniform indicating that the additives and clay-based soil had been thoroughly mixed. Ghea Sistem chose a universal rotor fitted with 200 carbide-tipped tools. The tools can be quickly replaced and are in a chevron pattern for maximum breakout force. The kicker paddles placed on each stand-off also helped improve mixing during soil stabilisation.
The results were positive for Ghea Sistem. “We previously tried a Cat Rotary Mixer, and the machine performed superbly, so we decided to purchase another for this project,” said Alessio Cagnola. “The machine has been giving us excellent results.”
