By Vincent Ho and Steven Chang
Stretches of the Yi-Lan-I Road in Taiwan are located more than 3,200ft (1,000m) above sea level. It’s an important road that provides the primary access to the National Taipingshan Forest Recreation Area. But during a particularly rainy 75-day period (mid-July through September 2005), the region experienced abundant rainfall in the wake of 7 powerful typhoons.
According to Taiwan’s weather bureau, 39in. (1m) of rain fell during this period. Among the results were dangerous situations throughout the island’s infrastructure, and especially so for an elevated road such as Yi-Lan-I. Portions of the road’s supporting slope became unstable. Earthslides occurred, the road was damaged, and use was restricted for more than 6 months.
Calling for reinforcement
The slope required reinforcement. In this case, a wraparound facing was selected.
The designers chose to construct a 230-ft-(70-m)-long slope along the original road line. The reinforced slope height ranged from 72ft (22m) to 100ft (30.5m) and followed the in-site landforms—a rather steep slope of 1V:0.5H).
The reinforcement was carried out over 6 stages along the varying road slope. Each stage measured more than 15ft (5m).
The embedment lengths for these stages were designed at 59ft (18m), 59ft, 52ft (16m), 39ft (12m), 29ft (9m), and 20ft (6m). The project team specified geogrids made of multifilament polyester yarns and coated with PVC. Three products were chosen to handle the various needs of the stages. The ultimate design strengths for the geogrids were 200 kN/m, 150 kN/m and 100 kN/m.
Two piles were driven under the base of the reinforced slope’s first stage. The piles were 29ft (9m) long and 1ft (0.3m) in diameter, and were driven 3-6ft (1-2m) into the bedrock. Soon after, a thorough drainage system—filter fabric, geopipe, and fill—was installed to secure the entire construction against similar future rain events.
The project was completed in 60 days, with the facing system—vegetated soil bags—installed to complete the job.
The end look is a green and well-engineered solution that blends into the hilly countryside.
After nearly 2 years in use, the asphalt remains unbroken, and no significant settlement has been detected.
Working in Taiwan
Taiwan’s hilly geography and dynamic weather make it a difficult place to obtain the type of granulated soil stones prefered for such a project. That type of fill is just not naturally abundant in the area.
Obtaining good-quality fill is, thus, quite expensive, especially for delicate reinforcement projects. And yet, the fill is obviously critical, as is the drainage system.
For the Yi-Lan-I Road project, fill soil was actually acquired from the season’s river dredging. This “cheaper” fill helped keep the construction costs down, but it also proved to be a shrewd design choice. The riverbed gravel had the proper control and density for the project’s terrain and precipitation. It worked wonderfully in this engineering scheme.
Add to that the stability of the geogrid reinforcement and the result was not only a project completed on time but one that exemplifies the way Taiwan’s mountain road infrastructure can be improved efficiently, affordably, and for the long term.