Application of Complex Reinforced Structure Near Fault Zone

ACE Geosynthetics

Taiwan’s Ministry of Transportation and Communication came up with the temporary solution of placing horizontal drainage pipes along with collector wells to drain out the excess water in the slope; sheet piles had also been utilized along both roadsides to stabilize the road and maintain single-lane transportation. However, on June 8, 2007, a great landslide up to 80 meters wide and 30 meters high was caused by a torrential rainstorm. The existing anchored slope was severely damaged with the main structure being pulled out and exposed after the landslide, forming a huge heave on the toe of the slope. The regional geologic map shows that the several fault zones cut through the job site area, making rock mass fragile and weak. According to the on-site boring hole results, colluvial lays 5 meters below the hole, crushed shale rocks 5 meters to 35 meters below, more complete stiffer shale rocks 35 meters to 60 meters below, groundwater exists between the colluvium and crushed shale rocks and there is a fault gouge in the crushed shale layer. Several tension cracks, allowing rainfall to permeate into soil, were found after the on-site investigation. All information given above indicates the job site has fragile geology and poor drainage colluvium near the fault zone. When a heavy rainstorm hits, water quickly penetrates into soil causing the groundwater to rise and soil stability drop, eventually leading to great scale of landslides. Therefore a drainage solution must be the first priority in roadway repair projects.

Since the damaged area is located inside the water supply reserve of Reservoir Ming-Tan, the foundation of the structure confronts the problem of water level change in the reservoir itself. The on-site boring hole report shows there is no more complete stiffer shale rock until 35 meters underneath the current roadway. After the stability analysis, a design was adopted to drill concrete piles of 1.5-meter diameter and 20 meters and 30 meters long installed every 2.5 meters beneath the 8.5 meter high reinforced concrete (RC) retaining wall. Three stages of soil anchors, bond length 10 meters, free length 30 meters, anchorage load 60 tonnes, were also installed from the wall surface every 2 meters, both horizontally and vertically. Above the RC retaining wall, a wrap-around reinforced structure was constructed: 10 meters and 8 meters long of ACEGrid® GG200-II installed within the structure, slope inclination 1V:0.3H, wall height 17 meters dividing into four stairs. A full interior and exterior drainage system was installed in the reinforced structure to effectively resolve the poor drainage problem. In-site soil (crushed shale mixed with silty soil) was adopted as filling material.

Mountains take up more than 70 percent of area in Taiwan and a massive transportation system is essential in order to save time and cost in cargo transportation. This roadway repair project is on the slope near the fault zone, and its fragile geology, with poor drainage and influence of storms, was causing massive landslides. Before the damage, only the soil anchor method was adopted to repair the structure; however, the soil anchor durability not only depends on soil anchor module control but also superior grouting quality, and the grouting requires precise judgment from experienced engineers. Therefore the installation complication, labor cost and grouting material cost are making it far more difficult and inconvenient than a flexible structure-reinforced soil retaining wall. The wraparound reinforcement method is adopted by the design company taking advantage of its basic required installation equipment, easy installation without topographic constraint and low labor cost. With the help of dredged gravel back-fill, combined with a complete drainage system, piles and soil anchors were installed to stabilize the toe of the slope. Landslides are successfully controlled based on the inclinator and water level indicator surveillance. This project is one perfect reference of mountain roadway repair for civil engineers.

Project details

Location: Shuili Town, Nantou County, Taiwan
Fabrics: ACEGrid GG (woven geogrid PET yarn coated with durable polymer) by ACE Geosynthetics