Reinforced Earth Slope, Tianliao Interchange, National Freeway No. 3, Taiwan

The site is located at the south bound exit of Tianliao Interchange, National Freeway No. 3. It is also situated at the junction of Nan’an and Xide villages in Tianliao District. The traffic volume is huge in this area because of the congested location. In addition, the site is located in hilly terrain and often has concentrated torrential rainfall, thus the prevention of slope instability is very important.

The geological formation at the site is soft mudstone and the vegetation was very poor at the surface of the slope. Concentrated heavy rainfall has also become more and more frequent in this region in recent years because of the influence of the extreme climate. The mudstone is easily vulnerable to heavy rainfall, which then causes serious erosion. The Infiltration of rain water further tends to trigger the mudstone to swell and to disintegrate. The authority had installed some retaining secant piles for protection. However, these piles were insufficient to safeguard the slope. The strength of the mudstone has degraded due to long-term weathering and finally caused the slope to slide. The collapse has seriously jeopardized the traffic safety at the interchange, and immediate rehabilitation thus must be conducted.

Based on multiple comparisons for several proposed approaches, the owner and the designer finally decided to adopt reinforced earth technology consisting of ACEGrid^® and ACESandbag™ for slope rehabilitation. The best solution has to consider the overall slope stability, the limitation of the proximity for construction, and the needs of landscape planting for aesthetics. The reinforced earth slope was constructed as a terrace using 2 tiers with heights of 7 m and 11 m. The averaged embedded length of ACEGrid^® was 12.5 m. The horizontal step width on the first tier was up to 8 m to ensure the overall stability of the slope. The design strength of the ACEGrid^® for the slope was 200 kN/m. The depth of ACEGrid^® for the foundation was 1 m with an embedded length of 12.5 m. The excavation must be carefully controlled within a limited range because of the site constraint and the soft geological formation.
During excavation, the engineer found that the moisture contents of the soil were higher than those found in geological exploration. Although the excavated cohesive materials presented self-supporting behaviors, the depth of the excavation for the second tier of reinforced earth structure was up to 11 m. The excavation must be cautiously conducted by steps with limited volumes to avoid the risk of collapsing. Furthermore, to prevent the possible sliding caused by the unexpected torrential rainfall during excavation, additional steel rail piles with steel sheets were installed at the toe of the slope to serve as temporary retaining structure.
Considering the surface erosion and the needs of vegetation, ACESandbag™(high erosion resistance sandbag) was selected as the wraparound facing. It was not only conducive to the shaping of the facing, but also improved the erosion resistance of the structure’s surface. It also facilitated the surface for vegetation as well.
To account for the heavy rainfall and the abundant groundwater condition at the site, the drainage system also was particularly designed for this project. Firstly, a layer of gabion was set up at the toe of the excavated slope to drain out the groundwater at greater depth. Secondly, drainage boards together with GeoSandbags filled with gravel are stacked on the excavated face to develop a cutoff system to efficiently intercept the seepage on the back of the reinforced earth structure. The reinforced earth slope was built using fine-grained materials. Therefore, all the drainage elements including gabions, GeoSandbags, and drainage pipes have been covered around with a layer of functioning geotextile filters to prevent the loss of fines, and the possible subsequent settlement of the structure.

The designer decided to use reinforced earth technology to restore the collapse of the slope. The excavated unsuitable soils were not allowed to be disposed of the site due to environmental regulations. Therefore, either highly weathered, mudstone colluvium or fine-grained clayey, silty soils must be used as the slope fill. To reduce the risk of using poor materials, the contractor conducted rigorously soil compaction with accurate moisture content. The site materials have been successfully placed with sound quality. Such construction has maximized the use of local materials and substantially reduced the cost of disposal of unsuitable materials and import borrow fills. In addition, it not only significantly expedited the construction schedule, but also solved the problem of limited construction space and the piling difficulty for unsuitable soils. The merit of directly using local materials for reinforced earth slope has become the greatest benefit in this project.
The construction of the entire project was less than five months, which significantly reduced the inconvenience of the local traffic. At present, the completed slope is in good condition and totally has blended with the environment due to growing vegetation. The slope also shows no damage after several torrential rainfalls and proves its excellent safety.
This project is located at the exit of freeway interchange, thus the structural design must pay attention to the effect of aesthetics. The shape of the slope adopted a beautiful and smooth curvature design for driving convenience and also gives passers-by a good visual aesthetic feeling. Eco-friendly environment and aesthetic landscaping are the current project construction goals. Use of reinforced earth technology for landslide rehabilitation is undoubtedly the best paradigm of the perfect combination of safety, nature-like landscape and ecological protection.