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Emergency Slope Repair Challenge at Freeway Interchange

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Freeways have contributed to a country’s economic growth, which not only increases the traffic volume but also leads to the daily living sphere. Thereby, the interchange location is thoroughly crucial. When choosing its site, it requires considering the proper terrain, geological conditions, its positive effect on surrounding towns, and the lineament if gentle enough to qualify for the regulated sighting distance. 

Due to the influence of the extreme climate, concentrated heavy rainfall has become frequent in recent years that severely threatens the transportation facilities, especially for areas with poor geological and vegetation conditions. Once the disaster attacks the freeway, decreasing traffic impact is the priority task. This case we share not only solves the road safety issue in the short restoration period; moreover, its rehabilitation is undoubtedly the perfect combination of nature-like landscape and ecological protection. 

Background
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.

Problem
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.
 
Solution
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.
Schematic Diagram of the Reinforced Soil Slope
 
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.

Result
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. 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.

ACE Geosynthetics provides economical and safe solutions for customers, and works with customers to develop new application systems that manage to solve difficult engineering problems. Ace your project with ACE team!

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