Application of Geogrid Reinforced Structure for Abutment Construction, Taichung, Taiwan

In order to diminish the transit time from Houli District of Taichung to metropolitan circular freeway system for the residents, Taichung City Government is planning to build a connecting road from the Shengang Interchange of National Highway No. 4 extending to Houli District. This connecting road needs to cross Taiwan’s fourth-largest river, Dajia River, and a bridge section will be built with total length 2K+973.3.

When the river suddenly rises during rainy and typhoon seasons, Dajia River with steep slopes will bring a lot of soils and rocks and driftwood to scour along the line of bridge and bridge pier. It frequently causes the bridge pier foundation to be exposed and even directly devastates the bridge. Additionally, because the downstream riverbed has gradually lowered down year by year, and the deep channel area is unpredictable, if there is no proper protective measure, the bridge is easy to topple down due to the scouring of the bridge foundation. Besides the aforementioned scouring threat of floods, since the bridge abutment of this connecting road is located in Dijia River shore, a large amount of soil excavation and filling needs to be conducted during the construction. If the excavation surface is not well protected, it is prone to cause the slope surface to collapse due to the rainwater or external force, further influencing the abutment’s safety. The filling also requires to be stabilized because of its large amounts.

With the purpose of increasing the stability and anti-scour ability of bridge abutment, it adopts a compound structure that the abutment pier and its foundation are constructed by reinforced concrete, and the peripheral is installed by wrapped-around reinforced earth structure. In addition to serving as the abutment’s protective layer, it is also able to enhance its bearing capacity and service life. This reinforced earth structure is constructed of ACEGrid® geogrids and in-situ excavated soils and rocks. The design height is 20 meters which has 4 stages in total and 5 meters for each stage; each stage is installed with the drainage layer to efficiently and rapidly eliminate a great amount of water. The whole structure presents in U shape, completely covering the rigid abutment. The structure also protects the peripheral bare slopes from scouring brought by rainwater and external force. Besides, there is no stress concentration problem in the reinforced structure. Therefore, the design is especially suitable for this project, which is located in the area adjacent to two earthquake zones.

This reinforced structure can fortify the stability of the bridge’s abutment and simultaneously withstand the scouring of Dajia River, also supporting the vehicle loads above the bridge. It significantly enhances the bridge’s structure safety and serviceability. The abutment structure construction utilizes a large amount of excavation soils to backfill, which can greatly save construction costs. The overall exterior of the structure is integrated with onsite terrain; the surface of wrapped-around reinforced structure can provide vegetation to grow so that the appearance of bridge abutment can be mixed with the surroundings, thus presenting landscapes with nature and beauty.

The city government is satisfied with the structure, which significantly enhances the bridge’s safety and serviceability. The results can be reviewed in the following aspects:
• Safety: The reinforced earth structure strengthens the stability of the abutment’s foundation and the slope simultaneously.
• Cost-effectiveness: The installation process is simple and short, and installation quality can be controlled easily. Large amounts of excavated soils and rocks are filled back into the reinforced structure and slash the cost drastically. Compared with the design of reinforced concrete structure, the cost of the project is reduced by 2.06 million NT dollars.
• Pleasant landscaping: Green vegetation in the surface of the reinforced wrap-around structure is integrated harmoniously into the natural surroundings and beautifies the outlooking of the abutment. The surface of the structure turns exuberant green within one year after project completion. The appearance of bridge abutment can be mixed with the surroundings, thus presenting landscapes with nature and beauty.
• Eco-Friendliness: Adopting in-situ soil and rocks in the reinforced retaining wall helps to lower down the carbon emission. Compared with the design of reinforced concrete structure, the carbon emission of the project is reduced by about 967 t-CO2e, 65% of the total amount.