Northern Thailand, which is hilly, experiences distinctive wet and dry seasons throughout the year. Here, the Province of Maehongson experiences mean annual rainfall of about 1200 mm, the vast majority of which falls in the wet season between April and October. During the wet season, there are many failures of earth slopes and hillsides. In this area, roads are often carved into slopes in some locations while the embankment sections are filled with the excavated soil. The fills that support the roads normally have little compaction. Consequently, during the wet season groundwater flows easily penetrate these fills, causing instability, with many slope failures resulting. Pang Oong, in Maehongson Province, is a village that is accessible only by road through hilly terrain and is rated one of the most romantic tourist destinations in Thailand, known for the beautiful misty lake and mountain pines located adjacent to the village.
A major fill slope failure occurred along the access road into the village during a period of heavy rainfall. An expedient solution to repair the slope failure using gabions was initially implemented as there was an urgency to protect the half of the road that had not failed. However, this solution also failed with a second slip failure occurring after further heavy rain. The client then decided to adopt a long-term solution by fully restoring the failed slope section. The most important constraint for the design of the slope restoration was that the single lane of the road remaining after the slope failure had to be kept functional as this road was the only access to many villages in the area. This also meant that any design involving excavation that could further jeopardize the integrity of the road above had to be excluded.
The final design resulted in a slope restoration in two parts. The lower part was rebuilt at a shallow slope angle of 1V:1.5H, with compacted residual soil benched into the existing good ground. This was done such that a 15 m wide platform would be created for the construction of the 13 m high upper part of the slope consisting of reinforced fill. Within the fill in the lower part of the slope horizontal drainage pipes were installed to drain out any accumulating groundwater at the rear of the compacted fill zone. At the base of the reinforced fill slope a horizontal drainage blanket was constructed using single sized aggregate sandwiched between two layers of a MIRAFI® Polyfelt TS geotextile filter.
The upper part of the restored slope consists of 3 benched tiers of reinforced fill, each having a slope face angle of 2V:1H. The lower tier is 4 m high and reinforced with a combination of 2 layers of MIRAGRID® GX300 geogrid reinforcement, 4 layers of MIRAGRID GX250 geogrid reinforcement and 2 layers of MIRAGRID GX130 geogrid reinforcement. These lower tier reinforcements span 13 m in length. MIRAGRID GX geogrid reinforcements are composed of high strength, high modulus polyester yarns within a robust polymer coating.
The middle-reinforced fill tier is 6 m high and reinforced with a combination of 4 layers of MIRAGRID GX130 geogrid reinforcement and 8 layers of MIRAGRID GX80 geogrid reinforcement. These middle tier reinforcements span 8.5 m in length. At the mid levels of the primary geogrid vertical spacing, MIRAGRID GX60 geogrid reinforcement is used as secondary reinforcement. The upper reinforced fill tier is 3 m high and reinforced with 3 layers of MIRAGRID GX80 geogrid reinforcement of 5 m reinforcement length.
At the face of the reinforced slope, the geogrid reinforcements are wrapped around soil bags and tucked back into the slope at the next reinforcement level. The soil bags serve as forms to shape the steep slope profile and enable fairly heavy compaction to be applied close to the slope face. The jute bags also serve to prevent surface erosion during the initial phase of surface vegetation.
To monitor the performance of the reinforced slope the Geodetect fiber-optic strain monitoring system was incorporated into the slope with the geogrid reinforcement. The monitoring results 7 months after construction showed that horizontal strains were small, less than 1%. At 15 months after construction there was negligible difference in the horizontal strains.
Completed reinforced slope with vegetation growth on the face
Initial excavation at the toe of the failed slope
Installation of the drainage blanket at the base of the reinforced slope
Soil bags with MIRAGRID GX geogrid wraparound facing
Spreading locally obtained reinforced fill over MIRAGRID GX
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