Application of complex reinforced structure near fault zone

This project, submitted by Ace Geosynthetics, won an Outstanding Achievement Award in the 2011 International Achievement Awards(IAA) / Geosynthetics presented by the Industrial Fabrics Association International. Each project in its own special way, exhibits outstanding innovation, technical skill and design excellence.

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Introduction

The Taiwan Ministry of Transportation had devised a temporary solution of placing horizontal drainage pipes along with collector wells to drain out excess water in a steep slope in Nantou County in central Taiwan. Sheet piles were also used along both roadsides to stabilize the road and maintain single-lane traffic.

However, on June 8, 2007, a destructive landslide up to 80m wide and 30m high was caused by a torrential rainstorm. The existing anchored slope was severely damaged, and the main structure was pulled out and exposed after the landslide, forming a huge heave on the toe of the slope.

The regional geologic map shows that several fault zones cut through the jobsite area, making rock mass fragile and weak. According to the on-site boring hole results, colluvial soil was 5m below the hole, crushed shale rocks 5–35m below, more complete and stiffer shale rocks 35–60m below, groundwater between the colluvium and crushed shale rocks, and there was a fault gouge in the crushed shale layer. Several tension cracks, allowing rainfall to permeate into soil, were found after the on-site investigation.

This description indicated the jobsite had fragile geology and poor drainage colluvium near the fault zone. When a heavy rainstorm hit, water quickly penetrated into soil, causing the groundwater to rise and soil stability to drop, eventually leading to great scale of landslides. Therefore, a drainage solution was the first priority in this roadway repair project.

Design

Since the damaged area was located inside the water supply reserve of the Mingtan Reservoir, the foundation of the structure confronts the problem of water level change in the reservoir itself.

The on-site boring hole report showed no more complete stiff shale rock until 35m underneath the current roadway. After the stability analysis, a design was adopted to drill concrete piles of 1.5-m diameter and 20m and 30m long installed every 2.5m beneath the 8.5m-high reinforced concrete (RC) retaining wall.

Three stages of soil anchors—bond length 10m, free length 30m, anchorage load 60 tonnes—were also installed from the wall surface every 2m both horizontally and vertically. Above the RC retaining wall, a wraparound reinforced structure was constructed—10m and 8m long with geogrid installed within the structure, slope inclination 1V:0.3H, wall height 17m divided into four stairs.

A full interior and exterior drainage system was installed in the reinforced structure to effectively resolve the poor drainage problem. In-situ soil (crushed shale mixed with silty soil) was used as fill material (see cross section in Figure 1).

Mountains cover more than 70% of Taiwan and a massive transportation system is essential to save time and cost in cargo transportation. This roadway repair project is on a 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 depends not only on soil anchor module control but also on superior grouting quality, which requires precise judgment from experienced engineers.

Summary

The installation complications, labor costs, and grouting material cost made it far more difficult and inconvenient than a flexible structure-reinforced soil retaining wall.

The wraparound reinforcement method was adopted by the design company to take advantage of its basic required installation equipment, easy installation without topographic constraint, and lower labor cost.

With the help of dredged gravel backfill, 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 serves as a good reference on mountain roadway repair for civil engineers.