Excessive water pressure at the backside of tunnel lining degrades the stability of a tunnel. Uses of geosynthetic materials have been increased for filtration and drainage of tunnels. However, the design criteria and standard specification of tunnel drainage in Korea restrict the usage of geosynthetics to non-woven fabrics only for tunnel drainage. The design criteria of various geosynthetic drains such as Thermal Heat Bonded Geotextiles by hot-press have not suggested in the designs of tunnel drainage layers.
In this study, the serviceability of geosynthetics drain and the effects of calcium carbonate (CaCO3) substances on hydraulic characteristics of geosynthetic materials in tunnels are evaluated by performing the effective opening size test, the gradient ratio test and the permeability test using geosynthetics drains of seven different types including the needle punched geotextiles being currently used in the tunnel drainages and the thermal heat bonded geotextiles that are not designated in the tunnel drain materials.
According to the results of effective opening size test, gradient ratio test and permeability test, the performance of Geocomposite Type A, B and C, all of which are the Thermal Heat Bonded Geotextiles, was good.
In addition, the discharge capacity of the selected geotextiles are evaluated by changing the cell pressure. The hydraulic performance of Geocomposite type drains and the influence of calcium carbonate (CaCO3) substances on hydraulic characteristics of geosynthetic materials for tunnel drainage materials are examined. Geocomposite drains of three different types and needle punched geotextiles are used for performance comparison. In the case of the needle punched geotextile, the discharge capacity test was performed by multiplying the layer of the geotextile.
The results of discharge capacity test with the thickness change of the needle punched geotextiles showed that discharge capacity increased with the increased thickness of needle punched geotextiles. However, when calcium carbonate (CaCO3) was present in the percolated water, discharge capacity of the needle punched geotextiles was decreased to large extent with the time elapsed, regardless of the thickness of geotextiles.
The results of discharge capacity test for the geocomposite drains showed that discharge capacity was found to be significantly higher than that of the needle punched geotextiles. When calcium carbonate (CaCO3) was present in the percolated water, the discharge capacity of geocomposite drains was decreased with the elapsed time but the discharge capacity of geocomposite drains was still significantly higher than that of the needle punched drain. It was expected that geocomposite drains have better performance as the tunnel drainage materials with respect to preventing the clogging the drains by the calcium carbonate substances.