Los Teques Metro Line 2, Venezuela – ground treatment for tunnel cavern in graphitc schist
For the Caracas region’s Metro Line 2, currently in progress, a tunnel cavern (cross-section 290 m, length 302 m) was excavated in saturated weathered graphitc schist. The upper part was driven through transition material between soil and weathered rock with very high SPTs in almost impenetrable ground. Hardness increased quickly with depth of predominantly weathered rock in the lower part of the tunnel cross-section. There were small karstic cavities with concentrated water inflow. The post-excavation behavior of ground around the tunnel vault was that of a graphitc silt soil turning into mud on contact with water and excavation. Treatment varied greatly depending on ground response to drilling, from horizontal jet grouting to forepoling with manchette valve and schedule tube, face-nailing with injected fiberglass bars, anchor bolt with bulb jet grouting, and deep internal vacuum drainage.