STABILITY PROBLEMS IN WATER TUNNELS CAUSED BY EXPANDABLE MINERALS - SWELLING PRESSURE MEASUREMENTS AND MINERALOGICAL ANALYSIS

To improve the water supply in Bogota, Colombia, 38 km of tunnels were excavated through the Cretaceous and Tertiary sedimentary rocks. Afte r four months in operation, a number of fall-outs and slides in the sh otcrete-lined parts of the longest tunnel stopped the water flow compl etely. The majority of the fall-outs occurred in the Fomeque formation , which is dominated by black shales with interbedded layers of siltst one and some limestones. Several rock samples from the Fomeque formati on have been analyzed with respect to swellability and petrography/min eralogy. Swelling pressures varying from 0.14 MPa (siltstone) to 4.5 M Pa (shale) have been measured on intact rock specimens. The siltstone does not show cracking or slaking after storage in water for several w eeks. The shale disintegrates almost immediately when submerged. Repre sentative samples of the two rock types were chosen for comprehensive mineralogical and textural analyses. The rocks show a marked differenc e in textures. The siltstone is homogeneous, massive nad cryptocrysall ine to fine grained, while the shale is microcrystalline to cryptocrys talline with a pentrative slaty cleavage. Before the tunnel was filled with water, extensive draining and drying-out of the rock masses alon g the tunnel periphery took place. This resulted in heavy cracking and fissuring, especially along the cleavage planes of the shales. The cr acking reduced the general strength of the rocks, but, more importantl y, caused a dramatic increase in the permeability and in the exposed r ock surface area. When the tunnel was filled, water entered all new fi ssures in the shales and the exposed and partly dehydrated illite/smec tite started to swell. The swelling of the mixed layered silicates is believed to be the initiating factor causing most of the fall-outs and slides in the Chingaza tunnel.