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. After four months in operation, a number of fall-outs and slides in the shotcrete-lined parts of the longest tunnel stopped the water flow completely. The majority of the fall-outs occurred in the Fomeque formation, which is dominated by black shales with interbedded layers of siltstone and some limestones. l rock samples from the Fomeque formation have been analyzed with respect to swellability and petrography/mineralogy. Swelling pressures varying from 014 MPa (siltstone) to 45 MPa (shale) have been measured on intact rock specimens. The siltstone does not show cracking or slaking after storage in water for several weeks. The shale disintegrates almost immediately when submerged. Representative samples of the two rock types were chosen for comprehensive mineralogical and textural analyses. The rocks show a marked difference in textures. The siltstone is homogeneous, massive and cryptocrystalline to fine grained, while the shale is microcrystalline to cryptocrystalline with a penetrative slaty cleavage. the tunnel was filled with water, extensive draining and drying-out of the rock masses along the tunnel periphery took place. This resulted in heavy cracking and fissuring, especially along the cleavage planes of the shales. The cracking reduced the general strength of the rocks, but, more importantly, caused a dramatic increase in the permeability and in the exposed rock surface area. When the tunnel was filled, water entered all new fissures in the shales and the exposed and partly dehydrated illite/smectite 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.