The Quito metro is part of the ‘Transportation Plan for the Mobility of Quito’. It calls for 20km of main tunnelling, carried out by 9.4m-diameter EPBM; 15 top-down rectangular box stations with dimensions 140m x 30m x 20m, with 30m-deep diaphragm walls; and 27 shafts for various service purposes, which are 8.2m in diameter and descend to an average depth of 30m. The Metro begins in El Labrador station (north Quito), and ends in Quitumbe station (south Quito).
GEOLOGY
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Site investigations were undertaken by the contractor and client following a GBR. The local geology is complex, comprising of different super positioned layers of rigid clayey silts above compact silty sands (and vice versa). Blocks and boulders are present at a 5m layer some 25-30m below the surface, which is the tunnel alignment depth. Additionally, the water table is 10m above the crown, reaching 20m in places.
It was this water table and the presence of fines along the alignment that primarily influenced the choice for mechanised tunnelling. For shaft and station construction, vertical spans of 2m will be excavated at a time to ensure control of muck removal and ring installation for each cycle.
Extensive ground treatment has been required, specifically jet grouting around connections between the shafts, galleries and main tunnel, mainly super-jet grouting. Additionally, where the tunnel cover is relatively thin, the teams have carried out compensation injections via horizontal tube-a-manchettes pushed out from small shafts of about 5m deep. And finally wherever geology has dictated special treatment is necessary.
LINING
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By GlobalDataThe final lining of the tunnel consists of universal precast segmental rings, 0.32m-thick and 9.07m outer diameter. The concrete is reinforced with 100kg of conventional steel rebar per m3 of concrete, which is further reinforced by 25kg/m3 of steel fibre.
For water control in the tunnel, hydrophilic gaskets plus elastomeric joints were used, located at the outer third of the segment thickness. In the shafts, hydro-expansive joints of bentonite rubber were used, positioned in the centre of the segment thickness of the horizontal joints. In the stations, nothing beyond the wall concrete is thought to be needed
