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2013FallShotcreteEMag

Goin’ Underground onto the membrane after it had cured for just “For soft ground tunnels, the traditional under 30 minutes; and approach of a temporary primary sprayed c) A layer of Masterseal 345 (accelerated), mea- concrete lining is very wasteful and, suring 4 mm (0.15 mm) thick, was sprayed with current technology, unnecessarily onto Test Panel 3 with a dry sprayed concrete conservative” mix sprayed onto the membrane after it had cured for just under 20 minutes. Table 2: Possible lining thicknesses for different lining options* Results—From the three test panels at Hager- Sealing Primary bach, the following results for bond strength Lining option layer lining Secondary lining Total were obtained: 350 mm inside a The tests showed that good bond strength can be DSL 75 mm 325 mm 750mm achieved with spraying concrete onto relativelysheet membrane 300 mm inside a young sprayed membrane, particularly the Meyco CSL-no bond 75 mm 325 mm 700mm TSL 865 (refer to Fig. 10). The latter could bespray-on membrane classified as a “medium” strength TSL, according to CSL-bonded 75 mm 325 mm 650mm Yilmaz’s groupings.10 While further testing would250 mm inside a spray-on membrane be required to prove that this could be achieved on a *This refers to a large diameter, shallow tunnel in soft ground regular basis, this opens up the possibility for a single- shell tunnel lining with sprayed membrane/sealing layer or mortar followed by a sprayed membrane and then the sprayed concrete structural lining. Discussion—The trials carried out on-site, spraying the TSL 865 onto London Clay, demonstrated that a single shell should be considered successful, and that a progressively strengthening bond was achieved between the TSL and the London Clay even though the conditions were not conducive to rapid curing. The testing carried out at Hagerbach demonstrated that a bond can be achieved between the waterproof membrane and the sprayed concrete after a minimum curing time of the waterproofing membrane of 30 minutes. In comparison with what can be achieved under laboratory conditions, as shown in Figure 11, it is clear that further optimization of this process Fig. 10: Bond strength vs. age from Hagerbach trial is possible, and further testing of this process should be carried out in particular to determine: • Optimal curing time of the thin skin liner to achieve an acceptable bond strength to the sprayed concrete compared to construction sequence requirements; • How accelerators affect curing time of the thin skin line compared to bond strength achieved with the sprayed concrete; and • Whether an alternative product could be devel- oped that could be optimized to fulfill both the sealing layer and waterproofing properties. conclusion For soft ground tunnels, the traditional approach of a temporary primary sprayed concrete lining is very wasteful and, with current technology, unnecessarily conservative. Over the last 15 years, Fig. 11: Bond strength vs. age under laboratory conditions a series of pioneering projects in the UK has revo- 52 Shotcrete • Fall 2013


2013FallShotcreteEMag
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