Effect of Soil Conditioning on Performance of an EPB TBM: from Laboratory to Mahmutbey Mecidiyekoy Metro Tunnels in Istanbul
The basic purpose of this study supported by the Scientific and Technological Research Council of Turkey (TUBITAK, Project No: 213M487) is to analyze the effects of soil conditioning on the field performance of an EPB TBM used for excavation of the Mahmutbey-Mecidiyekoy metro tunnels having a total length of around 18.5 km (double tube) in Istanbul. Soil samples were obtained during the excavation of Kazim Karabekir Station in where the silty sand lithological unit of Gungoren Formation was dominant.Natural water content, specific gravity, grain size distribution (sieve + hydrometer), consistency limits (liquid limit and plastic limit of only the fines) of the silty sand sample were determined to characterize the sample in the laboratory. Mixing (with power measurement), cone penetrometer, vane shear and flow table tests were performed to find out optimum soil conditioning parameters for the mixture of soil and foam at natural water content (17%), Foam Concentration Ratio (CF) of 3% and Foam Expansion Ratio (FER) of 16 by varying Foam Injection Ratio (FIR) between 5% and 30%. The laboratory studies indicated that optimum conditioning of the sample were obtained at FIR of 20-25%. Then, the field performance of the EPB TBM working in closed mode (instantaneous penetration rate, cutterhead torque, and TBM thrust force) was recorded, observed and analyzed for the rings between 790 and 920. It was seen that the average CF of 1.5%, FER of 18 and FIR of between 50-75% was the conditioning parameters preferred in the field (half of the CF and 2-3 times the FIR defined on the laboratory). It was determined that there were good and strong relationships between the field excavation performance (penetration rate and TBM thrust force) and FIR and FER values applied in the field. Any relationship between the cutterhead torque and FIR and FER could not be detected. EPB TBM, Soil conditioning, Excavation performance, Optimization.