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PREDICTED AND OBSERVED GROUND DEFORMATIONS DUE TO TBM
TUNNEL EXCAVATIONS ON THE IZMIR METRO PROJECT (STAGE 1)
Conference Paper May 2009
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PREDICTED AND OBSERVED GROUND DEFORMATIONS DUE TO TBM TUNNEL EXCAVATIONS ON THE IZMIR METRO PROJECT (STAGE 1)
Ahmet Unlutepe1,*, Volkan Tellioglu2,*, Basar Arioglu3
1Bogazici University, KOERI, Geodesy Department, Istanbul, 34680, Turkey firstname.lastname@example.org
2Tunnel Engineer, Parsons Brinckerhoff, 69 Ann Street, Brisbane, 4001, Australia email@example.com
3Managing Director, Yapi Merkezi Construction Co., Camlica, 34676, Istanbul, Turkey
* formerly with Yapi Merkezi Construction Co., Istanbul, Turkey Keywords: Soft ground, EPB (earth pressure balance) TBM tunnel, Surface settlements ABSTRACT Stage 1 metro construction for the city of Izmir, which has the third highest population in Turkey, has been successfully completed. It included a 2.75 km tunnel excavated by tunnel-boring machine (TBM) using the earth pressure balance (EPB) method in soft ground conditions with shallow overburden. This paper revisits the surface settlement values predicted by the empirical approach, adds numerical analysis, describes the geotechnical instrumentation for field performance and then compares the observed deformations with empirically and numerically predicted ground deformations. INTRODUCTION With a population of 3.5 million, Izmir is the third biggest metropolis and one of the most historically significant cities in Turkey. The total length of planned rail transport in the Izmir Bay area is 45 km, according to the transportation master plan. The Yapi Merkezi AdTranz Consortium has successfully completed Stage 1 of metro construction for Izmir Metropolitan under a design-and-build turnkey contract. With a total length of 11.5 km, the alignments peak capacity will be 45,000 passengers per hour per direction in 2010. Total contract value was US$600m. The project includes 2.75 km of tunnels excavated by tunnel-boring machine (TBM) using the earth pressure balance (EPB) method. Yapi Merkezi subcontracted construction of the TBM tunnels to YAPTAG JV (Yapi Merkezi Wayss & Freytag AG Joint Venture). Of the 10 stations on the project route, four are underground, and three are connected by TBM twin tunnels. The TBM tunnel alignment is situated in soft ground with shallow overburden, and crosses populated and historic built environment. The bore diameter of the completed TBM tunnels is 6.56 m. This paper compares the tunnelling-induced, observed ground deformations with values predicted by empirical methods and numerical analysis. It also describes the geotechnical instrumentation used for field performance and, finally, compares the observed deformations with empirically and numerically predicted deformations. PROJECT ALIGNMENT AND BUILT-ENVIRONMENT The 11.5-km-long project alignment was divided into its major elements: 1.7 km of mined tunnels, 1.4 km of TBM twin tunnels, 1.1 km of cut-and-cover tunnels, and the remainder was surface works. Konak, Cankaya and Basmane stations were constructed using the cut-and-cover method. Each station is approximately 200 m long, with excavation depths of 17 m, 20 m and 16 m respectively. These underground stations were connected by EPB-type TBM twin tunnels. Four
tunnels were bored using a single EPB TBM that was launched from Basmane and Konak, then received at Cankaya (Fig.1). Civilisation dates back to 3000 BC in the Izmir area, and the majority of the TBM tunnel route had to be constructed underneath this populated urban and historic environment in soft ground conditions the most challenging section of the whole project. Konak Station is close to a historic tower clock, and one of the tunnels was excavated 10 m from this structure with 8 m to 9 m of soil cover. Basmane Station was constructed next to the historic State Railways Station. In addition, along a section of the alignment from Basmane towards Cankaya, twin tunnels had to be excavated under a street lined on both sides with old masonry buildings of two- to three-storey, as well as six- to nine-storey-high relatively new buildings with reinforced-concrete frames; some of the buildings also had basements.
Figure 1: Project alignment (EPB twin tunnels are Figure 2: Alignment and typical cross-section for twin tunnels situated between Konak and Basmane stations) TUNNEL CHARACTERISTICS TBM tunnel construction formed 12% of the total alignment in each direction. Four single tunnels were excavated by one Herrenknecht EPB type of single-shield TBM that was launched from Basmane and Konak, then received at Cankaya. Overburden depth above the tunnel crown varies from 8 m to 13.5 m, which is approximately 1.3 to 2 times the bore diameter. The clearance between the two side-by-side twin tunnels varies between 5.4 m and 69 m approximately (Fig.2). Tunnel drive 3 and drive 4 are separated by a large distance because it was proposed that an 18-m-deep underground pump house be constructed by another contractor between the two tunnels. A total of 2,292 rings were installed during TBM tunnel operations for four drives. The TBM machine is 74.30 m long and weighs 550 tonnes. Total installed power is 1280 kW with 5000 kNm maximum cutterhead torque. Table 1 shows the alignment characteristics and advance rates. The maximum advance rate was achieved in Drive 4 - 25 rings per day, which equates to 30 m per day (Fig.3). After each drive, the advance rate was observed to increase. This happened for three major reasons. Firstly, the shield crew advanced along a learning curve in TBM operation, and gained experience in the site-specific soil conditions. Secondly, the grout mix design was revised to get the optimum performance. Thirdly, soil conditioning usage was optimised, improving the advance rates.
Table 1: The alignment characteristics and advance rates
TBM advance rate
0 50 100
Drive 1 Drive 2 Drive 3 Drive 4 Figure 3: TBM advance rate for all four drives
The bore diameter is 6560mm, external shield diameter is 6520mm and finished lining intrados is 5720mm. The width of the lining ring is 1.2 m. Lining thickness is 300mm, and concrete class is C45 (fck = 45MPa). All segments are conventionally reinforced. Each ring comprises 6 segments and a key. Table 2 shows the details of geometric characteristics of the EPB shield and segmental lining.
Table 2: Geometric characteristics of the EPB shield and segmental lining GROUND CONDITIONS The project area has complex topography and geology that consists of various soft ground formations, from gravelly sands, sandy silts, and silty clays to very soft unconsolidated clays with no boulders or bedrock formation. There are also sandy gravel beds with high artesian potential. Water content of clay units, particularly between Konak and Cankaya, is nearly at the liquid limit. Soils at Cankaya and Basmane are dominated by gravelly sands, sandy silts and silty clays. Konak and Cankaya are close to land reclaimed over many centuries from the sea with a deep artificial fill up to 6 m. Figure