MRT NEL C706, Design of Bored Tunnels Along Race Course Road
Total Page：16
File Type：pdf, Size：1020Kb
MRT NEL C706, Design of Bored Tunnels along Race Course Road. Dr. Oskar Sigl Dr. Georg Atzl Design Manager, C706 Bored Tunnels, Senior Geotechnical Engineer, C706 Bored Tunnels, SYNOPSIS: The northern section of the bored tunnels of Contract C706 of the Singapore MRT North East Line is located in varying soil conditions ranging from cemented Old Alluvium to very soft marine clay. The bored tunnels will run along Race Course Road, underneath a residential area underpassing several religious temples. The tubes will be constructed by earth pressure balanced shield machines. The paper is presenting the key approaches adopted for the design of the tunnel lining, the assessment of ground movements under the particular circumstances as well as the basic design considerations for the intersection of MRT tunnels with the future tunnels of the Singapore Underground Road System (SURS). In this context, the effect of water pressures in terms of sectional lining forces was studied in detail. A design chart has been developed to determine the sectional forces due to the water pressure load and in relation to the subgrade reaction modulus. An approach to consider the influence of the second tunnel has been developed and is presented in this paper. 1. INTRODUCTION hydrophilic gasket, both placed close to the Contract C706 of the Singapore MRT North outside of the segments. The segments are East Line (NEL) has been awarded to connected with straight galvanized bolts in Hyundai–Züblin Joint Venture and comprises both joint directions. Two Herrenknecht EPB the tunnels and stations between Dhoby Ghaut shield machines will be used. station (DBG) in the south and Boon Keng In the Jurong Formation of the southern station (BNK) in the north. The approximately bored tunnel section, it is expected that the 750 m long southern bored tunnel section starts TBM can be run in open mode over some from the start shaft at Kandang Kerbau station length due to good ground conditions. In the (KDK) and reaches DBG after underpassing northern section, however, ground conditions hills of sandstones and mudstones of the and building settlement control will require Jurong Formation with varying degrees of that the TBM is operated in EPB mode along weathering. In the middle section, from KDK the entire tunnel length. In this area, sections to Farrer Park station (FRP), a cut and cover with soft marine clay are located underneath section will be constructed in Old Alluvium Race Course Road, a densely overbuild and in soft clay sediments of the Kallang residential area. This paper is describing Formation. The northern bored tunnels section aspects of the tunnel lining design carried out starts from the start shaft at FRP station and for C706 and concentrates on the northern reaches BNK after approximately 915 m. This tunnelling section. tunnel section is located in Old Alluvium and The tunnel design in this area is soft clay sediments of the Kallang Formation. distinguishing basically two different design The tunnel concrete lining, with 5.8m i.d., cases. First, the regular tunnel construction and 1.5m width and 250mm thickness, consists of 5 secondly the construction of tunnels of the segments and 1 key stone. For sealing purpose, Singapore Underground Road System (SURS) each segment is fitted with one EPDM and one directly above the, at that time operating, MRT Oskar Sigl and Georg Atzl, C706 Design of Bored Tunnels along Race Course Rd. 1 tunnels. Whereas the first case is considered to units based on the unified soil classification Figure 1 Typical Geotechnical Sections in Race Course Road derive design values for sectional tunnel lining system (USC) and SPT N300 values. The and verification of expected surface natural water level is generally located settlements, the second case is putting approximately 1m - 3m below ground level. emphasis on the assessment of measures to Hence, ground water head above the tunnel limit and control the heave of NEL tunnels during SURS Table 1 Basic Soil Design Parameters construction. Formation Fill Kallang Old Alluvium Geotechnical Unit A F1 F2 M RSOA CWOA OA Unified Soil SC, SM CH, CL CH SC, SM, CL, ML 2. GROUND CONDITIONS Classification In the northern section, SPT N300 – values 5 - 30 0 – 5 0 - 5 5 - 25 25 - 100 > 100 between FRP and BNK, the Bulk unit weight, 18 20 20 16.50 21.5 21.5 21.5 tunnels are located in the kN/m3 alluvial-coastal plain of the Undrained 50 30 50 167.5 333 cohesion, kPa Rochor and the Kallang River, Undrained Young's 10 6 20 67 133 where the young deposits of the modulus, MPa Kallang Formation overlie the Poisson's ratio 0.5 undrained, 0.3 drained sediments of the Old Alluvium. Drained angle of 30.0 30.0 22.0 22 30.0 35.0 35 The upper layers of the Old friction, ° Alluvium are completely Drained cohesion, 10.0 0.0 0.0 0 0.0 5.0 20 kPa weathered (CWOA) to residual Drained Young's 12.5 11.0 8515.0 50.0 100.0 soil (RSOA). The stratigraphic modulus, MPa units of the Old Alluvium and UCS, measured, 0-70 0-600 17-800 76-2800 kPa the Kallang Formation are Permeability, m/s 10-4 to 10-7 to 10-8 to 10-5 to 10-8 subdivided into geotechnical 10-7 10-10 10-11 Oskar Sigl and Georg Atzl, C706 Design of Bored Tunnels along Race Course Rd. 2 crown ranges between 0.15 2.00 12 to 19m and the soil 0.10 1.90 overburden above tunnel fn crown is between 15 to 0.05 1.80 20m. The distance 0.00 fm 1.70 between the tunnel axes -0.05 1.60 varies between 10.5 and -0.10 1.50 22m. fn - Normal Force Factors In the area of Race fm - Bending Moment Factors -0.15 1.40 1 10 100 1000 Course Road, the bored Kr - Modulus of Subgrade Reaction [MN/m3] tunnels intersect two fmcrown fmbench fminvert buried valleys, which fncrown fnbench fninvert were eroded into the Old M = fm * γw * r3 N = fn * γw * r2 Alluvium and γw = 10 [kN/m3] N = Lining Thrust [kN/m] subsequently filled with r = Lining Radius [m] M = Bending Moment [kNm/m] soft marine clay Figure 2 Factors for Sectional Forces due to Water Pressure reaching a maximum Loads thickness of up to 30m. The marine clay may locally overlie a layer 3. Tunnel Design of estuarine peat and fluvial sands, which were Lining design calculations have been carried laid down directly on the weathered materials out according to the bedded beam model after of the Old Alluvium. These buried valleys of Duddeck and Erdmann (1982). The approach marine clay represent the major tunnelling was applied considering tangential slip challenge in this area both, in terms of lining between the lining and the subsoil. This yields design as well as settlement control and slightly larger bending moments, which was building protection. Table 1 is presenting the considered more appropriate to deal with the basic geotechnical design parameters. Figure 1 varying ground conditions of marine clay and is showing typical geotechnical sections in this Old Alluvium. According to the design criteria area. a surcharge, q, had to be applied to the ground ) PASSAGE NORTHBOUND TUNNEL 1.20 2 1.00 3 1 0.80 0.60 PASSAGE NORTHBOUND TUNNEL 1 ) ( MN FORCE NORMAL 1 2 0.02 3 3 10.00 Ø 6.30 0.00 NORTHBOUND SOUTHBOUND TUNNEL TUNNEL 2 -0.02 BENDING MOMENT ( MNm BENDING 1 10 100 1000 10 000 Oskar Sigl and Georg Atzl, C706 Design of Bored Tunnels along Race Course Rd.DAYS 3 Figure 3 Typical calculated History of Normal Forces and Bending Moments in the PASSAGE PASSAGE SOUTHBOUND TUNNEL NORTHBOUND TUNNEL 4.50 4.00 3.50 A F1 3.00 2 1 M 2.50 3 35 4 2.00 RSOA 5 1.50 1 4 2 F1 OA PORE PRESSURE ( bar ) ( bar PRESSURE PORE 1.00 CWOA 10.00 Ø 6.30 0.50 NORTHBOUND SOUTHBOUND TUNNEL TUNNEL 0.1 1 10 100 1000 10 000 DAYS Figure 4 Typical Calculated History of Pore Water Pressures surface. The surcharge has been introduced in analytically calculated sectional lining forces. the in the Erdmann approach by an average In the case of settlement calculations, the “effective” soil weight according to formula numerical results served the purpose to below. Calculations are carried out considering calibrate input parameters such as ground an average “effective” unit weight (γ’av): losses for analytical settlement calculations. γ’av = (σ’z(at tunnel axis)+q)/H In the numerical calculations, the soil is σ’z … effective stress at tunnel axis modelled as an elasto-plastic material with a H … Depth of tunnel axis constitutive law according to the Mohr- All Erdmann calculations have been carried Coloumb yield criterion using drained out based on effective stresses. Groundwater parameters. To consider the decrease of the pressures have been considered entirely shear modulus due to shear strains, zones with separate. Water pressures have been split into different soil stiffness have been modelled two parts, a uniform pressure and a pressure around the tunnel. The calculation results show gradient. Calculation factors for sectional that in Old Alluvium, 250 mm of plain concrete forces have been derived based on the results (Grade C60) are sufficient. In the marine clay, of a detailed plain frame analyses. Results have the close proximity of the two tunnels to each been compiled in to a design chart, which is other and the relatively shallow depth required presented in Figure 2.