Assessment of Tunnel Stability with Geotechnical Monitoring

Transcription

1 Assessment of Tunnel Stability with Geotechnical Monitoring G. Güngör, A. Şirin, S. Kiziroglu, H.D. Altuntas, A. Durukan General Directorate of Turkish Highways, Ankara, Turkey T. Solak Temelsu International Engineering Services Inc., Ankara,Turkey ABSTRACT: Application of tunnel design to site conditions requires continuous monitoring of deformation, review and revision of design considerations. A twin tube road tunnel has been constructed through the heterogeneous weak rock conditions involving sedimentary and volcanic units. Close distance between the tubes due to highway alignment constraints, lowering the water table due to excavation contributed excessive deformations in the tunnel. This paper includes the assessment of various factors influencing tunnel stability; geological condition, secondary stress development due to approach of second tube construction and ground water lowering. The monitoring data including 3D displacement measurements inside the tunnel, settlement profiles of ground surface are presented with emphasizing influencing factors. The efficiency of measures such as rapid closure of support ring, bolting at the invert, are also indicated and discussed. 1 INTRODUCTION General Directorate of Turkish Highway is constructing Konak tunnel, that was designed as twin tube road tunnel and will serve city traffic of İzmir. Due to the requirement of binding the tunnel route to existing road alignments with a large rotary bridge crossing, selecting a route with favorable condition was not possible. Tubes with a width of 11 m are located with a lateral distance of 1 tunnel width at the portal zone. Portal excavation and first 100 m of tunnel advance were completed in heterogeneous weak rock conditions. Geotechnical monitoring involving displacement measurement in tunnel section and settlement measurement at the surface were performed. Monitoring data with tube advance enables to control tunnel stability and directs time and type of additional measures. Measurement data from the tunnel station located near the entrance and related surface station is presented in the paper. Influence of tunnel advance and efficiency of additional measures on tunnel stability are evaluated. 2 GEOTECHNICAL DATA EVALUATION The concerned section is located at a distance of 45 m from the tunnel entrance. Geological mapping of top heading face shows that tunnel section is at the contact of sedimentary and volcanic units. Left part of the tunnel face is governed by medium weathered-partially weathered clay-stone with sand-silt-gravel. Medium weathered hard tuff starts to be observed at the right upper part of the tunnel face (Figure 1,2). The contact consists of damp and irregular shear surfaces, weak clay zone and clay-sandsilt band. Water inflow with an approximate discharge rate of 2 lt/ min was encountered from tunnel face and crown. Considering the geological data the proposed behavior of the tunnel is expressed as squeezing rock mass, C2 according to the ÖNORM 1993 classification. 1

2 Figure 1.Photo from tunnel face Figure 2.Geological mapping of tunnel face 2

3 2.1 Monitoring Data During tunnel excavation measurement stations were settled at close distance and 3 D displacement measurements were taken. Measurements were evaluated with combination of tunnel advance data. The presented graph is from the measurement station located in the first tube 45 m from entrance portal under 50 m overburden (Figure 3). It gives vertical displacement of Point 3, right shoulder which is the nearest location to second tube. After top heading excavation and support application displacement curve followed an expected trend at the first period of measurements. Then it accelerated indicating a destabilization. To achieve the stabilization at the top heading elephant foot application with flat or curved temporary invert was proposed by Designer. Displacement vectors in the tunnel section are shown in Figure 4. In the crown it was directed to left due to the contact zone located at the right upper part. The general tendency of the displacement vectors are as settlement. In the first periods of the measurement, convergence was observed. In the next periods direction of displacement vectors were dominated by settlements especially at the bench excavation. The factors causing the observed deformation behavior are evaluated as low strength of surrounding media, fault zone, inadequacy of applied support to form a support arch around tunnel section especially at the lower part of wall and invert. Increase in secondary stresses due to the other tube advance was also a primary factor bringing the high magnitude of and acceleration of displacement. vertical displacement (mm) time (day) influence of bench excavation influence of second tube influence of additional bolting displacement data top heading excavatiom bench excavation invert advance 2.tube advance progress (m) Figure 3.Vertical Displacement of Point 3 Site supervision preferred to apply rapid closure of support ring with shortening the distance between top heading-bench-invert. After closure of support ring displacement curve become level. However it tends to increase with probable influence of tunnel advance although the tunnel face was at a distance of 40 m from the measurement station. With the approach of the other tube to the concerned tunnel section displacements were accelerated. Signs of excessive loading on rock bolts were observed at lower part of the right wall in addition to cracks at shotcrete. Shotcrete invert was inspected and cracks at invert-wall intersection and heaving at the bottom was observed. 3 Figure 4.Displacement Vectors at the Section At the ground surface, settlements were measured along a line perpendicular to tunnel

4 axis. The settlement-time graphs for the points on this line follow a similar trend with tunnel displacement data. Acceleration in the settlements was observed parallel to the destabilization in tunnel section. Settlement value at the point coinciding the tunnel axis reached to the value measured in the tunnel. (Figure 5). Surface settlement above the tube located in sedimentary units and contact zone was considerably higher than that above the tube in volcanic units (Figure 6) is evaluated. Based on the settlement profile by Peck and Schmidt equation Vs, volume of the settlement through per unit length of tunnel, is determined by the following formula (Atkinson and Potts 1977). Vs = 2π * i *smax where i is the parameter to define the width of settlement. settlement (mm) time (day) progress (m) Figure 5.Surface Settlement Graph wrt Time P1 P2 P3 P4 P5 top heading advance Figure 6.Surface Settlement Curve Considering the maximum settlement ( s max ) at the ground surface and basic equations the factor leading high ground surface deformation Parameter i can be estimated by K*Z where K is an empirical coefficient and can be taken as 0.5 for stiff clay (O Reilly and New 1982). 4

5 Volume of lost ground is normalized with respect to tunnel size and Vs is expressed as a percentage of excavated tunnel volume (Vexc=π*D 2 /4).Vs/Vexc is calculated as %12, which is high compared to the values given in several references O Reilly and New 1982 and Aoyagi The relationship between maximum surface settlement and crown settlement is described with the following equation (Atkinson and Potts 1977). sc / s max = 1.0 α ( C / D) where sc is crown settlement, smax is maximum surface settlement; C is the overburden, D tunnel diameter, α is constant. For overconsolidated kaolinite α is defined as For C=50 m, D=11 m, 40 % of crown settlement was expected at the surface according to the equation. of surrounding ground due to the change in the groundwater level with tunnel excavation. 3 MEASURES FOR STABILITY Considering the development of displacements in the tunnel and defects in supports, one of the causes for the destabilization was evaluated as the inadequacy of tunnel support at the bottom of tunnel wall and invert. To stabilize the tunnel section several measures were determined and are listed below (Figure 7). Strengthening the invert with steel beam and additional shotcrete layer (a). Removing shotcrete invert, additional excavation and applying an invert with steel beam and shotcrete to obtain a round and deep invert (b), Repair and strengthening invert-wall connection with steel mesh-steel bars and additional bolting at invert and bottom of tunnel wall (c) Figure 7.Measure to Stabilize Tunnel The factors contributing high surface settlement and ground loss were evaluated as destabilization in the tunnel and consolidation Additional rock bolting at invert and bottom of tunnel wall was applied at the site and displacements at the tunnel section become level (Figure 3) 5

6 4 CONCLUSION Continuous monitoring of deformation with advance data enables to evaluate the tunnel stability and take measures in time. They also provide information about the failure mechanism and related factors. In Konak Tunnel close distance between the tubes due to highway alignment constraints and groundwater lowering were evaluated as primary factors bringing displacements in and above tunnel, which were higher than design values. Measures to strengthen support at the invert with additional bolting were effective to achieve stability. ACKNOWLEDGEMENTS The deformation measurements were carried out by the Engineers of Construction Company, Ege Asfalt. Their contributions are acknowledged. REFERENCES Temelsu International Engineering Services, , Konak Tunnel Design Reports Atkinson, H. and Potts, M Settlement above shallow tunnels in soft ground. Journal of Geotechnical Engineering ASCE O Reilly, P and New, B. 1982, Settlement above tunnels şn the United Kingdom-theiragnitude and prediction Proceedings of Tunneling Symposium. Aoyagi, T, Representing Settlement for Soft Ground Tunneling, MSc Thesis at MIT. 6