Optimization of Supports in a Road Tunnel through Conglomerate during Construction

Transcription

1 Optimization of Supports in a Road Tunnel through Conglomerate during Construction Tülin SOLAK, Temelsu International Engineering Services Inc., Turkey, tulin.solak@temelsu.com.tr Bülent ULUKAN, Temelsu International Engineering Services Inc., Turkey, b.ulukan@temelsu.com.tr Göktuğ SIREL, Temelsu International Engineering Services Inc., Turkey, gsirel@temelsu.com.tr Summary Ground characterization during design and construction of a tunnel determines support applications. For the formations such as conglomerate exhibiting intact rock and discontinuities application of standard characterization methods brings difficulties. Due to the uncertainties in ground characterization, the re-design process has to continue during the construction. In this paper the adaptation of tunnel design to construction in a road tunnel is given emphasizing the characteristic of tunneling through conglomerate. The design considerations of the Tunnel are checked during construction. Back analyses are performed with the first deformation readings to determine the representative ground parameters. As a result of further analysis support system is modified in terms of bolt length and size of steel ribs to optimize tunnel construction. Keywords: observational method, back analysis, conglomerate, ground characterization 1. Introduction Observational method in geotechnical field consists of review of the design considerations during construction with the aid of geotechnical measurements and modifications of design to meet actual conditions. Application of the method in tunneling is widened parallel to progress of NATM applications. According to the main concept of NATM the ground is allowed to deform enabling to activate its carrying capacity with flexible support systems. Conglomerate is a clastic sedimentary rock which contains clast of rock particles in matrix. This formation is generally characterized with particle composition of clast, matrix and their bond. While the formation does not include intact rock and discontinuities its characterization and design parameter determination bring difficulties. Gori Tunnel is located on Liakhvi Bridge-Ruisi Road Section in Georgia and being constructed under the supervision of Temelsu International Engineering Services Inc. The requirement of the adaptation of support system during construction in the Gori Tunnel, which is presented as a case in the paper, has both technical and financial reasons. Due to the budget constraints of the Client- Georgian Road Authority, Temelsu International Engineering Services Inc. had reviewed as consulting, construction process and tunnel design and recommended adaptation of support system to actual condition. The paper consists of related evaluation and analysis performed by the consultant engineer. In the first part of the paper the experiences in tunneling through conglomerate from several projects are summarized and the design concept of the Gori Tunnel in Georgia is given. In second part the conditions encountered in tunnel construction and results of back analysis based on the first deformation measurements are presented. Finally the modifications in tunnel support and auxiliary measures are determined for an optimum tunnel construction. The criteria for ground characterization are given to determine the excavation and support classes in the Tunnel.

2 2. Tunnelling through conglomerate 2.1 Cases from design and construction Characterization of the ground is the first step of the tunnelling through any media. The tools used for the ground characterization should meet the requirements of formation. For rock masses exhibiting discontinuities or soil formations ground characterization methods differ from each other. There are also difficulties to characterize formations in between such as conglomerate. There are not straightforward methods or techniques to handle the design in such formations. The experiences and findings from several projects are summarized firstly to have a general view. In San Vicente Pipeline Tunnel (USA) a part of tunnel is located in mixed face conditions that includes Friars Formation Conglomerate overlying granitic rock. The conglomerate exhibits a wide range of strength characteristics depending on the clast compositions, degree of matrix cementation, and grain size distribution. The parameters used to characterize the conglomerate were: the density of the formation, the gradation of the clasts, the amount and degree of cementation of the matrix, the strength of the clasts, and the groundwater conditions. Stand up time tests performed for the nearby Tunnel excavated in conglomerate were analyzed. The observations generally indicated that stand up time would be good above the groundwater level and less favorable below the groundwater level. Firm to slow raveling ground was anticipated above the groundwater table and slow to fast raveling ground was anticipated below the groundwater table. Where the conglomerate matrix is very weak, cobble and boulder clasts are not firmly held in place by the matrix, and it was expected that the clasts would fall from the tunnel roof, sidewalls, and face if not supported. Ground support for this class includes shotcrete and lattice girder structural support with presupport measures including drain holes, spiling, self-drilling tube spiles, and pipe umbrellas as required. During construction support system and advance length is arranged according to the conditions. Probe holes were drilled with minimum of 20 feet ahead of the face at all times. These probe holes also acted as drain holes. Groundwater levels have been drawn down by the tunnel excavation ahead of the tunnel face and this drawdown has tended to improve tunnel stability. [1]. In design of the Devil s Slide Tunnel, (USA) the conglomerate is described as light to dark gray, weathering to dark brown, medium to coarsegrained with lithic fragments, strong, hard, subangular to well-rounded clasts consisting of granodiorite, quartz, sandstone, siltstone, and claystone. Conglomerate is moderately to intensely weathered. The behavior types expected are defined with two failure modes. First one is progressive stress induced failure mode, where the rock mass undergoes progressive failures from shear stresses, resulting in progressive deformations as the load bearing capacity of the rock mass is exceeded. Second one is groundwater induced failure mode ahead of the tunnel face showing progressive failure induced by stresses ahead of the tunnel face (fast raveling ground) [2]. The experience gained from tunnel constructions through conglomerate shows that the ground behaviour or failure modes are closely related to the composition and compactness of conglomerate. Water inflow is a major factor determining the behaviour and makes drainage measures essential. The observed failure modes through conglomerate are generally stress induced failure and instabilities of blocks of conglomerate. Beside the primary supports presupport elements such as forepolings are effectively used. 2.2 Design considerations in the Gori Tunnel Gori Tunnel on Liakhvi Bridge-Ruisi Road Section in Georgia is a twin road tunnel with a length of 780 m, a width of 12 m, and 40 m distance between centerlines of tubes (Fig. 1). The geological formation through the Tunnel is described as weakly cemented conglomerate (boulders and cobbles in clay matrix) as result of site investigations during design process (Fig. 2). Fourteen borings were drilled and classification tests were performed. Cement of conglomerate is clay with liquid limit of 40 %, plasticity index of 20% and moisture content of %. Strength properties could not be determined with tests while sampling was not possible. Groundwater table is not observed in boreholes [3].

3 Fig. 1 Cross section of the Tunnel [3] BH-3 BH-4 BH-6 BH-5 EGE-2 EGE-2 BH-8 BH-8A EGE BH EGE-2 and CUT AND COVER TUNNEL CUT AND COVER BOULDERS AND COBBLES CLAY HARD WITH GYPSUM EGE-1 EGE-2 CLAY HARD WITH WITH LEAN CLAY CRYSTALS AND CONGLOMERATE LENSES GYPSUM CRYSTALS CONGLOMERATE ON CLAY WEATHERED, HARD PROBABLE BH:BOREHOLE CEMENT SLIGHTLY PROBABLE FAULT UNIT BOUNDARY 810 Fig. 2 Geological Profile of the Tunnel [3] Although the standard procedure of rock classification methods could not be followed due to the nature of formation, conglomerate is evaluated as weak rock and RMR=25 GSI=20 was assigned. The design parameters were determined by formulations related to the classification systems. The expected rock mass behavior is expressed according to the ÖNORM 1993 as C3 at portal areas, C2 and B3 at the tunnel. The support system consisting of 30 cm thick shotcrete, I 200 steel arch, rock bolts (pattern 1.5x1.0 m for B3, 1.0x1.0 m for C2, 1.0x0.8 m for C3) were determined according to the numerical analysis (Fig. 3) [3].

4 Fig. 3 Support Class C2 -Design [3] 3. Optimization of supports during construction 3.1 Evaluation of site conditions Excavation and support application at the tunnel have been started according to the design. Conglomerate at the tunnel face is described as fairly cemented and it consists of cobbles and boulders in clay matrix (Fig.4). As the construction proceeds and the first deformation measurements were taken at top heading excavation the adaptation of the support system to actual conditions is required. The measurements after top heading excavation shows that displacements finalized after 10 days with a maximum value around 1 cm (Fig. 5). 3.2 Back analysis Fig. 4 Photo of conglomerate at the tunnel Considering the measurement data, excavation stages and applied support elements, back analysis have been performed to check the design assumptions and mainly design parameters of the ground for C2 class. The main principles of plain strain modelling are followed in the two dimensional numerical analysis performed with Rocscience software of Phase2. Elastoplastic material behavior for the ground is considered. Core replacement procedure is followed to consider stage construction of three dimensional tunnel constructions. For support application composite lining of shotcrete (C20/25) and I beam, rock bolts (6-9 m long bolts at top heading, 6 m long bolts at bench) are considered.

5 GORI TUNNEL EAST PORTAL NORTH TUBE STATION 3 KM:7+862 vertical displacement displacement (mm) time (day) p1-y p2-y p3-y horizontal displacement displacement (mm) time (day) p1-x p2-x p3-x Fig. 5 Deformation data from measurement station The parameters of the conglomerate giving a deformation value similar to measured one at the site are determined. The deformation value from the site is accepted as 1 cm. considering the displacements occurred until starting the measurements at the site, the measured displacements are accepted as 70 % of the displacement. The parameters for conglomerate giving a total vertical displacement of 1.34 cm after top heading excavation at numerical analysis are determined; E=720 MPa, c=110 kpa, φ= 37, γ=22 kn/m3,ν=0.25. Yielding zone, which is defined as the zone where the stresses after excavation exceeds shear strength of the ground, has a thickness of 4 m at shoulders. More than half of the 9 m long bolts are drilled in rock where there is no failure (Fig. 6). There are no yielding at shotcrete and rock bolts. Maximum bolt force is 0.09 MN; compared to the bolt capacity (0.19 MN) they are subjected to lower forces. The maximum force occurs near excavation. Considering the maximum shear force and moments calculated at the most critical sections, shotcrete and steel arch are subjected to stresses rather lower than their capacities. Fig. 6 Outputs of back analysis-displacementsyielding points-bolt loads

6 3.3 Further Analysis Numerical analyses have been performed with ground parameters obtained from back analysis considering the maximum overburden over tunnel (50 m). Length of 9 m long rock bolts are decreased to 6m. Size of the steel arches was decreased from I200 to I160. The results show that yielding zone has a thickness of 5 m at shoulders. 6 m long bolts go beyond the yielding zone (Fig. 7). Considering the forces of rock bolts, moments and axial forces of shotcrete and steel arch, the support system is adequate to fulfill its functions. Additional analyses have been performed for the other excavation and support classes (B3 and C 3) to determine modifications. Fig. 7 Outputs of further analysis-displacementsyielding points-bolt loads 3.4 Recommended Procedure for Ground Characterization While a common geological mapping and rock classification could not be followed in the Tunnel, a procedure based on Austrian Guideline [4] is suggested to determine the support and excavation classes. The content of the procedure for ground characterization is given below. Conglomerate belongs to clastic rocks in basic rock types. Considering data from borings and excavation face in tunnel, rock material is coarse grained and no bedding is observed. Accordingly significant key parameters can be determined as grain size, cementation, strength properties, matrix/component ratio according to Austrian Guideline Factors influencing the rock mass behaviours in the tunnel are expressed as rock mass bond or cementation ratio (loose or not loose), joint water, which reduced joint friction (dry, minor inflow, major inflow). Considering those factors, the distinctive criteria for three support classes are given in Table 1. Considering the present advance of tunnel through conglomerate and borings, the main support class through tunnel is expected as C2. Increase in compactness in conglomerate and dry conditions will lead to B3 class. Water inflow can result in C3 class. Table 1 Support Classes Key parameters Low matrix /component ratio, dominating grains cobbles and boulders Medium matrix /component ratio, dominating grains cobbles, gravel High matrix /component ratio, dominating grains gravel Rock mass bond Not loose rock mass Water inflow dry Support Class raveling B3 Loose rock mass minor inflow squeezing C2 Loose rock mass major inflow high squeezing C3

7 3.5 Recommendations for Construction Activities In a tunnel the construction activities affect also the performance of support elements. Attention for construction activities will provide benefits rather than increasing the capacity of the support system by increasing bolt lengths or changing bolt pattern etc. Ring closure with the application of temporary shotcrete invert at top heading base will enable the redistribution of ground loads. The optimum distance between top heading, bench and invert excavation and support should be kept for the ring closure of the whole support system in the tunnel. Water inflow is the major factor which leads to washing out of the cement of conglomerate and degradation. Drainage measures from tunnel face will prevent loosening of formation and related instability problems. Low cementation in conglomerate will also bring failure at crown and face of the tunnel. Such failures can be prevented by adaptation of forepoling systems in terms of spacing, length and used material i.e. pipes. 4. Conclusion Tunneling is a process starting from site investigation and continuing through design stage to construction. With information gained from construction, the design considerations should be checked and adapted to natural condition enables to optimize tunnel construction. The tools used for this purpose are evaluation of measurement data, geological conditions and construction activities and back analysis using the results of those evaluations. Consultancy during tunnel construction enables to take required actions for an optimized construction; economic and safe. Considering the characteristics of conglomerate formation the criteria for the selection of support classes are composition of conglomerate, rock mass bond and groundwater inflow. Construction activities; ring closure of support system, drainage and forepolings have positive influence on the behavior of the tunnel through conglomerate. [1] KURILC M. (2007), MURRAY J., MUCREA M., SCHULER K., Construction of a Mixed Face Reach Through Granitic Rocks and Conglomerate, RETC PROCEEDINGS, pp [2] ILF CONSULTANTS, INC. AND EARTH MECHANICS, INC. (2005) Geotechnical Baseline Report Devil s Slide Tunnel Project [3] AKIN PROJECT (2009), Gori Tunnel Design Reports and Drawings [4] ÖGG (2001), Guideline for the Geomechanical Design of Underground Structures with Conventional Excavation