The excavation of a circular tunnel in a bedded argillaceous rock (Opalinus Clay): Short-term rock mass response and FDEM numerical analysis

Abstract

a b s t r a c tThe Opalinus Clay formation is currently being investigated as a potential host rock for the deep geological disposal of radioactive waste in Switzerland. Recently, a test tunnel was excavated at the Mont Terri underground rock laboratory (URL) as part of a long-term research project (''Full-scale Emplacement (FE) experiment'') aimed at studying the thermo-hydro-mechanical (THM) effects induced by the presence of an underground repository. The objective of this paper is twofold. Firstly, the results of the rock mass monitoring programme carried out during the construction of the 3 m diameter, 50 m long FE tunnel are presented, with particular focus on the short-term deformation response. The deformation measurements, including geodetic monitoring of tunnel wall displacements, radial extensometers and longitudinal inclinometers, indicate a strong directionality in the excavation response. Secondly, the deformational behaviour observed in the field is analyzed using a hybrid finite-discrete element (FDEM) analysis to obtain further insights into the formation of the excavation damaged zone (EDZ). The FDEM simulation using the Y-Geo code is calibrated based on the average short-term response observed in the field. Deformation and strength anisotropy are captured using a transversely isotropic, linear elastic constitutive law and cohesive elements with orientation-dependent strength parameters. Overall, a good agreement is obtained between convergences measured in the field and numerical results. The simulated EDZ formation process highlights the importance of bedding planes in controlling the failure mechanisms around the underground opening. Specifically, failure initiates due to shearing of bedding planes critically oriented with respect to the compressive circumferential stress induced around the tunnel. Slippageinduced rock mass deconfinement then promotes extensional fracturing in the direction perpendicular to the bedding orientation. The simulated fracture pattern is consistent with previous experimental evidence from the Mont Terri URL.

Displacement characteristics of high-speed railway tunnel construction in loess ground by using multi-step excavation method

“…Loess, which is derived from unconsolidated materials (Zhu, 2012), is sensitive to sliding upon wetting and being broken down due to its metastable structure (Wen and Yan, 2014). Loess has a significant character of randomly open and loose particle with high porosity (Tonon, 2011;Graziani and Boldini, 2012;Ng et al, 2012;Lisjak et al, 2015;Smalley and Derbyshire, 1991;Assallay et al, 1997;Jefferson et al, 2004). When a large span tunnel or underground space is constructed in loess ground, it inevitably disturbs the in situ stress field, and then causes large ground displacements (Sharifzadeha et al, 2013).…”

Stability analysis of underground oil storage caverns by an integrated numerical and microseismic monitoring approach

“…A motivation is to combine advantages of both continuous and discontinuous approaches proposed by Munjiza et al (1995), Li et al (2004), Wang et al (2013), etc. These combined methods include FDEM (FEMDEM) (Munjiza et al, 1995;Munjiza, 2004;Mahabadi et al, 2010Mahabadi et al, , 2012Lisjak et al, 2014Lisjak et al, , 2015 and Continuous-Discontinuous Element Method (CDEM) (Li et al, 2004(Li et al, , 2008Ma et al, 2011;Wang et al, 2013;Li et al, 2015). In the study of Cai et al (2007), the continuum-based software FLAC was coupled with the discontinuum-based software PFC to investigate acoustic emissions in large-scale underground excavations.…”

“…In the study of Cai et al (2007), the continuum-based software FLAC was coupled with the discontinuum-based software PFC to investigate acoustic emissions in large-scale underground excavations. Lisjak et al (2015) used the FDEM method to model the crack evolution due to the excavation of a circular tunnel in a bedded argillaceous rock. The progressive failure of rock masses was simulated by a cohesivezone approach (Munjiza et al, 1995;Munjiza, 2004;Mahabadi et al, 2010Mahabadi et al, , 2012Lisjak et al, 2014Lisjak et al, , 2015.…”

“…Lisjak et al (2015) used the FDEM method to model the crack evolution due to the excavation of a circular tunnel in a bedded argillaceous rock. The progressive failure of rock masses was simulated by a cohesivezone approach (Munjiza et al, 1995;Munjiza, 2004;Mahabadi et al, 2010Mahabadi et al, , 2012Lisjak et al, 2014Lisjak et al, , 2015. The method of CDEM not only can deal with both continuous and discontinuous problems, but also can reproduce the progressive damage and failure evolution of materials from a continuous state to a discontinuous state .…”

Experimental Study on the Softening Characteristics of Argillaceous Slate W Ith Water

“…By three axial compression test, Nguyen T S et al [6][7][8][9] established the argillite constitutive model, and revealed the relationship between mechanical parameters of slate such as peak strength and elastic modulus and confining pressure, moisture content and jointed relationship, proof the slate rock has anisotropic mechanical characteristics, and proved that the slate rock mass has anisotropic mechanical characteristics. In the slate rock tunnel stability, Lisjak A et al [10,11], by using the numerical analysis software, for the excavation of slate tunnel with different stratification plane directions, carried out numerical analysis, explained the characteristics of stratification plane direction effecting on the stability of the tunnel. Su Zhendao [12], in the deformation and stress analysis of the tunnel lining, based on the field test of the argillite tunnel, he recommended that the radial grouting behind the lining of the tunnel should be taken to reduce the pressure of the lining so as to enhance the stability of surrounding rock.…”