Rock fracturing mechanisms around underground openings

Shen, Baotang,Barton, Nick Techno-Press 2018 Geomechanics & engineering Vol.16 No.1

This paper investigates the mechanisms of tunnel spalling and massive tunnel failures using fracture mechanics principles. The study starts with examining the fracture propagation due to tensile and shear failure mechanisms. It was found that, fundamentally, in rock masses with high compressive stresses, tensile fracture propagation is often a stable process which leads to a gradual failure. Shear fracture propagation tends to be an unstable process. Several real case observations of spalling failures and massive shear failures in boreholes, tunnels and underground roadways are shown in the paper. A number of numerical models were used to investigate the fracture mechanisms and extents in the roof/wall of a deep tunnel and in an underground coal mine roadway. The modelling was done using a unique fracture mechanics code FRACOD which simulates explicitly the fracture initiation and propagation process. The study has demonstrated that both tensile and shear fracturing may occur in the vicinity of an underground opening. Shallow spalling in the tunnel wall is believed to be caused by tensile fracturing from extensional strain although no tensile stress exists there. Massive large scale failure however is most likely to be caused by shear fracturing under high compressive stresses. The observation that tunnel spalling often starts when the hoop stress reaches $0.4^*UCS$ has been explained in this paper by using the extension strain criterion. At this uniaxial compressive stress level, the lateral extensional strain is equivalent to the critical strain under uniaxial tension. Scale effect on UCS commonly believed by many is unlikely the dominant factor in this phenomenon.

Multi-Region Boundary Element Analysis for Coupled Thermal-Fracturing Processes in Geomaterials

Shen, Baotang,Kim, Hyung-Mok,Park, Eui-Seob,Kim, Taek-Kon,Wuttke, Manfred W.,Rinne, Mikael,Backers, Tobias,Stephansson, Ove Springer-Verlag 2013 Rock mechanics and rock engineering Vol.46 No.1

Modelling the effect of ice swelling in the rock mass around an LNG underground storage cavern using FRACOD

Shen, Baotang,Jung, Yong-Bok,Park, Eui-Seob,Kim, Taek-Kon Informa UK (Taylor Francis) 2015 Geosystem engineering Vol.18 No.4

Development and applications of rock fracture mechanics modelling with FRACOD: a general review

Baotang Shen 한국자원공학회 2014 Geosystem engineering Vol.17 No.4

Rock failure is often controlled by fracture initiation, propagation and coalescence, especially in hard rocks where explicit fracturing rather than plasticity is the dominant mechanism of failure. Prediction of the explicit fracturing process is therefore necessary when the rock mass stability is investigated for engineering purposes. However, the fracture mechanics approach is rarely used in practical rock engineering design partly due to the inadequate understanding of complex fracturing processes in jointed rock mass and partly due to the lack of tools which can realistically predict the complex fracturing phenomenon in rock mass. Since 1990s, a new approach to simulating rock mass failure problems has been developed using a numerical code namely FRACOD. FRACOD is a code that predicts the explicit fracturing process in rocks using fracture mechanics principles. Over the past three decades, significant progress has been made in developing this approach to a level that it can predict actual rock mass stability at an engineering scale. The code includes complex coupling processes between the rock mechanical response and thermal process and hydraulic flow, making it possible to handle coupled problems often encountered in geothermal, hydraulic fracturing, nuclear waste disposal and underground Liquefied Natural Gas (LNG) storage. During this period, numerous application cases have been conducted using FRACOD, which includes: borehole stability in deep geothermal reservoir, pillar spalling under mechanical and thermal loading; prediction of tunnel and shaft stability and excavation disturbed zone, etc. This paper summarises the theoretical fundamentals of the fracture mechanics approach with FRACOD and the most recent developments. Several selected application cases are also discussed briefly in this paper to demonstrate the effectiveness of this approach.

균열 암반의 복합거동해석을 위한 열-수리-역학적으로 연계된 파괴역학 수치해석코드 개발

김형목(Hyung-mok Kim),박의섭(Eui-Seob Park),Baotang Shen,신중호(Joong-Ho Synn),김택곤(Taek-Kon Kim),이승철(Seong-Cheol Lee),고태영(Tae-Young Ko),이희석(Hee-Suk Lee),이진무(Jin-Moo Lee) 한국암반공학회 2011 터널과지하공간 Vol.21 No.1

암반 내 균열 생성, 진전, 파괴 등과 같은 지하 암반의 역학적 거동과 이들 균열을 통한 지하수 유동 및 온도 변화에 기인한 열응력이 역학적 거동에 미치는 상호작용을 모델링하기 위한 수치해석코드를 개발하였다. 개발된 수치해석코드에서는 기존의 2차원 FRACOD(Shen & Stephasson, 1993)에 열-역학 및 수리-역학 상호거동을 모델링하기 위한 해석모듈을 개발하여 추가하였다. 열-역학 연계를 위해서는 가상열원법과 시간전진기법을 도입하였으며, 수리-역학 연계에서는 양해법에 의한 반복기법을 적용하였다. 수치해석결과와 해석해와의 비교를 통해 개발된 해석모듈의 유용성을 검증하고 해석사례를 통해 온도변화에 따른 열균열 발생 및 수압파쇄과정에서의 균열 진전 양상과 같은 절리암반 복합거동을 잘 표현할 수 있음을 확인하였다. In this study, it was aimed to develop a thermal-hydraulic-mechanical coupled fracture mechanics code that models a fracture initiation, propagation and failure of underground rock mass due to thermal and hydraulic loadings. The development was based on a 2D FRACOD (Shen & Stephasson, 1993), and newly developed T-M and H-M coupled analysis modules were implemented into it. T-M coupling in FRACOD employed a fictitious heat source and time-marching method, and explicit iteration method was used in H-M coupling. The validity of developed coupled modules was verified by the comparison with the analytical result, and its applicability to the fracture initiation and propagation behavior due to temperature changes and hydraulic fracturing was confirmed by test simulations.

Temperature change around a LNG storage predicted by a three-dimensional indirect BEM with a hybrid integration scheme

Jingyu Shi,Baotang Shen 한국자원공학회 2018 Geosystem engineering Vol.21 No.6

We employ a three-dimensional indirect boundary element method (BEM) to simulate temperature change around an underground liquefied natural gas storage cavern. The indirect BEM (IBEM) uses fictitious heat source strength on boundary elements as basic variables which are solved from equations of boundary conditions and then used to compute the temperature change at other points in the considered problem domain. The IBEM requires evaluation of singular integration for temperature change due to heat conduction from a constant heat source on a planar (triangular) region. The singularity can be eliminated by a semi-analytical integration scheme. However, it is found that the semi-analytical integration scheme yields sharp temperature gradient for points close to vertices of triangle. This affects the accuracy of heat flux, if they are evaluated by finite difference method at these points. This difficulty can be overcome by a combination of using a direct numerical integration for these points and the semi-analytical scheme for other points distance away from the vertices. The IBEM and the hybrid integration scheme have been verified with an analytic solution and then used to the application of the underground storage.

Thermo-mechanical simulations of pillar spalling for in-situ heater test by FRACOD

Hee-Suk Lee,Baotang Shen,Rinne Mikael 한국암반공학회 2003 한국암반공학회 학술대회 및 세미나 자료집 Vol.- No.-

FRACOD를 이용한 취성 암석의 손상 및 파괴에 대한 경계요소 해석

이희석(Hee-Suk Lee),Baotang Shen,Ove Stephansson 한국암반공학회 2004 터널과지하공간 Vol.14 No.4

응력 증가에 의한 취성 암석의 손상은 미세균열의 개시로부터 시작하여 각 개별 균열들의 전파 및 결합에 의해 거시적인 파괴면을 발생시킨다. 전통적으로 암반의 손상 및 파괴현상을 설명하기위해 거시적인 파괴 기준이나 탄소성 모델과 같은 연속체적인 접근법이 주류를 이루어왔다. 하지만 개별적인 균열들의 개시와 전파 과정을 명시적으로 고려할 수 있다면 현상론적인 관점에서 보다 실제에 가까운 암석 손상 및 파괴 과정을 재현할수 있을 것이다. 본 연구에서는 암석의 균열 진전 모델링을 위해 개발된 경계요소 코드인 FRACOD를 이용하여 암석의 손상 및 파괴 과정을 모사한 결과를 제시한다. 수치일축압축시험을 통해 개발된 모델의 적정성을 검증하고 암반의 치수효과를 고려한 현실적인 암석 파괴 과정을 재현하였다. 또한 이러한 접근법의 적용 사례로서, 실제 굴착이 진행중인 심부 수갱 암반 주변에서 심도와 암반 특성에 따라 균열 진전과 이에 따른 암반 손상의 범위를 예측한 결과를 제시하였다. 이 접근법은 취성도가 큰 암반에서 발생하는 안정성 문제에 대한 공학적인 해법을 찾는데 기여를 할 수 있을 것으로 기대된다. Damage in brittle rock due to stress increase starts from initiation of microcracks, and then results in failure by forming macro failure planes due to propagation and coalescence of these discrete cracks. Conventionally, continuum approaches using macro-failure criteria or a number of elasto-plastic models have been major solution to implement rock damage and failure. However, actual brittle failure processes can be better described in phenomenological approach if initiation and propagation of discrete fractures are explicitly considered. This study presents damage and failure process of rock using a boundary element code, FRACOD, which has been developed to model fracturing process of rocks. Through a series of numerical uniaxial compressive tests, the feasibility of the developed model was verified, and realistic rock failure process was reproduced considering scale effects in rocks. In addition, the fracturing process and the corresponding rock damage in the vicinity of deep shaft in rock mass were presented as an application of this approach. This approach will be expected to contribute to finding better engineering solutions for the analysis of stability problems in brittle rock masses.

Simulation of hydraulic fracturing and its interactions with a pre-existing fracture using displacement discontinuity method

Xie, Linmao,Min, Ki-Bok,Shen, Baotang Elsevier 2016 Journal of natural gas science and engineering Vol.36 No.2

<P><B>Abstract</B></P> <P>The coupled hydro-mechanical (HM) function has been implemented in the existing fracture mechanics modelling code FRACOD (Fracture Propagation Code) where the Displacement Discontinuity Method (DDM) is used to simulate rock fracturing process. This new advance allows the code to model hydraulic fracturing in the presence of natural fractures. Verification was performed by simulating hydraulic fracturing process against the KGD model of the propagation of a single fracture driven by fluid injection. Numerical results show a good agreement with the analytical solutions and the main features of hydraulic fracturing process in intact rock have been successfully captured. The simulator is used to model the interactions of induced and natural fractures, where the hydraulic fracture shows to divert into only one branch of the pre-existing sealed fracture after the intersection.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Develop HM coupling function in FRACOD code. </LI> <LI> Perform verification cases by hydraulic fracturing simulation. </LI> <LI> Model interactions of hydraulic fracture and natural fracture. </LI> </UL> </P>

Application of an Inverse BEM to the Modelling Fracturing Process at the Borehole Pillar by Excavation and Thermal Loading

Lee, Hee-Suk,Jing, Lanru,Shen, Baotang 한국암반공학회 2003 Geosystem engineering Vol.6 No.4

A two-dimensional boundary element code, FRACOD$^{2D}$, has been developed to simulate brittle fracturing in rock-like material including fracture initiation, propagation and coalescence under tensile, shear and mixed mode. Since the code is based on the displacement discontinuity solution it is very difficult to implement standard thermo-mechanical boundary element algorithms in its formulation. To simulate the thermo-mechanical process without changing the current structure of the code, an alternative technique has been developed by reconstructing the stress field from other thermo-mechanical models without fracturing process. In this paper an inverse boundary element formulation is presented to reconstruct the complex stress fields by equivalent boundary conditions. Truncated singular value decomposition technique is used for inverse solution. Results from predictive modeling with the reconstructed stresses during the in-situ heater test between the deposition holes are also presented. With successful application, this approach can be a good alternative for modelling fracturing processes under complex loading conditions with boundary elements.s.