Rock failure is often governed by the initiation,propagation,and coalescence of fractures,particularly in hard rocks where fracturing,rather than plastic deformation,is the dominant failure mechanism.Therefore,predict...Rock failure is often governed by the initiation,propagation,and coalescence of fractures,particularly in hard rocks where fracturing,rather than plastic deformation,is the dominant failure mechanism.Therefore,predicting the explicit fracturing process is crucial when assessing rock mass stability for engineering applications.However,fracture mechanics are seldom employed in practical rock engineering design,primarily due to the limited understanding of complex fracturing processes in jointed rock masses and the absence of tools capable of accurately simulating these phenomena.Since the 1990s,a novel approach to modelling rock mass failure has emerged,utilizing a numerical code called FRACOD.This code,based on fracture mechanics principles,predicts the explicit fracturing processes in rocks.Over the past three decades,substantial progress has been made in advancing this method to the point where it can reliably predict rock mass stability at an engineering scale.FRACOD incorporates complex coupled processes,including thermal effects,rock mechanical response,and hydraulic flow,enabling it to address coupled problems commonly encountered in geothermal energy extraction,nuclear waste disposal,hydraulic fracturing,and underground LNG storage,etc.Numerous applications of FRACOD have been conducted over the last thirty years,including studies on borehole stability in deep geothermal reservoirs,pillar spalling under mechanical and thermal loading,and the prediction of tunnel and shaft stability,as well as the excavation disturbed zone(EDZ).This paper reviews the theoretical foundations of the fracture mechanics approach employed by FRACOD and highlights the most recent developments.It also presents several validation cases to demonstrate the accuracy of this approach.Additionally,a case study on geothermal energy development in the Cooper Basin,Australia,is included to illustrate the practical applications of this method.展开更多
HYDROCK method aims to store thermal energy in the rock mass using hydraulically propagated fracture planes.The hydraulic fractures can interact with the pre-existing natural fractures resulting in a complex fracture ...HYDROCK method aims to store thermal energy in the rock mass using hydraulically propagated fracture planes.The hydraulic fractures can interact with the pre-existing natural fractures resulting in a complex fracture network,which can influence the storage performance.This study investigates the interactions between hydraulic and natural fractures using a fracture mechanics approach.The new functionality of the fracture mechanics modelling code FRACOD that enables crossing of hydraulically driven fracture by a pre-existing fracture is presented.A series of two-dimensional numerical models is prepared to simulate the interaction at different approach angles in granitic rock of low permeability.It is demonstrated that multiple interaction mechanisms can be simulated using the fracture mechanics approach.The numerical results are in agreement with the modified Renshaw and Pollard analytical criterion for fracture crossing.The results show that for large approach angles,the hydraulic fracture crosses the natural fracture,whereas for small approach angles,the hydraulic fracture activates the natural fracture and the wing-shaped tensile fractures are propagated from its tips.Thus,the presence of fractures with low dip angles can lead to the growth of more complex fracture network that could impair the thermal performance of the HYDROCK method.展开更多
The authors investigate the failure modes surrounding over-stressed tunnels in rock.Three lines of investigation are employed:failure in over-stressed three-dimensional(3D) models of tunnels bored under 3D stress,fail...The authors investigate the failure modes surrounding over-stressed tunnels in rock.Three lines of investigation are employed:failure in over-stressed three-dimensional(3D) models of tunnels bored under 3D stress,failure modes in two-dimensional(2D) numerical simulations of 1000 m and 2000 m deep tunnels using FRACOD,both in intact rock and in rock masses with one or two joint sets,and finally,observations in TBM(tunnel boring machine) tunnels in hard and medium hard massive rocks.The reason for 'stress-induced' failure to initiate,when the assumed maximum tangential stress is approximately(0.4-0.5)σ_c(UCS,uniaxial compressive strength) in massive rock,is now known to be due to exceedance of a critical extensional strain which is generated by a Poisson's ratio effect.However,because similar 'stress/strength' failure limits are found in mining,nuclear waste research excavations,and deep road tunnels in Norway,one is easily misled into thinking of compressive stress induced failure.Because of this,the empirical SRF(stress reduction factor in the Q-system) is set to accelerate as the estimated ratio σ_(θmax)/σ_c >> 0.4.In mining,similar 'stress/strength' ratios are used to suggest depth of break-out.The reality behind the fracture initiation stress/strength ratio of '0.4' is actually because of combinations of familiar tensile and compressive strength ratios(such as 10) with Poisson's ratio(say0.25).We exceed the extensional strain limits and start to see acoustic emission(AE) when tangential stress σθ ≈ 0.4σc,due to simple arithmetic.The combination of 2D theoretical FRACOD models and actual tunnelling suggests frequent initiation of failure by 'stable' extensional strain fracturing,but propagation in 'unstable' and therefore dynamic shearing.In the case of very deep tunnels(and 3D physical simulations),compressive stresses may be too high for extensional strain fracturing,and shearing will dominate,both ahead of the face and following the face.When shallower,the concept of 'extensional strain initiation but propagation' in shear is suggested.The various failure modes are richly illustrated,and the inability of conventional continuum modelling is emphasized,unless cohesion weakening and friction mobilization at different strain levels are used to reach a pseudo state of yield,but still considering a continuum.展开更多
文摘Rock failure is often governed by the initiation,propagation,and coalescence of fractures,particularly in hard rocks where fracturing,rather than plastic deformation,is the dominant failure mechanism.Therefore,predicting the explicit fracturing process is crucial when assessing rock mass stability for engineering applications.However,fracture mechanics are seldom employed in practical rock engineering design,primarily due to the limited understanding of complex fracturing processes in jointed rock masses and the absence of tools capable of accurately simulating these phenomena.Since the 1990s,a novel approach to modelling rock mass failure has emerged,utilizing a numerical code called FRACOD.This code,based on fracture mechanics principles,predicts the explicit fracturing processes in rocks.Over the past three decades,substantial progress has been made in advancing this method to the point where it can reliably predict rock mass stability at an engineering scale.FRACOD incorporates complex coupled processes,including thermal effects,rock mechanical response,and hydraulic flow,enabling it to address coupled problems commonly encountered in geothermal energy extraction,nuclear waste disposal,hydraulic fracturing,and underground LNG storage,etc.Numerous applications of FRACOD have been conducted over the last thirty years,including studies on borehole stability in deep geothermal reservoirs,pillar spalling under mechanical and thermal loading,and the prediction of tunnel and shaft stability,as well as the excavation disturbed zone(EDZ).This paper reviews the theoretical foundations of the fracture mechanics approach employed by FRACOD and highlights the most recent developments.It also presents several validation cases to demonstrate the accuracy of this approach.Additionally,a case study on geothermal energy development in the Cooper Basin,Australia,is included to illustrate the practical applications of this method.
基金The financial support from Aalto Doctoral Programme in Engineeringthe International Collaboration Project on Coupled Fracture Mechanics Modelling-Phase 3 (project team consisting of CSIRO,SDUST,Posiva,KIGAM,KICT,CAS-IRSM,DUT/Mechsoft,SNU, LBNL,ETH,Aalto Uni.,GFZ and TYUT)
文摘HYDROCK method aims to store thermal energy in the rock mass using hydraulically propagated fracture planes.The hydraulic fractures can interact with the pre-existing natural fractures resulting in a complex fracture network,which can influence the storage performance.This study investigates the interactions between hydraulic and natural fractures using a fracture mechanics approach.The new functionality of the fracture mechanics modelling code FRACOD that enables crossing of hydraulically driven fracture by a pre-existing fracture is presented.A series of two-dimensional numerical models is prepared to simulate the interaction at different approach angles in granitic rock of low permeability.It is demonstrated that multiple interaction mechanisms can be simulated using the fracture mechanics approach.The numerical results are in agreement with the modified Renshaw and Pollard analytical criterion for fracture crossing.The results show that for large approach angles,the hydraulic fracture crosses the natural fracture,whereas for small approach angles,the hydraulic fracture activates the natural fracture and the wing-shaped tensile fractures are propagated from its tips.Thus,the presence of fractures with low dip angles can lead to the growth of more complex fracture network that could impair the thermal performance of the HYDROCK method.
文摘The authors investigate the failure modes surrounding over-stressed tunnels in rock.Three lines of investigation are employed:failure in over-stressed three-dimensional(3D) models of tunnels bored under 3D stress,failure modes in two-dimensional(2D) numerical simulations of 1000 m and 2000 m deep tunnels using FRACOD,both in intact rock and in rock masses with one or two joint sets,and finally,observations in TBM(tunnel boring machine) tunnels in hard and medium hard massive rocks.The reason for 'stress-induced' failure to initiate,when the assumed maximum tangential stress is approximately(0.4-0.5)σ_c(UCS,uniaxial compressive strength) in massive rock,is now known to be due to exceedance of a critical extensional strain which is generated by a Poisson's ratio effect.However,because similar 'stress/strength' failure limits are found in mining,nuclear waste research excavations,and deep road tunnels in Norway,one is easily misled into thinking of compressive stress induced failure.Because of this,the empirical SRF(stress reduction factor in the Q-system) is set to accelerate as the estimated ratio σ_(θmax)/σ_c >> 0.4.In mining,similar 'stress/strength' ratios are used to suggest depth of break-out.The reality behind the fracture initiation stress/strength ratio of '0.4' is actually because of combinations of familiar tensile and compressive strength ratios(such as 10) with Poisson's ratio(say0.25).We exceed the extensional strain limits and start to see acoustic emission(AE) when tangential stress σθ ≈ 0.4σc,due to simple arithmetic.The combination of 2D theoretical FRACOD models and actual tunnelling suggests frequent initiation of failure by 'stable' extensional strain fracturing,but propagation in 'unstable' and therefore dynamic shearing.In the case of very deep tunnels(and 3D physical simulations),compressive stresses may be too high for extensional strain fracturing,and shearing will dominate,both ahead of the face and following the face.When shallower,the concept of 'extensional strain initiation but propagation' in shear is suggested.The various failure modes are richly illustrated,and the inability of conventional continuum modelling is emphasized,unless cohesion weakening and friction mobilization at different strain levels are used to reach a pseudo state of yield,but still considering a continuum.
