| Concrete faced rockfill dam(short for CFRD)is a new type of dam developed in the late 1980s.Because of the characteristics of small cross-section,water permeability,convenient construction and excellent adaptability in practice,CFRD is often the preferred type of dam.After 40 years of development,the modern CFRD becomes mature with the symbol of layered filling and thin-layer vibrated rolling technology.In China,a number of CFRDs higher than 150m have been built.With the accumulation of dam construcntion experience and the improvement of dam design level,the height of CFRD is still increasing.A number of CFRDs higher 250 are in plan.China has built and proposed a number of high CFRDs located in the western high seismic intensity area.Once these high dams damaged owing to seismic shaking,the consequences will be disastrous.As the main anti-seepage structure of CFRD,the safety and integrity of face slab are important foundations to ensure the effective operation of the dam.Under strong seismic shaking,the main failure modes of the face slab are crushing damage along the vertical joint of face slab,and the horizontal structural cracks.These damage modes are closely related to the stress and deformation characteristics of face slab.Therefore,it is very important to realize the seismic response characteristics of face slab,reveal the seismic damage mechanism,and propose the effective anti-seismic countermeasures.Numerical analysis is one of the important methods to study the dynamic response of the face dam,reproduce the catastrophic process,reveal the failure mechanism,evaluate the seismic performance of the dam and demonstrate the effectiveness of the seismic measures.In the past,there are some limitations in the research work.1)Research are mainly focused on single dam project owing to various dam material,site conditions,dam geometries,input motion characteristics and other factors.Therefore,the characteristics of dynamic stress distribution in face slab can not be drawn.2)Although the study of constitutive model of rockfill in recent years has gradually developed to the elasto-plastic model,the research of slab nonlinear analysis method is still lagging behind.The linear elastic model is still used to simulate the slab concrete,and the evaluation of slab safety and failure is still based on the concrete uniaxial strength criterion.On the one hand,the method of seismic safety evaluation of slab based on concrete uniaxial strength criterion can not reflect the true strength characteristics of concrete under multi-axis stress,and it is difficult to evaluate the dynamic damage of the slab and failure mechanism accurately.On the other hand,although the linear elastic model has the advantages of computational efficiency and easy realization of the program,the model can not reflect the nonlinear stress-strain relationship after cracking of quasi-brittle and ductile materials.On the basis,a series of three-dimensional finite element numerical analysis was performed to investigate the dynamic response characteristics of face slab during the operation period.The distribution law and range of the high stress area of the slab were delineated,and the weak area of the slab was clarified.The elasto-plastic analysis method of the progressive failure of the slab was established combining the generalized plastic model of rockfill and contact surface.The criterion and evaluation method of slab seismic safety were improved by introducing the multi-axial strength criterion of concrete.The dynamic damage mechanism of slab was expounded from aspect of the stress state in slab.Furthermore,the co-axial rotating cracking model was introduced in the elastic-plastic analysis method.The progressive cracking progress of face slab during earthquake was modeled,and mechanism of the dynamic cracking failure was expounded from the aspect of the dam deformations.The main contents of the paper include the following aspects:(1)Through the large-scale and systematic three-dimensional finite element calculation of the concrete faced rockfill dam,the distribution law of the high stress zone of the slab was proved,and the weak area of the slab was clear.The influences of the dam geometries and ground motion input characteristics on the high stress distribution area of the slab were clarified through the calculation of nearly a hundred design conditions.The results showed that the dynamic tensile overstress within slab(more than 3MPa)distributed in the area ranging from 0.6/H?0.8H(His dam height)in vertical direction,and 0.4Z?0.6L(L is dam axis length)in the dam-axis direction.And this vulnerable area in face slab was recommended as the key area in anti-seismic design for CFRDs.(2)An anti-seismic engineering measure was proposed to reduce the dynamic tensile stress in slab by setting the permanent horizontal seismic joint in the high tensile stress area in face slab.The validity of the measure was quantitatively evaluated by numerical analysis.The measure was adopted by the Houziyan CFRD(223m)design.A series of three-dimensional finite element calculation were carried out to clarify the influence of the position of horizontal seismic joints on the dynamic stress distribution of the slab.It was suggested the reasonable area of horizontal seismic joints should be set up for the seismic design of the project.The results showed that the horizontal tensile strength could be effectively reduced with the magnitude more than 40%,in the case that set the joint in the range of 0.75H-0.9H along the dam.(3)Introducing the Ottosen multi-axis strength criterion,safety evaluation method of slab was developed.The results showed that the force of the slab exhibited obvious multi-axial force characteristics both before and after the earthquake.The traditional evaluation method based on the uniaxial strength criterion would overestimate extrusion damage of lower slab,and underestimate the occurrence of cracking and extrusion damage of the upper slab under the earthquake.(4)Based on the multi-axial strength criterion of concrete,the extrusion failure mode and damage evolution process of the slab were elucidated from aspect of stress state in face slab.The results showed that the pull-compressive stress state is one of the main stress states when the slab was subjected to seismic extrusion damage.Furthermore,the effectiveness of horizontal anti-seismic joint on mitigating the extrusion damage was evaluated.The results indicated that the setting of the horizontal joint not only could significantly reduce the seismic stress of the slab,but also can significantly improve the slab pull-pressure adverse stress state,and reduce the possibility of slab extrusion damage.(5)The co-axial rotating cracking model was introduced in the elastic-plastic analysis method.The two-dimensional finite element asymptotic cracking failure calculation of the 200m-level face dam was carried out under the seismic loading.The development of cracks in the middle slab and the evolution of stress were investigated.The results showed that the macroscopic cracks were likely to occur in the vulnerable area of the slab,and the slab exhibited obvious brittle fracture characteristics during earthquake.The deformation of embankment moving towards downstream would generate frictional force in the interface between the face slab and cushion.The research results indicated that the friction force was the main reason for the cracking failure of the slab.(6)Based on the design idea of material modification,a UHTCC-reinforced concrete slab structure(new type slab)was proposed.By introducing the strain hardening constitutive relation of UHTCC,the nonlinear static and dynamic analysis method of UHTCC-reinforced concrete face rockfill dam was realized for the first time,and the seismic resistance of new slab was evaluated quantitatively.The results showed the new type face slab exhibited ductile failure characteristics with non-penetrating cracks during earthquake.(7)Using Gushui CFRD(245m)as a benchmark,the anti-seismic effectiveness of the horizontal joint and the UHTCC-reinforced concrete face slab was verified from the practical engineering point of view.Compared with the seismic performance of face slab without joint,the maximum tensile stress and the cracking area in slab with joint at EL.223 5m approximately reduced 50%and 38%respectively.Distinguished from penetrating cracking characteristics of traditional reinforced concrete face slab,the UHTCC-reinforced concrete face slab effectively prevented the cracks in concrete substrate penetrating towards upstream.The abovementioned results indicated that the two types of anti-seismic measures effectively improved the seismic performance of face slab,and it was suggested to consider the use of anti-seismic joints and UHTCC-reinforced concrete face slab,in the case of building high CFRD in the high seismic intensity area. |
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