Air tightness of compressed air storage energy caverns with polymer sealing layer subjected to various air pressures

During the operation of compressed air storage energy system,the rapid change of air pressure in a cavern will cause drastic changes in air density and permeability coefficient of sealing layer.To calculate and properly evaluate air tightness of polymer sealing caverns,the air-pressure-related air density and permeability must be considered.In this context,the high-pressure air penetration in the polymer sealing layer is studied in consideration of thermodynamic change of the cavern structure during the system operation.The air tightness model of compressed air storage energy caverns is then established.In the model,the permeability coefficient and air density of sealing layer vary with air pressure,and the effectiveness of the model is verified by field data in two test caverns.Finally,a compressed air storage energy cavern is taken as an example to understand the air tightness.The air leakage rate in the caverns is larger than that using air-pressure-independent permeability coefficient and air density,which is constant and small in the previous leakage rate calculation.Under the operating pressure of 4.5-10 MPa,the daily air leakage in the compressed air storage energy cavern of Yungang Mine with high polymer butyl rubber as the sealing material is 0.62%,which can meet the sealing requirements of compressed air storage energy caverns.The air tightness of the polymer sealing cavern is mainly affected by the cavern operating pressure,injected air temperature,cavern radius,and sealing layer thickness.The cavern air leakage rate will be decreased to reduce the cavern operating pressure the injection air temperature,or the cavern radius and sealing layer thickness will be increased.

Analytical prediction for time-dependent interaction of a circular tunnel excavated in strain-softening rock mass

Viscoelastic plastic solutions for tunnel excavation in strain-softening rock mass and tunnel-rock interaction are proposed based on the Mohr-Coulomb and the Generalized Zhang-Zhu(GZZ)strength criterion considering stress path.The solutions are verified by numerical simulations,results show that the theoretical solutions are close to the simulated data.The evolutions of rock stresses,strains,displacements and support pressure were investigated and the influences of residual strength parameter,support stiffness,support timing,initial support pressure and viscosity coefficient on the rock deformation and the support pressure are discussed by proposed solution.It is found that strain-softening results in large deformation and high support pressure,with stiffer support and a larger viscosity coefficient contributing to even greater support pressure.Ductile support is recommended at the first stage to release the energy and reduce the support pressure by allowing a relatively large deformation.The support pressure,especially the additional support pressure at the second stage will be much smaller if a higher initial support pressure is applied at the first stage.This can not only control the displacement rate of surrounding rock and improve the tunnel stability at the first stage by exerting sufficient support pressure immediately after tunnel excavation,but also greatly reduce the pressure acted on permanent support and improve the structure stability at the second stage.Therefore,to avoid the instability of support structure,ductile support,which could not only deform continuously but also provide sufficient high support pressure,is recommended at the first stage.

Viscoelastic plastic interaction of tunnel support and strain-softening rock mass considering longitudinal effect

A simplified two-stage method was employed to provide an explicit solution for the time-dependent tunnel-rock interaction,considering the generalized Zhang-Zhu strength criterion.Additionally,a simplified mechanical model of the yielding support structure was established.The tunnel excavation is simplified to a two-stage process:the first stage is affected by the longitudinal effect,while the second stage is affected by rheological behavior.Two cases are considered:one is that the rigid support is constructed during the first stage,and the other is that constructed at the second stage.Distinguished by the support timing at the seconde stage,different kinds of the“yield-resist combination”support method are divided into three categories:“yield before resist”support,“yield-resist”support,and“control-yield-resist”support.Results show that the support reaction of“control-yield-resist”is much higher than that of“yield before resist”if the initial geostress is not very high,but the effect is not obvious on controlling the surrounding rock deformation.So,the“yield before resist”support is much more economical and practical when the ground stress is not very high.However,under high geostress condition,through applying relatively high support reaction actively to surrounding rock at the first stage,the“control-yield-resist”support is superior in controlling the deformation rate of surrounding rock.Therefore,in the high geostress environment,it is recommended to construct prestressed yielding anchor immediately after excavation,and then construct rigid support after the surrounding rock deformation reaches the predetermined deformation.

Frost heave and freezing processes of saturated rock with an open crack under different freezing conditions

Frost heave experiments on saturated sandstone and tuff with an open crack are conducted under uniform and unidirectional freezing conditions.Frost heave of crack in sandstone with high permeability is more significant under uniform freezing condition than that under unidirectional freezing condition.However,frost heave of crack in tuff with low permeability is more significant under unidirectional freezing condition.To illustrate the reasons for this phenomenon,a numerical model on the freezing processes of saturated rock with an open crack considering the latent heat of pore water and water in crack is proposed and confirmed to be reliable.Numerical results show that a frozen shell that blocks the migration of water in crack to rock develops first in the outer part of the rock before the freezing of water in crack under unifonn freezing condition.However,the migration path of water in crack to the unfrozen rock under freezing front exists under unidirectional freezing condition.The freezing process and permeability of rock together determine the migration of water in crack and lead to the different frost heave modes of crack for various permeable rocks under different freezing conditions.The frost heave modes of crack in rock with low or high permeability are similar under uniform freezing condition because water migration is blocked by a frozen shell and is irrelevant to rock permeability.For high permeability rock,the frost heave of crack will be weakened due to water migration under unidirectional freezing condition;however,the frost heave of crack would be more significant for low permeability rock because water migration is blocked under unidirectional freezing condition.Therefore,the freezing condition and rock permeability determine the frost heave of rock with crack together,and this should be concerned in cold regions engineering applications.

Tunnels in squeezing clay-rich rocks

Squeezing ground conditions,which can lead to severe loads in tunnels,have historically been associated with the presence of clay minerals in the ground.Over the years,many methodologies have been proposed to predict squeezing in tunnels based on tunnel depth,in situ stress,ground mineralogy,and ground strength and deformation behavior.This paper presents a comprehensive review of methodologies to predict tunnel squeezing in clay-rich rocks.A new methodology is proposed where ground conditions and squeezing potential are assessed based on the Stress History and Normalized Soil Engineering Properties(SHANSEP)approach adapted to clayrich rocks,Peck’s stability number and Hoek&Brown’s(1997)Geological Strength Index(GSI).A squeezing number S is suggested to classify ground conditions based on the level of squeezing that the ground may experience in response to tunneling.Finally,it is demonstrated that by combining the proposed classification system and an existing classification system for ground squeezing condition,an accurate estimate of tunnel strain can also be obtained.The proposed method is applied to four case studies of tunnels in squeezing ground in shale and mudstone.

Modified ground response curve(GRC)in strain-softening rock mass based on the generalized Zhang-Zhu strength criterion considering over-excavation

The ground response curve(GRC)depicts the relationship between support reaction force and ground displacement,which improves the understanding of ground-support interaction and provides important references to the tunnel design.However,it is difficult to anticipate the tunneling-induced large deformation with sufficient reliability in soft rock with high geostress since the small strain theory is not applicable.When large deformation occurs,the tunnel needs to be over-excavated.Thus,the GRC should be modified considering the enlarged excavation radius since the actual excavation radius is usually greater than the designed one.To overcome the shortcomings of small strain theory in recognizing ground-support interaction under large deformation circumstances,a new large strain numerical approach for modifying the GRC was proposed considering over-excavation in strain-softening rock masses based on the generalized Zhang-Zhu strength criterion.A case study was conducted based on the Lianchengshan tunnel in China.The modified GRC was employed to investigate the ground-support behavior for different support schemes and to explore the applicability of the stress release measures.Combined with field tests,the proposed approach was validated.By comparing with GRCs proposed by previous work,the present modified GRC was proved to be superior to others.Parametric studies were conducted and it is found that over-excavation,for example,reserving a very large clearance between the surrounding rock and the support,is necessary to reduce ground pressure to a large extent.The yielding supports which can provide high support pressure during the process of deformation are highly recommended when tunneling in high geostress environment.However,if the initial geostress is not very high,it is not necessary to pursue unwarranted overexcavation since the ground pressure applied on the support is mainly the loosening stress when the deformation is large.Ample support stiffness should be provided in the process of deformation to prevent uncontrolled large deformation of surrounding rock.