Study On The Creep Mechanical Properties Of Badong Formation Mudstone And Its Application On The Time-dependent Deformation Analysis Of The Slope

The Badong formation mudstone is a kind of red layer soft rock,which is known for poor engineering properties.Since red layer soft rock is facility-sliding strata,it is significant to study the mechanical properties of Badong formation mudstone.In this paper,taking the mudstone as the main study object,a series of tests were taken including instantaneous mechanical and creep tests.The mechanical properties of Badong formation mudstone are studied,and the nonlinear creep model is established.The parameters are calculated by least square method.A numerical program is established to verify the model.In the end,the time-dependent deformation behaviors of slope before and after protected are analyzed by the second development creep procedure.Based on the analysis above,the main results of this paper are as follows:?1?Based on the material composition of mudstone,the disintegrative and dilatability of the Badong formation mudstone is because of the positive ion,such as Ca²⁺,Mg²⁺,K⁺and these positive ion can be adsorbed on the rock surface,making the rock hydrophilic.Since the mudstone contains kaolinite,illite and montmorillonite,it disintegrate with water which reduce the capbility of binding and enlarge the porosity of mineral.Meanwhile due to the effect of the external force,microcrack inside the mudstone extend gradually and the the rock failed eventually.?2?Taken the uniaxial compression and triaxial compression test,the changing energy characteristic during rock failure is analyzed.Improved partition method of the axial stress difference is used to divide the rock deformation stage,and the stress of crack closure and crack initiation in natural and saturated condition is determinated by this method.The rock deformation can be divided into 5 stage.The effect of water on the strength and deformation of rock is remarkable.The uniaxial and triaxial compression strength decreased,the value of peak strain is increased in saturated condition.The failure mode of rock in uniaxial compression test is fracture failure,in triaxial compression test is shear failure with ductility damage.?3?The Badong formation mudstone show brittle characteristic in direct shear test.The peak value of shearing strength declined obviously.The repeat cutting during shear test make the shearing strength fluctuating which make the mutli-peak shearing value.So it is hard to identify the peak value of shearing strength.A quadratic parabola?4?function is adopted to match the in-situ shearing test data.Compared with the nonlinear criterion,the shear strength is overestimated by the linear Mohr-Culomb criterion at lower and higher stress level and underestimated at medium stress level.?5?The creep test of Badong formation mudstone show:instantaneous strain is major strain under different stress level.The creep strain increases with the stress level.The creep strain in the damage level is higher than other levels.There are initial creep and steady-state creep at low stress level,and there are three typical stages?initial creep,steady-state creep and accelerated creep?at high stress level.The strain rate changed small at low stress level.The steady creep strain rate increase with stress level as exponential rate.The steady creep rate evolve significantly under low confining pressure,this means confining pressure has negative effect on the creep strain.The steady strain rate is higher at high stress level than low stress level.This means the increase of stress accelerate the rock creep.The total strain include instantaneous elastic strain and creep strain.The strain in creep test is obviously lower than triaixal compression strain,this means creep weaken the rock strength.The method of volume dilatation is applied to determined long-term compression strength under different confining pressure,the long-term shearing strength can be determined by Mohr-Culomb criterion.In comparison to short term strength,the internal friction angle and cohesion of the long-term strength decrease by 49.5%and 37.8%,which means rock creep decreased the rock strength obviously.The long-term strength is slightly higher than yield stress,the strain hardening is the probable reason.?6?Taking strain hardening into consideration,an nolinear viscoelastic-plastic creep model is established.This model include a viscoplastic dashpot considering the threshold value of yield stress and long-term strength.The one-dimensional and three-dimensional creep constitutive equation is deduced.Using the nonlinear matching function,the parameters of creep function are identified.The relationship of parameters,confining pressures and stress level is analyzed.The effect of the parameters of accelerate creep stage on the total creep time is studied.FEM model of creep constitutive equation is compiled by UMAT.To verify the program of established model,an numerical model is developed and the result show this model can fit the creep behavior of rock well.?7?The time-dependent behavior of the slope deformation show:the resultant displacement based on the rock creep is larger than that on the elastoplastic analysis.The displacement can be observed not only on the surface but also inside the slope;the deformation process is divided into two stages:the initial deforming stage and the steady deforming stage.At the initial deforming stage,the displacement increase gradually but the increasing rate of displacement tend to decrease gradually;at the steady deforming stage,the displacement increase very slow.The time of creep duration on soft rock slope is longer than hard rock slope.The resultant stress distribute more unevenly inside the slope.?8?After the slope is protected by the anti-slide pile,the time-dependent deformation behavior of the slope show:the distribution of displacement is similar with the slope before protected but the value of displacement can be controlled by the anti-slide pile.However,the resultant displacement is much larger than the result of elastoplastic analysis.Considering the rock creep,the phenomenon of stress concentration on the front of the slope is obvious.The resultant stress on the front of slope after the slope is protected is smaller than before the slope be protected.There exists large displacement on the top and bottom of the anti-slide pile and the sheering stress and the blending moment change with the creep time.The longer of the creep time of duration before the slope being protected,the larger of the resultant stress on the front of slope,this is,the sooner of the anchoring,the best of the effect of the anti-slide pile.