Model Test And Study On The Tunnel Excavation And Supporting Of Outwash Sediment In Nyingchi Prefecture Of Tibet

In China's southwest region,glacial stages and interglacial stages of Quaternary Pleistoceneaffected the landform and landscape today to a large extent.Outwash sedimentwidely distributed in nyingchi prefecture of Tibet is usually vast,which inevitably has close relation with engineering construction and results in a important influence to the human production and livingunder the background of China'svigorous support to westdevelopment.In a long period of geological history,outwash sediment is unceasingly changed by natural environmentsince the formation.How the properties are,what the differences between common debris are and how mechanical response is after the excavationneed to solve.The stability and excavation supporting method researchcan provide a guarantee for the engineering safety in future.Most of the related research of physical simulation is about rock tunnel,not about outwash sediment.This paper studies basic characteristics,deformation and destruction and supporting method after excavation and loose surrounding rock pressure of outwash sedimentthrough the methods of field investigation and experiment study.Based on the full investigation to distributioncharacteristics of outwash sedimentin Nyingchi Prefecture,tunnel physical simulation experiment is carried on.Abundant data is obtained through a dial indicator in the earth's surface and micro earth pressure cellsburied in the soil.On the basic of these data,deformation and failure characteristics of the tunnel are analyzedunder different supporting conditions.PFC is used to simulationprogressive failure after excavation without support,which shows final damage status of the tunnel under different buried depth.Finally,we discusses the deposit in ice water and buried depth of tunnel form stable pressure arch,and put forward the loosely based on the theory of the mt sand body of surrounding rock pressure improved formula.In order to insight intoa series of basic characteristics of outwash sediment,we combine remote sensing and field survey to analyse the distribution law of outwash sediment in Nyingchi Prefecture,namely article takes the ribbon and fan shapes,exposed section vertical,and top of the slope is slow.Selecting typical deposit in situ heavy,screening and penetration test,found that the density of water accumulation generally between 2.05 ~ 2.30 x 103 kg/m3,and with big grain and coarse grain composition,nonuniform coefficients greater than 30,the curvature coefficient between 1 ~ 3,belongs to the gradation of good soil.Detailed describes the material composition and the structure characteristics of the deposit in ice water,the last sample for the research of water accumulation indoor test,as a complement of field work.According to the principle of similar model test,the test of surrounding rock and advanced support,modulus of concrete,such as model material,designed the experimental device and measurement system,has carried on the tunnel excavation under different supporting conditions.Results show that the buried depth has a direct effect on the stability of final hole,when the tunnel buried deeply 15 cm,in the process of excavation,the vault cave,cave expanded gradually,the overall tunnel instability,finally the surface settlement cracks rapidly and well versed in,surface subsidence hole diameter,width slightly larger than tunnel collapse wall is nearlyupright;Buried deeply 20 cm and 25 cm,can form stable cave arch tunnel in the end.Analysis of the effect of various support measures,surface subsidence as judgment basis,grouting pipe roof support for ice debris tunnel effect is best.PFC is used to simulationprogressive failure after excavation,which shows that under different depth,tunnel vault destroys firstly and badly without support,and two side wall only a small amount of stripping of soil loss.Less than 20 m depth,vault cave has been developing to the surface,forming type through destruction;Buried depth is more than 20 m,vault fall not unlimited development,it forms a stable cave arch.The pressure arch is naturally occurring in the tunnel,and the formation of it has certain conditions.Criterion based on the results of numerical calculation and stress arch boundaries,quantitatively calculated the scope of the pressure arch to determine water accumulation critical embedment depth of the tunnel arch form stable pressure of 20 m,as the tunnel buried depth.Finally,based on the theory of too loose sand surrounding rock,put forward correction formula,compared with the results show that the modified formula can better reflect the deposit in ice water and the actual pressure of surrounding rock of the tunnel.Outwash sediment in Nyingchi Prefecture,is different from general debris,its composition and structure characteristics of the particularity of the mechanical characteristics of different,natural moisture content condition water accumulation showed different degrees of stress strain curve of softening phenomenon,and the confining pressure,the greater the softening phenomenon is more obvious.Under MAO hole excavation condition,tunnel eventually damage has direct relationship with buried depth,and under different support method,grouting pipe roof support for ice debris tunnel effect is best.Model test is on the basis of extensive investigation and full testing,so the conclusion is of high reliability,but due to the smaller physical model,as well as the lining is embedded installation,it has bigger difference with the actual.In the future study,can consider different influences on the stability of tunnel excavation methods,at the same time,also can decorate a stress line in a physical model,in the tunnel after excavation surrounding rock stress change rule.