| In China,permafrost regions and seasonally frozen regions account for about 20%and 55% of the total land area,which are mainly distributed in the western,northeastern China and Qinghai-Tibet Plateau.In recent years,more and more engineered infrastructures were built in seasonally frozen regions.Highway tunnels,as one of the common engineered structures,are commonly affected by the frost damage.After the excavation,the temperature,stress and even water fields change due to the heat exchange between the lining structure and the air inside the tunnel.As we known,the physical and mechanical properties of frozen soils(rocks)are closely to their temperature conditions and moisture contents.Hence,with the changes in temperature and moisture fields,the mechanical properties of frozen soils will also change significantly.For the tunnels in seasonally frozen regions,there are frost deformations and moisture migrations in the surrounding rocks due to the heat exchange with the cold are in winter,leading to deformation,cracking and destruction of the lining structure.The problems related to freezing of the lining structures and surrounding rocks generally seriously affect the safe operation and the traffic of the tunnel.Engineering practices indicated that the frost damage to the tunnels in seasonal frozen regions was serious and common.Thus,it is necessary to study the frost damage mechanisms and prevention methods to the tunnels in seasonal frozen regions.In this paper,the Altun Mountain highway tunnel was taken as the research object.In order to observe the air temperatures inside the tunnel,corresponding measurement sensors and a weather station were installed inside and outside the tunnel.According to the design of Altun Mountain highway tunnel,geological survey report and monitoring meteorological data,a coupling model considering the damage of tunnel lining structure and the freeze-thaw hysteresis phenomenon of surrounding rock body was established.With the model,the distribution of temperature field of surrounding rock body under different insulation layer thicknesses and different insulation materials was systematically studied by using numerical method.The insulation materials commonly used in cold region tunnels were compared and analyzed,and finally the equivalent indoor experimental freeze-thaw damage model was used.With the model,the stress field of the lining structure after excavation was investigated.The main conclusions are as follows:(1)Based on field observed results gained from the weather station and temperature sensors installed in the tunnel entrance section,it can be found that the air temperatures changing with time at all monitoring sections basically follows sinusoidal patterns.With the distance from the tunnel entrance increases,the annual average temperature gradually increases and the amplitude gradually decreases.This indicates that the influence of the temperature outside the tunnel to the air temperature inside the tunnel decreases with the increase of the distance from the tunnel entrance.Both the annual average temperature and amplitude reach constant values at 1500 m far from the tunnel entrance.Then,the annual average temperature and amplitude do not change with the distance increases further.(2)Under different insulation laying methods,the freezing and thawing conditions at the tunnel entrance sections differs significantly.Without insulation layers(WTTIL),the maximum freezing depth of surrounding rock reaches 1.6 m.The remaining three laying methods included insulation laying on the inner surface of the tunnel second lining(SSL),insulation laying between the tunnel second lining and the initial lining(BPSL),and the double-layered method(DTIL).When of the thickness of insulation layer was 5 cm,the freezing depth was basically 0 m.However,below the foot of the tunnel arch,the freezing and thawing of the tunnel under SSL was most serious,followed by the DTIL laying method,and the freezing depth of the BPSL laying method was the best,indicating that the BPSL laying method was the best laying method among the four laying methods.However,considering the wear and tear of the insulation layer after construction and the replacement during operation,the SSL laying method should be the best laying method.With the SSL laying method,the maximum frozen depth of the surrounding rock body gradually decreases with the increase of insulation layer thickness.When the insulation layer thickness is 0 cm,the maximum freezing depth of the surrounding rock body is about 1.6 m.When the insulation layer thickness increases to 5 cm,the maximum freezing depth of the surrounding rock body above the foot of the tunnel arch is close to 0 m.however,there is still a thicker surrounding rock body below the foot of the arch experiencing seasonal freezing and thawing.Therefore,for the SSL laying method,the part of the tunnel below the foot of the arch needs to be laid with additional protection measures.(3)The analysis of the maximum frozen depth at the typical locations of the tunnel shows that,when the thickness of the insulation layer was 0 cm,the frozen depth at the vault(A1)is the largest,with a frozen depth of 1.64 m.The frozen depth at the arch hance(B1)is considerably smaller than that at A1 because it is closer to the foot of the arch and its frozen depth is affected by the shape of the tunnel.Due to no insulation material laid below the foot of the arch under SSL,the frozen depth at the inverted arch(C1)point is not affected by the thickness of the insulation layer.It should be noted that,with the increase of insulation thickness,the change of frozen depth at point A1 is much larger than that at point B1,because point B1 is near the foot of the arch and the insulation layer is not laid below the foot of the arch.Thus,the frozen depth at point B1 is less affected by the thickness of insulation than at that point A1.(4)Analysis on the seven common insulation materials for tunnel projects in seasonally frozen regions shows that,rigid polyurethane material is much better than other six insulation materials(expanded polystyrene foam,Foley Kay insulation board,phenolic foam,rigid polyvinyl chloride foam,rock wool and dry aluminosilicate fiber material)in thermal insulation effect.Numerical simulated results shows that the minimum thicknesses of the seven materials that can prevent the lining structure from frost damage are 3.5,8.1,4.7,5.3,7.7,9.1 and 6.9 cm,respectively.Combined with the performance index tool called PI in engineering management,the above seven materials were analyzed and compared with consideration of the thermal conductivity of insulation materials coefficient,compressive strength,market price and minimum laying thickness.The results showed that material A is the best choice of insulation material for tunnels in seasonally frozen regions without consideration of the market price.However,dry aluminosilicate fiber material is the best choice when the market price is considered.(5)The numerical results show that in the first year of operation,the stress distribution of the tunnel lining structure under SSL is basically same as that under WTTIL.Under SSL,the initial lining structure only has tensile stresses at the foot of the arch and the side walls,with a maximum tensile stress of 0.16 MPa.While under WTTIL,the initial lining structure has tensile stresses near the foot of the arch,with a maximum tensile stress of 0.22 MPa,which is greater than the tensile stress at the foot of the arch under SSL.In the fifth year of operation,the stress distribution results are significantly different between the two laying methods.The location of the maximum stress in the initial lining structure is also different under the two laying methods,mainly because 5-cm-thick insulation material is only laid at the upper part of the foot of the slope under SSL,leading to uneven distribution of the temperature field in the upper and lower parts of the tunnel arch. |
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