| It is a inevitable phenomenon that cracks are often made at surface due to the construction of tunnel in loess. The formation mechanism, distribution and propagation law are systematically investigated and summarized by use of a lot of methods, such as, data collection, field investigation, In-situ tests, model tests and numerical simulations. The main achievements obtained in the thesis are as follows:(1)A series of tests were performed in lab to investigate the physical and mechanical properties of loess. The results and the further analysis revealed the macroscopic anisotropy characters of loess in mechanical property.The cohesion and tensile strength of loess are larger than sandy clay, however, collapse behavior can be observed when wetting. Experimental results revealed that shear strength of loess along the cracks is almost equal to its residual strength. It is also found that strength of loess with cracks is smaller than which of homogeneous loess, the difference of which increase with the decrease of confined pressure with range from34.2%to13.8%. The uniaxial compressive peak strength of loess is influenced by its compaction greatly, which increase with the density, while decrease with the water content.Horizontal compression modulus is17.4%larger than the vertical one, while the collapsibility coefficient of loess in vertical direction is36.8%larger than that in horizontal direction. It also can be found that horizontal cohesion force is larger than vertical one. However, the friction angle almost keeps content. Model tests show that the behavior for failure and deformation caused by unload at vertical and horizontal respectively are similar. Both of the failure forms are shear slippage damages with strain of0.5%-3.5%.(2)The manner of surface crack propagation during construction of loess tunnel is obtained.The manner of evolution of surface cracks are collected from several projects, such as, Zheng-xi passenger special line, the Longhai Railway,the Bao-Zhong Railway, Bao-Lan Second Line, Hou-Yue Railway, Taizhongyin Railway, and several highway tunnels. Several rules, including the time surface cracks initiation, the effects of buried depth of tunnel and location, depth of cracks, the relationship between deformation of tunnel and cracks are obtained by model tests, FEM and DEM.①The time for surface crack initiation For the shallow-buried tunnel at loess, it is about half a month that the longitudinal cracks occurred on the entrance slope and both sides of tunnel. For the flat roof loess tunnel is without bias, one or two cracks appear which parallel to the midline of the tunnel30-45days after excavation. And cracks develop following excavation. If the excavation is suspended for more than3days, the corresponding front of tunnel face will appear one transverse crack connected longitudinal crack, forming embrace transverse crack.②The Location of surface crack initiationThe longitudinal cracks appear at both sides of midline of the tunnel roof inclined inwards. There is some transverse crack between two longitudinal cracks. The front of excavation work face has visible cracks, which continuously extends forward in tunnel excavation. Surface crack is vertical in the shallow ground, gradually, incline to the surface tunnel center forming curved surface, the inclination angle of fracture is57°～73°(angle between crack with the tunnel bottom).③The Location of surface crack initiationThe longitudinal cracks appear at both sides of midline of the tunnel roof inclined inwards. There is some transverse crack between two longitudinal cracks. The front of excavation work face has visible cracks, which continuously extends forward in tunnel excavation. Surface crack is vertical in the shallow ground, gradually, incline to the surface tunnel center forming curved surface, the inclination angle of fracture is57°～73°(angle between crack with the tunnel bottom).④The depth of surface crackThe maximum width of crack is at the surface, width of crack decreases gradually with the depth. The tested visible crack depth is limited, about3～15m. When the loess tunnel buried depth is less than1times the tunnel diameter, surface cracks trends to connect with tunnel.⑤The surface deformationWhen the surface crack appears, surface settlement is mostly above100mm during construction.⑥The relationship of collapsed funnel and surface crackIf the support is not given in timely, the surrounding rock of loess tunnel occur to collapse easily, the most of collapse surfaces are nearly vertical, along the maximum excavation span of tunnel, collapse forming the wedge sliding surface rarely.(3)The formation mechanism of the surface cracks during loess tunnel construction is revealed. ①It is the external cause that the sliding trend of soil above tunnel after excavated which lead to the tensile stress in surface layer.During the construction in shallow loess tunnel, the displacement and deformation in the surface and soil layer are caused with the ground stress state change and adjustment. This deformation can lead to the non-uniform settlement groove within a certain range of the surface in a short time. The wedge with sliding trend appear above the tunnel excavation surface, the horizontal displacement come into being in the both sides of surface of tunnel midline during construction. The direction of horizontal displacement is to tunnel centerline, where exists the location of maximum horizontal displacement.The sliding wedge moves to the excavating surface, when tensile stress and shear stress was greater than the strength of soil in the sliding band, the actual failure is formed. With the sliding wedge sliding down, the gravity of sliding body generates pushing effect on the soil of both sides. As a result, there is no crack observed at deep place.②It is the structural characteristics and anisotropy of loess that is internal cause for surface crack.There exists weak structural surface in loess, such as level, vertical joints etc, after tunnel excavation, the unstable structure is formed around the tunnel, which lead to collapse. After shallow loess tunnel excavation, the surface formation sliding trend due to failure of soil, because tensile stress exceeds the tensile strength of soil. At the same time, as a result of the anisotropic mechanical properties of loess in the horizontal and vertical direction, especially the existence of crack and joints, which lead to horizontal tensile stress in these locations are almost zero. As results, cracks are easily initiated at the surface. The no crack loess strength is higher, so the crack is upright well and forms visible crack. Because of the stress release and stress accumulation with construction after cracking, crack formed continually.(4)The criterion to determine the wedge sliding and forming of surface crack are proposed for FEM.①Surface crack:for near-surface region, there is certain point where the minimum of horizontal stress coincidence with the maximum of horizontal displacement. In FEM, these are adopted as the criterion to determine the wedge sliding and surface crack initiation.②The wedge sliding surface:it is found that the maximum of shear stress coincident the maximum of horizontal displacement coincidence, so the contact area between the shear stress maximum and the horizontal displacement maximum is used as the shear failure criterion.(5)The criteria for surface subsidence deformation to prevent surface crack during loess tunnel construction is proposed.When the support closed distance is less than1times of the tunnel span, ground surface settlement is less than80mm during construction, surface crack can be effectively prevented. When the supporting closed distance is less than2times of the tunnel span, ground surface settlement is less than100mm during construction, the surface crack will not generate.(6)The treatment method is perfected to the surface cracks during construction.The3:7lime-soil replacement method, slurry perfusion, or plain soil cover face backfill method may be used as main treatment method. From the viewpoint of economic and environmental protection, and using the permeation properties of compacted loess, a new idea named the plain soil cover face backfill method is proposed.Due to the particularity of loess, there are questions in research for the surface crack formation mechanism and law in this dissertation, need to be further studied. |
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