| With the increasing severity of traffic congestion in our country,the traditional two-lane highway tunnels are gradually developing toward large sections such as three-lane or even four-lane highway tunnels.When large-section mountain tunnels are constructed in weak rock strata,they often encounter engineering problems such as instability of face and surface subsidence,which greatly increase the difficulty and cost of construction,and also cause huge economic damage to the country.Therefore,in order to find out the root causes of the above-mentioned engineering problems,reveal the failure mechanism of surrounding rock instability,and then put forward effective prevention and control measures,this paper takes the Great Wall Mountain Tunnel as the engineering background,and proceeds from the perspective of tunnel construction process mechanics,through the combination of theoretical research,numerical simulation and on-site monitoring,studies the spatial effect of unloading and deformation of surrounding rock during the excavation of large-section soft rock tunnel.The main conclusions are as follows:(1)Based on the summary of a large number of literature,the definition of large-section soft rock tunnel suitable for the current development of tunnel construction in China is summarized.Tunnels with excavated cross-section area greater than 100m~2,surrounding rock of IV grade and below,and with low compressive strength,high weathering degree,water-prone swelling and softening are called large section soft rock tunnels.At the same time,the main research results of unloading and spatial effect of deformation of surrounding rock in tunnel excavation are briefly introduced,which provides a theoretical basis for the study of this paper.(2)During the excavation of large-section soft rock tunnel,the unloading process of surrounding rock can be roughly summarized as three stages,the stage of concentrated load,the stage of rapid unloading and the stage of unloading stability;the unloading situation of surrounding rock around the tunnel is quite different,and the unloading degree of the vault and the vault bottom is greater than that of the shoulder and the side wall;The failure of surrounding rock begins at the vault,and develops gradually to the shoulder and side wall with the increase of unloading degree,which eventually leads to the overall instability of surrounding rock.By means of orthogonal test,the influence degree of three factors on unloading of surrounding rock is analyzed as follows:grade of surrounding rock>size of section>construction method.(3)The excavation of large-section soft rock tunnel can not be regarded as a simple plane strain problem,but a three-dimensional analysis model should be established.The influence range of tunnel excavation on the spatial effect of surrounding rock deformation is double diameter before and after tunnel face.The deformation of surrounding rock corresponds to the unloading process,which mainly undergoes three stages,namely,pre-deformation stage,sharp deformation stage and deformation stability stage.The settlement of the vault and the rebound deformation of the vault bottom are generally larger than the convergence deformation of the side wall.The disturbance range of the large section soft rock tunnel excavation on the rock mass above the vault is the largest,which is also one of the main reasons leading to the surface collapse of the shallow tunnel.(4)Through on-site monitoring test of relying on engineering,it is concluded that the deformation of surrounding rock has a certain spatial effect with the advancing of the face.When the excavation passes through the monitoring section,within the range of about 0.5times the diameter of the tunnel,the growth rate of surrounding rock deformation is relatively large,and its cumulative deformation value reaches more than 80%of the total deformation.It shows that the stress adjustment of surrounding rock is the most drastic in this stage,and support parameters such as advance reinforcement can be optimized according to the spatial influence range of the process,so as to prevent rock mass from destabilization and failure;then,with the distance of the face of the palm,the change process becomes a slow growth stage and finally reaches a stable state;from the monitoring analysis results,the settlement and deformation of the vault top is the largest,followed by the arch.Shoulder,and finally sidewall position.(5)The causes of instability and collapse of the face of The Great Wall ridge Tunnel are studied and analyzed,which can be attributed to four aspects:serious weathering of rock mass,inclusion of soft mud layer,shallow burial depth and improper construction.Based on the relevant conclusions of the spatial influence mechanism of large section tunnel excavation and the actual situation of the site,the comprehensive reinforcement measures are put forward,namely,backfilling of surface collapse cavity,shotcreting closure of tunnel face,pre-grouting reinforcement of arch roof in a certain range and adjustment of support parameters by changing construction scheme.The rationality of the reinforcement scheme is verified by on-site monitoring,which has certain guiding significance for practical engineering. |
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