Research And Application Of New Anti-floating Anchor In Anti-floating Reinforcement Of Existing Underground Structure

With the progress of urbanization,a large number of underground structures have been built in the north of China since the early 1990 s.Due to the low rainfall and over-exploitation of groundwater in the north at that time,the groundwater level was generally low,and many underground structures did not consider the problem of anti-floating stability in the construction.With the significant increase of rainfall and the protection of groundwater resources in northern China in recent years,the groundwater level in northern China has increased year by year,especially in the rainy season in many areas.As a result,many underground projects float up as a whole due to the rise of the groundwater level or the waterproof floor is damaged due to insufficient force,and such cases have been increasing in recent years.Therefore,the anti-floating reinforcement of this kind of existing underground structure has become a new topic,and the anti-floating anchor has been widely used because of its convenient construction,simple operation,small damage to the existing structure and low cost.When anti-floating anchor is used for anti-floating reinforcement,as the waterproof bottom plate is an existing component,its thickness and reinforcement are generally small,and it cannot withstand the pre-tension exerted by the anchor rod.When prestressing is applied,the prestress will change the stress mode of the original basement bottom plate,resulting in insufficient reinforcement at the bottom of the waterproof plate,or crack and damage due to excessive deflection.Engineering designers should pay more attention to the adverse effects of conventional anti-floating anchor on existing structures.In this paper,aiming at the problems such as pretension of anchor bolt acting on the original basement waterproof bottom plate,large displacement of the bottom plate and insufficient reinforcement,a new anti-floating anchor suitable for the anti-floating reinforcement of existing underground structures is proposed,which has been successfully applied to practical projects,and basic experiments and side resistance tests are conducted on it.At the same time,MIDAS/GTS finite element analysis software is used to model the anti-floating anchor and basement foundation and floor,and the influence of conventional anti-floating anchor and new anti-floating anchor on the stress and deformation of the existing floor is compared and analyzed,and countermeasures are proposed,which provides reference for the anti-floating engineering design of such existing basement structure.The main research work and achievements of this paper are as follows:(1)Review and summarize the existing literature on the anti-floating of underground engineering structures and the current regulations on the anti-floating of structures,and summarize the research status of the anti-floating design of underground structures and the anti-floating anchor.(2)Based on the actual project,the unit plate span is calculated and the conventional anti-floating anchor is used for anti-floating reinforcement,which may cause problems in the original basement waterproof floor.The results show that if the tension type non-prestressed anti-floating anchor is used,the basement waterproof floor will be deformed greatly under the action of high water level buoyancy,which is easy to crack,leakage and other engineering diseases,affecting the normal use of the structure,and it is not suitable for the anti-floating reinforcement project.If the conventional pressure type prestressed anti-floating anchor is used,the reinforcement of the waterproof bottom plate can not meet the requirements at both high and low water level,while the gap between the reinforcement of the bottom plate is large at low water level.(3)According to the characteristics of the existing underground structure as independent foundation with waterproof plate,a new type of pressure prestressed anti-floating anchor is proposed,and the basic experiment and side resistance test of the new anchor are carried out.The research shows that the new type of prestressed anti-floating anchor has high pulling capacity,and the steel pipe and the grouting body are similar to the concrete filled steel tube structure,and the compression performance is better than that of the conventional pressure type anchor.The new bolt solves the problem of insufficient reinforcement at the bottom of the waterproofing floor of the existing underground structure,and is a good way to strengthen the existing underground structure against floating,which can be popularized and applied in similar projects.(4)The anti-buoyancy anchor is adopted to strengthen underground structure.MIDAS/GTS finite element software is used to simplify and simulate the basement floor and anti-buoyancy anchor.The comparison and analysis show that the new type of anchor,conventional prestressed anchor and non-prestressed anchor have large uplift bearing capacity and are more suitable for anti-buoyancy reinforcement of existing underground structures.Comparing the two kinds of prestressed bolts,the maximum positive bending moment of the bottom plate is only 38% of that of the conventional prestressed bolts,and the maximum negative bending moment is only 25% of that of the conventional prestressed bolts.The reinforcement of the bottom plate by the new bolt can greatly reduce the amount of reinforcement in the new plate,and at the same time can avoid excessive deformation of the bottom plate under low water level,resulting in cracking and water seepage.