Study On The Role Of Cluster Loads In Land Subsidence

At present,land subsidence has become one of the most important geological disasters in the process of urbanization,resulting in huge socio-economic losses and threatening the safety of people's lives and property.For a long time,groundwater exploitation has been considered to be the main reason for land subsidence,and groundwater-related subsidence has received considerable attention from government research organizations and the academic community.Recently,land subsidence caused by the load from buildings has been recognized as another important subsidence factor but research in this sector has lagged.In the past research of land subsidence caused by the load from buildings,the load from buildings was commonly treated as a single load in land subsidence studies.The effect of building loads on land subsidence was underestimated in many cases owing to the narrow scope and the limited depth of the additional stress of a single load in stratum.However,groups of buildings can impose what is called a cluster load and additional stresses can be superimposed in the cluster load.In these cases,the land subsidence can be much greater than is the case under a single load.The study of the cluster load land subsidence is more in line with the actual land subsidence caused by building loads.But so far,the calculation method of the cluster load land subsidence is too idealistic,which is certain different from the reality.On the basis of predecessors' work,this paper generalizes the analytical algorithm of the cluster load land subsidence to the calculation at any point of rectangular buildings group with different horizontal and vertical distance between buildings.And the calculation is realized by MATLAB programming.Taking the typical buildings in the study area as an example,the land subsidence calculation is carried out.By analyzing the relationship of building height,building density and plot ratio in the cluster load land subsidence,we find using building height combine with building density to measure the land subsidence caused by the cluster load is a better way than only using plot ratio.In order to overcome the shortcomings of analytical algorithm in calculating complex interdependent large-scale cluster load land subsidence,based on the Biot consolidation theory,this paper selected the Mohr-Coulomb model to describe the elastoplastic constitutive relationships in the soil,and established a complete fluid-solid coupling model for the cluster load land subsidence by FLAC30 and calculated the land subsidence caused by the cluster loads.It is found that in the cluster load area with high building density,because the additional stress of the adjacent building is superimposed,the cluster load shows more significant characteristics of large overall building load.Because of the effect of stresses from adjacent buildings,there is a great deal of primary consolidation in the soil caused by the cluster loads and the land subsidence in intensively built-up areas is larger than subsidence in areas where the buildings are more isolated.Where the high building density cluster loads are imposed,pore water migration distances are longer so that consolidation takes longer.A high building density also increases the contribution that the deeper stratigraphic layers make to land subsidence.In addition to calculating the cluster load land subsidence in the study area,this paper also analyzes the change of groundwater level at different depth aquifer group in the study area,and calculates the land subsidence caused by the decline of groundwater level in the mid-deep aquifer of the study area.Comprehensive analysing of land subsidence caused by the cluster load,land subsidence caused by groundwater level decline,and land subsidence monitoring data,this paper finds that the main reasons for the land subsidence at different times are different.According to the calculation results and monitoring data of Monitor#1 and Monitor#2,the proportion of land subsidence caused by the cluster load in the total land subsidence during the period from 2004 to 2014 is 49.43%and 55.06%respectively.It is one of the main reasons for land subsidence.The proportion of the cluster load land subsidence in the total land subsidence is 69.70%and 80.86%,respectively,which is the main reason for land subsidence in the study area during the period from 2004 to 2010.After 2010,the construction load is basically completed,and the mid-deep groundwater level again decline.The proportion of the cluster load land subsidence in the total land subsidence of the two monitoring sites is reduced to 15.96%and 23.13%.The decline of the groundwater level is the main reason for the land subsidence in study area.Moreover,because the distribution of building loads is heterogeneous,the cluster loads can easily cause differential settling and differential settling can increase the occurrence of secondary disasters such as building damage,ground fissures,city waterlogging and so on.In summary,the cluster load play an important role in land subsidence,especially in the rapidly urbanized areas,which should be aroused sufficient attention and take a positive response.