An Analysis On The Effect Of Soil Excavation On The Nearby Buildings In Loess Soil

Deep pit excavation is an important procedure in high-rise building and subway station construction.Excavation changes the stress state of soil,which will result in unloading in horizontal direction of pit side wall soil,as well as unloading in vertical direction of pit bottom soil.Under the self-weight combined with other loads,the pit bottom soil is forced to heave and the pit side wall soil is forced to move laterally into the pit.The pit may collapse or excessive displacements may be induced within the near ground surface if unsuitable supporting measures were taken.This will also cause negative effects on adjacent buildings and underground pipelines.Collapsible loess is a special unsaturated soil with high strength under natural water content and density conditions.However,its strength will significantly decrease accompanied by the collapse of the natural micro-structure if flooded.This will in turn weaken the safety of pit excavation in loess.A few studies have been conducted on the behaviors of loess foundation pit excavation till now.Firstly,based on the soil condition of a proposed project site in Xi'an High-tech Zone,taking pile-anchor support as an example,the development of horizontal displacements of supporting structure and ground settlements near the pit during the excavation was analyzed to determine the distribution of the ground surface settlements and the influence region caused by the excavation.And also,the relationship of the maximum ground surface settlement between the excavation depth as well as the effect of the flood on the stability of the pit were studied.It was shown that under the condition of the pile-anchor supporting system,the distribution of the settlements outside the pitcaused by the excavation in loess is concave shape,and the maximum settlement point is about(0.3-1.2)times the excavation depth away from the edge of the pit.Then,taking a planned project in Xi'an High-tech Zone as an example,an analysis was performed on the influence of the excavation on an existing two-story building and an adjacent road.The analysis was based on two different conditions,(1)the existing building foundation soil is reinforced by jet grouting method,and(2)the existing building foundation soil is reinforced by concrete piles.Whatever the two conditions,the existing road was left unreinforced.For the two mentioned circumstances,studies mainly focus on the settlements and horizontal displacements of the building foundations and the additional stress in pavement caused by excavation.The results show that the building is safe under the two conditions during excavation,while the settlements and the differential settlements induced in building foundations when using jet grouting method are less that that by using the concrete piles method.And also,the induced additional stresses in pavement for condition(1)are less than that for condition(2)stated above.Finally,the variation of the axial force of piles installed in pit bottom soil was analyzed during excavation.It was shown that tensile force will be caused in the pile cross section by the excavation and the tensile force increases gradually with the proceeding of the excavation.The bigger the excavation depth,the greater the tensile force.