Analysis And Research Of Prestressed Anchor Retainning With Stiffen Pile In Deep Excavation

With the development of cities,urban buildings become more and more dense,and the area of constructable land is gradually reduced.The full development and utilization of underground space has become an inevitable trend.At present,the depth of excavation is getting deeper and deeper,the construction environment around the excavation is becoming more and more complex,urban buildings are dense,underground pipeline network is densely distributed,and the boundary of excavation is often close to the original building.At the same time,due to the narrow construction site,it is impossible to adopt the traditional rigid retaining such as large-scale cast-in place pile or diaphragm wall,and flexible support is often difficult to meet the requirements of adjacent buildings for excavation deformation,and the stability of excavation is also difficult to guarantee.Based on the above problems,prestressed anchor support with stiffen pile is proposed.Because of the advance retaining function of the stiffen pile,the deformation of rock mass was restrained,and the problems of excessive deformation of prestressed anchor flexible retaining and exposure of adjacent building foundations caused by free surface during excavation are solved.In this paper,the basic composition and construction method of the prestressed anchor retaining with stiffen pile are introduced in detail,the mechanism of its action is analyzed theoretically,the design and calculation method of prestressed anchor support with stiffen pile is discussed.Based on the three-dimensional continuum finite element method,the mechanical properties,deformation law and stability of the retaining structure are analyzed,and compared with prestressed anchor flexible retaining and pile-anchor support.Through the single factor analysis method,the influence of anchor prestress,anchor spacing and I-beam type in stiffen pile on the internal force of retaining structure and the deformation of excavation is analyzed.The conclusions are as follows:(1)Compared with the prestressed anchor flexible retaining,the prestressed anchor retaining with stiffen pile can significantly reduce the horizontal displacement of excavation,the surface settlement and the plastic zone of the soil behind the excavation;The potential slip surface position of the support system is transferred to the deep pit of the foundation pit,and the overall stability of the support structure is improved.(2)Compared with the pile-anchor retaining,the internal force of the retaining pile is significantly reduced.The bending moment value is about 10% of the cast-in-place pile in the pile-anchor retaining,and the shear value is about 21% of the cast-in-place pile in the pile-anchor retaining.The stiffen pile do not require a large size to meet the strength requirements.The deformation modes of the two supporting structures are similar.The control effect of the prestressed anchor retaining with stiffen pile on the deformation of the foundation pit is close to the pile-anchor retaining,which can also meet the strict requirements of the foundation pit deformation.(3)Anchor prestress can effectively reduce the deformation of excavation and the distribution of plastic zone.When the prestress value reaches the 62.5% standard value of anchor axial tension,continue to increase the prestress value,the control effect on deformation is weakened.Further,it is considered that the threshold prestress of the prestressed anchor retaining with stiffen pile is 75% of the standard value of the axial tensile force.(4)The maximum horizontal displacement of the sidewall of the foundation pit increases with the increase of the spacing of the anchor.When the spacing of the anchor is 1.8 m,the maximum horizontal displacement of the sidewall of the foundation pit increases.(5)By changing the type of I-beam in the stiffen pile,the deformation of the excavation is basically unchanged.Under the premise of meeting the strength requirements,I-beam can achieve the desired supporting effect by designing according to the structural requirements.