地铁上穿工程中既有隧道卸荷附加应力系数

A Study on the Unloading Additional Stress Coefficient Acting on the Existing Tunnel in the Subway Overpass Project

推导了新建地铁上穿施工引起的既有隧道结构的卸荷附加应力系数解析解,利用Maple软件对卸荷面积、卸荷中心距既有隧道水平距离、卸荷平面至既有隧道顶部距离、卸荷宽长比等关键卸荷参数与附加应力系数的关系进行研究,旨从弹性卸荷理论的角度提出了地铁上穿工程中如何减小既有隧道结构卸荷效应的措施。研究表明:随着卸荷面积的增大,底板下方不同深度处的附加应力系数逐渐增大,应尽量减少一次卸荷面积,减少开挖进尺;当既有隧道处于卸荷面积边界附近时,卸荷附加应力系数急速衰减,应对开挖卸荷长度范围内的既有线结构周围土体进行重点加固;附加应力系数随着夹层土厚度的增加逐渐减小,应尽量加大新旧结构之间的距离;随着宽长比的增大,卸荷中心下的附加应力系数逐渐减小,建议采用长条形的土体开挖方式;在相同的卸荷面积条件下,泊松比对附加应力系数的影响较小。

The elastic unloading calculating model of the subway overpass project is established and the analytic expression for the vertical additional stress subject to rectangular uniform load in the interior of semi-infinite solid is derived. The relationship between the key unloading parameters,such as unloading areas,the horizontal distance between the unloading center and the existing tunnel,the distance between the unloading plane and the top of the existing tunnel,the unloading length-width ratio,and the additional stress coefficient α is discussed,and some countermeasures are posed to reduce the unloading effects acting on the existing tunnel based on the unloading elastic theory. The study results show that α at different depth under the bottom of the existing tunnel increases gradually with the increase of the unloading areas,and unloading areas should be lessened,large excavating section should be divided into the smaller,excavating distance should be cut down also. When the existing tunnel is situated nearby the border of the unloading area,the α drops rapidly,therefore,it is suggested that the surrounding soil of existing tunnel structure within the scope of the unloading length should be reinforced. The α reduces with enlargement of the thickness of interlayer soil,consequently,the distance between the newly-built subway tunnel and the old one should be enlarged as soon as possible. The α under the unloading center decreases gradually with the increase of the lengthwidth ratio,and rectangular shape soil excavation is suggested. The Poisson’s ratio’s influence for the α is negligible under the case of same unloading areas.