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Fig. 4: Free end support method. A wall panel and its loading
Fig. 5: Net loading in the wall and moment diagram to find embedment length.
Springs support method: Using this method, several lines of bracing for the wall can be considered. With the help of computers, the beam can be pided into pieces and transformed into a system of joints and members of equal or different length. The beam can be loaded with active, water and surcharge pressures and supported by springs where it shows negative displacements, in other words, passive pressure. Each spring is represented in terms of its rigidity by the subgrade moduli k that corresponds to the portion of soil being exposed to passive pressure. Figure 6 shows the spring representation for a wall braced with 2 levels of struts.
Fig. 6: Springs model for a two-levels braced diaphragm wall.
Through this analysis, construction stages can be simulated and the embedment length can be found when the displacements at the bottom of the wall are zero. The position of the struts or the anchors can be modified conveniently in order to minimize bending moments or shear forces and the axial loads in the bracing elements can be obtained.
Finite elements method: With this approach, soil-structure interaction can be studied by incorporating a mesh of elements representing the soil and two-noded elements representing the walls and structures attached to it as roofs or slabs. Finite element analysis gives solutions based on stress-strain relations, boundary conditions and constitutive relations. Two general approaches are followed. In the first, assumptions about the soil-structure behavior (stress and deformations) are made and then corrected via instrumentation of the walls during construction. In the second approach, parametric studies of factors influencing wall behavior are introduced to the analysis for different wall functions. Figure 7 shows a finite elements mesh for an excavation braced at three points retaining different soil layers and a hydrostatic load from water behind it.
It is important to say that the four methods briefly described in this paper are just some approaches to consider when facing a problem of retaining open excavations with diaphragm walls. Since more sophisticated methods such as finite elements give more flexibility and better representation of geometry and different materials interacting as a whole, it is always good to have in mind more simple methods such as free earth support or fixed earth support. In this way, a rough idea of wall dimensions and the number and location of the bracing system can be obtained before analyzing the excavation by using more complex methods.
Fig. 7: Finite elements mesh for a 40 ft open excavation.
4. Case study: Central Artery/Third Harbor Tunnel Project.
The Central Artery/Third Harbor Tunnel Project (CA/T project) is one example of the current practice in the construction of diaphragm walls. Three million square feet of diaphragm walls were constructed to retain various below-grade structures and the walls of a tunnel. This tunnel is projected to solve the traffic problem in the city of Boston, Massachusetts. These walls have not only a retaining function; they also serve as a bearing support for an 8-to-10 lane elevated highway.
The geometry and soil configuration of the open excavation are shown in figure 7. A system of three levels of struts was considered to support both sides of the excavation. Instead of using reinforcing cages made of structural steel for concrete, long steel columns were introduced in the panels to provide moment and shear resistance. The water level was controlled via embedding the walls to the bedrock level, which in some areas reached 140 ft. Figure 8 shows the struts system and figure 9 the sequence of construction of the panels.
简介在索瓦尔帕莱一个用于5公里隧道的建设方案使用饱和砂土，智利是采用到地板和天花板楼板连接层地下连续墙，形成“防渗盒子”中，电气车会从内部山谷到海边循环运输人和不同的制成品。