10.3.3 Cross walls

The arrangement of cross walls is schematically shown in Figure 10.5.One constructs a wall connecting the two opposing retaining walls before excavation is started.It can be constructed using ground improvement techniques（such as jet grouting or deep mixing）.To obtain a better construction quality or compressive strength，the cross wall can also be constructed by unreinforced diaphragm walls（the unconfined compression strengths of treated soils are usually between 10 and 20 kg／cm2 whereas that of concrete diaphragm wall can achieve 280 kg／cm2 with a minimum of 100 kg／cm2）.

Figure 10.5 Configuration of cross walls

The mechanism of the cross wall for reducing wall deformation is quite different from that of ground improvement.The designing principle of ground improvement is to enhance the strength of soils in front of the wall，considering them too soft to have enough passive resistance.The cross wall should be viewed as a strut，which exists before excavation and owns a great deal of compression strength（especially for unreinforced diaphragm walls）.Theoretically，the locations where cross walls have been placed are less susceptible to deformation because they are restrained from moving.The effects of the cross wall on reducing lateral deformation of a retaining wall are quite obvious.

To ensure that cross walls can effectively reduce deformation of the retaining wall，the design of cross walls has to consider compression strength，depth，and span of two cross walls.Since the behaviors of the diaphragm wall with cross walls are three dimensional，neither the traditional two⁃dimensional plane strain analysis nor the beam on elastic foundation method can simulate behaviors of cross walls.Thus，to design cross walls，one has to resort to the three⁃dimensional finite element method or successful case histories.

Ou and Lin（1999）conducted a series of parametric studies on the basis of 14.5 m deep excavations and 31 m deep diaphragm walls to explore the behaviors of cross walls.The results indicate that the deformation of a retaining wall is the smallest at the location where cross walls are constructed，similar to counterfort walls.To save the cost，moreover，cross walls are best constructed between the excavation surface and 5 or 6 m below it.