10.2.1 Reducing the unsupported length of the reta...
Figures 10.1（a）and（b）are identical in the final excavation depth，the number of excavation stages，the number of strut levels，and the location of struts.The only difference between them is the distance between the location of strut and the excavation surface.
In Figure 10.1（a），each level of struts is installed 0.5 m above the excavation surface.When the first stage of excavation is completed，the unsupported length of the wall is 2 m.When the first level of struts is installed at the depth of GL⁃1.5 m and excavation proceeds to GL－5.5 m，the unsupported length of the wall is 4 m.When the second level of struts is installed and excavation proceeds to the third stage，the unsupported length of the wall is also 4 m.With the completion of the third level of struts，the unsupported length is 4.5 m.
In Figure 10.1（b），each level of struts is installed 1.5 m above the excavation surface.As a result，during the first stage of excavation，the unsupported length of the wall is 3 m.Evaluated similarly，the unsupported lengths of the wall at the second and the third stages are both 5 m.The unsupported length after completion of the final stage of excavation is 4.5 m.
As elucidated above，except for the last stage of excavation where the unsupported lengths in the two cases are identical，the unsupported lengths illustrated in Figure 10.1a are smaller than those in Figure 10.1b at other stages of excavation，with the same earth pressure on the back of the retaining wall.Actually，the total deformation of the retaining wall or ground settlement is the accumulated deformation at every stage of excavation.Thus，the deformation in Figure 10.1a should be smaller than that in Figure 10.1b.That is to say，the location of struts should be as close to the excavation surface as possible.Generally，considering the convenience of strut installation，the distance between struts and the excavation surface is about 0.5 m.
Figure 10.1 Influence of the distance between the strut and the excavation surface on the unsupported length