10.3.2 Counterfort walls

Figure 9.6（c）diagrams the wall type of ground improvement，which usually contacts the retaining wall without forming a whole structure with it.Thus，when the retaining wall is bent and deformed，there will be produced a relative displacement between the wall⁃type soil body and the retaining wall.That is to say，the wall⁃type soil body does not increase the moment⁃resistance of the retaining wall though it increases the strength of soils in front of the retaining wall.If the wall in Figure 9.6（c）is constructed as a counterfort（also called buttress），that is，constructed with the diaphragm wall method（either reinforced or not）and forms a whole structure with the diaphragm wall（retaining wall），like a T⁃beam in reinforced concrete structures，the counterfort will greatly enhance the capability of moment⁃resistance.

The location of the counterfort can be arranged either at the inner or at the outer side of the retaining wall as shown in Figure 10.4.When arranged at the inner side，it would be mostly subjected to tensile force.Theoretically，the counterfort should be reinforced under such a condition.The actual cases using counterforts，nevertheless，show that unreinforced counterforts are useful as well in increasing the moment⁃resistance stiffness.The counterfort constructed at the inner side of the retaining wall should be dismantled with the increase of the excavation depth.

Figure 10.4 Locations of counterfort walls

If no property rights are involved，the counterfort can be constructed on the outer side of the wall.The reason is that the counterfort will be subjected to compression force when it is bent or deformed.There is no need to place reinforcements in the counterfort.Besides，it also saves the trouble of dismantling it during the excavation process.If the counterfort need not be dismantled as the excavation is proceeding，it can offer stiffness throughout the whole course of excavation and the construction of the basement.

Theoretically，the counterfort can be cast into a whole structure with the retaining wall.However，there exists the problem of collapse near the concave comers of trenches.Jet grouting in this area is one of the measures to avoid the collapse of trenches.

Ou and Wang（1997）suggest that the deformation behavior of the counterfort diaphragm wall has much to do with whether it penetrates into the hard soil stratum（sandy or gravelly soils）or not.In other words，it relates to whether the wall bottom is effectively restrained or not.Besides that，the counterfort can only restrain the wall deformation at the part where it is placed.If the deformation of the whole diaphragm wall，that is，along the excavation border，is to be reduced，several counterforts have to be placed evenly along the excavation border.In practical design，the span of the counterforts can be determined referring to successful case histories or may be obtained from the three⁃dimensional finite element analysis.