3.3.6 Effect of preloading
To stiffen the support system，higher preload has been used in an attempt to reduce wall deflections and around settlements.The wall may be pushed back under this load.But one should be careful to avoid excessive preload，which can cause passive failure of the soil behind the wall，and large bending moment can be induced in the wall.Singapore Post Center used 100% preload.Most MRT stations along the NEL required a minimum of 50%.Some（O’Rourke，1974）consider little benefit in introducing the additional load.The way of preloading includes the following three types，as shown in Figures 3.11 to 3.13，showing the effectiveness of preloading using different methods.
Figure 3.11 Three types of applying preload
In general，strut forces reduce under increasing preloads，but with a diminishing efficiency.The forces at the top and bottom strut levels again agree well the reference APD；whereas forces at the intermediate strut levels are seen to exceed by as much as 100%.
Figure 3.12 Effectiveness of preloading using shim plate method
Figure 3.13 Effectiveness of preloading using locking type hydraulic jacks