4.2.5 Selection of the retaining system
The selection of retaining walls has to consider the excavation depth，geological conditions，groundwater conditions，adjacent building conditions，the site size，the construction period，and the budget，etc.Table 4.1 offers the application ranges of various retaining walls for design or construction reference.Table 4.2 illustrates the nominal stiffness per unit length where the value of moment of inertia is not reduced.A stress or deformation analysis，however，has to consider the decrease of stiffness of a soldier pile or a steel sheet pile because of repetitive use.A discount of 8 0% of the nominal stiffness is often suggested.
The
⁃value ofa PIP pile isabout170 kg／cm2 whilethat ofaconcretepile，areverse circul ationdrill pile，ora diaphragm wallis about 210 kg／cm2 or more.Their Young'smodulus can be derived from E＝15 000
kg／cm2.Considering the cracks in the retaining wall due to bending moment，the moment of inertia may be reduced by 30%⁃50%.Since the compressive strength of a mixed pile is very small，about 5 kg／cm2，its stiffness can be estimated solely on the basis of the stiffness of the H⁃steel.Listed in the sixth column of Table 4.2 are the stiffness ratios where the inverse values represent the deformation ratios under the same construction condition（i.e.same excavation depth，strut location，and strut stiffness，etc.）.For example，the deformation of a SP⁃Ⅲsteel sheet pile is 6.63（5.3／0.8）times as great as a 50 cm thick diaphragm wall.
Table 4.1 Application conditions for retaining walls
Notes：◎：good ○：acceptable×：not good
1.Should be applied along with special drill and striking device.
2.If driven into soil by static vibrating process，noise and vibration can be reduced.
Table 4.2 Nominal stiffness（before reduction）
continued
Notes：
1.The distance between H⁃steels is 1.00 m.The stiffness of the soldier pile can be reduced by 20% in analysis.
2.The stiffness of the sheet pile can be reduced by 20% in analysis.
3
for PIP piles is about 170 kg／cm2；f′c for reinforced concrete column piles is about 280 kg／cm2；in the table f′c is assumed to be 210 kg／cm2，which can be reduced by 30%⁃50% in analysis.
4.
for MIP piles is about 5 kg／cm2，that is，the stiffness of the MIP can be ignored and only the stiffness of H⁃piles is to be considered.The distance between H⁃piles in the table is 40 cm.
5.
is assumed to be 210 kg／cm2.The stiffness can be reduced by 30%⁃50% in analysis.