4.2.2 Sheet piles
Sheet piles can be driven into soil by striking or static vibrating，and interlocked or connected with one another.Figure 4.3 shows the front view of sheet piles and Figure 4.4 is a photo showing the sheet piles in an excavation.There are several shapes of sheet pile sections.Some commonly used include the U⁃section，Z⁃section，and the line⁃section，as shown in Figure 4.5.If the interlocking is well done，sheet piles can be quite efficient in water sealing.If not，leaking may occur at the joints.In clayey soils having low permeability，sheet piles do not necessarily require perfect connection to prevent leaking.On the other hand，if they are used in sandy soils with high permeability，any breach in sheet piles may well cause leaking.If leaks occur，sand in back of the retaining walls will very possibly flow out，which may cause settlement in turn.If leakages are too great，the excavation might be endangered.The construction method for sheet piles can be described as follows：
Figure 4.3 Steel sheet pile method
Figure 4.4 Photo of the steel sheet pile method（Ou，2006）
Figure 4.5 Sections of steel sheet piles
①Drive sheet piles into soil by striking or static vibrating.
②Proceed to the first stage of excavation.
③Place wales in proper places and install horizontal struts.
④Proceed to the next stage of excavation.
⑤Repeat procedures 3 and 4 till the designed depth.
⑥Complete excavation and begin to build the foundation of the building.
⑦Build the inner walls of the basement.Dismantle the struts level by level and build the floor slabs accordingly.
⑧Complete the basement.
⑨Dismantle the sheet piles.
The merits of the steel sheet piles method are as follows：
a.It is highly watertight.
b.It is reusable.
c.It has higher stiffness than soldier piles.
The drawbacks of the steel sheet piles method are as follows：
a.Lower stiffness than column piles or diaphragm walls.
b.Susceptible to settlement during striking or dismantling in a sandy ground.
c.Not easy to strike piles into hard soils.
d.A lot of noise is caused during striking.
e.Leaks cannot be completely avoided but sealing and grouting are probably necessary if leaks occur.