4.2.3 Column piles
The column piles method is to construct rows of concrete piles as retaining walls by either the cast⁃in situ pile method or the precast pile method.The cast⁃in situ method can be divided into three subtypes according to their construction characteristics：
①Packed in place piles.The packed in place pile method，also called PIP pile method，can be described as follows：dig to the designed depth with a helical auger；while lifting the chopping bit gently，fill in prepacked mortar from the front end to press away the loosened soil to the ground surface；after grouting is finished，put steel cages or steel H⁃piles into the hole.The diameter of a PIP pile is around 30⁃60 cm.It often happens that PIP piles are not capable of being installed completely vertically，so connections are not always watertight，and connection voids often cause leaks of groundwater.Thus，if the PIP pile is adopted for the retaining wall in sandy soils with high groundwater level，sealing and grouting are often required.Figure 4.6 illustrates the construction of a PIP pile.
Figure 4.6 Construction procedure of a packed in place（PIP）pile
②Concrete piles.The construction of concrete piles can be described as follows：drill a hole to the designed depth by machine，put the steel cages into it，and fill it with concrete using Tremie tubes.The reverse circulation drill method（also called the reverse method），which is to employ stabilizing fluid to stabilize the hole wall during drilling，is the most commonly used construction method for concrete piles.It is also feasible to build following the all casing method，which is to drill with simultaneous casing⁃installment to protect the hole wall.Since the wall is protected by casings，stabilizing fluid is not required.The cost of the all casing method is rather high.Nevertheless，it can be easily applied to cobble⁃gravel layers or soils with seepage；whereas the reverse method cannot.The diameters of the concrete piles are around 60⁃200 cm.
③Mixed piles.Mixed piles are also called MIP piles（mixed in place piles）or SMW（soil mixed wall）.The method is to employ a special chopping bit to drill a hole with the concrete mortar sent out from the front of the bit to be mixed with soil.When the designed depth is reached，lift the bit a little，keeping swirling and grouting simultaneously，and let mortar mix with soil thoroughly.After pulling out the drilling rod，put steel cages or H⁃piles into the hole if necessary.Figure 4.7 illustrates the construction process for a mixed pile.Figure 4.8 shows MIP piles with H steels.
Figure 4.7 Construction procedure of a mixed in place（MIP）pile
Figure 4.8 Soil mixed wall（SMW）
As illustrated in Figure 4.9，the patterns of column piles include the independent pattern，the S pattern，the line pattern，the overlapping pattern，and the mixed pattern.They can be described as follows：
①Independent pattern：As shown in the figure，sealing is impossible with the independent pattern.Thus，excavation in permeable ground with high groundwater level requires dewatering in advance.The pattern，nevertheless，is especially fitting for soil with high strength，such as gravel soils.
②S pattern：The second row of piles fits into gaps in the first row，which have been arranged in the independent pattern.This pattern is also called the tangent piles.The method is relatively simple in construction.On the other hand，the sealing capability is weak because of the less orderly layout.Grouting is necessary for sealing.
Figure 4.9 Layouts of column piles
③Line pattern：Being one type of tangent piles，the piles are connected to one another to form a line.It is also one of the most commonly used patterns.Since sealing capability is not good，chemical grouting is often required to make up for the problem.
④Overlapping pattern：As shown in the figure，after piles No.1，2，and 3 are completed，before they congeal，construct piles No.4，5，and 6 piles in order.The latter will cut through the former.They，therefore，are also called secant piles.The overlapping pile can only be cast in the field.Precast piles cannot be used for this type of pattern.The pattern is good for sealing.
⑤Mixed pattern：The mixed pattern is the independent pattern along with jet grouting or MIP piles.If grouting quality is guaranteed，the sealing capability of the mixed pattern will be fairly good.
The merits of column piles are as follows：
①Less noise or vibration than produced by the installation of soldier piles or steel sheet piles.
②Adjustable pile depth.
③Greater stiffness than soldier piles or steel sheet piles.
④When equipped with a special bit，they are also applicable to cobble⁃gravelly soils.
⑤Easier construction on sandy ground.
The drawbacks of column piles are as follows：
①Without lateral stiffness in the direction parallel to the excavation side，no arching effect to prevent wall deformation exists.
②Longer construction period than that for the soldier pile method or the steel sheet pile method.
③Lower stiffness than diaphragm walls.
④Highly susceptible to construction deficiency.