10.3.4 Micro piles

Micro piles are also called soil nails.Since they were first applied in Europe to strengthen or underpin existing buildings，they have been used for more than 40 years.They have also been applied to property protection in many countries.Some successful case histories have been documented in the literature（Woo，1992）and many more unsuccessful cases were not recorded.

Because the mechanisms of micro piles for property protection are indirect and systematic studies are also lacking，though successfully applied in some excavations，most of them are designed on an empirical basis and without theoretical support.

In practice，the diameter of a micro pile varies from 10 to 30 cm.The reinforcements can be steel bars，steel rails，H steels，or even steel cages.The construction process of micro piles is as follows.First，bore to the designed depth with casings or by other drilling measures and then place reinforcements into the bores.Inject cement mortar into bores under a certain pressure.Pull out the casing little by little and add more mortar.The micro piles are usually arranged in a single row or multiple rows.The distance between piles is three to five times the pile diameter，depending on the soil strength.Whether the arrangement is the single row or the multiple row type，they should be intermingled by 5⁃30 degrees as shown in Figure 10.6.

Figure 10.6 Mechanism of micro piles

There are two design principles for use of micro piles.The first is they have to pass the potential failure surface so that the shear strength of micro piles and the pull⁃out resistance can restrain the failure of soils and reduce the possibility of ground settlement accordingly，as shown in Figure 10.6.The potential failure surface，however，is usually rather large and the micro piles passing it are limited in number.The shear strength and pull⁃out resistance of the micro piles are not too large.Thus，whether the design is useful remains to be evaluated.

The other principle is to design many small⁃diameter micro piles enveloping the retaining wall to reinforce the soils through the process of steel placing and grouting.Especially in sandy soils，grouts may permeate into soils extensively and reinforce a larger area of soils.They may even form a quasi⁃gravity retaining wall.Figure 10.7（a）diagrams the micro piles in two rows，which perform as gravity retaining wall⁃like structure，as shown in Figure 10.7（b）.The method may be capable of stabilizing the retaining wall and reducing the active earth pressure on the retaining wall or increasing the resistance to the failure surface.Thus，ground settlement would be reduced.

Figure 10.7 Mechanism of micro piles