9.3.4 Compaction grouting method
The compaction grouting method has been applied in property protection for more than 50 years.Compaction Grouting is a technique used mainly for treating granular material（loose sands），where a soil mass is displaced and densified by a low⁃slump mortar（usually a blend of water，sand，and cement）injected to form continuous grout bulbs.
The method costs low and has been successful in many cases.It is the only grouting method designed specifically not to penetrate soil voids or blend with the native soil.It is also a good option for improving granular foundation materials beneath existing structures，for it is possible to inject from the sides or at inclined angles to reach beneath them.Therefore，compaction grouting can make soils in the vicinity of injection points compact and improve them as a result.It can also uplift a building to restore it from over⁃settlement.The application of building rectification will be introduced in Section 10.5.1.
The compaction grouting method injects pressurized cement mortar with high consistency，low slump，and low plasticity into soils.The cement mortar does not easily flow into the voids of soils and will therefore form a grouting bulb，which will in turn make an interface with the soils around.Squeezing the cement mortar into soils continuously，the grouting bulb will expand and press the soils around to compact them.Besides，the grouting bulb expands in radiation though faster in lateral direction（because the lateral stress of soils is smaller than the vertical stress）.When the soils are compacted to a certain degree，nevertheless，the expansion in the lateral direction will stop.Instead，the grouting bulbs begin to expand upward and generate an upheaving force，which will make a cone⁃shaped shear zone above.When the upheaving force exceeds the weight of overburden or the building，the phenomenon of ground surface heave or building upheaval will occur as shown in Figure 9.5.
Figure 9.5 Compaction grouting method
The diameter of grout bulbs r can be estimated from Eq.9.1：
where
k＝permeability of soil；
β＝viscosity ratio of grout and water；
h1＝grouting pressure head；
r0＝grouting pipe radius；
t＝grouting time；
n＝porosity of soil.
The materials used in the compaction grouting method have to be designed to achieve high consistency，low slump，and low plasticity.Otherwise，they may flow into the voids of soils.On the other hand，they should have a certain fluidity to be well transported.Sand mortar and cement mortar are two widely used grouting materials.Fly ash and bentonite are widely adopted additives.
Drawing from the above elucidation，compaction grouting can make soils in the vicinity of injection points compact and improve them as a result.It can also uplift a building to restore it from over⁃settlement.