10.3.5 Underpinning
This section will introduce another method，underpinning，which may not be helpful to decrease retaining walls deflection but protect buildings from exceeded settlement.
Underpinning is to strengthen the existing foundations of a building，to improve the soils，or，to replace the original foundations.The applications of underpinning are quite extensive，including property protection in excavations，the prevention of natural settlement of heavy buildings，the strengthening of the foundations of buildings which have been unsuitably designed or constructed，and the underpinning of the bottoms of buildings through which a new tunnel has just been built，etc.
The methods of underpinning commonly applied in property protection in excavation are as follows：
（1）Improve the soils beneath the foundations of existing buildings
As shown in Figure 10.8，to prevent over⁃settlement of the adjacent buildings near excavations，the soils beneath the foundation are treated before excavation.Once excavation is started，the soils behind the retaining wall come to the active condition and the maximum deformation usually occurs near the excavation surface.The potential failure surface outside the excavation zone normally develops from below the excavation surface.Thus，to underpin existing buildings，the depth of the soil improvement should extend from below the foundations to outside the failure surface.
When carrying out soil improvement，the over⁃pressurized grouting often heaves the building.It also disturbs the soil structure below the building foundations and reduces the soil strength.If the method is not implemented with prudence，it may not only turn out to be a failure in strengthening the foundation soils but may also worsen slanting conditions and damage buildings.Thus，a detailed plan referring to successful case histories is required before implementation.
Figure 10.8 Underpinning an existing building by soil improvement
（2）Add an extra foundation to existing buildings
For fear of the insufficient bearing capacity of the foundations of existing buildings，an extra foundation could be constructed near the original foundation before excavation.Such an underpinning measure is usually implemented before excavation.The implementation，without weakening the original foundations，does not need to be accompanied by the measure of load transfer，which will be introduced in the next.
Figures 10.9 and 10.10 are some possible underpinning methods.Figure 10.9（a）shows a method of constructing new piles beside the retaining wall to support the foundations of existing buildings.Figure 10.9（b）shows a method of constructing a protruding part to the side of the foundations of existing buildings to join the newly constructed piles.Figures 10.10 uses the underpinning methods adopted in the construction of the Singapore mass transit system（Huang，1992）.In Figure 10.10，the government building is a brick building with wood piles of 4.5 m deep and 50⁃100 mm diameter.The mass transit system passes near the government building.The construction follows the open cut method with an excavation depth of 27 m.The retaining system consists of steel sheet piles and soldier piles.The outside column of the government building is only 3 m from the soldier piles.To protect the building during the construction of the mass transit system，an underpinning measure was used，which was as follows：four micro piles were constructed next to the outside columns.The micro piles were constructed through the pile caps up to 26⁃28 m below the ground surface，which was 5 m below the 45° failure surface.The resulting void in the ground was filled fully with cement mortar.
Figure 10.9 Underpinning an existing building
Figure 10.10 Underpinning foundations of a building near an excavation
（3）Construct new foundations under existing buildings
Figure 10.11 diagrams the underpinning method in which an additional foundation is constructed beneath the building's original one.With the new foundation constructed，the original one may weaken or even become useless.The method is as follows：
①Excavate an operation space beside the footing and below the foundation.
②Construct new foundations（piles）.
③Install temporary supports.
④Load transfer operation⁃transfer the weight of the building to the new foundation or the strengthened one.This step is a crucial point in determining whether the underpinning will succeed.A precise preload control and measurement of the behavior of tile structure are to be carried out.The procedure of the load transfer operation is as shown in Figure 10.12.
Figure 10.11 Underpinning an existing building by constructing new foundation
Figure 10.12 Schematic diagram of load transfer operation
⑤Dismantle the temporary supports.Proceed to grouting and backfilling.
As shown in Figure 10.12，the procedure of load transfer can be described as follows：
①Set jacks and a steel plate on the new foundation.
②Lay sand mortar or concrete between the building bottom and another steel plate，which is set on tile top of the jack.
③Preload tile steel plate or the foundation using jacks.Preloading usually takes the building as the reaction frame.The direct adding of the weight of the building onto the new foundation usually produces settlement.The aim of preloading is to have the new foundation acted on by a preload in advance to accelerate settlement.
④Place temporary supports between the two steel plates.
⑤Hammer wedges into the voids of the temporary supports.
⑥Dismantle the jacks and fill the operation space with concrete.The underpinning is thus completed.