10.4.3 Underpinning

The principle of the underpinning for rectifying buildings is similar to that of the underpinning method introduced in Section 10.3.5.The only difference is that the latter aims at prevention before excavation，transferring the load of the building to a new foundation which is not to be influenced by excavation.Therefore，jacks are not necessarily required to uplift the building.The former is to solve an existing problem，correcting a building problem due to excavation.Since the building has already settled，jacks are required to uplift the building.The procedure of the underpinning method is as shown in Figure 10.16.First excavate a sufficient working space，where underpinning piles are to be constructed，around the mat foundation or the individual footing under the column，then set a jack between the pile cap and the mat slab to uplift the building，and last proceed to the load transfer operation.

Figure 10.16 Rectification of buildings by the underpinning method

The underpinning rectification method can be applied to mat foundations and individual footings.Its strength is that the target or range of rectification is specific，without the problem of grout fleeing，which frequently occurs in grouting methods，or worrying about wrongly uplifting a column.The shortcoming is that it costs much more and requires advanced techniques.If not designed well，the building might settle instead due to the settlement of the bearing pile induced by underpinning.A mat foundation that has been underpinned might become a partial pile foundation and change its designed stress distribution.