UNDERPINNING - Stabilizing the Foundations of a Building

Underpinning is the process of  strengthening and stabilizing the  foundations of an existing building.

It may be required for any of several reasons: The existing foundations may never have been adequate to carry their loads, leading to excessive settlement of the building over time. A change in building use or
additions to the building may over-load the existing foundations. New construction near a building may disturb the soil around its foundations or require that its foundations be carried deeper. Whatever the cause, underpinning is a highly specialized task that is seldom the same for any two buildings. Three different alternatives are available when foundation capacity needs to be increased: The foundations may be enlarged; new, deep foundations can be inserted under shallow ones to carry the load to a deeper, stronger stratum of soil; or the soil itself can be strengthened by grouting or by chemical treatment. Figures 2.53 and 2.54 illustrate in diagrammatic form some selected concepts of underpinning.

Two methods of supporting a building while carrying out underpinning work beneath its foundation, each shown in both elevation  and section. (a) Trenches are dug beneath the existing foundation at intervals, leaving the majority of  the foundation supported by the soil. When portions of the new foundations have been completed in the trenches, using one of the types of underpinning shown in Figure 2.54, another set of trenches is dug between them and the remainder of the foundations is completed. (b) The foundations of an entire wall can be exposed at once by needling, in which the wall is supported temporarily on needle beams threaded through holes cut in the wall. After underpinning has been accomplished, the jacks and needle beams are removed and the trench is backfi  lled
Figure 2.53 Two methods of supporting a building while carrying out underpinning work beneath its foundation,
each shown in both elevation  and section. (a) Trenches are dug beneath the existing foundation at intervals, leaving the majority of  the foundation supported by the soil. When portions of the new foundations have been completed
in the trenches, using one of the types of underpinning shown in Figure 2.54, another set of trenches
is dug between them and the remainder of the foundations is completed. (b) The foundations of an
entire wall can be exposed at once by needling, in which the wall is supported temporarily on needle
beams threaded through holes cut in the wall. After underpinning has been accomplished, the jacks
and needle beams are removed and the trench is backfi  lled
Three types of underpinning. (a) A new foundation wall and footing are constructed beneath the existing foundation. (b) New piles or caissons are constructed on either side of the existing foundation. (c) Minipiles are inserted through the existing foundation. Minipiles do not generally require excavation or temporary support of the building.
Figure 2.54 Three types of underpinning. (a) A new foundation wall and footing are constructed beneath the existing
foundation. (b) New piles or caissons are constructed on either side of the existing foundation. (c)
Minipiles are inserted through the existing foundation. Minipiles do not generally require excavation or
temporary support of the building.

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