14.
0 Underpinning
OUTLINE
1.1 DEFINITION ....................................................................................................................................... 2
1.2 SITUATIONS IN WHICH UNDERPINNING WOULD BE REQUIRED.................................................. 2
1.3 METHODS OF UNDERPINNING ....................................................................................................... 2
1.3.1 Wall underpinning ............................................................................................................................. 3
1.3.1.1 Traditional wall underpinning ............................................................................................. 3
1.3.1.2 Jack or Miga Pile underpinning .......................................................................................... 5
1.3.1.3 Needle and piles .................................................................................................................... 6
1.3.1.4 Pynford stooling.................................................................................................................... 7
1.3.1.5 Cerfax hoopsafe method ..................................................................................................... 8
1.3.1.6 Double angle root piling ...................................................................................................... 9
1.3.2 Column underpinning ......................................................................................................................10
REFERENCES .................................................................................................................................................12
1.1 Definition
• The process of strengthening and stabilizing the foundation of an existing building or other
structure.
o It can also be used to replace an existing weak foundation
• It is accomplished by extending the foundation in depth or in breadth so it either rests on a
more supportive soil stratum or distributes its load across a greater area.
1.2 Situations in which underpinning would be required
• The original foundation is simply not strong or stable enough.
• There is need to increase the loadbearing capacity of the foundation because:
o The usage of the structure has changed (increase in imposed load)
o There is need to construct an extra storey
• The properties of the soil supporting the foundation may have changed
o possibly through subsidence caused by the action of tree roots, or
o were mischaracterized during design.
• The construction of nearby structures (e.g. basements or installation of deep services)
necessitates the excavation of soil supporting existing foundations.
1.3 Methods of Underpinning
a) Wall underpining
i. Traditional wall underpinning
ii. Jack or Miga pile underpinning
iii. Needle and piles
iv. Pynford stooling
v. Cerfax hoopsafe
vi. Double angle root piling
b) Column underpinning
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1.3.1 Wall underpinning
1.3.1.1 Traditional wall underpinning
• Excavate in stages alongside and underneath the existing foundation
• Cast a new foundation
• Build up to the underside of the existing foundation in brickwork or concrete
• Pin between the old and new work with a rich dry mortar
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Figure 1 Traditional wall underpinning. (Source: Greeno and Chudley, 2006: page 196)
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1.3.1.2 Jack or Miga Pile underpinning
• Used where a suitable bearing capacity is too deep (making the traditional wall underpinning
uneconomical)
• Short precast concrete piles are jacked into the ground until a suitable subsoil is reached
• The space between the top of the pile and the underside of the existing foundation is filled
with a pinned concrete cap
• The existing foundation must be in good condition because it will act as a beam spanning
the piles
Figure 2 Jack or miga pile underpinning (Source: Greeno and Chudley, 2006: page 197)
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1.3.1.3 Needle and piles
• Applicable if the foundation is too weak to act as a beam
• Uses a pair of jacks or usually bored piles in conjunction with an insitu reinforced concrete
beam or needle placed above the existing foundation
Figure 3 Needle and pile underpinning (Source: Greeno and Chudley, 2006: page 199)
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1.3.1.4 Pynford stooling
• Portions of brickwork are cut away above the existing foundation
• Precast concrete or steel stools are inserted and pinned
• The intervening brickwork is removed leaving the structure supported entirely by the stools
• Reinforcing bars are threaded between and around the stools and caged to form the ring
beam reinforcement
• The beam is cast
o This beam replaces the existing strip foundation
o From it other forms of deep underpinning can be carried out
Figure 4 Pynford stooling method (Source: Greeno and Chudley, 2006: page 200)
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1.3.1.5 Cerfax hoopsafe method
• Applicable in areas experiencing differential settlement
• Procedure:
o Excavate to expose the substructure wall to a depth just above the foundation level
o Cast an insitu concrete beam around the building’s perimeter
The beam will have longitudinal voids created with plastic conduits within
which steel stressing tendons will be placed for post tensioning to bind the
wall into a solid unit
Figure 5 Hoopsafe foundation support (Source: Greeno and Chudley, 2006: page 201)
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1.3.1.6 Double angle root piling
• Consists of the installation of concrete angle piles from inside and outside of the building
Figure 6 Angle piled foundation stabilization (Source: Greeno and Chudley, 2006: page
202)
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1.3.2 Column underpinning
• Brick or stone columns
o Can be underpinned by inserting a series of stools, casting a reinforced concrete
base, then underpinning by the methods already described
• Structural steel or reinforced concrete columns
o Must be relieved of their load first
Method:
• Collar steel or precast concrete members are fixed around the
perimeter of the column
• (Steel collars are welded to the structural member; concrete columns are chased to a
depth of 25 to 50 mm to receive the support collar)
• The column loading is transferred from the collar to cross beams or
needles
• These then transmit the loads to the ground at a safer distance
o (Cantilever techniques that transfer the loading to one side
are possible)
Underpinning of the column foundation can now be carried out by the
methods already described
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Figure 7 Typical column underpinning (Source: Greeno and Chudley, 2006: page 204)
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References
Greeno R., and R., Chudley (2006). Advanced Construction Technology, Fourth Edition. Harlow: Pearson
Education Limited
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