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Underpinning

In addition to protecting an existing structure, which may be threatened by undermining due to excavation for an adjacent project, underpinning may also be used as a remedial measure to correct faulty foundation structures or to increase the capacity of an existing foundation, which may be required due to a proposed structural addition. The critical nature of such work requires a prerequisite background knowledge of soil mechanics and structural engineering along with specific experience in the design and installation of a variety of underpinning techniques. AGRA Foundations Limited has developed the necessary experience in many underpinning projects successfully executed over the past 40 years.

Underpinning can usually be accomplished by one or a combination of the
following methods:

  1. Panel Construction – This is basically the extension of an existing footing to a new depth. A series of adjacent panels are excavated, one at a time, under the existing footing down to the required new depth. Panel widths are generally not more than 1200 mm wide and each panel is reinforced and concreted before the adjacent panel is excavated.
  2. Jacked Pile – A group of pipe or H-piles are installed under a footing or foundation wall by hydraulically jacking a series of short pile lengths into the ground until satisfactory penetration resistance is obtained. Short pile lengths are welded one on top of the other as installation proceeds, providing a continuous pile once the proper penetration resistance is reached. Reaction to jacking is provided by the structure itself, since the pile is installed by working in a small excavated area directly under the footing or wall that is to be underpinned. Upon completion the pile is tightly wedged to the existing foundation by short beams to ensure proper load transfer.
  3. Secant or Tangent Pile Wall – For secant pile walls a series of intersecting reinforced bored concrete piles are installed at multiples of pile diameter center to center spacing along the side of the structure to be underpinned or site to be shored. Secondary (squeeze) piles are constructed once primary piles gain sufficient strength. Overlap is typically 75 mm. For tangent pile walls there is no pile overlap. A tieback system may be installed along the top of the wall to prevent deflection away from the existing foundation or movement into the excavation.

Advantages:

  • Alignment flexibility.
  • Can be installed in difficult ground conditions.
  • Greater wall stiffness characteristics.
  • Minimal construction noise.

Disadvantages:

  • Vertical alignment difficult to maintain in deep piles.
  • Difficult to obtain watertight structure.
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