Types of underpinning Traditional

The main traditional forms of underpinning are brick with concrete footings, and mass concrete. They usually have a maximum depth of about 3 m and when complete are generally continuous over the length

Table 7.5 Main types of modern underpinning methods

Method Characteristics

Grouting This involves injecting a viscous cement or resin grout into the ground to 'stiffen'

and/or expand the soil under the wall affected by subsidence. It is more suited for use in non-cohesive soils such as gravely and sandy soils. Needle beams ladder Usually used in conjunction with piles under the main ring beams. The needles link the ring beams to form a ladder frame under the building. This forms a continuous underpinning system. Suitable for shallow underpinning work (i.e. 1.5-3.5 m). The piers are installed at about 2-3 m centres - depending on the loadings and depth of beam. See type 4 underpinning in Figure 7.29. Pre-cast reinforced concrete jacks, installed in sections under the affected foundation. Jack-down piles can be used if the scarcement of the existing footing is wide enough to take the pile cap head. Bored in-situ piles (can be cantilevered if piling directly next to existing building may disturb the structure) with reinforced concrete needle beam. 'Palo radice' underpinning uses 'a series of small-diameter piles, rotary drilled through the existing masonry and taken to an adequate depth in the ground below' (Lizzi, 1993). These raked mini-piles are installed in pairs at an angle through drilled holes in the existing substructure, typically at 1-1.5 m centres. 75-100 mm diameter cast aluminium helical piles drilled into the scarcements of the foundation in 1m long sections to the required depth, such as 'Brutt Helical Pile'. Pre-cast concrete stools inserted into the substructure above the foundation. This can be used in conjunction with in-situ reinforced concrete beams to link the stools within the body of the wall. A reinforced concrete raft slab with downstand edge beam can be formed within part of the depth of the defective wall to provide a floor and some degree of underpinning.

This is a modern method of using water pumped into the soil so that it expands back to its near original level. Suitable for some low-rise buildings on clayey soils. The controllability of this technique, however, is uncertain. 'Safehoop' system This involves installing a prestressed reinforced concrete girdle around the affected building. Usually suitable for relatively small- or low-rise dwellings of square or rectangular plan shape.

of wall being underpinned. The concrete is poured in staggered bays about 1.2 m wide and is taken up to within about 50 mm of the underside of the existing foundation - the gap being filled latter with a lean mix grout in a process called 'pinning-up'. This form of underpinning is, however, very expensive. It is also time-consuming and disruptive to install.

Modern

Table 7.5 lists the main forms of modern underpinning, some of which involve specialist proprietary techniques. Four common versions of modern underpinning techniques are shown in Figure 7.29.

Modern piles are either continuous or discontinuous. Continuous underpinning involves supporting the affected walling throughout its entire length. With discontinuous underpinning, on the other hand, a space is left between the new substructure supports.

The pier and beam system is continuous under the length of the underpinned wall. A more recent method using a continuous ladder frame of concrete needle beams may be required in very old buildings with thick walls. In contrast, the pile and beam method is discontinuous in that it does not cover the complete length of the foundation being underpinned.

frame Pier and beam

Concrete piles

Metal piles 'Pynford' system

Raft floor

Rehydration

Pier And Beam Foundations Sandy Soil

Notes

1. Continuous mass concrete legs cast in bays about 1.2 m wide under existing footing in 'hit and miss' sequence.

2. Shallow reinforced concrete beam and mass concrete pier underpinning.

3. Deep piles and needle beam underpinning. The needles may be installed either directly under the existing footing or through existing wall. They may also be cantilevered if there are problems of access or disturbance on either side of the wall.

4. Cantilevered to avoid internal work.

Figure 7.29 Four common types of underpinning

Helical metal piles can provide a quick and effective treatment for minor subsidence. The piles are fixed to the existing structure using a variety of methods and configurations. The capping generally involves using 8 mm stainless steel rods (such as 'Brutt Bar') grouted in to provide a shear and tensile connection to the existing structure. The 'Brutt Bar' cage is then usually connected back to the HeliPile by encasing it all in concrete.

Soil stabilization involving injection materials is becoming increasingly popular as an alternative to piling and traditional underpinning. For example, 'Uretek' is a proprietary soil stabilization technique using a deep injection non-compressible structural resin that provides up to 500 per cent increase in ground-bearing capacity. Multiple injections to 3 m in depth can be made in the identified weak soil layer beneath strip and rafted foundations and flooring. The resin expands about 30 times its liquid volume, exerting in excess of 50 tonnes/sq. m expansive pressure on surrounding soils and compresses any weak soil layer around it to prevent further subsidence. It expels ground water with lightweight material reducing weight on lower strata.

According to the manufacturer the URETEK system can be applied in either radial or linear form to create columns with similar characteristics to piled foundations. It's verifiable by penetrometer testing and other techniques. At face value it offers a non-disruptive solution - house foundations and floors can be treated within 1-2 days by the installer.

The lack of full control during installation and uncertain or limited durability of soil stabilization techniques are their main drawbacks. Injecting a stabilization substance into a soil can be difficult to control in terms of depth and extent of penetration. As yet, the long-term durability of soil stabilization materials remains unproven. There are unlikely to have the same service life as traditional masonry or modern concrete underpinning systems. The duration of any guarantee relating to a soil stabilization technique therefore is unlikely to exceed 30 years. Moreover, it's unlikely to be effective in cohesive soils.

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Responses

  • Kirsty
    What is traditional brick underpinning?
    2 years ago
  • Destiny
    What are the types of underpinning?
    2 years ago
  • Gregory
    How to underpinned strip foundation using traditional method?
    2 years ago

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