Traditional Underpinning

Underpinning is a method of creating support below the ground for an existing structure. This is required when a structure is either sinking or is showing signs of subsidence. These can appear as cracks in the masonry or concrete of the structure. Underpinning can also be used to create a void below an existing building, or to reinforce existing buildings.

A small area below the existing foundation or wall base is excavated, then filled with concrete. We dry pack with non shrink grout, before excavation of the next section is commenced. This can be done in several horizontal layers, one below the other. It is like building an underground concrete wall, from the top down, whilst supporting the structure above.

Underpinning is accomplished by extending the foundation in depth or in breadth so it either rests on a stronger soil stratum or distributes its load across a greater area. Mini piling methods are used in underpinning.

Underpinning may be necessary for a variety of reasons:

• The original foundation is simply not strong or stable enough, e.g. due to decay of wooden piles under the foundation

• The usage of the structure has changed

• The properties of the soil supporting the foundation may have changed (possibly through subsidence)

• The construction of nearby structures necessitates the excavation of soil supporting existing foundations

• It is more economical, due to land price or otherwise, to work on the present structure's foundation than to build a new one

Pile & Beam Underpinning

This method stabilises wall foundations by using piles installed either side of an existing wall. A small excavation is made below the ground level and a reinforced concrete needle beam is used to connect the piles and support the wall.

Reducing the distance between needle beams can accommodate very high loads. However, the bearing capacity of the underlying strata will determine the number, diameter and spacing of piles used.

This system is used where traditional underpinning is not appropriate due to the existing foundations being too deep, or that good bearing strata is so deep that it is uneconomical to dig. Piles are installed in pairs at 1.0m - 1.5m intervals and approximately 1.0m - 1.5m apart.

Advantages of this system are:

• Suitable for restricted access

• Needle beam constructed at a higher level if existing foundation is too deep

• Faster than traditional underpinning

• High load capability

• Less disruption, less spoil generated and completed quickly