Underpinning may be defined as the process of strengthening and stabilising the foundation of an existing building or structure.

Underpinning

Underpinning may be necessary for a variety of reasons, the most common of which are:

  •      The existing foundation is not strong or stable enough.
  •      The usage of the structure has changed.
  •      The properties of the soil supporting the foundation may have changed or may have been mischaracterised during planning / construction.
  •      The construction of nearby structures necessitates the excavation of soil supporting existing foundations.

The oldest and most well known form of underpinning is ‘Traditional Underpinning’.  Traditional Underpinning stabilises the existing walls by excavating beneath the existing foundations in a ‘Hit & Miss’ fashion, to a depth where firm load bearing strata exists.  The excavated material is replaced with mass concrete and can be reinforced, or un-reinforced depending on the application.

Traditional Underpinning is usually applied when the existing foundations are at a shallow depth.  It is economical if load bearing strata can be found at a depth up to 1.5m.  If load bearing strata is found to be at a depth greater than 1.5m then a minipiled solution will be more economical, safer, less obtrusive and faster.

Due to advances in minipiling installation and minipiled underpinning techniques traditional underpinning has become less common.

Larsen Foundations Ltd offer several cost effective minipiled underpinning solutions.  We offer a surveying service and our panel of professionally qualified engineers can provide cost effective underpinning design solutions. 

Our most popular restricted access minipiled underpinning solutions are outlined below.

 

Simply Supported Pile & Needle Beams

This method supports the wall / foundations by installing piles at either side of an existing wall (internally & externally) or at either side of a corner (both piles externally).

A pocket is created either in the masonry, above the foundation (if site dimensions allow), or beneath the foundation (if the depth from ground level to top of foundation is shallow – less than 400mm).  A cage of structural steelwork is inserted through the pocket and placed over the piles.  The needle is then cast in-situ using high strength concrete.

The advantages of this system are:

  • Suitable for restricted access situations.
  • Considerably less excavation than for a traditional underpinning solution, thus a reduction in material to be taken off site.
  • High load capacities possible, depending on the diameter of the piles installed and the spacing of the needle beams.
  • Quicker than traditional underpinning.  The needles are spaced at intervals and there is no requirement for a ‘hit & miss’ approach.  
  • Depending on the walls condition, the needles can be at up to 1200mm centres, this reduces site time, plant and labour costs.

The disadvantage of this system is:

  • The requirement to have piles both inside and outside means excavation and disruption both internally and externally.  It is likely that the homeowners would be required to vacate the premises during the works.

For an example of Single Pile Needle Beams please click here.

Cantilever Needle Beams 

The construction of the cantilever needle beam is as per the simply supported needle beam.  It varies however as both piles are installed either externally or internally (usually externally).  The pile installed closest to the wall is a compression pile and the pile furthest away from the building is a tension pile.

The advantages of this system are:

  • Suitable for restricted access situations.
  • Considerably less excavation than for a traditional underpinning solution, thus a reduction in material to be taken off site.
  • High load capacities possible, depending on the diameter of the piles installed and the spacing of the needle beams.
  • Quicker than traditional underpinning.  The needles are spaced at intervals and there is no requirement for a ‘hit & miss’ approach.  
  • Depending on the walls condition, the needles can be at up to 1200mm centres, this reduces site time, plant and labour costs.
  • The cantilever needle can be created externally allowing the owner to remain in the property whilst works are being carried out.

The disadvantage of this system is:

  • The external working space requirement, perpendicular to the wall, is greater than for the simply supported needle beam as a cantilever needle beam can be up to 2.0m long, depending on the load to be carried.

For an example of Cantilever Needle Beams please click here.

Single Pile Cantilever Needle Beam

A single pile cantilever needle beam is a shorter version of the cantilever needle beam, that relies on the tension capacity of one pile.  It can be installed either internally or externally (usually externally), within a space of 750mm from the wall to be underpinned (including rig access)

The advantages of this system are:

  • Considerably less excavation than for a traditional underpinning solution, thus a reduction in material to be taken off site.
  • High load capacities possible, depending on the diameter of the piles installed and the spacing of the needle beams.
  • Quicker than traditional underpinning.  The needles are spaced at intervals and there is no requirement for a ‘hit & miss’ approach.  
  • Depending on the walls condition, the needles can be at up to 1200mm centres, this reduces site time, plant and labour costs.
  • The single pile cantilever needle can be created externally, allowing the owner to remain in the property whilst works are being carried out.
  • The single pile cantilever needle can be constructed within a space of 750mm making it ideal for highly restricted access situations.

The disadvantage of this system is:

  • As the system relies on a single pile, the load bearing capacity of the needle is less than the simply supported needle or the cantilever needle.
  • Due to the reduced load capacity there is a requirement for more needles at closer centres.

Internal Piled Raft Underpinning System

This underpinning method supports existing walls to individual rooms or entire buildings.  Minipiles are installed internally using low vibration piling equipment and the piles are incorporated into a series of needles, which are in turn incorporated into a new reinforced concrete structural floor slab.  The needle beams project from the raft slab at intervals into the internal and external load bearing walls below ground level.

Depending on the height of finished floor level, in relation to finished ground level, we can offer a dropped edge raft solution that ensures the ends of the needles are not visible from outside.

For highly loaded structures the spans between the needles are reduced and the needle reinforcement increased.  Multi storey buildings of solid masonry construction can be underpinned up to 6 storeys high.

The advantages of this system are:

  • The system provides a new floor slab upon completion of the underpinning works.
  • The system is carried out from the inside only and removes the requirement to cause disruption to external paths, gardens, drainage etc.
  • Where the property to be underpinning has suspended timber floor construction, a minimum amount of excavation may only be required thus reducing construction time.
  • Timber floors can be replaced with solid concrete floors which can be left at a reduced level to facilitate the installation of insulation or the provision of an under floor heating system.

The disadvantage of this system is:

  • The work is carried out from inside only and generally requires the homeowner to vacate the property for the duration of the works.

For an example of Internal Piled Raft Underpinning please click here.

Pin Pile Underpinning System

Raft foundations and internal raft floors are common practice, however when they fail can be difficult to support.  It is a common mistake to support the external perimeter of a property constructed off a raft slab, as no allowance is made for the ability of the raft to span internally.  Differential settlement can be induced as the raft is supported around the perimeter, but does not have the ability to span the length and breadth of the property internally, as the ground beneath the slab is no longer capable of offering support.

Larsen Foundations “Pin Pile System” solves this problem by installing piles through the raft internally, at a grid of centres, determined by the reinforcement in the slab and calculating the rafts ability to span between pile positions.

Holes are diamond cored in the raft and pile caps created beneath the slab.  The size of the pile cap depends upon the diameter of pile used and the loading requirements.  The pile is installed through the core and cut off within the pile cap.  The pile, pile cap and cored hole are then grouted.  Stability is provided by new pile cap and the bond created between the core and the grout.

A Larsen liquid Damp Proof Membrane is poured over the stabilised raft and the deviation from level made up using one of Larsen’s self levelling compounds.

Find out more about these and other Larsen products here.

The advantages of this system are:

  • No requirement to remove the raft floor thus undermining the internal walls.
  • The raft is supported uniformly which minimises differential settlement.
  • Depending on the thickness and reinforcement in the slab the pile centres can be large thus reducing the number of piles required.
  • If necessary the raft can be jacked off the piles to minimise the severity of the subsidence.

The Disadvantages of the system are:

  • The internal fixtures and fitting s must be removed and the floor levels reduced to expose the top of the raft slab.  This means that the homeowner must vacate the premises during the underpinning works.

For an example of Internal Pin Piling please click here.

For more information please contact your local Larsen Office.