What is Underpinning? - Part 1 - Traditional Underpinning

There is not a single method of underpinning which will be suitable in every circumstance and to be effective, careful considerations of things such as dead and imposed loads, ground conditions, depth of excavation, site restraints including access and ultimately costs will help to determine the most appropriate method of underpinning to use

Source: http://www.davocal.ie/

I have previously written two articles on the subject of ground movement a term more commonly referred to as subsidence, Subsidence – Part 1 – Not all it’s cracked up to be! and Subsidence – Part 2 – Factors that contribute to subsidence and what to look for. The first article explains how the very word (subsidence) seems to strike fear into most people due to the misconception that ALL instances are both disruptive and expensive to rectify.  Whilst this is true in some circumstances, in the vast majority of cases subsidence can be rectified reasonable quickly and without excessive expense.  In order to establish what correct remedial measures are necessary it is crucial to accurately diagnose the factors causing any episodes of ground movement and whether movement is continuing or has stopped.  In my previous articles I explained that when I first graduated from University I joined a Property Consultancy who’s main area of business was dealing with subsidence insurance claims as Loss Adjusters.  My role was to visit site with a more experienced Structural Engineer, who would make an initial assessment, and then I would be required to manage the claim to a conclusion.  This often involved crack monitoring to establish whether any movement was historic (had now stopped) or progressive (was still continuing).  This was a crucial part of the process as it is pointless dealing with the effect of subsidence until movement has been stopped.  On the occasions where movement was found to be progressive, it was often first necessary to undertake substantial remedial work such as underpinning to stabilise and stop any movement.  In this article I plan to explain what underpinning is so that anyone who is affected by subsidence where underpinning is suggested will have an understanding of what is happening.

So what is underpinning?  Well in simple terms underpinning is a method of strengthening weak foundations whereby a new, stronger foundation is placed beneath the original.  The whole concept of underpinning can seem mind boggling when you consider that this often involves excavating underneath of an existing foundation to strengthen whilst at the same time ensure that the building or structure that is being supported does not disappear even further into the ground!  There are a number of different methods of underpinning including traditional underpinning, needle beam underpinning, cantilever beam underpinning and an underpinning raft. The method of underpinning selected in each situation will be determined by a number of different factors, which I will discuss in this and in next week’s article.  There is not a single method of underpinning which will be suitable in every circumstance and to be effective careful considerations of things such as dead and imposed loads, ground conditions, depth of excavation, site restraints including access and ultimately costs will help to determine the most appropriate method to use.

Source:   Source: Chudley R. & Greeno R (2005), Building Construction Handbook

Traditional Underpinning - This method stabilises existing foundations by digging under the present foundation in sequenced bays to a depth where firm strata exists and replacing the excavated material with mass concrete.  This method of underpinning is used when the existing foundations are at a shallow depth.  ‘Bays’ are usually excavated in 1.0m to 1.5m in length and generally 0.6m wide.  This method can be used to depth of circa 2.5m however due to cost an safety issues it may be worth considering a ‘mini-piled’ solution for depths in excess of 1.5m, something that will be discussed in next weeks article.

Advantages of traditional underpinning compared to other methods is that is simple engineering and generally easily understood, suitable for heavy loads and large structures, occupants can remain in the property as work can be undertaken from outside, method can be used in restricted access areas (does not require plant or machinery), it is a low cost solution at shallow depths and it creates minimal disruption and is not noisy.

Source: http://buildingandengineering.blogspot.co.uk/

You can see from the diagram on the right that each bay is excavated in sequence and then filled with concrete, which is allowed to cure (harden), before the next bay is excavated.  This is crucial to the stability of the building/structure during the process.  Once the concrete has cured the next bay in sequence is excavated, filled with concrete and cured and so the process continues until all of the bays are complete.  This method therefore provides a new deeper foundation over a short period of time but not in one operation. In order to carry out traditional underpinning it is first necessary to understand the ground conditions (via a ground investigation) to know what depth to excavate for solid ground and the to plan the sequence for the underpinning bays.

Although traditional underpinning is the simplest and most commonly used method of underpinning it is still necessary to obtain professional advice in order to ensure that the selected method of underpinning is designed and completed correctly.  It is also necessary to engage a contractor who is familiar with traditional underpinning and will take into account the various safety aspects of providing temporary support where required, supporting excavations where necessary, working in confined spaces and so on.  Excavating a bay and filling with concrete may seem a very simple process to most people and you may wonder how anyone could get this wrong, however never be surprised with the fallibility of human nature.

A common example I give to my students relates to my previously explained role as a graduate Building Surveyor, working with Property Consultancy who’s main area of business was dealing with subsidence insurance claims as Loss Adjusters.  Late one evening we received a call to a mid-terraced property in the Moseley area of Birmingham from a man who had contacted his insurance company (who then contacted us), where he had arrived home to find a problem with his house.  He had left his house in the morning as usual, shut his from door and proceeded to work.  When he arrived home he put his key in the door, turned the lock the door would not open.  After a number of attempts he promptly shoulder barged the door, which eventually opened.  When he got inside the house he realised that the front door was the least of his problems. There was major vertical cracking in three locations on the party wall (a shared wall with his neighbour), which was so bad he could see through to next door's lounge, the suspended timber ground floor had significantly dropped near the junction of the party wall, there was a 50mm horizontal crack at the junction of the top of the ground floor lounge wall and ceiling and all of the ground floor internal door frames were distorted and none of the internal doors would close properly. Not exactly what you want or expect when you come home from work!  On investigation it was established that his neighbour was in the process of an extensive refurbishment, which included underpinning of the party wall between the two properties. For reasons which we never got to the bottom of the contractor excavated underneath the party wall in one operation, rather that in sequenced bays, and unsurprisingly the whole party wall had dropped as the excavation was part way through!  How somebody was not seriously injured or even killed was a miracle.  The cost of rectifying the works and in addition the costs of temporary accommodation while the work was being carried out were calculated in the region of £75,000.  This demonstrates the significant impact of what can happen if underpinning is not carried out correctly.

In next week's article I will discuss other methods of underpinning such as, needle beam underpinning, cantilever beam underpinning and an underpinning raft.

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