In sites of poor ground, piles can be used to transfer the loads from the building into deeper stratum. They transfer these loads via friction, end bearing or a combination of both.
If the piles were to fail, there can be very serious consequences; the most notable local example being an apartment block in Wilton which had to be demolished about ten years ago.
Piles can fail for a number of different reasons; communication is vital between all parties during the construction process, particularly on sites with variable ground conditions.
Failure modes include buckling where the slenderness ratio is high in very poor soils or punching failure where an end bearing pile ‘punches’ through a strong formation into an underlying weak formation.
This property in Cork City, constructed over twenty years ago, is suffering from foundation movement due to the failure of the pile(s) supporting the corner. A trial hole revealed a 800mm*800mm*800mm pile cap supporting a 400*400 ground beam.
The site was located on an obsolete Waulsortian limestone quarry which was made up over the years; hence the piles. Considering that Waulsortian limestone is a geohazard, it may have had something to do with the pile failure.
In modern construction, most if not all internal walls are constructed on traditional strip foundations. Along with other functions, these walls provide support to the roof if it is of traditional ’cut’ construction. One key difference between a ‘cut’ roof and a ‘trussed’ roof is that the trussed roof transfers its load to the external walls only and does not need intermediate support, unlike cut roofs.
In some bungalows with trussed roofs built in the last century, the internal walls were built on the floor slab and constructed in masonry or timber stud. As very little load was applied to these walls, the theory was that strip foundations were not required. This approach has been completed in both one off and estate housing. However, this type of construction requires very good ground conditions and a high standard of construction in order for the floors to perform over the buildings intended design life.
The tell-tale signs for floor settlement in dwellings with trussed roof construction are normally represented by horizontal cracking or gaps at the junction of the internal walls and the ceiling. And this is certainly the case at this property in Co. Kilkenny where there is moderate cracking throughout the property at this particular location. An internal trial hole revealed that the floor was constructed on made up ground full of broken tiles and broken concrete, clearly unsuitable for any type of floor construction.
Tilt is a term that is loosely used in the construction industry but it in fact a very rare occurrence.
When buildings constructed on traditional strip foundations move, the building will become distorted and cracking will occur. This cracking is usually very noticeable. However, in raft foundations, which are much stiffer, the failure mode is quite different as the whole of the building may move as one unit without any obvious signs of distortion. This is the definition of tilt. Of course, if movement progresses, occupiers will notice floors becoming more and more off level or doors swinging open.
The Building Research Establishment, a UK independent research organisation, has offered guidance in relation to acceptable levels of tilt for low rise housing but it is noticeability of 1/250 to 1/200 where problems normally begin when tilt is identified. At this point, investigation works should be completed to determine cause and monitoring considered, particularly if the tilt has surpassed 1/200. If the building has reached a tilt of 1/100, then some form of remedial action is required and feasibility comes into play. Therefore, it is important to address the concerns as quickly as possible.
This building in Co. Clare has in fact reached the value of 1/100 and underpinning works is required. The cause of the movement was identified as a leaking supply pipe in the kitchen. The design approach becomes more challenging as the building is sited in the middle of a terrace.
When building extensions, professionals differ in how the new build is or is not going to be fixed to the original structure. Nowadays, some designers require wall starters which are mechanically and / or chemically fixed to the original blockwork and the installation of a galvanised steel or UPVC movement bead at the junction between both buildings. Others just require the bead. Wall starters are quick and easy to use and allow for very minor settlement or shrinkage of the new blockwork. Of course, if the finish is in brickwork, constructing a seamless joint between both buildings takes a lot more time and effort.
Traditionally, some builders would tooth into the existing blockwork by cutting or breaking out alternate blocks and installing new blockwork to create a continuous course. However, this is very labour intensive and difficult to create a new bond with the original building.
If ever the extension was to fail, and of course the risk should be negligible, it would normally fail at this location. However, in this property, it appears the toothing in on the external leaf was completed so well that when the extension began to move it pulled beyond the toothing in and into the original structure. However, this extension is so poorly constructed, that demolition is the most economical approach.
Limestone bedrock is the most common rock found in Ireland and is formed of the disseminated fragments of shells of calcareous animals that lived on or above the sea floor about 350 million years ago.
Limestone is subject to karst which is solution by rain or groundwater. The most famous example of this is located in the Burren, Co. Clare. Karst landscapes provide many interesting features such as caves, sinking streams and enclosed depressions or sinkholes, known locally in east Waterford as ‘falls of ground’ or ‘breaking ground’. These landforms are a geohazard to construction causing subsidence if not adequately addressed during design and construction.
One limestone type, known as Waulsortian, can be particularly subject to karstification. This rock can be found from Farnanes, heading east through Ballincollig, Cork city and Midleton as far as the coast and from Rathcormack through Tallow and onto Dungarvan. It also crops up (pardon the pun) in other areas such as Cloyne and Youghal (see light blue shading in GSI mapping). The photos of the sinking steam were taken near Cappoquin during ground investigation works for a new domestic dwelling.
Subsidence can occur in many ways such as unidentified sinkholes or punching failure of driven piles. It is also a geohazard for our poor probing rods where we slide along a pinnacle during probing until refusal is met at the base. If you live in any of these areas, contact Substruck for a professional approach to ground investigation.
The provision of adequate surface water disposal is not always paid enough respect in one off house construction. If there isn’t any local stream or drain to divert water to, then a soakaway is needed. Soakaways are large holes, sited away from the building, constructed in a manner to receive surface water and allow it to filter down through the underlying soils over a particular time period. Traditionally, they were filled with rubble or large stones and are still completed today in this manner with great success.
Modular cells are cuboid thermoplastic units, which look like milk crates, that are stacked together to form a type of soakaway. This system is a relatively new alternative and offer the great advantage in terms of a significantly higher void ratio of 95%, about three times greater than traditional rubble or stone. Therefore, the volume of the soakaway can in effect be one-third of the size, creating great savings in resources. Suppliers also provide silt traps which keep the soakaway free of silt and thus preserving its lifespan.
In this property in East Cork, the dwelling was not even provided with a soakaway and the drain was simply diverted from the building and covered over. Needless to say, this system was never going to perform over the design life of the building. The main signs of soakaway failure are blocked gullies which overflow during periods of rainfall. If you have any concerns, please contact Substruck for further investigations.
The location of access points, relative to soil pipes, needs careful consideration during design and construction. Soil pipes and direct connections from WCs discharge to much higher velocities than gullies and whilst carrying solids, pose a much greater risk of deposits accumulating within the chamber due to splashing and thus causing blockages. Furthermore, as some of the discharge to any drain will always be forced upstream, the risk of solids accumulating in the channel must also be considered.
Although Technical Guidance Document H – Drainage and Wastewater Disposal of Building Regulations 2010 recommends that a foul wastewater system should minimise the risk of blockages, it does not provide any further guidance on addressing splashing. In the UK, the NHBC (National House Building Council), the UKs version of Homebond, offers some guidance on the matter.
The NHBC recommends that the primary channel entry connection is used for all high velocity discharges and in some cases in conjunction with swept or long radius bends. However, they refer to the design of the connections rather than the location of the access point itself.
Anyhow, such guidance has not been applied at this property in Cork City and although there are items that should not be entering the drainage system, the result is splashing and the accumulation of deposits and persistent blockages. The only course of action is to change the layout of the drainage locally with respect to the above guidance.
Many older buildings were constructed of thick stonework and lime mortar. This lime mortar is relatively weak and would have been used primarily for bedding as opposed to bonding of the stone.
Lateral restraint was provided by the floor joists, roof and internal spine walls; the floor joists were normally built into the walls but the mortar would break down over time and the restraining abilities would be reduced. In some buildings, you might see restraint provided by buttresses. In modern construction, ceiling joists are mechanically fixed to the walls with galvanised steel joist hangers which provides greater stiffness to the building.
Cracking or bulging walls are the main sign of lateral restraint failure and this property in North Cork was suffering from serious vertical cracking along both gables. The opening up of previous repairs suggested that damage was progressive with movement being compounded at one gable where the stairway was located.
Unfortunately for Substruck, this cracking is not foundation movement related. Remediation should primarily focus on the root cause as opposed to the affects. There are several solutions to arrest the movement; a more traditional approach would be tie bars with pattress plates installed to both elevations to provide enough restraint. These ties may be installed parallel to the floor joists at first floor level.
If you have concerns about any type of cracking, please drop us an email with some photos on firstname.lastname@example.org.
Leaking drains in UPVC pipework is normally attributed to poor workmanship and poor supervision. One of the most common defects associated with such leakage is related to the rubber sealing rings or the absence of.
There is always anecdotal evidence of construction workers removing the seals prior to installation as this accelerates progress on site. Such seals have to be carefully cleaned and lubricated before installation and tested to ensure that they are in fact watertight. During subsidence remediation works at this property in County Cork, we observed numerous seals deposited in the excavations during the original drainage works.
Under normal working conditions such defects may not contribute to significant leakage but however, in the event of a blockage, can readily allow exfiltration for long periods before becoming noticed by the homeowner. The ability to withstand such pressures is one of the primary reasons why drains are tested in the first place.
Wavin pipework was considered the ‘crème de la crème’ of UPVC drainage for many years. The fittings were provided with fixed sealing rings which could not be easily removed or dislodged during installation and the triple seal provided great tolerates to deviation in trenches. However, with the onset of the last recession, these premium fittings have been phased out in order to compete with other brands. The existing brand, known as the economy fitting, the name of which speaks for itself, is no where near the standards of the premium fittings. Extra care is required for such fittings.
Whatever type of fitting we use at Substruck, every piece of drainage is carefully installed and tested during and after installation. Our final tests are always a formality for the many professionals we work with every day.
Sometimes we are asked about the cost of underpinning a house, but we are provided with very little other information. In some instances, when we survey the property, the damage is in fact not subsidence related.
The cost for underpinning a house will be dependant on many factors such as the type of construction, ground conditions, extent of damage, etc. There are so many variables that need consideration it is not always possible to even provide a ‘ballpark’.
In order to provide a firm quotation, we would need some if not all of the following information: structural survey, drain test report, specification and / or ground investigation report.
In most cases, subsidence works will be covered by home insurance, assuming that the sum insured is adequate. However, a policy excess will normally apply which may be in the region of €500-€1000.
Of course, you won’t get any joy for the stress you may suffer due to the long time it may take to settle a claim. This type claim can be very complex and requires different professionals of various disciplines to represent both the insured and the insurance company.
If you do suspect subsidence, the first thing you should do is contact Substruck and we will take it from there. Being there from the start allows us to build up a rapport with our clients and we are never more than a phone call away to provide the necessary advice and support.