|This subsidence project was completed on behalf of a private client via an ‘escape of water’ peril clause on a house insurance policy. The extension to the rear of a terraced property was found to be rotating away from the original dwelling where cracking of over 8mm had formed through previous repairs between the junction of both buildings. |
The primary constraint in executing the works was access. Not only was the property terraced, but it was located on a busy narrow street in the inner city. Therefore, the only route to the works area was through the front door. Considering these limitations, grundomat piles was chosen as they could be easily and safely installed in such restricted access areas and the required loadings were very low.
Grundomat uses impacts of relatively high frequency but low intensity using a man portable bottom driven cylindrical air hammer. It can be applied where rig access is wholly restricted and/or loads are very low (≤50kN) and/or where the pile lengths will be less than about 8 meters. The piles were driven to refusal and supported a series of reinforced concrete straddle beams constructed beneath the foundation.
All defective drains in the vicinity of the area of distress were repaired or replaced to current standards and tested as per Technical Guidance Document H Drainage and Waste Water Disposal. Hard-standing surfaces were made good locally on completion.
Diagnosing subsidence or foundation movement through a simple present condition survey may sometimes prove difficult without further investigation.
The most common sign of foundation movement is cracking. As a foundation moves, the building will become distorted, particularly around openings in walls.
The construction type of the building can also have an influence on how the building will react to such movement. Foundation movement in this conservatory in Co. Cork has resulted in the window frames becoming so distorted that the glass has cracked!!!!
Extensions to existing buildings are always vulnerable to cracking where they meet the main structure, normally due to shrinkage or differential settlement. Such cracking is normally superficial and is not expected to deteriorate further.
Nowadays, movement joints are placed at specific locations in order to allow for such movement. Most movement joints in Ireland are in fact contraction joints as concrete blockwork will always shrink first. The degree to which shrinkage will occur will depend on a few factors, but particularly the time and conditions to which blocks are allowed cure.
These movement joints, normally placed on the external leaf of a traditional cavity wall, are constructed with a proprietary joint bead to allow movement of ± 3mm. Homebond provide good details for the design and construction of movement joints in different forms of construction.
However, this extension in Capwell, Cork city has failed due to an escape of water from a vitreous clay drain at the rear. Services which are leaking over long periods gradually reduce the load bearing capacity of the soils by removal of the finer particles in granular soils.
Although sometimes diagnosing subsidence through a simple present condition survey may sometimes prove difficult without further investigation, the degree of cracking and evidence of a previous repair strongly suggests that movement is ongoing. Note the failure of the traditional ‘toothing in’ below eaves level. Movement of about 15mm was also noted internally.
This cracking is typical of subsidence in an extension where a vertical crack forms between both components; the crack being wider at the top as the extension rotates away from the original structure; the corner of the extension acting point to foundation movement.
The approach to this problem is to replace the drain and underpin the rear elevation through micropile supported reinforced concrete ground beams.
Homebond identify foundations as ‘the most important structural element of any building and great care great care should be taken to ensure that they are constructed properly.’ Design and construction of foundations must be completed by competent persons and supervised accordingly.
Superstructure cracking is caused by a variety of different reasons but rarely as a result of foundation movement. The measure sought to address this cracking on a commercial property in Kilkenny City by one engineering professional was the installation of a movement joint!!!
However, most proprietary joints only allow for movement of ±3mm as the building expands and contracts over time. If the building exceeds this movement, then there is a more serious problem.
As cracking continued to deteriorate beyond the scope of a movement joint, the Client became more concerned and a second opinion was sought. Crack monitors were installed and confirmed that movement was progressive in one direction.
Substruck Ltd. was employed to complete further investigation in the form of trial holes and ground investigation. Trial holes revealed poorly formed foundations on non-engineered fill material. Dynamic probing and sampling identified the presence of boulders in poorly consolidated fill material to a depth of 3 metres. Probing to further depths was completed to for micropile design.
It was now clear that the building was moving due to poorly designed and constructed foundations and underpinning works was required. This was specified as a series of 12 nr micropile supported straddle and cantilever ground beams beneath foundations along the area of distress.
Substruck Ltd. complete many types of drainage investigation, the nature of which will depend on the client brief or requirements. Generally, there are three types of investigation:
reactive – in response to a problem such as a blockage,
subsidence – investigation into a potential cause (i.e. leaking drains) or to provide information to form part of a scope of works where the drainage will have to be removed and replaced, or
general – in response to a recommendation of a house pre-purchase survey.
Our Phase 1 reports document a non-destructive investigation of the drainage system using one or a combination of direct observation, hydrostatic testing, CCTV surveying / inspection and drain tracing techniques.
On arrival to site, direct observation will tell you a lot about the current condition of the drainage and the standard of work applied during construction before any testing or surveying is carried out. We examine the system to determine compliance in relation to construction and siting of access points, ventilation, trip hazards, traps, etc.
Direct inspection of two vitreous clay drainage systems in Pouladuff and Lower Friars Walk in Cork tells us immediately that sections of each system is wholly not serviceable and will have to be replaced.
The photos show fractured and/or collapsed gullies and access junctions.
Interceptor traps can be found in many Irish properties constructed to up the 1970’s. It is normally located at the final chamber nearest the public sewer and may be provided with a fresh air inlet.
The interceptor trap, or ‘winser trap’ as defined by the Water Research Centre, is a development of the buchan trap; the key difference being that the trap was now constructed in a chamber and access was provided both upstream and downstream. The original buchan trap could only be rodded from ground level through a rodding eye to clean the trap itself. Access to the trap would be provided by an inspection chamber downstream.
Buchan traps were originally used in Victorian times to prevent sewer gases entering the atmosphere or dwelling using a water seal. It was patented by a Scottish sanitary engineer, W.P Buchan, in 1875. Another function of the trap was to prevent (or rather limit) the ingress of our furry friends. However, as the winser trap was susceptible to blockages, many householders removed the stopper to the rodding eye allowing easy access for rats.
Winser and buchan traps are no longer installed due to much improvement in the design and construction of drainage systems, particularly from a ventilation point. Such traps can be a common cause of blockages so it’s worth while keeping an eye on it from time to time.
This trial hole was completed as part of a subsidence investigation project on behalf of Cork City Council in December 2018. It identified a traditional strip foundation founded at 0.95m with a projection of 0.33m-0.44m and a thickness of 0.3m.
The local gully was found to be installed incorrectly resulting in a continual escape of water from the first day of construction!! There was no actual pipe installed between the gully and the hopper!!! The presence of water was evident in the subsoil and leakage could be observed when water was placed in the gully!!!
In many cases, a crowbar is used by the designer to prise the ground locally to give an indication of the bearing capacity of the soil. There isn’t a lot of science to this procedure but if the crowbar can be easily pushed into the soil for its full length, then this would be a cause for concern!!!!!!!!!!
To provide some science to the bearing capacity of the soils, a dynamic probe was employed which showed very little resistance at formation level and to a depth of about 2.5m with no value exceeding N10H=1 before bedrock was encountered between 3 and 4 metres. For Dynamic Probe Heavy, values of N10H=1 in till are indicative of bearing pressures of less than 20kN/m2!!!!!
A window sample showed that the building was founded on very soft SILT; Homebond recommend great care in the design of foundations in such soils.
The design to address this problem was a reinforced concrete raft, needled through the rising walls, and supported by 20 nr R38-500 DYWI® micropiles drilled one metre into rock with a SLS of 100kN, with each needle dry-packed on completion with a proprietary non-shrink grout. All works were designed, constructed, tested and certified by Substruck Ltd.