What Went Wrong? Terrace and Mezzanine Failures in Landscape Projects
This is the fifth in a series of articles, showing that it is easier to learn from someone else’s mistakes than your own, each following a similar format, analysing project failures as recorded by expert witnesses commissioned to produce court compliant statements for use by the court system.
All locations and names have been anonymised to avoid conflict. All cases shown have been settled through proceedings.
The same modus operandi – how the analyses are conducted is used throughout.
All expert witness comments are made using either Industry Best Practice or British Standards where they apply when writing reports in the absence of any documentation or working practices mentioned as specification by a contractor.
TERRACES AND MEZZANINES – Building Regulations and BS7533:101 and 102 may apply, also Defective Premises Act 1974.
The most common complaints and reasons for project failures in terrace and mezzanine construction stem from the same two errors in design and installation involving a) sub-base and laying base preparation, and b) failure to ensure that the retaining wall supporting the project has been properly constructed to prevent lateral movement in the area between the foundations and vertical structures.
If the retaining or supporting wall has shifted, the infill material will spread to fill any gaps in the volume or space under the horizontal surface, leading to collapse or subsidence.
This subsidence will result in breaking any bonds between the decorative surface – e.g. paving slabs – and the laying bed, causing cracking in the various layers, which allows water to enter the surface and create further subsidence due to washing fine material in the sub-base down to the bottom of the raised area.
In most cases, the only issues will be with loose slabs, with some ponding on the surface due to minor subsidence, but in other, major collapse may occur due to the deteriorating nature of the loosening of the infill material as the fines progress downwards.
Where a slip-plane occurs between the walling blocks used to raise levels and the supporting concrete foundations has been compromised by the use of solid concrete blocks laid flat, with no key securing them in place, the whole wall may shift sideways due to compressive weight and the effects of vibrating machines used to compact the infill material.
The cross-sectional drawing shows hollow concrete blocks laid with reinforcing bars set into the concrete foundations and into the hollows in the blocks, which are filled with rammed concrete, thereby preventing any lateral movement.
It also shows the method of laying a concrete foundation under the paving units, which is tied onto and into the retaining wall, which prevents any subsidence from taking place.
Other causes of failures in terraces and retaining walls involve the absence of internal tanking (waterproofing the back of the walls), lack of adequate drainage and weep-holes to prevent water build causing hydrostatic pressure from becoming a major problem.
Case One – Detached property in Hampshire
This was a large property, on heavy clay soil. Retaining walls had been built at various levels across the garden, with areas of terracing serving different parts as patios and mezzanines.
The retaining walls had been constructed using solid concrete blocks to a height of 1200mm, which had slipped from their foundations, causing the infill material to subside over the course of a few months.
The walls were rendered, and lacked any weep-holes. As the paving began to settle and become separated, so more and more rainwater entered the base, exacerbating the collapse.
The image shows the retaining wall to the left of the photograph, with the settlement clearly shown by the gap under the spirit level. Notice too, that the bubble is showing a fall away from the edge and towards the house walls. Every one of the paving slabs – over four hundred metres – was found to be loose and debonded.
Case Two – Birmingham
This was a semi-detached house in Birmingham, with a long sloping rear garden, which had been terraced into three separate levels by use of timber ‘sleepers’ by a building contractor.
All three ‘walls’ had failed due to poor construction and reliance on untreated timbers that were only 100mm wide. Each wall had been built ‘stack-bond’ with no horizontal bonding at all – simply one timber placed on top of the other, and secured with two timber screws and 100mm2 vertical backing supports.
The image shows the upper level, which had been laid to concrete paving slabs 600mm x 600mm x 35mm, laid over a weak building sand and cement laying bed.
All of the slabs were loose, having no key to their base, and were spreading as the timber wall shifted forwards, out of upright, causing the infill material to subside.
The whole project was literally, falling apart, and becoming dangerous. This is not a case of What Went Wrong? There was no part of this project that met any standards in landscaping or building.
The owners had arranged for the garden to be landscaped in order to sell the property and move out of the area. As a result of the work, the house was unsaleable.
The contractor closed their business, and went into voluntary liquidation.
Summary
All projects involving retaining walls and raised platforms require careful planning and evaluation before commencing work on producing quotations.
Without accurate information and full specification, it is not possible to price a scheme.
Evaluations and calculations should be made by a qualified and insured structural engineer (SE) for all retaining walls over 600mm to be safe when providing a quotation. Whilst you do not need to engage an SE for walls at 600mm, you can follow standard industry guidance (Good Building Guide 27), without specific engineering qualifications, although you will still need to work out quantities of materials.
Approximately 20% of disputes include an element of subsidence in raised paving projects, even those only a few centimetres high due to subsidence and lack of proper retention of the infill material.
