By Gordon Reid
Last month I gave a presentation to over 100 members of the Royal Institution of Chartered Surveyors on the topic of ‘Structural Movement in Buildings’
The talk was well received, and this was evidenced by most of the attendees remaining awake throughout, as well as the feedback received afterwards, so I thought I’d share some of the key points on this blog.
The subject matter covers such a wide area that I decided to review the most common courses of movement and hence damage to properties.
- Inadequate foundations
- Volumetric changes
- Point loading
- Embankment stability
- Lack of lateral restraint
- Thermal and moisture related movement
- Dissolution features
When inspecting a property in attempt to ascertain a cause its essential to thoroughly recorded the evidence with experience and be able to recognise patterns of cracking. These can help to point towards particular causes. to give a guide, following simple rules of thumb are helpful when making an initial appraisal.
Diagonal cracking – most often a sign of downward or upward movement.
Diagonal crack wider at the top – symptomatic of downward movement (subsidence at settlement)
Diagonal cracks wider at the bottom – symptomatic of upward movement (heave)
Vertical cracks – most commonly associated with horizontal movement (thermal and moisture related)
Bulging and bowing – regularly associated with lack of lateral restraint
Horizontal cracks – routinely associated with corrosion (of wall ties for example)
I include a link to my presentation which gives more detail about all of these matters. When assessing damage, it is so important to ascertain the cause prior to implementing any repairs. It’s equally important to determine whether or not the movement is progressive, cyclical or has stopped. Only when these key points have been totally understood is it sensible to implement repairs.
As the presentation was in the North East which has a proud mining history this was a subject of particular interest. I ran through a case study of an investigation I undertook a few years ago. A terrace of a dwellings has experienced severe damage to the point where it had to be evacuated. Downward movement in the centre of the terrace was recorded at more than 300mm. After researching all of the possible mechanisms which could be responsible for such movement I finally decided that the damaged had been caused as a result of deep seated movement associated with extensive mining activity in the 1950’s.
The terrace was underlain by a coal seam approximately 1m thick at a depth of 42m below ground. I determined that the seam had been worked and obtained the abandonment plans. The plans showed that workings had been undertaken by pillar and stall techniques whereby pillars of coal were left in place to support the ground above and the remaining coal around these pillars was removed. Our analysis indicated that the cause of the subsidence was attributable to failure of one of these pillars beneath the property.
By good fortune open cast mining activity had just started in the same seam less than half a mile away. I visited the open cast site and determined that the actual pillars in the workings were in several locations smaller than shown in the abandonment plan and therefore prone to overstressing and potential collapse. The issue became a matter for the Land Tribunal and at a hearing the Tribunal decided they agreed with my assessment of the cause. The owners of the properties were fully reimbursed the full market value of their properties and the houses were subsequently demolished. I don’t often come across cases where the structural movement is quite so extreme. It does show, however, that it pays to keep an open mind when investigating the cause of damage.
My colleague Neil Dely has just presented on the same topic in Dunblane. We’ve also got five more talks later this year where we will be presenting on solutions to some of the defects we’ve highlighted here. It’s always better to focus on solutions rather than problems. For more information on the talks planned, please click here
Image: ©Paul Vincent