It’s well known within the survey industry that underground mapping surveys completed at the pre-demolition stage of a project yield all-round benefits. However, in our experience many clients don’t understand the importance of the survey work being completed pre-demolition. This can result in demolishing more than just their site, but also the quality of their survey results.
A key part of a utility survey is being able to understand the site and its prior use. Knowing the environment that the surveyors will be working in is critical. Whether it’s the project estimating in the office or undertaking the survey itself. The site could be a relatively low-risk greenfield site, or a much higher risk site such as a hospital. But knowing what they will be working with will dramatically improve the quality of the survey deliverable.
What are the benefits of a pre-demolition survey?
At the pre-demolition stage the site information is only available whilst it has been untouched. Simply by walking the site, a surveyor can use reasonable judgment to build a picture of what is underground. This would be from identifying features such as manhole covers, ground scars and lamp columns. Even identifying frosted glass on buildings (indicating a toilet block) suggests there will be evidence of foul drains in the area. These factors should be considered, as they will be lost after demolition. The benefit of this information is that more detail can be added to the final deliverable. This gives the client a better picture of what’s going on underground and reduces the risks associated with the next stages of construction.
Why are people still commissioning surveys post-demolition?
Many clients tell us that the cost of a post-demolition survey can be lower than that of a pre-demotion survey. However, it is our experience that these clients are often unaware of the actual reason for this reduction in cost. It is because a post-demolition survey limits what is provided to the client. Unlike a pre-demolition survey where the surveyor can deliver drawings which show a network of pipes and cables with detailed references; it is often the case in a post-demolition survey, that the client will receive a limited drawing containing spurious findings. As well as this, large numbers of unknown buried features can be due to the lack of information obtainable from a post-demolition site.
As such, SUMO always encourages clients to have utility survey work completed pre-demolition. This mitigates the significant risk associated with features that haven’t been located.
The equipment and working on post-demolition sites:
There is a common thought of ‘why do you need to visit my site before demolition, can’t the equipment still find buried features, post demolition?’ Well yes, in theory it can, but only if the remaining ground conditions are suitable. However, the likelihood is that the ground conditions will become severely limited as soon as the demolition has occurred. Thus, increasing the risks associated with the next stages of the project.
There are a number of reasons why there could be equipment limitations on a post-demolition site. Including:
The equipment may be unusable due to the lack of infrastructure. Manhole chambers may have been buried and lampposts destroyed, so in these circumstances, there would be nothing for the surveyor to clamp Electromagnetic Location equipment to. In these site conditions where the equipment relies on infrastructure being present, the surveyor will be limited to carrying out a much less effective ‘free hand sweep’ with poorer results.
The post-demolition ground conditions may have become unsuitable for many types of survey equipment. This could the use of Ground Probing Radar (GPR). This is is essential for finding non-metallic pipes and cables. They require a smooth and flat surface to produce optimal results, limiting the survey further.
So, what are you risking by getting your survey completed post-demolition?
It is likely that planned demolition sites are going to be re-developed. Therefore, the ground opened up during later building processes. By taking unnecessary risks to save on initial survey costs, you may end up spending considerably more over the course of the project. This could mean potential fines from hitting buried utilities. Other risks also include:
Personal injury - this could range from minor to fatal incidents.
Consequential loss claims due to downtime.