Civil Engineering March 2021 | Vol 29 No 2

Geotechnical information is often under- valued within the project cycle. It can be regarded as a simple tick box exercise or a process to get one value – the allowable bearing pressure. While the allowable bearing pressure is a very useful number to structural engineers and is largely used to define the type and size of foundation required, it is only a fraction of the infor- mation that should be considered from the report. It is essential to understand the nature of the ground you are building on so that you can appreciate and plan for the prob- lems you might encounter both during excavation and over the long term. PROBLEMATIC GROUND CONDITIONS If you have a high water table, the vi- ability of your basement floors could be affected. This means that the waterproofing requirements, the crack control and jointing on your walls can become more costly. If your soils have a high heave potential, you might need to replace large areas of material, move to a raft foundation, implement water con- trolling systems, or even a combination of all of these. If you find collapsible soils you need to understand what can be done in the short term. This includes wetting and compacting the ground prior to placing foundations, and exploring the potential for long term localised collapse. When building in shales, you need to understand how they can react when exposed to the air and how water can flow through the rock. This strongly affects the construction sequencing and can have time and cost implications if it is not con- sidered up front as part of the planning. DOLOMITIC AREAS One of the common concerns when building in Gauteng is the possibility of working on dolomite. While many structural engineers are aware of the potential hazards of building on dolomite, not all are aware of the variability of the material or the potential “dolomite tax”. Projects may end up paying this tax to accommodate dolomite considerations on earthworks, foundations, services and occasionally even superstructure. From a structural consideration side, dolomite needs to be accommodated in the foundation and this can be complicated and require numerous design iterations to reach an optimal solution. In addition, the ramifications extend far further than just the foundations, and this is frequently forgotten. As there are limitations on locating services within the ground on dolomitic sites, the above ground structure needs to accommodate considerably more complicated services routing and the trenches, sleeves and voids associated with such. All this needs to be designed for up front in your structure. Collaboration These are just a few of the standard prob- lems one can encounter on sites anywhere in the country. There are numerous poten- tial concerns within the soil profile that, as a structural engineer, you need to be aware of and plan for, even if you do not have the same level of in-depth knowledge that a geotechnical specialist would have. It is not reasonable to expect a structural engineer to simultaneously be a geotechnical expert. That is why, in any large design project, the structural, civil, and geotech- nical professionals must work together as a team. A good geotechnical report provides a lot of information and a springboard from which to progress the design, but a discus- sion between the three parties can go a long way to promoting a more detailed under- standing of the site and alleviating potential misunderstandings. Geotechnical investigations should be required on all sites. The extent and detail of the investigation might vary depending on the nature and size of the planned development, but even the simplest single- story structure can be rendered useless within a year by a misinterpretation of the underlying soils. We cannot avoid problems under the ground, but clear and detailed geotechnical information is the best way to plan for surprises that might occur. Money spent on geotechnical investigations frequently leads to money being saved during design and construction. Ultimately, good geotechnical information minimises the negative conse- quences of problematic ground conditions and enables projects to run smoother.  The value of good geotechnical information within structural design Kim Timm Pr Eng Executive Structural Engineer AECOM As a structural engineer, one of the most vital pieces of information you can have on a project is a good geotechnical report. What happens below ground can shape the entire building and project. If this is not considered at the start, you are likely to encounter numerous surprises as you start to excavate, which invariably lead to time delays and cost overruns. Civil Engineering  March 2021 33