The analysis of the make-up of soil and rock, or “geotechnics”, is useful – and even essential – whenever you build or modify structures. For work in an urban setting, geotechnical studies require special considerations. The presence of the public, the proximity of buildings and the existence of public utility infrastructure are among the considerations that the study must take into account. Here is an overview of the particularities of urban geotechnics.
PREVENTION FROM THE GET-GO
Well before any civil engineering works take place in a downtown, the geotechnical engineer has to make sure that the site is suitable, examining the construction plan, visiting neighbouring buildings, determining the condition of the site, assessing different risks, and so on.
These risks include nuisances more common in the urban environment that must be considered: noise and vibrations, discomfort for the residents, possible damages to adjoining structures, traffic problems, etc. Once these nuisances identified, protective measures or preliminary work that will prevent problems along the way need to be decided on and implemented.
Controlling project costs is of capital importance in all types of geotechnical work. Study data is updated, but it often includes a certain level of uncertainty. How sensitive is a building to soil deformations? Are the calculations 100% reliable? Does a structure have to be secured before the work begins? Are ground slopes stable? Numerous questions have to be asked and, based on the answers, costs and cost variants related to possible situations are established.
Estimating the costs of an urban geotechnical project is based not just on the nature of the work but also on all the parameters related to urban issues. Securing existing buildings, defining conditions that are potentially dangerous to the public, estimating the bearing capacity of existing structures – buildings, bridges, overpasses, parking lots, etc. -, recommending corrective measures to protect infrastructures as well as the public: these elements and many more have to be part of urban geotechnical studies. They are also indispensable for controlling project costs.
Geotechnical work in an urban environment
Roughly, the overall objectives of the steps in a geotechnical study are to determine the general and specific conditions of the soil and the infrastructures on a site in order to be able to restore a structure or build a new one with a maximum assurance of safety, productivity and economy.
Then it’s time for the fieldwork. What exactly does it entail?
In the city, drilling and sampling are required in order to obtain accurate knowledge of the soil and its characteristics for the erection of different structures, roads, public utility systems, etc. This drilling and sampling has to be supervised, and supervision is also on the list of urban geotechnical tasks.
Restrictions may also be formulated with respect to planned site grading if they are essential for situating the civil engineering works.
The samples taken during sampling and drilling are sent to the lab for in-depth study. The main laboratory tests are:
- water content;
- Atterberg limits;
- granulometric analysis by screening or sedimentometry;
- undrained shear strength tested by Swedish cone;
- consolidation oedometer tests;
- soil permeability;
- core sample compression tests.
Once the testing is finished, samples and related data must be kept, often for a minimum of one year. The company providing urban geotechnical services therefore needs to have the needed expertise and instruments for the studies, fieldwork and lab tests.