Information about the sub-soil conditions of a site is necessary for the architect and specialist consultants (structural and civil engineers primarily) to complete the design and tender documentation. The more detailed and accurate the information provided, the better the chances are that the tender documentation will capture site conditions and design/document for it, thus reducing the unknowns and risk for variations during construction.
Evaluation of sub-soil conditions
'Sub-soil' comprises all the various layers of soil and rock beneath the site which may affect the design and construction of the project.
Sub-soil conditions vary dramatically with clays, sands, silts, various forms of rock and combinations of one or more possible on any given site.
These sub-soil conditions react differently depending upon their geographic location and the prevailing climatic conditions.
While architects may be familiar with the peculiarities of a particular location, it is essential to investigate the sub-soil conditions using a specialist consultant at the pre-design stage.
Sub-soil conditions can have a significant impact on the design and construction costs of a building and, in some instances, may be the dominant influence in the structural design of a building.
Outcomes of sub-soil evaluation
The principal reason for investigating the sub-soil conditions of a site is to provide the architect and relevant specialist consultants with information regarding the nature of the sub-soil, its stability, bearing capacity, profile and other relevant facts concerning its ability to support the substructure of the building. The information gathered can be used to prepare construction documents that mitigate claims by contractors for additional costs associated with the construction of the substructure.
Sources of information
Sub-soil conditions can be determined by techniques which include:
- visual examination of the site
- reference to existing geological-survey data
- geotechnical investigations which can include drilling of bore holes, site excavations, analysis of samples and reports on the results together with recommendations on bearing pressures, substructure systems, methods of excavation and precautions which should be taken during excavations
- observation of general sample site excavations
- references to previous uses of the site
- reference to known soil conditions and environmental site conditions
- previous site surveys and existing data
- consultation with local building authorities, contractors, and engineers or others who have worked on the site or sites in close proximity.
Architect's responsibility
It is the architect's responsibility as part of the design process to recommend to the client that the nature of the sub-soil conditions on the site (including possible seasonal variations) are properly and thoroughly investigated by specialists so that the building can be correctly designed. It may be necessary to consider sub-soil conditions on surrounding land in case this may have an effect upon the sub-soil under the proposed building.
In the majority of cases, it will be necessary to engage a reputable geotechnical consultant to investigate and provide a report on the sub-soil conditions The structural engineer should advise or ideally engage the geotechnical engineer to ensure the reports contain the information they require to complete the design. If the engagement of this consultant is not covered in the client-architect agreement, this matter should be discussed and agreed with the client prior to the engagement of the geotechnical consultant.
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