Soils vary from one property to another. In some locations such as Toronto’s west end near High Park, soils are typically very sandy. This provides good drainage, but usually low bearing capacity. Conversely, locations in Toronto’s Beach neighbourhood typically have dense, clay-filled soils which support structures well, but also retain water.
Soils-testing is conducted by a professional geotechnical engineer. He will make recommendations for foundation type, sizes and details. Similar to legal surveys, soils testing or other geotechnical investigations are considered ‘existing conditions.’ An architect or structural engineer cannot directly engage a geotechnical engineer on behalf of an owner as they cannot take liability for the conditions of the property prior to the new work. However, an architect may help arrange, evaluate and review the geotechnical investigations in assistance to the owner.
Generally there are two options for home owners as to when to gather this information: pre-construction and during construction. For large new homes, previously undeveloped land, multi-family units or large residential developments, geotechnical investigations are recommended before construction starts. This information is sometimes gathered even prior to engaging the architect.
Prior to construction the geotechnical engineer will use a boring machine to dig test holes at various locations on the property - typically in the approximate location of the proposed new house or development. This test will provide three results: 1) it defines soil conditions, types and classifications, 2) it determines the bearing capacity of the soil, and 3) locates the height of the water table at time of the test. The machines used to dig these test holes are typically very large. The smallest bore-hole drilling machine is 30” wide and 12’ long making maneuvering very difficult in small spaces (e.g. between existing houses). This machine also has limited power and if the property is known to have much clay or rock present in the soil this machine will not work as it cannot dig through the dense or hard materials. An even larger machine will be required.
For this type of test, the samples removed from the holes are sent to an accredited soils testing laboratory for analysis. Results are sent back to the engineer for interpretation and recommendation of foundation types and sizes. These recommendations will be presented in a report. The whole process of conducting this type of pre-construction test can take 4 to 5 weeks or even longer. The mobilization of the crew and machinery can take 1 to 3 weeks, while drilling usually takes only 1 day for a small property. For a small residential property this process starts at approximately $3,000.
For the typical urban house, this is not necessary. For renovations, additions and new construction on existing sites the easiest method of soils testing is for the bearing test to be done at the exact location of the foundations during construction. This is facilitated by the contractor who has already mobilized and is getting ready to pour foundations. The contractor digs test pits either in the location of the footing or in the case of an existing house, inside the existing basement through the floor closest to where the addition will be placed. In the case of this test, the geotechnical engineer will determine the bearing capacity of the soils and make recommendations for any revisions to the footings as they have already been designed. If revisions are required the geotechnical engineer – or the original structural engineer – can make revised footing details that are stamped for City approval. This method ensures that construction is not delayed.
For testing during construction, a geotechnical engineer needs only 24 hours’ notice to arrive on site. The testing is done in situ and a report confirming the findings is released following another 24 hours. As the contractor has already dug the holes, the cost is much less. Usually in the range of $350 to $400.
Soils testing is an important part of the design and construction process for any residential project. Whether it is undertaken before or during construction depends on the size of development, access to the property and relative risk of unknown conditions. An architect may assist with this process to help the owner determine what type of testing is right for them.
If you are interested in learning more about geotechnical soils testing, or have any questions about the process, feel free to drop us a note.
- All stated costs are approximate based on rates in the City of Toronto as of June 2014. Confirm exact costs through written quotations with your selected geotechnical company.
- All time lines described above are approximate. Confirm actual schedules with the geotechnical company undertaking the work on your project.