In our modern world, we typically take for granted that buildings and other structures were constructed on stable foundations. But many of our cities and towns were established in places where the soil may be soft and unsuitable for supporting structures and heavy loads. Lessons have been learned the hard way when structures of the past sank on these soft soils.
Urban centers like Boston, Seattle, and New Orleans have all experienced drastic settlements, and had to rebuild or underpin buildings. Why? They were all built in areas where various layers of sand, silt, clay, and organic materials have been deposited over millennia. Tidal pools, river valleys, lakes, bayous, bogs, and marshes can all play a role in this.
For various reasons, buildings and other infrastructure often need to be sited in these places. For instance, wastewater treatment facilities and pumping stations are often located in low-lying areas, riverbanks, and coastal areas where the soil and groundwater conditions can be problematic. Port facilities are another example; they are typically constructed in the water, and along the shoreline where the soils tend to be wet and unstable. The underlying soil deposits in these areas can vary widely depending on the geologic history of the site.
How can we avoid the mistakes of the past?
Engineers must have a clear understanding of the underlying soil profile to properly design and construct foundations in these areas. A thorough soil investigation should always be performed at these sites. The need to understand the soil profile and groundwater conditions cannot be overemphasized. A good soil investigation should include multiple borings advanced to adequate depth to understand the underlying soil profile, and if feasible, the depth to bedrock. Soil samples should be collected at regular intervals during drilling, and should be classified, recorded, preserved, and taken to a lab for further analysis. Groundwater conditions should be observed and recorded. A licensed geotechnical engineer should prepare a well-written report of the site’s soil conditions and underlying geology.
The foundation engineer must read and understand the geotechnical report prior to selection and design of the foundation system. The engineer must carefully weigh all factors that could possibly influence the foundation design. Some of these factors may include type and magnitude of loading, cost, constructability, potential settlement or sliding, earthquake, flooding, depth to bedrock, corrosion, etc. The foundation engineer should then bring forward the two or three foundation alternatives and perform a cost comparison.
Foundation contractors are a good resource for information and assistance with cost estimating. But the engineer should request input from at least three different contractors, because each will have a different opinion based on their resources and experience. It’s up to the foundation engineer to decide which system is most practical and cost effective for the project.
Foundation engineering is not an exact science; rather, it is a carefully considered strategy. Project stakeholders should be educated on the importance of a thorough geotechnical investigation and continued collaboration with the geotechnical engineer throughout the project. The foundation designer must have a good understanding of the geotechnical report and must consider potential alternatives. Specialty foundation contractors should be consulted for constructability and cost. By following these guidelines, the project stakeholders can provide a solid foundation that avoids the mistakes of the past.
