Within a couple of months, we will have completed the installation of just under 500 caissons–36” to 66” in diameter–across two projects. Both are urban infill and on third and fourth generation use. One site presents brownfield conditions.
This work got me thinking about the need for added stability on some job sites. When do you need deeper foundations vs. a shallow foundations system?
- When buildings cannot be supported on near surface soils due to potential for excessive settlement.
- When building loads are so high that a shallow footing foundation will not support them.
- When settlement tolerances, as in measured in microns, require a foundation that is unyielding.
This week I visited with two of our project managers involved with these projects and Uday Bhate, a geotechincal engineer whom we first worked with years back on a project using deep dynamic compaction, but that is for another post. Here are 3 of most important action tasks we discussed for successful caisson construction:
1. Subsoil investigation and foundation design.
- Ensure there are sufficient numbers of borings to an appropriate depth to define potential design and construction issues. There is no substitute for good evaluations to determine soil or rock behavior.
- It is best not to make decisions based upon the lowest-cost of sub soil investigation proposals, as this may not provide all the necessary information. As a result, it could raise foundation cost significantly during construction.
- The extent of field exploration is dependent upon the nature of the structure, loads and settlement tolerances, along with the owner’s risk tolerance for future building performance.
2. Plans and specifications.
- There is no substitute for a project specific foundation specification to minimize conflicts and change orders. It is a preemptive strike against delays conflict and potential cost overruns.
- Make sure that the engineers are seasoned with gray hair. It seems during the recession, institutional memory is leaving the industry and some practical knowledge is not being transferred.
3. Point of Caution.
- Like most things, it’s an ounce of prevention. Resolve the documentation required for each cassion before the start of the job.
- Determine any needed documentation up front for earth, rock, test holes, concrete, water conditions, and concrete quantities, reinforcing steel placed and allowable bearing.
- Ensure documentation is signed by the geotechnical engineer, contractor and drill shaft contractor. And more importantly, it should be prepared and signed after each shaft is completed.