Lessons Learned

ECS Lessons Learned

After over 30 years in the industry, we’ve amassed some great takeaways pertaining to the work we do. We felt this key information was important to share, so in 1996, Lessons Learned was created as a mailed piece sent out to our peers. Now, we are pleased to offer Lessons Learned electronically once a month, accessible on this page or delivered directly to your inbox when you sign-up.

After over 30 years in the industry, we’ve amassed some great takeaways pertaining to the work we do. We felt this key information was important to share, so in 1996, Lessons Learned was created as a mailed piece sent out to our peers. Now, we are pleased to offer Lessons Learned electronically once a month, accessible on this page or delivered directly to your inbox when you sign-up.

Selecting the Correct Foundation System

Geotechnical Engineering

Everyone has surely heard of the “Leaning Tower of Pisa”!  Was the foundation system used to construct the tower correct for the soil conditions at the site?  It doesn’t take a geotechnical engineer to answer that question, but what about your project?  Our philosophy as geotechnical engineers is to provide our clients a foundation system, that provides adequate performance with the lowest overall cost.  First, let’s look at foundation systems and define what we mean by the “correct” foundation system.

The foundation system supports a structure constructed over soil or rock.  A “good” foundation system adequately supports the structure without damage to the structure from underlying movements of the soil or rock.  While you may have constructed a “good” foundation system, was it the “correct” foundation system?  Most people consider the correct foundation system to be the least expensive in dollars that provides adequate support.  However, that is not always true.  In this fast-paced world, time is money and the faster the building is built, the quicker the income stream begins to pay off the debt load.  Therefore, time for installation of the foundation system can be a factor.  Constructability is something to consider as well.  If the local contracting firms are unfamiliar with a technically feasible foundation system, then there may be a premium added to the project after the project starts.

Sometimes a “cheap” part makes other components more expensive.  Is the foundation one that results in the cheapest building “system”?  Spread footings for buildings below ground, into the water table, require continuous dewatering.  This increases operational costs for the building “system”, even though the spread footings appear cheaper.  Similarly, does the “cheapest” foundation have off site impacts that result in higher project costs?  Sometimes the more “expensive” foundation eliminates underpinning, site stabilization, or expensive backfilling.  Successful engineers, architects, owners and contractors must look at the whole building “system” and not merely the cost of the “part” in ultimately selecting the correct foundation.

What types of foundation systems are available?  Certainly the two that come to mind quickly are the most common, spread footing foundations and deep foundations (piles or drilled piers).  The distinction between these two systems is that the spread footings are generally termed “shallow” foundations, because the bearing surface is generally within several feet of the building subgrade, while piles or drilled piers are generally termed “deep” foundations, because the support of the foundation system is usually deep within the subsurface strata below the structure.  Not-withstanding some of the “system” considerations discussed above, deep foundations are usually required when there is a soft layer directly below the building that will cause excessive settlement.

Experienced geotechnical engineers are well versed in a variety of support methods and apply that knowledge to the many ground situations that are faced by developers and owners today.  While it is desirable that every project be supported by spread footings, the reality of the situation is that many of the “good” sites suitable for spread footings have already been developed, leaving marginal to poor sites.

Does the presence of soft soil layers, or even existing uncontrolled fill at the spread footing subgrade automatically result in the need to resort to the (usually) more costly deep foundations?  Not necessarily.  In the last two decades, there have been significant advances in “ground modification” techniques, whereby the existing subsurface conditions are improved to a level where conventional spread footings can be used for support of the structure.  While the ground modification techniques certainly add cost to a project, the cost increase can be less than the total cost for a deep foundation system.  Some common ground modification techniques include; dynamic compaction, vibrated or rammed stone columns, compaction grouting, geosynthetically-reinforced earth mattresses, wick drains with preloading, and chemical modification.  Generally, these ground modification techniques require a specialty contractor.  Once completed, they result in a site condition that allows conventional footings.

The selection of the correct foundation system can have a significant impact on a project.  Cost impacts, both planned and unplanned, can significantly affect the overall outcome of a project.  Generally, the largest risk for cost overruns on a project lies within the subsurface conditions beneath the site.  With a proper understanding of the options available, appropriate levels of subsurface exploration, and an appreciation of the factors that are a function of your site, expenses can be reduced and the “correct” foundation system can be designed and constructed in a cost effective and timely manner.

We hope this Lessons Learned will help you arrive at a practical and cost-effective decision regarding the foundation system for your next project. For additional information, please contact your nearest ECS office for more info and support.

ECS Group of Companies

The ECS Group of Companies (ECS) is an employee-owned engineering consulting firm with more than 2,000 employees providing geotechnicalconstruction materialsenvironmental and facilities consulting services. ECS has grown to over 75 locations and five subsidiaries spread across the Mid-Atlantic, Midwest, Southeast and Southwest. The firm is ranked 68 in Engineering News-Record’s Top 500 Design Firms (April 2021) and 49 in Zweig Group’s 2021 Hot Firms (June 2021).

 

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