On Solid Ground
Nov 1, 2001 12:00 PM, By Mark Loes
Most commercial buildings begin to show their age after 20 years. At that point, owners must renovate to maintain appearance and functionality or sell to someone else who will renovate or use the building for another purpose.
That is not the case with educational buildings. K-12 buildings, as well as college and university structures, must endure from 30 to 75 years, despite modest and sometimes unpredictable maintenance and renovation budgets.
Add to this the fact that educational buildings provide functions that go far beyond what is expected of commercial buildings. For example, many school structures enclose swimming pools, support long-span roofs for gyms, move across varying elevations and accommodate shifting load conditions, including any number of double-loaded walls. Such designs require information on how the soil and bedrock will respond to these extraordinary foundations and structural features.
School buildings have unique requirements with regard to the number of underground utilities. Extra sanitary and water lines are needed for the numerous bathrooms in schools. Buss duct lines for the electrical, cable and Internet access lines are required for classrooms and offices. A commercial building has the option of placing some utilities under raised floor systems. Schools typically bury the utilities under slabs that could affect adjacent foundations and floor slabs.
The special performance characteristics of educational buildings demand the sturdiest of foundations, in terms of the building structure itself and the ground beneath the structure. Ensuring the foundation of an educational building requires comprehensive geotechnical services that precede and extend through the construction process.
Unearthing potential problems
Educational administrators preparing for new rounds of construction need to understand the critical role that geotechnical services have come to play in modern construction.
Five years ago, for example, a school district expanded several of its campuses, adding large new building wings. The designs placed numerous new tall walls against shorter walls. No geotechnical engineering or construction services were included in the construction process.
Within two years, the new walls had begun to rotate away from the existing walls. The school district commissioned a geotechnical firm, which found that the soil beneath the site of each structure contained an unusual amount of organic material. This condition, combined with a poor drainage scheme, allowed water to pond and saturate the soils beneath the foundations. The lack of proper drainage across each site softened the foundation soils and caused the new walls to fail.
Repairing the problem involved installing a new drainage system, underpinning the footings of the structures, and repairing cracked slabs and exterior walls of the buildings, all at a cost of tens of thousands of dollars.
If the district had used geotechnical engineering services prior to and during construction, the district could have prevented these problems.
But how much would those services have cost? Geotechnical engineering exploration and construction services cost between one-quarter and one-half of 1 percent of the construction budget. For a $20 million high school, that amount would be between $50,000 and $100,000.
Schools should consider this price tag a low-cost insurance premium on their investment. It will not necessarily cut construction costs, but it may reduce the severity of problems that could create larger costs later.
Then again, geotechnical services can save money on construction. Geotechnical investigations can turn up findings that allow the use of less expensive construction techniques.
Geotechnical short course
Geotechnical services involve five critical steps. First, preliminary sitework must be done. A geotechnical engineer collaborates with a civil engineer to map grades, slopes and any modifications that may have been made to the site. The engineers will determine appropriate sites and depths for taking soil samples. Boring sites will include areas where filling and cutting may occur during construction. In addition, engineers will develop preliminary concepts for drainage structures that may be needed to direct water away from buildings.
The second step, exploration, begins with drilling for soil samples. A geotechnical engineer will take a series of soil samples from each of the defined boring locations, paying special attention to foundation-bearing materials and subgrades for slabs and pavements.
Laboratory tests are the third stage. Engineers evaluate soil samples for strength, moisture, density and other features. Are the soils on the site undisturbed by human activity? Were they deposited naturally through glaciers, chemical reaction, water or wind? How will existing subsurface soil and bedrock such as shale, limestone or sandstone perform as bearing materials for the planned construction?
What about soils that have been manipulated previously? What type of fill was used? Was it uncontrolled fill that has been dumped, but not compacted, or structural fill that has been compacted? Does the fill include buried debris? How can the contractor modify this material to ensure the load-bearing characteristics?
A geotechnical report, the fourth step in the process, answers these questions. It provides a detailed analysis of subsurface conditions across the site. The report's recommendations will guide the construction process. Recommendations cover issues in site preparation and drainage, as well as in foundation, footing and slab design.
Sample recommendations might, for instance, include removing uncontrolled fill that contains deleterious material incapable of supporting a structure. Geotechnical engineers will define how much material must be removed from particular areas of the site. They also will define the characteristics of the structural fill needed to replace such a cut.
Last, but equally important, geotechnical services include monitoring and consulting during site preparation and development. Geotechnical engineers understand how the soil beneath a structure must perform, and they have the training, equipment and knowledge of soil characteristics to make engineering judgments about potential soil problems.
Making the call
Because the primary function of geotechnical services is to help ensure the integrity of a building, schools generally pay close attention to advice about structural foundations. However, officials have to guard against the temptation to shave costs when it comes to parking lots.
Poor parking lot subgrades will lead to maintenance costs in the future, just as surely as a poor building foundation. In fact, parking lots represent another difference between educational and commercial construction. Commercial owners typically need only a 10-year design. But schools usually select the 20-year design, believing that a long-term design will cut maintenance costs.
In fact, a 20-year parking lot design will reduce maintenance costs as long as the soil beneath the parking lot will provide 20 years of support.
The best asphalt, concrete and steel structures in the world will not perform when built on top of bad soil. For this reason, geotechnical firms have, over the past decade, become key members of design and construction teams. Architects, other engineers and contractors provide the high-quality structure, while the geotechnical firm ensures that this high-quality structure is on a solid foundation.
Loes is marketing manager with GeoSystems Engineering, Inc., Lenexa, Kan.
