Subsurface problems are, by definition, hidden. They shouldn't come as a surprise, but often do. On the whole, the old adage, “An ounce of prevention is worth a pound of cure,” holds true.
The same geotechnical engineers that you would consult were you to encounter deep subsurface problems, such as sinkholes, should also be asked to examine the soil at your construction site-before you build.
“Geotechnical subsurface investigation is a tool to communicate unforeseen site conditions, as well as design and construction recommendations, prior to breaking ground,” said Valerie Giangiulio-Moody, president and CEO of CMT Services Group of King of Prussia, Pa. “Owners, architects, engineers, and general contractors associated with any construction project should require preconstruction investigations to increase their confidence that the site will be able to meet the project plans and specifications.”
Giangiulio-Moody and others perform initial geotechnical engineering studies to determine soil and subsurface conditions. For a successful building project, it’s important to understand soil type, depth to rock, depth to the water table, and if any contaminants are present that need remediation. A common study involves soil borings taken through a standard penetration test, or SPT. Other methods include ground-penetrating radar, which provides soil and rock profiles, and electrical resistivity, which is used to map soil layers and moisture content.
The information learned from testing will help determine the best places to locate the building and help with cost estimates for developing the site.
Preconstruction soil testing gives owners, design engineers, architects, and contractors an opportunity, early in the design phase, to discover and manage the potential issues caused by poor soil conditions,” said John Celot, director of estimating at High Construction Company in Lancaster, PA. “With this knowledge upfront, the project team can proactively approach these issues rather than reacting to them later.”
Water, Water, Water.
Moisture has a major effect on the site's load-bearing capacity and compaction.
You may also encounter trouble if excavations reach or exceed the depth of the water table. The initial soil borings give you clues to finding this depth, as can installing a piezometer, which is a slotted pipe that allows you to monitor the water level.
In some cases, builders and developers may first consider a site in the winter. Although frozen wet soil can mimic drier, well-compacted soil, proper testing will uncover this potentially damaging surprise.
If borings are taken in a dry fall and construction doesn't happen until spring, the groundwater conditions may change due to seasonal weather fluctuations. Consistent monitoring is the key. Geotechnical engineers can, in essence, help de-water a site.
Is it Always Necessary?
Experience has taught many lending institutions to demand a geotechnical study before committing to financing. In addition, many municipal codes include the need for these studies as part of their permitting process.
Although a sophisticated developer or owner will automatically schedule such testing, novices learn quickly. The money saved by not doing a comprehensive geotechnical study before site work may easily be wiped out by extremely high costs later.
Reliable preconstruction evaluations serve to not only provide quality control and quality assurance, but can prevent a ‘Pandora’s Box’ from being opened that could severely impact the trajectory of your project.
Rich Pagano is a 12 year veteran in commercial construction in the mid-Atlantic region and director of business development for High Construction Company, Lancaster, PA.