|
Current Issue
Geotechnical
Preliminary Analysis of Subsurface Conditions:
Reading the Landscape & Other Methods
Many of us think that the first step in analysis of site subsurface conditions is to hire a contractor to drill test borings or excavate test pits. But wait! There are several preliminary steps, such as review of readily available geologic and aerial information and “Reading the Landscape”, that need to be completed in preparation for fieldwork. These steps can also provide a preliminary site analysis early in project planning, for which some key foundation issues can be determined. These methods provide only very rough approximations of subsurface conditions and should not be a substitute for the site-specific information obtained during a field exploration program.
There is a plethora of available public documents, including USGS topographic, surficial geology, and elevation top of bedrock maps; aerial photographs; and USGS well and test boring logs.
The USGS surficial geology maps show the major soil units (e.g. man-placed fill, organic soils, clay, outwash sand, glacial till and bedrock) that occur just below the thin surface layers of topsoil and subsoil. This wealth of geologic information has been compiled over many years using available test boring and well logs, inferences about soil contacts from topographic plans, drainage maps, and aerial photographs.
Other USGS maps provide information about depth to bedrock, areas of significant soil borrow, depth to groundwater, and total thickness of overburden soils above bedrock. Much of the USGS information is color coded on 7.5-minute maps that cover about 55 square miles.
The State of Connecticut has compiled information on a state surficial geology map,which also includes summary test boring data.
Aerial photographs provide information on land forms that can be brought to three-dimensional views using stereoscopes. Since the early 1930s, the State of Connecticut has maintained a program of periodic statewide flights that document the changes in landform. The effects of excavations and fillings can be traced through the years by viewing these scaled photographs from successive years.
Visiting the site and reading the landscape provides some useful information about subsurface conditions. For example, certain trees may indicate the presence or absence of water. Oak and maple trees require considerable water, which must be available near the surface. This condition may also suggest that clay soils are near the surface. The tops of young pine trees also suggest certain subsurface conditions. If pointed, they are growing fast and their root system is probably not restricted by an impervious layer, such as hardpan. On the other hand, flat tops suggest that the rate of growth is slower, and the root system is inhibited from development because of a limiting hardpan layer.
These and other methods are used by the geotechnical engineer to develop preliminary information about a site and to appropriately develop site specific subsurface exploration programs. If used early in the planning, this information can be used by a potential buyer or owner in making important financial decisions about building foundations.
Contact David Freed for further information at:
860 388.1224 ext.125 or freed@gncbengineers.com
[ Top ]
|
