PHASES OF A SOILS INVESTIGATION
The scope of a soils investigation depends on
- The
type, size, and importance of the structure;
- The client and Economy;
- The engineer’s familiarity with the soils at the site; and
- Local building codes.
Structures that are sensitive to settlement such as machine foundations and high-use buildings
usually require a more thorough soils investigation than a foundation for a house. A client
may wish to take a greater risk than normal to save money and set limits on the type and
extent of the site investigation.
You should be cautious about any attempt to reduce the
extent of a soils investigation below a level that is desirable for assuming acceptable risks
for similar projects on or within similar ground conditions.
If the geotechnical engineer is
familiar with a site, he/she may undertake a very simple soils investigation to confrm his/
her experience. Some local building codes have provisions that set out the extent of a site
investigation.
It is mandatory that a visit be made to the proposed site.
A soils investigation has following components.
- The first component is done prior to design.
- The second component is done during the design process.
- The third component is done
during construction.
- The second and third components are needed for contingencies.
- The
first component is generally more extensive and is conducted in phases.
Phases of SOIL INVESTIGATION are as
follows:
- 1. DESK STUDY
- 2.PRELIMINARY RECONNAISSANCE OR A SITE VISIT
- 3.DETAILED SOIL EXPLORATION
- 4.LABORATORY TESTING
- 5.WRITE A REPORT
Phase I. DESK STUDY
This phase is sometimes called “desk study.” It involves collection of available
information such as a site plan; type, size, and importance of the structure; loading conditions; previous geotechnical reports; maps, including topographic maps, aerial photo-
graphs, still photographs, satellite imagery, and geologic maps; and newspaper clippings.
An assortment of maps giving geology, contours and elevations, climate, land use, aerial
photos, regional seismicity, and hydrology are available on the Internet. Geographical information system (GIS)—an integration of software,
hardware, and digital data to capture, manage, analyze, and display spatial information—
can be used to view, share, understand, question, interpret, and visualize data in ways that
reveal relationships, patterns, and trends. GIS data consist of discrete objects such as roads
and continuous fields such as elevation. These are stored either as raster or vector objects.
Google Earth can be used to view satellite imagery, maps, terrain,
and 3D structures. You can also create project maps using Google Earth.
Phase II. PRELIMINARY RECONNAISSANCE OR A SITE VISIT
Preliminary reconnaissance or a site visit to provide a general picture of the topography and geology of the site. It is necessary that you take with you on the site visit all
the information gathered in Phase I to compare with the current conditions of the site.
Your site visit notes should include:
- ■ Photographs of the site and its neighborhood.
- ■ Access to the site for workers and equipment.
- ■ Sketches of all fences, utility posts, driveways, walkways, drainage systems, and so on.
- ■ Utility services that are available, such as water and electricity.
- ■ Sketches of topography including all existing structures, cuts, flls, ground depression,
ponds, and so on.
- ■ State of any existing building at the site or nearby. Your notes should include exterior
and interior cracks, any noticeable tilt, type of construction (e.g., brick or framed stucco
building), evidence of frost damage, molds, and any exceptional features.
- ■ Geological features from any exposed area such as a road cut.
- ■ Occasionally, a few boreholes, trenches, and trial pits may be dug to explore the site.
Phase III. DETAILED SOIL EXPLORATION
Detailed soils exploration. The objectives of a detailed soils exploration are:
■ To determine the geological structure, which should include the thickness, sequence,
and extent of the soil strata.
■ To determine the groundwater conditions.
■ To obtain disturbed and undisturbed samples for laboratory tests.
■ To conduct in situ tests.
Phase IV. LABORATORY TESTING
Laboratory testing. The objectives of laboratory tests are:
■ To classify the soils.
■To determine soil strength, failure stresses and strains, stress–strain response, permeabilities, compactibility, and settlement parameters.
Not all of these may be required for a
project.
Phase V. WRITE A REPORT
Write a report. The report must contain a clear description of the soils at the site,
methods of exploration, soil strati-graphy, in situ and laboratory test methods and results,
and the location of the groundwater. You should include information on and/or explanations of any unusual soil, water-bearing stratum, and any soil and groundwater conditions
such as frost susceptibility or waterlogged areas that may be troublesome during
construction.
Key points
1. A soils investigation is necessary to determine the suitability of a site for its intended
purpose.
2. A soils investigation is conducted in phases. Each phase affects the extent of the
next phase.
