Page 419 - Trenchless Technology Piping Installation and Inspection

Planning and Safety Considerations for Trenchless Installation Methods 369

performed. Investigations for complex installations should comprise
two phases: a general geotechnical review and a geotechnical survey. A
geotechnical survey alone may be sufficient for simpler installations.

General Geotechnical Review
A general geologic review involves examining existing geological
data to determine what conditions might be encountered in the vicin-
ity of the installation. Existing data may be available from past project
records in the area of the trenchless project (buildings, piers, bridges,
levees, and so on). Such an overall review will provide information
that may not be otherwise developed from exploratory borings. This
step allows the geotechnical survey to be tailored to the anticipated
conditions at the site, thus enhancing the effectiveness of the geotech-
nical survey.

Geotechnical Survey
For trenchless installations, it is necessary to know the actual soil
stratification at a given site, the laboratory test results of the soil sam-
ples obtained from various depths, and the observations made dur-
ing drilling exploratory bores. The steps for subsoil investigation
should include the following:

• Determining the nature of soil at the site and its stratification
• Obtaining disturbed and undisturbed soil samples for visual
identification and appropriate laboratory tests
• Determining the depth and nature of bedrock, if encountered
• Performing in situ field tests, such as field density and stan-
dard penetration tests (SPT)
• Observing surface drainage conditions from and into the site
• Assessing any special construction problems with respect to
the existing structures nearby
• Determining groundwater levels, sources of recharge, and
drainage conditions
The main methods of geotechnical surveys are as follows:

• Hand augers: Suitable only for shallow depths, only disturbed
or mixed samples of soil can be obtained in this method.
• Test shaft/pits or trenches: This method is suitable for shallow
depths only, but allows visual observation over a larger area
than is possible with samples from vertical soil borings.
• Boring test holes and sampling with drill rigs: This is the princi-
pal method for detailed soil investigations. Sampling interval
and technique should be set to accurately describe the soil
characteristics, taking into account the site-specific conditions.