Page 420 - Trenchless Technology Piping Installation and Inspection

370 Cha pte r Ni ne

Typically, split spoon samples will be taken in soft soil at 5-ft-
deep intervals in accordance with ASTM D1586.
• Ground penetration radar: Useful in gravels and sands.
• Acoustic (sonar): Useful for determining depth of rock, inter-
faces between soft and hard deposits and buried objects.
• Geophysical methods: Variations in the speed of sound waves
or in the electrical resistivity of various soils are useful
indicators of the depth of water table and bedrock.

As said earlier, it is essential that a thorough subsurface investi-
gation is carried out in the design phase to identify the geologic con-
ditions along the pipeline alignment. The anticipated geologic
conditions comprise the most important factor in the selection of an
appropriate trenchless installation method for a specific project.
Groundwater conditions will have an important influence on the
behavior of the ground and constitute a major factor for loss of
ground. Groundwater levels should be determined, and pumping
tests or other field tests should be conducted to estimate the perme-
ability of soil to see if dewatering is necessary and feasible. Contrac-
tors must investigate groundwater conditions just before start of
trenchless installation, as water table conditions may have changed
due to change in season when the design investigations were com-
pleted. In summary, the following is a list of recommended informa-
tion to be obtained for a trenchless installation project:

• Soil information based on the unified soil classification
system (USCS).
• Gradation curves on granular soils.
• Standard penetration test values where applicable (generally
in unconsolidated ground).
• Particle-size distribution, including presence of cobbles and
boulders.
• Shear strength of soil.
• Atterberg limits (liquid, plastic, shrinkage limits and plastic-
ity index).
• Moisture content.
• Depth and movement of water table.
• Permeability.
• Cored samples of rock with description, rock quality designa-
tion, and percent recovery. It should be noted that rock hard-
ness (Moh’s hardness or Vickers test), texture, tenacity, and
formation will determine penetration rate of drilling and tun-
neling equipment.