Page 421 - Trenchless Technology Piping Installation and Inspection

Planning and Safety Considerations for Trenchless Installation Methods 371

• Unconfined compressive strength for representative rock
samples (frequency of testing should be proportionate to
the degree of variation encountered in rock core samples).
Rock should be cored in accordance with ASTM D2113 to the
maximum depth of the proposed trenchless boring.
• Presence of contaminated soils (such as hydrocarbons and
the like).
• Climatic data (such as temperature ranges).
• Information on quality of water [e.g., pH values (acidity or
alkalinity), salinity, to determine possibility for pipe material
corrosion].
• Special investigations for possible existence of swelling clays,
or particular chemical conditions.
It is essential that subsurface investigations be conducted by geo-
technical engineers who have knowledge of regional geology and
hydrogeology as well as an experience in trenchless technology and
boring/tunneling. If during the exploration of the underground a
potentially difficult ground is found (such as mix-face conditions
with sloped layers, presence of boulders and cobbles larger than one-
third diameter of proposed borehole, swelling clays, running sands,
gravelly soil, and the like), additional investigations must be carried
out. The complete geotechnical report must be provided to potential
bidders so they can submit realistic bids.

Settlement Potential
Surface settlement is mainly a result of loss of ground during tunnel-
ing and dewatering operations that cause subsidence. During a
trenchless installation project, loss of ground may be associated with
soil squeezing, running, or flowing into the tunnel boring machine
(TBM) or the cutterhead; settlement due to large overcut size; and
steering adjustments. Mixed ground conditions, unstable ground
over stable ground (sand over clay), are the most common reasons for
overexcavation. The actual magnitudes of these ground losses are
largely dependent on the type and strength of the ground, ground-
water conditions, size, and depth of the pipe, equipment capabilities,
and the skill and experience of the operator in operating and steering
the machine. If passive earth pressure is exceeded, heave of ground
surface may occur, causing damage to nearby utilities, pavement,
and other nearby structures.

Geotechnical Baseline Report
Subsurface conditions can vary, sometimes significantly, within a
project site. Geotechnical engineers do not possess the ability to
predict all possible ground variations in advance of trenchless