Geotechnical News June 2011
25
GEOTECHNICAL INSTRUMENTATION NEWS
Displacement Monitoring by Terrestrial
SAR Interferometry for Geotechnical
Purposes
Paolo Mazzanti
Acronyms Used in this Article
• GPS: Global Positioning System
• RTS: Robotic total stations
• SAR: Synthetic Aperture Radar.
• SInSAR: Satellite SAR Interferom-
etry
• TInSAR: Terrestrial SAR Interfer-
ometry
• TLS: Terrestrial Laser Scanner
• DTM: Digital Terrain Model
Introduction
The geotechnical community is
looking with increasing interest at
emerging technologies. Innovative
techniques able to solve problems that
have been unsolved for decades are
now available. However, geotechnical
engineers and engineering geologists
must be confident on their effectiveness
before applying them, and especially
on the reliability of collected data.
Remote sensing techniques are one
of the main innovations in the field
of geotechnical monitoring, since
they are changing the philosophy
from “contact” to “non-contact”
monitoring. In other words, by remote
sensing techniques, some geotechnical
parameters are collected by equipment
located away from the investigated
area. However, ground-based remote
sensing instruments such as manual or
robotic total stations and GPS (Global
Positioning Systems) cannot be defined
as fully “non-contact” instruments
since they need targets or sensors
installed on the monitored ground or
structure. Among the ground-based
techniques, only Terrestrial Laser
Scanner (TLS) and the Terrestrial
SAR Interferometry (TInSAR) can be
considered completely “non-contact”
remote sensing techniques.
The first prototypes of TInSAR were
developed at the end of the 1990s, and
the first commercial equipment dates
back to 4-5 years ago. Seven years ex-
perience with TInSAR has allowed me
to follow this technique from its first
steps to the first long-term and success-
ful applications for complex geotechni-
cal problems.
In what follows, the basic principles
of this technique, together with a de-
tailed description of its performance,
main advantages and limitations and
lessons learned from real cases will be
discussed.
Theoretical Basis and
Performance
The Terrestrial SAR Interferometry
(Bozzano et al. 2010; Luzi 2010) is a
displacement monitoring technique
basedon the same operational principles
of Satellite SAR Interferometry
(Massonet & Fiegl 1998). The SAR
principle consists of a combination of
several radar images collected while
the emitting and receiving antennas
move along a predefined trajectory
(an orbit for a satellite, a route for
an airplane or a rail in the case of
terrestrial equipment) (Figure1). The
Assigned Subcontractor and
Assigned Supplier
When the
assigned subcontract
method is used for installing
instruments and collecting data, the
project owner or designer negotiates
with specialist firms, selects one firm
using a qualifications-based selection
procedure, and assigns the contract
to the construction contractor for
administration. Payment is made on
the basis of actual work done, and
the cost is included in the total bid
price. A line item in the bid schedule
is designated as an
allowance item
and
Provide Services of Specialist
Field Instrumentation Personnel
is
entered in the description column.
The cost estimate is included in the
bid schedule. An explanation of this
procedure is included in the contract
documents. After contract award, the
construction contractor is instructed
to enter into a subcontract with the
assigned subcontractor, and payment
is made to the subcontractor via the
construction contractor under the
allowance item. The construction
contractor’s
monthly
payment
requests to the owner are supported
by including copies of subcontractor
invoices. The cost estimate should
not be regarded as a not-to-exceed
figure, and the contract price should be
increased by change order if needed.
Opposition to this procedure some-
times includes the concern that the
subcontractor, who has been selected
by the project owner or designer, is
under contract with the construction
contractor, hence is there uncertainty
about contractual commitment? In
my experience, with appropriate peo-
ple-communication, this has never
been a problem in the field.
When the
assigned supplier
method is used for buying instru-
ments a similar procedure is used
with another allowance item,
Fur-
nish Instruments.
The specification
states that, after contract award, the
owner’s representative will deter-
mine instrument descriptions, sourc-
es, quantities, and prices and will
provide this information to the con-
struction contractor. The contractor is
then required to place orders, within a
specified time period, and the instru-
ment suppliers become assigned sup-
pliers.
