Environmental Engineering Reference
In-Depth Information
4.2.2
Surveying
GPS (Global Positioning System)
Applications
GPS is one of the most important instrumentation systems to have been developed in the
past 20 years. Its applications in geotechnical engineering will increase as the system sen-
sitivity increases. At present it is used to monitor slope movements (
Chapter 9)
and sur-
face movements in seismically active areas (
Chapter 11).
Method
GPS is a worldwide radio-navigation system at present made up of a constellation of 24
satellites and their ground stations. GPS receivers have been miniaturized to just a few
integrated circuits. Positioned on a surface, the antennas measure the travel time of radio
signals from at least three of the 24 satellites. Antenna locations are determined by trian-
gulation and by applying complex geometric relationships and very accurate timing to the
radio signals from the satellites. Antenna positions are calculated accurately to a matter of
meters with standard equipment, and with advanced forms of GPS the positions can be
calculated to about 1 cm.
The GPS receiver locations and elevations can be read when the data are received,
or the data can be sent by a radio modem and antenna to a personal computer
where a graphical output of vertical and horizontal movements are stored and can be dis-
played.
Survey Nets
Applications
Survey nets making use of optical systems are used to monitor deflections of slopes, walls,
buildings, and other structures, as well as ground subsidence and heave.
Methods
Ranges, accuracy, advantages, limitations, and reliability of various surveying methods
are summarized in
Table 4.5.
Procedures
Reference points are installed to provide a fixed framework. They include ground monu-
ments, pins in structures, and immovable bench marks.
Close-distance surveys
are usually performed with a first-order theodolite, and critical
distances are measured by chaining with Invar or steel tapes, held under a standard
tension.
Long-distance surveys
, such as for slope movements, are more commonly performed with
the geodimeter (electronic distance-measuring unit or EDM) because of a significant
reduction in measurement time and increased accuracy for long ranges. Distances are
determined by measuring the phase difference between transmitted and reflected light
beams, using the laser principle. Calculation and data display can be automated by using
an electronic geodimeter with a punched-tape recorder. The accuracy for slope monitoring
is about 30 mm (as compared with 300 mm for the theodolite) (Blackwell et al., 1975),
depending on the sight distance. An example of a scheme for geodetic control of a land-
slide is given in
Figure 4.3.
Sequential plotting of the data reveals the direction and veloc-
ity of the movements.
