Geotechnical News • September 2017
25
GEOTECHNICAL INSTRUMENTATION NEWS
and have placed nearly 2-3 meters of
asphalt over the down sliden block
in order to keep the highway level.
Since January 2016, recently installed
inclinometers
and piezom-
eters have been
actively moni-
toring ground
movement and
groundwater
elevations to help
develop a land-
slide response
rate to precipita-
tion. This instru-
mentation has
measured ground
movement at
depths as deep as 34 meters below
ground surface, with movement rates
approaching 2.5 cm/month during the
rainy season. The groundwater eleva-
tion appears to rise by as much as 4
to 5 meters between the dry (summer)
and wet (winter) seasons, with ground
movement accelerating when the
groundwater elevation stays elevated
over extended time periods.
The GNSS system deployed at the
site includes one base station receiver,
four rover receivers and a post-pro-
cessing gateway. The raw GNSS data
is first logged at all rover and base
receivers and then transmitted to the
gateway via local radio network. The
data are then post-processed using
carrier-phase ranging algorithms with
network adjustment at the gateway to
produce high accuracy geographical
positions of each rover receiver. From
the gateway, which is connected to the
Internet via a cellular network, the cal-
culated results are sent to the remote
server and accessible to end users via
a web-portal. Because the calculating
module is removed from the receiver
and the antenna is integrated into the
receiver, both the hardware cost (per
monitor point) and power consump-
tion (0.5 w in this case) are effectively
reduced. In addition, by combining
public Internet connection and local
radio network with mesh topology, it
allows for a highly flexible deploy-
ment of GNSS node and the gateway
(as shown in the schematic of Fig.3).
In April 2016 two rover receivers were
installed in the sliding zone (#2 and
Figure 1. Site photo 1.
Figure 2. Site photo 2.
Figure 4. Layout of GNSS receivers on site (red line shows the outline of the
landslide).
Figure 3. Schematic of GNSS monitoring system.
