2189
Characteristics of Ground Motion on Colluviums Slope Induced by Heavy Rainfall
Caractéristiques du déplacement du sol sur la pente de colluvions induit par la pluie violente
Jeng C.J., Sue D.Z.
Huafan University, Taiwan
ABSTRACT: In this study, a slope covered with various depths of colluvium soil and set up with monitoring system has been studied.
The results from hundreds of settlement and displacement observation marks were taken into account. It was found that the maximum
settlement and displacement were concentrated around the buildings of Hui-tsui, Zhian and Wu-Ming, and coincided with rainfall
records for the area. The direction and distribution of displacement and surface cracks supports previous findings of a sliding block.
The slope stability analysis for this study was carried out with the STABL program. A drainage system with additional stability
measures was proposed to prevent an unstable slope caused by the rising up of the ground water table during rainfall. Finally, a curve
showing the relationship between rainfall and slope stability is established and presented.
RÉSUMÉ : Dans cette étude, une pente couverte de colluvions de différentes profondeurs et équipée d’un système de surveillance a
été étudiée, ayant recours aux remarques tirées des observations sur les affaissements et les déplacements de la pente. Nous avons
découvert que les plus grands affaissements et déplacements de pente, coïncidant avec les précipitations dans la région, avaient lieu
autour des bâtiments Hui-tsui, Zhian et Wu-Ming. La direction et la répartition des déplacements de pente ont confirmés nos
recherches précédentes sur un bloc coulissant. Cette recherche a été faite avec le programme STABL. Un système d’évacuation des
eaux avec des mesures de stabilité a été proposé pour protéger une pente instable contre les eaux souterraines qui montaient lors des
averses. À la fin de cette étude, une courbe a été présentée pour expliciter la relation entre les précipitations et la stabilité de pente.
KEYWORDS: colluviums slope, ground monitoring, settlement and displacement, slope stability analysis, threshold value of rainfall.
1 INTRODUCTION
Rainstorms frequently trigger colluvium landslides. For
example, in November 1993, more than 800 landslides were
triggered by rainstorms on Lantau Island, Hong Kong (Dai &
Lee 2002). In Taiwan, colluvium slope disasters related to
rainfall are very common (Jeng et al, 2007; Jeng and Lin, 2011;
Pan et al, 2008
).
The threshold values of typhoon disasters were
studied extensively by Wang and Yeh(2011) and Hu and Liao
(2010). This paper focuses on the effects of heavy rainfall on
the colluvium slopes in northeastern Taiwan. The results from
295 points of settlement and displacement monitoring marks
and their significance to ground motion were evaluated. Finally,
a threshold value curve for typhoon rainfall is proposed.
2 BASIC INFORMATION OF THE FIELD CASE
The field case discussed in this research is the campus of
Huafan University in northeastern Taiwan. The University was
constructed on the slope. For risk management and research on
slope stability, a monitoring system was set up and data was
collected for over ten years. The monitoring system includes the
inclinometer, building tilt measurement, tiltmeters, crack
gauges, water level observation wells, settlement and
displacement monitoring marks, rebar strain gauges, concrete
strain gauges and rain gauges. According to the records
obtained from the rain gauges from 2003 to 2010,
the average
annual rainfall is about 4000mm, most of which is attributed to
torrential rainfall concentrated in the period of typhoon season.
The maximum monthly rainfall record is in October 2010 with
1208mm.
2.1 Geological Condition
The base stratum of the site consists of the Miocene Mushan
Formation. On the ground surface is 10 to 20 meters of
colluvium comprised of a bottom layer of sandstone (SS) and
thin alternating layers of sandstone and shale (SS-SH). The
attitude of the bedrock is strike in the east-west direction with
the dip-anchor 10
o
to 20
o
toward south.
2.2 Ground Motion Monitoring Results
2.2.1. Settlement and Displacement Monitoring
Since 2001, hundreds of settlement and displacement
monitoring marks were set up and recorded every six months.
Some additional marks were gradually included over the years.
Data was recorded until January 2011, and after ten years of
monitoring, a total of 295 marks were collected. The accuracy
of the investigation was controlled to within 1/5000 by the
plane triangulatuin method with GPS (TRIMBLE 4800) for six
fixed station points. Four traverse points were then laid within
the survey area and double-checked for accuracy to be within
1/10000. The coordinates and elevation of each observation
mark were then surveyed based on those traverse points. The
displacement and settlement values were obtained by comparing
the coordinates and elevation results of each survey to the initial
results from the first survey. The data marks were then divided
into two categories for buildings, and for roads and land.
2.2.2 Settlement and Displacement Distribution
Figure 1 shows the results of the settlement distribution. The
settlement marks around buildings are shown in solid triangle
and the settlement marks for roads and other land are shown in
solid circles, while the heaving points are shown in hollow
circles. The values of settlement or heaving are discriminated
into five levels from 5 to 25mm and are represented by different
size circles. As indicated in Fig. 1, the maximum annual
settlement is more than 20mm, distributed around the Hui-tsui
building(B1),
Chih-an
building(B2)
and
Wu-Ming
