552
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
The complex investigations at the location have been made,
including borings and sampling, laboratory tests, water level
measurements and monitoring of church wall movements.
The thorough engineering geology investigations were also
performed, and, in order to obtain reliable geotechnical model
of landslide, the so-called RNK method was used.
2 RNK METHOD - FUNDAMENTAL NOTIONS AND
BASIC DEFINITIONS
The
RNK method
(RNK-the acronym in Croatian language)
or
the
Reference Level of Correlation Method
(Ortolan 1996) is a
fully developed method for engineering-geological and
geotechnical modelling. It is primarily intended for the landslide
recognition and the analysis of the slope stability of soils and
soft rock formations. However, the “sedimentation fingertip”
obtained by geotechnical correlation column can be also used
for reliable association of other test results in clayey sloppy
profiles (Ivsic et al., 2005)
The RNK (Reference Level of Correlation)
is defined as an
unequivocally recognizable and visually identifiable (or
graphically defined!) bedding plane or any other reference plane
within a structural feature, in relation to which an altitude of all
studied profiles can be unambiguously defined, with individual
point analysis of any material property. Such plane is a part of a
single vertical lithostratigraphical i.e. engineering geological
and/or geotechnical sequence (engineering-geological and/or
geotechnical correlation column).
The importance of correlation for the slip-surface and/or
slip-zone determination is emphasized by Ortolan (1990).
The plasticity index has proven to be a reliable strength
indicator for cohesive materials (Ortolan 1996, Ortolan &
Mihalinec 1998, Ortolan et al., 2009). The highest values of
plasticity index, but also the liquid limit, correspond to the
lowest expected values of friction angle. This fact allows a new
approach to exact geotechnical modelling. Therefore, testing of
Atterberg plasticity limits on point samples can be
recommended for the identification of zones with lowest shear
strengths. The sample length should not exceed 10cm
(sometimes it should be aslittle as several centimetres, and even
several millimetres). The sampling interval of 0.5–1.0m is
usually considered sufficient.
The correlation between the plasticity index and angle of
internal friction is given in Figure 3, as developed by various
authors, systemized by Ortolan & Mihalinec (1998) and
enriched by new carefully obtained data.
46,0
119
118
117=120
116
111
108
106
104
102
101
100
96
95
92
91
88
68
87
54
115
110
89
90
51
84
19
74
7
82
25
72
26
71
70
14
73
1
75
76
24
3
22
18
31
59
21
30
44
49
9
20
8
36
33
35
69
16
37
77
60
17
80
47
2
32
15
45
48
46
6
83
67
61
42
13
38
50
53
56
52
55
98
58
62
93
39
65
11
12
66
64
40
78
34
4
29
94
41
79
23
5
10
27
28
43
63
86
85
97
99
103
112
113
105
107
109
114
0
5
10
15
20
25
30
35
40
0 10 20 30 40 50 60 70 80 90 100 110 120 1
PLASTICITY INDEX -
PI (%)
PEAK
-
P
OR RESIDUAL FRICTION ANGLE
-
R
(
30
O
)
Residual friction angle (Ortolan & Mihalinec, 1998)
Peak friction angle (Ortolan & Mihalinec, 1998)
Residual friction angle (1998-2006)
Landslide Hospital Merkur in Zagreb ( 2005/2006)
Landslide Jarpetar near Buje - Istra (2002)
Landslide Česmički west in Zagreb (2002)
Landslide Zalesina: Triassic clays and shales (Ortolan, 1996)
Residual friction angle: Landslide Gorica Svetojanska
Very sensitive clays:
4-8 (OTAWA-KANADA: 8)
Cucaracha
Shale: (15)
Allophane:
JAVA
Halloasyte:
JAVA
Halloasyte
(29-32):Clay fromCarboniferous
Shales andMudstones
(25-27):Materials
ContainingHydrousMica
SoftClays (47-49)
(20-24):Materials containing
montmorillonite
(100-107):Triassic
clays and shales
P
R
Figure 3. Correlation between index of plasticity and angle of internal
friction – both peak and residual.
The following supporting documents are most often used in
the study of landslides: general geological map of the wider area
under study, geotechnical correlation column, and engineering-
geological map with slip-plane contour lines and with clearly
delineated slip areas and hydro-isohypses or hydro-isopiestic
lines at the slip-plane level (Ortolan 1996, 2000).
The
geotechnical correlation column
is a consistent
engineering-geological and/or geotechnical soil model (design
cross section) in which adequate parameters (defined in
laboratory or in situ either by point method or continuously) can
reasonably be allocated to every defined layer (and portions of
such layers) along the entire height of the vertical sequence of
formations covered by the study. From such geotechnical
correlation column we may in principle differentiate zones of
minimum residual shear resistance, with their thicknesses and
continuities, but also layers with different moisture content,
hydraulic conductivity, natural compaction, compressibility, etc.
The engineering-geological and/or geotechnical correlation
column of an analyzed area is the "key" to the interpretation of
overall engineering-geological and/or geotechnical relationships
in a required number of profiles selected at will for 2D and
spatial analysis, which is especially significant in 3D analysis of
stability.
The consistent use of the RNK-method in the period from
1995 to the present day has resulted in the elaboration of three-
dimensional geotechnical models for some fifty landslides. In
all of these cases the following parameters were successfully
defined: sliding body geometry, pore pressures and shear
strength parameters for materials along zones of minimum shear
resistance. In combination with existing topographical
documents, this enabled accurate stability analyses and
definition of optimum improvement procedures. The Podsused
landslide may be described as one of the most complex urban
landslide projects in the world (Ortolan 1996, 2000). It is
precisely on this project that the RNK-method has been
developed in full detail, and the reliability of the model was
confirmed with photogrammetric measurements (Ortolan et al.
1995) as well as with three-dimensional stability analyses
(Mihalinec & Stanić, 1991).
Most of the studied landslides have been stabilized, in all
cases with great success, and the supervisory work conducted
during remedial works provided positive feedback information
about the correctness of adopted engineering-geological and
geotechnical landslide models, (e.g. at the Granice landslide;
Jurak et al., 2004), and about reliability of the engineering-
geological and geotechnical correlation column (design cross
section). On some projects the reliability of the model was
checked and confirmed by appropriate inclinometer, piezometer
and benchmark measurements.
3 DESCRIPTION OF THE LANDSLIDE AND GEO-
TECHNICAL PROPERTIES OF MATERIALS
The topographic presentation of the neighboring terrain in
Gorica Svetojanska with the contour of the landslide is given in
detailed engineering geology map of the area (Figure 4). Results
of laboratory and in situ investigations, presented in form of
geotechnical correlation column are presented in Figure 5.
Plasticity chart with encircled critical geotechnical zone-2 is
presented in Figure 6. Formations found on the landslide
(calcitic clays and clayey marls) date back to the Pontian
.
4 ANALYSES
4.1
Wall movements
The investigation program included the measurements of
relative rotation of church walls using several horizontal and
vertical tilt meters, and, also the change of crack widths during
monitoring period (originally found cracks were 15-20mm
wide). The particular results are shown in Figure 7.
