2753
Technical Committee 212 /
Comité technique 212
medium to coarse angular to semi rounded gravels with soft
sandy-clayey filler. Its thickness varies from 0.50 m in BH 1 to
2.30 m in BH 3.
Table 2. Physical and mechanical properties of the layers
Layer
No
Unit wt.
γ,
кN/m
3
Angle of
internal
friction, φ
Cohesion,
c
кN/m
2
1
23,0
35
0,00
2
20,5
14,9
13,3
3
22,7
16,3
15,8
4
21,6
12,1
14,5
5
19,3
9,8
10,2
Layer 4 - Lower Cretaceous marls. Layer 4 is established
under the Quaternary cover at various depths below the surface
of the terrain - from 3.90 m in dynamic probing 1 (DP 1) to
10.80 m in BH 2. The whole thickness of marls is not exceeded
in exploratory drillings. According to visual macroscopic
description marl is gray with a brownish tint. The texture is
layered and it is built by calcite and clay minerals with some
single quartz grains.
Layer 5 - Landslide masses. Landslide body is made of
highly mixed clays with some gravels. The landslide movement
caused violation of soil structure, changing natural water
content and consistency. For the purposes of slope stability
calculations three soil samples from slip surface were tested.
2.7. Hydro geological conditions
The region characterizes by middle ground water abundance.
The presence of cracked karst limestone creates appropriate
conditions for the formation of fissure-karst groundwater. They
are drained underground in the Yantra river alluvium or by
springs in contact zone between limestone and marl.
Groundwater is accumulated in the Lower Cretaceous karst
and cracked limestone and Quaternary sand and gravel layers
and lenses. Deep drainage of groundwater has been determined
by highly dissected topography. The aquifer is confined with
low to medium water pressure at the bottom of the slope
depending on the position of ground water table. Groundwater
flow is directed northeast to the Yantra.The feeding of ground
water is performed by infiltration of precipitation in areas of
outcropping of rocks. Groundwater levels are strongly
influenced by the seasonal distribution of precipitation and
levels of Yantra River. At high river water levels the
groundwater upraise, where a sharp drop in river flow creates
high-gradient groundwater and hence hydrostatic and
hydrodynamic pressure in the slope.
3 SLOPE STABILIZATION
3.1. Overall stability of the slope
To compile a design model for determination of actual stability
stage and its alternation due to different destabilizing factors
have been reviewed geological and geomorphologic
characteristics of the slope, physical and mechanical soil’s
parameters, sliding mechanism, etc. Main factors in landslide
activation have been River Yantra’s erosion, increasing of the
water table levels and dynamic impact of the vehicles on the
road. Additionally the slope stability is influenced from the
restraining of gravitational movement of surface and ground
waters due to the positioning of road embankment, the lack of
effective drainage in the foot of the slope, weathering of the
down part of the slope with high river levels, deterioration of
shear strength of soils due to vibrations from heavy vehicles.
Position of the most unfavorable sliding surfaces is
determined as following boundary conditions have been
accepted: obtained main slope of the landslide, swelling on the
landslide terrain and established sliding surfaces in drilling
boreholes. The form of the sliding surfaces is circular. Janbu’s
“effective stresses” method is applied under consideration of
following conditions: slope stability in natural and dry state and
under a dynamic loading.
Table 3. Alternation of Factor of safety (Fs,min
)
for a different
design states and for all of investigated geological profiles
Design state
Fs,min
After landslide activation, in natural
state of the slope
0,94 - 1,04
Dynamic impact of the road traffic
0,91 – 0,99
Lower ground water level
1,05 – 1,20
Main conclusions from slope stability analyses can be
generalized as follow:
During the active stage at the time of in – situ investigations
of landslide, the slope is in the state of limit equilibrium,
near to the further movement.
Under dynamic loading from the road traffic, the slope
exhibits additional decrease of stability measured by the
minimum coefficient of safety less than 1.
Lowering of the ground water levels leads to the increase of
its stability.
Analyses of slope stability show that the design of effective
drainage system shall not be enough for ensuring the minimum
values of factor of safety through all testing profiles, prescribed
in national standard for construction in unstable slopes,
including the value of Fs,min=1.1 set in National annex of EC7.
To achieving the standard overall stability prescriptions the
landslide should be strengthened by a retaining structure set in
an upper part of slope near to the road (Kolev, 2006).
3.2. Landslide strengthening and drainage works
For recovering of the damaged road section, cantilever
reinforced retaining wall on driving pile foundation and
additional trailing plate has been designed (fig. 3). The retaining
wall has a length of 60 m and it is divided into 12 sections long
5 m each with 2 cm gap between them. The height of retaining
wall above the foundation is 4 m. Each section’s foundation is
composed by 8 driving piles 30/30/900 cm. Due to large
horizontal loading from landslides materials at the foundation
level the additional “trailing” plate has been designed (fig. 4).
The transverse limit state design of pile group has to be done
considering of the weight of the backfill above the plate and the
activated friction beneath the plate and ground base. The width
of trailing plate is 3.5 m and its height is 0.3 m. The piles are
designed as end – bearing and embedded into a strong soil layer.
The drainage of landslide has been performed by deep
horizontal interception drainage toward the slope. From the
bottom of drainage have been constructed drainage shafts 2 m
deep, in 10 m from each other. The shafts cross the
impermeable soil layer and embedded into lower layer with
high seepage capacity. The drainage of high ground water
decreases the hydrostatic pressure on the landslide materials.
Through the shafts water flows in drainage concrete pipes and
then in concrete culvert beneath the road.
The soil beneath the foundation of retaining wall at a depth
of 0.5 m is replaced with gravel connected with three deep
trench drains with branches. Draining water flows
gravitationally into the river.
