1510
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
35 Hz. Therefore it is relevant to use contour diagrams for a
load period of 0.1s and with N=10 and N=100, respectively.
For a degree of mobilization of 0.6, 100 load cycles, and an
average strain of 4.5%, Figure 3 give a cyclic strain,
c
of 0.12%
and a normalized cyclic stress
cy
/s
u
of 0.47.
The lab tests show that ratio of the initial shear modulus,
,
to the undrained shear strength, , is approximately 800, giving
a cyclic stress,
cy
of 6.8 kPa.
To account for the cyclic strain level the shear modulus was
reduced from initial value of 11.5 MPa to a value 5.6 MPa. The
reduced shear modulus corresponds to a local shear wave
velocity of 56 m/s.
With the determined cyclic strain to cause failure and the
reduced shear wave velocity, the necessary vibration velocity on
the surface to cause local failure in the clay is calculated, by
using Eq. (1), to
(2)
The same procedure has been applied for other mobilization
degrees and number of loading cycles and Table 3 summarizes
the resulting cyclic strains and corresponding vibration
amplitudes that will lead to local failures in the clay.
Table 3 Vibration velocities to cause local failure for two different
mobilization degrees, number of cycles and initial shear wave velocity,
c
s
, of 80 m/s.
Mobilization degree, (
a
/s
u
)
0.6
0.7
Number of cycles, N
10 100 10 100
Cyclic shear strain,
cy
[%]
0,22 0,12 0,13 0,1
Normalized shear stress,
cy
/s
u
0,68 0,47 0,47 0,4
Shear modulus corresponding
cyclic strain [MPa]
4,5 5,6 5,2 5,8
Vibration amplitude [mm/s] to
cause local failure in clay.
110 67
70
57
6 DISCUSSION
The vibration amplitudes in Table 3 is the basis for developing
recommended vibration limits for safe blast operations, to avoid
initiation of sliding in low stability quick clay slopes. In this
process it is necessary to take into account that there are
uncertainties and simplifications in the performed analysis,
some are conservative and some are non-conservative.
There is a lack of knowledge of how large a local failure
zone must be to initiate a progressive global failure and to
initiate a quick clay landslide. Therefore a conservative choice
was made to set the limit as to avoid a local failure.
Conservative aspects are:
If a local failure of certain extent could be accepted,
higher vibration amplitudes could be allowed.
The estimated vibration velocities correspond to failure
during the creep phase of the laboratory tests. For local
failure to take place during the cyclic loading phase, a
20-30% higher vibration amplitude and/or higher
mobilization degree is necessary.
The maximum shear strain occurs only in part of the
highly affected zone in the clay. The mean cyclic strain
in the highly affected zone are about half of the
maximum strains used in the analysis.
Non-conservative aspects are:
When measuring the vibrations in the field during blast
operations it is unlikely that the sensor is put at the
location of the peak value, and since the vibrations
reduce quickly with distance from the rock-clay
boundary, the measured amplitudes are lower than the
peak value.
The effect of thin soft sand and silt layers often present
in quick clays have not been considered when
estimating the velocities.
The mobilization degrees were selected to correspond to
overall factors of safety, in reality the mobilization
degree can be higher closer to the rock-clay boundary
and then the estimated vibration velocities are on the
high side.
The whole calculation procedure does not contain any
safety margins.
7 CONCLUSIONS AND RECOMMENDATIONS
There is a lack of knowledge of how large a local failure
zone must be to initiate a progressive global failure and to
initiate a quick clay landslide. Therefore a conservative choice
was made to set the limit as to avoid a local failure.
Taking into account uncertainties and simplifications in the
analysis, spatial variability of vibrations and desired safety
margins we recommend a vibration limit of 25 mm/s.
Peak value monitoring during blast operations of vibrations
on quick clay deposits should be done in two locations e.g. at
distances of 5 and 10 m from the rock-clay boundary and the
highest vibration amplitude in any of the vertical and the two
orthogonal horizontal directions should be lower than the
vibration limit.
Since blasting vibration amplitudes show a large variability
it is important to use vibration monitoring actively during blast
operations to adjust the blast design to avoid exceeding the
vibration limit.
To better understand the behaviour of quick clay with thin
silt and sand layers, it is recommended to perform further
laboratory tests and also to perform more numerical simulations
with more realistic soil profiles looking at the effect of such silt
layers, soil stiffness increasing with depth, a stiff dry crust etc.
This could improve our understanding and possibly allow for
further adjusting vibration limits, probably to somewhat higher
values.
8 REFERENCES
Andersen, K.H., 2009, Bearing capacity under cyclic loading
—
offshore, along the coast, and on land. The 21st Bjerrum Lecture
presented in Oslo, 23 November 2007.
Canadian Geotechnical
Journal
46, 513-535.
Bjurström G. and Broms B. 1975, The landslide at Fröland, June 5,
1973, In Symposium on Slopes on Soft Clays, Swedish
Geotechnical Institute Report No. 17, Linkoping, 113-126.
Ekström, J. 2012, Personal Communication, Trafikverket.
NGI, 1999, Finneidfjord, Measurements of excess pore pressure and
dynamic stresses during blasting, Report 983009-1, September. (in
Norwegian).
NPRA, 2011, HB018, Vegbygging (normaler) Nr. 018 i Statens
vegvesens håndbokserie
NTNU, Skredet i Kattmarkvegen i Namsos 13. mars 2009, Report of
commission appointed by The Ministry of Transport and
Communications. (in Norwegian)
