618
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
observed as the resistance monotonically increases up to 100
mm movement. By analyzing the obtained movements-
resistance curves it can be noted that values of shaft resistance
for Vendian clays can be taken as corresponding barrette shaft
resistance for 40 mm (according to RF building codes)
movement.
Design shaft resistance values provided for assessment of
pile bearing capacity by SP24.13330.2011 at depths greater than
40 m are assumed constant and equal to 50 kPa, which is 4.6 ...
6.4 times less than the values obtained in static Barrette test. For
comparison, ultimate values of shat friction were also calculated
for the most common approaches worldwide: the α-method
(Skempton 1959) using undrained strength parameters and
Mohr-Coulomb law. In calculation with the alpha method lower
(α=0.3) and upper bound (α=0.6) values of alpha were
examined. Resulting specific values of shaft friction values
were 250 and 500 kPa respectively. For calculation by Mohr-
Columb earth pressure at rest coefficient K
0
were taken for non-
consolidated soil by well-known Jaky equation and as for
overconsolidated soil with OCR=2,5. Resulting K
0
values were
0.66 and 0.99 respectively. Factor of 0.5 for interface strength
also applied according to SP24.13330.2011. Specific shaft
friction values obtained by this calculation method were 270
kPa for K
0
=0.66 and 460 kPa for K
0
=0.99.
Thus, the lower limit of the specific shaft friction, calculated
using the mechanical properties of soils were within the values
obtained by the results of static Barrette tests, and the upper
limit value – was higher on average of 1.7 times. One of the
reasons for this discrepancy may be that the central parts of
barrette during the first stage of test (lower part testing) has the
5...7 mm upward movement, during which partial mobilization
of shaft friction forces in opposite direction were observed.
6 CONCLUSIONS
Trial works confirmed the technical feasibility of barrettes
construction with cross-section of 1.5 x 3.0 m length of 85 m by
the standard "slurry wall" technology in difficult sub-soil
conditions of St. Petersburg.
It is advised to implement Osterberg testing technique (by
cast in pile submerged jack) for the deep foundation of high-rise
buildings. The method allows to use pile parts as an anchor
system and to clearly determine the values of unit shaft friction
and base resistance. It is recommended to install two levels of
jacks in a pile: one near the base of the pile, and the second in
the middle of the main bearing layer. It is critical to install
several levels of strain gauges in the pile along its length.
Pile testing at construction site should be seen not only as
method to determine pile bearing capacity but as an effective
method to calibrate design parameters of adopted soil model,
and to assess its applicability and adequacy. Soil parameters
provided by ground investigation can by checked and adjusted
if necessary. During design process of tower foundation,
obtained results of unit shaft friction and base resistance should
be used as the control values, against which the calculation
results are checked.
The calculations made on the basis of experimental data
showed what the values of the mechanical properties of soils
determined by the laboratory testing has severely
underestimated soil strength and deformability parameters due
to sample disturbance, the influence of the scale factor & etc.
As a result of the tests it was found that the Vendian clays
can provide high values of shaft friction and base resistance.
The experimental values of shaft friction and base resistance
exceed the ultimate values provided by codes by 4 ... 6 and 1.6
timesrespectively.
7 REFERENCES
Aspects of pile testing for high-rise buildings on the example of ODTS
«Okhta» tower.High-rise buildings journal 6 (2012).96-
99.PetrukhinV.P., ShuliatevO.A., BokovI.A., ShuliatevS.O.
Cast-in-situ Bored Piles in London Clay, Geotech,Vol.9.Skempton,
A.W., and Northey, R.D. (1952)
