52
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
Recent practical applications include a fleet of 40 wind-turbines
at Borkum West II (German North Sea) which employ the tripod
design shown in Fig. 49 and each rely on three 2.48m diameter
piles driven in (mainly) very dense sands; Merritt et al 2012.
Another application of the laboratory derived ‘ABC’ approach
involved manned oil platforms founded on pile groups driven in
very hard sandy glacial tills: Jardine et al 2012.
Fig. 49. Wind-turbine tripods in fabrication yard;
-
technology.com/projects/borkum-farm/borkum-farm3.html
The fully analytical cyclic assessment route shown as the
central path through Fig. 48 may also be followed. Laboratory
testing can provide the detailed information required for
modelling the sands’ complex behaviour including: stiffness and
shear strength anisotropy; non-linearity and progressive yielding;
grain crushing; time effects/creep; and cyclic loading responses.
Similarly, the laboratory and field model pile stress
measurements can guide the specification (or modelling) of the
effective stress regime set up around the driven piles and show
how this may change under static/cyclic loading conditions. The
stage is now set for numerical modelling that can capture field
behaviour far more accurately than was previously possible.
10 SUMMARY AND CONCLUSIONS
The key aim of the lecture was to demonstrate the special
capabilities and practical value of the Advanced Laboratory
Testing promoted by Bishop and TC-101. New insights have
been offered through static and cyclic experiments with the
apparatus and techniques they advocated, including highly
instrumented stress-path and high pressure triaxial tests as well
as hollow cylinder, ring-shear interface and micro-mechanical
experiments. Emphasis has been placed also on integrating
laboratory research, field observations, numerical analysis and
calibration chamber model pile studies to advance understanding
and prediction of the complex behaviour of driven piles in sands.
The experiments investigated sand behaviour under a wide
range of conditions. Aspects highlighted for consideration in
ongoing and future constitutive modelling include:
1. The strong non-linearity, marked in-elasticity and time
dependency seen from small-to-large strains.
2. Markedly anisotropic behaviour within the large scale
classical critical state soil mechanics (Y
3
) yield surface.
Sands also show Phase Transformation (Y
4
) over a wide
range of states. These features may occur in either soil
continua, or during shearing against interfaces.
3. Behaviour can only be considered elastic within a very
limited kinematic true (Y
1
) yield surface that is dragged with
the current effective stress point, growing and shrinking with
the mean effective stress p΄ and changing in shape with
proximity to the outer, Y
3
surface; stiffness is anisotropic
within Y
1,
following patterns that evolve with K = σ΄
r
/σ΄
z
.
4. Plastic straining commences once Y
1
is engaged and
becomes progressively more important straining continues
along any monotonic path.
5. An intermediate Y
2
kinematic surface may be identified in
either continuum or interface shear tests that signifies: (i)
potentially marked changes in strain increment directions (ii)
the onset of important strain-rate or time dependency and
(iii) a threshold beyond which permanent strains (and mean
effective stress reductions in constant volume tests)
accumulate significantly in cyclic tests.
6. Creep tests and experiments that combine drained creep and
low level cycling show that the Y
2
process is both time
dependent and affected by cyclic perturbations.
7. Undrained cyclic tests taken to large numbers of cycles tend
to show continuous rates of p΄ reduction, even under
relatively small strain cycles. These trends may be modified
considerably by overconsolidation, ageing or pre-cycling.
8. Particle breakage develops under large displacement
interface shearing as well as high pressure compression and
triaxial conditions. Breakage leads to continuous evolution
of the index properties and critical state e-p΄ relationships.
Conclusions regarding piles driven in sand include:
1. Conventional approaches for capacity and load-displacement
assessment have generally poor accuracy and reliability.
2. It is possible to improve predictions considerably through
numerical analyses that capture the observations made with
advanced laboratory stress-strain and interface shear tests.
3. Such predictions rely critically on assumptions regarding the
stresses set up around the piles during and after installation.
4. Laboratory and field tests highlight the importance of plastic
and time-dependent straining which becomes progressively
more important as stress and strain levels rise.
5. The Calibration Chamber model pile tests demonstrate key
physical features of the pile-soil mechanics, including the
extreme stress changes and grain breakage experienced
during installation. Micro-mechanical laboratory analysis
and high pressure triaxial and ring shear tests allow the
properties of the modified material to be studied in detail.
6. Laboratory model pile experiments demonstrate that radial
stress maxima develop at some distance from the pile shafts.
This feature can also be predicted analytically in studies that
address grain breakage. Taken together with the creep trends
discussed above, this feature offers a mechanism for the
growth in shaft capacity of piles driven in sand over time.
7. Axial cyclic pile tests show broadly similar modes of
Stable,
Metastable
and
Unstable
behaviour in full scale field tests
and model experiments in Calibration Chambers.
8. Local stress measurements made on the ICP and Mini-ICP
piles give profound insights into the mechanisms of cyclic
degradation, demonstrating features of kinematic yielding
and interface shear failure that can be tracked in triaxial,
HCA and ring shear laboratory experiments.
Advanced laboratory testing is critical to advancing all
difficult geotechnical engineering problems where the outcomes
depend critically on the detailed constitutive behaviour of the
ground. Tatsuoka 2011, for example, described advanced testing
directed towards the performance of large bridge foundations
and the compaction of reinforced earth retaining wall backfills,
while Kovacevic et al 2012 describe novel analyses of very large
submarine slope failures that employed models derived also
from detailed and advanced laboratory studies.
