47
Honour Lectures /
Conférences honorifiques
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Fragments appear to choke available void spaces after large
displacements (8m), preventing lower friction angles
persisting with coarser sands and upper interfaces. The ring
shear trends converge, but do not conform fully to the
uniform δ = 29
o
CUR 2001 recommendation.
The Calibration Chamber model studies reported in Section
5 testified to the extreme stresses developed beneath advancing
pile tips. Stresses rose and fell around the shaft (at any given
depth) by almost two orders of magnitude as the tip penetrated to
greater depths. Such changes in stress level, combined with
particle breakage, affect the sand’s constitutive behaviour.
Altuhafi and Jardine 2011 conducted tests to investigate these
features using the high pressure apparatus shown schematically
in Fig. 35 to subject medium-dense NE 34 to the effective stress
paths set out in Fig. 36.
Fig. 35. High pressure triaxial apparatus employed to test crushing NE34
sand. System described first by Cuccovillo and Coop 1998
The key test stages were:
K
0
compression to p΄ = 9 MPa, simulating the pile tip
advancing towards the sand element from above.
Drained compression under constant σ΄
r
until apparent
‘critical states’ were reached with σ΄
1
> 20 MPa, simulating
failure beneath the conical pile tip. Tests that stopped
abruptly developed large creep strains. The displacement
strain rates therefore were slowed progressively to reduce
residual creep effects prior to unloading. The ‘critical state’
e-p΄ relationships depend on time.
Drained unloading to q = 0 under constant σ΄
r
before
isotropic unloading to p΄ values between 150 and 500 kPa
(giving ‘OCRs’ of 40 to 140 in terms of vertical stresses),
simulating the sharp unloading experienced as the tip passes.
Renewed drained shearing to failure at constant σ΄
r
in
compression (or at constant p΄ in extension) to assess the
shear strength and dilatancy of the ‘heavily
overconsolidated’ and partially crushed sand.
See Fig. below
for low pressure
test stages
Fig. 36. Effective stress paths followed in high-low pressure triaxial tests
on NE 34 sand, showing high pressure stages (top) and overconsolidated
low pressure stages (below): Altuhafi and Jardine 2011
The results obtained are illustrated in Fig. 37, plotting
mobilised angles of shearing resistance φ΄ against axial strain.
The upper plot (a) shows the generally ductile-contractant
response seen in six similar high pressure tests, with peak φ΄
only slightly greater than the ‘critical state’ (30
o
) angle. The
lower plot (b) summarises the ‘overconsolidated’ response
observed on recompression after unloading. All three
‘overconsolidated’ samples dilated as they sheared, developing
peak φ΄ ≈ 42
o
, well above the ultimate angles (around 33
o
)
developed after large shear strains and diminished dilation.
It is clear that the sand’s behaviour alters radically on
unloading as the pile tip advances by a few diameters, changing
from being contractant, ductile, highly prone to creep and
