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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
methodologies have been carried out. A general description is
indicated in Table 1.
Table 1. Piling Trials.
Trial
Dates
Piles
TOPIC
2001
CFA, Bored, Displacement
RuFUS
2001-2006 CFA, Bored
RaPPER
2007-2009 CFA, Driven
3.2
Pile Installation
A number of different pile types (commercial and
developmental) were installed at the site over 10 years. This
paper focuses only on those types in current use, tested at an age
of up to 5 months, but mostly 2.5-3.5 months, by static
incremental maintained load testing (ML). Other papers
describe piles tested for other purposes (Skinner et al 2003,
Fernie et al 2006, Powell and Brown 2006, Powell and Skinner
2006, Butcher et al 2008, Brown and Powell 2012) Tables 1 and
2 show the piles used in this study.
Table 2. The test piles.
Pile
Date Dia.
(mm)
Effective
Length (m)
CFA (T33)
2001 300
10
CFA (T34)
2001 300
10
CFA (T14)
2001 300
10
CFA (T13)
2001 400
7
CFA (T15)
2001 400
7
CFA (MC1)
2007 450
9.5
*
CFA (MC2)
2007 450
9.5
*
Bored (T16)
2001 300
10
Bored (T40)
2001 300
7
Bored (T46)
2001 300
5.8
**
Bored (T47)
2001 300
5.8
**
Screw Dispt (T30)
2001 300/600 7
Displacement (T35)
2001 300
9
Displacement (T36)
2001 300
7
Driven (TP1)
2007 275
10
Driven (TP2)
2007 275
10
* 1.5-11 m, ** 5.2-10.m
3.3
Pile testing
The Topic and RuFUS pile tests were undertaken using a
combination of BRE load frames and a remotely operated
hydraulic loading and control system The loading system
utilised closed loop control of the hydraulic jack, monitoring
displacement transducers and a load cell. Load was applied in
incremental steps; increments of 25kN were each held for a
minimum 1 hour and until the settlement rate reduced to
0.1mm/ hr. Using this procedure it was hoped that the load at
which rupture of the skin friction occurred would be approached
relatively slowly. Tests were terminated once failure was clearly
defined, generally indicted by runaway displacement.
For the RaPPER programme the test equipment was similar
to that described above but operated on site. The test method
used complied with the ICE Specification for Piling and
Embedded Retaining Walls 2nd ed. (ICE 2007); the procedure
was similar to that above but using 125kN increments
throughout firstly up to 500kN, then an unload/reload loop
before continuing until failure was established. The increments
during loading were maintained for a minimum 30mins and
until the rate of settlement reduced to 0.1mm/hr. This criteria
works well until failure is approached.
In the RaPPER project testing was also conducted by
constant rate of penetration, dynamic and rapid load or
statnamic means and is described elsewhere (e.g. Butcher et al,
2008, Brown and Powell 2012).
3.4
Test Results
3.4.1
Definition of shaft failure capacity
The majority of the CFA and bored piles, at 300 to 450mm were
anticipated to be essentially friction piles. These piles had
relatively high length to diameter ratios; additionally no attempt
was made to clean the bases of the bored piles. As failure was
achieved at relatively small displacements, typically less than
5/6mm this would appear to be a reasonable assumption. In
these cases, capacity was taken to be the maximum ‘stable’ load
achieved. Given the tendency in brittle London clay for the pile
to ‘shed’ load down its length as failure is initiated in the upper
part
s then a ‘stable’ load was tak
en to be either the last
increment applied if this was maintained for some time before
significant displacements occurred or the next to last increment
if the pile failed rapidly soon after application. The
interpretation of the failure load increment was more difficult
for larger increments. The screw displacement piles, at 600mm
external diameter, and the driven piles were expected to
demonstrate rather more base capacity. For the driven piles
capacity was taken to be again the maxi
mum ‘stable’ load but
with an allowance for base capacity based on eqn (2). For the
larger ‘displacement’ piles failure was taken as the load at
which significant creep started to occur under load and this was
also checked based on Chin and Fleming constructions.
Based on the failure criteria discussed above, the ultimate
capacity of each pile is shown in Table 3.
4 PILE DESIGN
4.1
Design by calculation
In the UK, it is common to use a total stress method for the
calculation of pile capacity in clay soils. For the purposes of the
present paper the model for pile capacity has been taken
considering undrained behaviour and to be the sum of shaft
(Q
su
) and base (Q
bu
) where:
Q
su
= Σ(q
su
ΔL A
s
)
(1)
Where; q
su
is ultimate unit shaft friction; ΔL the appropriate
section of pile length; A
s
is surface area per unit length of pile
Here :
q
su
= α c
u
where:
α is an empirical factor; c
u
is the average shear strength
over the length ΔL
and base capacity as: Q
bu
= A
b
N
c
c
u base
(2)
where: A
b
is the area of the base of the pile, N
c
is the undrained
bearing capacity factor generally taken as 9, c
u
base is the
undrained shear strength at the base of the pile.
This is used to calculate the pile capacity under BSEN1997-
1 (7.6.2.3), to which model and partial factors are applied to
identify the design pile resistance. Estimates of pile capacity for
the different types of piles have been made for all of the piles
using the α–
c
u
method.
4.1.1
Results
–
alpha values and soil parameters
There is an intimate link between selection of a value of alpha
(α)
and soil strength. One has to ensure that when
α
values are
selected from the literature then the same method of shear
strength derivation has to be used (sampling methods, sample
sizes and testing). Typically a design line for shear strength has
been a
‘mean’ value
and that is what has been adopted here.
All
values for α
(Table 3) were those based on shear strength
profiles from CPTs to the piles correlated to UU triaxial.
Although the main test area described was very uniform, the
area where the RaPPER piles were located was a little distance
away and seems to have undergone desiccation in the upper
layers although the CPTs come together below 5m.
