1. 19th European Young Geotechnical Engineers’ Conference 3-5 September 2008, Győr, Hungary
The Bearing Capacity of Bored Belled Piles in Subsiding
Soils under the Dead Weight
D. Karpenko
Affiliation: The State Scientific Research Institute of Building Constructions, Kiev, Ukraine
ABSTRACT
The article represents the results of full-scale investigation concerning the interaction of the bored belled
piles in loessial soils that subside under the dead weight in artificial saturation of soil. The bored pile
capacity along the pile shaft and belly capacity are individually determined based on the static test data.
The available results are comparised using the numerical simulation. A conclusions are made concerning
the behavior of such pile under negative or additional loading friction.
Keywords: pile, negative friction, subsiding soil.
In industrial and civil engineering on the loessial soils, the bored belled piles are widely used as
a foundation. For determination of their bearing capacity, the standard tests are conducted and
the theoretical methods are used. The prediction of the bearing capacity of the bored belled
piles should be made taking into account the pulldown of the loessial soil resistance due to
artificial saturation and negative (additional loading) skin friction.
Nevertheless, some factors affecting the reliable estimate of the pile capacity stay insufficiently
known by now. To these factors belong: load application speed, method of artificial saturation of
soil, piling technology, check of design strength of soil along pile shaft for the bored piles in
loessial soils, the contact of concrete and soil, etc.
The insufficiently studied interaction of the bored belled piles and subsiding loessial soil requires
a detailed investigation of processes that take place during piling and load transfer from
buildings.
To this end, the bored belled piles were tested both in compressive and pulling load and analyzed
on a building site of Ukraine.
To approximate the behavior of the tested piles to that in actual operating conditions, it was
saturated the whole loessial soil layer up to Sr > 0,8 [DSTU, 1996]. Before and during the test the
subsiding soil layer was saturated after the piling. For the soil saturation acceleration, water was
delivered trough four drain holes of Ø350mm joined by an air drain in the upper part.
On the test ground the subsiding loessial clay sands and loams occur at depth of 20,5m. Firm
foxy clays underlay the loessial soil layer. During the test and the piling the subsoil waters were
lack within the loessial layer. For the geological succession of the test ground and the soil
properties, see Figure 1.
Being saturated, the soil layers of EGL – 4 to 8 display the subsiding properties under the dead
weight and being loaded externally. According to the laboratory research the total value of
inherent settlement Ssl.g is 41,8sm.
The tested piling was made by a ”dry” method using a setting CO-2 and an auger with diameter
of 500 mm having the following parameters:
- P1 and P2 with a diameter of 500 mm and a belly diameter of 1600 mm;
- P5 with a diameter of 500 mm and a belly diameter of 1400 mm;
- P3 and P4 with a diameter of 500 mm.

2. 19th European Young Geotechnical Engineers’ Conference 3-5 September 2008, Győr, Hungary
Filled soil -
Loessial loams, silty, firm,
subsiding -
Loessial loamy sands silty, firm,
subsiding -
Light loessial loams, silty, firm,
subsiding -
Loessial loamy sands silty,
subsiding -
Loessial loams silty,
subsiding -
Loessial loamy sands silty,
firm to half-firm, subsiding -
Loessial loams heavy, silty,
subsiding-
Clays silty, firm -
Sands of middle size, containing low
humidity, compact -
Clays firm, with sand and loamy
sand layers -
Legends Lytological types
of soil
P-5P-3, P-4P-1, P-2
Fig. 1 - Engineering-geological model of the subsiding base with the tested piles P1 to P5
For the real design of the bearing capacity of the tested piles in subsiding soils that significantly
transform their properties being saturated during building and construction operation, the piles
P3 and P4 were tested for the static axial pulling load (excluding the pile weight) both in
naturally saturated soils and in artificial water saturation of subsiding soil (P3*
, P4*
). The piles
P1, P2 and P5 were tested for the static axial compressive load in preliminary inundated soils.
For the data of the pile static test for compression and pull, see fig. 2 and 3.

3. 19th European Young Geotechnical Engineers’ Conference 3-5 September 2008, Győr, Hungary
Fig.2 S=f(N) relation curves based on the results of the single pile test for
compression (P1, P2, P5)
Fig.3 h=f(P) relation curves based on the results of the single pile test for pull (P3 and P4)
According to the modern scientific conceptions the maximal approximation of the calculation
scheme of the system “pile-subsiding base” to real-life environment may be obtained using the
nonlinear analysis based on elasto-plastic problem solving. In this problem, the boundary state of
two groups is being analyzed while one soil model is studied. The solutions of such art make
possible to design a deflected mode (DM) that satisfies the desired physical conditions in each
point of the system “pile-subsiding base”.
A single belled pile was analyzed using the numerical simulation in the program complex
“PLAXIS 8.2” in the axis that is position symmetric (see fig. 4).
The boundary conditions in the lower part of the model are represented as total pinch and the
vertical walls as hinged movable supports. In this case is applied Mor-Coulomb elasto-plastic
model. The nonlinear model includes five parameters that are known in most cases:
- E is a deformation module;
- ν is a Poisson ratio;
- c is a specific cohesion;
- φ is a angle of internal friction;

4. 19th European Young Geotechnical Engineers’ Conference 3-5 September 2008, Győr, Hungary
- λ is a dilatation factor.
The pile is modeled by elements of the deflected mode within an elastic linear-strained model.
The pile material has the following design characteristics:
- deformation module Е=3,0x107
kPа;
- Poisson ratio ν = 0,20.
The interaction of pile and soil is taken into account through an interface meaning a contact area.
Interfaces make possible to specify the pulldown of negative friction along the pile pillar. The
interface properties are dependent on structural behavior of soil through strength retrogression
factor. It means, that at prescribed value, the values of interface friction and cohesion are lower
in comparison with a friction angle and a specific cohesion of bordering soil.
During bored belled piling, the design of the DM base included the following stages. Before the
design were determined the initial conditions meaning water pressure generation. Then, the
initial stresses under soil dead weight was being generated. Having derived the initial data, a soil
excavation process was designed at the second stage. Soil excavation had a few stages, the
boundaries of those were designed using geometrical lines. The third stage included the design of
belly process in the lower part of pile. At the fourth stage the process of borehole concreting was
designed using both the activation and decontamination model parameters. The external load
applied stepwise in the form of concentrated force down the axis up to the node of the foundation
upper bound formed the fifth stage.
Fig.4 Finitely element model and strained scheme “Pile – subsiding base”
The designed results are presented as follows: for the ysopols of pile displacement, see fig. 5.
Using the numerical simulation it was determined that the process of taper water infiltration into
soil and its saturation acts upon a pile in two ways. First, as water penetrates deep into the
loessial soils it comes to water saturation of more and more soil amount and the more part of a
pile pillar subjects to the overhanging soil action. Second, with water saturation reduces the
cohesion force in the contact area of the pile and soil. It means that in saturation of the loessial
soils, the structural cohesion and soil cohesiveness are equal to zero and the internal friction
angle reduces for 25 to 50%.

5. 19th European Young Geotechnical Engineers’ Conference 3-5 September 2008, Győr, Hungary
a b
Fig.5 Ysopols of pile displacement in the direction a being Uy and b being Ux
For the value comparison of bearing capacity of bored belled piles obtained by design and in
static test of full-scale piles on the test ground with subsiding under the dead weight soils, see
Table 1.
Table 1. Data comparison of the full-scale test of belled piles and numerical simulation.
Pile
number and
its
parameters
Soil
conditions as
to
subsidence
Soil
type
under
the
belly
Bearing
capacity of
pile Fd, kN
according to
the numerical
simulation in
PLAXIS
Bearing
capacity of
pile
according to
the static test
when
S=16mm,
Fd, kN
Design load,
allowable for pile
according to the data taking
into account friction force
with additional loading
based on results
NР
P
/
NР
С
PLAXIS
NР
P
Static test
NР
C
=Fu/1.2,
kN*
P – 1
l = 23m
d=0.5m
D=1.6m
In saturation
subsides
under the
dead weight
Fine
sand
3654 3970 1094 1190 0.92
P – 2
l = 23m
d=0.5m
D=1.6m
The same
The
same
3576 3883 1037 1117 0,93
P - 5
l = 23m
d=0.5m
D=1.4m
The same
The
same
3438 3651 876 924 0.95
* according to 8.10 and expression (32) of standards [CNiP,1985].

6. 19th European Young Geotechnical Engineers’ Conference 3-5 September 2008, Győr, Hungary
Based on the complex experimental and numerical investigations, it may be concluded the
following:
- under loading of the bored belled piles a soil compaction region forms around the belly.
A diameter of the compaction region of the soil massif in a draft is approximately 3 to
3,5 pile diameter (D) and app. 0,5d under the pile vertically.
- In the compaction region under the belly the module of the soil deformation increases for
40 to 60% in comparison with the natural soil, the specific cohesion of soil comes to 40%
while the internal friction angle stays nearly the same. Growth of these factors depends
on a belly diameter.
- The comparison of the values obtained by design and by test points out the difference in
subsidence values of the single bored belled pile when vertically loaded being equal 5 to
8% that makes possible to predict pile base deformations during simulation.
- The simulated technological stages of bored belled piling indicated the change of the DM
base at all stages that points out the necessity of taking into account the base relief stages
during the cavity formation and its bellying;
- The conducted investigation displayed that the use of numerical methods and the
experimental test results make it possible to design the DM base and to predict the
behavior of the system “pile-subsiding base” in wide range of loads effectively.
REFERENCES
СNiP 2.02.01–83 Building and construction bases. – М.: Stroyizdat, 1985. – 40pp.
СNiP 2.02.03–85 Pile foundations ( change №1 to СNiP 2.02.03-85 // Building and standartization. –
2001.– № 4). – М.: Stroyizdat , 1985. – 45pp.
DSTU B V.2.1-1-95 (GОСТ 5686 – 94). Soils. In-situ testing methods using piles: introd.
01.04.96. – К.: Ukrarchbudinform, 1996. – 57pp.
PLAXIS Theoretical Manual.