1) The document discusses numerical modeling to determine negative friction forces on pile side surfaces during static load testing in settling soils. It presents modeling of single piles and pile clusters to approximate these forces. 2) Key findings from the modeling include that negative friction forces depend on the soil saturation case, soaked layer depth, and soil properties. Soaking from parallel settling layers below poses more risk than top soaking. 3) The proposed simplified modeling method allows preliminary analysis of pile behavior in loose settling soils under self-weight, accounting for negative friction forces with sufficient accuracy for design. Modeling results were confirmed in a construction project.

1. Peculiarities of implementation of primary
numerical modeling when determination of the
forces of negative friction on pile side surface in
condition of its test with static load
The State Scientific Research Institute of Building
Constructions, Kiev, Ukraine
Junior researcher - D. Karpenko
Proceedings of the 23rd European Young Geotechnical
Engineers Conference, Barcelona 2014

2. From experience we know that several factors (the type and location of possible
sources of soaking the soil; value of settlement thickness and level of soil settlement
because of their own weight; physical and mechanical characteristics of the
underlying soil; permissible uneven of base deformation; vertical nature of section;
depth of laying of ground beam and other) should be taken into account additionally
in the design of foundations.
The necessity to consider the additional
actions on the piles (negative or downdrag
friction force on the lateral surface of the
pile) arising in case of outrunning
settlements of surrounding soil is the main
feature of pile foundations designing in
settling soil because of its own weight when
soaking or additional overload. Prognosis of
bearing capacity of piles should be made
taking into account the possibilities of
decreasing of weak soil resistance during
soaking and arising of negative friction
forces on the lateral surface.

3. There are three most typical design cases for taking into account of soaking influence:
1) Soaking source is located directly within the pile foundation when the surrounding
soil in contact area of the pile with soil under settlement is in water-saturated state (see
Figure 1, a);
2) The source is located at some distance from the pile foundation, therefore a soil has
a natural or prescribed humidity in the upper part along all contact and it has humidity
close to full water saturation in bottom part (see Figure 1, b);
3) The gradual rise of the groundwater level, which leads to increasing of soil
moisture up to full water saturation, therefore the pile interaction with soil at the top is
determined by natural or prescribed humidity, and by complete water saturation of the soil
at the bottom (see Figure 1, c).
Figure 1. Cases of pile foundation soaking: 1 – piles; 2 – soaking source; 3 – curve of
distribution of water to the sides of soaking source; 4 – rise of the groundwater level;
5 – impermeable soil layer.

4. In addition to loads arising under the structure
action, the pile may be subject to actions arising
under the additional movement of the soil in which
it is installed. This phenomenon is known as
Europe's term "negative friction on the lateral
surface of the pile" when we say about further
consolidation of soil around the pile. This
consolidation leads to additional forces of friction
on pile lateral surface directed down along pile and
it reduces the pile bearing capacity in general.
The soil moving relative to the pile in other areas
(e.g., upward or horizontally) can cause lifting,
stretching or lateral displacement of the pile.
Standard EN 1997-1 requires using one of the two
listed below ways for neutralization of movements:
1) to analyze the interaction of piles with the soil
when taking into account the actions of the "soil-
upper structure"; 2) bearing capacity of
the pile should be considered as the upper limit of its value when taking into account
the resistance of the soil as an equivalent of direct action, which is defined separately.
The direct methods are absent in these Norms for such detailing.

5. Everyone knows that the most reliable data on the bearing capacity of drilling
piles can be obtained only when they are tested in static compressing load. Test of
the piles are generally carrying out for the natural state of the soils. The negative
friction is taking into account when possible flood of territory and when the piles
are testing for withdraw load. Such tests are difficult (time consuming, long time
and expensive) if loess thickness is 25...30 m, and the design results are
approximate. Especially since the gotten data are the result of the instantaneous
load of piles, and they do not permit to analyze the change of the stress-strain state
of a weak base in time or they are characterizing a specific area or engineering-
geological section only. It complicates the formulation of the general laws of pile
behavior.
In practice, it is possible to solve the problem of calculation of pile foundations
on base of experimental data if we use well-developed numerical methods.

6. Many studies were performed according to the arrangement
of the early experimental bored piles at the construction site.
Piles dimensions were taken on the recommendations of the
simulation.
The value of the negative forces of friction Pn was simulated
as follows. The withdraw load was applied to top of single pile
without extension. This pile was installed between a day ground
surface and mark corresponding to the calculated depth of
action of negative friction of soil (soil in natural moisture and
water saturation of settling soil). The withdraw load was
gradually increased up to time when software complex PLAXIS
gives message "about development of plastic deformation of the
soil". This moment will correspond to the maximum axial
withdraw load (the amount of negative friction).
Modeling of single located pile cluster from 4 piles was
carried out (as it was foreseen in design) in order to
approximate the initial evaluation of the work of single pile
taking into account predefined negative friction forces on the
lateral surface of the pile.

7. Í àñèï í èé ãðóí ò (ñóãëèí êè
í åî äí î äí î ð³äí ³ â ñóì ³ø êó ç
áóä³âåëüí èì ñì ³òòÿì äî 10%)
Ñóãëèí êè ëåñî â³ , ï èëóâàò³
æî âòî -áóð³, òâåðä³, ï ðî ñ³äàþ ÷³
Ñóï ³ñêè ëåñî â³ ï èëóâàò³,
êàðáî í àòí ³, òâåðä³, ï ðî ñ³äàþ ÷³
Ñóãëèí êè ëåñî â³ ëåãê³, ï èëóâàò³,
æî âòî -áóð³ ç ÷åðâî í èì â³äò³í êî ì ,
êàðáî í àòí ³, òâåðä³, ï ðî ñ³äàþ ÷³
Ñóï ³ñêè ëåñî â³ ï èëóâàò³ ï àëåâî -æî âò³,
ç ï ðî æèëêàì è êàðáî í àò³â,
ï ðî ñ³äàþ ÷³
Ñóãëèí êè ëåñî â³ ï èëóâàò³ , ç âêëþ ÷åí í ÿì
êàðáî í àò³â (IL= -0.5; E=23Ì Ï à),
ï ðî ñ³äàþ ÷³
Ñóï ³ñêè ëåñî â³ ï èëóâàò³, êàðáî í àòí ³,
â³ä òâåðäèõ äî í àï ³âòâåðäèõ (I L= -0.25;
E=21Ì Ï à) ï ðî ñ³äàþ ÷³
Ñóï ³ñêè ëåñî â³ òÿæê³ , ï èëóâàò³, ç
âêëþ ÷åí í ÿì êàðáî í àò³â (IL= -0.14;
E=21.5Ì Ï à)
Ãëèí è ï èëóâàò³ ÷åðâî í î -áóð³,
òâåðä³, ç âêëþ ÷åí í ÿì êàðáî í àòí èõ
ñòÿæî ê (IL= -0.27; E=19Ì Ï à)
Ãëèí è ï èëóâàò³ áóðî âàòî -ñ³ð³,
òâåðä³, ç âêëþ ÷åí í ÿì êàðáî í àò³â
(IL= -0.27; E=19Ì Ï à)
Ï ³ñêè ñåðåäí ³ ç ë³í çàì è äð³áí èõ,
æî âòî -ñ³ð³, ç ï ðî ø àðêàì è ñóï ³ñê³â,
ñóãëèí ê³â, ì àëî âî ëî ã³ ³ âî ëî ã³, ù ³ëüí ³
(E=41Ì Ï à)
Ãëèí è ñ³ð³, çåëåí î âàòî -ñ³ð³,
òâåðä³, ç ï ðî ø àðêàì è ï ³ñê³â ³
ñóï ³ñê³â (E=18Ì Ï à)
Óì î âí ³
ï î çí à÷åí í ÿ
˳òî ëî ã³÷í ³ âèäè
ãðóí ò³â
1-1
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
Figure 2. Embarking of a pile
foundation on geologic section.
Modeling of single
located pile cluster from
4 piles was carried out (as
it was foreseen in design)
in order to approximate the
initial evaluation of the work
of single pile taking into
account predefined negative
friction forces on the lateral
surface of the pile.

8. Figure 3. Finite-element 3D model "pile foundations-settling base".

9. Figure 4. Generation of water pressure, kN/m2
.

10. Figure 5. Iso-fields of soil displacement in vertical direction Uy, m (section 1-1).

11. Figure 6. Iso-fields of soil displacement in vertical direction Uy, m (section 2-2).

12. Figure 7. Iso-fields of soil displacement in vertical direction Uy, m (section 3-3).

13. Table. Comparison of field tests of piles with expansion according to
numerical studies.

14. According to the calculations of numerical simulations, the bearing
capacity of the pile cluster, taking into account the forces of negative
friction on the lateral surface of the pile is 4468 kN, provided that the
working design is 4000 kN under prescribed conditions.
In the numerical simulation there was also found that the process of
gradual infiltration of water into the soil and its watering operates on a pile
in two ways. From the first side, the water saturation has place for increased
soil volume and most part of the pile cluster is under overhanging soil action
as the flow of water is action on loess soil. From the second side, the
cohesion forces on contact of pile with soil are decreased when the water
content is reduced.
I.e., the specific cohesion and angle of internal
friction is reduced by more than 10...15%. when
soaking the loose (loess) soil The value of the
bearing capacity of piles is under influence of
changes in soil under expansion due to soil
compaction. Thus the values of strength and
deformability can be increased by 10% or more.

15. 2) The negative friction force depends on the case of base soaking, depth of soaked
layer, the properties of settling soil of base and other factors. The soaking by parallel
layers of settling loose base below is more dangerous for the drilling piles (including the
piles with extension) then top soaking;
3) The proposed simplified method allows to research preliminary the drilling piles
work (including piles with extension) in structure of clusters) in loose settling soil due
to its own weight in view of the negative forces of friction for different cases of soaking
with necessary accuracy to select the structural measures for design. This method has a
strict definition, i.e. it provides the reliable estimates in determining the allowable load
on pile foundations of the specified type under construction architectural designs of the
project;
4) The results of these studies were used in the construction of the shopping center
wholesale and retail trade in Zaporozhe, which confirmed the reliability of the
calculations.
GENERAL CONCLUSIONS
The following conclusions may be done on the basis of detailed
analysis of the interaction of the pile with surrounding settlement of
loess massif:
1) Comparison of the calculated values of the obtained negative
friction forces on the lateral surface of the pile with the experimental
data shows that the difference of these values is 8 ... 13% ( in terms
of the construction site);

16. THANK YOU FOR YOUR ATTENTION
Peculiarities of implementation of primary numerical
modeling when determination of the forces of
negative friction on pile side surface in condition of
its test with static load