Negative friction in pile operation

Assoc.Prof.Dr.Dao Van Toai

The bearing capacity of a pile is the maximum load that the pile can receive and ensure that the structure still operates under normal conditions.
In soil, the bearing capacity of the pile is created by friction acting on the surface around the pile and the reaction force of the ground acting on the pile tip.
Under normal conditions, under the effect of load, the pile will move downwards more than around the pile and then the frictional force of the soil around the pile will appear to prevent that displacement of the pile and move upwards - that friction is called positive friction - it creates the load-bearing capacity of the pile. That frictional force at a certain segment i is equal to (u*l _i *f _si ). With: u is the cross-sectional circumference of the pile; l _i - is the length of the pile segment in the segment i; f _si - the average calculated frictional resistance of the soil in that segment (Figure 1).

However, in reality, the operation of the pile does not always happen smoothly like that, but in many cases in some segments of the pile, the soil around the pile, for various reasons, moves down more than the pile. At that time, the soil does not want to sink down more, so it "holds" the pile, that "holding" force is negative friction - it is directed downward.
Negative friction reduces the bearing capacity of the pile (Figure 2).
When does negative friction appear? Of course, negative friction appears during the construction period. During the construction phase, whether driven or pressed, the movement of the pile downwards is many times faster than the movement of the soil, so there is no way that negative friction can appear.
So in reality, what factors are the causes of negative friction? These are all factors that cause the soil around the pile to settle more than the pile itself. In particular, when the soil layers through which the pile penetrates have weak soil layers, negative friction easily appears in the pile section from the weak soil layer up.
Specifically, negative friction appears when:
- The ground surface around the pile bears an additional load with intensity q ≥ 20kN/m ² (Figure 3).

- Thickness of the leveling layer ≤ 1m.
- Due to lowering of groundwater level. Currently, groundwater exploitation in urban areas is taking place at high intensity, so the groundwater level has been lowered a lot. At that time, in this area of lowered groundwater level, the effective self-weight of the soil increases by approximately 10kN/m ³ - it acts as an additional load that continues to cause the soil to sink further and create negative friction.
- Due to the load of the newly built adjacent building, the ground of the old building sinks further.
- Because the new land for construction has not yet completed the consolidation process...
As mentioned above, negative friction reduces the load-bearing capacity of piles. Depending on the magnitude of this force, it can: lightly cause the structure to subside, the structure to crack, severely cause the connection between the pile and the pile cap to weaken or be completely destroyed, even break the pile and depending on the level of damage, it will lead to different construction incidents.
To avoid unwanted incidents to the construction caused by negative friction, this issue has been studied by many scientists and recommendations have been made, cases where negative friction must be considered. Below, we only present the conditions stated in “Pile foundation – Design standards” – TCVN 10304-2014.
- The layer of earth filling is thicker than 1.0m;
- Useful load on warehouse floor exceeds 2kN/m ² ;
- Place equipment with a useful load from the equipment over 100 kN/m2 on the floor next to the foundation;
- Increase effective stress, eliminate buoyancy load of water due to lowering groundwater level in the soil;
- Consolidation of soil of recent sediments and artificial sediments has not been completed;
- Compact loose soils by dynamic loading;
- Subsidence due to flooding;
- When building a new building near an existing building
So when there is negative friction, how will the calculation be carried out? Up to now, there have been many research works and calculation methods proposed on this issue. We can mention the methods of Joseph E.Bowler (1996), GHPoulos & EHDavis (1980), Braja M.Das (2007), Terzaghi and Peck (1967), Brinch Hansen (1968) ... As well as in the design standards of Vietnam, Japan, the United States, the European community (Eroucode7) ... Below we would like to introduce the method of Braja M.Das.
The author distinguishes the following two cases:
Case 1:

The backfill layer is clay lying on loose soil layer (Figure 4). Unit negative friction force acting on the pile:
(1)
In there: - coefficient of lateral pressure of soil, =K0=1-sinj';
- Effective vertical stress caused by self-weight at depth z;
d' - External friction angle between soil and pile ( d' » (0.5÷0.7) j';
The total frictional force acting on the pile is:
P _n = (2)

In which: H _f – Height of the embankment layer;
- Effective unit volume weight of the embankment;
u - Cross-sectional perimeter of pile;
Case 2:
The backfill layer is a layer of sand lying on top of a layer of clay (Figure 5).
In this case negative friction appears on the pile section from z=0 to the neutral plane ^(*[1]) . Depth of the neutral plane, according to Bowler (1982):
(3)

(*[1]) The plane at depth z=L1 where there is no relative displacement between the pile and the soil is called the neutral plane. The part of the pile above the neutral plane is subjected to negative friction while the part below is subjected to positive friction.
In there: - are respectively the effective unit volume weights of the sand and clay fill.
The unit negative friction force is also defined according to (1), but
The total negative friction force acting on the pile is:
P _n = (4)

To reduce the effects of negative friction can be done by the following measures:

- Reduce the external friction angle d' between soil and pile by sweeping a layer of bitumen onto the surface of the pile section subject to negative friction. To protect this bitumen layer during the pile driving process, it can be done by drilling or using a belt to separate the soil.
- Reduce ground settlement before pile construction by soil compaction solutions such as compaction, preload reduction combined with vertical drainage to increase consolidation speed and shorten loading time.
- Isolate the pile from the ground by placing the pile in a pipe with a larger diameter, so that the negative friction force does not directly affect the pile. This method is quite expensive and complicated, the simplest and most effective method is lubrication.
Conclude:
Because it depends on many factors, taking into account the negative friction effect in the design of a project is a complex problem. Although it has been studied quite a lot, it still needs to be further improved. Negative friction can cause serious problems for the project, so when calculating and designing a project located on weak soil areas with a large thickness of the ground layer, special attention must be paid.
References
1. R. Whitlow, soil mechanics, Education Publishing House, Hanoi 1997;
2. Braja M.Dá, Principles of Foundatio Engineering, the USA, 2007;
3. CBYxob…, Soil mechanics, foundations and soils, ACB Publishing House, Moscow, 1994;
4. Nguyen Ngoc Thanh, analysis and evaluation of the effectiveness of solutions to reduce negative friction of piles in weak geological areas. Master's thesis, 2016;
5. TCVN 10304-2014. Pile foundation and design standards, Hanoi, 2014.