Importance of soil cohesion on the stability of retaining wall

Abstract

Cohesion is a material property that is independent of the normal stress. Coherent or bound materials have strength on shear at normal stresses equal to zero.
For retaining reinforced concrete walls incoherent materials such as gravel or crushed stone is used as a backfill material behind the wall. We use materials with the cohesion for backfill behind the retaining wall during the construction of the retaining wall on inaccessible terrain.
Based on geostatic calculations of reinforced concrete retaining wall for different sizes of cohesion from 0 kN/m2 to 15 kN/m2, we got the analysis of the external stability of retaining wall. The study shows the influence of cohesion on the size of an active soil pressure on retaining wall and the effect of cohesion on the size of the tensile stress in the soil behind the retaining wall, and the stability of the overturning and sliding of retaining wall.

Full text

17th International Research/Expert Conference
”Trends in the Development of Machinery and Associated Technology”
TMT 2013, Istanbul, Turkey, 10-11 September 2013
IMPORTANCE OF SOIL COHESION ON THE STABILITY
OF RETAINING REINFORCED CONCRETE WALL
Prof. PhD. Nedim Suljić, Civil Engineer
University of Tuzla, Faculty of Mining, Geology and Civil Engineering Tuzla
Ul. Univerzitetska broj 2, Tuzla
Bosnia and Herzegovina
ABSTRACT
Cohesion is a material property that is independent of the normal stress. Coherent or bound materials
have strength on shear at normal stresses equal to zero.
For retaining reinforced concrete walls incoherent materials such as gravel or crushed stone is used
as a backfill material behind the wall. We use materials with the cohesion for backfill behind the
retaining wall during the construction of the retaining wall on inaccessible terrain.
Based on geostatic calculations of reinforced concrete retaining wall for different sizes of cohesion
from 0 kN/m2 to 15 kN/m2, we got the analysis of the external stability of retaining wall. The study
shows the influence of cohesion on the size of an active soil pressure on retaining wall and the effect
of cohesion on the size of the tensile stress in the soil behind the retaining wall, and the stability of the
overturning and sliding of retaining wall.
Keywords: cohesion, retaining wall, active soil pressure, external stability.
1. THE TERM OF SOIL COHESION
Cohesion is a physical property of matter that is reflected in the intermolecular drag towards between
molecules of the same. Cohesion is a very important feature of a coherent or bound soil and occurs
under the influence of electrochemical forces between particles.
Shear strength of the soil is defined via Coulomb's law, which is:

tgc 
c-soil cohesion for total stresses,
-total normal stress,
-angle of internal friction for total stresses.
Coherent soils have a certain size of cohesion and it is usually classified according to the limits of
plasticity. Also, coherent soils have more than 50% weight of solid particles of dust or clay. The
difference between coherent and incoherent soils can best be seen when the soil is completely dry. In
that case an incoherent soil is the batch of dispersed particles (grains of sand, gravel, etc.), and a
coherent soil has a structure that can be shaped.
So, cohesion represents the soil resistance to shear when the normal stress is zero. Cohesion of the
soil is not constant and depends mainly on the moisture 2.
The size of the cohesion in the soil, beside the soil moisture, is influenced by following factors:
- particle size and their mineral composition,
- the distance between neighboring particles characterized by the coefficient of pore,
- electrochemical composition of pore water.

Based on this we can see that cohesion is not a constant value as in the practice of conventional
thinking. Therefore, the study and definition of the shear strength of coherent material is very
complex 3.
2. SOIL COHESION INFLUENCE
If we look at a concrete retaining wall with the backfill of coherent soil (clay, for example), then the
expression of active soil pressure on retaining wall is:
aaza kckp 2

(1)
Figure 1. Distribution of stress in the coherent soil for active state.
Figure 1 shows the distribution of pressure and tensile stress on the retaining wall. On the diagram of
the total active soil pressure we see that for coherent material there is a vertical stress
z

at depth zc
from the surface of terrain. For depths less than zc the tensile stress occurs in the soil behind the
retaining wall.
At a depth of zc there are tensile stresses and cracks occur in the soil behind the retaining wall at that
depth. Geostatic calculation ignores the part of an active soil pressure which is related to tension. The
depth at which there is tension in the soil, and at which the cracks occur in the soil behind retaining
wall, we get when we equal the expression (1) to zero, and then we have:
aaza kckp 2

= 0
cz z

a
ck
c
z


2
(2)
Expression (2) can be used to estimate the possible depth of vertical crack in the soil behind the
retaining wall. In practice, this term is the depth of the excavation pit in the clay ground that can stand
vertically without substructure support. By the increase of depth in the soil and, in particular, with the
penetration or infiltration of surface water into the ground, when cohesion is significantly reduced, the
vertical excavation sides plunge pressed by the mass of soil. The force intensity of active soil pressure
is corresponding to the surface of triangle of the pressed part of the active soil pressure diagram
behind retaining wall, and then we have 1:
aaa kckHE 2
2
1

 
'1mbH p
(3)
b-unit width of retaining wall.

Cracks formed in the zone of tension can be filled with water over time. When the backfill material
behind the retaining wall is relatively impermeable then the water will not disappear quickly so we
need to count on the hydrostatic pressure at the depth of the crack in the tense area 4.
3. IMPORTANCE OF SOIL COHESION ON THE EXTERNAL STABILITY OF
RETAINING WALL
To determine the effect and importance of cohesion size on the external stability of reinforced
concrete retaining wall, we analyzed the case of the same cross-sectional geometry of the reinforced
concrete retaining wall with the same soil parameters (, ), but with different values of soil cohesion
c=0 kN/m2, c=5 kN/m2, c=10 kN/m2 and c=15 kN/m2. For given conditions we analyzed safety
coefficients on overturning and sliding of the reinforced concrete retaining wall and established the
algorithm of functional dependency of coefficients on safety and overturning compared to the change
in soil cohesion.
Figure 2. The cross section of the analyzed retaining wall with soil parameters adopted
Based on geostatic calculations we created a diagram of vertical effective stresses in the soil as well as
diagrams of active soil pressure depending on the size of cohesion in the soil, as shown in Figure 3.
Figure 3. Diagram of vertical stresses and diagrams of active soil pressure depending on cohesion
By further geostatic calculations we got the force intensities of active soil pressure and the weight of
retaining reinforced concrete structures. Then we checked the stability of the retaining wall on
overturning and sliding, and the obtained results we can show in the corresponding diagrams.

Also, considering the size of cohesion in the soil the values of active soil pressure on retaining wall
are given in the form of the following diagram, shown in Figure 4.
Figure 4. Diagram of the active soil pressure changes in the function of cohesion in the soil
Also, based on the size of soil cohesion we calculated the values and impacts of cracks in the soil
behind the retaining reinforced concrete wall as well as the values of the active part of pressed height
of the retaining wall.
Figure 5. Diagram of changes in the depth of cracks in the soil (blue line) and pressed height of the
retaining wall (red line) in function of soil cohesion
Following tables show the values of safety factor of analyzed retaining reinforced concrete wall on
sliding and overturning, in function of the soil cohesion size.
Figure 6. Values of safety factor on sliding and overturning in function of the soil cohesion size
4. CONCLUSIONS
By conducted geostatic analysis we conclude that cohesion has a positive effect on the stability of
retaining walls. With an increase of soil cohesion, the active soil pressure on the wall decreases
linearly. Also, increasing the size of cohesion in the soil leads to a linear growth of cracks in the soil
behind the retaining wall, and with it there is a linear decrease in the pressed height of the retaining
wall. The existence of cohesion in the soil reduces the resultant of active soil pressure which leads to a
reduction in active soil pressure force arm resulting in decreasing of the destabilizing overturning
torque on the retaining wall. The increase in soil cohesion leads to a significant increase in safety
factor on overturning and sliding.
5. REFERENCES
1 Suljić N.: Supporting structures, university textbook, University of Tuzla, 2010.
2 Maksimović M.: Soil mechanics, AGM book Belgrade, 2008.
3 Suljić N.: Modern materials for the supporting structures, IGK Planjax Tešanj, 2005.
4 Stević M.: Soil and rocks mechanics, University of Tuzla, 1991.