Lateral Earth Pressures
Soil
Mechanics Lateral Earth Pressures page 1
Contents of this chapter :
CHAPITRE 7.
LATERAL EARTH PRESSURES.......................................................................... 1
7.1 INTRODUCTION............................................................................................................................... 1
7.2 LATERAL EARTH PRESSURES IN GRANULAR SOILS – RANKINE THEORY............................................. 2
7.2.1 POINT
A : ACTIVE
SOIL PRESSURE.................................................................................................. 2
7.2.2 POINT
B : PASSIVE
SOIL PRESSURE................................................................................................. 4
7.2.3 SUMMARY..................................................................................................................................... 5
7.3 LATERAL EARTH PRESSURES IN COHESIVE SOILS – RANKINE THEORY............................................... 6
7.4 EXERCISE....................................................................................................................................... 6
7.5 LATERAL EARTH PRESSURE IN PRESENCE OF GROUNDWATER......................................................... 6
7.6 LATERAL EARTH PRESSURES IN CASE
OF INCLINED GROUND SURFACE OR FRICTION AT WALL-
GROUND INTERFACE.............................................................................................................................. 9
7.6.1 EXERCISES................................................................................................................................... 9
7.7 TECHNIQUES USED TO RESIST LATERAL EARTH
PRESSURES................................................................ 10
7.7.1 GRAVITY WALL............................................................................................................................ 10
7.7.2 SHEET PILE WALL......................................................................................................................... 10
7.7.3 CANTILEVER WALL....................................................................................................................... 11
7.7.4 SECANT OR TANGENT
(ALSO
CALLED CONTIGUOUS) PILE WALL......................................................... 11
7.7.5 ANCHORED WALL........................................................................................................................ 12
7.7.6 SOIL NAILING.............................................................................................................................. 13
7.7.7 SOIL-STRENGTHENED.................................................................................................................. 13
7.7.8 DIAPHRAGM WALL........................................................................................................................ 14
7.7.9 TIMBERED TRENCH...................................................................................................................... 15
Chapitre 7. Lateral Earth Pressures1
7.1
Introduction
The
pressure at any point in a fluid such as water is the same in all directions.
Thus the lateral pressure on a vertical surface retaining water is equal to gw.h where h = the height of water
above the point considered. Fig. 1 shows the lateral pressure diagram on a wall
of height H retaining water.
The
total force P per unit length of wall will be equal to the area of the pressure
diagram.
P=
½ gw.H² and this force will act at the
centroid of the diagram, i.e. at 2H/3 from the surface.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
h
|
gw.h
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
H
|
|
|
2/3 H
|
Ko.s'
|
|
s'v.z
|
z
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
gw.H
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Figure 1 : lateral pressure in
water
|
Figure 2 :
lateral pressure in soil
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Soil
Mechanics Lateral Earth Pressures page 2
In the case of soil, which. unlike
water, possesses resistance to shearing, the lateral pressure at any point will
not be the same as the vertical pressure at that point (Fig.2).
In a homogeneous natural soil
deposit, the ratio s’h/s’v is a constant known as
coefficient of earth pressure at rest (K0).
For
normally consolidated clays and granular soils, K0» 1 – sin f’
In order to design soil-retaining
structures such as retaining walls2 and sheet pile walls3, it is necessary to determine the
magnitude of the lateral pressures to which the structure is subjected.
The lateral pressure behind a wall
will vary depending on whether the wall is going away from soil or towards
soil.
7.2
Lateral Earth
Pressures In Granular Soils – Ran kine theory
Hypothesis
: the ground surface is horizontal and there is no friction between the wall
and the soil.
Let's imagine a sheet pile wall
(Fig3.) and let’s look at the soil elements A and B during the wall movement
caused by the earth pressures at the right of the wall.
7.2.1 Point A : Active soil pressure
In A, the earth pressure is called
"active", because the soil in A is responsible of the wall movement.
Wall movement
|
|
|
|
|
|
|
Wall moves
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
away from soil
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
z
|
|
sv’
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
sh’
|
|
|
|
|
|
|
Wall moves
|
|
|
|
A
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
towards soil
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
B
|
|
|
|
|
|
Initial
|
|
|
|
|
|
|
|
|
|
position of
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
the wall
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Figure 3 : Active soil pressure
Initially, there is no lateral
movement, thus, at this time, the Mohr circle (Fig.4) has two principal
stresses
sv’
= g.z and
sh’
= K0 sv’ = K0 g.z
As
the wall moves away from the soil,
·
sv’
remains the same and
2
Murs
de soutènement
3
rideau
de palplanches
Soil
Mechanics Lateral Earth Pressures page 3
·
sh’ decreases till failure occurs.
The
Mohr circle changes thus during the movement and at failure, it is tangent to
the Mohr-Coulomb failure line (Fig.4).
t
active earth
|
Mohr-Coulomb
|
At failure
|
|
failure line
|
|
pressure
|
(Active limit state)
|
|
|
|
Initially (Rest state)
s’h,active
K0.sv’ sv’ s
decreasing
sh’
Figure 4 : Mohr circle under Active Soil Pressure
As we have seen
in chapter 6 (exercise 1), the failure plane is at 45° +
t
Failure plane is at 45° + f/2 to horizontal
|
45°+ f/2
|
|
|
r
|
|
f
|
90°+ f
|
|
|
|
|
sv’
|
|
|
s’h,active
|
|
|
x
|
|
f/2 to horizontal (Fig.5).
sv’
sh’
A
s
Figure 5 : Mohr circle at Rankine Active Limit State
s 'h ,active = K Asv
Where KA is called the Rankine’s
coefficient of active earth pressure
|
|
|
|
|
|
|
Lateral Earth Pressures
|
page 4
|
|
|
|
s 'h ,active
|
|
x - r
|
1-
|
r
|
|
|
1 - sin f ' = tan 2 (45 -f '/ 2)
|
|
|
|
|
|
x
|
|
|
|
K A
|
=
|
=
|
=
|
|
|
=
|
|
|
|
|
|
|
|
|
|
|
|
s v '
|
|
x + r
|
1
|
+
|
r
|
|
|
1+ sinf '
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
x
The displacement of the wall
necessary to reach the active Rankine state is about H/1000, where H is the
height of the wall.
7.2.2 Point B : Passive soil pressure
Now, let’s look at the soil
element B during the wall movement caused by the earth pressures at the right
of the wall.
The
soil in B resists to the wall movement, thus the earth pressure in B is called
"passive".
Initially,
there is no lateral movement, thus the Mohr circle (Fig.6) has two principal
stresses
sv’
= g.z and
sh’
= K0 sv’ = K0 g.z
As the wall moves away from the
soil,
·
sv’
remains the same and
·
sh’
increases till failure occurs.
At
failure, the Mohr circle is tangent to the Mohr-Coulomb failure line (Fig.6).
t
Mohr-Coulomb failure line
Initially (Rest
state)
K0.sv’ sv’ increasing sh’
sv’
sh’ B
At failure (Active limit state)
passive earth pressure
s’h,passive
s
Figure 6 : Mohr circle under Passive Soil Pressure
As we have seen
in chapter 6 (exercise 1), the failure plane is at 45° - f/2 to horizontal (Fig. 7).
Soil
Mechanics Lateral Earth Pressures page 5
t
sv’
Failure
plane is at
|
|
|
|
45° - f/2 to horizontal
|
sh’
|
|
|
B
|
|
|
|
|
|
|
|
|
45° - f/2
|
r
|
|
f
|
90°+ f
|
|
|
|
|
s
|
|
sv’
|
s’h,passive
|
|
x
Figure 7 : Mohr circle at Rankine Passive Limit State
s 'h , passive = KPs v '
Where KP is called the Rankine’s
coefficient of passive earth pressure
|
|
s 'h ,
passive
|
|
x + r
|
1+
|
|
r
|
|
|
1 + sin f ' = tan 2 (45 +f '/ 2)
|
|
|
|
|
x
|
|
|
KP
|
=
|
=
|
=
|
|
|
|
=
|
|
|
|
|
|
|
|
|
|
|
s v '
|
|
x - r
|
1
|
-
|
|
r
|
|
|
1- sinf '
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
x
The displacement of the wall
necessary to reach the Rankine passive state is about H/100, where H is the
height of the wall. The necessary movement of the wall is thus 10 times greater
than the one necessary to reach the active state.
7.2.3 Summary
t
|
s
|
|
2
|
(45+f '/ 2
|
|
|
' = s ' tan
|
|
|
|
1
|
3
|
|
|
|
|
s
|
|
2
|
(45-f '/ 2
|
|
|
' = s ' tan
|
|
|
|
3
|
1
|
|
|
|
f'
|
|
|
|
s'
|
|
s' 3
|
s' 1
|
|
|
|
|
Figure 8 : Mohr circle at Rankine Limit States
In the active state, s'1 = s'v and s'3 = s'h.
In the passive state, s'1 = s'h and s'3 = s'v.
Soil
Mechanics Lateral Earth Pressures page 6
7.3
Lateral Earth
Pressures In Cohesive Soils – Ran kine theory
By
similar developments, we find, in case of cohesive soils : t'
|
|
s ' = s
|
'
|
2
|
(45+ f '/
2)+
|
2c
|
' tan(45+ f
|
'/ 2
|
|
|
|
tan
|
|
|
|
1
|
3
|
|
|
|
|
|
|
|
|
|
s ' = s
|
'
|
2
|
(45- f '/ 2)-
|
2c
|
'
tan(45- f
|
'/ 2
|
|
|
|
tan
|
|
|
|
3
|
1
|
|
|
|
|
|
|
|
f'
|
c'
|
|
|
|
s'
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
s'3
|
s'1
|
|
|
|
|
|
|
|
|
c'.cot f'
Figure 9 : Mohr circle at Rankine States in cohesive
soils
In
the active state, s'1 = s'v and s'3 = s'h.
In
the passive state, s'1 = s'h and s'3 = s'v.
In practice, for long term
calculations, the cohesion is not taken into account in the calculations,
because it is impossible to be sure that it will be always present. It is thus
safer to neglect it.
7.4
Exercise
A
retaining vertical wall 6 m high supports cohesionless dry soil of dry unit
weight 16.3 kN/m³, effective angle of friction 35° and void ratio 0.68 . The
surface of the soil is horizontal and level4 with the top of the wall.
Neglecting wall friction, determine the total active earth thrust5 on the wall per metre of wall and
at what height above the base of the wall the thrust acts.
7.5
Lateral Earth
Pressure In Presence Of Groundwater6
If due to poor drainage, the water
table is 2.5 m below the ground surface in the previous exercise, determine the
resulting lateral thrust on the wall and at what height above the base of the
wall the thrust acts. Note : when the soil is saturated, the friction angle is
30°.
4
de
niveau
5
poussée
6
nappe
d'eau souterraine
Soil
Mechanics Lateral Earth Pressures page 7
Step one: Calculation
of Saturated Unit Weights. (Assume a volume of solid of 1m³)
|
|
|
|
|
|
|
|
Distribution by Volume
|
Distribution by Weight
|
Distribution by Weight
|
|
|
|
|
|
|
|
|
|
|
|
(m³)
|
for the dry soil
|
for the saturated soil
|
|
|
|
|
|
|
|
|
|
|
|
|
(kN)
|
(kN)
|
|
|
Voids
|
|
|
|
|
Vv = e.Vs= e = 0.68
|
0
|
Wv= Vv . gw = 6.8
|
|
|
Solid
|
|
|
|
|
|
|
1
|
Ws
|
Ws
|
|
TOTAL
|
|
|
|
|
|
|
1.68
|
Ws
|
Ws+6.8
|
|
g
|
|
=
|
Ws
|
kN
|
= 16.3 kN / m3
|
|
|
|
dry
|
|
|
|
|
|
|
|
1.68 m3
|
|
|
|
|
|
|
|
Hence Ws = 27.384
kN
|
|
|
|
|
and
|
|
g
|
|
=
|
Ws + 6.8kN
|
|
= 20.35 kN / m3
|
|
|
|
|
sat
|
|
|
|
|
|
|
|
|
|
|
1.68 m3
|
|
|
|
|
Step two: calculations of the pressures on the wall
The
calculations are done, taking into account that :
·
The earth pressures on the wall are calculated, under
the water table, from the effective stresses
·
The water pressure is added because it is present
between the soil particles and acts on the wall independently of the earth
pressure
|
soil
|
|
|
Effective
|
|
|
horizontal
|
|
Wall
|
stress
|
|
|
|
Water pressure in the
voids
Figure 7 : Lateral pressures on the wall
·
Below the water table the angle of friction changes,
so does Ka. This explains the step at 2.5m depth in the active pressure diagram
because the horizontal effective stress is different in the dry soil and in the
wet soil at that depth.
Soil
Mechanics Lateral Earth Pressures page 8
|
|
|
|
q
|
|
|
|
|
|
|
|
|
|
|
|
|
Active
pressure (kN/m ²)
|
|
|
|
|
|
|
|
0.0
|
10.0
|
20.0
|
30.0
|
40.0
|
|
|
|
|
|
|
0.0
|
|
|
|
|
|
|
|
|
H1
|
|
1.0
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
2.0
|
|
|
|
|
|
|
Hw
in layer 2
|
|
H2
|
(m)
|
3.0
|
|
|
|
active
pressure
|
|
|
|
depth
|
|
|
|
water pressure
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
4.0
|
|
|
|
|
|
|
|
|
|
Hw
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
H4
|
|
|
|
5.0
|
|
|
|
|
|
|
|
H3
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
6.0
|
|
|
|
|
|
|
DATA
|
|
|
|
|
|
|
|
|
|
|
H1
|
2.5
|
m
|
|
|
|
|
|
|
|
|
g1
|
16.3
|
kN/m³
|
Ka1
|
0.27099005
|
KA = tan
2 (45 -f '/ 2)
|
|
|
f1
|
35
|
°
|
|
|
|
|
|
|
|
|
H2
|
3.5
|
m
|
|
|
|
|
|
|
|
|
g2
|
20.35
|
kN/m³
|
Ka2
|
0.33333333
|
|
|
|
|
|
f2
|
30
|
°
|
|
|
|
|
|
|
|
|
Hw
|
3.5
|
m
|
|
|
|
|
|
|
|
|
q
|
0
|
kN/m²
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Active pressures
|
|
|
|
|
|
|
Forces
|
|
|
|
|
|
|
|
d(m)
|
|
sv,i(kN/m²
|
sv,tot(kN/m²
|
u(kN/m²)
|
s'V(kN/m²)
|
s'H(kN/m²)
|
Fi(kN)
|
xi(m)xtoe(m)
|
|
Mi (kNm)
|
|
|
DRY
|
|
0.0
|
0.0
|
0.0
|
0.0
|
0.0
|
0.0
|
|
|
|
|
|
|
|
DRY
|
|
2.5
|
40.8
|
40.8
|
0.0
|
40.8
|
11.0
|
13.8
|
0.8
|
|
4.3
|
59.8
|
|
|
WET
|
|
2.5
|
0.0
|
40.8
|
0.0
|
40.8
|
13.6
|
0.0
|
0.0
|
|
3.5
|
0.0
|
|
|
WET
|
|
6.0
|
71.2
|
112.0
|
35.0
|
77.0
|
25.7
|
68.7
|
1.6
|
|
1.6
|
107.9
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Water pre
|
|
2.5
|
|
h(m)
|
0
|
|
|
|
|
|
|
|
|
|
active
side
|
|
6
|
|
3.5
|
35.0
|
|
|
61.3
|
|
|
1.2
|
71.5
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
TOTAL Active
side
|
|
|
|
|
|
|
143.7
|
|
|
|
239.1
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
x toe (m)
|
|
1.66374632
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Soil
Mechanics Lateral Earth Pressures page 9
7.6 Lateral Earth Pressures in case of
inclined ground surface or friction at wall-ground interface
By now, we have considered the
wall as perfectly smooth and the ground surface as horizontal.
In
practice, a perfectly smooth wall is not realistic because some friction is
developing between the wall and the ground.
The
amount of shear stress, which can be mobilised at the wall-ground interface is
determined by the wall-ground interface parameter d.
A
concrete wall or steel sheet pile wall supporting sand or gravel may be assumed
to have a design wall ground interface parameter d=
k.f .
Where :
·
k £ 2/3 for
precast concrete or steel sheet piling
·
k = 1,0 for concrete cast against soil
Values of the earth pressure
coefficients may be taken from figures C.1.1 to C.1.4 for Ka and C.2.1 to C.2.4 for Kp. These figures are taken from the
Annex C of the Eurocode EN 1997-1 and are in a separate PDF file on the Moodle
website.
7.6.1 Exercises
1.
Looking at the figures of the Annex C of the Eurocode
EN 1997-1, try to find how will change the earth pressure, if friction is taken
into account in case of active pressure and passive pressure behind a wall
retaining a horizontal ground surface. (answer this question on Moodle)
2.
Find the total horizontal force acting, per linear
meter of wall, on the precast concrete gravity wall (efficient drainage is
present behind the wall)
|
|
|
|
b = 18°
|
|
|
|
|
g=
18 kN/m³
|
|
|
|
|
|
10 m
|
|
|
f= 30 °
|
|
|
|
|
|
|
|
Soil
Mechanics Lateral Earth Pressures page 10
7.7
Techniques used
to resist lateral earth pressures
The most frequent structure used
to retain soil is the retaining wall. Various types of retaining walls are
described herebelow.
7.7.1 Gravity wall7
Gravity
walls depend on the weight of their mass (stone, concrete or other heavy
material) to resist pressures from behind and will often have a slight
inclination to improve stability by leaning back into the retained soil.
For
short landscaping walls, they are often made from mortarless stone or segmental
concrete units.
Figure 8 : gravity wall
7.7.2 Sheet pile wall
Sheet pile walls are often used in
soft soils and tight spaces. Sheet pile walls are usually made out of steel
sheet piles driven8 into the ground.
As
a rule of thumb: 1/3 third of the sheet pile is above ground, 2/3 below ground.
Figure 9 : sheet pile wall
Soil
Mechanics Lateral Earth Pressures page 11
7.7.3
Cantilever wall9
Cantilever walls are made from a relatively thin stem
of steel-reinforced, cast-in-place concrete (often in the shape of an inverted
T). These walls cantilever loads (like a beam) to a large, structural footing,
converting horizontal pressures from behind the wall to vertical pressures on
the ground below. Sometimes cantilevered walls include a counterfort on the
back, to improve their stability against high loads.
These
walls require rigid concrete footings below frost depth.
This type of wall uses much less material than a
traditional gravity wall.
Figure 10 : cantilever wall
7.7.4 Secant10 or Tangent11 (also called contiguous) pile
wall
Secant pile walls are formed by constructing
intersecting reinforced concrete piles.
The
piles are reinforced with either steel rebar12 or with steel beams and are
constructed by either drilling under mud or augering13.
Primary piles are installed first with secondary
piles constructed in between primary piles once the latter gain sufficient
strength.
Pile
overlap is typically in the order of 8 cm.
In
a tangent pile wall, there is no pile overlap
as the piles are constructed flush14 to each Figure 11 : Secant
Pile wall other.
The
main advantages of secant or tangent pile walls are:
1. Increased
construction alignment flexibility.
2. Increased
wall stiffness compared to sheet piles.
9
Mur
cantilever
10
Secant
pile = pieu sécant
11
Tangent
pile = pieu tangent
12
Barre
d'armature
13 Forage
à la tarière
14
à
ras
Soil
Mechanics Lateral Earth Pressures page 12
3. Can be
installed in difficult ground (cobbles/boulders).
4. Less noisy
construction.
The main disadvantages of secant
and tangent pile walls are:
1. Verticality
tolerances may be hard to achieve for deep piles.
2. Total
waterproofing is very difficult to obtain in joints.
3. Increased
cost compared to sheet pile walls.
For a
more comprehensive explanation of the building technique of such a wall, see
the separate pdf file on the Moodle website.
7.7.5 Anchored wall15
This
version of wall uses cables or other stays anchored in the rock or soil behind
it. Usually driven into the material with boring, anchors are then expanded at
the end of the cable, either by mechanical means or often by injecting
pressurized concrete, which expands to form a bulb in the soil. Technically
complex, this method is very useful where high loads are expected, or where
the
wall itself has to be slender and would otherwise be too weak. Figure
12 : anchored wall The wall itself can be a sheet pile
wall or secant pile wall.
Figure 13 : anchored sheet pile wall
Note : These two pictures were taken
at the
Guillemins station works in Liège.
Figure 14 : anchored secant pile wall
Soil
Mechanics Lateral Earth Pressures page 13
7.7.6 Soil nailing16
Soil
nailing is a technique in which soil slopes, excavations or retaining walls are
reinforced by the insertion of relatively slender elements - normally steel
reinforcing bars. The bars are usually installed into a pre-drilled hole and
then grouted17 into place or drilled and grouted simultaneously.
They are usually installed untensioned at a slight downward inclination. A
rigid or flexible facing (often sprayed concrete) or isolated soil nail heads
may be used at the surface.
|
|
Figure 15 : Soil nailing
|
|
|
7.7.7 Soil-strengthened
|
|
|
|
|
|
|
|
|
A
number of systems exist that do not simply
|
|
|
|
|
|
|
|
|
consist
of the wall itself, but reduce the earth
|
|
|
|
|
pressure
acting on the wall itself. These are
|
|
|
|
|
usually used in combination with
one of the
|
|
|
|
|
other wall types, though some
may only use it
|
|
|
|
|
as facing (i.e. for visual
purposes).
|
|
|
|
|
Gabion meshes18
|
|
|
|
|
This type of soil strengthening,
often also used
|
|
|
|
|
without an outside wall,
consists of wire mesh
|
|
|
|
|
'boxes' into which roughly cut
stone or other
|
|
|
|
|
material is filled. The mesh
cages reduce some
|
|
|
|
|
internal movement/forces, and
also reduce
|
|
|
|
|
erosive forces.
|
Figure 16 : Gabion meshes
|
|
|
|
|
|
|
|
|
|
|
Mechanical stabilization
Mechanically stabilized earth19 is soil reinforced by layered
horizontal mats (geosynthetics). Other options include steel straps,
also layered.
The wall face is often of precast
concrete units that can tolerate some differential movement. The reinforced
soil's mass, along with the facing, then acts as an improved gravity wall. The
reinforced mass must be built large enough to retain the pressures from the
soil behind it. Gravity walls usually must be a minimum of 50 to 60 percent as
deep or thick as the height of the wall, and may have to be larger if there is
a slope or surcharge on the wall.
Figure
17 : soil reinforcement by layered horizontal mats
16
Sol
cloué
17
injectée
18
gabions
19
Terre
armée
Soil
Mechanics Lateral Earth Pressures page 14
Figure 18 : soil reinforcement by geotextiles
7.7.8 Diaphragm wall20
The
continuous diaphragm wall (also referred to as slurry wall in the US) is a
structure formed and cast in a slurry trench.
The
trench excavation is initially supported by either bentonite21 or polymer based slurries that
prevents soil incursions into the excavated trench. The term "diaphragm
walls" refers to the final condition when the slurry is replaced by
tremied concrete that acts as a structural system either for temporary
excavation support or as part of the permanent structure. This construction
sequence is illustrated in Figure 19.
Figure 19 : Diaphragm wall
20
Paroi
moulée
21
La
bentonite est un argile
Soil
Mechanics Lateral Earth Pressures page 15
7.7.9 Timbered trench22
This
kind of wall retains the soil by means of struts23 and sheetings24. It is widely used for trenches.
.
Figure 20 : timbered trench
22
Tranchée
blindée
23
entretoise
24
coffrage