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CHAPTER THREE
LATERAL EARTH PRESSURE
3 Introduction
A retaining wall is a structure that is used to support a vertical or near vertical slopes of soil. The resulting horizontal stress from the soil on the wall is called lateral earth pressure. To determine the magnitude of the lateral earth pressure, a geotechnical engineer must know the basic soil parameters – that is, unit weight γ , angle of friction φ , and cohesion c – for the soil retained behind the wall. In the evaluation of the magnitude of this lateral earth pressure, it is assumed that the soil behind the wall (called backfill soil) is on the verge of failure and obeys some failure criterion, for example, the Mohr-Coulomb failure criterion. When you complete this chapter, you should be able to:
Understand lateral earth pressure.
Determine lateral earth pressure.
3.1 Definitions of Key Terms
At rest earth pressure coefficient (k 0 ) is the ratio between the lateral and vertical principal effective stresses when an earth retaining structure is at rest (or is not allowed to move at all). Active earth pressure coefficient (ka) is the ratio between the lateral and vertical principal effective stresses when an earth retaining structure moves away from the retained soil. Passive earth pressure coefficient (kp) is the ratio between the lateral and vertical principal effective stresses when an earth retaining structure is forced to move against a soil mass.
3.2 Lateral Earth Pressure at Rest
Consider a vertical wall of height H, as shown in Fig. 3.1, retaining a soil having a unit weight of γ. At any depth z below the ground surface the vertical effective stress is:
σ v '^ = γ z − u (3.1)
If the wall is not allowed to move at all either way from the soil mass or to the soil mass (or in other words if there is no lateral expansion or compression in the backfill soil), the lateral pressure is called at rest earth pressure. In this case, the lateral earth pressure σ (^) x ' at a depth z is:
σ x ' = k 0 σ^ ' z (3.2)
where k 0 is coefficient of at rest earth pressure. You must remember that k 0 applies
FIGURE E3.
3.3 Active and Passive Lateral Earth Pressures
The lateral earth pressure condition described in section 3.2 involves walls that do not yield at all. In this section, we will discuss other conditions that involve movement of the wall and consequently failure of the backfill soil. Failure of the backfill soil occurs by two mechanisms depending on the direction of wall displacement. If the displacement of the wall is away from the backfill soil the resulting failure is called active and the lateral pressure exerted on the wall by the backfill soil is called active lateral earth pressure or simply active earth pressure. A passive failure occurs if the wall is displaced towards the backfill soil until the limiting displacement is achieved. In this case, the wall exerts a pressure on the backfill soil, and the passive resistance provided by the backfill soil against the wall displacement is called passive earth pressure. In the next sections, we will deal with two active and passive earth pressure theories: one proposed by Rankine (1857) and the other by Coulomb (1776).
3.4 Rankine Active and Passive Earth Pressures
Consider a vertical frictionless (smooth) wall retaining a soil mass in both front and back of the wall as shown in Fig. 3.2a. If the wall remains rigid and no movement occurs, then the vertical and horizontal (lateral) effective stresses at rest on element A, at the back of the wall, and B, at the front of the wall are given by Eqns. (3.1 and 3.2) in section 3.2. Mohr’s circle for the at rest state is shown by circle ① in Fig. 3.2b. Let us now assume a rotation about the bottom of the wall sufficient to produce slip planes in the soil mass behind and in front of the wall (Fig. 3.3). The rotation required, and consequently the lateral displacement or strain, to produce slip planes in front of the wall is much larger than that required for the back of the wall, as shown in Fig. 3.4. The soil mass at the back of the wall is assisting in producing failure, thus
it is in the active pressure state while the soil mass at the front of the wall is resisting failure, thus in the passive pressure state.
Figure 3.2: a) A smooth retaining wall, b) Mohr’s circles for at rest, active and passive states.
Figure 3.3: Failure planes within a soil mass near a retaining wall.
Figure 3.4: Rotation required to mobilize active and passive pressures.
principal stress (^) σ 1 ' and the vertical effective stress (^) σ ' z becomes the minor principal stress (^) σ 3 ' as shown in Fig. 3.2 b (Mohr’s circle ③). Therefore, using Eqn. (1.16) in Chapter 1, the Rankine passive lateral effective stress is:
z p p
p z
k c k
c
1 sin '
2 '^1 sin '
1 sin '
1 sin '
'
' '
σ
φ
φ φ
σ σ φ (3.10)
where,
tan ( 45 '
1 sin '
1 sin ' 2 φ φ
φ (^) = +
=^ +
k p (3.11)
is called the passive earth pressure coefficient. Based on Eqns. (3.9 and 3.11), we can easily get the following relation for the active and passive earth pressure coefficients:
a
k p k
Equations (3.8) and (3.10) indicate that, for a homogeneous soil layer, the lateral earth pressure varies linearly with depth z.
Figure 3.5: pressure distribution in c- φ ' soil: a) c- φ ' soil, b) active, c) passive state.
Figure 3.5 shows the active and passive lateral stress distribution for a smooth wall retaining a c- φ ' soil. In the active state case, the soil at depth z = 0 is subjected to a tensile stress as shown in Fig. 3.5 b. Soils do not have tensile strength, as a result tension cracks will occur down to a depth z 0 , where the tensile stress becomes zero. At depth z 0 (known as depth of tension crack), the stress is zero, thus,
a
a a k
z k c k z c
0 ' 2 '^2 '
The lateral earth force is the area of the lateral stress diagram (Fig. 3.5), which for the Rankine active state, is:
a a a
H
Pa ( zka 2 c ' k ) 21 k ' H^22 c ' H k
0
= '^ − = −
∫γ^ γ (3.14)
and, for the Rankine passive state, is
p p p
H
Pp ( zkp 2 c ' k ) 21 k ' H^22 c ' H k
0
= '^ + = +
∫γ^ γ (3.15)
For most retaining wall construction, a granular backfill is used and c’ = 0, therefore, for granular soils Eqns. (3.13) and (3.14) can be rewritten as:
Pa = 21 ka γ' H^2 (3.16)
and
2
P p = 21 kp γ' H (3.17)
EXAMPLE 3.
Draw the active pressure diagram per meter length of an 8 m high smooth vertical retaining wall. Also, calculate a) tension crack depth and b) the resultant (total) active force and its location. The properties of the backfill soil are c’ = 20 kPa,
φ '=25^0 and γ =17.5 kN/m^3. Note that the tension zone is usually ignored for
finding the magnitude and location of the resultant force.
3.5 Lateral Earth Pressure due to Surcharge
Surfaces stresses (due to surcharge) also impose lateral pressure on retaining walls as illustrated in Fig. 3.6 d. A uniform surface stress, qs, will transmit a uniform active lateral earth pressure of kaqs and a uniform passive lateral earth pressure of
kpqs. The active and passive lateral stresses due to the soil (i.e. c’, φ 'soil), and the
uniform surfaces stresses are then:
σ a ' = k a γ' z + kaq s (3.18)
and σ p ' = k p γ' z + kpqs (3.19)
The corresponding active and passive lateral forces are also given by:
pressure) to the lateral earth pressure. For example, if the groundwater level is at a distance hw from the base of the wall as shown in Fig. 3.6, the hydrostatic pressure is,
u = γ w h w (3.26)
and the hydrostatic force is:
Pw = 21 γ whw^2 (3.27)
3.7 Summary of Rankine Lateral Earth Pressure Theory
- The lateral earth pressures on retaining walls are related directly to the vertical effective stress through two coefficients ka and kp.
- Substantially more movement is required to mobilize the full passive earth pressure than the full active earth pressure.
- A family of slip planes occurs in the Rankine active and passive states. In the active state, the slip planes are oriented at ( 450 + φ '/ 2 ) to the horizontal, and while for the passive case they are oriented at ( 450 - φ '/ 2 ) to the horizontal.
- The lateral earth pressure coefficients, developed so far are only valid for a smooth, vertical wall supporting a soil mass with a horizontal surface; and must be applied to effective stresses only.
EXAMPLE 3. For the frictionless wall retaining a stratified soil and shown in Fig. E3.2, determine: (a) The active lateral earth pressure distribution with depth. (b) The passive lateral earth pressure distribution with depth. (c) The magnitude and location of the active and passive forces. (d) The resultant force. (e) The ratio of passive moment to active moment.
FIGURE E3.
Strategy There are two layers. It is best to treat the layers separately. Neither ka nor kp should be applied to the pore water pressure. You do not need kp for the top soil layer. Since the water level on both sides of the wall is the same, the resultant hydrostatic force is zero. However, you are asked to determine the forces on each side of the wall; therefore, you have to consider the hydrostatic force. A table is helpful to solve this type of problem.
3.8 Rankine Active & Passive Earth Pressure for Inclined Granular Backfill
If the backfill of a frictionless retaining wall is a granular soil (c = 0, φ ') and rises at an angle β ( β ≤ φ ') with respect to the horizontal (Figure 3.7), the Rankine active earth pressure coefficient ka is expressed in the form:
2 2 '
2 2 '
cos cos cos
cos cos cos cos
β β φ
β β β φ
ka (3.28)
The Rankine active stress on the wall is:
σ (^) a ' = γ' zk a (3.29)
and the Rankine active lateral force is:
Pa = 12 ka γ' H^2 (3.30)
Note that, the direction of the lateral force Pa is inclined at an angle β to the
horizontal and intersects the wall at a distance of H/3 from the base of the wall.
Figure 3.7: Rankine Active Earth Pressure for Inclined Granular Backfill
The Rankine passive pressure coefficient kp for a wall with a granular sloping
in the passive state. Figure 3.8 illustrates direction of active and passive forces when wall friction is present.
Figure 3.9: Retaining wall with slopping back, wall friction, and sloping soil surface for use with Coulomb’s method for active state.
Based on Coulomb’s theory, a condition of limit equilibrium exists through which a wedge of a soil mass behind a retaining wall will slip along a plane inclined at an angle θ to the horizontal. Figure 3.9 illustrates a retaining wall with slopping back, wall friction, and sloping soil surface for use with Coulomb’s method for active state. Based on the equilibrium of the forces acting on the wedge (Fig. 3.9), Coulomb proposed the following equation to determine the active lateral force,
2 Pa = 21 kac γ' H (3.32)
where kac is Coulomb’s active pressure coefficient, which is determined by the following expression.
2 2
2
sin( )sin( )
sin sin( ) 1 sin( ' )sin( ' )
sin ( ')
β δ α β
β β δ φ δ φ α
β φ
k ac (3.33)
Note that the line of action of the active force Pa will act at a distance H/3 above the base of the wall and will be inclined at angle δ to the normal drawn to the back of the wall. In the actual design of retaining walls, the value of the wall friction angle, δ , is assumed to be between φ^ ' 2 and 23 φ '. Retaining walls are generally constructed of masonry or mass concrete. Table 3.1 shows the general range of the values of δ for various backfill materials.
Table 3.1: General range of wall friction angle for masonry or mass concrete walls
Backfill material Range of δ in degrees Gravel 27 – 30 Course sand 20 – 28 Fine sand 15 – 25 Stiff clay 15 – 20 Silty clay 12 – 16
Coulomb’s passive earth pressure is determined similarly, except that passive pressure inclination at the wall and direction of the forces acting on the wedge will be as shown in Fig. 3.10. Coulomb’s passive earth pressure is given by:
Pa = 21 kac γ' H^2 (3.34)
where kpc is Coulomb’s passive pressure coefficient, which is determined by the following equation.
2 2
2
sin( )sin( )
sin sin( ) 1 sin( ' )sin( ' )
sin ( ')
β δ α β
β β δ φ δ φ α
β φ
k pc (3.35)
Figure 3.10: Retaining wall with slopping back, wall friction, and sloping soil surface for use with Coulomb’s method for passive state.
EXAMPLE 3. What is the total active force per meter of wall for the soil-wall system, shown in Fig. E3.4 using the Coulomb equation? Where does Pa act?
