Engineering and Construction

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a. Criteria forselecting earth pressures.

     (1) The equivalent  fluid method should be used  for  estimating  active earth pressures on retaining structures up to 20  feet high, with  the  addition  to  earth pressures resulting from backfill compaction (fig 14-8).
     (2) For walls higher than  20  feet,  charts, equations, or graphical solutions should  be  used  for computing  lateral  earth  pressures,  with the addition of earth pressures resulting from backfill compaction.
     (3) Use at-rest pressures for rigid retaining structures resting on rock  or batter piles.   Design cantilever walls  founded on rock or restrained  from lateral movement  for at-rest pressures near the base of the wall, active pressures along the upper portions of the wall,  and compaction-induced earth pressures  from  the top  to the depth at which they no longer increase lateral earth pressures (fig 14-8).  Generally,  a linear variation in earth pressure coefficients with depth  may  be assumed between the sections of wall.
     (4) Consider passive pressures  in  the design if applied loads  force  the  structure  to  move against the soil.  Passive pressures in  front of retaining walls are partially effective in resisting horizontal sliding.

Figure 14-8.  Estimates of increased pressure induced by compaction.

b. Overturning.  Calculate the factor of safety, FS, against overturning, defined as  the  ratio of  resisting moments to the overturning  moments.  Calculate the resultant force using load diagrams shown in figure 14-1, as well  as  other loadings that may be applicable.  Use only half of the ultimate passive resistance  in calculating the safety factor.  The resultant of all forces acting on the retaining wall should fall within the middle third to provide a safety  factor with respect to overturning equal to or greater than 1.5.

Figure 14-1.  Load diagrams for retaining walls.

c. Sliding.
     (1) The factor of safety against sliding, calculated as the ratio of forces resisting movement to the horizontal component of earth plus water pressure on the back wall, should be not less than 2.0.  If soil in front of the toe is disturbed or loses its strength because of possible excavation, ponding, or freezing and thawing, passive resistance at the toe, Pp,  should be neglected and the minimum factor of safety lowered to 1.5; but if the potential maximum passive resistance is small, the safety factor should remain at 2.0 or higher.
     (2) For high walls,  determine the shearing resistance between the base of wall and soil from laboratory direct shear tests in which the adhesion between the concrete and the undisturbed soil is measured.  In the absence of tests,  the coefficient of friction between concrete and soil may be taken as 0.55 for coarsegrained soils without silt, 0.45 for coarserained soils with silt, and 0.35 for silt.  The soil in a layer beneath the base may be weaker, and the shearing resistance between the base of wall and soil should never be assumed to exceed the soil strength.  Consider maximum uplift pressures that may develop beneath the base.
     (3) If the factor of safety against sliding is insufficient, increase resistance by either increasing the
width of the base or lowering the base elevation.  If the wall is founded on clay, the resistance against sliding should be based on su for short-term analysis and φ’ for long-term analysis.

d. Bearing capacity.   Calculate from the bearing capacity analysis preously.  Consider local
building codes or experience where applicable.

e. Settlement and tilting.   When a high retaining wall is to be founded on compressible soils, estimate total and differential settlements using procedures outlined previously.  Reduce excessive total settlement by enlarging the base width of the wall or by using lightweight backfill material.  Reduce tilting induced by differential settlement by proportioning the size of the base such that the resultant force falls close to the center of the base.  Limit differential settlement to the amount of tilting that should not exceed 0.05H.  If settlements are excessive,  stabilize compressible soils by surcharge loading or a support wall on piles.

f. Deep-seated failure.   Check the overall stability of the retaining wall against a deep-seated foundation failure using methods of analysis outlined previously.  Forces considered include weight of retaining
wall, weight of soil, unbalanced water pressure, equipment, and future construction.  The minimum safety factor is 1.5.

g. Use of piles.  When stability against bearing capacity failure cannot be satisfied or settlement is excessive, consider a pile foundation.  Use batter piles if the horizontal thrust of the lateral earth pressure is high.

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a. General.  Retaining walls must be designed so that foundation pressures do not exceed allowable bearing pressures, wall settlements are tolerable, safety factors against sliding and overturning are adequate, and
the wall possesses adequate structural strength.

Methods for evaluating earth pressures on retaining walls and design procedures are summarized herein for cohesionless backfill materials, which should be used whenever practicable.

b. Forces acting on retaining walls.   Forces include earth pressures,  seepage and uplift pressures, surcharge loads,  and weight of the wall.  Typical load diagrams for principal wall types are shown in figure 14-1.  The magnitude and distribution of active and passive earth pressures are developed from the earth theory for walls over 20 feet high and from semiempirical curves for lower walls.  The subgrade reaction along the base is assumed linearly distributed.

Figure 14-1.  Load diagrams for retaining walls.

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In order for the active wedge to be developed, there must be sufficient movement of the retaining wall. There are many cases where movement of the retaining wall is restricted. Examples include massive bridge abutments, rigid basement walls, and retaining walls that are anchored in non-yielding rock. These cases are often described as restrained retaining walls.

For earthquake conditions, restrained retaining walls will usually be subjected to larger forces as compared to those retaining walls that have the ability to develop the active wedge. Provided the soil is not weakened during the earthquake, one approach is to use the pseudostatic method in order to calculate the earthquake force, with an increase to compensate for the unyielding wall conditions, or:

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The pseudostatic method has been previously discussed and it is applicable for slope stability and retaining wall analyses. The advantages of this method are that it is easy to understand and apply. Similar to earthquake slope stability analyses, this method ignores the cyclic nature of the earthquake and treats it as if it applies an additional static force upon the retaining wall. In particular, the pseudostatic approach is to apply a lateral earthquake force upon the retaining wall. To derive the lateral force, it can be assumed that the force acts through the centroid of the active wedge. The pseudostatic lateral force PE is calculated by using Eq. 13.12 (i.e., PE = kh W) and it has units of pounds per linear foot of wall length or kilonewtons per linear meter of wall length.

Note that an earthquake could subject the active wedge to both vertical and horizontal pseudostatic forces. However, the vertical force is usually ignored in the standard pseudostatic analysis.

This is because the vertical pseudostatic force acting on the active wedge usually has much less effect on the design of the retaining wall. In addition, most earthquakes produce a peak vertical acceleration that is less than the peak horizontal acceleration, and hence kv is smaller than kh.

The only unknowns in the pseudostatic method are the weight of the active wedge (W) and the seismic coefficient kh. Because of the usual relatively small size of the active wedge, the seismic coefficient kh can be assumed to be equal to amax /g. Using Fig. 11.3, the weight of the active wedge can be calculated as follows:



Using Eq. 14.1, the final result is as follows:


Note that since the pseudostatic force is applied to the centroid of the active wedge, the location of the force PE is at a distance of 2/3H above the base of the retaining wall. Seed and Whitman (1970) developed a similar equation that can be used to determine the horizontal pseudostatic force acting on the retaining wall, as follows:


Note that the terms in Eq 14.3 have the same definitions as the terms in Eq. 14.2. In comparing
Eqs. 14.2 and 14.3, the two equations are identical for the case where

According to
Seed and Whitman (1970), the location of the pseudostatic force from Eq. 14.3 can be assumed to act at a distance of 0.6 H above the base of the wall.

Mononobe and Matsu (1929) and Okabe (1926) also developed an equation that can be used to determine the horizontal pseudostatic force acting on the retaining wall. This method is often referred to as the Mononobe-Okabe method. The equation is an extension of the Coulomb approach and is as follows:

where PAE is the sum of the static (PA) and the pseudostatic earthquake force (PE). The equation for kAE is shown in Fig. 11.4. Note that in Fig. 11.4, the term y is defined as follows:

 The original approach by Mononobe and Okabe was to assume that the force PAE from Eq. 14.4 acts at a distance of 1/3 H above the base of the wall.

For the analysis of sliding and overturning of the retaining wall, it is common to accept a lower factor of safety (1.1 to 1.2) under the combined static and earthquake loads. Thus the retaining wall would be considered marginally stable for the earthquake sliding and overturning conditions. Note in the above table that the factor of safety for overturning is equal to 2.35 based on the Mononobe-Okabe method. This factor of safety is much larger than the other two methods. This is because the force PAE is assumed to be located at a distance of 1/3 H above the base of the wall. Kramer (1996) suggests that it is more appropriate to assume that PE is located at a distance of 0.6 H above the base of the wall (i.e., PE = PAE – PA, see Eq. 14.4).

Although the calculations are not shown, it can be demonstrated that the resultant location of N for the earthquake condition is outside the middle third of the footing. Depending on the type of material beneath the footing, this condition could cause a bearing capacity failure or excess settle- ment at the toe of the footing during the earthquake.

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The purpose of this section is to describe retaining wall design considerations for earthquakes. The performance of retaining walls during earthquakes is very complex. As stated by Kramer (1996), laboratory tests and analyses of gravity walls subjected to seismic forces have indicated the following:

1. Walls can move by translation and/or rotation. The relative amounts of translation and rotation depend on the design of the wall; one or the other may predominate for some walls, and both may occur for others (Nadim and Whitman, 1984; Siddharthan et al., 1992).

2. The magnitude and distribution of dynamic wall pressures are influenced by the mode of wall movement, e.g., translation, rotation about the base, or rotation about the top (Sherif et al., 1982; Sherif and Fang, 1984a,b).

3. The maximum soil thrust acting on a wall generally occurs when the wall has translated or rotated toward the backfill (i.e., when the inertial force on the wall is directed toward the backfill). The minimum soil thrust occurs when the wall has translated or rotated away from the backfill.

4. The shape of the earthquake pressure distribution on the back of the wall changes as the wall moves. The point of application of the soil thrust therefore moves up and down along the back of the wall. The position of the soil thrust is highest when the wall has moved toward the soil and lowers when the wall moves outward.

5. Dynamic wall pressures are influenced by the dynamic response of the wall and backfill and can increase significantly near the natural frequency of the wall-backfill system (Steedman and Zeng, 1990). Permanent wall displacements also increase at frequencies near the natural frequency of the wall-backfill system (Nadim, 1982). Dynamic response effects can also cause deflections of different parts of the wall to be out of phase.

This effect can be particularly significant for walls that penetrate into the foundation soils when the backfill soils move out of phase with the foundation soils.

6. Increase residual pressures may remain on the wall after an episode of strong shaking has ended
(Whitman, 1990).

Because of the complex soil-structure interaction during the earthquake, the most commonly used method for the design of retaining walls is the pseudostatic method.

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INTRODUCTION
As discussed previously, the actual rupture of the ground due to fault movement could damage a structure. Secondary effects, such as the liquefaction of loose granular soil and slope movement or failure could also cause structural damage. This chapter will discuss some of the other earthquake-induced effects or structural conditions that can result in damage to foundations and retaining walls.

Topics will include earthquake-induced settlement and foundation alternatives to mitigate earth-quake effects.

Earthquakes throughout the world cause a considerable amount of death and destruction.

Earthquake damage can be classified as being either structural or nonstructural. For example, the Federal Emergency Management Agency (1994) states:

Damage to buildings is commonly classified as either structural or non-structural. Structural damage means the building’s structural support has been impaired. Structural support includes any vertical and lateral force resisting systems, such as the building frames, walls, and columns. Non-structural damage does not affect the integrity of the structural support system. Examples of non-structural damage include broken windows, collapsed or rotated chimneys, and fallen ceilings. During an earthquake, buildings get thrown from side to side, and up and down. Heavier buildings are subjected to higher forces than light-weight buildings, given the same acceleration. Damage occurs when structural members are overloaded, or differential movements between different parts of the structure strain the structural components. Larger earthquakes and longer shaking durations tend to damage structures more. The level of damage resulting from a major earthquake can be predicted only in general terms, since no two buildings undergo the exact same motions during a seismic event. Past earthquakes have shown us, however, that some buildings are likely to perform more poorly than others.

There are four main factors that cause structural damage during an earthquake, as follows:

1. Strength of shaking. For small earthquakes (magnitude less than 6), the strength of shaking decreases rapidly with distance from the epicenter of the earthquake. According to the United States Geological Survey (2000b), the strong shaking along the fault segment that slips during a earthquake becomes about half as strong at a distance of 8 mi, a quarter as strong at a distance of 17 mi, an eighth as strong at a distance of 30 mi, and a sixteenth as strong at a distance of 50 mi.

In the case of a small earthquake, the center of energy release and the point where slip begins is not far apart. But in the case of large earthquakes, which have a significant length of fault rup- ture, these two points may be hundreds of miles apart. Thus for big earthquakes, the strength of shaking decreases in a direction away from the fault rupture.

2. Length of shaking. The length of shaking depends on how the fault breaks during the earthquake. For example, the maximum shaking during the Loma Prieta earthquake lasted only 10 to 15 sec But during other magnitude earthquakes in the San Francisco bay area, the shaking may last 30 to 40 sec. The longer the ground shakes, the greater the potential for structural damage. In general, the higher the magnitude of an earthquake,  the longer the duration of the shaking ground (see Table 13.2).



3. Type of subsurface conditions. Ground shaking can be increased if the site has a thick deposit of soil that is soft and submerged. Many other subsurface conditions can cause or contribute to structural damage. For example, as discussed in Sec. 13.4, there could be structural damage due to liquefaction of loose submerged sands.

4. Type of building. Certain types of buildings and other structures are especially susceptible to the side-to-side shaking common during earthquakes. For example, sites located within approxi- mately 10 mi (16 km) of the epicenter or location of fault rupture are generally subjected to rough, jerky, and high frequency seismic waves that are often more capable of causing short buildings to vibrate vigorously. For sites located at greater distance, the seismic waves often develop into longer period waves that are more capable of causing high-rise buildings and buildings with large floor areas to vibrate vigorously (Federal Emergency Management Agency, 1994).

Much like diseases will attack the weak and infirm, earthquakes damage those structures that have inherent weaknesses or age-related deterioration. Those buildings that are nonreinforced, poorly constructed, weakened from age or rot, or underlain by soft or unstable soil are most susceptible to damage. The next section will discuss some of these susceptible structures.

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Figure 11.19 shows a picture of a temporary retaining wall that consists of steel I-beams, wood lagging, and tieback anchors. The temporary retaining wall is being used to support the excavation during the construction of the foundation. This type of temporary retaining wall consists of steel I-beams that are either driven into place or installed in pre-drilled holes with the bottom portion cemented into place. During the excavation of the interior area, wood lagging is installed between the steel I-beams and the earth pressure exerted on the wood lagging is transferred to the steel I-beam flanges. In order to reduce bending moments in the steel I-beams and to reduce lateral deflections, tieback anchors are often installed as the excavation proceeds.

In some cases, the steel I-beams will be braced with struts and the earth pressure distributions would be similar to those as shown in Fig. 11.18. For cantilevered steel I-beam and wood lagging retaining walls, there will be sufficient movement of the top of the wall to develop the active earth pressure (i.e., Eq. 11.1).

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Temporary retaining walls are often used during construction, such as for the support of the sides of an excavation that is made below-grade in order to construct the building foundation. If the temporary retaining wall has the ability to develop the active wedge (Fig. 11.3), then the basic active earth pressure principles described in the previous sections can be used for the design of the temporary retaining walls.

Especially in urban areas, movement of the temporary retaining wall may have to be restricted to prevent damage to adjacent property. If movement of the retaining wall is restricted, the earth pressures will typically be between the active  kA and at-rest  ko values. Common types of temporary retaining systems and braced excavations are shown in Fig. 11.17.

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In terms of the internal stability of MSE walls, Zornberg et al. (1998) have shown that there is very good agreement between the actual location of the slip surface and the location of the critical slip surface predicted by slope stability analyses (i.e., method of slices). To check the stability of the mechanically stabilized zone, a slope stability analysis can be performed where the soil reinforcement is modeled as horizontal forces equivalent to the allowable tensile resistance of the geogrid. In addition to calculating the factor of safety, the pullout resistance of the reinforcement along the slip surface should also be checked.

The analysis of mechanically stabilized earth retaining walls is based on active earth pressures.

It is assumed that the wall will move enough to develop the active wedge. Similar to concrete retaining walls, it is important that building footings or other load carrying members are not supported by the mechanically stabilized earth retaining wall and the active wedge, or else they will be subjected to lateral movement.

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The analysis for the external stability is similar to a gravity retaining wall. For example, Figs. 11.12 and 11.13 (adapted from AASHTO, 1996) present the design analysis for external stability for a level backfill condition and a sloping backfill condition. In both Figs. 11.12 and 11.13, the zone of mechanically stabilized earth mass is treated in a similar fashion as a massive gravity retaining wall.

The following analyses must be performed:

1. Allowable bearing pressure. The bearing pressure due to the reinforced soil mass must not exceed the allowable bearing pressure.

2. Factor of safety of sliding. The reinforced soil mass must have an adequate factor of safety for sliding (F = 1.5 to 2).

3. Factor of safety of overturning. The reinforced soil mass must have an adequate factor of safety (F = 2) for overturning about Point O.

4. Resultant of vertical forces N. The resultant of the vertical forces N should be within the middle 1/3 of the base of the reinforced soil mass.

5. Stability of reinforced soil mass. The stability of the entire reinforced soil mass (i.e., shear failure below the bottom of the wall) would have to be checked.

Note in Fig. 11.12 that two forces (P1 and P2) are shown acting on the reinforced soil mass. The
first force P1 is determined from the standard active earth pressure resultant equation (i.e., Eq. 11.1).

The second force P2 is due to a uniform surcharge Q applied to the entire ground surface behind the mechanically stabilized earth retaining wall (i.e., Eq. 11.3). If the wall does not have a surcharge, then P2 is equal to zero.

Figure 11.13 presents the active earth pressure force for an inclined slope behind the retaining wall. Note in Fig. 11.13 that the friction d of the soil along the back side of the reinforced soil mass has been included in the analysis. The value of kA would be obtained from Coulomb’s earth pressure equation (Fig. 11.4). As a conservative approach, the friction angle d can be assumed to be equal to zero and then PH = PA. Note in both Figs. 11.12 and 11.13 that the minimum width of the reinforced soil mass must be at least 7/10 the height of the reinforced soil mass.

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The construction of a mechanically stabilized earth retaining wall is shown in Fig. 11.9 to 11.11, as follows:

1. Excavation of key and installation of drainage system (Fig. 11.9). This photograph shows the
first step in the construction of a mechanically stabilized earth retaining wall. A key (slot) has
been excavated into the natural ground and a drainage system is being constructed at the back of
the key.

2. Construction of mechanically stabilized earth retaining wall (Fig. 11.10). A mechanically stabilized earth retaining wall is composed of three elements: (1) wall facing material, (2) soil reinforcement, such as strip- or grid-type reinforcement, and (3) compacted fill between the soil reinforcement, as follows:

a. Wall facing element. The wall facing elements, which are precast concrete members, are being installed as shown in Fig. 11.10. Other types of wall facing elements are often utilized, such as wood planks or concrete interlocking panels. The wall facing members are first aligned and then connected together by using steel dowels.

b. Geogrid. Figure 11.10 shows the black geogrid, which is a type of geosynthetic. Geogrids are usually composed of a high strength polymer used to create an open grid pattern. The large arrows in Fig. 11.10 indicate the width of the geogrid, which is considered to be the zone of mechanically stabilized earth. Geogrid is transported to the site in rolls, which are then spread out and cut as needed. Geogrid has different tensile strengths along the roll versus perpendicular to the roll and it is important that the highest strength of the geogrid be placed in a direction that is perpendicular to the wall face. The small arrow in Fig. 11.10 points to a splice, where the geogrid is overlapped. A smaller width of geogrid is used at the end of the wall (location of small arrow) because the required resistance is less at the wall perimeter. The geogrid is often attached to the wall facing elements and the vertical spacing of the geogrid is equal to the thickness of the wall facing elements.

c. Compaction of fill. The final step is to place fill on top of the geogrid and the fill is then compacted. Since this mechanically stabilized earth retaining wall only has a drain in the key, granular (permeable) soil is being used to prevent the buildup of pore water pressure behind or in the mechanically stabilized earth zone. For geogrid installed in cohesive (plastic) soil, a backdrain system is often installed.



The geogrid and compacted fill derive frictional resistance and interlocking resistance between each other. When the mechanically stabilized soil mass is subjected to shear stress, the soil tends to transfer the shear stress to the stronger geogrid. Also, if high stress concentrations develop in the mechanically stabilized soil mass, such as along a potential slip surface, then the geogrid tends to redistribute stresses away from areas of high stress.

Lightweight equipment must be used to compact the fill that is placed on top of each layer of geogrid. Heavy compaction equipment could push the wall facing elements out of alignment.



3. Final constructed condition (Fig.11.11). This photograph shows the final constructed condition of the mechanically stabilized earth retaining wall. The arrow in Fig. 11.11 points to a gap in the wall facing element, which can be used to install plants which will grow over the wall facing elements and eventually blend the wall into the surrounding vegetation.

The design analysis for a mechanically stabilized earth retaining wall is more complex than for a cantilevered retaining wall. For a mechanically stabilized retaining wall, both the internal and external stability must be checked as discussed in the next two sections.

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As mentioned in the previous section, in order for the active wedge (Fig. 11.3) to be developed, there must be sufficient movement of the retaining wall (Table 11.1). There are many cases where movement of the retaining wall is restricted. Examples include massive bridge abutments, rigid basement walls, and retaining walls that are anchored in non-yielding rock. These cases are often described as restrained retaining walls.

In order to determine the earth pressure acting on a restrained retaining wall, Eq. 11.1 can be utilized where the coefficient of earth pressure at rest (ko) is substituted for kA. The value of ko can be estimated from Eqs. 4.19 and 4.20. A common value of ko that is used for restrained retaining walls is 0.5. Restrained retaining walls are especially susceptible to higher earth pressures induced by heavy compaction equipment and extra care must be taken during the compaction of backfill for restrained retaining walls.


For either a line load or point load acting on the ground surface behind the restrained retaining wall, Fig. 11.8 can be used to determine the additional horizontal force PH acting on the restrained retaining wall and the location of this horizontal force above the base of the wall. The upper plots in Fig. 11.8 can be used to determine the horizontal pressure distribution acting on the retaining wall due to the line load or point load.

If there is a uniform surcharge Q acting upon the entire ground surface behind the wall, then there would be an additional horizontal pressure exerted upon the restrained retaining wall equal to the product of ko
times Q. Thus the resultant force P Q, in pounds per linear foot of wall or kilonewtons per linear meter of wall, could be calculated by using Eq. 11.3 and substituting ko for kA.


In summary, for the retaining wall shown in Fig. 11.5, the values of the earth pressure acting on the wall are as follows:

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There are many construction factors that can result in excessive lateral movement, bearing capacity failures, sliding failures, or failure by overturning of the retaining wall. Common causes can include inadequate subsurface exploration or laboratory testing, incorrect design, improper construction, or unanticipated loadings. Typical construction related problems are discussed below:

1. Clay backfill. A frequent cause of failure is because the wall was backfilled with clay. As previously mentioned, clean granular sand or gravel is usually recommended as backfill material.

This is because of the undesirable effects of using clay or silt as a backfill material. When clay is used as backfill material, the clay backfill can exert swelling pressures on the wall (Fourie, 1989; Marsh and Walsh, 1996). The highest swelling pressures develop when water infiltrates a backfill consisting of clay that was compacted to a high dry density at low moisture content. The type of clay particle that will exert the highest swelling pressures is montmorillonite. Because the clay backfill is not free draining, there could also be additional hydrostatic forces or ice-related forces that substantially increase the thrust on the wall. Fig. 11.7 shows the collapse of retaining wall that has a clay backfill.

2. Inferior backfill soil. To reduce construction costs, soil available on-site is sometimes used for backfill. This soil may not have the properties, such as being a clean granular soil with a high shear strength, that were assumed during the design stage. Using on-site available soil such as clays, rather than importing granular material, is probably the most common reason for retaining wall failures.

3. Compaction induced pressures. As previously mentioned, one reason for applying a factor of safety (F) to the active earth pressure (Eq. 11.1) is because larger wall pressures will typically be generated during compaction of the backfill. By using heavy compaction equipment in close proximity to the wall, excessive pressures can be developed that damage the wall. The best compaction equipment, in terms of exerting the least compaction induced pressures on the wall, are small vibrator plate (hand-operated) compactors such as models VPG 160B and BP 19/75 (Duncan et al., 1991).

The vibrator plates effectively densify the granular backfill, but do not induce high lateral loads because of their light weight. Besides hand-operated compactors, other types of relatively light-weight equipment (such as a bobcat) can be used to compact the backfill.

4. Failure of the back-cut. There could also be the failure of the back-cut for the retaining wall.

A vertical back-cut is often used when the retaining wall is less than 5 ft (1.5 m) high. In other cases, the back-cut is usually sloped. The back-cut can fail if it is excavated too steeply and does not have an adequate factor of safety.

Retaining wall movement is often gradual and the wall deforms by intermittently tilting or moving laterally. It is also possible that a failure can occur suddenly, such as when there is a slope-type failure beneath the wall or when the foundation of the wall fails due to inadequate bearing capacity.

These rapid failure conditions could develop if the wall is supported by soft clay and there is an undrained shear failure beneath the foundation.

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Retaining walls are sometimes constructed at the top of slopes. In this case, there is a descending ground surface in front of the retaining wall and Eq. 11.5 cannot be used to determine the passive earth pressure coefficient. For either a descending slope (−β) or ascending slope (+β ) in front of the retaining wall, the following equation can be used to determine the passive earth pressure coefficient kp:


where: f is the friction angle of the soil in front of the retaining wall (degrees) and b is the slope inclination measured from a horizontal plane, where a descending slope in front of the retaining wall has a negative b value (degrees).

Although not readily apparent, if β = 0, Eq. 11.10 will give exactly the same values of kp as Eq. 11.5. For example, substituting β = 0° and f = 30° into Eq. 11.10, kp = 3 which is exactly the value obtained from Eq. 11.5.

Suppose that a retaining wall is constructed at the top of a 2:1 (horizontal:vertical) slope. In this case, the slope inclination is β = -26.6°. Assuming f = 30° for the soil comprising the slope, then according to Eq. 11.10, the value of kp = 1.12. Thus for the case of a 2:1 descending slope in front of the retaining wall, the passive resistance will be significantly reduced (kp decreases from 3 to 1.12). Many retaining walls tilt or deform downslope because the condition of a sloping ground surface in front of the retaining wall significantly reduces the passive resistance for the wall foundation.

There can be additional factors that contribute to the failure of retaining walls constructed at the top of slopes. In some cases, there may be a low factor of safety of the entire slope that is exacerbated by the construction of a top of slope wall. In other cases, where the soil in front of the retaining wall is clayey, there can be slope creep that causes a soil gap to open up at the front of the retaining wall footing. In these cases of a descending clayey slope in front of the retaining wall, the depth of creep of the outer face of the slope must be estimated and then the passive pressure should only be utilized below the depth of surface slope creep.

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The previous section dealt with simple retaining walls. This section will provide an additional discussion of the design and construction of retaining walls. Figure 11.6 (from NAVFAC DM-7.2, 1982) shows several examples of different types of retaining walls. Figure 11.6A shows gravity and semigravity retaining walls, Fig. 11.6B shows cantilever and counterfort retaining walls, and the design analyses are shown in Fig. 11.6C.

Although the equations in Fig. 11.6C include an adhesion value  ca, as previously mentioned, the adhesion is often neglected. This is because active pressures develop when there is translation (movement) of the footing that would tend to break the adhesive resistance.

1 Retaining Walls at the Top of Slopes   Retaining walls are sometimes constructed at the top of slopes. In this case, there is a descending ground surface in front of the retaining wall and Eq. 11.5 cannot be used to determine the passive earth...

2 Retaining Wall Failures   There are many construction factors that can result in excessive lateral movement, bearing capacity failures, sliding failures, or failure by overturning of the retaining wall...

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A retaining wall is defined as a structure whose primary purpose is to provide lateral support for soil or rock.

In some cases, the retaining wall may also support vertical loads. Examples include basement walls and certain types of bridge abutments.

Some of the more common types of retaining walls are gravity walls, counterfort walls, cantilevered walls, and crib walls. Gravity retaining walls are routinely built of plane concrete or stone and the wall depends primarily on its massive weight to resist failure from overturning and sliding.

Counterfort walls consist of a footing, a wall stem, and intermittent vertical ribs (called counterforts) that tie the footing and wall stem together. Crib walls consist of interlocking concrete members that form cells, which are then filled with compacted soil. Common types of retaining walls are shown in Fig. 11.1.

Although mechanically stabilized earth retaining walls have become more popular in the past decade, cantilever retaining walls are still probably the most common type of retaining structure.

There are many different types of cantilevered walls, with the common features being a footing that supports the vertical wall stem. Typical cantilevered walls are T-shaped, L-shaped, or reverse L-shaped (Cernica, 1995a, 1995b).

Clean granular material (no silt or clay) is the standard recommendation for backfill material.

There are several reasons for this recommendation:

1. Predictable behavior. Import granular backfill generally has a more predictable behavior in terms of earth pressure exerted on the wall. Also, expansive soil related forces would not be generated by clean granular soil. If clay backfill should be used, the seepage of water into the clay backfill could cause horizontal swelling pressures well in excess of at-rest values. For example, Fourie (1989) measured the swell pressure of compacted clay for zero lateral strain to be 8800 psf (420 kPa). Besides the swelling pressure induced by the expansive soil, there can also be groundwater or perched water pressure on the retaining or basement wall because of the poor drainage of clayey soils.

2. Drainage system. To prevent the buildup of hydrostatic water pressure on the retaining wall, a
drainage system is often constructed at the heel of the wall. The drainage system will be more
effective if highly permeable soil, such as clean granular soil, is used as backfill.

3. Frost action. In cold climates, frost action has caused many retaining walls to move so much that they have become unusable. If freezing temperatures prevail, the backfill soil can be susceptible to frost action, where ice lenses will form parallel to the wall and cause horizontal movements of up to 2 to 3 ft (0.6 to 0.9 m) in a single season (Sowers and Sowers, 1970). Backfill soil consisting of clean granular soil and the installation of a drainage system at the heel of the wall will help to protect the wall from frost action.

Movement of retaining walls (i.e., active condition) involves the shear failure of the wall backfill and the analysis will naturally include the shear strength of the backfill soil. Similar to the analysis of strip footings and slope stability, for most field situations involving retaining structures, the backfill soil is in a plane strain condition (i.e., the soil is confined along the long axis of the wall). As previously mentioned, the friction angle f is about 10 percent higher in the plane strain condition as compared to the friction angle f measured in the triaxial apparatus. In practice, plane strain shear strength tests are not performed, which often results in an additional factor of safety for retaining wall analyses.


Previously will discuss the basic retaining wall equations for a simple retaining wall without and with wall friction. The following sections will then discuss in more detail the design and construction of retaining walls, restrained retaining walls, mechanically stabilized earth retaining walls , sheet pile walls, and temporary retaining walls, which are often needed to support foundation excavations. The final is devoted to moisture migration through retaining walls. Geotechnical earthquake engineering analyses for retaining walls will be covered later.