Retaining Wall Technical Guidance

7/27/2019 Retaining Wall Technical Guidance

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Retaining Wall Technical Guidance

Please look at the information and related

sources for Retaining Walls in the

publications or software links. Or, post a

question in the Geotechnical Forum.

Comprehensive step by step calculations for retaining wall analysis are provided below, or click:

Example Problems for Retaining Wall Analysis

On this page, you will find an abundance of information relating to:

Retaining wall and lateral earth pressure variables,

Rankine analysis,

Coulomb analysis,

Graphical methods,

Log spiral theory, Sliding, and

Overturning

Retaining Wall Variables

Magnitude of stress or earth pressure acting on a retaining wall depends on:

height of wall,

unit weight of retained soil,

pore water pressure,

strength of soil (angle of internal friction),

amount and direction of wall movement, and other stresses such as earthquakes and surcharges.

Lateral Earth Pressure Variables

Lateral earth pressures are analyzed for either "Active," "Passive" or "At-Rest" conditions.

Active conditions exist when the retaining wall moves away from the soil it retains.

Passive conditions exist when the retaining wall moves toward the soil it retains.

At-Rest conditions exist when the wall is not moving away or toward the soil it retains.

Conditions for active, passive and at-rest pressures are usually determined by the structural engineer. Basically,

at-rest pressures exist when the top of the wall is fixed from movement. Active and passive pressures are

assumed when the top of the wall moves at least 1/10 of 1% of height of wall in the direction away from , andtoward the soil it retains, respectively. Some theorize that at-rest pressures develop over time, when a retaining

wall is constructed for the active case.

Retaining Wall Analysis Methods

Lateral earth pressures are typically analyzed, as presented below, from one of the following methods:

Rankine Analysis

Coulomb Method

Log Spiral Theory

After determining lateral earth pressures, retaining wall analysis and design also includes:

http://www.geotechnicalinfo.com/retaining_walls_publications.html


http://www.geotechnicalinfo.com/retaining_walls_software.html



http://www.geotechnicalinfo.com/geotechnical_forum.html



http://www.geotechnicalinfo.com/retaining_walls_technical_guidance.html#retainingexamples








Sliding

Overturning

Bearing capacity and settlement

Structural design of wall

Rankine Analysis

Basically, lateral earth pressures are derived from the summation of all individual pressure (stress) areas behindthe retaining wall. These pressure areas are triangular in shape with the base of the triangle at the base of the

wall for the soil component and pore water component. Pressure areas for surcharges are rectangular in shape,

and earthquake pressures are usually analyzed with a nearly 'upside-down' triangle. See the RANKINE

ANALYSIS link for an excellent presentation of determining lateral earth pressures using the Rankine Analysis.

For the Rankine analysis, assumptions include:

horizontal backfill

vertical wall with respect to the retaining soil

smooth wall (no friction)

Resultant Lateral Earth Pressure, R

The resultant lateral earth pressure, R, is the summation of all individual lateral earth pressure components.

R = Ps + Pw + Pq + Pe kN/m2 (lb/ft

2)

Where,

Ps = 1 K H2

kN/m2

(lb/ft2

) earth pressure due to soil2

Pw = 1 wH2 kN/m

2 (lb/ft

2) earth pressure due to pore water

2 Pq = qKH kN/m

2 (lb/ft

2) earth pressure due to surcharge (i.e. building, vehicle load)

Pe = 3 K hH2 kN/m

2 (lb/ft

2) earth pressure due to earthquakes

8

and,

Ps = lateral earth pressure due to soil

Pw = lateral earth pressure due to pore waterP

q = lateral earth pressure due to surcharge (i.e. building, vehicle load)

Pe = lateral earth pressure due to earthquakes

K = K A, K P or K o lateral earth pressure coefficient

K A = (1 - sin ) coefficient for active conditions

(1 + sin )

K P = (1 + sin ) coefficient for passive conditions

(1 - sin )

K o = 1 - sin coefficient for at-rest conditions

K h = 3 K earthquake coefficient

4 = effective unit weight of soil medium kN/m2(lb/ft

2)

http://www.geotechnicalinfo.com/lateral_earth_pressures.pdf




http://www.geotechnicalinfo.com/soil_unit_weight.html








w = 9.1 kN/m2

(62.4 lb/ft2) = unit weight of water

= angle of internal friction degrees

H = height of retaining wall m (ft)

q = surcharge on soil, if any kN/m2(lb/ft

2)

water table

Engineering judgment should allow for some pore water pressure behind a retaining wall due to stormwater or

other water source. For a water table behind the wall, why would you analyze a partially submerged backfill?

You could reasonably expect for almost every situation that a partially submerged backfill will become fully

inundated during the life of the wall. The following lateral earth pressure equation is for a water table at the top

of the wall. This equation is composed of a soil component plus a pore water component. Add the above

surcharge and earthquake components if necessary.

P = 1/2 K subH2 + 1/2 wH

2 (lb/ft

2)

sub = submerged soil unit weight (lb/ft3)

= sat - w

sat = saturated soil unit weight (lb/ft3)

w = unit weight of water (lb/ft3)= 62.4 lb/ft

3

See the following link for an excellent presentation of determining lateral earth pressures using the Rankine

Analysis

RANKINE ANALYSIS

Coulomb Method

The Coulomb Method:

Allows for friction between the retaining wall and soil

May be used for non-vertical walls

Allows for non-horizontal backfill (inclined), but must be planar

Backfill must be cohesionless for inclined backfill

Assumes a planar slip surface, similar to Rankine

Is used for Active and Passive (see above) conditions only

Assumes a homogeneous backfill

Any surcharge must be uniform and cover entire surface of driving wedge

P = 1 1 KH2 kN/m

2 (lb/ft

2)

2 sin cos

where,

K = K A or K P lateral earth pressure coefficient;

K A = active, K P = passive (see above)

K A = sin2 ( + ) cos

sin (sin - )[1 + SQRT[(sin ( + ) sin ( - ))/(sin ( - ) sin ( + ))]]2

K P = cos2

http://www.geotechnicalinfo.com/angle_of_internal_friction.html













[1 - SQRT[(sin sin ( - ))/(cos )]]2

= effective unit weight of soil medium kN/m2(lb/ft

2)

= angle of internal friction degrees

H = height of retaining wall m (ft)

= 2/3 = angle of wall friction degrees

= angle of wall face from horizontal (90 degrees for vertical wall) degrees

= angle of backfill (0 degrees for horizontal backfill) degrees

Graphical Methods

Graphical methods are more in-depth than the Rankine or Coulomb Analysis. Until some examples are presented on this website, look for more information in the following downloadable publication:

NAVFAC 7.02 - Foundations and Earth Structures. This publication has a graphical solution for lateral earth

pressure analysis. Other publications with Coulomb solutions may be found in the publications section of thiswebsite.

Log Spiral Theory

Since a planar slip surface, as assumed for both Rankine and Coulomb Methods, is reasonable for active earth

pressure conditions, this assumption may yield unreasonable results for passive earth pressure conditions. The

Log Spiral Method assumes a curved slip surface, and therefore should be used for all passive earth pressure

conditions.

Horizontal backfill is required for this method. If backfill is not horizontal, then it may be reasonable to use

engineering judgment and include the sloping portion of the backfill as a surcharge.

Geotechnical Info .Com does not currently have procedures and examples for the Log Spiral Method. Please

check the retaining wall publications section of this website for additional resources that may have information

on the Log Spiral Method.

Sliding

Sliding failure is a result of excessive lateral earth pressures with relation to retaining wall resistance therebycausing the retaining wall system to move away (slide) from the soil it retains.

See a depiction for calculating the factor of safety for retaining wall sliding from the following link:

SLIDING ANALYSIS

The following factors of safety (F.S.) are typically used for analyzing sliding:F.S. = 1.5 for active earth pressure conditions.

F.S. = 2.0 for passive earth pressure conditions.

(R SL/R H) > F.S.

R SL = Resistance to sliding= ( Wi + R V)tan + cAB when a key is not used







http://www.geotechnicalinfo.com/navfac_foundations_and_earth_structures.pdf


http://www.geotechnicalinfo.com/geotechnical_publications.html




















= ( Wi + R V)tan + cAB + PP when a key is used

R H = R cos = horizontal component of resultant lateral earth pressure (kN/m

2) (lb/ft

2)

R V = R sin = vertical component of resultant lateral earth pressure (kN/m2) (lb/ft

2)

R = Ps + Pw + Pq + Pe (see Rankine Analysis above) PP = Ps (use Rankine where K is passive)

= Soil pressure exerted on key using passive earth pressures

Wi = summation of weights (see this link ), that includes:

o weight of footing

o weight of wall

o weight of soil directly above the entire width of the footing

soil

= effective unit weight of soil medium kN/m3(lb/ft

3)

concrete = unit weight of concrete = 23.6 kN/m3 (

150 lb/ft3)

A = area of soil or concrete unit (see this link ) m2(ft

2)

= angle of internal friction (deg)

= external friction angle (deg)

= (2/3)

cA = adhesion (kN/m2)(lb/ft

2)for concrete on soil only

= c, for c = (23.9 kN/m2) (500 lb/ft

2) or less

= 0.75c, for c = (47.9 kN/m2) (1000 lb/ft

2)

= 0.5c, for c = (95.8 kN/m2) (2000 lb/ft

2)

= 0.33c, for c = (191.5 kN/m2) (4000 lb/ft

2)

c = cohesion (kN/m2) (lb/ft

2)

B = footing width (m) (ft)


SLIDING ANALYSIS

Overturning

Overturning failure is a result of excessive lateral earth pressures with relation to retaining wall resistance

thereby causing the retaining wall system to topple or rotate (overturn). Sliding governs the design of retainingwalls most of the time, especially for walls less than 8 feet in height. However, overturning must be analyzed.

See a depiction for calculating the factor of safety for retaining wall overturning from the following link:

OVERTURNING ANALYSIS

Factor of safety (F.S.) is typically 1.5 when analyzing overturning

( Wixi + R VxV)/(R Hy) > F.S.

where:

Wixi = summation of moments about the retaining wall toe. (see this link ), that includes:
















http://www.geotechnicalinfo.com/external_friction_angle.html



http://www.geotechnicalinfo.com/cohesion.html






















o weight of footing

o weight of wall

o weight of soil directly above the entire width of the footing

o distance between toe of wall and centroid of specific weight

Wi = A weight of individual soil or concrete component (see this link ) (kN) (lb)

xi = distance from toe of the retaining wall system to the centroid of

each individual weight in the x-axis direction (horizontal) (m) (ft)

R V = R sin

= vertical component of resultant lateral earth pressure (kN/m2) (lb/ft

2)

xV = distance from toe of the retaining wall system to the centroid of

the resultant vertical earth pressure (R V) in the x-axis (horizontal) direction (ft)

(see this link )

R H = R cos

= horizontal component of resultant lateral earth pressure (kN/m2) (lb/ft

2)

y = distance from the bottom of the retaining wall to the

resultant earth pressure location in the y-axis (vertical)

direction (m) (ft)

R = Ps + Pw + Pq + Pe (see Rankine Analysis above)

soil = effective unit weight of soil medium kN/m3(lb/ft

3)

concrete = unit weight of concrete = 23.6 kN/m3 (

150 lb/ft3)

A = area of soil or concrete unit (see this link ) m2(ft

2)


OVERTURNING ANALYSIS

Bearing Capacity and Settlement

Bearing capacity and settlement for wall foundations can be determined in the same manner as buildingfoundations. Technical guidance for these analyses can be found on this website under the following headings:

Bearing Capacity

Settlement Analysis

Example Problems for Retaining Wall Analysis

Example #1: Using the Rankine analysis, determine the individual lateral earth pressures, and resultant

lateral earth pressure on a 2.1 m (7 ft) rigid concrete retaining wall. The free draining gravel backfill hasa soil unit weight, , of 21.2 kN/m

3 (135 lb/ft

3), and an angle of internal friction, , of 36 degrees. There

will be vehicle surcharges of 14.4 kN/m2 (300 lb/ft

2). The retaining wall will be constructed for passive

conditions.

Given

unit weight of soil backfill, = 21.2 kN/m3 (135 lbs/ft

3) *see typical values

vehicular surcharge, q = 14.4 kN/m2 (300 lbs/ft

2) *from wall use determination

angle of Internal Friction, = 36 degrees *see typical values

wall height, H = 2.1 m (7 ft)

passive case (wall moves toward retained soil)















http://www.geotechnicalinfo.com/bearing_capacity_technical_guidance.html


http://www.geotechnicalinfo.com/settlement_analysis_technical_guidance.html



















Solution

Soil parameters, and , are determined from laboratory testing. Engineering soil properties from a known

granular material source is sometimes used. Some engineers use conservative soil parameters based on the soil

classification without laboratory testing. It is good practice to avoid cohesive soils, and use gravel type materials

for retaining wall backfill.

From the Rankine Analysis equation provided above, the resultant (total) pressure exerted on a retaining wall is:

R = Ps + Pw + Pq + Pe kN/m2 (lb/ft

2)

coefficient for passive conditions

K = K P = (1 + sin ) = (1 + sin 36) = 3.85

(1 - sin ) (1 - sin 36)

lateral earth pressure due to soil

Ps = 1 K H2

2

= 1 3.85(21.2 kN/m3 m

2 = 180.0 kN/m metric

2

= 1 3.85(135 lb/ft3

2 = 12,734 lb/ft standard

2

The soil pressure component is triangular behind the retaining wall. This means that the theoretical lateral earth

pressure due to soil is minimum (zero) at the top of the wall, and maximum (K H) at the bottom of the wall. The

resultant soil pressure, area of the triangle = 0.5K H2, acts at the bottom 1/3 of the wall (i.e. centroid of the

triangle). In this case, the resultant location is H/3, or 0.7 m (2.3 ft) from the bottom of the wall.

lateral earth pressure due to pore water pressure

Pw = 1 wH2 = 0 because backfill is above water table

2

The pore water pressure component is also triangular, similar to the soil component. The resultant location is

H/3 from the bottom of the wall.

lateral earth pressure due to surcharge

Pq = qKH

= 14.4 kN/m

2

(3.85)(2.1 m) = 116.4 kN/m metric = 300 lb/ft2 (3.85)(7 ft) = 8085 lb/ft standard

The surcharge pressure component is rectangular behind the retaining wall. This means that the theoretical

lateral earth pressure due to the surcharge (qK) is the same at both the top of the wall, and bottom of the wall.

The resultant surcharge pressure, area of the rectangle = HqK, acts in the middle of the wall (i.e. centroid of the

rectangle). In this case, the resultant location is H/2, or 1.05 m (3.5 ft) from the bottom of the wall.

lateral earth pressure due to earthquakes

Pe = 3 K hH2

8 K

h = 3 K = 3 (3.85) = 2.89 earthquake coefficient



4 4

Pe = 3 K hH2

8

= 3 (2.89)(21.2 kN/m3(2.1 m)

2 = 101.3 kN/m metric

8

= 3 (2.89)(135 lb/ft3(7 ft)2 = 7169 lb/ft standard

8

The earthquake pressure component is nearly an upside down triangle behind the retaining wall. The resultant

earthquake pressure, area of the triangle = 3/8(K hH2, acts at the upper 1/3 of the wall (i.e. centroid of the

triangle). In this case, the resultant location is H/3, or 0.7 m (2.3 ft) from the top of the wall.

resultant lateral earth pressure

R = Ps + Pw + Pq + Pe

R = 180.0 kN/m + 0 + 116.4 kN/m + 101.3 kN/m = 398 kN/m metric

R = 12,734 lb/ft + 0 + 8085 lb/ft + 7169 lb/ft = 27,990 lb/ft standard

The position of the resultant pressure, y, is determined by taking the moments of each individual pressure about

the base of the wall:

R(y) = Ps(H/3) + Pw(H/3) + Pq(H/2) + Pe(2H/3)

y = 180.0kN/m(0.33(2.1m)) + 0 + 116.4kN/m(0.5(2.1m)) + 101.3kN/m(0.67(2.1m))

398 kN/m

= 0.98 m from bottom of wall metric

y = 12,734lb/ft(0.33(7ft)) + 0 + 8085lb/ft(0.5(7ft)) + 7169lb/ft(0.67(7ft))

27,990 lb/ft

= 3.2 ft from bottom of wall standard

Conclusion

The resultant pressure behind the retaining wall is 398 kN/m (28 kips/ft) at a distance of 0.98 m (3.2 ft) from the

bottom of the wall.

***********************************

Example #2: Using the results from the Rankine analysis in example problem #1, determine the factor of

safety for the concrete retaining wall to resist sliding due to lateral earth pressures exerted on the wall.

The wall foundation is on soils with a cohesion of 23.9 kN/m2 (500 lb/ft2). The retaining wall is not

threatened by earthquakes, so omit the dynamic component. The retaining wall dimensions are provided

below.

Given

unit weight of soil backfill, = 21.2 kN/m3 (135 lbs/ft

3) *see typical values

vehicular surcharge, q = 14.4 kN/m2 (300 lbs/ft



= ()2/3 = 24 degrees

c = 23.9 kN/m2 (500 lb/ft

2) = cohesion


wall thickness, h = 0.30 m (1 ft)

footing thickness, t = 0.30 m (1 ft)















footing width, B = 2.1 m (7 ft)

distance from the footing edge (toe) to face of wall in front of wall, 0.46 m (1.5 ft)

R = 398 kN/m (27,990 lb/ft) from example problem #1

Solution

F.S. = 2.0 for passive earth pressure conditions.

(R SL/R H) > F.S.

R SL = Resistance to sliding

= ( Wi + R V)tan + cAB when a key is not used

= ( Wi + R V)tan + cAB + PP when a key is used

R H = R cos

= (398 kN/m)cos 24 = 364 kN/m metric

= (27,990 lb/ft)cos 24 = 25,570 lb/ft standard

R V = R sin

= (398 kN/m)sin 24 = 162 kN/m metric

= (27,990 lb/ft)sin 24 = 11,385 lb/ft standard

Wi = summation of weights (see this link ) for a depiction

W1 = soil(width of soil block above footing)(height of soil block above footing)= 21.2 kN/m

3(1.68 m)(1.83 m) = 65.1 kN/m metric

= 135 lbs/ft3(5.5 ft)(6 ft) = 4455 lb/ft standard

W2 = concrete(width of wall)(height of wall above footing)

= 23.6 kN/m3

(0.253 m)(1.83 m) = 10.9 kN/m metric = 150 lbs/ft

3(0.83 ft)(6 ft) = 750 lb/ft standard

W3 = concrete(width of footing)(height of footing)= 23.6 kN/m

3(2.13 m)(0.30 m) = 15.1 kN/m metric

= 150 lbs/ft3(7 ft)(1 ft) = 1050 lb/ft standard

Wi = W1 = W2 = W3 = 91.1 kN/m (6,255 lb/ft)

cA = c for c = (23.9 kN/m2) (500 lb/ft

2) or less

= 23.9 kN/m2 (500 lb/ft

2)

B = 2.13 m (7 ft)

F.S. = R SL/R H = (214 lb/ft)/(364 kN/m) = 0.6 metric

F.S. = R SL/R H = (14,824 lb/ft)/(25,570 lb/ft) = 0.6 standard

Conclusion

The factor of safety with relation to retaining wall sliding is 0.6. This factor of safety is unacceptable. In order to

increase the F.S., we can design a number of combinations including adding a key beneath the footing,

increasing the footing width, and using tie-backs. Also, note that soil above the footing in front of the wall was

not accounted for in this problem. Depending on the footing depth, this soil aids in the sliding resistance.

***********************************

Example #3: Using the results from the Rankine analysis in example problems #1 and #2, determine the











factor of safety for the concrete retaining wall to resist overturning due to lateral earth pressures exerted

on the wall. The retaining wall dimensions are provided below.

Given

unit weight of soil backfill, = 21.2 kN/m

3

(135 lbs/ft

3

) *see typical values vehicular surcharge, q = 14.4 kN/m2 (300 lbs/ft



= ()2/3 = 24 degrees

c = 23.9 kN/m2 (500 lb/ft

2) = cohesion


wall thickness, h = 0.30 m (1 ft)

footing thickness, t = 0.30 m (1 ft)

footing width, B = 2.1 m (7 ft)

distance from the footing edge (toe) to face of wall in front of wall, 0.46 m (1.5 ft)

R = 398 kN/m (27,990 lb/ft) from example problem #1

y = 0.98 m (3.2 ft) from example problem #1

Solution

Factor of safety (F.S.) is typically 1.5 when analyzing overturning

( Wixi + R VxV)/(R Hy) > F.S.

Wixi = summation of the moments (see this link ) for a depiction

W1 = soil(width of soil block above footing)(height of soil block above footing)

= 21.2 kN/m3(1.68 m)(1.83 m) = 65.1 kN/m metric


W2 = concrete(width of wall)(height of wall above footing) = 23.6 kN/m

3(0.253 m)(1.83 m) = 10.9 kN/m metric


W3 = concrete(width of footing)(height of footing) = 23.6 kN/m

3(2.13 m)(0.30 m) = 15.1 kN/m metric

= 150 lbs/ft3(7 ft)(1 ft) = 1050 lb/ft standard

x1 = (width of footing in front of wall) + (width of wall) + (1/2 of width of soil block above footing)

= 0.457 m + 0.253 m + 0.5(1.676 m) = 1.55 m metric

= 1.5 ft + 0.83 ft + 0.5(5.5 ft) = 5.1 ft standard

x2 = (width of footing in front of wall) + (1/2 of wall width)

= 0.457 m + 0.5(0.253 m) = 0.583 m metric

= 1.5 ft + 0.5(0.83 ft) = 1.9 ft standard

x3 = (1/2 width of footing)

= 0.5(2.13 m) = 1.07 m metric = 0.5(7 ft) = 3.5 ft standard

Wixi = W1x1 + W2x2 + W3x3

= (65.1 kN/m)(1.55 m) + (10.9 kN/m)(0.583 m) + (15.1 kN/m)(1.07 m) = 123.4 kN metric

= (4455 lb/ft)(5.1 ft) + (750 lb/ft)(1.9 ft) + (1050 lb/ft)(3.5 ft) = 27,821 lb standard

R V = R sin



















= (398 kN/m)sin 24 = 162 kN/m metric

= (27,990 lb/ft)sin 24 = 11,385 lb/ft standard

xV = (width of footing in front of wall) + (width of wall)

= 0.457 m + 0.253 m = 0.71 m metric

= 1.5 ft + 0.83 ft = 2.3 ft standard

R H = R cos = (398 kN/m)cos 24 = 364 kN/m metric

= (27,990 lb/ft)cos 24 = 25,570 lb/ft standard

y = 0.98 m (3.2 ft)

F.S. = ( Wixi + R VxV)/(R Hy)

= 123.4 kN + (162 kN/m)(0.71 m) = 0.7 metric

(364 kN/m)(0.98 m)

= 27,821 lb + (11,385 lb/ft)(2.3 ft) = 0.7 standard

(25,570 lb/ft)(3.2 ft)

Retaining Wall Technical Guidance

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