Earth Pressure and Retaining Wall Basics for Non-Geotechnical Engineers
Instructor: Richard P. Weber, P.E.
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PDH Course C155
Earth Pressure and Retaining Wall Basics for Non-Geotechnical Engineers Richard P. Weber
Content Section 1 Retaining walls are structures that support backfill and allow for a change of grade. For instance a retaining wall can be used to retain fill along a slope or it can be used to support a cut into a slope as illustrated in Figure 1.
Retaining Wall to Support a Fill.
Retaining Wall to Support a Cut.
Figure 1 – Example of Retaining Walls Retaining wall structures can be gravity type structures, semi-gravity type structures, cantilever type structures, and counterfort type structures. Walls might be constructed from materials such as fieldstone, reinforced concrete, gabions, reinforced earth, steel and timber. Each of these walls must be designed to resist the external forces applied to the wall from earth pressure, surcharge load, water, earthquake etc. Prior to completing any retaining wall design, it is first necessary to calculate the forces acting on the wall.
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PDH Course C155
This course is not intended to be exhaustive nor does it discuss a wide range of surcharge loads or other lateral forces that might also act on a wall such as earthquake. There are many textbooks and publications that explain loading conditions in depth including: • • • Foundations and Earth Structures, NAVFAC, Design Manual 7.2 Retaining and Flood Walls, Technical Engineering and Design Guides As Adapted from The US Army Corps Of Engineers, No. 4, ASCE Standard Specifications for Highway Bridges, AASHTO
In the following sections, we will first discuss basic considerations necessary for calculating lateral earth pressure and then how to apply these pressures in developing the force. We will illustrate how the lateral forces are combined with vertical forces to calculate the factor of safety with respect to sliding, overturning and bearing capacity. These three components are important elements in retaining wall design. Structural design of a retaining wall is beyond the scope of this course.
Content Section 2 Categories of Lateral Earth Pressure There are three categories of lateral earth pressure and each depends upon the movement experienced by the vertical wall on which the pressure is acting as shown in Figure 2 (Page 4). In this course, we will use the word wall to mean the vertical plane on which the earth pressure is acting. The wall could be a basement wall, retaining wall, earth support system such as sheet piling or soldier pile and lagging etc. The three categories are: • • • At rest earth pressure Active earth pressure Passive earth pressure
The at rest pressure develops when the wall experiences no lateral movement. This typically occurs when the wall is restrained from movement such as along a basement wall that is restrained at the bottom by a slab and at the top by a floor framing system prior to placing soil backfill against the wall. The active pressure develops when the wall is free to move outward such as a typical retaining wall and the soil mass stretches sufficiently to mobilize its shear strength. On the other hand, if the wall moves into the soil, then the soil mass is compressed, which also mobilizes its shear strength and the passive pressure develops. This situation might occur along the section of wall that is below grade and on the opposite side of the
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PDH Course C155
retained section of fill. Some engineers might use the passive pressure that...