Under normal conditions, soil generates vertical and horizontal pressure due to Poisson’s effects. Terzaghi defined the lateral earth pressure coefficient as the ratio of effective horizontal stress to effective vertical stress.

**In this article, *the terms* stress and pressure are interchangeable.*

To compute lateral earth pressure, we must first compute vertical effective pressure. Then, multiply the vertical pressure by the lateral earth pressure coefficient for the relevant state of soil movement.

The magnitude of lateral earth pressure depends on the lateral flexibility of the retaining wall. An active condition (Ka) is applied at one extreme when the wall is freely moved. The at-rest pressure state (Ko) applies when the wall is rigid against the soil. When the wall moves to the soil, passive condition (Kp) is applied at another extreme on the verge of failure. As a general rule, Ka < Ko << Kp.

The relationship between the wall movement and the value at various states is shown in Fig. 3.12 (Clayton, 2013)

AT-REST PRESSURE

A well-known empirical formula to estimate at-rest pressure for cohesionless soil was proposed by Jaky (1944)

For overconsolidated soils, Meyerhof(1976), Mayne, and Kulhawy (1982) modified the above equation as

Meyerhof (1976)

Mayne and Kulhawy (1982)

For sloped ground surfaces (with Beta angle), Eurocode 7 gives the following formula.

Table 3.1 (Clayton et al., 2013) provides the value of Ko at various OCR values for soils with phi' = 20 and 30 degrees compared to Ka and Kp.

In summary, Active and at-rest Lateral earth pressure decreases when the soil strengthens. Passive pressure gets larger when the soil gets stronger.

DESIGNER's GUIDE

REFERENCES

C Clayton, RI Woods, AJ Bond, and J Milititsky, Earth Pressure and Earth-Retaining Structures, 3rd ed., CRC Press, 2013