LATERAL EARTH PRESSURE:
This is the pressure that we can expect in the horizontal direction.
It has three states: Active, Passive and At-Rest.
At-Rest: The soil is in equilbirum and does not have any forces acting on it.
Active: The soil is “mobilized” and moves away and expands from its natural state and into the shoring.
Passive: The soil is being compressed by a wall, such as tie backs.
Aside from moisture content, the amount of lateral earth pressure seen is dependent on vertical pressures – such as soil weight and surcharges.
The deeper the excavation, the more pressure the soil has to support above it (think of a column of dirt). Weaker soils will be less able to support the soil above and begin to fail; in other words, the internal friction breaks down and soil starts to shift laterally.
This loading is measured in Pounds per Square Foot (PSF).
As you go deeper, pressures increase in proportion to the depth. For example, moist soil to 10’ deep:
10′ deep x 35 PSF = 350psf
Vertical pressure increases as well:
10′ deep x 120 PCF = 1200lbs
PCF: Pounds per Cubic Feet