Lateral Earth Pressure and Retaining Structures

Study Notes

Lateral earth pressure is the force exerted by soil on a retaining wall.
Structures that need to resist lateral earth pressure include:
- Retaining walls
- Basement walls
- Tunnels
- Trench walls
- Sheet pile walls
- Slopes
- Buried structures
Gravity retaining walls are typically made of:
- Concrete
- Stone
- Masonry
Cantilever sheet pile walls can support heights up to 7.5 m.
Anchored sheet pile walls are also known as bulkheads.
Cantilever sheet pile walls are stabilized by the weight of the soil behind the wall, and by the interlock between the sheets.
Retaining walls that support backfill and allow for a change of grade are known as recessed walls.

Soils can be classified as coarse-grained or fine-grained.
Coarse-grained soils include gravels and sands, often referred to as cohesionless soils.
Fine-grained soils include silts and clays, with clays often referred to as cohesive soils.
Grain size distribution for coarse-grained soils can be obtained through sieve analysis (ASTM D 422).
Grain size distribution for fine-grained soils can be obtained through hydrometer analysis (ASTM D 422).

Two parameters are determined from the grain size distribution curves of coarse-grained soils:
- Uniformity coefficient (Cu): Calculated as D60 / D10.
- Coefficient of graduation or curvature (Cc): Calculated as (D30)^2 / (D60 * D10).
D10, D30, and D60 represent the diameters corresponding to 10%, 30%, and 60% finer, respectively.

Void ratio (e) is the ratio of the volume of voids to the volume of solids.
Porosity (n) is the ratio of the volume of voids to the total volume of the soil, expressed as a percentage.
Water content (w) is the ratio of the weight of water to the weight of solids, expressed as a percentage.
Degree of saturation (S) is the ratio of the volume of water to the volume of voids, expressed as a percentage.
Bulk (or total) unit weight (γ) is the weight of the soil per unit volume.
Dry unit weight (γd) is the weight of the solids per unit volume.
Specific gravity (Gs) is the ratio of the weight of a substance to the weight of an equal volume of water.

Liquid and plastic limits are routinely determined for cohesive soils.
Atterberg limits are based on the moisture content of the soil.
The plastic limit (PL) is the moisture content at which the soil transitions from a semi-solid to a plastic state.
The liquid limit (LL) is the moisture content at which the soil transitions from a plastic to a viscous fluid state.

The plasticity index (PI) is a measure of the plasticity of a soil.
PI is the difference between the liquid limit and the plastic limit (PI = LL - PL).
Soils with a high PI tend to be clay, while those with a lower PI tend to be silt.