natural foundations and subsoils

what are the 2 main functions of foundations?

1. safely spread the load exerted by the building over a sufficiently wide area

2. divert/transfer the load to a strata deep in the soil

bearing pressure

the total load per unit area (exerted by the live, dead and wind loads)

bearing capacity

the load per unit of ground the soil can safely carry

what units are used for bearing pressure and bearing capacity?

KN/m²

what relationship must bearing pressure and capacity have to ensure the soil isn’t overloaded?

BP<BC

typical range of depth of topsoil

150-230mm

what are the characteristics of granular soils?

• relatively high/good bearing capacity

• low risk of long-term settlement

• easy extraction

• finer particles can be affected by freezing conditions

• relatively quick initial settlement

examples of coarse-grained granular soils

gravel and sand

what is the average initial settlement period of coarse granular non-cohesive soils?

3-4months

average initial settlement period of fine grained cohesive soils?

3-4 years

general characteristics of cohesive soils

• low bearing capacity

• prone to shrinkage and swelling

• susceptible to frost

examples of fine grained cohesive soils

silt and clay

characteristics of clays

• stability and strength affected by moisture content

• volume changes as moisture content does - swells/shrinks

• wide range of bearing capacities and colours

• very slow initial settlement

• can contain sulfates which attack concrete

4 main causes of compression of soil

1. elastic deformation of soil particles - immediate settlement

2. decrease in the number of voids

3. plastic deformation of soil

4. lateral movement of soil particles

particle size and bearing capacity of gravel

particle size > 2mm

bearing capacity up to 600Kn/m²

bearing capacity of dense, medium dense and loose gravel

dense > 600KN/m²

medium dense 200-600KN/m²

loose > 200KN/m²

particle size and bearing capacity of sand

particle size 0.06-2mm

bearing capacity up to 300+KN/m²

bearing capacity of dense, medium dense and loose sand

dense >300KN/m²

medium dense 100-300KN/m²

loose <100KN/m²

particle size and bearing capacity of silt

particle size 0.002-0.06mm

bearing capacity up to 75KN/m²

particle size and bearing capacity of clay

particle size <0.002mm

bearing capacity 75-300KN/m²

bearing capacity of stiff vs firm clay

stiff: 150-300KN/m²

firm: 75-150KN/m²

void ratio

• the ratio between the volume of voids and the volume of solids in soil

• void ratio = voids/solids

• coarse: larger but fewer voids

• fine: smaller but more voids

compressibility

particles of soil being squeezed or pushed together

permeability

the rate at which water under pressure can flow through interconnected voids in the soil

• flows quicker in granular than cohesive due to larger voids

cohesion

• the molecular attractive forces that bind soil particles together

• determines the shear strength of cohesive soils

order main soil types from biggest to smallest

gravel, sand, silt, clay

examples of rock

granite, basalt, limestone, sandstone

pros and cons of solid rock strata

+excellent base - high bearing capacity

+can use blinding to provide level base - cavities must be filled

- potential differential settlement if other subsoils support foundation

-cost of levelling and excavating outweighs good natural base