Soils and Foundations

Foundation Loads A foundation supports a number of different kinds of loads: Dead Load – of the building which is the sum of the weights of the frame, the floors, roof, and walls. Live Load – which is the sum of the weights of the people in the building, the furnishings and equipment they use, and snow, ice and water on the roof

Foundation Loads Wind Loads – which can apply lateral downward and uplift loads to a foundation Horizontal pressures – of earth and water against sub building walls Horizontal thrusts – from arches, rigid frames, domes, and vaults Buoyant uplift force – from underground water, identical to the force that cause a boat to float Earthquake forces – horizontal and vertical forces caused by the motion of the ground relative to the building.

Satisfactory Foundation A satisfactory foundation for a building must meet three general requirements. The foundation including the underlying soil and rock must be safe against a structural failure that could result in collapse. During the life of the building, the foundation must not settle by an amount that could damage the structure or impair its function. The foundation must be feasible both technically and economically and practical to build without adverse effects to surrounding property

Foundation Settlement All foundations settle to some extent as the soil adjust around and beneath them because of the load No settlement (unnoticeable) Uniform settlement Differential settlement

Soil Classified for Engineering Purposes Rock – continuous mass of solid mineral material Soil – general term referring to earth material Boulder – two hands to pick up Cobble – one hand to pick up

Soil Classified for Engineering Purposes Course Gravel Soils gravel – can be lifted with thumb and forefinger sand – can be seem but cannot be pick up .25 in - .002in. Fine gravel soils Silt – equal dimension range in size from .002 in. - .00008in Clay – play shape and smaller then .0008 in.

Cohessive vs. Frictional Clay Soils – cohesive – retain a measurable shear resistance in the absence of confining forces Sand and Silt soils – fictional or cohessiveless – the shear resistance is directly proportional to the confining force pushing particles together.

Other Issues Peat and other organic soils are not suitable for the support of buildings foundations Water passes readily through clean gravels and sans, slowly through very fine silts and sands, and almost not at all through many clays. An excellent way to keep a basement dry is to surround it with a thick layer of clean gravel or crushed stone. Water passing through the soil toward the building cannot reach the basement without first falling to the bottom of the gravel layer.

Subsurface Exploration and Soil Testing Prior to designing a foundation for any building larger than a single family house, it is necessary to determine the soil and water conditions beneath the soil and water conditions beneath the site. Test pits are useful when foundation is not expected to extend deeper than about 8 ft. Test water level and soil contents Boring machines use on deeper soil test.