Chapter 2b Foundations Shallow & Deep Foundations

Major Building Parts SuperstructureSubstructureFoundation

FOUNDATIONS Purpose; –Transfer Building Loads to the Earth Basic Types –Shallow: Transfer Base of Substructure –Deep: Penetrate Unsuitable Soil to Reach Competent Soil or Rock

PRIMARY FACTORS AFFECTING FOUNDATION CHOICE Subsurface soil Ground water conditions Structural requirements

SECONDARY FACTORS AFFECTING FOUNDATION CHOICE Construction access, methods & site conditions Environmental factors Building Codes & Regulations Impact on surrounding structures Construction schedule Construction risks

SHALLOW FOUNDATIONS Requirements –Suitable soil bearing capacity –Undisturbed soil or engineered fill Basic types or configurations –Column footings –Wall or strip footings

EXAMPLE OF SPREAD FOOTINGS FOR COLUMNS

TOP OF SPREAD FOOTING

EXCAVATION AND REINFORCING FOR TRENCH FOOTING, OR GRADE BEAM

CONCRETE GRADE BEAMS, WITH STEEL REINFORCING TO CONNECT WITH UPPER WALLS

COMBINATION SPREAD & STRIP FOOTING

Shallow Foundations  Concrete Slab on grade with edge turned down onto grade beam. The form at right makes a smooth finish to the exterior.

Shallow foundations that are placed on inert, compacted soil that is not expansive will be stable as long as surface drainage of water does not undermine the foundation bearings. Shallow foundations for low and medium rise buildings, must be protected from unequal settlement due to expansive soil. Provisions must be made in the structure to resist upheaval forces, even if concrete piers are extended to rock formations.

Two Techniques are used to add strength to structural foundations in Low-Rise Buildings: Post-tensioned concrete slabs: Post-tensioned concrete slabs: Concrete beams are cast on the bottom side of the floor slab and reinforced with steel that does not bond to the concrete. After the concrete is set, the steel is stretched and tied, causing a stiffened resistance to forces perpendicular to the slab. Cardboard forms for footings: Cardboard forms for footings: Where grade beams are supported by drilled concrete piers at regular intervals. The grade beam transfers load to the piers, which bear on rock or high capacity soil below. After the piers are in place, but before the grade beam is poured, cardboard planks designed for the purpose are placed in the bottom of the excavation with the concrete grade beam poured on top.

EXAMPLE OF POST-TENSIONING A CONCRETE FLOOR SYSTEM

Deep Foundations transfer building loads deep into the earth Basic types –Drilled (& poured) –Driven

AN AUGER USED TO DRILL AN EXCAVATION FOR A DEEP PIER FOUNDATION