Foundation Types and Functions Quiz

Foundations

• Foundations reduce load intensity, evenly distributing the load and providing a level surface.
• They ensure lateral stability and protection against undermining, scouring, and soil movements.
• Essential requirements of a good foundation include sustaining dead and imposed loads, transmitting these loads to the subsoil without causing excessive settlement, maintaining stability of the building and adjacent structures, and minimizing differential settlement.
• Foundations should be taken sufficiently deep to prevent damage or stress from swelling or shrinkage of the soil.
• The location of the foundation should not be affected by unexpected future influences.

Types of Foundations

• Shallow foundations, according to Terzaghi, are those where the depth is equal to or less than the width.
• Deep foundations have a depth much greater than their width.
• Types of shallow foundations include spread footings, combined footings, strap footings, and raft or mat foundations.
• Spread footings distribute the load over a larger area.
• Combined footings support multiple columns.
• Strap footings connect footings of two columns with a beam.
• Raft or mat foundations span the entire area beneath the structure.
• Types of deep foundations include deep strip, rectangular or square footings, pile foundations, pier foundations (or caissons), and well foundations.

Shallow Foundations: Spread footings

• Spread footings distribute the load over a larger area.
• Types of spread footings include single footings, stepped footings, sloped footings, wall footings, stepped footings for walls, and grillage foundations.

Shallow Foundations: Isolated footing

• Footings for a single column are called isolated footings.
• Single footings have various configurations (single, stepped, and sloped).

Shallow Foundations: Wall footings

• Wall footings have different types, one without a step, and stepped.

Shallow Foundations: Grillage foundation

• A grillage foundation is a type of shallow foundation that supports columns or walls using multiple layers of steel beams.

Combined Footings

• A combined footing supports two or more columns.
• Types include rectangular combined footings, trapezoidal combined footings, and combined column and wall footings.
• Footings are designed so that the center of gravity of the column loads coincide with the centroid of the footing area.
• If the columns have unequal loads, the footing's shape will be trapezoidal.

Mat Foundations (Raft Foundations)

• A raft foundation (also called a mat foundation) covers the entire area beneath a structure, supporting all columns and walls.
• They are used when the allowable soil pressure is low or building loads are heavy.
• Considering using a raft footing when spread footings would cover more than half the area.

Strap Footings

• Strap footings connect independent footings of two columns with a beam.
• Strap footings are used when the distance between the columns is large, making a combined trapezoidal footing impractical.

Deep Foundations (Pile Foundation)

• Pile foundations use vertical members (timber, concrete, or steel) to transfer loads to a lower level.
• Types of pile foundations based on load transfer include end bearing piles, friction piles, combined end bearing and friction piles, and compaction piles.

Pile Foundations: End Bearing Piles

• Used to transfer loads to suitable bearing strata below water or soft soil.
• Often used in multi-story buildings to minimize settlement.

Pile Foundations: Friction Piles

• Transfer loads to a depth where friction along the pile's length resists the load.
• Commonly used in granular soils where the hard stratum is deep.

Pile Foundations: Combined End Bearing and Friction Piles

• Combine both end bearing and friction load-carrying characteristics.
• Common when end-bearing piles pass through granular soils.

Pile Foundations: Compaction Piles

• Designed to compact loose granular soils, increasing their bearing capacity.
• Compaction piles are composed of weaker materials such as timber or bamboo.

Types of Piles (Construction Process)

• Bored piles are constructed by excavating a hole and filling it with concrete.
• Driven piles are hammered or driven into the ground.

Types of Piles (Materials)

• Pile types include: wooden piles, concrete piles (precast and cast-in-situ), steel piles, and composite piles.

Pier Foundations

• Pier foundations consist of a large-diameter cylindrical column used to transfer heavy loads to firm strata.
• Pier foundations transfer loads only through bearing.
• Used in locations where the top soil layer is composed rock and overlying a stratum of sound rock.
• Drilled caissons are a type of pier foundation, used when hard strata are found at shallow depths.

Well Foundations

• Box-like structures, rectangular or circular, sunk to a desired depth.
• Much larger in diameter than pier or drilled caissons.
• Wells are hollow, filled with sand, and plugged at the bottom.

Subsoil Exploration

• Used for new structures to select the foundation type and depth, evaluate bearing capacity, predict settlements, determine ground water levels.
• Used for existing structures to evaluate safety, predict settlements, determine remedial measures.

Site Exploration

• Aims to gather reliable, detailed information about soil and ground water conditions for safe, cost-effective foundation design.

Methods of Soil Exploration

• Methods include: open excavation/trial pits, boring, sub-surface sounding, and geo-physical methods.

Bearing Capacity of Soil

• The bearing capacity of soil is its ability to support loads.
• Key concepts include gross pressure intensity, net pressure intensity, ultimate bearing capacity, and allowable bearing pressure, important in foundation design.

Methods for Estimating Bearing Capacity

• Methods include analytical methods, plate load tests, penetration tests, and building codes.

Depth of Exploration

• Exploration depth depends on the expected load, likely settlement, and potential for shear failure.
• Rules of thumb are often used to estimate required depth.

Design of Shallow Foundation

• The width of shallow footings is often calculated based on soil bearing capacity considerations.
• Types of shallow foundation design include: simple strip, stepped strip, isolated/pad footings. These are categorized based on the geometry of the supporting structure and its load.