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Chapter 2 Foundations


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The portion of a building that has the sole purpose of transmitting structural loads from the building into the earth.
dead load
The weight of the building itself.
live load
The weight of snow, people, furnishings, machines, vehicles, and goods in or on a building.
wind load
A force on a building caused by wind pressure and/or suction.
A lateral or inclined force resulting from the structural action of an arch, vault, dome, suspension structure, or rigid frame.
foundation settlement
The amount, usually measured in millimeters, that all foundations settle as the soil around and beneath them adjusts itself to the loads of the building.
uniform settlement
Subsidence of the various foundation elements of a building at the same rate, resulting in no distress to the structure of the building.
differential settlement
Subsidence of the various foundation elements of a building at differing rates.
A continuous mass of solid mineral material that can only be removed by drilling and blasting. It is never completely monolithic, but is crossed by a system of joints that divide it into irregular blocks. Despite these joints, it is generally the strongest and most stable material on which a building can be founded.
A general term referring to earth material that is particulate.
An individual particle of soil that is too large to lift by hand, or requires two hands to lift.
A particle that takes the whole hand to lift.
A particle that can be easily lifted with thumb and forefinger. Classified as having more than half their particles larger than 0.25 inch in diameter.
The individual particles can be seen but are too small to be picked up individually. Particles range in size from about 0.25 - 0.002 inch.
coarse-grained soil
Sand and gravel are considered this type of soil.
Particles are approximately equidimensional and range in size from 0.002 to 0.00008 inch. Because of their low surface-area-to-volume ratio, which approximates that of sand and gravel, the behavior of this particle is controlled by the same mass forces that control the behavior of the coarse-grained soils.
Plate shaped rather than equidimensional and smaller than silt particles, less than 0.00008 inch. Because of their smaller size and flatter shape, they have a surface-area-to-volume ratio hundreds or thousands of times greater than that of silt.
The arrangement of particles and pores.
A soil such a clay whose particles are able to adhere to one another by means of cohesive and adhesive forces. They retain a measurable shear resistance in the absence of confining forces. More directly, clay soil sticks together.
cohesionless or frictional
The opposite of cohesive particles. The ability of this type of soil such as sand or silt to support a building depends on the friction between its particles.
Superimposed layers of different soils that lies beneath every building.
The ability of a soil to retain its engineering properties under the varying conditions that may occur during the lifetime of the building.
test pit
A procedure done prior to designing a foundation for any building larger than a single-family house that is necessary to determine the soil and water conditions beneath the site. Useful when the foundation is not expected to extend deeper than about 8 feet, which is the maximum practical reach of small excavating machines
test boring
A procedure done prior to designing a foundation for any building larger than a single-family house that is necessary to determine the soil and water conditions beneath the site.
water table
The level at which the pressure of water in the soil is equal to the atmospheric pressure; effectively, the level to which groundwater will fill an excavation.
liquid limit
The water content at which the soil passes from a plastic state to a liquid state.
plastic limit
The water content at which the soil loses its plasticity and begins to behave as a solid.
A device for testing the resistance of a material to penetration, usually used to make a quick, approximate determination of its compressive strength.
Some excavation is required for every building. Involves the removal of organic topsoil and then further digging to place the footings out of reach of water and wind erosion and below that level at which the ground freezes in winter.
ice lenses
Under certain soil and temperature conditions, upward migration of water vapor from the pores in the soil can result in the formation of these, thick layers of frozen water crystals that can lift buildings by even larger amounts.
A stiff material used to retain the soil around an excavation; a material such as polyethylene in the form of very thin, flexible sheets.
soldier beam
Steel wide-flange sections driven vertically into the earth at close intervals around an excavation site before digging begins.
Planks placed between soldier beams to retain earth around an excavation.
sheet piling
Vertical planks of wood, steel, or precast concrete that are placed tightly against one another and driven into the earth to form a solid wall before excavation begins.
slurry wall
A more complicated and expensive type of sheeting that is usually economical only if it becomes the permanent foundation wall of a building.
guide wall
Shallow poured concrete walls used to define the location and thickness of the slurry wall.
clamshell bucket
A special narrow bucket mounted on a crane that is used to excavate the soil from between the guide walls. This is done after the formwork has been removed from the guide walls.
A large funnel with a tube attached, used to deposit concrete in deep forms or beneath water or slurry.
precast slurry wall
A slurry wall made of a mixture of water, bentonite clay, and portland cement. The wall is prestressed and is produced in sections in a precasting plant, then trucked to the construction site.
soil mixing
A technique of adding a modifying substance to soil and blending it in place by means of paddles rotating on the end of a vertical shaft.
Diagonal members, either temporary or permanent, installed to stabilize a structure against lateral loads.
crosslot bracing
Horizontal compression members running from one side of an excavation to the other, used to support sheeting.
A horizontal beam used to support sheeting or concrete formwork.
A sloping brace for supporting sheeting around an excavation.
heel block
Temporary footings used to support rakers when the excavation is too wide for crosslot bracing.
A tie, one end of which is anchored in the ground, with the other end used to support sheeting around an excavation.
rock anchor
A posttensioned rod or cable inserted into a rock formation for the purpose of tying it together.
soil nailing
The process of inserting a steel reinforcing bar into a nearly horizontal hole drilled deep into the soil. Grout is then injected into the hole to bind the soil nail to the surrounding soil. Large numbers of closely spaced nails are used to knit a large block of soil together so that it behaves more like weak rock than particulate soil.
A process that is done to remove water from the excavation site when the excavation work is carried out below the water table in the surrounding soil.
A pit designed to collect water for removal from an excavation or basement.
well point
Vertical pieces of pipe with screened openings at the foot that keep out soil particles while allowing water to enter. They are closely spaced and are driven into the soil around the entire circumference of the excavation. Commonly used to depress the water table in an excavation.
watertight barrier
A wall used to keep water out of the excavation site when well points are not possible to use. Examples include a slurry wall and sheet piling.
The above-ground portion of a building.
The occupied, below-ground portion of a building.
shallow foundation
One of the 2 basic types of foundations. Foundations that transfer the load to the earth at the base of the column or wall of the substructure.
deep foundation
One of the 2 basic types of foundations. Foundations, either piles or caisons, that penetrate through upper layers of incompetent soil in order to transfer the load to competent bearing soil or rock deeper within the earth.
The widened part of a foundation that spreads a load from the building across a broader area of soil.
column footing
A square block of concrete, with or without steel reinforcing, that accepts the concentrated load placed on it from above by a building column and spreads this load across an area of soil large enough that the allowable bearing stress of the soil is not exceeded.
wall or strip footing
A continuous strip of concrete that serves the same function for a load bearing wall as a column footing does for a building column.
engineered fill
Earth compacted into place in such a way that it has predictable physical properties, based on laboratory tests and specified, supervised installation procedures.
slab on grade
A concrete surface lying upon, and supported directly by, the ground beneath.
A space that is not tall enough to stand in, located beneath the bottom of a building.
A space that is tall enough to stand in, located beneath the bottom of a building.
tie beam
A reinforced concrete beam cast as part of a masonry wall, whose primary purpose is to hold the wall together, especially against seismic loads, or cast between a number of isolated foundation elements to maintain their relative positions.
combined footing
Since footings cannot legally extended beyond a property line, and if the outer toe of the footing were cut off at the property line the footing would not be symmetrically loaded by the column or wall and would tend to rotate and fail, a combined footing is used to solve the problem. This is done by tying the footings for the outside row of columns to those of the next row in such a way that any rotational tendency is neutralized.
cantilever footing
Since footings cannot legally extended beyond a property line, and if the outer toe of the footing were cut off at the property line the footing would not be symmetrically loaded by the column or wall and would tend to rotate and fail, a cantilever footing is used to solve the problem. This is done by tying the footings for the outside row of columns to those of the next row in such a way that any rotational tendency is neutralized.
mat or raft foundation
A single concrete footing that is essentially equal in area to the area of ground covered by the building.
floating foundation
Used where the bearing capacity of the soil is low and settlement must be carefully controlled. It is essentially the same as a mat, but is placed beneath a building at a depth such that the weight of the soil removed from the excavation is equal to the weight of the building.
Similar to a column footing in that it spreads the load from a column over a large enough area of soil that the allowable stress in the soil is not exceeded. It differs from a column footing in that it extends through strata of unsatisfactory soil beneath the substructure of a building until it reaches a satisfactory bearing stratum, such as rock, dense sands and gravels, or firm clay.
belled caisson
Practical only where the bell can be excavated in a cohesive soil (like clay) that can retain its shape until concrete is poured and where the bearing stratum is impervious to the passage of water, to prevent flooding of the hole.
socketed caisson
Drilled into the rock at the bottom, rather than belled. Its bearing capacity comes not only from its end bearing, but from the frictional forces between the sides of the caisson and the rock as well.
drilled pier
A term sometimes used in literature to describe the foundation unit
Distinguished from a caisson by being driven into the earth rather than drilled and poured.
timber pile
The simplest kind of pile, made of a tree trunk with its branches and bark removed. It is supported small end down in a piledriver and beaten into the earth with repeated blows of a very heavy mechanical hammer.
Used to support a timber pile.
end bearing pile
A pile driven until its tip encounters firm resistance from rock, dense sands, or gravels.
friction pile
A pile driven only into softer material, without encountering a firm bearing layer. It will still develop a considerable load-carrying capacity through the frictional resistance between the sides of the pile and the soil through which it is driven.
pile cap
A thick slab of reinforced concrete poured across the top of a pile cluster to cause the cluster to act as a unit in supporting a column or grade beam.
grade beam
A reinforced concrete beam that transmits the load from a bearing wall into spaced foundations such as pile caps or caissons.
pile hammer
Massive weights lifted by the energy of steam, compressed air, compressed hydraulic fluid, or a diesel explosion, then dropped against a block that is in firm contact with the top of the pile. Single-acting hammers fall by gravity alone, while double-acting hammers are forced downward by reverse application of the energy source that lifts the hammer.
Tall vertical rails at the front of a piledriver that the pile hammer travels on.
One of 2 types of steel pipes used to make piles. They are special hot-rolled, wide-flange sections, 8-14 inches deep, that are approximately square in section. Used mostly in end bearing applications. Displace relatively little soil during driving.
pipe pile
One of 2 types of steel pipes used to make piles. 8-16 inches in diameter. They are stiff and can carry heavy loads. Displace relatively large amounts of soil during driving, which can lead to upward heaving of nearby soil and buildings.
The forcing upward of ground or buildings by the action of frost or when many piles are driven in close together. Is a particular problem in urban sites, where it can lift nearby buildings.
precast concrete pile
Concrete cast and cured in a position other than its final position in the structure. They are square, octagonal, or round in section. Advantages of this type of pile include high load capacity, an absence of corrosion or decay problems, and in most situations, a relative economy of cost.
sitecast concrete pile
Concrete that is poured and cured in its final position in a building. Made by driving a hollow steel shell into the ground and filling it with concrete.
A stiff steel core placed inside the thin steel shell of a sitecast concrete pile to prevent it from collapsing during driving.
pressure-injected footing
Share characteristics of piles, piers, and footings. Highly resistant to uplift forces, which is useful for tall, slender buildings which have the potential to overturn, and for tensile anchors for tent and pneumatic structures.
base isolator
Buildings are sometimes placed on these, which allow the ground to move laterally back and forth beneath the building while the substructure and superstructure remain more or less at rest and free from damage.
The process of placing new foundations beneath an existing structure.
retaining wall
A wall that resists horizontal soil pressures at an abrupt change in ground elevation.
earth reinforcing
An economical alternative to conventional retaining walls in many situations. Soil is compacted in thin layers which stabilize the soil in much the same manner as roots of plants.
A synthetic cloth used beneath the surface of the ground to stabilize soil or promote drainage.
Removal of water. Reduces the amount of water that reaches the foundation.
waterproof membrame
Acts as a barrier to the passage of water through the basement wall.
A coating applied to the outside face of a basement wall as a barrier to the passage of water.
bentonite clay
An absorptive, colloidal clay that swells to several times its dry volume when saturated with water.
A synthetic rubber or bentonite clay strip used to seal joints in concrete foundation walls.
protection board
Semirigid sheet material used to cushion the outside of a foundation wall, and particularly its waterproofing layer, from damage caused by rocks in the backfill material.
blind-side waterproofing
An impervious layer or coating on the outside of a foundation wall that, for reasons of inaccessibility, was applied before the wall was constructed.
drainage mat
Involved in the installation of blind-side waterproofing. A drainage mat is applied to the slope retention sheeting, which will be left in place permanently, and the membrane is applied to the drainage mat.
Earth or earthen material used to fill the excavation around a foundation constructed within a benched excavation site only. An open, fast-draining soil like sand is preferred for this because it allows the perimeter drainage system around the basement to work properly. The act of filling around a foundation.
controlled low-strength material (CSLM)
A concrete that is purposely formulated to have a very low but known strength, used primarily as a backfill material.
mud slab
A slab of weak concrete placed directly on the ground to provide a temporary working surface that is hard, level, and dry.
up-down construction
A sequence of construction activity in which construction proceeds downward on the sublevels of a building at the same time as construction proceeds upward on the superstructure.

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