PPP Notes - Foundations

FOUNDATIONS

 (These notes are compiled from AGS)

GENERAL

 

• Foundations, because they are hidden below the surface, are often overlooked and their importance minimized by the design team.
• A great deal of scientifically guided creativity is often necessary to produce a foundation that supports the loads of the structure in such a way as to economically maintain the aesthetics and function of a facility.
• The wide variety of soil types and conditions across the United States – from bedrock near the surface in New York to the sinkholes and coral in Florida to the deep soft clays in the Midwest and South, and expansive soils in the Southwest to the seismically active areas of the country such as the West Coast – pose a challenge to the design team.
• The most popular and economical foundation solution is the spread footing.
• Spread footings are typically shallow, simple to design and construct, and perform well under many conditions.
• When properly designed, load is spread from a column to the soil at a bearing pressure that causes neither excessive settlement nor failure of the soil.
• Should the soil conditions near the surfaces be weak, poorly compacted, filled with debris or organic material, or too compressible, deep foundations are warranted.
• The effect is to extend the foundation through the weak strata to a soil type that can withstand the loadings with tolerable settlement.
• Deep foundations come in several types and, depending on the soil conditions, may include driven piles, bored piles (bored, augered or drilled) or caissons.
• Deep foundations resist imposed loads either by end bearing or side friction or some combination.
• It is not necessary to drive or drill a deep foundation to rock, only to the depth required to reach a suitable stratum.
• A spread footing is not always appropriate, such as when the property line limits the extent of the foundation in one direction, or when the soil conditions are very weak and suitable soil strata too deep to reach with a deep foundation.
• In these cases, other types of special foundations (such as combined footings, strap footings, and raft foundations) are sometimes required.
• Two basic criteria should be met for all foundations:
• Soil strength (bearing capacity): The ability of a soil to support a load without experiencing failure is known as the bearing capacity and is a function of the foundation size as well as the inherent strength properties of the soil.
• If the pressures exerted by a foundation exceed the strength of a soil, the soil mass experiences a shear failure leading to gross movements of both the soil and the supporting foundation element.
• Limitations of Settlement: Settlement can happen either immediately (foundations on sands), or over period of time, short of long term (foundations on clays).
• Some settlement is expected, over various parts of the country, typical and acceptable settlement is usually less than 1 in.
• Settlement is not as important with a solitary structure, but becomes more important when:
• 1. Buildings adjacent to an existing structure need to be interconnected.
• 2. Long utility runs need to be connected to the structure.
• 3. There is sensitive equipment in the building.
• Uniform settlement is somewhat better tolerated than differential settlement that is uneven across several columns.
• Differential settlement distorts the structure and causes cracking of the exterior skin and interior partitions, broken windows, and doors that don’t open.
• Allowable differential settlement may be dependent on the material of the skin and structure; for example, brick and concrete masonry buildings tolerate less differential settlement than curtain wall buildings.
• Differential settlement of ¼ in. is typically considered tolerable for most building types.
• The importance of proper foundation design and detailing cannot be overemphasized. 
• Working with the geotechnical engineer, familiar with the soil conditions in the area, and a structural engineer, familiar with the proposed design and detailing of the foundation, will help ensure the building functions as intended for its life cycle.

 

SETTLEMENT AND DIFFERENTIAL SETTLEMENT

 

• Often, settlement governs the allowable bearing pressure, which is set at an intensity that will yield a settlement within tolerable levels for the building type.
• Allowable settlement is typically building and use-specific.
• Total and differential settlement, as well as the time rate of the occurrence of the settlement, must be considered when evaluating whether the settlement is tolerable.
• For example, in the case of a conventional steel frame structure, in typical practice a total maximum settlement of 1 in. is usually acceptable, and differential settlement of one-half of the total settlement is also usually tolerable.

 

ANGULAR DISTORTION

 

• Settlement tolerance is commonly referred to in terms of angular distortion in the building or settlement between columns.
• Typically, an angular distortion of 1:480 is used for conventional structure.
• This equates to 1 in. in 480 in., or 1 in. in 40 ft.
• Depending on the type of structure, the allowable angular distortion might vary from 1:240 for a flexible structure (such as a wood frame, single-story structure) to 1:1000 for a more “brittle” or sensitive structure.

 

EFFECTS OF SOIL TYPES

 

• When load is applied to granular soils, the grains of soils are able to respond almost immediately, and they will densify as the packing of the grains becomes tighter.
• Clay soils exhibit a time-dependent relationship associated with the consolidation of the clay soil.
• In order for the clay to consolidate, and the overlying soil or structure to settle, the excess pressures that are induced in the water in the clay must dissipate, and this takes time because of the low permeability of the clay.
• Depending on the drainage characteristics of the clay, the time required for 90 percent of the consolidation (and settlement) to occur may vary from a few months to a several years.
• If there is a high frequency of sand layers or seams within the clay mass, then the consolidation will be quicker, because the excess pore water pressure can be dissipated faster.
• Both sand and clay soils have a built-in “memory” that, in effect, remember the maximum load that was applied to the soil at some time in the past.
• This memory is referred to as the preconsolidation pressure.
• If 10 ft. of soil has been removed (by excavation or erosion) from a soil profile then the equivalent weight of that 10 ft. of soil (approximately 1250 lbs per square foot) could be reapplied to the soil profile without the soil below sensing any difference.
• Depending on the process that deposited the soil, weathering processes, past climatological changes, or human activities, the preconsolidation pressure of the soil may be far in excess of the pressures induced by the current soil profile.
• When that is the case, settlement of conventional structures is rarely a significant concern. 
• But when the soil has not been preconsolidated, the addition of any new load may result in excessive settlement.

 

SHALLOW FOUNDATIONS

 

• Shallow foundations are typically the most economical foundations to construct where soil and loading conditions permit.
• Coordination with local codes for frost depth and with the underground utilities is required.
• The thickness of the footing has to be coordinated with anchor bolt and dowel embedment. 
• Typically, one layer of steel in the bottom of the footing is required to resist the bending of the footing caused by soil-bearing pressures.
• Axial loads, combined with shear and overturning forces can be resisted by spread footings.
• The combination of axial load and moment forces on the foundation need to be balanced to keep the calculated loads on the footing less than the allowable bearing pressure of the soil as determined by the geotechnical engineer.
• Minimum sizes of shallow footings are specified by the geotechnical engineer, to reduce the possibility of local soil failures by punching shear of an overall movement of soil mass.
• Maximum sizes of spread footings keep the non-uniform bearing pressure from becoming extreme and overstressing the soil.

 

PILE CAPS

 

• Typically, more than one deep foundation element is required to resist the gravity and lateral loads; in order to distribute the loads from the single point column to the multiple foundation elements, a pile cap is required.
• Pile caps are thick, reinforced concrete blocks that distribute the load from the column to the foundations through a combination of flexure and shear.
• Other applications of the pile caps include providing a method of connecting the columns to the foundations, and easing the construction tolerance issues that occur when installing deep foundations.
• These pile caps are designed and detailed to encase a small portion of the deep foundation, and transition to the column support elevation, thus providing a convenient location to position anchor bolts and column dowels.

 

FINISH AND FLOOR FLATNESS

 

• In general, concrete floor slabs are monolithically finished by floating and troweling, to achieve a smooth and dense surface finish.
• ACI 302 provides guidance for appropriate finishing procedures to control achievable floor flatness.
• ACI 302, ACI 360, ACI 117 provide guidance for flatness selection, as well as techniques by which flatness and levelness are produced and measured.
• Floor finish tolerance is measured by placing a freestanding 10 ft. straightedge on the slab surface, or by F-Numbers.
• The preferred method of measuring flatness and levelness is the F-Number System.
• Special finishes are available to improve appearance, as well as surface properties.
• These properties include sprinkled (shake), finishes or high-strength toppings, either as monolithic or separate surfaces.

 

PROTECTIVE AND DECORATIVE COATINGS

 

• Concrete surfaces may require a sealer or coating for the following:
• To protect against severe weather, chemicals, or abrasions
• To prevent dusting of the surface layer
• To harden the surface layer
• To add a decorative finish
• Sealers are usually clear and are expected to penetrate the surface without leaving a visible film.
• Coatings are clear or opaque, and, though they may have some penetration, they leave a visible film on the surface.
• Sealers and coatings should allow vapor emission from the concrete but, at the same time, prevent moisture from penetrating after curing.
• Decorative coatings usually protect as well, and are formulated in a wide selection of colors.
• Decorative coatings include the following:
• Water-based acrylic emulsion
• Elastomeric acrylic resin
• Liquid polymer stain
• Solvent-based acrylic stain
• Portland cement-based finish coating
• Water-based acidic stain (a solution of metallic salts)