Shallow Foundations Civil Engineering Notes
Shallow Foundations
A shallow foundation is a type of foundation which transfers building loads to the earth very near the surface, rather than to a subsurface layer or a range of depths as does a deep foundation. Shallow foundations include spread footing foundations, mat-slab foundations, slab-on-grade foundations, pad foundations, rubble trench foundations, and earthbag foundations.
A frost protected shallow foundation (FPSF) is a building technique whereby insulation is used to reduce heat loss from a foundation. Reducing this heat loss prevents the frost line from penetrating below the insulation and under the foundation and floor slab, allowing for the construction of a much shallower foundation footing, which saves on excavation and material costs. The thickness and location of the insulation are dependent on the severity of the winter in a given location. In many northern locations, builders can save hundreds of dollars in construction costs on a single house. Shallow foundations can be used for heat and unheated buildings.
Difference between deep and shallow foundation?
Shallow foundation: -A type of foundation that is used when the earth directly beneath a structure has sufficient bearing capacity to sustain the loads from the structure
Deep foundation:-A type of foundation that is used when the soil near the ground surface is weak.
1. Light, flexible structure: older residential construction, residential construction which include a basement, and in many commercial structures,
2. Nice soil condition: hard, uniform soil.
3. Cheaper than deep foundation
4. Easier construction
5. Typically types: spreading footing foundation, slab-on-grade foundation, pad foundation, strip foundation, and raft foundation.
6. Spreading footing foundation: controlled by several factors: lateral and vertical capacity, penetration through near surface layers likely to change volume due to frost heave or shrink-swell.
7. Mat-slab foundation: the distribution of loads in a mat slab helps reduce differential settlement due to the non-uniform building loads
1. Heavy, rigid structure: other uncommon building, such as large bridge, tower, and the empire state building.
2. Poor soil condition: liquefaction, soft clay and sands.
3. Typically more expansive
4. More complex to construct and more time than shallow foundation.
5. Typically types: battered piles, bearing piles, caissons, and friction piles.
6. Friction piles: Friction piles obtain a greater part of their carrying capacity by skin friction or adhesion. This tends to occur when piles do not reach an impenetrable stratum but are driven for some distance into a penetrable soil. Their carrying capacity is derived partly from end bearing and partly from skin friction between the embedded surface of the soil and the surrounding soil.
7. End bearing piles: End bearing piles are those which terminate in hard, relatively impenetrable material such as rock or very dense sand and gravel. They derive most of their carrying capacity from the resistance of the stratum at the toe of the pile.
Shallow, Insulated Foundations Lower Construction Costs
Most cold-climate building codes require you to place foundation footings below the frost line. That can be 3- to 4-feet deep in the northeastern United States. The goal is to protect foundations from frost heaving. There is an exception to this standard: many codes permit foundations to lie above the frost line as long as they’re “protected from frost.” However, approval depends on local code officials, and may require special engineering. The 1995 edition of the Council of American Building Officials (CABO) One and Two-Family Dwelling Code may simplify life for builders and code officials alike. The CABO code, which serves as the basis for the country’s other three model codes, includes simplified guidelines for building slab-on-grade homes with shallow foundations that are protected from frost by rigid foam insulation.
The technology cited by CABO-called the Frost Protected Shallow Foundation (FPSF) not only saves energy, but slashes construction costs as well. FPSFs can be used beneath heated or unheated buildings. The technology itself is nothing new. It has been used in Scandinavia for more than 40 years (where it’s now standard practice), and the National Association of Home Builders (NAHB) has aggressively promoted it in the U.S. for more than a decade. An FPSF often improves the energy efficiency of a typical home, because it requires more foundation insulation than many codes. (The exceptions are states with especially strong energy codes, like Washington and Oregon.)
How It Works
The concept is simple. Instead of placing footings below the frost line, the FPSF uses insulation and drainage techniques to raise the frost line to just below the surface. “We basically make the footings think they’re in Florida,” quips Bill Eich, a builder from Spirit Lake Iowa, and a nationally known proponent of the system. Even in the coldest climates, this permits footing depths as shallow as 12 inches.
Compressive strength is an important characteristic for these below-grade applications. Compressive strength is related to foam density. Extruded polystyrene used for sheathing above-grade walls is typically 1.5 pounds per cubic foot (pcf). Naturally, foundation applications require stouter material. CABO permits 2.0 pcf extruded polystyrene for horizontal insulation and 2.0 pcf extruded or expanded polystyrene for vertical insulation. The compressive strength of 2 pcf extruded is 40 pounds per square inch (psi) or 3600 pounds per square foot (psf). This exceeds the underlying soil’s bearing capacity of around 2500 psf. If you prefer a margin of safety, higher density foams are available.
Designing Good Drainage
Insulation is only half the equation. The other half is drainage and moisture control. To keep surface water from soaking in around the foundation, all roof runoff must be directed away from the house. This means putting effective gutters all around the building and sloping the final grade away from the foundation at least 5 inches in the first 10 feet. (That’s 1/2 inch per foot.) To protect the footing from subterranean water, it must bear on at least 4 inches of a non frost-susceptible material such as washed gravel or rock.
Tests Prove Efficiency
NAHB confirmed the system’s efficiency by placing test probes around five homes in Vermont, Iowa, North Dakota and Alaska. Instruments recorded ground, foundation, slab, indoor and outdoor temperatures. The insulated footings kept the soil above freezing even in the coldest weather. When probes that Eich buried three feet below uninsulated ground measured temperatures below freezing, those at the base of nearby shallow foundations checked in at 37°F to 40°F.
A Win/Win System
An FPSF can benefit both builder and homeowner. Shallow foundation ditches are easier to work around. The FPSF uses less concrete than a 4-foot deep stemwall. Smaller ditches require less backfill material and the backfill settles less over time. Since a shallow ditch is less likely to disturb root systems, you can leave shade trees closer to the house. (Eich has built within three feet of large trees.)
Eich credits FPSF’s with saving an average of $1500 in construction costs for a typical home. “Even homes with full basements usually have walkout portions or attached garages” he notes, “so we routinely plan on a shallow foundation for everything we do.”
The technology has even made some customers reconsider what they want in a house. In Eich’s market, everyone used to build full basements, since the footing had to go down 4 feet anyway. Now he finds more people building larger homes on the main level and forgetting about the basement. In a 131-unit Denver, Colorado, housing project, the U.S. Department of Housing and Urban Development was able to save $3000 per unit by substituting traditional stem walls with FPSFs. NAHB estimates that, given a realistic market penetration, the system could save nearly $300 million in annual construction costs.
How does insulation stop frost heave from occurring?
“Frost heave can only occur when all the following three conditions are present: 1) the soil is frost susceptible (meaning it contains more than 5 percent silt), 2) sufficient moisture is available (soil is above approximately 80 percent saturation) and 3) subfreezing temperatures are penetrating the soil. Removing one of these factors will negate the possibility of frost damage.
“Insulation as required in this design guide will prevent underlying soil from freezing. Soil has an insulating value ranging between R-1 per foot and R-3 per foot. (Yes, these values are in feet, not inches.) An inch of polystyrene insulation, R-4.5, has an equivalent R-value of about 4 feet of soil on average. The use of insulation is particularly effective on a building foundation for several reasons. First, heat loss is minimized while storing and directing heat into the foundation’s soil-not out through the vertical face of the foundation wall. Second, horizontal insulation projecting outward will shed moisture away from the foundation further minimizing the risk of frost damage. Finally, because of the insulation, the frost line will rise as it approaches the foundation. Since frost heave forces act perpendicular to the frost line, heave forces, if present, will act in a horizontal direction and not upwards.”