PDHonline Course S231 (4 PDH)Basics of Frost Protected ShallowFoundation DesignInstructor: Andrew J. Bibb, PE2012PDH Online PDH Center5272 Meadow Estates DriveFairfax, VA 22030-6658Phone & Fax: 703-988-0088www.PDHonline.orgwww.PDHcenter.comAn Approved Continuing Education Provider

www.PDHcenter.comPDH Course S231www.PDHonline.orgBasics of Frost Protected Shallow Foundation DesignAndrew J. Bibb, P.E.INTRODUCTION:Section 1805.2.1 of the International Building Code 2003 and 2006 editions requires foundations for all but the most minorof structures to be constructed in such a manner that they will not be adversely affected by freezing of the surroundingand supporting soils. This protection can be achieved by constructing the foundations on bedrock, constructing thefoundations so the bottom of footing is below the frost depth of the locality, or by protecting the foundation per therequirements of ASCE 32. This course will cover the basics of the last method. The primary advantage in using this typeof construction is, as is so often the case, lower construction cost. Strategically placed rigid insulation allows shallow frostprotected foundations to be build at depths as little as 16 inches in areas that would normally require excavations of 48inches or more using a conventional design.ASCE 32-01, “Design and Construction of Frost-Protected Shallow Foundations”, contains several different, codeapproved, methods to design shallow foundations of various types. The reference booklet you downloaded for thiscourse, the HUD “Revised Builder’s Guide to Frost Protected Shallow Foundations” contains design methods for the mostcommon foundation types. The methods in the builders guide are all contained in ASCE 32 and therefore meet therequirements of the IBC code.For this course ASCE 32 is an optional reference. The HUD builders guide is required.BEHAVIOR OF SOILS AS THEY FREEZE:Most materials will contract in volume as they become colder, and dry soil is no exception. Even water will contract as itbecomes colder, however it has the somewhat unique property of expanding just as it freezes. This volume increase isapproximately 9%.Recall that the porosity of soil is defined as the void volume divided by the total volume, and is most commonly expressedas a percentage. Typical soils have a porosity in the range of 20% to 30%. Consider then the volume change 1 cubicfoot of saturated soil with a porosity of 25% as it freezes. Because the soil is saturated, 25% (or .25 ft3) of the initialvolume is water, and 75% (or .75 ft3) is soil. Neglecting the contraction of the soil as it freezes the total change in volumeof the initial 1 cubic foot is then:Volume after freezing .75 ft3 1.09(.25ft3) 1.02 cubic feetThis small volume change is not significant. For frost heave problems to occur some other mechanism has to come intoplay; that something is the addition of water to the freeze front. This water can come from the surface, but morecommonly it comes from below the freeze front via capillary action. Water rises in the soil by capillary action and formsice lenses at the freeze front as illustrated below. It is the volume of these ice lenses which continue to grow during thefreezing season, which cause heaving problems, not the freezing and expansion of the initial moisture in the soil mass. Andrew J. Bibb, P.E.Page 2 of 16

www.PDHcenter.comPDH Course S231www.PDHonline.orgIn order to obtain the quantity of water necessary for the growth of large ice lenses soils must be both permeable andhave voids small enough to support capillary rise. Coarse sand has very poor capillary rise, but very good permeability.Clays have very good capillary rise, but poor permeability. Silts have both good capillary rise and good permeabilitywhich is why silts are generally considered frost susceptible.There are three requirements that must be met simultaneously for soils to heave.1.The soils must be frost susceptible.It takes a very small amount of fine material to contaminate an otherwise good granular soil. ASCE 32 defines a“small amount” as 6% or more by weight passing a number 200 sieve. A pure clay should theoretically not befrost susceptible due to its low permeability, however any type of clay or clayey soils are generally consideredfrost susceptible.2.There must be a ready source of water available, usually from below but it could be from surface water.3.Freezing air temperaturesThe soil must experience sub-freezing temperatures of a duration where any water in the ground can freeze.Remove any one of these three and soil will not experience frost heave. This means that if there is no moisture availableeven a frost susceptible soil in a frozen state will not heave. It also means that a good coarse granular material, even ifsaturated and frozen, will not heaveFROST DAMAGE TO FOUNDATIONS:There are three primary mechanisms where frost can damage a structure foundation.1.Frost heave under the footing.The force exerted by expansion of ice can easily exceed the weight of any reasonable size structure. There is nopractical way to resist these forces, and if frost heave is allowed to occur below footings the structure will be lifted.2.AdfreezeThis is loosely related to heaving under a footing, but with this mechanism the ice adheres to the side of afoundation. As free water in the ground freezes and expands it can adhere to the side of a foundation and lift thestructure. This is sometimes seen in utility poles embedded into the ground. During the freezing season they arelifted by adfreeze. Soil will slough into the void created at the bottom of the hole. When the soils thaw in springthe pole will tend to settle back, but the soil that has sloughed into the hole will keep the pole from quite goingback to its original level. Over time the pole can be jacked completely out of the ground.3.Thaw WeakeningCohesive soils in a moist state can be prone to weakening while thawing. This process generally causessignificant settlement.HOW THE PROTECTION SYSTEM WORKS:It is generally not practical to ensure that the soils below and around a structure are completely non-frost susceptible. It isalso not practical to ensure that the soils are completely dry, and more importantly that water can never migrate into themfrom below via capillary action. The frost protection scheme attacks the third criteria, exposure to sub-freezingtemperatures.A building will have a “heat bulb” stored in the soil below it. The heat comes from conduction of warm summer air, heatingof the interior space, and heat from deep soil under the structure. This stored “heat bulb” will exist even below anunheated structure. This stored heat will tend to raise the freeze line in the vicinity of a building (refer to Figure 2 in theHUD Builders guide). Rigid insulation, properly designed and placed around the building perimeter, will raise the frost lineeven further. It is the insulation retarding the flow of stored heat, which in turn raises the frost line right at the building,which allows the extremely shallow footing depths to be used.Added benefits to the system are reduced adfreeze against the foundation and reduced potential for thaw weakening.Both are due to lower depth of the freeze line against the building. Andrew J. Bibb, P.E.Page 3 of 16

www.PDHcenter.comPDH Course S231www.PDHonline.orgThe thermal conductivity of soils is dependent upon the type of soil, and the moisture content of the soil. Because of allthese variables there is no single R value for soil, and it is somewhat irrelevant anyway as all soils are poor insulators. Inround numbers, an inch of polystyrene with R 4.5 is more or less equivalent in insulation value to 4 feet of soil.Polystyrene can also have remarkable allowable bearing capacities. The types of insulation used in frost protectedfoundations have allowable bearing capacities ranging from 1,200 to 4,800 pounds per square foot making it suitable foruse under slabs, continuous strip footings, and isolated column footings.TERMS USED: Air-Freezing Index.Mathematically the Air-Freezing index is the sum of the freezing degree days during a winter.A.F.I. Σ ( ½ (Tmax Tmin) – 32oF)Tmax maximum daily air temperatureTmin minimum daily air temperature NFS soil or fillNFS is Non-Frost Susceptible soil. Defined by ASCE 32 as a granular soil with less than 6% by weightpassing a number 200 sieve. MATMean Annual Temperature at a site. This number is used to calculate insulation requirements for unheatedbuildings.PARTS OF THE SYSTEM:The design method only applies to sites where the ground is seasonally frozen. It cannot be used in areas where theground is permanently frozen (permafrost). ASCE 32 further limits it to localities with mean outdoor air temperaturesgreater than 32oF and to areas with air freezing indexes less than 4,500Fo-days. By way of example, Elmendorf Air ForceBase in Anchorage, Alaska has a mean annual temperature of 35oF and an air freezing index of 3,430Fo-day. Even theclimate of southern Alaska is in the range where this method of design can be utilized.The significant cost savings associated with limiting excavation will not generally be seen in southern states asconventional foundations there can already be shallow. Southern states also can have a high probability of termiteinfestation. Below grade foam is not recommended in these areas unless special provisions have been made to protectthe foam.The polystyrene foam must comply with ASTM C578, Standard Specification for Rigid, Cellular Polystyrene ThermalInsulation. The polystyrene can be either expanded (EPS) or extruded (XPS). EPS type IV and XPS types IV to VII allhave published allowable bearing capacities. EPS type II and XPS type X have no allowable bearing capacity andtherefore are only suitable for vertical applications. In general XPS has a higher strength and a higher R value. ASTMC578 does not address or test for possible degradation of the insulating properties for below grade use, so the Nominal Rvalues are reduced approximately 10% for insulation placed vertically and 20% for insulation placed horizontally. Thesereduced values to be used for design are shown in columns 5v and 5h “Max. Effective R-value” in table 2 of the HUDbuilders guide.The insulation must be protected. Above grade insulation must have a covering to protect against both mechanical andultraviolet damage, and the protection must extend for at least 6” below grade. Horizontal insulation must be firmlybedded on a smooth ground surface. The top of the insulation should be protected by a hard layer, and the HUD buildersguide requires it for insulation that extends 24 inches or more from the foundation. The HUD Builders guide, appendix Ihas more information on sources of protective coatings.ASCE requires an approved foundation drainage system or a layer of screened and washed gravel or crushed stone thatis daylighted between the subgrade and insulation for subgrade soils other than GW, GP, SW, SP, GM and SM. As isgood practice for a conventional foundation, the grade around the building should have a positive slope away from thebuilding to drain away surface water. Andrew J. Bibb, P.E.Page 4 of 16

www.PDHcenter.comPDH Course S231www.PDHonline.orgOne of the most important things to watch out for in detailing are cold bridges. A cold bridge occurs when a material witha very low thermal resistance (R value) is directly exposed to the outside air. These are very easy to detail into a building,particularly if it has concrete block walls above the base slab or brick veneer. In conventional construction Architects andEngineers generally try to conserve interior heat. This is certainly a concern with a shallow frost protected foundation, butwith this type of foundation we are primarily trying to conserve the heat stored in the ground below the building. A coldbridge can completely negate the beneficial effects of the insulation placed around the foundation, resulting in anincreased potential for frost heave. ASCE 32 forbids a cold bridge unless it is accounted for in the design. The diagramsbelow show some typical cold bridges, and ways to remedy them. Both the HUD Builders guide (figure 9) and ASCE 32(figure 11) have examples of others.For heated structures, assuming cold bridges have been eliminated from the details, vertical insulation is generallyrequired along the foundation and horizontal insulation is often required. In the simplified method the HUD builders guiderequires vertical insulation wherever a shallow frost protected foundation is designed. ASCE 32 allows no insulationwhere the air freezing index is 500Fo-days or less. This is basically south of a line from Wilmington, DE, along theKentucky-Tennessee border, along to the Kansas-Oklahoma border, through the upper third of Arizona and New Mexico,and then north west on the Nevada side of the California border through the western ¼ of Oregon and Washington. Andrew J. Bibb, P.E.Page 5 of 16

www.PDHcenter.comPDH Course S231www.PDHonline.orgHorizontal wing insulation is generally not required where the air freezing index is 2,000 Fo-days or less. This contourruns through the southern 1/3 of Maine, about midway through New Hampshire and Vermont, through the middle of NewYork and Michigan. It drops a little further south past the Great Lakes to follow the Wisconsin-Illinois border, throughcentral Iowa and along the South Dakota, Nebraska border to where it turns a little north going into Montana. It then turnssouth for the Rocky Mountains. The Air-Freezing map shows all this in detail, the description is merely to show that thereare large areas of the country that many would consider “cold” where horizontal wing insulation is not required using thismethod.Where horizontal wing insulation is required there is always a wider zone or increased footing depth used at the buildingcorners. The heat loss from a building is intensified at a corner because of the heat loss from the two