BUILDING CONSTRUCTION 1 ARCH 205 | a Foundation Is a Structure That Transfers Loads to the Ground
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BUILDING CONSTRUCTION 1 ARCH 205 | A foundation is a structure that transfers loads to the ground. Foundations are generally broken into two categories: 1. shallow foundations and 2. deep foundations. 2 3 Shallow foundations of a house versus the deep foundations of a Skyscraper 1- SHALLOW FOUNDATIONS Shallow foundations are usually dug a meter or so into suitable soils. One common type is the spread footing which consists of strips or pads of concrete (or other materials) which extend below the frost line and transfer the weight from walls and columns to the soil or bedrock. Another common type is the slab- on-grade foundation where the weight of the building is transferred to the soil through a concrete slab placed at the surface. 4 SHALLOW FOUNDATION 1. 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. 2. Shallow foundations include: 1. spread footing foundations, 2. mat-slab foundations, and 3. slab-on-grade foundations 5 1- SHALLOW FOUNDATIONS SPREAD FOOTING FOUNDATION | Spread footing foundations consists of strips or pads of concrete (or other materials) which transfer the loads from walls and columns to the soil or bedrock. Embedment of spread footings is controlled by several factors, including development of lateral capacity, penetration of soft near-surface layers, and penetration through near-surface layers likely to change volume due to frost heave or expansion and contraction. | These foundations are common in residential construction that includes a basement, and in many small commercial structures. 6 SHALLOW FOUNDATIONS SPREAD FOOTING FOUNDATION In ground reinforced concrete block perimeter footing 7 SPREAD FOOTING FOUNDATION IN-GROUND-FOUNDATION | 8 Wall-foundation SHALLOW FOUNDATIONS MAT-SLAB FOUNDATIONS | Mat-slab foundations are used to distribute heavy column and wall loads across the entire building area, to lower the contact pressure compared to conventional spread footings. Mat- slab foundations can be constructed near the ground surface, or at the bottom of basements. In high-rise buildings, mat-slab foundations can be several meters thick, with extensive reinforcing to ensure relatively uniform load transfer. 9 1- SHALLOW FOUNDATIONS SLAB-ON-GRADE FOUNDATION | Slab-on-grade foundations are a Structural Engineering practice whereby the concrete slab that is to serve as the foundation for the structure is formed from a mold set into the ground. The concrete is then placed into the mold, leaving no space between the ground and the structure. This type of construction is most often seen in warmer climates, where ground freezing and thawing is less of a concern and where there is no need for heat ducting underneath the floor. | The advantages of the slab technique are that it is relatively cheap and sturdy, and is considered less vulnerable to termite infestation because there are no hollow spaces or wood channels leading from the ground to the structure (assuming wood siding, etc., is not carried all the way to the ground on the outer walls). 10 1- SHALLOW FOUNDATIONS SLAB-ON-GRADE FOUNDATION | The disadvantages are: y a very low elevation that may expose the building to flood damage in even moderate rains. y Remodeling or extending such a structure may also be more difficult. y Over the long term, ground settling (or subsidence) may be a problem, as a slab foundation cannot be readily jacked up to compensate; | proper soil compaction prior to pour can minimize this. y the lack of access from below for utility lines, the potential for large heat losses where ground temperatures fall significantly below the interior temperature, and | The slab can be decoupled from ground temperatures by insulation, with the concrete poured directly over insulation (for example, styrofoam panels), or heating 11 provisions (such as hydronic) can be built into the slab (an expensive installation, with associated running expenses). 1- SHALLOW FOUNDATIONS SLAB-ON-GRADE FOUNDATION | Slab-on-grade foundations are commonly used in areas with expansive clay soil, particularly in California and Texas. While elevated structural slabs actually perform better on expansive clays, it is generally accepted by the engineering community that slab-on-grade foundations offer the greatest cost-to-performance ratio for tract and semi-custom homes. Elevated structural slabs are generally only found on large custom homes or homes with basements. 12 SHALLOW FOUNDATIONS SLAB-ON-GRADE FOUNDATION | Care must be taken with the provision of services through the slab. Copper piping, commonly used to carry water and galvanized steel piping used for natural gas, reacts with concrete over a long period, slowly degrading until the pipe fails. Copper pipes must be run well below the slab, run through a conduit, or piped into the building above the slab. Electrical conduits through the slab need to be water-tight, as they extend below ground level and can potentially expose the wiring to groundwater. 13 SHALLOW FOUNDATIONS SLAB-ON-GRADE FOUNDATION Slab on Grade house foundation 14 SHALLOW FOUNDATIONS SLAB-ON-GRADE FOUNDATION 15 Example of slab on grade foundation 16 2- DEEP FOUNDATIONS | Deep foundations are used to transfer a load from a structure through an upper weak layer of soil to a stronger deeper layer of soil or to carry loads to bedrock. | Different types of deep foundations include: y piles, y drilled shafts, y caissons, y piers, and y earth stabilized columns. y The naming conventions for different types of foundations vary between different engineers. Historically, the first piles used were wood, later steel, reinforced concrete, and pre-tensioned concrete were introduced. 17 2- DEEP FOUNDATIONS | Deep foundations are distinguished from shallow foundations by the depth they are embedded into the ground, by their shape, the level of design engineering required and their much higher bearing capacity. | There are many reasons a geotechnical engineer would recommend a deep foundation over a shallow foundation, but some of the common reasons are: y very large design loads, y a poor soil at shallow depth, y or site constraints (like property lines). | There are different terms used to describe different types of deep foundations including piles, drilled shafts, caissons, and piers. The naming conventions may vary between engineering disciplines and firms. Deep foundations can be made out of timber, steel, reinforced concrete and pre-stressed concrete. | Deep foundations can be installed by either driving them into the ground or drilling a shaft and filling it with concrete, mass or reinforced. 18 DEEP FOUNDATIONS DRIVEN FOUNDATIONS | Prefabricated piles are driven into the ground using a pile driver. Driven piles are either wood, concrete, or steel. Wooden piles are made from trunks of tall trees. Concrete piles are available in square, octagonal, and round cross-sections. They are reinforced with rebar and are often pre- stressed. Steel piles are either pipe piles or some sort of beam section (like an H-pile). Historically, wood piles were spliced together when the design length was too large for a single pile; today, splicing is only common with steel piles, though concrete piles can be spliced with difficulty. Driving piles, as opposed to drilling shafts, is advantageous because the soil displaced by driving the piles compresses the surrounding soil, causing greater friction against the sides of the piles, thus increasing their load-bearing capacity. 19 DEEP FOUNDATIONS DRIVEN FOUNDATIONS- PILE FOUNDATION SYSTEMS | Pile foundation systems: Foundations relying on driven piles often have groups of piles connected by a pile cap (a large concrete block into which the heads of the piles are embedded) to distribute loads which are larger than one pile can bear. Pile caps and isolated piles are typically connected with grade beams to tie the foundation elements together; lighter structural elements bear on the grade beams while heavier elements bear directly on the pile cap. 20 DEEP FOUNDATIONS DRIVEN FOUNDATIONS- PILE FOUNDATION SYSTEMS DEEP FOUNDATIONS DRIVEN FOUNDATIONS- PILE FOUNDATION SYSTEMS Pile driving operations 22 DEEP FOUNDATIONS DRIVEN FOUNDATIONS- PILE FOUNDATION SYSTEMS 23 Pipe piles being driven into the ground. DEEP FOUNDATIONS DRILLED PILES - PILE FOUNDATION SYSTEMS 24 DEEP FOUNDATIONS DRILLED PILES | Also called drilled piers or Cast-in-drilled-hole piles (CIDH piles) or Cast-in-Situ piles. Rotary boring techniques offer larger diameter piles than any other piling method and permit pile construction through particularly dense or hard strata. Construction methods depend on the geology of the site. In particular, whether boring is to be undertaken in 'dry' ground conditions or through water- logged but stable strata - i.e. 'wet boring'. | 'Dry' boring methods employ the use of a temporary casing to seal the pile bore through water-bearing or unstable strata overlying the suitable stable material. Upon reaching the design depth, a reinforcing cage is introduced, concrete is poured in the bore and brought up to the required level. The casing can be withdrawn or left in situ. 25 DEEP FOUNDATIONS DRILLED PILES 26 zA pile machine DEEP FOUNDATIONS DRILLED PILES | 'Wet' boring also employs a temporary casing through unstable ground and is used when the pile bore cannot be sealed against water ingress. Boring is then undertaken using a digging bucket to drill through the underlying soils to design depth. The reinforcing cage is lowered into the bore and concrete is placed by tremmie pipe, during which, extraction of the temporary casing takes place. 27 DEEP FOUNDATIONS DRILLED PILES | In some cases there may be a need to employ drilling fluids (such as bentonite suspension) in order to maintain a stable shaft. Rotary auger piles are available in diameters from 350 mm to 2400 mm or even larger and using these techniques, pile lengths of beyond 50 metres can be achieved.