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Engineering Geology: Footings and Foundation

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* Purpose: Transfer the building load to the soil. * The size of the foundation is determined by the weight of the building and the load bearing capabilities of the soil. * When the load bearing capabilities of the soil is low, a footing is usually used. Types of Foundation 1. Shallows Foundations * Shallow foundations are those founded near to the finished ground surface; generally where the founding depth (Df) is less than the width of the footing and less than 3m. Shallows foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils, such as poorly-compacted fill, peat, recent lacustrine and alluvial deposits, etc.

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2. Deep Foundations * Deep foundations are those founding too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished ground level. Deep foundations are used when there are weak (“bad”) soils near the surface or when loads are very high, such as very large skyscrapers.

Shallow Foundation Types 1. Pad or column footings (Isolated or Combined) Isolated – used to support single columns. This is one of the most economical types of footings and is used when columns are spaced at relatively long distances. Combined – usually supports two columns, or three columns not in a row.

Combined footings are used when two columns are so close that single footings cannot be used or when one column is located at or near a property. . Cantilever or strap footings-consist of two single footings connected with a beam or a strap and support two single columns. This type replaces a combined footing and is more economical. 3. Continuous footings – support a row of three or more columns. They have limited width and continue under all columns. 4. Wall Footings – are used to support structural walls that carry loads for other floors or to support non-structural walls. 5. Mat (Raft) footings (Thickened slabs) * consists of one footing usually placed under the entire building area.

They are used, when soil bearing capacity is low, column loads are heavy single footings cannot be used, piles are not used and differential settlement must be reduced. * Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. Deep Foundation Types: 1. Pile foundations – are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. 2.

Piers – are foundations for carrying a heavy structural load which is constructed in a deep excavation. 3. Caissons – are a form of deep foundation which are constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson. 4. Compensated foundations – are deep foundations in which the relief of stress due to excavation is approximately balanced by the applied stress due to the foundation. The net stress applied is therefore very small. A compensated foundation normally comprises a deep basement. Types of Loads: 1. Dead Load 2.

Live Load 3. Wind Load 4. Horizontal Pressures Below Grade 5. Forces (Uplift) 6. Earthquake Push Forces * The primary pushing force is frost heave. * Can be a serious problem * Some building codes allow footings above the frost line, but they must be insulated according to building code. Foundation Systems * Four main types of foundation systems. * Crawl space * Full basement * Slab at grade * Elevated slab * Foundation Requirements Three primary foundation requirements: * Safe against Structural Failure * Not Impairing Function of Building * Technically & Economically Feasible Footings The job of the footing is to transfer the weight of the building to the soil and provide a level platform for the foundation.

* Building load diminishes with depth. * An additional concern is the location of the foundation wall on the footing. Foundation Components How to Install Concrete Block Foundation Walls * Allow concrete footing to properly cure * Install first course of concrete block on bed of mortar * Continue to lay block on top of first course until desired height reached * Install horizontal reinforcement as courses proceed * Install vertical reinforcement at significant heights Install grout into cells with vertical reinforcement * Tool joints and clean masonry * Install waterproofing Piled Walls These methods of retaining wall construction employ bored piling techniques – normally CFA or rotary.

They provide special advantages where available working space dictates that basement excavation faces be vertical. Both methods offer technically effective and cost efficient temporary or permanent means of retaining the sides of bulk excavations even in water bearing strata. When used in permanent works, these walls can be designed to ccommodate vertical loads in addition to moments and horizontal forces. Construction of both methods is the same as for foundation bearing piles. Contiguous walls are constructed with small gaps between adjacent piles. The size of this space is determined by the nature of the soils. Secant piled walls Secant pile walls are constructed such that space is left between alternate ‘female’ piles for the subsequent construction of ‘male’ piles. Construction of ‘male’ piles involves boring through the concrete in the ‘female’ piles in order to key ‘male’ piles between them.

The male pile is the one where steel reinforcement cages are installed, though in some cases the female piles are also reinforced. Secant piled walls can either be true hard/hard, hard/intermediate (firm), or hard/soft, depending on design requirements. Hard refers to structural concrete and firm or soft is usually a weaker grout mix containing bentonite. All types of wall can be constructed as free standing cantilevers, or may be propped if space and sub-structure design permit. Where party wall agreements allow, ground anchors can be used as tie backs. Slurry walls

A slurry wall is a non-structural barrier built underground using a mix of bentonite and water to prevent the flow of groundwater. A trench that would collapse due to the hydraulic pressure in the surrounding soil does not collapse as the slurry balances the hydraulic pressure. Deep Mixing/Mass Stabilization Techniques Cement, lime/quick lime, flyash, sludge and/or other binders (sometimes called stabilizer) are mixed into the soil to increase bearing capacity. The result is not as solid as concrete, but should be seen as an improvement of the bearing capacity of the original soil.

The technique is most often applied on clays or organic soils like peat. The mixing can be carried out by pumping the binder into the soil whilst mixing it with a device normally mounted on an excavator or by excavating the masses, mixing them separately with the binders and refilling them in the desired area. The technique can also be used on lightly contaminated masses as a means of binding contaminants, as opposed to excavating them and transporting to landfill or processing. Classification of Pile With Respect To Type Of Material As the name implies, timber piles are made of wood.

Historically, timber has been a plentiful, locally-available resource in many areas. Today, timber piles are still more affordable than concrete or steel. Compared to other types of piles (steel or concrete), and depending on the source/type of timber, timber piles may not be suitable for heavier loads. A main consideration regarding timber piles is that they should be protected from rotting above groundwater level. Timber will last for a long time below the groundwater level. For timber to rot, two elements are needed: water and oxygen.

Below the groundwater level, oxygen is lacking even though there is ample water. Hence, timber tends to last for a long time below groundwater level. It has been reported that some timber piles used during 16th century in Venice still survive since they were below groundwater level. Timber that is to be used above the water table can be protected from decay and insects by numerous forms of wood preservation using pressure treatment (ACQ, CCA, creosote, etc. ). Steel Pipe piles are a type of steel driven pile foundation and are a good candidate for battered piles.

Pipe piles can be driven either open end or closed end. When driven open end, soil is allowed to enter the bottom of the pipe or tube. If an empty pipe is required, a jet of water or an auger can be used to remove the soil inside following driving. Closed end pipe piles are constructed by covering the bottom of the pile with a steel plate or cast steel shoe. In some cases, pipe piles are filled with concrete to provide additional moment capacity or corrosion resistance. In the United Kingdom, this is generally not done in order to reduce the cost.

In these cases corrosion protection is provided by allowing for a sacrificial thickness of steel or by adopting a higher grade of steel. If a concrete filled pipe pile is corroded, most of the load carrying capacity of the pile will remain intact due to the concrete, while it will be lost in an empty pipe pile. The structural capacity of pipe piles is primarily calculated based on steel strength and concrete strength (if filled). An allowance is made for corrosion depending on the site conditions and local building codes.

Steel pipe piles can either be new steel manufactured specifically for the piling industry or reclaimed steel tubular casing previously used for other purposes such as oil and gas exploration. Prestressed concrete piles Concrete piles are typically made with steel reinforcing and prestressing tendons to obtain the tensile strength required, to survive handling and driving, and to provide sufficient bending resistance. Long piles can be difficult to handle and transport. Pile joints can be used to join two or more short piles to form one long pile.

Pile joints can be used with both precast and prestressed concrete piles. Composite piles “Composite pile” is a pile made of steel and concrete members that are fastened together, end to end, to form a single pile. It is a combination of different materials or different shaped materials such as pipe and H-beams or steel and concrete ————————————————- Design Foundations are designed to have an adequate load capacity with limited settlement by a geotechnical engineer, and the footing itself may be designed structurally by a structural engineer.

The primary design concerns are settlement and bearing capacity. When considering settlement, total settlement and differential settlement is normally considered. Differential settlement is when one part of a foundation settles more than another part. This can cause problems to the structure the foundation is supporting. It is necessary that a foundation not be loaded beyond its bearing capacity or the foundation will “fail” Driven Pile Foundation Piles are used to transfer surface loads to a competent soil or rock at depth when the surface layer is not adequate or is not economically feasible to use.

This load transfer may be by vertical distribution of the load along the pile shaft (skin friction) or a direct application of load to a lower stratum through the pile point (end-bearing). Piles can also serve to carry lateral, uplift and overturning loads. Driven piles are typically delivered to construction sites prefabricated and consist of either reinforced, pre- or post-tensioned concrete, timber, open- or closed-ended steel pipe or H-beams. Diesel hammers are the most common driving mechanism used to drive piles although other driving methods are seldom used.

A primary drawback to driving piles is the large noise and vibration associated with this form of deep foundation construction. Driven piles are typically of small diameter and are often grouped into clusters or rows. The pile groups are then tied together using pile caps or grade beams that carry the column and wall loads. An indicator pile program can be conducted at the onset of construction using a pile driving analyzer, to help correlate driving energies and capacities, and optimize design SUBSURFACE CONDITIONS

Bedrock: The bedrock in the area consists of argillite from the Cambridge formation. The condition of the bedrock varies considerably with location, even within a given site. Evaluation of rock core samples indicates that the rock is typically in a soft and weathered condition and contains a significant amount of fracturing. However, hard and sound bedrock was found at some locations. Glacial Soils: The glacial soils were deposited during the last glaciation approximately 12,000 years ago. These deposits include glacial till, and glaciomarine, glaciolacustrine, and glaciofluvial soils.

Till is characterized by a mass of unsorted debris that contains angular particles composed of a wide variety of grain sizes, ranging from clay-sized particles to large boulders. Glaciomarine or glaciolacustrine deposits generally consist of clay, silt, and sand, whereas glaciofluvial deposits contain coarser grained sand and gravel. The glacial soils are typically dense in nature as indicated by high standard penetration test (SPT) resistance, and the piles were typically terminated in these deposits. Marine Soils: Marine soils were deposited over the glacial soils during glacial retreat in a quiescent deepwater environment.

The marine clay layer, as shown in figures 2 through 5, is the thickest unit in the profile, but was encountered only to a limited extent at the Charlestown site. The clay is generally overconsolidated in the upper portions of the layer and is characterized by relatively higher strengths. The overconsolidation is a result of past desiccation that occurred during a period of low sea level. By comparison, the deeper portions of the clay layer are much 6 softer and penetration of the SPT split spoon can sometimes occur with just the weight of the drilling rods alone.

Inorganic Soils: Inorganic silts and sands are typically encountered overlying the marine soils. These soils were deposited by alluvial processes. Organic Soils: The organic soils that are encountered below the fill generally consist of organic silt and may contain layers of peat or fine sand. These soils are the result of former tidal marshes that existed along the coastal areas. Fill Soils: Fill material was placed in the more recent past to raise the grade for urban development. The fill layer is highly variable in its thickness and composition, ranging from silts and clays to sands and gravels.

The consistency or density is also variable as indicated by the SPT blow counts. The variability in the fill is attributed to the characteristics of the particular borrow source material and the methods of placement. Drilled Piles (also known as drilled piers) are a kind of pile wall made using the stabilator system. Drilled piles are of high technical quality and are a durable, long lasting solution. They can be assembled quickly and effectively without the need for vibrating or pneumatic machinery. Different kinds of method : Rotary Bored Piling

Rotary boring is a method which can be used to create larger diameter piles than other piling methods. Rotary piling can also be used through tough strata. Depending on the natural features of a worksite different rotary boring techniques can be used. * Dry Boring Techniques are effective for use on ground with dry conditions. When using this method the pile is sealed in a temporary casing to protect the bore from any moisture or unstable layers of earth. When the bore reaches the desired depth a cage structure is introduced to stabilize the bore and concrete is poured.

The concrete helps the bore to rise to the correct level where it is stabilised. The casing can be left on or removed as desired. * Wet Boring Techniques are used on ground that is sodden but still stable. Wet boring methods also involve a temporary casing to protect the bore against moisture. The bore is then positioned using a digging bucket to reach through the layers of strata to the desired depth. Similar to dry boring techniques a cage is positioned around the bore and concrete is poured in. In this method removing the temporary casing is also optional.

Specialty piles Specialty piles are generally considered “problem solver” deep foundation systems. Many projects are faced with extensive construction constraints. In urban environments, vibration or noise may be an issue. Pile driving hours may be strictly limited. Construction is often planned adjacent to sensitive unreinforced masonry buildings, high tech equipment buildings, hospitals, or entertainment studios, where vibration is an issue. The site may be low overhead. The site may be found to be contaminated, making off haul and spoil disposal costly.

When you factor in the indirect cost of these outside factors, a specialty pile often proves to be the most economical solution on a total project cost basis. At Foundation we offer augercast piles and our FDP-EX series (Foundation Displacement Pile Series). FDP-EX Series consists of the following: 1. FUNDEX Pile (Cast in place Displacement Pile) 2. TUBEX GI (Grout Injection) Pile (A torqued in displacement pile with an outer grout/soil coverage) 3. TUBEX Pile (A torqued in displacement Pile with an oversized tip. ) 4. EDTTEX Pile (A torqued in displacement Pile without an oversized tip) TUBEX GI PILES (GROUT INJECTION)

The TUBEX GI piles(Grout Injecion) are steel and concrete pipe composite piles that are screwed into the ground under very high torque and down-pressure. As the piles are installed, a high pressure grouting system is used to pump grout through the pile tip to create a soil-cement grouted zone around the outside of the pipe. TUBEX GI piles are true soil displacement piles. Unlike conventional drilling methods TUBEX GI piles do not produce drill spoil, which creates off haul costs, and reduces skin friction capacity. Tubex are soil displacement, and the grouting actually increases the pile’s effective diameter.

TUBEX GI piles are ideal for ontaminated sites to avoid spoils, in low overhead conditions, in restricted site conditions, and on sites where noise and vibration aren’t tolerated. TUBEX PILES The TUBEX piles are steel and concrete pipe composite piles that are screwed into the ground under very high torque and down-pressure. TUBEX piles are true soil displacement piles. Unlike conventional drilling methods TUBEX piles do not produce drill spoil, which creates off haul costs, and reduces skin friction capacity. Tubex are soil displacement, and the over sized tips increases the pile’s surface are to allow for higher compression and tension capacities.

TUBEX piles are ideal for contaminated sites to avoid spoils, in low overhead conditions, in restricted site conditions, and on sites where noise and vibration are not tolerated. EDTTEX PILES The EDTTEX piles are steel and concrete pipe composite piles that are screwed into the ground under very high torque and down-pressure. EDTTEX piles are true soil displacement piles. Unlike conventional drilling methods EDTTEX piles do not produce drill spoil, which creates off haul costs, and reduces skin friction capacity. EDTTEX are soil displacement with the same size diameter tips as the outside diameter of pipe.

EDTTEX piles are ideal for contaminated sites to avoid spoils, in low overhead conditions, in restricted site conditions, and on sites where noise and vibration aren’t tolerated. FUNDEX PILES Sometimes referred to as Full Section Soil Displacement concrete piles, Fundex piles differ from augercast piles in that structural concrete and full length reinforcing of any configuration are used to construct the pile that is fully inspectable. A high torque Fundex rotary table forces a mandrel into the ground to the bearing layer.

A sacrificial boring tip fitted to the mandrel base prevents soil and water from entering the mandrel. Upon reaching the required depth, a reinforcing cage is placed up to full length of the pile, and structural concrete with aggregate sizes up to 1”—not grout– is placed in the pile. The mandrel is then extracted leaving the tip, cage and concrete in place. There is no possibility of soil intrusion into the structural concrete, and the cage is fully inspectable for placement and code-required concrete coverage prior to placing the concrete.

Installation produces no vibration, noise or spoils. AUGERCAST PILES An augercast pile is formed by drilling into the ground with a hollow stemmed continuous flight auger to the required depth or degree of resistance. A fine aggregate (fine to course sand) is then pumped into the hollow stemmed auger under pressure. Auger cast piles have specific advantages for some installations with difficult excavation requirements, in environmentally sensitive areas and the right soil conditions. These piles, however, are not suited for contaminated or rocky soils.

Cite this Engineering Geology: Footings and Foundation

Engineering Geology: Footings and Foundation. (2016, Oct 20). Retrieved from https://graduateway.com/engineering-geology-footings-and-foundation/

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