MAT FOUNDATIONA mat foundation is primarily shallow foundation. In essence, it is an expanded continuous footing and is usually analyzed in the same way. Mat foundations are sometimes referred to as raft foundations. Mat foundations are selected when:1. The area covered by the individual footings exceeds 50% of the structural plan area This is usually the case for buildings higher than 10-stories, and/or on relatively weak soils where q< 3 ksf=150 kPa2. The building requires a deep basement, below the phreatic surface. For example, to build several levels of parking, for mechanical systems, access to subway stations, etc3. The Engineer wishes to minimize the differential settlement in variable (that is, heterogeneous) soils, or if pockets of extremely weak soils are known to be present4. The Engineer wishes to take full advantage of the soils increasing bearing capacity with depth by excavating basements, and thereby seek a fully or a partially compensated foundation.In most tall and large buildings, the mat thickness T varies with the load. Therefore, the Engineer may desire to separate the various sections of the structure. Mats have been used for centuries:

Assyrians joined ceramic blocks with asphalt.Chinese joined large stones with keys of molten lead. Romans joined stones with hydraulic cements. Today, we exclusively use reinforced concrete.

ADVANTAGES OF MAT FOUNDATION Raft or mat foundation is economic due to combination of foundation and floor slab. It requires little excavation. It can cope with mixed or poor ground condition. It reduces differential settlement.

DISADVANTAGES OF MAT FOUNDATION Mat foundation requires specific treatment for point loads. Edge erosion occurs if not treated properly.

SOILS THAT MAY NECESSITATE THE USE OF MAT FOUNDATION1. Compressible soils, occur in highly organic soils including some glacial deposits and certain flood plain areas. Problems involved are excessive settlements, low bearing capacity, and low shear strength.2. Collapsing soils, settlement in loose sands and silts primarily. May occur in sandy coastal plain area, sandy glacial deposits.3. Expansive soils, containing swelling clays, mainly Montmorillite, which increase in volume when absorbing water and shrink when loosing it. Climate is closely related to the severity of the problem. Foundation supports should be placed below the active soil zone.

TYPES OF MAT FOUNDATION Flat Plate Mat The mat is of uniform thickness. A flat plate mat is used for fairly small and uniform column spacing and relatively light loads. A flat plate type of mat is suitable when the soil is not too compressible.

Plate Thickened under Columns - For columns subjected to very heavy loads usually the flat plate is thickened under columns as shown in Fig 2 to guard against diagonal shear and negative moments.

Two-way Beam and Slab - When the column spacing is large and carries unequal loads it would be more economical if a two-way beam and slab raft as shown in Fig 3 is used. This type of mat is particularly suitable when underlying soil is too compressible.

Plates with Pedestals The function of this mat is same as that of flat plate thickened under columns. In this mat pedestals are provided at the base of the columns.

Rigid Frame Mat This type of mat is used when columns carry extremely heavy loads. When the depth of beam exceeds 90 cm in simple beam and slab mat, a rigid frame mat is referred. Fig 4 shows a typical rigid frame mat.

Piled Raft In this type of construction the mat is supported on piles as shown in fig 5. This type of mat is used where the soil is highly compressible and the water table is high. This type reduces settlement and control buoyancy.

Caisson, Pier, Drilled shaft, and Drilled pierare -often used interchangeably in foundation engineering; all refer to a cast-in-place pile generally having a diameter ofabout 750 mmor more, with or without steel reinforcement and with or without an enlarged bottom. Sometimes the diameter can be as small as 305 mm.

Advantages:1. A single drilled shaft may be used instead of a group of piles.2. Constructing drilled shafts in deposits of dense sand and gravel is easier than driving piles.3. Drilled shafts may be constructed before grading operations are completed.4. When piles are driven by a hammer, the ground vibration may cause damage to nearby structures. The use of drilled shafts avoids this problem.5. Piles driven into clay soils may produce ground heaving and cause previously driven piles to move laterally.6. There is no hammer noise during the construction of drilled shafts; there is during pile driving.7. Because the base of a drilled shaft can be enlarged, it provides great resistance to the uplifting load.8. The surface over which the base of the drilled shaft is constructed can be visually inspected.9. The construction of drilled shafts generally utilizes mobile equipment, which, under proper soil conditions, may prove to be more economical than methods of constructing pile foundations.10. Drilled shafts have high resistance to lateral loads.

Disadvantages: 1. The concreting operation may be delayed by bad weather and always needs close supervision. 2. As in the case of braced cuts, deep excavations for drilled shafts may induce substantial ground loss and damage to nearby structures.

Types of Drilled shaft1. Straight Shaft - It extends through the upper layer(s) of poor soil, and its tip rests on a strong load-bearing soil layer or rock. The shaft can be cased with steel shell or pipe when required (as it is with cased, cast-in-place concrete piles). For such shafts, the resistance to the applied load may develop from end bearing and also from side friction at the shaft perimeter and soil interface.

2. Belled Shaft - A belled shaft (see Figures b and c) consists of a straight shaft with a bell at the bottom, which rests on good bearing soil. The bell can be constructed in the shape of a dome (see Figure b), or it can be angled (see Figure c). For angled bells, the underreaming tools that are commercially available can make to angles with the vertical.

Construction ProceduresThere are three major types of construction methods: the dry method, the casing method, and the wet method.1. Dry Method of ConstructionThis method is employed in soils and rocks that are above the water table and that will not cave in when the hole is drilled to its full depth. Step 1.The excavation is completed (and belled if desired), using proper drilling tools, and the spoils from the hole are deposited nearby.Step 2.Concrete is then poured into the cylindrical hole. Step 3. If desired, a rebar cage is placed in the upper portion of the shaft. Step 4.Concreting is then completed, and the drilled shaft will be as shown in Figure 12.2

2. Casing Method of Construction - This method is used in soils or rocks in which caving or excessive deformation is likely to occur when the borehole is excavated. Step 1. The excavation procedure is initiated as in the case of the dry method of construction. Step 2.When the caving soil is encountered, bentonite slurry is introduced into the borehole. Drilling is continued until the excavation goes past the caving soil and a layer of impermeable soil or rock is encountered.Step3. A casing is then introduced into the hole. Step 4.The slurry is bailed out of the casing with a submersible pumpStep 5. A smaller drill that can pass through the casing is introduced into the hole, and excavation continues. Step 6.If needed, the base of the excavated hole can then be enlarged, using an underreamer. Step 7. If reinforcing steel is needed, the rebar cage needs to extend the full length of the excavation. Concrete is then poured into the excavation and the casing is gradually pulled out. Step 8. Figure 12.3h shows the completed drilled shaft

3. Wet Method of Construction-This method is sometimes referred to as the slurry displacement method. Slurry is used to keep the borehole open during the entire depth of excavation. (See Figure 12.4.) Following are the steps involved in the wet method of construction:Step 1. Excavation continues to full depth with slurry. Step 2.If reinforcement is required, the rebar cage is placed in the slurry.Step 3.Concrete that will displace the volume of slurry is then placed in the drill hole.Step 4. Figure 12.4d shows the completed drilled shaft. Figure 12.5 shows a drilled shaft under construction using the dry

Equipment1. Rotary Table-the rotary table is a machine that transmits energy from a power unit to rotate Kelly bar.

2. Kelly Bar-usually attached to a crane, or built into a truck or modified excavator.

MaterialsRebar cage- is constructed of longitudinal bars bent into rungs or continuous spirals.Slurry- is sometimes used during the drilling. The purpose of this fluid is to support the walls of the excavation. Grout - is used for both core holes and CSL pipes.

Shallow Foundations - are those that transmit structural loads to the near-surface of soils. These include spread footing foundations and mat foundations.

SPREAD FOOTINGSA spread footing (also known as a footer or simply a footing) is an enlargement at the bottom of a column or bearing wall that spreads the applied structural loads over a sufficiently large soil area. Spread footings are by far the most common type of foundation, primarily because of their low cost and ease of construction. They are most often used in small to medium-size structures on sites with moderate to good soil conditions, and even on some large structures located at sites underlain by exceptionally-good soil or shallow bedrock.

TYPES OF SPREAD FOOTING:a) Square Spread Footings (or simply Square Footing) have plan dimensions of BxB. The depth from the ground surface to the bottom of the footing is D and the thickness is T. Square footings usually support a single, centrally-loaded column.

b) Rectangular Spread Footings have plan dimensions BxL, where L is the longer dimension. These are useful when obstructions prevent construction of a square footing with sufficiently large base area and when large moment loads are present.

c) Circular Spread Footings are round in plain view. These are most frequently used as foundation for light standards, flagpoles, and power transmission lines.

d) Continuous Spread Footings (also known as Wall Footings or Strip Footings) are used to support bearing walls.

e) Combined Footings are those that support more than one column. These are useful when columns are located too close together for each to have its own footing.

f) Ring Spread Footings are continuous footings that have been wrapped into a circle. This type of footing is commonly used to support the walls of above-ground circular storage tanks. In the preceding figure, the load carried by the column is eccentric, and as such, the footing may rotate and produce undesirable moments and displacements.One solution is to use a strap footing or cantilever footing, which consists of an eccentrically loaded footing under the exterior column connected to the first interior column using a grade beam.

MATERIALS:Before mid-nineteenth century, almost all spread footings were made of masonry as shown below. Dimension-stone footings were built of stones cut and dressed to specific sizes to fit together with minimal gaps, while rubble-stone footings were built from random size materials joined with mortar (Peck, et. al., 1974). The steel-grillage footings used in the ten-storey Montauk Block Building in Chicago in 1882, may have been the first spread footings designed to resist flexure, as they included several layers of railroad tracks, but was later modified into I-beams. They prevailed until the advent of reinforced concrete in the early twentieth century.CONSTRUCTION METHODS:Contractors usually use a backhoe to excavate spread footings. Once the excavation is open, it is important to check the exposed soils to verify that they are comparable to those used in the design. Inspectors often use a 9-mm (3/8 in) diameter steel probe. If the soil conditions are not anticipated, especially if they are too soft, it is necessary to revise the design accordingly.Pouring a neat footing involves pouring the concrete directly against the soil such as when the soil has sufficient strength to stand vertically until the pouring process.

Sometimes, shallow wooden forms are placed above the excavation, so the top of the footing is at the proper elevation. A formed footing is designed if the soil will not stand vertically, such as with clean sands or gravels, wherein it is necessary to make a larger excavation and build a full-depth wooden form.

Methods of placing concrete in footings:a) Neat excavationb) Neat excavation with wooden formsc) Formed footing with full- depth wooden forms

PILE FOUNDATION

-A pile is a slender structural member made of steel, concrete or timber. In pile foundation, a pile is either driven into the soil and or formed in-situ by excavating a hole and filling it with concrete.

-steel pile have been used since 1800 and concrete pole since 1900.

-Piles made from these materials are driven, drilled or jacked into the ground and connected to pile caps. Depending upon type of soil, pile material and load transmitting characteristic piles are classified accordingly. A pile cap is a thick concrete mat that rests on concrete or timber piles that have been driven into soft or unstable ground to provide a suitable stable foundation. It usually forms part of the foundation of a building, typically a multi-story building, structure or support base for heavy equipment. The cast concrete pile cap distributes the load of the building into the piles.

FUCNTION OF PILES

-To transmit the buildings loads to the foundations and the ground soil layers whether these loads vertical or inclined-To install loose cohesion less soil through displacement and vibration.-To control the settlements; which can be accompanied by surface foundations.-To increase the factor ofsafety for heavy loads buildings. A structure can be founded on piles if the soil immediately beneath itsbasedoesnothaveadequatebearingcapacity.-Piles are a convenient method of foundation for works overwater, such as jetties orbridge piers.-Piles can be used in normal ground conditions to resist horizontal loads.CLASSIFICATION OF PILES

1. End bearing piles2. Friction or cohesion piles3. Combination of friction piles andcohesion piles

END BEARING PILES

-These piles transfer their load on to a firm stratum located at a considerable depth below the base of the structure and they derive most oftheir carrying capacity from the penetration resistance of thesoil at the toeofthe pile.

Nagative skin friction -the soil may adhere to the surface of the pile.

Cohesion Piles -these piles transmit most of their load to the soil through skin friction.

Friction Piles(Floating pile foundation) -the process of driving such piles does not compact the soil appreciably.

FRICTION OR COHESION PILES

-Carrying capacity is derived mainly from the adhesion or friction of the soil in contact with the shaft of the pile.-These piles also transfer their load tothe ground through skin friction. Theprocess of driving such piles does not compact the soil appreciably. These types of pile foundations are commonly known asfloating pile foundations.-These piles transmit most of their load to the soil through skin friction. This process of driving such piles close to each other in groups greatly reduces the porosity and compressibility of the soil within and around the groups. Therefore piles of this category are sometimes called compactionpiles.

Combination of friction piles andcohesion piles

-An extension of the end bearing pile when the bearing stratum is not hard, such as firm clay. The pile is driven far enough into the lower material to develop adequate frictional resistance. -A farther variation of the endbearing pile is piles with enlarged bearing areas. This is achieved by forcing a bulb of concrete into the soft stratum immediately above the firm layer to give an enlarged base.-A similar effect is produced with bored piles by forming a large cone or bell atthe bottom with a special reaming tool.

Bored piles -which are provided with a bell have a high tensile strength and can be used as tension piles.

PILE TYPES

1. Timber- temporary piles2. Precast (typically prestressed)3. Steel (H-pile)- permanent and temporary works4. Composite Piles

Precast (typically prestressed)-may be defined as a reinforced concrete pile which is moulded in circular, square, rectangular or octagonal form.

Steel (H-pile) - are dimensionally square structural beams that are driven in the ground for deep foundation applications - are also used in conjunction with sheet piles to add lateral stiffness and bending resistance where loads exceed the capacity of sheet piles alone.

Composite piles -are made of two different materials. A composite pile may consist of the lower portion of cast-in-situ concrete. Composite piles are used rarely as it is difficult to provide proper joint between two or more materials.