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.

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

SPREAD FOOTINGS A 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 formsa) Neat excavation with wooden formsb) Formed footing with full- depth wooden forms.

a) Neat excavation with wooden formsb) Formed footing with full- depth wooden forms.

c) Neat excavation with wooden formsd) Formed footing with full- depth wooden forms.Once the excavation has been made and cleaned, and the forms are in place, the contractor places the reinforcing steel bars. If the footing will support a wood or steel structure, threaded anchor bolts or steel brackets are embedded into the concrete. For concrete or masonry structures, dowels are placed such as that they extend above the completed footing, thus providing for a lap splice with the column or wall steel.