What is a Mat Foundation?

Definition:

A mat foundation, according to the Dictionary of Construction, is a continuous foundation with a thick slab that supports a whole structure. The foundation is shallow and poured over reinforcing mats.

Mat foundations are sometimes referred to as raft foundations.

What is the difference between raft and mat foundation?

Mat foundation is otherwise called as Raft Foundation and its working principle is similar to Shallow foundation. Mat foundation is preferred for large space where separate footings are not possible to construct due to overlapping condition. Generally, Mat foundation's footing will extend over a large area. The main reason why the Mat adopted by most Engineers is Mat will distribute loads equally to prohibit differential settlement. Mat is considered when the soil condition is so poor & weak.

Mat foundations are selected when:

• 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 kPa;


• 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,


• 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 present


• The Engineer wishes to take full advantage of the soil’s increasing bearing capacity with depth by excavating basements, and thereby seek a fully or a partially compensated foundation.

Types of Mat Foundation:

• Flat plate mat


• Plate thickened under columns


• Two-way beam and slab


• Plate with pedestal


• Rigid frame mat


• Piled raft

Flat Plate Mat:

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. A line sketch of this type of mat is shown in Fig 1.

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. In such design, basement walls act as ribs or deep beam. 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.

Method of Construction:

The raft slab generally projects for a distance of 30 cm. to 45cm. on all the sides of the outer walls of the structure as such the area of excavation is slightly more than the area of the structure itself. The excavation is made to the required depth and the entire excavated area is well consolidated. This surface, when dry, provides the base upon which the raft or mat slab is laid. All the precautions that are necessary to be observed during the reinforced concrete construction are strictly adhered to and further construction is started only after the curing of the raft has been fully done.

Problem Soils That May Necessitate the Use of Mat Foundations:

• Compressible soils occur in highly organic soils including some glacial deposits and certain floodplain areas. Highly plastic clays in some glacial deposits and in coastal plains and offshore areas there can be significant amounts of compressible soils. Problems involved are excessive settlements, low bearing capacity, and low shear strength.


• Collapsing soils, settlement in loose sands and silts primarily. Densification occurs by the movement of grains to reduce the volume. Typically includes shallow subsidence. May occur in a sandy coastal plain area, sandy glacial deposits, and alluvial deposits of Intermountain regions of the western United States.


• Expansive soils, containing swelling clays, mainly Montmorillite, which increase in volume when absorbing water and shrink when losing it. Climate is closely related to the severity of the problem. Semi-arid and semi-humid areas with swelling clays are the most severe because the soil moisture active zone has the greatest thickness under such conditions. Foundation supports should be placed below the active soil zone. Expansive soils are most prevalent on the Atlantic and Gulf coastal plain and in some areas of the central and western United States.

Analysis of Rigid Mats:

The analysis of a mat by assuming that it is rigid simplifies the soil pressures to either a uniform condition or varying linearly. This is attained by not permitting R (the resultant force) to fall outside the kern of the mat. Hence, the corner stress is: s = R / BL (1 ± 6 ex / L ± 6 ey / B)

[caption id="attachment_4746" align="aligncenter" width="361"] 

Analysis and Design Procedure for Rigid Mats:

• Calculate total column load, Q = Q1 + Q2 + Q3


• Determine the pressure on the soil q, at the mat’s invert, 


• Compare the resulting soil pressures with the allowable soil pressure


• Divide the mat into several strips in the x and y directions


• Draw the shear V and the moment M diagrams for each strip in both directions


• Determine the effective depth d of the mat by checking for diagonal tension shear at the columns (ACI 318 11.12.2.1c);


• From the all the moment diagrams of all the strips in one direction, choose the maximum positive and negative moments per unit of width


• Determine the areas of steel per unit width for the positive and the negative reinforcement in both directions.

Analysis of Semi-Rigid Mats:

Mats are rarely completely rigid since the cost would be prohibitive. Some differential settlement must be admissible, without making the mat so flexible that shear reinforcing becomes necessary. There are various methods of analysis for a semi-rigid mat:

1. as a rigid mat


2. as a row of independent strips or beams


3. as a grid


4. using an elastic mat theory (for example, Hetenyi’s),


5. employing a finite element software

Method (a) as a Rigid Mat is admissible when:

1. Column loads differ by less than 20% from each other


2. Column spacing is very similar throughout


3. The building superstructure is very rigid


4. The load resultant R falls within the kern

Method (b) of Independent Strips or Beams:

The mat is represented as strips along column centerlines and each strip is analyzed as an independent elastic beam. Each column contributes an equal load to each contiguous strip, and thus pressures vertical displacement compatibly.

Analysis of Semi-Rigid Strips (Conventional Method):

The pressure under the mat is assumed to be linear,

Consider each strip as a combined footing. Adjust so that equilibrium is satisfied with V = 0

between strips). Analyze as a beam on an elastic foundation.

For cases A, B and C (shown below), the reaction suggestion by Seiffert may be used.

Raft Foundation- Reinforcement Details:

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