Introduction

Foundation Engineering:

The art of selecting, designing, and constructing the elements that transfer the weight (Weight may also include horizontal loads in addition to vertical loads) of a structure to the underlying soil or rock.

·The term "foundation engineering" is used to include the design of foundations for buildings and other structures and also for such non-foundation problems as designs of retaining walls, bulkheads, cofferdams, tunnels, and earth dams, as well as the design of natural slopes, dewatering of soils, and stabilization of soils mechanically and chemically.

·A foundation is interfacing element between the superstructure and the underlying soil or rock. The loads transmitted by the foundation to the underling soil must not cause soil shear failure or damaging settlement of the superstructure.

Classification of Foundations

        Shallow Foundation                                        Deep Foundation

n The first consideration in the foundation design should be the subsurface investigation.

n  The data from such investigations should be evaluated to determine whether or not the use of a deep foundation is necessary.

n  If such studies, together with studies of the soil properties, reveal that detrimental settlement can be avoided by more economical methods, then deep foundations should not be used.

FOUNDATION  DESIGN APPROACH :

1-Determine the foundation loads to be supported.

2-Evaluate the subsurface exploration and elaborator testing     programs.

3-Prepare a final soil profile. Determine soil layers suitable or unsuitable for shallow foundations or deep foundations. Also consider if ground improvement techniques could modify unsuitable layers into suitable support layers.

4-Consider and prepare alternative designs.

    Shallow foundations or Deep foundations.

5-Prepar cost estimates for feasible alternative foundation designs including all associated substructure cost.

6-Select the optimum foundation alternative.

Q. Why worry about  FOUNDATONS?

A. They hold everything up !!

Q. What factors does the Geotechnical Engineer need to consider in selecting a proper foundation?

A.  1. Type of Structure    2. Loads (how heavy)

       3. Type of Ground (“good” soil vs. “bad” soil)

       4. Schedule                 5. Cost

       6. Accessibility

The foundation takes the load from a structure and transmits it to safely to the ground

Q. What kinds of loads do we need to worry about ?

A.  1. Compression (downward)

      2. Tension (upward)

      3. Lateral (sideways)

      4. Torsion (twisting)

      5. Combinations of the above

Deriving support at or near the ground surface and are usually used when the load is small

§       Shallow Foundation Types

1.     Pad or column footings (Isolated or Combined)

2.     Strip footings

3.     Trench footings

4.     Slab on grade with thickened edge

5.     Interior footings for bearing walls

6.     Mat (Raft) footings (Thickened slabs)

Common Types of Shallow Foundations

• Strip Footings

• Isolated Pad Footings

• Mat

Combined Footings

§       Mat (or Raft) Foundation

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.   

§       Mat (or Raft) Foundation

§       Mat (or Raft) Foundation

§       Mat (or Raft) Foundation

·        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 can be used to transfer the loading to a deeper, more competent strata at depth if unsuitable soils are present near the surface.

·        Deep foundations are used when there are weak (“bad”) soils near the surface or when loads are very high, such as very large skyscrapers.

·        Deep foundations derive their support from deeper soils or bedrock

Common Types of Deep Foundations are :

1.      Pile foundations

2.      Piers

3.      Caissons

4.      Compensated foundations

·  Pile foundations are the part of a structure used to carry and transfer the load of the structure to the bearing ground located at some depth below ground surface. The main components of the foundation are the pile cap and the piles.

·  Pile foundations are frequently needed because of the relative inability of shallow footings to resist inclined, lateral, or uplift loads and overturning moments.

·  Pile foundations are used in areas of expansive or collapsible soils to resist undesirable seasonal movements of the foundations.

·  Pile caps are thick slabs used to tie a group of piles together to support and transmit column loads to the piles.   

Pile Caps

       Piles are long and slender members which transfer the load to deeper soil or rock of high bearing capacity avoiding shallow soil of low bearing capacity

Piles 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. They are used when for economic, constructional or soil condition considerations it is desirable to transmit loads to strata beyond the practical reach of shallow foundations. In addition to supporting structures, piles are also used to anchor structures against uplift forces and to assist structures in resisting lateral and overturning forces.

Piles

Piers are foundations for carrying a heavy structural load which is constructed insitu in a deep excavation.

A pier is a drilled shaft varying in diameter and depth. After the pier hole is drilled, it is filled with concrete and Steel reinforcement is sometimes utilized for a portion of the length of the pier.

Piers

Caissons

•         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.                                                       

•         Caissons are usually used in construction of bridge piers and other structure where the foundation is under water.

Pneumatic Caissons

•         Compressed air is used to keep water out and allow installation and construction in dry conditions.

•         High air pressures have created dangerous air conditions for workers, who must use an air lock

Caissons

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.