Preparation of Foundation

Site Investigation

A site investigation is required for any engineering or building structure. The investigation may range in scope from a simple examination of the surface soils with or without a few shallow trial pits to a detailed study of the soil and ground water conditions to a considerable depth below the surface.

Three Geotechnical Categories

1. Category 1 - light structures such as buildings with column loads up to 250 KN or walls loaded to 100 KN/m, low retaining walls, single and or two-storey houses.

2. Category 2 - structures for which quantitative geotechnical studies are required with the involvement of qualified engineers with relevant experience. Conventional substructures such as shallow spread footing, rafts and piles, retaining walls, bridge piers and abutments, excavations and excavation supports and embankments. A two-stage investigation is required for this category. The first stage includes boreholes, in-situ tests, and laboratory tests. The second stage is a detailed study of the ground-water conditions is required.

3. Category 3 - structures include buildings with exceptional loads, multi-storey  basement, dams, large bridges and tunnels, heavy machinery foundations and offshore platforms. Structures located on expansive or collapsing soils or in areas of exceptionally high seismic activity.

Subsurface Soil Exploration Methods

1. Trial Pits. Trial pits are generally used for category 1 investigations. They are useful for examining the quality of weathered rocks for shallow foundations.

2. Hand & Mechanical Auger Borings. Hand & mechanical auger borings are also used for category 1 investigations in soils which remain stable in an unlimited hole. When  carefully done augering causes the least disturbance of any boring method.

3. Light Cable Percussion Borings. Light cable percussion borings are used in British practice. It is well suited to the widely varying soil conditions in Britain, including the very stiff or dense stony glacial soils and weathered rocks of soil-like  consistency.

4. Rotary Open Hole Drilling. Rotary Open Hole drilling is generally used in the USA, Middle East and Far Eastern Countries. The rotary drills are usually tractor or skid-mounted and are capable of rock drillings as well as drilling in soils. Hole diameters are usually smaller than percussion drilled holes and sample sizes are usually limited to 50 mm diameter.

5. Wash Borings. Wash borings are small holes 65 mm in diameter drilled by water flush aided by chiselling.

6. Wash Probings. Wash probings are used in over-water soil investigations. They consist of a small-diameter pipe jetted down and are used to locate rock head or a strong layer overlain by loose or soft soils, i.e., investigations for dredging.

Types of Foundations

1. Pad Foundations. Pad foundations are used to support structural columns. They may consist of a simple circular, square, or rectangular slab of uniform thickness or they may be stepped or haunched to distribute the load from a heavy column. Pad foundations to heavily loaded structural steel columns are sometimes provided with a steel grillage.

Types of Pad Foundations

a. Mass concrete for steel column
b. Reinforced concrete with s/loping upper surface
c. Plain reinforced concrete.
d. Stepped reinforced concrete.

2. Strip Foundations. Strip Foundations are normally provided for load-bearing walls and for rows of columns which are spaced so closely that pad foundations would nearly touch each other.

3. Wide Strip Foundation. Wide strip foundations are necessary where the bearing capacity of the soils is slow enough to necessitate a strip so wide that transverse bending occurs in the projecting portions of the foundation beam and reinforcement is required to prevent cracking.

4. Raft Foundations. Raft foundations are required on soils of low bearing capacity, or where structural columns or other loaded areas are so closed in both directions that individual pads would nearly touch each other. raft foundations are useful in reducing differential settlement on variable soils or where there is a wide variation in loading between adjacent columns or other applied loads.

5. Bearing Piles. bearing piles  are required where the soil at normal foundation level cannot support ordinary pad, strip or raft foundations or where structures are cited for deep filling which is compressible and settling under its own weight. Piled foundations are a convenient method of supporting structures built over water or where uplift loads must be settled.

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