civil engineering

Foundation Systems

The GeoTechnical Engineer in action...

The foundation system for any structure  is the critical link in the transmission of its loading down to the ground (on surface or beneath surface). With the load bearing directly on the soil, the foundation system must:

• Distribute vertical loads so the settlement of a building is either negligible or uniform under all parts of the building,
• Relatively high stresses in the superstructure have to be safely transferred to the much softer and weaker soil,
• Anchor the building’s superstructure to prevent uplifting due to wind and earthquake forces,
• The best solution would be to place the supports of the structure on solid rock, but this is seldom possible,
• In most cases solid rocks or bed rock lies deep inside the ground, with softer and weaker soil layers above it.

The most critical factor in determining the foundation system of a building is the classification and bearing capacity of the soil.

Loading and Settlements of Foundations:

• Types of loads: Dead, live, inclination thrusts and uplift, water table, wind and earthquake forces.
• Types of settlements: Uniform and differential (Differential settlement must be minimized, depends on the soil conditions at site and distribution of loads on columns which supporting the structure).
• Requirements of safety: Structure-foundation system safe against settlements that would lead to collapse (Foundation settlement should not damage the structure and must be technically and economically feasible).

Types Of Foundation
1) Shallow Foundation System

Shallow Foundation, Spread Footing in construction...

Shallow foundations system normally located just below the lowest part of the structure, means that it is placed relatively close to the surface of the ground. The loads were transferred from the building to the soil by providing large enough area of the foundation in order to reduce pressure below the ones allowed by the strength of the soil. This will prevent an excessive settlement and bearing failure of the structure.

Types: Spread Foundation, and Mat / Raft Foundation

2) Deep Foundation System

Deep Foundation, Caisson Pile in construction...

In the case of deep foundations, the means of support is normally drilled shaft, group of piles or a pier. Major buildings in areas underlain with thick cohesive soil deposits to carry the loads vertically to more competent strata or bedrock primarily to control settlement or gradually transmit the load to the soil by friction and at a greater depth below the structure.

Types: Pile, Pile walls, Diaphragham wall, and Caissons pile

Advantages of Deep foundation:

• Cost (at affordable price)
• Construction Procedure (simple to follow)
• Material (mostly using concrete)
• Labor (doesn’t need any expertise)

Designing A Foundation:

• Information on the Working Loads – get from Structural Engineers or Architects
• Information on Sub-surface conditions – get from Site Investigation report
• Established Design Criteria
• Foundations must be designed to satisfy 3 general criteria:

It must be located properly so as not to be adversely affected by outside influence,

It must be safe from bearing capacity failure,

It must be safe from excessive settlement.

Factors Affecting Foundation Choice
Primary Factors Affecting Foundation Choice:

• Sub-surface soil
• Ground water table conditions
• Building structural requirements

Secondary Factors Affecting Foundation Choice:

• Construction access, methods and site conditions
• Environmental factors
• Building Codes and Regulations
• Impact on surrounding structures
• Construction schedule
• Construction risks

Depth and Location of Foundations
The depth and the location of foundations are dependent on as follows:

Significant Soil Volume Changes:

• Some soils shrink and swell significantly upon drying and wetting respectively,
• The specific depth & volume change relationship for a particular soil is dependent on the type of soil and level of groundwater,
• Volume change is usually insignificant below a depth of 1.5 to 3.0 m and does not occur below the Ground Water Table (GWT),
• In general, soil beneath the center of a structure is more protected from sun and precipitation, hence moisture content changes and resulting soil movement are relatively smaller.

Adjacent Structures and Property Lines:

• Existing structures may be damaged by construction of new foundations nearby,
• After new foundations have been constructed, the load that they place on the soil may cause settlement of existing structures,
• Damage to existing structures by new construction may result in liability problems, thus new structures should be located and designed very carefully,
• In general, the deeper of the new foundation and the closer to the older structure, the greater will be the potential for damage and movement,
• A general rule is that a straight line drawn downward and outward at a 45^o angle from the end of bottom of any higher footing should not intersect any existing footing,
• As a footing is wider than the building it supports; therefore, part of the footing may extend across the property line and may encroach on adjacent land.

Groundwater:

• Presence of groundwater near a footing is undesirable because:
• Footing construction below GWT is difficult and expensive,
• Groundwater around a footing can reduce the strength of soils,
• It may also cause hydrostatic uplift,
• Frost action may increase,
• Waterproofing problems.

Underground Defects:

• This includes faults, caves and mines,
• Human-made discontinuities such as sewer lines, underground cables and utilities must be considered,
• Structures should never be built on or near tectonic faults (plate movements) that may slip,
• Survey upon the underground utility lines should be made before any excavation in order to avoid damages to utilities during excavation.

Type of Soils and Characteristics:

• Soils (particulate earth material): Boulder (too large to be lifted by hands), cobble (particle that can be lifted by a single hand), gravel aggregates (course grained particle larger than 6.4mm), sand (frictional, size varies from 6.4 to 0.06mm), silts (frictional, low surface-area to volume ratio, size varies from 0.06 mm to 0.002mm) and clays (cohesive – fine grained – high surface-area to volume ratio, size smaller than 0.002 mm)
• Rocks: Broken into regular and irregular sizes by joints
• Peat: Soils not suitable for foundations of any structure

Problems due to Settlement can Arise when:

• Soil property changes at different points under the same structure,
• When construction of the building proceeds fast (mostly in modern times cases),
• When an additional heavy load (e.g. a tower in old times – Pisa) is added to stabilize it,
• Ground water is pumped out; Notorious instances examples like Venice and Mexico-city tragedy.

The Conclusions
The decision of choosing the foundation type is selected in consultation with the geotechnical engineer. The factors to be considered are the soil strength, the soil type, the variability of the soil over the area and with increasing depth, and the susceptibility of the soil and the building to deflection. I personally consider foundation as the most critical part of any structure that requires very careful structuring before the next stage could be done…