Soil condition and construction of pile foundation Q1

Types of piles
D06 2013 Q1
Types of building foundation
D06 2015 Q1
D06 2016 Q2 Types of foundation with soil

(a)
Same as D06 2016 Q2

First, pile foundation is a form of Deep Foundation. The other 2 types of deep foundations are Diaphragm Wall and Caisson & Well Foundation. See classification below:

[Soil condition at site will definitely affect foundation type. In the first place, deep foundation is used in condition that the soil support is not ideal - sand, clay or ground affected by water.

In engineering, this is under structural or geotechnical engineers. Below is a simple and concise verbatim from Level, an organization in New Zealand, the authority on sustainable building. More reading at the site analysis here.]

Soil types of insufficient bearing pressure
Soil types that have insufficient bearing pressure include peat, sand and expansive clay.

Peat
Peat occurs in low-lying areas and consists of compressed dead vegetation that has been preserved from decay by acidic groundwater. Although the surface of the ground can appear stable and dry, peat may be present below the surface in a deep layer that will compress under the weight of a building.

If the presence of peat is suspected, consult a soil engineer. The extent and depth of the peat will need to be determined by drilling bore holes. Where there is only a thin layer of peat, it may be able to be removed to expose firmer soil below. Alternatively, a specifically designed raft foundation and floor slab may be needed.

Sand
Sands vary in particle size and in compaction, and some types of sand have low bearing capacity. If the soil type is sand, piles driven down to a good bearing layer may be required in conjunction with a concrete slab.

Expansive clay
Expansive clay increases significantly in volume when wet and shrinks again when dry.

When expansive clay extends a significant depth below the surface and particularly if it occurs at a depth where the water level fluctuates, substantial uplift of the ground’s surface may occur during wet periods, followed by subsidence during a dry periods. The amount of uplift will vary according to the clay content of the soil but may be up to 50 mm.

Building on a clay soil will affect the ground moisture content and result in a different pattern of expansion and contraction. Moisture content will also be reduced by large paved areas, tree planting and subsoil drainage.

If expansive clay is present, consult a structural or geotechnical engineer

Water table
A high water table means the water pressure in the soil is high and that the soil is likely to be correspondingly weaker. High water pressure will also adversely affect the stability of sloping ground and increase the loading on a wall retaining the sloping ground.

If the building site is surrounded by areas of higher ground, underground water will tend to flow to the site. This may cause pressure beneath a concrete floor slab or increased moisture levels beneath a timber floor. It can also cause water to be driven into timber piles. In this situation, subsoil drainage may be necessary.

Indications of a high groundwater table include:

• reeds or other wet area vegetation
• surface water or boggy ground
• springs.

A high water table is likely to mean the construction will be more difficult, and it may be necessary to pump excavations and provide drainage to remove the water, which will generally result in additional costs.
Ref:
http://www.level.org.nz/site-analysis/site-conditions-and-ground-stability/

(b)
Pile Foundation Functions
Piles are used if the bearing capacity of soil at shallow depth is not enough to withstand the structural load. So the piles are driven to transfer the loads from the structure to the high bearing capacity soil/rock stratum.

The main functions of using pile foundations are listed below in short.

1. To transfer the load from the structure to the foundation at a greater depth where the bearing capacity of soil is efficient to withhold the structure loads.
2. Pile controls the soil settlement which can be accompanied by surface foundations.
3. Piles are used to increase the safety factor for high loaded structures.
4. Piles enable the scope of construction in very low bearing capacity soils.

The diagram below explains the construction of pile foundation. It is obtained from 'DESIGN AND CONSTRUCTION OF PILE FOUNDATION'. Further reading is recommended from this site.

(b) Small-displacement piles, which include rolled steel sections such as H-piles and open ended tubular piles. However, these piles will effectively become large displacement piles if a soil plug forms.

(c) Replacement piles, which are formed by machine boring, grabbing or hand-digging. The excavation may need to be supported by bentonite slurry, or lined with a casing that is either left in place or extracted during concreting for re-use.

(d) Special piles, which are particular pile types or variants of existing pile types introduced from time to time to improve efficiency or overcome problems related to special ground conditions.
Ref:
4 Functions Of Pile Foundation – Why Piles Are Used, available at