LESSON 6: EXCAVATION FOR FOUNDATION, TYPES OF FOUNDATION AND TERMITE TREATMENT

Excavation is the equipment-heavy task of removing the earth from the allotted area in order to make room for the foundation.

Foundation is the lowest support of a structure. Foundations are classified as shallow and deep foundations.

Types of Shallow Foundations

1. Individual Footing or Isolated Footing

Individual footing or an isolated footing is the most common type of foundation used for building construction. This foundation is constructed for a single column and also called a pad foundation.

2. Combined Footing

Combined footing is constructed when two or more columns are close enough and their isolated footings overlap each other. It is a combination of isolated footings, but their structural design differs.

3. Spread footings or Strip footings and Wall footings

Spread footings are those whose base is wider than typical load-bearing wall foundations. The wider base of this footing type spreads the weight from the building structure over more area and provides better stability.


Spread footings

Spread footings and wall footings are used for individual columns, walls and bridge piers where the bearing soil layer is within 3m (10 feet) from the ground surface. Soil bearing capacity must be sufficient to support the weight of the structure over the base area of the structure.

These should not be used on soils where there is any possibility of a ground flow of water above bearing layer of soil which may result in scour or liquefaction.

4. Raft or Mat Foundations

Raft or mat foundations are the types of foundation which are spread across the entire area of the building to support heavy structural loads from columns and walls.


Raft or Mat Foundation

The use of mat foundation is for columns and walls foundations where the loads from the structure on columns and walls are very high. This is used to prevent differential settlement of individual footings, thus designed as a single mat (or combined footing) of all the load-bearing elements of the structure.

It is suitable for expansive soils whose bearing capacity is less for the suitability of spread footings and wall footings. Raft foundation is economical when one-half area of the structure is covered with individual footings and wall footings are provided.

These foundations should not be used where the groundwater table is above the bearing surface of the soil. The use of foundation in such conditions may lead to scour and liquefaction.

Types of Deep Foundation

5. Pile Foundations

Pile foundation is a type of deep foundation which is used to transfer heavy loads from the structure to a hard rock strata much deep below the ground level.


Pile Foundation

Pile foundations are used to transfer heavy loads of structures through columns to hard soil strata which is much below ground level where shallow foundations such as spread footings and mat footings cannot be used. This is also used to prevent uplift of the structure due to lateral loads such as earthquake and wind forces.

Pile foundations are generally used for soils where soil conditions near the ground surface is not suitable for heavy loads. The depth of hard rock strata may be 5m to 50m (15 feet to 150 feet) deep from the ground surface.

Pile foundation resists the loads from the structure by skin friction and by end bearing. The use of pile foundations also prevents differential settlement of foundations.

6. Drilled Shafts or Caisson Foundation

Drilled shafts, also called caissons, is a type of deep foundation and has an action similar to pile foundations discussed above, but are high capacity cast-in-situ foundations. It resists loads from structure through shaft resistance, toe resistance and/or combination of both of these. The construction of drilled shafts or caissons is done using an auger.

Fig: Drilled Shafts or Caisson Foundation (Source: Hayward Baker)

Drilled shafts can transfer column loads larger than pile foundations. It is used where the depth of hard strata below ground level is located within 10m to 100m (25 feet to 300 feet).

Drilled shafts or caisson foundation is not suitable when deep deposits of soft clays and loose, water-bearing granular soils exist. It is also not suitable for soils where caving formations are difficult to stabilize, soils made up of boulders, artesian aquifer exists.


Pre-construction damage (Termites):

  • Termites damage household articles like furniture, plywood, clothing, stationery, etc. by feeding on wood and other organic material having cellulose base
  • Subterranean termites make nest in soil and workers search for wood by burrowing through the ground and making tunnels of mud
  • They travel through masonry work, brick walls and conduits of the building to the source of food and cause damage to the property and belongings

Facts:

  • Subterranean termite colony may survive for many years and a single colony may develop population containing millions of individuals, thus control is very important as the damage caused by them is huge
  • Around 2761 species of termites in the world of which 350 species are found infesting wood in India

Common termite species:

Subterranean termites

  • Odontotermes spp (75-80%)
  • Coptotermes spp.
  • Heterotermes spp.
  • Reticulitermes spp.
  • Zootermopsis spp.
  • Drywood termites
  • Cryptotermes spp.
  • Incisitermes spp.
  • Marginitermes spp.

 

What is the solution to the problem?

  • Treating the soil beneath the building and around foundations when it is still in the process of construction is a good way to prevent termite infestation.
  • Pre-construction anti-termite treatment is attracting attention throughout the world.
  • The soil in immediate contact with the foundation and floor structure of a building is treated with an insecticide which kills or repels termites thus forming a toxic barrier which is lethal to termite entry.
  • Pre-construction anti-termite treatment is done in the early stages of construction so as to prevent termite infestation in the constructed building.

 

Summary:

What are broad classifications of foundation?

Building foundations are broadly classified as shallow and deep foundations.

What are Types of Shallow Foundation?

Types of shallow foundations are individual footing or isolated footing, combined footing, strip foundation, raft or mat foundation.

What are Types of Deep Foundation?

Types of deep foundations are pile foundation and drilled shafts or caissons.

What are difference between pile foundation and drilled shafts?

Drilled shafts has an action similar to pile foundations but are high capacity cast-in-situ foundations. It can transfer column loads larger than pile foundations. It is used where the depth of hard strata below ground level is located within 10m to 100m (25 feet to 300 feet).

What are difference between isolated and combined footing?

Combined footing is constructed when two or more columns are close enough and their isolated footings overlap each other. It is a combination of isolated footings, but their structural design differs.

When is Raft or Mat Foundations used?

The use of raft or mat foundation is for columns and walls foundations where the loads from the structure on columns and walls are very high. Rafts are used to prevent differential settlement of individual footings, thus designed as combined footing of all the load-bearing elements of the structure.

ACTIVITY 6

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