Shallow Foundation PDF

+ Shallow Foundation | 4 Types of Shallow Foundation Table of Contents Shallow Foundation 1. Types of shallow foundation o 1.1. Spread Footing ▪ a. Isolated Column Footings / Isolated Footings ▪ b. Combined Footing...

+ Shallow Foundation | 4 Types of Shallow Foundation Table of Contents Shallow Foundation 1. Types of shallow foundation o 1.1. Spread Footing ▪ a. Isolated Column Footings / Isolated Footings ▪ b. Combined Footings  ▪ c. Continuous Footings o 1.2. Strip Footing ▪ a. Inverted Arch Footing ▪ b. Eccentrically Loaded Footing o 1.3. Mat footing / Raft footing o 1.4. Grillage Footing Foundation is the substructure that transfers the load from the superstructure to the soil beneath. It is responsible for the stability of the whole structure. There are 2 types of foundation naming shallow foundation and deep foundation. In this article, we are discussing the shallow foundation. Note: SBC refers to the soil bearing capacity. Shallow Foundation Shallow Foundation is also called an open or wide foundation. Those, foundation which transfers the loads to subsoil at a point near to the ground floor of the building such as strips and rafts are called shallow foundations. For this type of foundation, the depth of the foundation is less than or equal to the width of the foundation. The features of a shallow foundation are as follows: a. Depth is less or sometimes equal to its width. b. It is placed immediately beneath the lowest part of the superstructures. c. It is spread more horizontal than vertical. d. It transfers the loads to subsoil at a shallow depth, close to the ground level. 1. Types of shallow foundation There are 4 types of shallow foundations. They are: 1. Spread footing 2. Strip footing 3. Mat foundation 4. Grillage foundation 1.1. Spread Footing This footing is also known as pad foundation. In this type of foundation, the base is made wider than the top to distribute the load from the superstructure over a large area. This type of foundation is suitable for walls and masonry columns. These foundations are constructed after opening the trenches to the required depth. It is economical for a maximum depth of 3 m. While constructing this type of footing trenches are opened to the required depth and the soil is rammed well. Then a plain concrete mix 1:4:8 is placed. Its thickness differs from 150 to 200 mm. The stone-masonry footing is constructed over this bed. It is constructed in courses and each course is projected 50 to 75 mm from the top course and the height of each course is 150 to 200 mm. In the case of wall footing, the projections are provided only in one direction while in the case of columns, they are provided in both directions. The projection of bed concrete from the lowest course of foundation masonry is usually 150 mm. Types of Spread Footing There are 3 types of spread footing. They are: a. Independent Footing / Isolated b. Combined Footing c. Continuous Footing a. Isolated Column Footings / Isolated Footings Separate footings are built for each column in isolated column footings. For distributing the load of the columns safely and uniformly over the soil, the size of the footing is kept according to the area required. Such type of footings is usually constructed over 100 or 150 mm concrete bed. Design engineers evaluate the required reinforcement and thickness of footing. Footing thickness may be uniform or sometimes varying. b. Combined Footings In this type of footings, two or more columns are supported by a single base. This type of footing is required when a column is extremely close to the boundary of the property and hence it is worthless to provide footing much beyond the column face. This footing may be rectangular or sometimes trapezoidal. The footing should be designed and constructed to transfer loads from both columns safely to the soil. A strap beam is provided for connecting two columns. c. Continuous Footings In continuous footings, a footing is common to more than two columns in a row. In case the columns in a row are nearer or if the SBC of soil is low, the continuous footing will be more applicable. 1.2. Strip Footing Strip footing is the independent footing of two columns connected by a beam. It is of four types. They are: a. Wall footing b. Inverted arch footing c. Eccentrically loaded footing d. Offset and strap (cantilever) footing a. Inverted Arch Footing This type of foundation is applicable for the areas where the SBC of the soil is incredibly poor and the load of the structure is through walls. Thus, inverted arches are built between the walls. End walls should be able to withstand the outward horizontal thrust due to arch action. So, it should be sufficiently thick and strong. The outer walls may be provided with buttress walls to strengthen them. b. Eccentrically Loaded Footing As far as practicable, the foundation should be so shaped and proportioned in such a way that center of gravity of the imposed loads coincides with the C.G of the supporting area of the base. But the footing which is so shaped that the center of gravity does not coincide with the C.G of the supporting area of the base is known as eccentrically loaded footing.  1.3. Mat footing / Raft footing Whenever the load on the column is extensive (multi-story column) or when the SBC of the soil is low, the foundations overlap each other. In such a situation, it is beneficial to provide common footing to several columns and this footing is called mat footing. Load distribution is uniform in this footing. It is also called raft footings. The raft foundation in which the beams are built in both directions over the footing slab for connecting columns may be called a grid foundation. The settlement is uniform in this type of footing and hence unnecessary stresses are not developed. Types of mat footing: a. Slab (solid)-up to 30 cm b. Slab and beam-slab > 30 cm c. Cellular-slab > 90 cm 1.4. Grillage Footing Most high-rise buildings are constructed with steel columns encased in concrete. Such types of columns carry a very heavy load and hence it requires special foundations for spreading the entire load to a larger area of soil. So Grillage foundation is one such special foundation that is used where the load of the structure is excessive and the bearing capacity of the soil is poor and a deep foundation is not possible. It has one tier or more tiers of I-section steel beams. The top tiers consist of fewer numbers but large steel sections while the lower tier consists of larger numbers but smaller size steel sections. Through the baseplate; column load is transferred to the top tier. The unpainted grillage beams are enclosed in concrete beyond the edge of steel sections with a minimum cover of 100 mm. A minimum clear space of 75 mm is required to be kept between the flanges of adjacent grillage beams which ensures proper concreting. Pipe separators are used to maintain spacing. Based on the material of the foundation, the grillage foundation is of two types. a. Timber grillage: It is mostly used for masonry wall foundations. It avoids differential settlement. b. Steel grillage: It is used to carry heavy loads from steel columns and distribute them into the soil having low bearing power. Types of Shallow Foundation: The various types of shallow foundations are as follows: 1. Wall Footings. 2. Combined footings. 3. Cantilever footings. 4. Mat or raft foundations. 5. Wall footing or Spread footing or strip footing. 6. Isolated footings. 7. Inverted arch footings. 8. Grillage footings or Grillage foundation. Foundations are classified as shallow and deep foundations. Types of foundations under shallow and deep foundations for building construction and their uses are discussed. It is advisable to know the suitability of each type of foundation before their selection in any construction project. Contents: [show] Types of Foundation and their Uses Following are different types of foundations used in construction: 1. Shallow foundation o Individual footing or isolated footing o Combined footing o Strip foundation o Raft or mat foundation 2. Deep Foundation o Pile foundation o Drilled Shafts or caissons 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. The shape of individual footing is square or rectangle and is used when loads from the structure is carried by the columns. Size is calculated based on the load on the column and the safe bearing capacity of soil. Rectangular isolated footing is selected when the foundation experiences moments due to the eccentricity of loads or due to horizontal forces. For example, Consider a column with a vertical load of 200 kN and a safe bearing capacity of 100 kN/m2 then the area of the footing required will be 200/100 = 2m2. So, for a square footing, the length and width of the footing will be 1.414 m x 1.414 m. 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. The shape of this footing is a rectangle and is used when loads from the structure is carried by the columns. 3. Spread footings or Strip footings and Wall footings Spread footings are those whose base is wider than a 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. Read More on Deep Foundations 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. Read More on Pile Foundation 6. Drilled Shafts or Caisson Foundation Drilled shafts, also called as 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 are 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. 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. Read More: Soil Investigation and Types of Foundations Based on Soil Properties