FOUNDATION

 

Foundation is the lowest part of a structure which provides a base for the super structure proper. This term includes the portion of the structure below the ground level as well as the artificial arrangement of concrete block, piles, raft, grillage, etc. provided to transmit the loads on the structure including the dead weight of the structure itself to soil below.

If a steel grinder provided in a structure is found to be weak and flexible, it can usually be strengthened by suitable means; if a column acting as a load bearing member in a building is found to be inadequate, it can be made safe by providing additional reinforcement or by introducing intermediate columns, but if the foundation of a building yields and ruptures the structure, then little can be done to improve the situation.

PURPOSE OF FOUNDATION

It is often misunderstood that the foundation is provided to support the load of the structure. In fact, it is a device to transmit the load of the structure to the soil below. Foundation is provided for the following four main purposes.

1.To distribute the weight of the structure over large area so as to avoid over loading of the soil beneath.

2. To load the sub stratum evenly and thus prevent unequal settlement.

3. To provide a level surface for building operations.

4. To take the structure deep into the ground and thus increase its stability, preventing overturning.

SITE EXPLORATION FOR FOUNDATIONS

The designer should have adequate information regarding the type and nature of soil available at different depths at the site for designing safe, sound and economical foundation for a structure. The aim of site exploration is to get as much information as possible about the physical properties and characteristics of the underlying material at site as well as details of other geological features of the area. The study of site exploration can be broadly sub divided in following groups:

1.Purpose of site exploration.

2. Site inspection and preliminary investigation.

3. Methods of site exploration.

4. Depth and spacing of trial pits and bore holes.

5. Choice of method of exploration.

 PURPOSE OF SITE EXPLORATION

The purpose of site exploration is to collect complete details of the site to enable the designer to take following decisions:

1.To fix the value of the safe bearing capacity of soil.

2.To select an economical yet safe type of foundation.

3.To fix the depth up to which the foundation must be taken inside the ground.

4.To predict the likely settlement of the selected foundation and to make allowance for the same in design.

5. To know the underground water level and if needed, to decide upon the method to be adopted to solve the ground water problem.

6. To forecast the difficulties which are likely to be encountered due to nature of the sub soil during construction and to take actions in this record.

 SITE INSPECTION AND PRELIMINARY INVESTIGATION

Inspection of the site is the first step which should be taken prior to the design and construction of a structure. This includes the study of all neighboring quarries or cuts made in the nearby areas for construction purposes, the study of existing structures and if possible, of their foundations. Information gathered in respect of other’s experience in making excavation in that locality may prove to be great help.

The other important information which can be gathered during the inspection is summarized below:

1.Whether the ground is soft, hard, marshy, water logged or made up type.

2. Classification of soil by visual examination.

3.Behaviour of the ground during changes in ground water level. Whether the rain water gets drained off or causes flooding of site.

4. Whether the sub soil water contains sulphates or other chemicals in quantities to cause damage to foundations.

 

CHOICE OF METHOD OF SITE EXPLORATION

The choice of method to be adopted for site exploration for a particular site depends upon the following factors:

1.Nature of ground

2. Topography

3. Cost of exploration

1.Nature of ground

·    In clayey soils borings are suitable for deep exploration and pits for shallow exploration.

·  In sandy soils boring is easy but special equipment is required to be used for taking representative samples below the water level. Trail pits, however, present difficulty in taking samples, when ground water table is high. In such cases, it is necessary to lower the ground water table to facilitate taking soil sample for testing.

·  In cases of rocks, boring is considered suitable in hard rocks and pits in soft rock. Core borings are suitable for identification in respect of joints and fissures in rock formation.

2. Topography

In case the site is located in hilly terrace, the choice between vertical openings and horizontal openings may depend upon the topography and geological structure of the sub strata. In case of steeply inclined strata and steep slopes, inclined boring or drifts are considered suitable, whereas for horizontal or gentle slopes, trial pits or vertical borings are considered suitable.

 3. Cost of exploration

From consideration of cost, deep exploration borings are preferred to deep shaft. In case of shallow exploration in soil, the choice between pits and borings will depend on the nature of ground and extent the information required. In case of shallow exploration in rock, use of core drilling can be justified, if number of bore holes required to be drilled are large. Alternatively, trial pits prove to be economical.

BEARING CAPACITY OF SOILS

The topic of bearing capacity is perhaps the most important of all the topics in soil engineering. Soils behave in a complex manner when loaded. Soil when stressed due to loading, tend to deform. The resistance to deformation of the soil depends upon factors like water content, bulk density, angle of internal friction and the manner in which load is applied on the soil.

When excessive load is transmitted to the soil by a structural foundation, the settlement of the foundation takes place which can endanger the stability of the structure. The settlement due to load is caused basically on account of two factors, namely 1. The soil below footings gets compressed by certain amount 2. Since the foundations cover only a limited area there is a possibility that the concentrated stresses developed are so high as to cause actual rupture and displacement of soil below.

Past experience shows that very often a structure fails due to unequal settlement or differential settlement. This happens when a part of building is founded on compressible stratum and the remaining part rests on firm soil strata. Thus the part of the building on compressible soil settles at a rate well in excess of the part of building is loaded much more than other or intensity of load is varying and is more than the bearing capacity of soil.

The various terms which are used in connection with the bearing capacity are summarised under:

1.Ultimate bearing capacity of soil: The intensity of loading, at the base of foundation, at which soil support fail in order.

2. Safe bearing capacity: The maximum intensity of loading that the soil will safely carry without risk of shear failure is called safe bearing capacity of soil. This is obtained by dividing the ultimate bearing capacity by a certain factor of safety, and it is the value which is used in the design of foundation.

3. Net Pressure intensity: Net Pressure Intensity refers to loading acting on the bottom of the foundation trench, in excess of the weight of soil removed from the trench. Net pressure intensity is normally considered in the design of foundation thereby utilizing the advantages of reduction of loading brought about on account of removal of soil from the foundation trench.

4. Allowable bearing pressure: It is the maximum allowable net loading intensity which can be applied to the soil taking into account the ultimate bearing capacity, the amount and kind of settlement excepted and the ability of the given structure to withstand the settlement. It is therefore dependent upon both the sub soil and the type of building proposed to be erected there on. The allowable bearing pressure adopted in the design of foundation is lesser of the two values:

1.The safe bearing capacity of soil or

2. The maximum allowable bearing pressure that the soil can take without exceeding the specified limits of permissible settlement.

TYPES OF FOUNDATIONS

Foundations can be broadly classified into two types i.e. 1.  Deep foundations and 2. Shallow foundations. Pile, cofferdams and caisson fall under the category of deep foundations.

SWALLOW FOUNDATIONS

When the foundation is placed immediately beneath the lowest part of the super structure, it is termed as shallow foundation. The object of this type of foundation is to distribute the structural loads over a wide horizontal area at shallow depth below the ground level. The various types of foundations which can be included under shallow foundation are:

1.Spread Footing

2. Eccentrically loaded footing

3. Mat on raft foundation

4. Grillage foundation

5. Combined footings

1.SPREAD FOOTING

As the name suggests, in case of spread footings, the base of the member transmitting load to the soil is made wider so as to distribute the load over wider area. Broadly speaking, all types of foundation mentioned above can be covered under the term spread. However, from design and construction point of view they have been treated separately. The various footings describe under this classification are:

1.Wall footings

2. Reinforced concrete footings

3. Inverted arch footings

4. Column footings.

Wall Footings: It consists of several courses of bricks, the lowest course being usually twice the breadth of the wall above. The increased base width of the wall is achieved by providing 5 cm offsets on either side of the wall. The depth of each course is usually 10 cm. in some cases, however the bottom courses are made 20 cm. in case of footings for stone walls, the size of offsets is slightly more than that of the brick wall footing. A bed of lean concrete of uniform thickness is first spread over the entire length of the wall. In any case, the depth of the concrete bed should never be less than its projection beyond the wall base.

Reinforced Concrete Footings: In places where the walls are subjected to relatively heavy loading and the bearing capacity of the soil on which the wall footing in above will work out to be massive in size. In such cases it is desirable to provide reinforced concrete footing below the wall. This appreciably reduces the volume of masonry work in footing and depth of bed concrete and as such proves to be economical. A 7 to 8 cm thick bed of lean concrete is usually provided below the reinforced concrete footing to perform the function of bed block.

Inverted arch Footings: This type of foundation used to be provided for multi-storeyed buildings in olden times. However, with the advent of reinforced cement concrete construction practice, inverted arch construction is rarely done these days. One of the drawbacks in this type of construction is that the end piers have to be especially strengthened by buttresses to avoid the arch thrust tending to rapture the pier junction. However, the advantage of inverted arch footing is that in soft soils the depth of foundation is greatly reduced.

Column Footings: An independent footing is one which is provided under a column or other similar member for distributing the concentrated loads in the form of uniformly distributed load on the soil below. The footing may be square, rectangular or circular in plan. Depending upon the load to be carried and the bearing capacity of the soil, independent footing may be of brick masonry, stone masonry, R.C.C, steel grillage etc.

2. ECCENTRICALLY LOADED FOOTING

As far as practicable, the foundation, should be so shaped and proportioned that the centre of gravity of the imposed loads coincide with the c.g. of the supporting area of base. However, when walls and columns are to be placed concentrically with the imposed loads without overlapping the property lines. Hence different methods are adopted to ensure the stability of the wall or column without encroaching the area outside the property lines of the building.

3. RAFT FOUNDATION

In made up ground, soft clay or marshy site having low value of bearing capacity, heavy concentrated structural loads are generally supported by providing raft foundation. Also if the structure is liable subsidence on account of its being located in mining area or due to uncertain behaviour of its sub soil water condition, raft foundation should be preferred. It provides an economical solution to difficult site conditions, where pile foundation cannot be used advantageously and independent column footing becomes impracticable.

Raft foundation consists of thick reinforced concrete slab covering the entire area of the bottom of the structure like a floor. The slab is reinforced with bars running at right angles to each both near bottom and top face of the slab.

4. GRILLAGE FOUNDATION

When heavy structural loads from columns, piers or stanchions are required to be transferred to a soil or low bearing capacity, grillage foundation is often found to be lighter and more economical. This avoids deep excavation and provides necessary area at the base to reduce the intensity of pressure within safe bearing capacity of soil. Depending upon the material used in construction grillage, foundation can be broadly divided in two categories:

1.Steel Grillage

2. Timber grillage

 Steel Grillage

Steel grillage foundation consists of steel beams also known as grillage beams which are provided in single or double tiers. In case of double tier grillage, the top tier is laid at right angles to the bottom one. The grillage beams of each tier are held in position by 20 mm spacer bars with 25 mm dia pipe separators. The beams are suitably spaced so as to provide facility for the placing and rodding of concrete between them. If beams are spaced more distance apart, there is a danger of the concrete filling not acting monolithically with the beams and such may result, in the failure of foundation.

Timber Grillage

Where the soil encountered is soft and is permanently water logged building walls can be economically supported by suitably designed timber grillage foundation. This type of foundation can be safely used for light builings by limiting the loading on the soil to 5.5 tonne/m^2. In this type of construction, the concrete block usually provided below the wall footing is replaced by timber platform.

 5. COMBINED FOOTINGS

A combined footing is so proportioned that the centre of gravity of the supporting area is in line with the C.G. of the two column loads.  Combined footing may be rectangular or trapezoidal in shape. Rectangular shape is only possible where loading condition is such that either the two columns are equally loaded or the interior columns carries greater load. On the other hand, in case of trapezoidal footing, no such condition is applicable.

CAUSES OF FAILURE OF FOUNDATION

1.Unequal Settlement of sub soil

Unequal settlement of foundation results in dangerous cracks which ultimately lead to the failure of structure. If the nature of soil and the loading conditions are uniform over the entire side of the building, the distribution of pressure can be safely assumed to the uniform and the danger of unequal settlement is minimized.

2. Unequal settlement of masonry

Mortar used as binding material in the masonry construction shrinks and gets compressed when loaded excessively before it has fully set. This defect may lead to the unequal settlement of masonry.

3. Horizontal movement of the soil adjoining the structure

This defect is very common in clayey and black cotton soils. Such type of shrinkable soils undergoes volumetric changes with the changes in atmospheric conditions. They swell excessively when and shrink excessively when dry.

4. Action of atmosphere

A part of rain water finds its way inside the ground. The rain water during its passage downwards brings down salts from the surface which react chemically with the material of foundation and cause it to disintegrate. Also if the foundation is not taken deep inside the ground, rain water may scour the soil above the foundation and expose it.