15

FOUNDATIONS

15.1 INTRODUCTION

A foundation is that part of the structure which is in direct contact with the ground and transmits the load of the structure to the ground. It includes the soil or rock of the earth’s crust or any special part of the structure which serves to transmit the loads into the soil or rock. The main purpose of the transmissions of load can be satisfied by a particular type of foundation that takes into account the properties of the supporting soil.

Thus the supporting soil plays a major role in the performance of foundation. Hence it is of prime importance to know the soil which is done by a proper soil investigation. It is necessary to know about the types of soils and their distribution to decide a particular type of foundation.

The structural support is actually being provided by a soil-foundation system. This combination of soil and foundation (now referred to as soil-structure interaction) can not be separated. Although engineers are aware of this relationship, it is common practice to consider the structure to be sound and to attribute the failure of the foundations to the failure of the supporting soil.

15.2 SOIL INVESTIGATION

Ground investigation refers to the methodology of determining surface and sub-surface features in the proposed construction area. Information of surface conditions is necessary for planning the accessibility of the site, for deciding the disposal of removed material, for removal of surface water in waterlogged areas, for movement of construction material and equipment and other factors that could affect construction procedures.

Information on surface and sub-surface conditions is a more critical requirement in planning and designing the foundations of structures, dewatering systems, shoring or bracing of excavation, the materials to be used in construction, and site improvement methods.

Thus the purpose of ground investigation is to:

1. Determine the geological condition of rock and soil formation.
2. Establish ground water level.
3. Select the type and depths of foundation.
4. Determine the bearing capacity of the site.
5. Evaluate the anticipated settlement of the structure.
6. Locate and select the materials of construction.
7. Locate suitable transportation routes.

15.2.1 Planning the Ground Investigation

The ground investigation, irrespective of the magnitude of the project, consists of four phases, which are discussed below.

1. Available Information

This is the first phase in which the collection of published geological and topographical information of the area, hydrological data, details of load regulations for construction activity, etc., are made.

2. Reconnaissance

At this stage a thorough study of the existing structures for the type of construction and defects such as cracks and settlement, availability of water, etc., for the building project are collected.

3. Preliminary Investigation

This is an important phase of the entire programme. As the sub-soils were formed under different geological environments, the first step towards a ground investigation is a thorough understanding of the geology of the site, which enables an efficient working out of the investigation programme. The second step is to obtain more details about the sub-soil strata (e.g., thickness of each stratum) from one or two exploratory drill holes. All further steps depend on the magnitude of the job and the character of the soil profile.

4. Detailed Investigation

Additional borings are planned from the data obtained from the preliminary borings. If the sub-soil is uniform in stratification, an orderly spacing may be planned. Often, additional borings are made to locate weak soil or rock zones, outcrops, etc., which may influence the design and construction of the project. Sufficient samples are procured to obtain relevant parameters for design and construction.

15.2.2 Methods of Exploration

Methods of exploration are indirect methods, semi-direct methods and direct methods of exploration.

1. Indirect Methods

Indirect methods consist of geophysical and sounding methods. In these methods, depths to the principal strata are established based on some physical properties of the material, and the measurements are made on the ground surface. No samples are obtained in the geophysical methods, but in some sounding methods, representative samples are obtained.

2. Semi-direct Methods

Semi-direct methods are common boring and drilling methods combined with intermittent sampling. The depths of different layers are ascertained by the rate of advertisement of boring tools or by means of non-representative samples obtained in the course of boring operations. Borings provide access to a particular layer of sampling. In general, only major changes in the character of the sub-surface materials can be detected by this method. The different boring methods are wash boring, rotary drilling and auger boring.

3. Direct Methods

Direct methods are boring and sampling methods which continuously provide representative or undisturbed samples. All accessible exploration, such as test pits, trenches, large diameter boring, shafts, drifts, etc., are grouped under direct methods. These accessible explorations allow direct examination of strata in-situ.

15.3 TYPES OF SOILS

Based on the method of formation, soil may be categorised as residual and transported soils. Residual soils have formed from the weathering of rocks and practically remain at the location of origin with a little or no movement of individual soil particles. Transported soils are those that have formed at one location (like residual soils), but transported and deposited at another location.

15.3.1 Residual Soils

Weathering (due to climatic effects) and leaching of water soluble materials in the rocks are the geological process in the formation of these soils. The rate of rock decomposition is more than the rate of erosion or transportation of weathered material, and results in the accumulation of residual soils.

15.3.2 Transported Soils

Weathered materials have been moved from their original location to new locations by one or more of the transportation agencies, viz., water, wind, gravity and glacier and deposited to form transported soil. Such deposits are further classified depending on the mode of transportation causing the deposit. For example, soils that are carried and deposited by river is called alluvial deposits, soils carried by wind and subsequently deposited are designated as aeolian deposits, gravity deposits are termed talus and the land formed after a glacier has receded is called a till plain.

15.3.3 Regional Soil Deposits of India

Among different types of soils spread over the Indian Peninsula, only six major deposits have been identified, viz., marine deposits, black cotton soils, lateritic soils, alluvial deposits, desert soils and boulder deposits.

1. Marine Deposits

They are generally formed in seawater areas. These deposits cover a narrow belt of tidal flats all along the coast from Porbandar in west to Puri in east. Marine deposits need a pre-treatment before application of any external load.

2. Black Cotton Soils

They are spread over a wide area of 300,000 sq km around Central India and in some parts of Tamil Nadu, Karnataka and Andhra Pradesh. The soil surface is hard in summer and becomes slushy during rainy season. Because of swelling and shrinking nature of the soils, there is a necessity for treatment of the soil. Special foundations are to be adopted in these soils to prevent failure of structures.

3. Lateritic Soils

In tropical regions of high moisture and temperature the lateritic soils are formed. These soils spread over an area of 100,000 sq km around Kerala, Tamil Nadu and West Bengal. Coarse grained soils of this type are called lateritic. The characteristic property of this type of soil is high strength when it is cut and dried in the sun.

4. Alluvial Deposits

The well-known alluvial deposits of India are in the Indo-Gangetic and Brahmaputra flood plains. North of Vindhya Satpura range is also covered with alluvial deposits. Alluvial deposits exhibit alternate layers of coarse and fine soils. The Bengal basin is another important alluvial deposit.

5. Desert Soils

Thar desert in Rajasthan covers most of the area which forms the desert soils of India. These are wind-blown deposits generally present in the form of sand dunes. Scarcity of water is a serious problem in these areas for any construction activity.

6. Boulder Deposits

Rivers flowing in hilly terrains and near foot hills carry large quantities of boulders. Such deposits are often found in the sub-Himalayan regions of Himachal Pradesh and Uttar Pradesh. The properties of these deposits depend on the relative proportions of the boulders and the soil matrix.

15.3.4 Re-claimed Soils

The term re-claimed soils comprises of all materials deposited on a site using various methods for different purposes. It is justifiable to name the re-claimed materials as soil, when it comes to the purposes of construction of structures on them considering them as foundation material. Industrial and commercial development of urban areas, development of navigation channels for ports and other waterfront structures require a large usable land which could be possible only by reclamation. Reclamations are also needed, though may be less important, for the disposal of garbage, industrial wastes, paper sludge, mine tailings, etc. Reclamations may be on unusable low level land areas or on large bodies of water. These reclamation especially near larger bodies of water lead to unsavoury odours, greater turbidity and toxicity of shore waters and affect in great extent the ecology of all marine life.

The geotechnical problems to be answered in these areas are control of settlement, increase of bearing capacity and biological stability of fill. Generally reclamation followed by ground treatment is preferable and economical than designing deep foundations.

The materials which are used in practice for reclamation purposes fall into five groups, viz., hydraulic fills of dredged soil, sanitary fill, paper sludge, flash including slag and rubbish sand debris.

15.4 BEARING CAPACITY

It is the earth that provides the ultimate support for most of the structures including buildings, bridges, dams, highways, etc. The behaviour of the supporting ground may therefore affect the stability of structures. The supporting ground is invariably the soil, particularly for buildings, which is weaker than any construction material like wood, concrete, steel or masonry. Soil is a particulate material, although weak, involved in carrying large loads.

15.4.1 Bearing Capacity Criteria

The design of foundation is primarily based on the concept of bearing capacity of the soil. Thus the bearing capacity is defined as the load or pressure developed under the foundation without introducing damaging movements in the foundation and in the super-structure supported on the foundation. Since damaging movements may result from foundation failure (collapse) as well as from excessive settlement, the following criteria should always be used in evaluating the bearing capacity:

1. Adequate factor of safety against failure by collapse.
2. Adequate margin against excessive settlement.

The bearing capacity after allowing a certain factor of safety over the ultimate bearing capacity against failure (collapse) is termed as safe bearing capacity.

The bearing pressure which is safe against failure and at the same time does not cause settlement more than the permissible settlement is called allowable bearing capacity or allowable soil pressure.

Safe bearing capacity or allowable soil pressure is not a fixed value for a given soil but depends on density, strength, depth of foundation, cohesion, etc. In general, for cohesionless soils it varies from 100 to 450 kN/m² and for cohesive soils from 150 to 450 kN/m². The problematic soils are loose dry fine sand and expansive clays (black cotton soils). However, a suitable foundation can be provided based on the field condition.

15.4.2 Factors Affecting Bearing Capacity

The following factors directly or indirectly affect the bearing capacity of soil:

1. Type of soil (i.e., homogeneous, layered, expansive, etc.) and its physical and engineering properties.
2. Initial stress condition of the soil due to pre-history and due to the existing structure in and around the proposed foundation.
3. Location of ground water in the soil and its fluctuations with time.
4. Type of foundation (i.e., shallow or deep), and other factors such as shape, size, and rigidity condition of the foundation.
5. Depth and location of foundation.
6. Allowable settlement of the foundation which shall not be detrimental to the functioning of the foundation.
7. Natural calamities such as earthquake, flood, heavy wind, etc., of the region where the structure has to be located.

15.4.3 Methods of Determining Bearing Capacity

Based on the theoretical approaches, bearing capacity of a soil can be found knowing the strength parameters, density, depth of foundation and water table possible. Terzaghi’s bearing capacity theory has been used mostly which suits most of the field conditions.

Field plate-load test can be conducted at the stipulated depth of foundation. Indirect methods such as penetration tests may be used.

Bureau of Indian Standards has given presumptive bearing capacity values, which is presented in Table 15.1.

Table 15.1 Safe bearing capacity

Source: IS: 1904–1986.

15.4.4 Improvement of Bearing Capacity of Soils

If unsuitable soil conditions are encountered at the foundation site of a proposed structure, one of the following three procedures may be adopted:

1. The unsuitable soil is bypassed by means of deep foundations extending to a suitable bearing material.
2. The poor material is replaced and either treated to improve and replaced or substituted by a suitable material.
3. The soil is treated in place to improve the properties.

Now-a-days various methods are available by which the characteristics of the construction site can be improved to facilitate construction operation, to allow increased bearing pressures or to reduce settlements. Soil improvement in its broadest sense is the alteration of any property of a soil to improve its engineering performance. The various techniques discussed are surface compaction, drainage methods, grouting and injection, chemical stabilisation, thermal stabilisation, soil reinforcement, and application of geotextiles and geomembrane.

1. Surface Compaction

One of the most widely used and the oldest technique of soil densification is compaction. Construction of a building on a loose foundation site needs a compacted base for laying the structures. If the depth to be densified is less then surface compaction may alone solve the problem. Surface compaction needs less skilled labour and is usually the most economical method.

2. Drainage Methods

Drainage method of densification of soil is lowering the water table temporarily or permanently by pumping using well-point systems.

3. Vibration Methods

Vibration methods comprise of vibro-compaction and vibro-displacement compaction. Another method of vibration method is heavy tamping.

The most basic and simplest way of compacting loose soil is by repeated dropping of a weight on the ground. This method, also known as deep dynamic compaction or deep dynamic consolidation, consists of allowing a very heavy weight (up to 400 kN) to fall freely on the ground surface from a height of 15–40 m. This leaves an impression on the ground. The tamping is then repeated either at the same location or over other parts of the area to be stabilised. In the case of non-cohesive soils, the impact energy causes liquefaction, followed by settlement as water drains. Fissures formed around the impact points sometimes facilitate drainage in some soils. This method can be adopted for densifying soils both above and below the water table. This method has been successfully used to treat various types of soils and fill deposits up to 20 m thick.

4. Pre-loading and Surcharge Fills

In this process, an earth fill or some other material is placed over the required site. The amount of fill is sufficient enough to produce a stress in the soil equal to the one anticipated from the final structure. The soft soil is allowed to consolidate prior to construction. Since the consolidation takes a very long time, the method is suitable only for stabilisation of thin layers.

The rate of pre-load and surcharge fill placement has to be controlled depending on the bearing capacity of the soil. If the bearing capacity of the soil is inadequate layers of fill can be placed only after a sufficient gain in shear strength is obtained. The two main requirements for preloading are enough space and availability of fill material. Heaping of fill is the most common method of pre-loading although pre-loading can be successfully effected by the weight of water or by lowering the water table. Among the fill materials, granular soil is the most desirable because it does not turn into mud during rains. Ores and industrial products are generally satisfactory, but clayey soils are less desirable.

5. Vertical Drains

For deep clay deposits, pre-loading alone will take more time because of the long drainage path available for consolidation. An efficient way to do this is by providing vertical drains. Vertical drains are continuous vertical columns of pervious materials installed in clayey soil for the purpose of collecting and discharging the water expelled during consolidation. Vertical drains in combination with pre loading will rapidly accelerate consolidation.

6. Grouting and Injection

Grouting is used for the following in connection with foundation:

1. Void filling to prevent excessive settlement.
2. Stabilising loose sands against liquefaction.
3. Strengthening existing foundation.
4. Reduction of machine foundation vibrations.

As discussed earlier suspension or solution grouts are used in the above cases depending on the field condition.

7. Chemical Stabilisation

Chemical stabilisation in the form of lime, cement, fly ash and a combination of the above is widely used in soil stabilisation to:

1. Increase bearing capacity
2. Decrease settlement
3. Expedite construction
4. Reduce permeability
5. Improve shear strength

Chemical stabilisation may be used for surface soils more successfully. Such a stabilisation technique is sparingly used for building foundation.

8. Soil Reinforcement

Soil reinforcement is the process of strengthening weak soil by providing high-strength thin horizontal membranes. The modern form of soil reinforcement was first applied by According to Vidal’s concept, the interaction between the soil and the reinforcing horizontal membrane is solely due to friction generated by gravity.

Reinforced soil is somewhat analogous to reinforced concrete. A wide variety of materials such as steel, concrete, glass fibre, rubber, aluminium and thermoplastic have been used successfully. High alloy steel, aluminium, glass-fibre reinforced plastics (GRP) and geosynthetics are non-corrosive and have long life.

15.5 FUNCTIONS OF FOUNDATIONS

A foundation by definition is that part of the structure which is in direct contact with the ground and transmits the load of the structure to the ground.

15.5.1 Load and Load Distribution

Foundations are subjected to three types of loading, viz., dead load, live load and wind load. Dead load is the self-weight of the various components of a building which include the proposed future expansion. Live load is not a constant load but a varying load, viz., weight of persons using the building, weight of material stored temporarily on the floor, weight of snow, etc. Wind load will be significant in tall buildings wherein the sides and roofs are exposed to wind pressure. Because of this the pressure on the wind-ward side is reduced and in lee-ward side is increased.

Foundation distributes the above loads to a large area (in shallow foundation) or through end-bearing and skin friction (in deep foundation) so that the intensity of stress and the settlement are within limits. It also provides a level surface for the super-structure to be raised.

15.5.2 Stability Requirements

Foundation imparts lateral stability to the super-structure by anchoring it to the ground. It also provides additional stability against sliding and overturning due to horizontal forces like wind, earthquake, etc.

15.5.3 Settlement Control

Settlement of a foundation may be classified as uniform (or total), tilt and non-uniform (or differential) settlement. Structures on rigid foundations undergo uniform settlement. When the entire structure rotates, the structure is said to be under uniform tilt. If foundations of different elements of a structure undergo varied settlements, the foundation is said to be under non-uniform or differential settlement. Foundations are capable of distributing the load evenly under non-uniform loading conditions and non-uniform soil conditions and thereby prevent differential settlement. This can be achieved by adopting suitable foundations such as combined footings, rafts, mats, etc.

15.5.4 Safety Against Natural Events

Foundations sustain large wind forces and earthquake forces and also provide safety against scouring or undermining by flood water or burrowing animals. Distress or failure due to seasonal variations causing volume changes in soils are minimised by providing special type of foundations.

15.5.5 Requirements of Good Foundation

Thus the foundation should satisfy the following requirements.

1. Depth of Foundation

1. Foundations should be carried well below the top soil, miscellaneous fill, abandoned foundation, debris or muck.
2. Foundation should be carried below the depth of weathering.
3. Foundation on sloping ground should have sufficient edge distance as protection against erosion.
4. Difference in elevation of foundation should not be so great as to introduce undesirable overlapping of stresses on soil.

2. Shear Failure of Foundation

Foundation should be safe against breaking into the ground (i.e., against shear failure). In order to satisfy this requirement an adequate factor of safety on the bearing capacity of the soil is provided.

3. Settlement of Foundation

Foundation should not undergo exercise total and differential settlements. The limiting total and differential settlement should satisfy the requirement specified by building codes for different structures and different soils.

15.6 SHALLOW AND DEEP FOUNDATIONS

Structural foundations may be grouped under two broad categories – shallow foundations and deep foundations. This classification indicates the depth of foundation installation.

A shallow foundation is one which is placed on a firm soil near the ground, and beneath the lowest part of the super-structure. A deep foundation is one which is placed on a soil that is not firm, and which is considerably below the lowest part of the super-structure.

15.6.1 Types and Suitability of Shallow Foundations

Shallow foundations are all suitable for building and are sub-divided into a number of types according to their size, shape and general configuration. They are discussed below.

1. Spread Footings

These footings are the most common of all types of footings with minimum cost and complexity of construction (Fig. 15.1(a)). It necessarily provides the function of distributing the column load over a wide area taking care of the strength and deformation characteristics of the soil. These types of footings are also known as pad footings, isolated footings and square or rectangular footings (for length of footing, L, and width of footing, B, ratio less than 5).

Figure 15.1 Types of shallow foundations

2. Combined Footing

These footings are formed by combing two or more equally or unequally loaded columns into one footing. This arrangement averages out and provides a more or less uniform load distribution in the supporting soil. Further distribution prevents variation of settlement along the footing. These footings are usually rectangular in shape. It may be modified to a trapezoidal shape so as to accommodate unequal column loadings or column close to property line. It may be provided with a strap to accommodate wide column spacing or columns close to property line (Fig. 15.1(b)).

3. Continuous Footing

These footings carry closely spaced columns or a continuous wall such that the load distribution is uniform and load intensity is low on the supporting soil (Fig. 15.1(c)). These footings are also named as strip footings or wall footings (for L/B ratio greater than 5).

4. Mat or Raft Foundation

These are characterised by the feature that columns frame into the footing in two directions. Any number of columns can be accommodated with as low as four columns (Fig. 15.1(d)). In the majority of the cases, mat foundations are used where the soil has low bearing capacity. By combining all individual footings into one large mat, the unit pressure in the sub-soil is reduced.

Since the bearing capacity increases with increasing depth and width of the foundation and the settlement decreases with the increasing depth of foundation, the advantage of mat foundation is two-fold. Mat foundation is also preferred when the total area of the footings exceeds 50% of the total plinth area.

15.6.2 Types and Suitability of Deep Foundations

The design and construction of deep foundations for transferring the weight of the super- structure through soft or weak soils, to deep load bearing strata is a challenging job for a civil engineer. Piles, piers and caissons are the most common types of deep foundations. For any system the mechanism of deriving support from the soil or rock below and adjacent to the foundation is similar. However, each system differs in its method of construction.

1. Pile Foundations

Piles are slender structural members normally installed by driving by hummer or by any other suitable means. The piles are usually placed in groups to provide foundations for structures. Piles may be classified according to their material composition, installation method, group effect and their function as a foundation.

(i) Classification Based on Materials

Under this classification, piles may be further classified as timber, steel, concrete or composite piles. Timber piles are the oldest types of piles made from tree trunk. The maximum length of pile is 20 m. The life of timber piles may be increased by treating them with preservatives (Fig. 15.2(a)).

Steel piles consist generally of either pipe piles or rolled steel H-section piles. Because of high strength there can be no restriction on length but steel piles are affected by corrosive agents such as salt, acid, moisture and oxygen. In order to prevent steel piles from corrosion, the thickness is increased, encased in concrete or chemical coating is applied (Fig. 15.2(b)). Concrete piles are precast to specified lengths and shapes with reinforcement. The reinforcement is provided to enable the pile to resist the bending moment developed during lifting and transportation. Concrete piles are also cast in-situ (Fig. 15.2(c)).

Figure 15.2 Classification based on materials

(ii) Classification Based on Installation Methods

Based on installation techniques piles are classified as driven piles and cast-in-situ piles. Driven piles may be concrete, steel or timber. Concrete piles are classified as driven pre-cast concrete piles, and bored cast-in-situ concrete piles. Driven precast concrete pile is the one casted in a casting yard subsequently driven to the required location.

Driven cast-in-situ pile is formed within the ground by driving a closed bottom casing and subsequently filling with concrete in the hole so formed with adequate reinforcement. Bored cast in-situ pile is formed within the ground by excavation or boring with or without the use of a temporary casing and subsequently filling it with plain or reinforced concrete.

(iii) Classification Based on Ground Effects

Piles are also used to compact soils and such piles are referred to as displacement or compaction piles. These piles displace a substantial volume of soil during installation. In granular soils, there is a tendency for compaction, whereas in clays heaving of the ground surface often results. Driven piles installed in pre-drilled holes are also called as non-displacement piles. Piles are also used to prevent the movement of earth slopes and to safeguard the foundation from damage due to shock.

(iv) Classification Based on Functions

Where the top soil is soft or too weak to support the super-structure, piles are used to transmit the load to the underlying bed rock, such piles are called end-bearing piles or point bearing piles. If the bed rock does not exist at a reasonable depth below the ground surface, the load is transferred through friction along the pile shaft such piles are called friction piles. Transmission towers, off-shore platforms, and basement mats are subjected to uplift forces and piles are used to resist the uplift forces, which are called uplift piles or tension piles. In order to resist horizontal and inclined forces in water and earth retaining structures batter piles are used. Application of piles for providing anchorage to sheet piles are called as anchor piles (Fig. 15.3).

Figure 15.3 Classification based on function

2. Pile Groups

Where piles are used for foundation support, they are always used in a group. This requirement is essential so as to assure that the imposed structural load lies within the support area provided by the foundation. As per the building codes at least three piles should be used to support a major column and two piles to support a foundation wall.

A pile cap is provided near the ground encompassing all the top ends of piles. Pile caps are almost invariably made of reinforced concrete. The axial and the lateral load carrying capacity of a pile group is significantly affected by a pile cap. In order to keep the stresses in the pile cap to a minimum the piles should be arranged in the most compact geometric form. Typical arrangement of one pile group is shown in Fig. 15.4.

3. Drilled Piers

Drilled piers are structural members of relatively large diameter massive struts constructed of concrete placed in a pre-excavated hole.

They are also called bored piles, large-diameter piles, foundation piers and drilled caissons. The shaft can be enlarged at the base resulting in a belled or under-reamed pier.

The common type of drilled pier is the straight shafted type (Fig. 15.5(a)). The shaft is taken through the upper soil layers and the end is placed on the firm ground or rock. Drilled piers which are provided with a broad base (called a bell) at the bottom of the straight shaft are referred to as belled piers. The bell may have a shape of a dome or it may be angled (Fig. 15.5(b)). The third type is the extended straight shaft or socketed pier in which the straight shafts are extended into the underlying rock layer (Fig. 15.5(c)).

4. Caissons

Caissons are structural boxes or chambers that are sunk in place through ground or water. The sinking is systematically done by excavating below the bottom of the unit which thereby descends to the final depth. These have large cross-sectional area and hence provide high bearing capacity. Two types of caissons, viz., open caisson and monolith caisson are shown in Fig. 15.6.

Figure 15.6 Two types of caissons

5. Well Foundation

Well foundation is a type of caisson. It is constructed either on dry ground or over an artificially formed island. The curbs are pitched in the current position and then sunk into the ground to the desired level by grabbing the soil through the dredge holes formed by the masonry or concrete the steining. In India, this procedure of initial sinking is referred to as the caisson method. Well foundations have all the advantages of open caisson. Figure 15.7 shows a typical cross section of a well foundation.

Figure 15.7 Typical section of a well foundation

15.6.3 Foundation under Special Conditions

For some structures or for soil conditions the routine method of providing foundations may not be suitable. Such foundations under special conditions are dealt below.

1. Grillage Foundation

Steel columns may be founded on concrete footings or footings of steel. Many a times the latter approach is preferred. This primarily consists of steel beams arranged in layers at right angles to one another and the beams are connected with each other by bolts in order to form a rigid unit. The entire assembly is embedded in concrete. This is known as Grillage foundation (Fig. 15.8).

The steel grillage foundations are adopted for structure having concentrated loads. Hence they are employed for the foundations of buildings such as theatres, factories, town halls, clock towers, etc.

1. In this method the depth of foundation is fixed to 1–1.5 m and the width is increased to satisfy the safe bearing capacity and permissible settlements.
2. The beams are R.S.Js which are fully embedded in concrete so as to protect them from atmospheric actions.
3. The bed of concrete should have a minimum thickness of 15 cm and nowhere the depth of concrete is less than 80 mm. The concrete filling does not carry any load but it maintains the R.S.Js in proper position.

Sometimes timber beams are used for temporary grillage foundation.

Figure 15.8 Typical grillage footing for steel columns

2. Stepped Foundation

In a sloping ground, it is uneconomical to provide the conventional type of foundation. In such cases, stepped foundation may be provided as given in Fig. 15.9.

Figure 15.9 Stepped foundation

The following points are to be taken care of:

1. The overlap between two layers of foundation concrete should be greater of the depth of foundation concrete or twice the height of the step.
2. In order to protect from weathering action a minimum depth of 80 cm should be provided at all points.
3. The depth of foundation concrete should be in even number.
4. The distance of the sloping surface from the lower edge point should be at least 100 cm for soils and 60 cm for rocks.
5. Stability of slope has to be checked if heavy load is expected on the foundation.

3. Foundations near Adjacent Structures

The horizontal location of a footing is often affected by adjacent structures and property lines. The existing adjacent structure may be damaged due to construction of new foundation because of vibration and shock due to blasting, caving in due to nearby excavation, lowering of water table or increasing stress.

The Indian Standards (IS: 1904, 1986) recommends the following for footings placed adjacent to a sloping ground or when the bases of footings are at different levels.

When the ground surface slopes downwards adjacent to a footing, the sloping surface should not encroach upon a frustum of bearing material under the footing, as shown in Fig. 15.10(a) and (b) for granular soils and clayey soils respectively.

The following norms have to be adopted to avoid any damage to the existing structure:

1. The footing should be founded at least at a distance S from the edge of the existing footing where S is the width of the larger footing.
2. The line from the edge of the new footing to the edge of the existing footing should make an angle of 45° or less.

Figure 15.10 Footings at different levels (Source: IS: 1904, 1986)

1. When a new footing is constructed lower than an old footing, the excavation for the foundation must be carefully done with a suitable bracing system so as to prevent damage to the existing structure (Fig. 15.11).

Figure 15.11 Footings for old and near structures (Source: IS: 1904, 1986)

4. Under-reamed Piles

Under-reamed piles are of bored cast-in-situ and bored compaction concrete piles with enlarged base. The enlarged base is termed as a bulb or under-ream. An under-reamed pile may have one, two or more bulbs. Accordingly, they are called as single-, double-, or multi-under-reamed piles. The bulb provides adequate bearing or anchorage. Under-reamed piles are used for a variety of field conditions, viz.,

1. To obtain adequate capacity for downwards, upward, and lateral loads and moments, e.g., transmission tower foundation.
2. To take the foundation to deeper structure in order to prevent the effect of seasonal changes, e.g., in expansive soils (black cotton soils)
3. To take the foundation, below scour level, e.g., in piers.

In deep deposits of expansive soils, the minimum length of piles (irrespective of any other factors) should be 3–5 m below ground level. In weak soil structure or in recently filled grounds, the pile should pass through such soils and be seated in strong bearing strata, Fig. 15.12 (IS: 2911-Part 3, 1980).

Figure 15.12 Single and double under-reamed Piles (IS: 2911–Part 3, 1980)

15.7 CAUSES OF FOUNDATION SETTLEMENT

Settlement of foundations may be caused due to the following reasons:

1. Elastic compression of the foundation and the underlying soil.
2. Plastic or in-elastic compression of the underlying soil.
3. Ground water lowering is another major cause for settlement. This is more adverse in granular soils due to repeated raising or lowering of the ground water. In clayey soil prolonged lowering of ground water may cause settlement.
4. Vibrations caused by pile driving, machinery, blasting, etc. This is more adverse in granular soils.
5. Other causes of settlement include volume change of soil, ground movement and excavation for adjacent structures, mining subsidence, etc.

15.8 SELECTION AND DESIGN OF SIMPLE FOUNDATIONS

15.8.1 Selection Procedure

The selection of a foundation suitable for the type of structure to be constructed or for a given size depends on several factors. Following are the general steps to be followed in choosing the type of foundation.

1. Necessary data about the type of structure and the loads anticipated to be carried by the structure are collected.
2. Adequate information about the sub-soil condition through a suitable soil investigation is got.
3. The possibility of constructing a different foundation keeping in mind the basic design criteria for a foundation is explored. During this exercise, all unsuitable types may be eliminated in the preliminary choice.
4. One or two types of foundations based on the preliminary studies which may be a shallow or deep foundation, are selected and more detailed studies regarding the stability of the foundation and super-structure are carried out.
5. Cost estimates of one or more chosen foundations are worked out.
6. Three types of foundations to satisfy all the requirements are finally decided.

15.8.2 Design Procedure

The following general steps have to be adopted in the design of foundations:

1. A soil investigation has to be carried out as discussed in Section 15.2.
2. It is necessary to compute the total load (both dead and live load) and the distribution has to be assessed.
3. It is to assess the total and differential settlement which the structure may under go.
4. Based on the type of soil and the structure and load the type of foundation is decided as discussed in Section 15.6.1.
5. The appropriate allowable soil pressure has to be determined for the selected type of foundation.
6. The type of material for the foundation has to be decided.
7. Alternate designs are to be made before finalization.
8. Cost estimate has to be made and any further modification may be made keeping in view economy and life of the structure.

15.8.3 Design of Shallow Foundations

Following guidelines may be taken while designing shallow foundation other than rafts and mats.

1. In case of wall footing the width of foundation should be computed based on the allowable soil pressure.
2. In case no footings are to be provided to the walls the width of foundation should be equal to three times the width of the wall.
3. In case of piers the width of foundation is equal to square roots of total load of the pier divided by the allowable soil pressure.
4. For unreinforced strip footings the thickness should not be less than the projection from the base of the wall. It should not be less than 150 mm where the foundations are laid at more than one level.
5. For unreinforced column footing the spread of footing may be 1 vertical to one horizontal.
6. As a general rule, the shallow foundation should be taken down to a depth where the allowable bearing capacity is adequate.
   1. As for as possible the foundation should be kept above the ground water table.
   2. In order to safeguard a against minor soil erosion, a minimum depth of 500 mm is provided for strip or column foundation
   3. The depth of foundation can be also determined by plotting the pressure distribution lines (Fig. 15.13).

      h₁, h₂ = Depth of footing, Depth of base concrete

      h = Depth of foundation

      Then h =

      

      Figure 15.13

   4. Minimum depth of foundation for loose soils may be obtained from Rankine’s formula, i.e.,

      

      Where h = Minimum depth of foundation in m

      w = Weight of soil in kg/m³

      ϕ = Angle of repose

      p = Load in soil kg/m²

      The depth of concrete block is given as in cm.

      where a = offset of concrete in cm

      f = safe modulus of rupture in kg/m²

15.8.4 Design of Piles

Following guidelines may be considered in design of piles

1. Direct vertical load coming on the pile should be considered.
2. In case of driven piles, the impact stresses induced due to pile driven is taken into account.
3. Bending stresses induced due to bending in piles and due to eccentricity to be accounted.
4. (iv) Lateral forces due to wind, waves, water, current, ice sheets, impact of ships are to be accounted
5. Forces due to uplift may also to be considered.
6. If the area is earthquake – prone area necessary modifications have to be made.
7. Load carrying capacity of pile is computed based on the type of pile. Pile load tests can be done for all type of piles. For driven piles, pile driving formulas can be used. One such formula is ENR formula which is derived on the basis of work-energy theory. The ENR formula has been modified by Hiley as the ultimate pile load, , is given as

Where = Hammer efficiency

W = Weight of hammer

h = Height of fall

S = Final set

= Efficiency of the blow.

C = Sum of the temporary elastic compression of the pile.

15.9 EXCAVATION FOR FOUNDATION

The foundations for most structures are invariably established below the surface of the ground. Thus they can not be constructed until the soil or rock above the base level of the foundations has been excavated. Open excavations are supported in some soils by lateral support called bracings. It is generally the engineer’s duty to decide the construction procedure proposed by the builder and to check the design of bracing. In previous soils, excavation below the water table usually requires drainage of the site either before or during construction. The general aspects of excavating and providing support for the sides of the pits or cuts are discussed in the following sections.

15.9.1 Shallow Excavations with Unsupported Slopes

Shallow excavations can be made if there is enough space is available to establish slopes at which the material can stand. As a general rule construction slopes can be made as steep as possible although a few small slides is generally not serious. But the steepness of slope depends on the type of soil or rock, climate and weather conditions, the depth of excavation and the time to which the excavation should stand.

The steepest slope that can be used in a particular location are decided based on the experience. However, in sandy soils, slopes of about 1 vertical to horizontal are usually considered. The maximum slope in a clayey soil depends on the depth of cut and the shearing resistance of the clay.

15.9.2 Shallow Excavations with Sheeting and Bracing

Many a times, building sites extend to the edges of the property lines or are adjacent to other sites, over which some structures may already be existing. Under these conditions, it is mandatory that the sides of the excavation must be made vertical and should be usually supported. Several methods are available in such occasions. Two common and simple methods are explained below.

If the depth of excavation is less than 4 m, it is common practice to drive vertical planks knowing as sheeting around the boundary of the proposed excavation. The depth of sheeting is kept near to the bottom of the excavation in progress. The sheeting is held in position by means of horizontal beams called wales. These wales are in turn are commonly supported by horizontal struts extending from side to side of the excavation. The struts are usually are of timber for the excavation not more than 1.5 m wide. For wider excavation metal pipes called trench braces are commonly used (Fig. 15.14).

If the excavation is too wide, the wales may be supported by inclined struts known as rakes. Rakes can be used to provide the supporting soil is firm enough to withstand the forces (Fig. 15.15).

Figure 15.14 Sheeting of shallow excavation

Figure 15.15 Shallow bracing

15.9.3 Deep Excavations

Excavation beyond depth of 1.5 m is generally categorised as deep excavation. The problems generally encountered in deep excavation are:

1. The collapsing of the sides of the trench.
2. The prevention of water entering the trench from the sides or from the bottom of trench.

Only the first aspect is treated in the sections treated below. The secured aspect is dealt separately elsewhere. Following methods of bracing are commonly employed.

1. Stay Bracing

This arrangement is similar to that followed for shallow excavations. This type of bracing is used in moderately firm ground and when the depth of exaction does not exceed 2 m. Here vertical sheets or poling boards are placed on opposite sides of the trench and they are held in position by one or two rows of struts. The sheets are placed at the spacing of 3–4 m and generally extend to the depth of trench. The thicknesses of poling boards are about 40–50 mm and of width 200 mm. The struts may be of 100 mm × 100 mm size for trench up to 2 m width and of 200 × 200 mm width for trench width exceeding 2 m (Fig. 15.16).

2. Box Sheeting

This arrangement is made for loose soil and when the depth of excavation does not exceed 4 m. Sheeting planks, wales and struts are used to form box like structure as shown in Fig. 15.17. In this arrangement the planks are placed closer or sometimes touching each other. Tow longitudinal rows of wales keep the sheets in position. Struts hold the wales in position.

In very loose soils additional bracings are provided. In this arrangement the planks are placed horizontally (in plan) and are supported by wales and struts as shown in Fig. 15.18.

3. Vertical Sheeting

In soft ground up to 10 m depth of trenches, the work is carried out in stages. This method is similar to box sheeting. Here at each stage of excavation one offset is provided for each stage separate vertical sheets, horizontal wales, struts and braces are provided. The offset is provided at 3–4 m depth and of 30–60 cm wide at each stage. Suitable working platform is provided (Fig. 15.19).

4. Runners

In situations where immediate support is needed, in case of very loose and soft ground, as the excavation progresses the special arrangement as shown in Fig. 15.20 is made. Here the runners are long thick wooden sheets with iron shoe at one of its ends is used to drive the runners. The wales and struts are provided as usual.

5. Sheet Piling

When the depth of excavation exceeds 10 m the use of vertical timber sheeting becomes generally uneconomical. In such situations other methods of sheeting and bracing are commonly employed. One such procedure is driving of steel sheet piling around the boundary of the excavation. As the soil is removed from the enclosure wales and struts are inserted.

The types of sheet piles commonly used are shown in Fig. 15.21.

The strength and stiffness of piling is in the increasing order as flat arch and z-piling. Flat and arch web types are used for shallow to deep excavation whereas z-type is used for deep to very deep excavations where the heaviest pressure is expected.

As the excavation proceeds wales and struts are inserted. The wales are commonly of steel, and the struts may be of steel or wood. Excavation is then proceeded to a lower level, and another set of wales and struts is installed. This process is continued until the excavation is completed. In order to prevent local heaves in most of the soils it is necessary to drive the sheet piles several cms below the bottom of excavation (Fig. 15.22).

15.10 CONSTRUCTION OF FOUNDATIoNS FOR BUILDINGS

The construction procedure adopted in each of the building foundations are briefly explained below.

15.10.1 Construction of Spread Footing

Spread footing is called as isolated or column footing. They are used to support individual columns. They can be of stepped type or provided with projections in the concrete base. Main reinforcement is placed at the bottom. In case of heavily loaded columns reinforcement is provided in both the directions in the concrete bed. The concrete mix is based on the strength requirement. In general 1:2:4 mix is used. Generally 15 cm offset is provided on all sides of the concrete bed. In case of brick masonry columns an offset of 5 cm is provided.

15.10.2 Construction of Combined Footing

Combined footings are designed keeping the following aspects in view:

1. The shape of the footing is so selected such that the centre of gravity of the column loads and of soil reaction remains in the same vertical line. Unusually a rectangular or trapezoidal shape of foundation will generally satisfy this requirement.
2. The area of the combined footing should be equal or greater than the ratio of the total load and the bearing capacity of the soil.
3. The combined footing is treated as an inverted floor, loaded by earth reaction and supported by columns.

Other procedure adopted for column footing may be adopted here too.

15.10.3 Construction of Continuous Footing

A wall footing is a typical case of continuous footing. This may have a base course of concrete or may be made of the same material as that used for the wall. This type of footing may be simple or stepped.

For light loads a simple base with a projection of 15 cm on either side is provided. As a general rule the base width of concrete bedding should be twice the width of the wall and the depth of the base concrete is at least twice the projections.

In another type of wall footing no base concrete is provided. In such a case in order to transmit the load gradually the width of the wall is gradually increased. This is adopted by projecting bricks regularly to a distance not greater than of a brick beyond the edge of the wall. Cement mortar is used in both the cases for walls. For foundation part a richer mix has to be used.

In another type of continuous footing series of columns in a line are provided with a footing. Here reinforced concrete slabs extends over the series of columns. In order to increase the stability a deep beam is constructed in between the columns. Such type of footings resist differential settlements.

15.10.4 Construction of Mat Foundation

As discussed earlier this type of foundation is used when the bearing capacity is low and total area of spread footing exceeds 50% of the total plinth area.

Mat foundation consists of rows of columns built monolithic with a continuous slab covering the entire foundation area, with or without depressions or openings.

A true mat is a flat concrete slab with uniform thickness throughout the entire area. This type is most suitable where the column spacing is fairly small and uniform and the column loads relatively small. For large column loads a portion of the slab under the column may be thickened. If bending stresses become large, thickened bands may be used along the column lines in both directions. Under extremely heavy column loads, two-way grid structure made of cellular construction may be used. Basement walls are also sometimes used as ribs or as deep beams.

The choice of mat type depends on one or more of the following factors:

1. For fairly small loading and uniform column spacing and the supporting soil is not very much compressible a flat concrete slab with uniform thickness of mat may be provided.
2. In order to provide adequate strength against shear and negative bending moment for heavy loaded columns the slab is thickened.
3. For unequal column loading and wide spaced columns beam and slab type of raft is more economical.
4. For heavy structures, cellular rafts or rigid frames may be adopted.

Example 15.1

A residential building is to be constructed on a sandy soil with a safe bearing capacity of 1.65 kg/cm² and the angle of shearing resistance (angle of repose) is of 30°, and the unit weight of the soil 1580 kg/m³. The thickness of wall is 30 cm. The total load transmitted is 11500 kg per metre length of the wall.

Solution:

Depth of foundation can be found using Rankine’s formula

Width of footing,

From practical consideration:

Hence B = 90 cm is adopted.

Load on wall/metre length = 11500 kg

Assuming 10% of load–Self Weight = 1150 kg

Total load on the soil = 11500 + 1150

= 12650 kg/m length

Pressure on soil

This is less than SBC of the soil, hence satisfied.

Using 1:4:8 concrete the modulus of rupture f = 2.45 kg/cm² and taking offset of concrete = 15 cm,

As the structure is lightly loaded a bed thickness of 15 cm is considered. Design features are shown in Fig. 15.23.

Figure 15.23

Example 15.2

Design an isolated footing to carry a brick-pillar of 300 mm square. The load transmitted at the top of footing is 140 kN. The bearing capacity of the soil at the location is 150 kN/m². The unit weight of soil is 18.5 kN/m³. The angle of repose, is . The base concrete is of 1: 3: 6 mix plain concrete.

Solution:

Load on the footing = 140 kN

Self weight (Taking 10% of the load) =

∴ Total load on the soil = 140 + 14 = 154 kN

Considering a square base,

Let the side be adopted as 110 cm. Considering an offset of 5 cm,

Width of bottom-most course of pillar footing = 700

∴

Which is less than the SBC of the soil, hence OK.

Using 1:3:6 plain cement concrete the modulus of rupture, f = 350 kN/cm²

A minimum thickness of 15 cm is adopted

Depth of foundation,

A minimum depth of 80 cm may be adopted (Fig. 15.24).

Figure 15.24

1. A foundation is that part of the structure which is in direct contact with the ground and transmits the load of the structure to the ground.
2. Information on surface and sub-surface conditions is a more critical requirement in planning and designing the foundations of structures, dewatering systems, shoring or bracing of excavation, the materials to be used in construction, and site improvement methods.
3. Planning the ground investigation comprises of (i) available information, (ii) reconnaissance and (iii) preliminary investigation and (iv) detailed investigation.
4. Methods of exploration are indirect methods, semi-direct methods and direct methods.
5. Indirect methods consist of geophysical and sounding methods. Semi-direct methods are common boring and drilling methods combined with intermittent sampling. Direct methods are boring and sampling methods.
6. Residual soils have formed from the weathering of rocks and practically remain at the location of origin with a little or no movement of individual soil particles.
7. Transported soils are those that have formed at one location (like residual soils) but transported and deposited at another location.
8. Regional soil deposits are marine deposits, black cotton soils, lateritic soils, alluvial deposits, desert soils and boulder deposits.
9. Re-claimed soils comprise of all materials deposited on a site using various methods for different purposes.
10. Bearing capacity is the pressure developed under the foundation without introducing damaging movements in the foundation and in the super-structure supported on the foundation.
11. The bearing capacity after allowing a certain factor safety over the ultimate bearing capacity against failure (collapse) is termed as safe bearing capacity.
12. The bearing pressure which is safe against failure and at the same time does not cause settlement more than the permissible settlement is called allowable bearing capacity or allowable soil pressure.
13. A shallow foundation is one which is placed on a firm soil near the ground and beneath the lowest part of the super-structure.
14. A deep foundation is one which is placed on a soil that is not firm and which is considerably below the lowest part of the super-structure.
15. Spread footing provides the function of distributing the column load over a wide area taking care of the strength and deformation characteristics of the soil.
16. Combined footings are formed by combining two or more equally or unequally loaded columns into one footing.
17. Continuous footings carry closely spaced columns or a continuous wall such that the load distribution is uniform and load intensity is low on the supporting soil. The footings are also named as strip footings or wall footings.
18. Mat or raft foundations are characterised by the feature that columns frame into the footing in two directions.
19. Piles are slender structural members normally installed by driving by hammer or by any other suitable means.
20. Bored cast in-situ pile is formed within the ground by excavation or boring with or without the use of temporary casing and subsequently filling it with plain or reinforced concrete.
21. Piles used to compact soils are called as compaction or displacement piles.
22. Driven piles installed in pre-drilled holes are called as non-displacement piles.
23. Where the top soil is soft or too weak to support the super-structure, piles are used to transmit the load to the underlying bed rock, such piles are called end-bearing or point-bearing piles.
24. If the bed rock does not exist at a reasonable depth below the ground surface, the load is transferred through friction along the pile shaft such piles are called friction piles.
25. Some structures are subjected to uplift pressure and piles are used to resist the uplift forces which are called uplift piles or tension piles.
26. In order to resist horizontal and inclined forces in water and earth retaining structures better piles are used.
27. Application of piles for providing anchorage to sheet piles is called as anchor piles.
28. Drilled piers are structural members of relatively large diameter massive struts constructed of concrete placed in a pre-excavated hole.
29. Caissons are structural boxes or chambers that are sunk in place through ground or water.
30. Well foundation is a type of caisson which is constructed either on dry ground or over an artificially formed island.

1. List the purpose of ground investigation.
2. How do you plan a ground investigation for a multi-storeyed building?
3. What are the methods of exploration?
4. Explain the regional soil deposits.
5. Distinguish between safe bearing capacity and allowable soil pressure.
6. Discuss the factors affecting the bearing capacity of soil.
7. Mention the techniques adopted for improving the bearing capacity of the soils.
8. What factors determine whether a foundation type is shallow or deep?
9. Indicate the circumstances under which combined footings are adopted.
10. What precautions are to be taken while locating a footing (i) on a slope and (ii) adjacent to an existing structure?
11. How piles are classified based on materials?
12. Enumerate the different types of piles and describe each type briefly. Give the advantages and disadvantages of each type.(UPSC)
13. Briefly explain the probable causes of failure of pile foundations.
14. What is a grillage foundation?
15. Mention the precautions to be taken to ensure safety of foundations on expansive soil.
16. Under what ground conditions under-reamed piles are recommended.
17. Under the following field conditions, what type of foundation you would suggest:

    (i) Soil is of soft nature and the load is uniform.

    (ii) A cavity is met during excavation.

    (iii) A made – up ground.

    (iv) A new structure with five-stories to be constructed next to an old structure.

18. What are the factors you consider in the selection of a type of foundation?
19. Describe with sketches the method timbering in a trench of size 180 cm deep × 120 cm wide for laying a foundation in a moderately firm ground.(AMIE)
20. How temporary supports are made for deep foundation excavation?
21. How construction of a mat foundation is done?
22. What are the factors considered in the selection of mat foundation?
23. A six-storeyed RCC framed building has to be constructed on an old tank bed with loose soil over 12 m deep and water-logged up to 1.2 m from ground level. What type of foundation would you adopt? Explain in detail the process of constructing out such a foundation. (AMIE)