FOUNDATION IN BUILDING

CIVIL_ENGINEERING

Foundation is the part of structure below plinth level up to the soil. It is in direct contact of soil and transmits load of super structure to soil. Generally it is below the ground level. If some part of foundation is above ground level, it is also covered with earth filling. This portion of structure is not in contact of air, light etc, or to say that it is the hidden part of the structure.

         

             Fig 1 :-  Parts of a foundation

Whenever construction workers begin work on a new building, they must first assess where and how they will build the foundation.

When engineers design the foundation of a building, they must keep in mind how much the soil will settle beneath it, as well as how much weight will go on top. If calculated incorrectly, the foundation may fail and place the entire structure in peril.

Bearing capacity

The bearing capacity of soil is the maximum average contact pressure between the foundation and the soil which should not produce shear failure in the soil.

• Ultimate bearing capacity – is the theoretical maximum pressure which can be supported without failure;
• Allowable bearing capacity-  is the ultimate bearing capacity divided by a factor of safety.

Sometimes, on soft soil sites, large settlements may occur under loaded foundations without actual shear failure occurring; in such cases, the allowable bearing capacity is based on the maximum allowable settlement.

The processes executed in the foundation works

• Excavation of earth work in trenches for foundation.
• Laying out cement concrete.
• Setting out for footing.
• Reinforcement for footing and column shaft and formwork for footing.
• Laying the footing in case of raft or column construction.
• Lying Anti termite treatment.
• Setting out and formwork for column shaft.
• Laying Column shaft work up to plinth level.
• Laying Damp proof course on the walls.
• Refilling of earth around the walls
• Refilling of earth in the building portion up to the required height according to plinth level

 

Fig 2:-  Procedure for foundation

LOADS ON BUILDINGS-

The occupant load describes the number of square feet allocated to each person within a building.

Dead loads

• Vertical loads due to weight of building and any permanent equipment
• Dead loads of structural elements cannot be readily determined b/c weight depends on size which in turn depends on weight to be supported initially weight must be assumed to make a preliminary calculation, then actual weight can be used for checking the calculation
• Easily calculated from published lists of material weights in reference sources
• IBC requires floors in office buildings and others with live loads of 80psf or less here partition locations are subject to change be designed to support a minimum partition load of 20psf & is considered part of the live load.

Total load = Dead load + Live/Imposed load

SETTLEMENT OF GROUND-

Settlement of a foundation that is caused by seasonal factors is especially noticeable during the hot dry summer months.  Below ground level, depending upon climate and environmental conditions. The drying of the soils occur because of both evaporation and transpiration (water being removed because of trees and shrubs). As the soils dry, they tend to consolidate; as they consolidate, many times, a slab-on-grade foundation settles.

Rankine’s Formula

d = 

?- Bearing capacity (safe)

W- Weight of a unit volume of earth

?- Angle of repose

d- Dept

Fig 3–Rankine’s Formula

Precautions while designing ‘Foundation’

• A foundation should be designed to transmit combined dead load, imposed load and wind load to the ground.
• Net loading intensity of pressure coming on the soil should not exceed the safe bearing capacity.
• Foundation should be designed in such a way that settlement to the ground is limited and uniform under whole of the building to avoid damage to the structure.
• Whole design of the foundation, super structure and characteristics of the ground should be studied to obtain economy in construction work.

Note: For structures being built in colder climates, engineers must consider frost heaves as well. Frost heaves occur              when moisture in the soil freezes, thereby changing the density of the building’s support. A frost heave can                     cause damage to the foundation, thereby compromising the structural integrity of the entire building.

Drier, warmer climates are not entirely exempt from such worries, however; certain soils will expand and                      contract when moisture is added or taken away, and engineers must factor in such movement when                                considering where and how to lay a foundation.

Precautions during Excavation of Foundation Work

The depth and width of foundation should be according to structural design.

• The depth of the foundation should not be less than 1 meter.
• The length, width and depth of excavation should be checked with the help of center line and level marked on the buries.
• The excavated material/ earth should be dumped at a distance of 1 meter from the edges.
• Work should be done on dry soil.
• Arrangement of water pump should be made for pumping out rain water.
• The bottom layer of the foundation should be compacted.
• There should be no soft places in foundation due to roots etc.
• Any soft/ defective spots should be dug out and be filled with concrete/ hard material

Fig 4–Excavation

Footing

Footings that support walls and isolated pad footings that support concentrated loads. The footings themselves are usually made of concrete and its wide bases placed directly beneath the load-bearing beams or walls.

Fig 5–Footing

THE FUNCTION OF FOOTINGS AND FOUNDATIONS

The function of a structure is to do nothing. The most successful structures stay still. We can look at footings and foundations as having two functions:

• Transfer Loads – To transfer the live and dead loads of the building to the soil over a large

enough area so that neither the soil nor the building will move.

• Resist Frost – In areas where frost occurs, to prevent frost from moving the building

Types of foundations and their uses

There are different types of foundation designs and each serves a different specific purpose, but generally, every one works to transfer the weight load of a structure to the soil beneath. Types of building, nature of soil and environmental conditions are the major determinant of type of foundation of the building.

Shallow foundation (spread FDN):

Most small and medium homes are built upon a shallow foundation (spread FDN). These are usually comprised of concrete strips that are laid about 3.3 feet (1 meter) beneath the soil, or of a single large concrete slab that is also set about 3.3 feet (1 meter) beneath the soil. The considerations for a shallow foundation, engineers must consider weight and settlement, as well as scour  water eroding soil beneath the structure. Can be classified as spread footings, wall and continuous (strip) footings, and mat (raft) foundations.

Deep foundation:

Larger buildings use a deep rather than a shallow foundation. A deep foundation uses long pylons of steel or concrete to penetrate beyond the weaker surface soils into the deeper and more stable soils or bedrock beneath. The load from the walls above is transferred deep into the earth, thereby providing support for the intense weight above.

Spread Foundation

1. Strip foundation–

• This is the most common type, it is mainly used where you have strong soil base and non-waterlogged areas. Most small buildings of just a floor are constructed with this type of foundation .
• Depends on the structural engineers recommendation , the depth of your foundation could be from 600mm to 1200mm mostly for small scale buildings .
• When the soil is excavated, a level at which the concrete will settle evenly is established, then concrete is poured this may be from 150mm(6”) thick to 450mm(18”) thick depending also on building after that block is set round the trenches at the center of foundation ,the foundation usually follows the block lines. The blocks are then layed to d.p.c level before another concrete is poured on top this is the german or oversite concrete. This type seems to be the cheapest.

Fig 6- Strip foundation

2. Pad foundation–

• This is where isolated columns (pillars) are casted from the foundation to carry a slab at the top of the ground.
• This is mostly used when you want to make use of the under of building as parking space or when the other space is not conducive to have foundation. This columns are thus isolated and their foundations are referred to as pad.

Fig 7- Pad foundation

     3. Raft foundation–

Raft foundation is a thick concrete slab reinforced with steel which covers the entire contact area of the                           structure like a thick floor. This concrete transfers loads from walls and columns to the underlying rock or soil.             That is laid on a soft ground consisting of an extended layer (soil are sandy and loose). It is also recommended             in waterlogged areas Sometimes area covered by raft may be greater than the contact area depending on the                 bearing   capacity of the soil underneath. The reinforcing bars runs normal to each other in both top and                       bottom layers of steel reinforcement.

Fig 8- Raft foundation

Combined footing

Combined footing is foundations supported more than one column and useful when its two columns are so close that single footings cannot be used or is located at or near a property line. Combined footing is usually support two or three columns not in a row. Combined footings are used where the bearing areas of closely spaced columns overlap.

Fig 9- Combined footing

Ramp Foundation

Ramps are an important feature in accessing a home or agricultural building. This applies not only to people who use wheelchairs but also to those who have difficulty climbing stairs, such as people who have arthritis or hemiplegic and those who use walkers, crutches or canes. To be safe and most effective, ramps should be built with a few basic guidelines in mind.

While constructed ramp the slope is extremely important because it affects how difficult it is to travel up and down the ramp. If the slope is too steep, the ramp may be too difficult for someone to use or may even be unsafe. The 1 to 12 slope should be seen as the steepest slope to be built and may be too steep for some people.

Pile foundation

The most expensive and the strongest type of foundation, this requires specialist engineering to do. The soil are bored deep down the earth and filled with concrete to be able to support loads of multistory building on top. Most skyscrapers are constructed with this foundation type, a waterlogged area of high building may also require this. It is the costliest hence it is used for high rise building mostly.

Type of Pile Foundation, Problems , Solution , Classification 

  Fig 10-  Drilled Piles, Drilled Shafts

Pier

Pier,  in building construction, vertical loadbearing member such as an intermediate support for adjacent ends of two bridge spans. In foundations for large buildings, piers are usually cylindrical concrete shafts, cast in prepared holes, while in bridges they take the form of caissons, which are sunk into position. Piers serve the same purpose as piles but are not installed by hammers and, if based on a stable substrate, will support a greater load than a pile. Especially adapted to large construction jobs, pier shafts having widths of more than 1.8 m (6 feet) have been excavated to depths greater .

Grillage foundation-

Stepped Foundation-

On sloping sites the so-called stepped foundation must be used, which is in fact just a special form of the strip foundation.

 

Cantilever Foundation-

Cantilever footings are designed to accommodate eccentric loads.

 

 

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ULTIMATE BEARING CAPACITY DUE TO VERTICAL ECCENTRIC LOAD

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Based on several laboratory model tests, Meyerhof (1953) has suggested a semiempirical procedure to determine the ultimate bearing capacity of shallow foundations due to eccentric loading condition. Eccentric loading of shallow foundations occurs when a vertical load Q is applied at a location other than the centroid of the foundation (Fig. 4.36a), or when a foundation is subjected to a centric vertical load of magnitude Q and a momentum M (Fig. 4.36b). In such a case, the load eccentricities may be given as

 

Figure 4.36: Eccentric load on shallow foundations.

and

 

where

e L,e B	=	load eccentricities, respectively, in the direction oflong and short axes of the foundation
MB,M L	=	moment about the short and long axes of the founda-tion, respectively

According to Meyerhof (1953), the ultimate bearing capacity q _uand ultimate load Q _u of an eccentrically load foundation (vertical load) can be given as

and

 

where

A ?	=	effective area = B ” L“
B ?	=	effective width
L ?	=	effective length

The effective area A ? is a minimum contact area of the foundation such that its centroid coincides with that of the load. For one-way eccentricity, that is, if e _L=0 (Fig. 4.37a), then

Figure 4.37: Case of one-way eccentricity of the load on the foundation.

However, if e _B=0 (Fig. 4.37b),…

ECCENTRICALLY LOADED FOOTING

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ECCENTRICALLY LOADED FOOTINGS.

The footings are so designed and proportioned that the C.G. of the superimposed load coincides with the C.G. of the base area, so that the footing is subjected to concentric loading, resulting in uniform bearing pressure. However, in some cases, it may not be possible to do so. For example, if the wall (or column) under construction is near some other property, it will not be possible to spread the footing to both the sides of the wall or column. Such a situation is shown in Fig. 3.7.

FIG. 3.7 ECCENTRIC LOADING.

 

Let this resultant load have an eccentricity e with respect to the centre of base width B. This eccentric weight
is equivalent to (i) a centrally placed load W and (ii) bending moment M = W . e.

Due to these two, a trapezoidal soil pressure diagram, having pressure intensities q1 and q2will result.

 The magnitude of q1 should not exceed the safe bearing pressure for the soil. Also in order that the footing may remain in contact with soil, q2 should be positive (i.e. no tension should be developed). In the extreme case, q2 = 0, when e = B/6. This gives the maximum value of eccentricity. In that case

average pressure on the foundation. If e is greater than B/6 tension will be developed, in which case, the end B of the footing will have loose contact with the soil.

TYPES OF SHALLOW FOUNDATIONS

VESICS BEARING CAPACITY THEORY

BEARING CAPACITY OF SHALLOW FOUNDATION

BEARING CAPACITY FOR SHALLOW FOUNDATIONS

The bearing capacity of a shallow foundation can be defined as the maximum value of the load applied, for which no point of the subsoil reaches failure point (Frolich method) or else for which failure extends to a considerable volume of soil (Prandtl method and successive).

Prandtl, has studied the problem of failure of an elastic half-space due to a load applied on its surface with reference to steel, characterizing the resistance to failure with a law of the type:

τ=c+σ·tanφ valid for soils as well

Prandtl assumes:

• Weightless material, therefore γ=0;
• rigid – plastic behaviour;
• resistance to failure stated as: τ=c+σ·tanφ;
• uniform vertical load applied to an infinitely long strip of width 2b (Plane strain case);
• no tangential stresses on interface between load strip and bearing surface;
• no overload the edges of the foundation (q’=0).

Upon failure the yield of the material within the space GFBCD is verified. Within the triangle AEB failure occurs according to two families of straight segments inclined by 45°+φ/2 to the horizontal.
Within the triangle AEB failure occurs according to two families of straight segments inclined by 45°+φ/2 to the horizontal.
Within zones ABF and EBC failure occurs along two families of lines, the ones made up of straight lines passing through points A and E, and the other consisting of arcs of families of logarithmic spirals. The poles of these are points A and E. In the triangles AFG and ECD failure occurs along segments inclined at ± (45°+ φ/2 ) to the vertical.

Solution of Prandtl

Having thus identified the soil tending to failure by application of the ultimate load, this can be calculated expressing the equilibrium between the forces acting in any volume of soil whose base is delimited by whichever slip surface.
Thus one reaches the equation q =b·c, where the coefficient B depends only upon the soil’s angle of friction φ.

b=cotgφ·[e^(π·tanφ)·tan^(2)(45+φ/2)-1]

For φ =0 coefficient b is 5.14, and therefore q=5.14 · c.

In the alternate case, namely that where the soil is cohesionless (c=0, φ≠0)  q=0, so that according to Prandtl it would not be possible to apply any load to cohesionless soils.
Based on this theory, admittedly of little practical value, all the various investigations and developments have proceeded.
Caquot proceeds from the same premises as Prandtl excepting that the load strip is no longer placed on the surface but at a depth of h ≤ 2b;  the soil between surface and depth h has the following characteristics: γ≠0, φ=0, c=0 i.e. that it is material with weight attribute but no resistance.

The equilibrium equations thus resolve to:

q=A·γ1+b·c

which is certainly a step forward but hardly reflects reality.

Terzaghi (1955), continues on the same lines as Caquot but adds modifications to take into account of the real characteristics of the foundation-soil system.
Under the action of the load transmitted by the foundation, the soil at the contact with the foundation tends to move laterally, but is restrained in this by the tangential resistances that develop between the soil and the foundation. This results in a change of the stress state in the ground placed directly below the foundation.
Terzaghi assigns to the sides AB and EB of Prandtl’s wedge, an inclination ψ to the horizontal, assigning to this a value as a function of the mechanical characteristics of the soil at the contact soil-foundation.

Thus γ2 =0 for soil below the foundation is reviewed assuming that the failure surfaces remain unaltered, the expression for ultimate load becomes:

q=A·γ·h+b·c+C·γ·b

in which, is a coefficient that is a function of the angle of friction φ of the soil below the footing and of the angle φ defined above; and b is the half width of the strip..

Further on the basis of experimental data, Terzaghi introduces factors due to the shape of the foundation. Again Terzaghi refines the original hypothesis of Prandtl who considered the behaviour of soil as rigid–plastic.Terzaghi instead assigns such behaviour only to very compact soils.

In these soils the curve loads/settlements is at first linear, followed by a short curved segment (elastic-plastic behavior). Failure is instantaneous and the value of the ultimate load is easily identifiable (general failure). In a very loose soil however the relation loads/settlements has an accentuated curved line even at low levels of load due to a progressive failure of the soil (local failure) and thus the identification of ultimate load is not so clear like for compact soils.. For very loose soils therefore Terzaghi introduces in the previous formula the reduced values for the mechanical properties of the soil:

tanφrid=(2/3)·tanφ e crid=(2/3)·c

Thus Terzaghi’s formula becomes:

Bearing-capacity equations by the several authors indicated

Meyerhof (1963) proposed a formula for calculation of bearing capacity similar to the one proposed by Terzaghi but introducing further foundation shape coefficients.
He introduced a coefficient sq that multiplies the Nq  factor, depth factors  di and inclination factors ii  depth factors di and inclination factors ii for the cases where the load line is inclined to the vertical. Meyerhof obtained the N factors by making trials on a number of BF arcs (see Prandtl mechanism) whilst shear along AF was given approximate values.

Shape, depth, and inclination factors for the Meyerhof bearing-capacity

Hansen’s (1970) formula is a further extension on Meyerhof’s; the additions consists in the introduction of bi that accounts for the possible inclination of the footing to the horizontal and a factor gi for inclined soil surface
Hansen’s formula is valid for whatever ratio D/B and therefore for both surface and deep foundations, however the author introduces coefficients to compensate for the otherwise excessive increment in limit load with increased depth.

Vesic (1975) proposes a formula that is analogous to Hansen’s with Nq ed Nc as per Meyerhof and Nγ as below:

Nγ=2·(Nq+1)·tanφ

Shape and depth factors are the same as Hansen’s but there are differences in load inclination, ground inclination and footing inclination factors.

Shape and depth factors for use in either the Hansen (1970) 
or Vesic (1975) bearing-capacity equations.

Table of inclination, ground, and base factors for the Hansen (1970) equations

Table of inclination, ground, and base factors for the Vesic (1975) equations

Brich-Hansen (EC7-EC8) “In order that a foundation may safely sustain the projected load in regard to general failure for all combinations of load relative to the ultimate limit state, the following must be satisfied: 

Vd≤ Rd

Where Vd the design load at ultimate limit state normal to the footing, including the weight of the foundation it self and Rd is the foundation design bearing capacity for normal loads, also taking into account eccentric and inclined loads. When estimating Rd for fine grained soils short and long term situations should be considered.”
Bearing capacity in drained conditions is calculated by:

R/A’=(2+π)·cu·sc·ic+q

Where:

A’ = B’·L’   design effective foundation area. Where eccentric loads are involved, use the reduced area at whose center the load is applied.

cu    undrained cohesion
q     total lithostatic pressure on bearing surface
sc      foundation shape factor
sc     1 + 0,2 (B’/L’) rectangular shapes
sc     1,2 square or circular shapes
ic     correction factor for inclination due to a load H

ic=0.5·[1+(1-H/(A’·cu)^0.5]

Design bearing capacity in drained conditions is calculated as follows:

R/A’ = c’ ·Nc ·sc ·ic + q’· Nq ·sq ·iq + 0,5· g’ ·B’ ·Ng ·sg·ig

Where:

Nc= same as Meyerhof (1963) above
Nq= same as Meyerhof (1963) above
Nγ=2·(Nq-1)·tanϕ

Shape factors
sq = 1+(B’/L’) ·sinϕ’ rectangular shape
sq = 1+sinϕ’ square or circular shape
sγ =1-0.3·(B’/L’) rectangular shape
sγ =0.7 square or circular shape
sc= (sq ·Nq-1)/(Nq-1) rectangular, square, or circular shape.

In addition to the correction factors reported in the table above will also be considered the ones complementary to the depth of the bearing surface and to the inclination of the bearing surface and ground surface (Hansen).

Sliding considerations
The stability of a foundation should be verified with reference to collapse due to sliding as well as to general failure. For collapse due to sliding, the resistance is calculated as the sum of the adhesion component and the soil-foundation friction component. Lateral resistance arising from passive thrust of the soil can be taken into account using a percentage supplied by the user. Resistance due to friction and adhesion is calculated with the expression:

FRd = Nsd ·tanδ+ca ·A’

In which Nsd is the value of the vertical force, δ is the angle of shearing resistance at the base of the foundation, ca is the foundation-soil adhesion, and A’ is the effective foundation area. There where eccentric loads are involved, use the reduced area at whose centre the load is applied.

Bearing capacity for foundations on rock
Where foundations rest on rock, it is appropriate to take into consideration certain other significant parameters such as the geologic characteristics, type of rock and its quality measured as RQD. It is the practice to use very high values of safety factor for bearing capacity of rock and correlated in some way with the value of RQD (Rock quality designator). For example for a rock whose RQD is up to a maximum of 0.75 the safety factor oscillates between 6 and 10. Terzaghi’s formula can be used in calculation of rock bearing capacity using friction angle and cohesion of the rock or those proposed by Stagg and Zienkiewicz (1968) according to which the coefficients of the bearing capacity are:

Nq=tan^6(45+φ/2)
Nc=5·tan^4(45+φ/2)
Nγ=Nq+1

These coefficients should be used with form factors from the formula of Terzaghi. Ultimate bearing capacity is a function of RQD as follows:

q’=qult(RQD)^2

If rock coring does not render whole pieces (RQD tends to 0) the rock is treated as a soil estimating as best the factors:  c and φ.

Extract of the technical report of  LOADCAP

HANSEN’S BEARING CAPACITY THEORY

Hansen’s Bearing Capacity Theory

Hansen 1970 proposed a general bearing capacity equation. This equation is widely used because the equation can be used for both shallow as well as deep foundation. Full scale test on footings has indicated that the Hansen equation gives better correlation than the Terzaghi’s equation. Terzaghi’s equation is known to give conservative results. However, it is still in wide use for its simplicity. The proposed form of the equation is:

Qult = ½ Bγ Nγ sγ dγ iγ gv bγ + qNq sq dq iq gq bq+ cNc sc dc ic gc bc   ———- (1)

Table 1 Shape, Depth, load Inclination, Ground and Base Inclination factors. Use term with prime factor when ɸ = 0

Shape Factors	Depth Factors	Inclination Factors	Other Factors Ground factors (base on slope)
Sc‘ = 0.2 B/L   Sc = 1+(Nq/Nc).(B/L)   Sc = 1 for strip	dc‘ = 0.4 k   dc = 1 + 0.4k k = D/B for D/B≤ 1, k = tan-1D/B for [D/B>1] k in radians	ic‘ = 0.5 – 0.5√(1-H/Afca) ic = iq – (1-iq)/(Nq – 1)	gc‘ = β0/1470   gc = 1 – β0/1470
Sq = 1 + B/L sinɸ	dq = 1+2tanɸ×(1-sinɸ)2k k defined above	iq = [1-(0.5H)/(V+ Afca cotɸ)]5	gq = gγ = (1-0.5 tanβ)5
Sq = 1-0.4B/L	dγ = 1 for all ɸ	iγ=[1-(0.7H)/(V+ Afca cotɸ)]5 iγ = [1-{0.7H –(η0/4500)5} /{V+ Afcacotɸ}]a2	Base factors (tilted base) bc‘= η0/1470 bc = 1- η0/1470 bq = e^(-2η tanɸ) bγ = e^(-2.7η tanɸ) η in radians

Where s, d, i, g, b are the shape, depth, inclination and ground factors. For pure cohesive soil the above equation takes the form of:

Qult = cNc(1 + sc +dc – ic – gc – bc) + q   ———- (2)

The bearing capacity factors are given by:

Nc = (Nq – 1) cotɸ

Nq = [e^(Ï€tanɸ)]tan2(450 – ɸ/2)

Nγ = 1.5(Nq – 1)tanɸ

Hansen’s shape, depth and other factors are given in Table 4.1 below. Hansen’s equation also takes into consideration of base tilting and footings on slopes. When the values used in the inclination equations has the horizontal load component H parallel to B, one should use B’ with the Nγ term in the bearing capacity equation and if H is parallel to L use L’ with Nγ. For a footing on clay with ɸ=O compute ic using H parallel to B and/or L as appropriate and note that it is a subtractive constant in the modified bearing capacity equation. When the base is tilted, the component H and V are perpendicular and parallel to the base respectively as compared with when it is horizontal. For footing on slopes gi factors are used to reduce the bearing capacity.

Note:

iq, iγ > 0

Af = Effective footing area B‘×L‘ for eccentric loading ­

Ca = Adhesion to base = cohesion or a reduced value

D = Depth of footing (Used with B and not B’)

eB, eL: Eccentricity of load with respect to center of the footing area

H = Horizontal component of the footing load with H ≤ Vtanδ + caAf

V = Total vertical load on footing

Î’ = Slope of ground away from base with downward (+)

Δ = Friction angle between base and soil, δ = ɸ for concrete on soil

η = Tilt angle of base from horizontal with (+) upwards as usual case

General Case

1. Do not use si in combination with ii
2. Can use si in combination with di, gi, and bi
3. For L/ B less than or equal to 2 use ɸu
4. For L/B> 2 use ɸps = 1.5 ɸu – 17
5. For ɸ <1 340 Use ɸps = ɸu