SHALLOW FOUNDATION

SHALLOW FOUNDATION Introduction – Location and depth of

foundation – Codal provisions – bearing capacity of shallow foundation on homogeneous deposits – Terzaghi‟s formula and BIS formula – factors affecting bearing capacity – problems – Bearing capacity from in-situ tests (SPT, SCPT and plate load)Allowable bearing pressure – Seismic considerations in bearing capacity evaluation. Determination of Settlement of foundations on granular and clay deposits – Total and differential settlement – Allowable settlements – Codal provision – Methods of minimizing total and differential settlements.

INTRODUCTION- (VNS Murthy-Advanced Foundation Engineering)Foundation is the part of the structure

which serves exclusively to transmit loads from the structure on to the sub-soil.

If the structure of soil lying close to ground surface possess adequate power to take loads –Foundations are laid at shallow depth

If the upper strata is too weak or loads need to be carried to deeper depths –Piles, piers etc

Two foundations- Shallow and deep

Shallow foundation Deep foundation

The ratio of depth of embedment to width of foundation does not exceed 1

D/B>15 or L/D >15

Load is transferred to the soil which lies immediately below the foundation.

Partly by skin friction and partly by point load

They are constructed in open excavation in visible manner

Installed in the interior of earth unaided by visible inspection

Extent of soil disturbance is limited to very small zone

Larger zone of soil is affected extending over entire length

Minimum dimension of foundation(Kaniraj)

Minimum width of footing 1. B> 2w+30 cm (B and w in cm)B- width of footingw- width of wall or columnD- depth of footingd- Thickness of footingde- Edge thickness of tapered footing

Maximum width from consideration of load transfer

1.B< w+d for brick and stone masonry2. B < w+(4/3)d for lime concrete3. B < w+2d for cement concreteMinimum thickness at the edge ofReinforced or plain concrete footing=15 cmMin depth of foundation – 50 cm(except on rock or weather resistant Ground)

Requirements for satisfactory foundation

Location and depth IS 1904-1986 CODE OF PRACTICE FOR DESIGN AND CONSTRUCTION OF FOUNDATIONS IN SOILS : GENERAL REQUIREMENTS

Stability or bearing capacity – Failures can be structural and soil

ruptureSettlement- Should not undergo excessive settlement

Foundation location and depth(VNS murthy -AFE)Location –should not affect future

expansion, and should not be affected by construction of adjoining structures

Depth of foundation depends upon sub soil strata , type of soil, size of structure, magnitude of loads, and environmental conditions

Location and depth of Foundation (ref CE 6302 hand out)

The following considerations are necessary for deciding the location and depth of foundation

As per IS:1904-1986, minimum depth of foundation shall be 0.50m.

Foundation shall be placed below the zone of The frost heaveExcessive volume change due to moisture

variation (usually exists within 1.5 to 3.5 m depth of soil from the top surface)

Topsoil or organic material Peat and Muck Unconsolidated material such as waste dump

Foundations adjacent to flowing water (flood water, rivers, etc.) shall be protected against scouring. The following steps to be taken for design in such conditions

Determine foundation type Estimate probable depth of scour, effects, etc. Estimate cost of foundation for normal and

various scour conditions Determine the scour versus risk, and revise the

design accordingly

• IS:1904-1986 recommendations

For foundations adjacent to slopes and existing structures • When the ground surface slopes downward adjacent to footing, the sloping surface should not cut the line of distribution of the load 2H:1V.•In granular soils, the line joining the lower adjacent edges of upper and lower footings shall not have a slope steeper than 2H:1V• In clayey soil, the line joining the lower adjacent edge of the upper footing and the upper adjacent edge of the lower footing should not be steeper than 2H:1V

Other recommendations for footing adjacent to existing

structuresMinimum horizontal distance between the

foundations shall not be less than the width of larger footing to avoid damage to existing structure

If the distance is limited, the principal of 2H:1V distribution should be used so as to minimize the influence to old structure

Proper care is needed during excavation phase of foundation construction beyond merely depending on the 2H:1V criteria for old foundations. Excavation may cause settlement to old foundation due to lateral bulging in the excavation and/or shear failure due to reduction in overburden stress in the surrounding of old foundation

Footings on surface rock or sloping rock faces

For the locations with shallow rock beds, the foundation can be laid on the rock surface after chipping the top surface.

If the rock bed has some slope, it may be advisable to provide dowel bars of minimum 16 mm diameter and 225 mm embedment into the rock at 1 m spacing.

A raised water table may cause damage to the foundation by Floating the structure, Reducing the effective stress beneath the foundation,Water logging around the building may also cause wet basements.

In such cases, proper drainage system around the foundation may be required so that water does not accumulate.

VNS murthy (AFE) Pg 108

Bearing capacityUltimate Bearing capacity: qu

Maximum gross intensity of loading that the soil can support against shear failure is called ultimate bearing capacity.Net Ultimate Bearing Capacity: qnu

Maximum net intensity of loading that the soil can support at the level of foundation.

qnu = qu - γ Df

Net Safe Bearing capacity: qns

Maximum net intensity of loading that the soil can safely support without the risk of shear failure.

qns = qnu / FOS

Gross Safe Bearing capacity:Maximum gross intensity of loading that the soil can safely support without the risk of shear failure

qgs = qns +γ DSafe Bearing Pressure:Maximum net intensity of loading that can be allowed on the soil without settlement exceeding the permissible limit.Allowable Bearing Pressure:Maximum net intensity of loading that can be allowed on the soil with no possibility of Minimum of capacity and shear failure or settlement exceeding the permissible limit.

Types of Failure

Plastic equilibriumA body of soil is said to be in a state of plastic equilibrium, if every part of it is on the verge of failure. So this can be visualized by a perfectly rigid plastic model where with a stress strain relationship if we assume that it is rigid and perfectly plastic. So here the stress strain behavior of the soil can be represented here by the rigid perfectly plastic idealization.

General Shear FailureExperiments have shown that foundations on dense sand with RD greater than 70 percent fail suddenly with pronounced peak when settlement reaches about 7 percent of foundation widthThis type of failure is seen in dense and stiff soil. The following are some characteristics of general shear failure.1. Continuous, well defined and distinct failure surface develops between the edge of footing and ground surface.2. Dense or stiff soil that undergoes low compressibility experiences this failure.3. Continuous bulging of shear mass adjacent to footing is visible.4. Failure is accompanied by tilting of footing.

5. Failure is sudden and catastrophic with pronounced peak in P – curve.6. The length of disturbance beyond the edge of footing is large.7. State of plastic equilibrium is reached initially at the footing edge and spreads gradually downwards and outwards.8. General shear failure is accompanied by low strain (<5%) in a soil with considerable ɸ (ɸ>36o) and large N (N > 30) having high relative density(ID > 70%).(ID- Density index or relative density) N- Standard penetration test N value

Local shear failureThis type of failure is seen in medium dense and hard to medium consistency soil. The following are some characteristics of Local shear failure.1. A significant compression of soil below the footing and partial development of plastic equilibrium is observed.2. Failure is not sudden and there is no tilting of footing.3. Failure surface does not reach the ground surface and slight bulging of soilaround the footing is observed.4. Failure surface is not well defined.5. Failure is characterized by considerable settlement.6. Well defined peak is absent in P – curve.7. Local shear failure is accompanied by large strain (> 10 to 20%) in a soilwith considerably low ɸ (ɸ <28o) and low N (N < 5) having low relativedensity (ID < 20%).

Punching Shear FailureThis type of failure is seen in loose and soft soil and at deeper elevations. The following are some characteristics of general shear failure.1. This type of failure occurs in a soil of very high compressibility.2. Failure pattern is not observed.3. Bulging of soil around the footing is absent.4. Failure is characterized by very large settlement.5. Continuous settlement with no increase in P is observed in P – curve.

General Shear Failure

Local Shear Failure

Occurs in dense/stiff soilɸ>36o, N>30, ID>70%, Cu>100 kPa

Occurs in loose/soft soilɸ<28o, N<5, ID<20%, Cu<50 kPa

Results in small strain (<5%)

Results in large strain (>20%)

Failure pattern well defined & clear

Failure pattern not well defined

Well defined peak in P- curve

No peak in P- curve

General Shear Failure Local Shear Failure

Bulging formed in the neighborhood offooting at the surface

No Bulging observed in theneighbourhood of footing

Extent of horizontal spread ofdisturbance at the surface large

Extent of horizontal spread ofdisturbance at the surface very small

Failure is sudden & catastrophic

Failure is gradual

Less settlement, but tilting failureobserved

Considerable settlement of footingobserved

TERZAGHI BEARING CAPACITY

Terzaghi’s bearing Capacity TheoryTerzaghi (1943) was the first to propose a comprehensive theory for evaluatingthe safe bearing capacity of shallow foundation with rough base.He extended the theory of PrandtlAssumptions1. Soil is semi infinite, homogeneous and Isotropic.2. The shear strength of soil is represented by Mohr Coulombs Criteria.3. The footing is of strip footing type with rough base. It is essentially a twodimensional plane strain problem.4. Elastic zone has straight boundaries inclined at an angle equal to ɸ to thehorizontal.5. Failure zone is not extended above, beyond the base of the footing. Shearresistance of soil above the base of footing is neglected.

6. Method of superposition is valid.7. Passive pressure force has three components (Ppc produced by cohesion, Ppqproduced by surcharge and Ppγ produced by weight of shear zone).8. Effect of water table is neglected.9. Footing carries concentric and vertical loads.10. Footing and ground are horizontal.11. Limit equilibrium is reached simultaneously at all points. Complete shearfailure is mobilized at all points at the same time.12. The properties of foundation soil do not change during the shear failureLimitations1. The theory is applicable to shallow foundations2. As the soil compresses, increases which is not considered. Hence fullyplastic zone may not develop at the assumed .3. All points need not experience limit equilibrium condition at different loads.4. Method of superposition is not acceptable in plastic conditions as the ground isnear failure zone.

1. Zone abc. This is a triangular elastic zone located immediately below the bottom of the foundation. The inclination of sides ac and bc of the wedge with the horizontal is ɸ(soil friction angle).2. Zone bcf. This zone is the Prandtl’s radial shear zone.3. Zone bfg. This zone is the Rankine passive zone. The slip lines in this zone make angles of (45 − ɸ/2) with the horizontal.

The ultimate load per unit area of the foundation (that is, the ultimate bearing capacity(qu) for a soil with cohesion, friction, and weight can now be given as

DETERMINATION OF BEARING CAPACITY OF

SHALLOW FOUNDATIONS

IS 6403-1981

Factors affecting bearing capacity

Effect of shape

Effect of water table

Effect of water table

Type of failure

EFFECT OF ECCENTRICITYSINGLE ECCENTRICITY- If the load has an

eccentricity eᶫ, with respect to the centroid of the foundation in only one direction, then

L’ = L – 2 eᶫA’ = L’ × B

DOUBLE ECCENTRICITY- If the load has double eccentricity (eᶫ and eᵇ )

L’ = L – 2 eᶫ B’ = B – 2 eᵇ

A’ = L’ × B’

IS Codal provision

W1

Shape, depth and inclination FACTORS

Problems

HW

BEARING CAPACITY FROM FIELD TEST

Plate load test

1. It is a field test for the determination of bearing capacity and settlement characteristics of ground in field at the foundation level.2. The test involves preparing a test pit up to the desired foundation level.3. A rigid steel plate, round or square in shape, 300 mm to 750 mm in size, 25 mm thick acts as model footing.4. Dial gauges, at least 2, of required accuracy (0.002 mm) are placed on plate on plate at corners to measure the vertical deflection.5. Loading is provided either as gravity loading or as reaction loading. For smaller loads gravity loading is acceptable where sand bags apply the load.6. In reaction loading, a reaction truss or beam is anchored to the ground. A hydraulic jack applies the reaction load.7. At every applied load, the plate settles gradually. The dial gauge readings are recorded after the settlement reduces to least count of gauge (0.002 mm) & average settlement of 2 or more gauges is recorded.8. Load Vs settlement graph is plotted as shown. Load (P) is plotted on the horizontal scale and settlement () is plotted on the vertical scale.9. The maximum load at which the shear failure occurs gives the ultimatebearing capacity of soil.

Reference can be made to IS 1888 - 1982.The advantages of Plate Load Test are1. It provides the allowable bearing pressure at the location considering both shear failure and settlement.2. Being a field test, there is no requirement of extracting soil samples. 3. The loading techniques and other arrangements for field testing are identical to the actual conditions in the field.

The disadvantages of Plate Load Test are1. The test results reflect the behaviour of soil below the plate (for adistance of ~2Bp), not that of actual footing which is generally very large.2. It is essentially a short duration test. Hence, it does not reflect the longterm consolidation settlement of clayey soil.3. Size effect is pronounced in granular soil. Correction for size effect isessential in such soils.4. It is a cumbersome procedure to carry equipment, apply huge load andcarry out testing for several days in the tough field environment.

Housel method based on Plate load test

Housel (1929) has suggested, based on extensive experimental investigations, a practical method of determining the bearing capacity of a prototype foundation in a foundation soil which is reasonably homogeneous in depth by means of two or more small-scale model tests. It is assumed that the load-carrying capacity of a foundation for a predetermined allowable settlement consists of two distinct components—one which is carried by the soil column directly beneath the foundation, and the other which is carried by the soil around the perimeter of the foundation. The first component is a function of the area and the second, a function of the perimeter of the foundation

Q = Am + Pnwhere Q = total ultimate loadA = bearing area of the foundation (m2), andP = perimeter of the foundation (m).m and n are constant

Based on Standard Penetration Resistance ValueThe standard penetration resistance shall

be determined as per IS:2131-1981 at a number of selected points at intervals of 75 cm in the vertical direction or change of strata if it takes place at earlier.

The Corrected value beneath each point shall be determined between the level of the base of the footing and depth equal to 1.5 to 2 times width of foundation

Tengs equation for bearing capacityFor Strip footing

Tengs equation based on allowable settlement

PROBLEM

Based on Static Cone Penetration valueThe static cone point resistance qc

determined as per IS:4968(part III)-1976 at number of selected points at intervals of 10 to 15 cm.

The observed values corrected for the dead weight of sounding rods

The ultimate bearing capacity of shallow strip footings on cohesionless deposits determined from following graph

Meyerhof formula based on SCPT for settlement of 25mmqa= qcs/30 for (B< 1.2 m)qa= (qcs/50)x B+0.3 B

For B>1.2 m

Bearing Capacity for cohesive Soil(when ø = 0)

Homogeneous LayerThe net ultimate bearing capacity

immediately after construction on fairly saturated homogeneous cohesive soils

qd = cNc sc dc icwhere Nc = 5.14

the value of c obtained from1. Unconfined compression

test2. Static cone test

Continuation…… If the shear strength for a depth of 2/3 B

beneath the foundation does not depart from the average by more than 50%, the average may be used in calculation

C from static cone test:

Allowable Bearing Capacity The allowable bearing capacity shall be

taken as either of the following, whichever is less:

a) Net ultimate bearing capacity divided by suitable factor of safety, that is, net safe bearing capacity

b) The net soil pressure that can be imposed on the base without the settlement exceeding the permissible values as given in IS:1904-1978 to be determined for each structure and type of soil,that is, safe bearing pressure

SettlementImmediate or elastic settlement (Si)Primary Consolidation (Sc)Secondary consolidation (Ss) Total Settlemt – Si+ Sc + Ss

(iii) Ground water lowering, especially repeated lowering and raising of ground water level in loose granular soils and drainage without adequate filter protection,(iv) Vibration due to pile driving, blasting and oscillating machinery in granular soils,(v) Seasonal swelling and shrinkage of expansive clays,(vi) Surface erosion, creep or landslides in earth slopes,(vii) Miscellaneous sources such as adjacent excavation, mining subsidence and underground erosion.

Footing in cohesionless soil reach almost final settlement during the construction itself due to high permeability. The water in voids expelled simultaneously with application of load and as such immediate and consolidation settlements in such soils are rolled into one

Methods to compute settlement1. Elastic settlementTheory of elasticityJambu et alSchmertmann’s methodPressuremeter method

2. Consolidation settlemente- log p curve from oedometer testSkempton –Bjerrum method

Construction Practices to Avoid Differential Settlement

(i) Suitable design of the structure and foundation ... desired degree of flexibility of the various component parts of a large structure may be introduced in the construction.(ii) Choice of a suitable type of foundation for the structure and the foundation soil conditions...e.g., large, heavily loaded structures on relatively weak and non-uniform soils may be founded on ‘mat’ or ‘raft’ foundations. Sometimes, piles and pile foundations may be used to bypass weak strata.(iii) Treatment of the foundation soil...to encourage the occurrence of settlement even before the construction of the structure, e.g., (a) Dewatering and drainage, (b) Sand drains and (c) Preloading. (iv) Provision of plinth beams and lintel beams at plinth level and lintel level in the case of residential buildings to be founded on weak and compressible strata.

Allowable settlementThe differential settlement should not exceed 75%

of the maximum settlementMaximum settlement range from 20 mm to 300 mm ρ> 150 mm damages the utilitiesIS 1904 (1966)- Permissible settlement Isolated footing On sand -40mm On clay -65 mmRaft On sand -40mm to 65mm

On clay – 65 to 100 mm

Differential settlement If differential settlement between two

columns placed at distance L is δ , the angular distortion or til is given by

T= δ/L

Settlement calculationSchleicher formula for elastic settlement for

footing on clays

q- uniformly distributed loadB- width of footingμ – Poisson’s ratio (0.5 for saturated clay)Es- modulus of elasticityI- influence factor

Consolidation settlement

Schmertmann method of calculating settlement in granular soil