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 footing w- width of wall or column D- depth of footing d- Thickness of footing de- Edge thickness of tapered footing

Maximum width from consideration of load transfer 1.B< w+d for brick and stone masonry 2. B < w+(4/3)d for lime concrete 3. B < w+2d for cement concrete Minimum thickness at the edge of Reinforced or plain concrete footing =15 cm Min 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 rupture Settlement- 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 heave Excessive 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 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 structures Minimum 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 capacity Ultimate 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 D Safe 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 equilibrium A 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 Failure Experiments 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 width This 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 failure This 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 soil around 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 soil with considerably low ɸ (ɸ <28o) and low N (N < 5) having low relative density (ID < 20%).

Punching Shear Failure This 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 soil ɸ>36o, N>30, ID>70%, Cu>100 kPa Occurs in loose/soft soil ɸ<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

Bulging formed in the neighborhood of footing at the surface General Shear Failure Local Shear Failure Bulging formed in the neighborhood of footing at the surface No Bulging observed in the neighbourhood of footing Extent of horizontal spread of disturbance at the surface large disturbance at the surface very small Failure is sudden & catastrophic Failure is gradual Less settlement, but tilting failure observed Considerable settlement of footing

TERZAGHI BEARING CAPACITY Terzaghi’s bearing Capacity Theory Terzaghi (1943) was the first to propose a comprehensive theory for evaluating the safe bearing capacity of shallow foundation with rough base.He extended the theory of Prandtl Assumptions 1. 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 two dimensional plane strain problem. 4. Elastic zone has straight boundaries inclined at an angle equal to ɸ to the horizontal. 5. Failure zone is not extended above, beyond the base of the footing. Shear resistance of soil above the base of footing is neglected.

6. Method of superposition is valid. 7 6. Method of superposition is valid. 7. Passive pressure force has three components (Ppc produced by cohesion, Ppq produced 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 shear failure is mobilized at all points at the same time. 12. The properties of foundation soil do not change during the shear failure Limitations 1. The theory is applicable to shallow foundations 2. As the soil compresses, increases which is not considered. Hence fully plastic 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 is near 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 DETERMINATION OF BEARING CAPACITY OF SHALLOW FOUNDATIONS

Factors affecting bearing capacity

Effect of shape

Effect of water table

Effect of water table

Type of failure

EFFECT OF ECCENTRICITY SINGLE ECCENTRICITY- If the load has an eccentricity eᶫ, with respect to the centroid of the foundation in only one direction, then L’ = L – 2 eᶫ A’ = L’ × B DOUBLE ECCENTRICITY- If the load has double eccentricity (eᶫ and eᵇ ) B’ = B – 2 eᵇ A’ = L’ × B’

IS Codal provision W1

Shape, depth and inclination FACTORS

Problems

Nc = 65.38, Nq = 49.38, N = 54 at = 36º by interpolation

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 ultimate bearing capacity of soil.

Reference can be made to IS 1888 - 1982. The advantages of Plate Load Test are 1. 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 are 1. The test results reflect the behaviour of soil below the plate (for a distance 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 long term consolidation settlement of clayey soil. 3. Size effect is pronounced in granular soil. Correction for size effect is essential in such soils. 4. It is a cumbersome procedure to carry equipment, apply huge load and carry 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 + Pn where Q = total ultimate load A = bearing area of the foundation (m2), and P = perimeter of the foundation (m). m and n are constant

Based on Standard Penetration Resistance Value The 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 capacity For Strip footing

Tengs equation based on allowable settlement PROBLEM

Based on Static Cone Penetration value The 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 25mm qa= 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 Layer Unconfined compression test Static cone test The net ultimate bearing capacity immediately after construction on fairly saturated homogeneous cohesive soils qd = cNc sc dc ic where Nc = 5.14 the value of c obtained from Unconfined compression test 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: Net ultimate bearing capacity divided by suitable factor of safety, that is, net safe bearing capacity 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

Settlement Immediate 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 settlement Elastic settlement Theory of elasticity Jambu et al Schmertmann’s method Pressuremeter method 2. Consolidation settlement e- log p curve from oedometer test Skempton –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 settlement The differential settlement should not exceed 75% of the maximum settlement Maximum settlement range from 20 mm to 300 mm ρ> 150 mm damages the utilities IS 1904 (1966)- Permissible settlement Isolated footing On sand -40mm On clay -65 mm Raft 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 calculation Schleicher formula for elastic settlement for footing on clays q- uniformly distributed load B- width of footing μ – Poisson’s ratio (0.5 for saturated clay) Es- modulus of elasticity I- influence factor

Consolidation settlement

Schmertmann method of calculating settlement in granular soil