SHALLOW FOUNDATION

FOUNDATION ENGINEERING
SHALLOW FOUNDATION

1. SHALLOW FOUNDATION MADE BY : MISS.DHARA DATTANI (ME TRANSPORTATAION) LECTURER AT ATMIYA INSTITUTE OF TECHNOLOGY AND SCIENCE FOR DIPLOMA STUDIES,RAJKOT.GUJRAT,INDIA . 1

2. CONTENTS Definitions Design criteria Methods to determine bearing capacity Modes of failures Effect of eccentric loading Settlement of foundation Bearing capacity of raft and its factor affecting Floating foundation Contact pressure 2

3. Bearing Capacity Of Shallow Foundation * A foundation is required for distributing the loads of the superstructure on a large area . * The foundation should be designed such that a) The soil below does not fail in shear b) Settlement is within the safe limits. 3

4. 1. FOOTING : Lowest part of foundation of the structure that transmits loads directly to soil 2. FOUNDATION It is a part of structure that transmits the load of super structure to the ground 3. BEARING CAPACITY Load carrying capacity of foundation soil Expressed as load per unit area kN /m 2 4. GROSS BEARING CAPACITY : (q) Total pressure acting on the base of footing including weight of superstructure ,self weight of footing and weight of fill over the footing called as gross bearing capacity 4

5. 5. NET BEARING CAPACITY : ( qN ) It is defined as gross bearing capacity minus the original overburden pressure at the foundation level. q n =q-Y *D 6. ULTIMATE BEARING CAPACITY ( qu ): It is defined as the gross pressure at the base of foundation at which the soil fails in the shear 7. NET ULTIMATE BEARING CAPACITY( qnu ) : It is defined as a net pressure in excess of the surcharge pressure at which soil fails in shear. It is equal to gross pressure minus surcharge pressure. 5 qnu = qu –  

6. 8. NET SAFE BEARING CAPACITY : ( qns ) It is defined as the maximum load per unit area in excess of surcharge pressure which the foundation soil or bed can carry safely load without risk or shear failure. Where , f= Factor of safety taken as 2 to 5. 9. Safe Bearing Capacity: ( qs ) It is the maximum gross pressure which the soil can carry safely load without shear failure. It is equal to net bearing capacity plus the original over burden pressure. 6 qnx   q s =  

7. 10. Allowable Bearing Capacity : ( qa ) It is the net loading intensity at which the foundation soil neither fails in shear nor there is excessive settlement ,to the structure. 7

8. 2. DESIGN CRITERIA For A Satisfactory Performance, A Foundation Must Be Following Criteria : 1. Location & Depth Criteria 2. Shear or Bearing Failure Criteria 3. Settlement Criteria 8

9. 1 . LOCATION AND DEPTH CRITERIA Foundation must be properly located and at a desired depth that its performance not affected by factors such as, Lateral expulsion of soil from beneath the foundation Seasonal volume changes caused by freezing and thawing 2. SHEAR OR BEARING FAILURE CRITERIA It is associated with plastic flow of the soil material beneath the foundation, and lateral expulsion of the soil from underneath the foundation. A adequate factor of safety is provided to preclude the bearing capacity failure. 3. SETTLEMENT CRITERIA The settlement of the foundation, especially the differential settlement must be in permissible limits . Excessive settlement may affect the utility of structure, spoil the appearance of structure and also damages the structure. 9

10. Methods of determining the bearing capacity Methods of determining the bearing capacity 10 I.S.CODE METHOD (I.S.:1904-1978) BEARING CAPACITY TABLES IN VARIOUS BUILDING CODES B) ANALYTICAL METHOD 1.PRANDTL’S ANALYSIS 2. RANKINE’S ANALYSIS 3. TERZAGI’S METHOD 4.SKEMPTON THEORY C) FIELD TEST METHOD PLATE LOAD TEST SPT SCPT DCPT PRESSURE METER

11. Prandtl’s A nalysis This theory is used to determine the ultimate bearing capacity of soil . The analysis is based on assumptions that a strip footing placed on ground surface sinks vertically downwards direction in to soil as failure, like a punch. Fig shows failure zones developed below footing Zone-1 : immediately under footing subjected with compressive stress . Zone -2: exerts pressure on side zones II and III Soil in zone II assumed as in plastic equilibrium Zone II pushes zone III upwards 11

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13. So prandtls gave an expression for ultimate bearing capacity for a strip footing using theory of plasticity. He assumed curved path of a slip surface of the shape of logarithmic spiral. For purely cohesive soils(ø=0) ,the spiral becomes circular arc and prandtls analysis gives the equation : c=cohesion of the soil. For cohesion less soil prandtls theory shows that UBC(ultimate bearing capacity) increases with the width B For cohesive soil of strip footing the, It is only applicable for footing with perfectly smooth base, but in actual footing have rough surface so theory does not gives exact results 13 qu =( π +2)c= 5.14*c q u = 5.14 c +  

14. Rankine’s analysis Rankine considered the plastic equilibrium of two soil elements ,one below the footing (element-I) and the other just outside the footing (element-II) at the base level of the foundation 14

15. When the load on the footing increases and approaches a value of qu , a state of plastic equilibrium is reached under the footing i.e. element I is on the verge of failure. For element I the vertical stress is the major principal stress (   σ 1 ) and lateral stress is minor principal stress (   σ 3 ).on the other hand , for element II, the lateral stress is major principal and vertical is minor principal stress. For the shear failure of element I, element II must fail by lateral thrust from Element-1,during the state of shear failure the following principal stress relationship exist, Tan 2 α + 2c tan α For cohesionless soil c=0 σ1=  σ 3 tan 2 α for element II = Tan 2 α + 2c tan α σ 3 = σ v = Tan 2 α   15

16. For element I σ 3 = σ h = σ 1 of the element II = Tan 2 α σ 1 =   σ 3 tan 2 α Tan 2 ( Tan 2 α) Tan 4 α But qu = DTan 4 α tan 2 α =   16 q u =  

17. Minimum depth of foundation obtained by q= intensity of loading at the base of foundation. 17 D min = [ ]  

18. MODES OF SHEAR FAILURE When horizontal strip footing resisting on the earth surface , homogeneous soil is subjected to a gradually increasing load, characteristic load settlement curves are obtained . The load settlement behaviour is found to be related to the soil characteristic. Three types of failure are considered: 1. GENERAL SHEAR FAILURE 2. LOCAL SHEAR FAILURE 3. PUNCHING SHEAR FAILURE 18

19. General shear failure If the soil properties are such that as the footing is loaded to failure a slight downward movement of the footing develops the fully plastic zone due to which the entire soil along a slip surface fails in shear and the soil is bulges out on the sides of the footing , this type of failure is called General shear failure. It usually occurs in dense sand or stiff clay. 19

20. Local shear failure If the soil properties are such that before the plastic zone are fully developed , large deformation occur immediately below the footing resulting in the shear failure of the soil in the potion just below footing is called a Local shear failure The local shear failure usually occurs in the case of sand or clayey soils of medium compaction 20

21. Punching shear failure It occurs when there is a high compression of soil under the footing,accompanied by shearing in the vertical direction around the edges of the footing. It is slightly inclined and never reaches to the ground surface. There is no tilting of the footing. It is mainly occurred in loose or soft clay. 21

22. Terzagi’s bearing capacity theory In 1943 terzagi gave an assumption on bearing capacity of soil Assumptions : The footing is long : i.e. L/B ratio is infinite The base of footing is rough The footing is laid at shallow depth i.e. D<B The load on footing is vertical and load is uniformly distributed The shear strength of the base is neglected Soil above the base is neglected by surcharge pressure The shear strength of the soil is governed by mohr -Columbus equation. 22

23. 23 TERZAGI ANALYSIS IMAGE OF STRIP FOOTING

24. Strip footing of width B subjected to the loading intensity qu to cause failure The footing is shallow i.e. the depth D of the footing is equal to or less than width of the footing According to the terzagi the loaded soil fails along the composite surface FGCDE as shown in fig. this region can be divide into five zones: Zone I , called zone of elastic equilibrium(ABC ) Zone II , called zone of radial shear Zone III , called zone of linear shear When the base of the footing AB sinks into the ground zone I wedge shaped immediately beneath the footing is prevented from undergoing any lateral yield by friction and cohesion between the soil and the base of the footing Thus zone I remains in state of elastic equilibrium and it acts as if it were the part of footing Its boundaries assumed as plane surface rising at an angle with the horizontal 24

25. Zone II (BCD and ACG) are called the zones of radial shear, because the lines that constitute one set of shear pattern are straight radial lines which radiate from the outer edges of the base of the footing, while the lines of the other set are logarithm spirals with their centres located at the outer edge of the base of the footing. Thus the boundaries AC,AG,BC and BD of these zones are the plane surfaces while the boundaries CG and CD are the arcs of a logarithm spiral Zone III AFG,BDE are called the zones of linear shear and are triangular in shape. These are passive rankine zone with their boundaries making an angle with the horizontal 25  

26. It is assumed that the failure do not extend above the horizontal plane passing through base of the footing. This implies that the shear resistance of the soil lying above the horizontal plane passing through the base of footing is neglected, and the effect of the soil above the plane is taken equivalent to a surcharge pressure The application of the zone surface qu on the footing tends to push the wedge of soil ABC into the ground with lateral displacement of zones II and III, but this lateral displacement is resisted by forces on the plane AC and BC 26

27. For ultimate bearing capacity of soil for strip footing q u =c* Nc + *Nq+0.5 *B*N Where, q u = UBC in kN /m2 C= cohesion/m2 = unit weight of soil kN /m3 D= depth of footing (m) B= width of footing(m) Nc,Nq,Ny = terzagi bearing capacity factors Nc ’,Nq’, Ny ’ depends on the angle of shearing resistance( ø )of the soil.   27

28. Nq= a= ) tanø Nc = [Nq-1] cotø Ny = - 1] Kp = passive earth pressure depending upon ø   28

29. For cohesiveless soil ø=0 Nc =1.5 π +1=5.7 Nq=1 Ny =0 Thus, for strip footing q u =5.7c+ *D If footing located at ground surface D=0 q u =5.7c   29

30. Equation for local for shear failure for strip footing can be written as When local shear failure ø<29 qu = * C* Nc ’+ *Nq’+0.5 *BN ’ Nc ’,Nq’, Ny ’ depends on the angle of shearing resistance( ø )of the soil. Or equal to ø’   30

31. Terzagi gave different equations for different footing Square footing: qu = 1.3cNc+ * Nq+ 0.4 *B*Ny B= dimension of side of footing Circular footing qu = 1.3c* Nc + * Nq+ 0.3 *B*Ny B= diameter o f footing Rectangular footing qu = 1.3cNc ( 1+ 0.3 *D*Nq+0.5 *B*N ( 1 – 0.2 ) B= width or diameter L= length of footing   31

32. EFFECT OF WATER TABLE ON BEARING CAPACITY Terzagi assumed the water table is at greater depth. If the W.T. is located close to the foundation ,the bearing capacity equation written as follows : qu= c.Nc + 1*D*Nq*Rw1+0.5 2*B*N *Rw2 Where, Rw1 and Rw2 are the reduction factors for the water table 1= average unit weight of the surcharge soil situated above water table 2= avg unit weight of the soil in the wedge zone, situated within the depth below the footing.   32

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34. Case 1 Water table above the base of footing Rw1 = 0.5 [ 1+ Rw2=0.5 Case 2: Water table below the base of footing Rw1 = 1 Rw2 = 0.5 [ 1+ Case 3: when water table at a depth equal to or greater than the width of footing below the base of foundation Rw1=1 Rw2= 1   34

35. Skempton’s analysis Skempton’s analysis used for purely cohesive soil ø=0 And for purely saturated soil Can be used for shallow footing as well as deep footing For pure cohesive soil : q u =c* Nc + *Nq+0.5 *B*N.. Given by terzagi ø=0 Nq=1 Nc =0   35

36. As per skempton UBC= cNc + Qu=c* Nc + *D The Net.U.B.C qnu = qu - D Different shape of footing: (A) STRIP FOOTING: Nc =5[1+0.2 [ ] If Nc value is greater than 7.5 , take Nc =7.5 (B) Square or Circular Footing: Nc =6[1+0.2 [ ] If Nc is greater than 9.0 take Nc =9 B= square of footing or dia of footing   36

37. Rectangular footing : For Nc =5[1+0.2 [ ] +[1+0.2 [ ] For Nc =7.5[1+0.2 [ ] Where, B= width of rectangular footing L= Length of rectangular footing   37

38. I.S. Code Method As per the Is-code 6403-1981: qnu= cNc * Sc *dc* ic+q (Nq-1) Sq * dq *iq+0.5 BN *S d i W’ Nc * Sc *dc* ic+q (Nq-1) Sq * dq *iq+0.5 BN S d i W’ Where, dc,dq,d = depth factors Iq,ic,i = inclination factors Nc,Nq,N = bearing factors recommended by Vesic (1973) q= effective pressure at base qnu = qu - *D = qu -q All notation defined earlier.   38

39. When W.t. is at below depth W’=1.0 If above W’=0.5 If same size or equal to depth W’=0.5[1+ ]   39

40. Settlement of Foundation A geotechnical engineer is called upon to predict the magnitude of settlement and rate of settlement of foundation due to structural loads There are mainly three types of settlements 1. Immediate settlement 2. primary consolidation 3. Secondary consolidation 40

41. Immediate settlement When a partially saturated soil mass is subjected to external loads, a decrease in volume occurs due to expulsion and compression of air in the voids. Decrease in volume of soil mass due to compression of solid particles called as immediate or initial settlement 41

42. Primary consolidation ( sp ) After the initial consolidation, further reduction in volume of soil mass due to expulsion of water from air voids, called as Primary consolidation. In fine grained soil, sp occurs for long time where as in coarse grain soil it takes less time due to high permeability. 42

43. Secondary consolidation The reduction in volume continues at a very slow rate even after the excess pore water pressure developed by the applied pressure is fully dissipated and the primary consolidation is complete. This reduction in volume called as Secondary consolidation. This settlements are important for peats and soft organic clay Settlement is expressed as S= si+sc+ss 43

44. Sources of settlement of foundation Elastic compression of the foundation soil, caused by giving rise to immediate settlement Plastic compression of the foundation soil, caused by loads giving rise to consolidation settlement of fine grained soils Repeated lowering and raising of ground water level in granular loose soils Under ground erosion may cause cavity in subsoil which collapse and cause settlement Seasonal swelling and shrinkage and expansion of soil Vibrations due to pile driving Surface erosion landslides 44

45. Due to some minerals in soil example-non cohesive soil,gypsum,may result the settlement of foundation and alsoresponsible for the intergranulars bond of soil Frost heave occurs if the structure is not found below the depth of frost penetration,when thaw occurs the foundation may settle 45

46. Bearing capacity of Raft A mat or raft foundation is a thick reinforced concrete slab which supports all the load of bearing walls columns of structure etc. When raft should be provided? Soil pressure is low Loads of structure are heavy When columns and walls are so close to structure that individual footing would overlap When there is large variation of load in individual columns When soil is non homogeneous and also when differential settlements chances are more. 46

47. FACTORS AFFECTING BEARING CAPACITY Position of ground water table Relative density or angle of shearing resistance Width of footing Depth of footing Unit weight of soil Eccentricity of load 47

48. Floating Foundation Floating foundation is defined as foundation in which the weight of the building is approximately equal to the full weight of soil and water removed from the site of the building 48

49. The principle of floating foundation is explained that a horizontal ground surface with a horizontal water table at a depth Dw below the ground surface shows an excavation made in groundto a depth D where D> dw ,fig shows that structure built in an excavation and completely filling it. If the weight of the building is equal to weight of soil and water removed from the excavation ,then it is evident that the total vertical pressure in the soil below depth D is same as before excavation Since the water level has not changed neutral pressure and effective pressure are therefore unchanged. Since the settlements caused by an increase in vertical pressure the building will not settle at all. 49

50. FLOATING FOUNDATION Example 1 Example 2

51. While dealing with floating foundation two type of soil are there: Soil type -1 Shear failure will not occur but settlements or differential settlements will occur due to building loads. Soil type 2 : Shear strength of foundation is low that the rupture of the soil would occur if the building were founded at ground leve l In absence of strong layer at a reasonable depth, the building can only built on a floating foundation which reduces the shear stresses to an acceptable value. Solving this problem solves the settlement problem 51

52. FLOATING FOUNDATION   Floating foundation was a necessary innovation in the development of tall buildings in the wet soil of Chicago in the 19th century, floating foundation was developed by John Wellborn Root .

53. Contact pressure A vertical pressure acting at the surface of the contact between the base of footing and the underlying soil mass. In design it is assumed that the load on footing is uniform also the distribution of contact pressure is uniform However this assumption is no always valid, because it depends on elastic properties of underlying soil mass. 53

54. Contact pressure on saturated clay It shows the qualitative contact pressure distribution under flexible and rigid footing resting on a saturated clay and subjected to uniformly distributed load q. when the footing is flexile it deforms into shape of a bowl, with the maximum deflection at the centre. If the footing is rigid the settlement is uniform. The contact pressure is more at the centre and maximum deflection at the edges 54

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56. Contact pressure on sand in this footing edges of flexible footing undergo a large settlement than at centre. The soil at the centre is confined and has large modulus of elasticity and deflects less for the same contact pressure. The contact pressure is uniform. In case of rigid footing settlement is uniform. the cp increases from zero at the edges to a maximum at centre. 56

57. PLATE LOAD TEST Plate load test  is a field test, used to determine the ultimate bearing capacity of soil the probable settlement under a given load. design of shallow foundation. For performing this test, the plate is placed at the desired depth, then the load is applied gradually and the settlement for each increment of load is recorded. At one point a settlement occurs at a rapid rate, the total load up to that point is calculated and divided by the area of the plate to determine the  ultimate bearing capacity  of soil at that depth. The ultimate bearing capacity is then divided by a safety factor (typically 2.5~3) to determine the  safe bearing capacity 57

58. Plate load test 58

59. Plate Load Test Equipment The following apparatus is necessary for performing plate load test. Test plate Hydraulic jack & pump Reaction beam or reaction truss Dial gauges Pressure gauge Loading columns Necessary equipment for loading platform. Tripod, Plumb bob, spirit level etc 59

60. Plate Load Test Procedure The necessary steps to perform plate load test is written below- Excavate test pit up to the desired depth. The pit size should be at least 5 times the size of the test plate ( B p ). At the center of the pit, a small hole or depression is created. Size of the hole is same as the size of the steel plate. The bottom level of the hole should correspond to the level of actual foundation. The depth of the hole is created such that the ratio of the depth to width of the hole is equal to the ratio of the actual depth to actual width of the foundation. A mild steel plate is used as load bearing plate whose thickness should be at least 25 mm thickness and size may vary from 300 mm to 750 mm. The plate can be square or circular. Generally, a square plate is used for square footing and a circular plate is used for circular footing. A column is placed at the center of the plate. The load is transferred to the plate through the centrally placed column. The load can be transferred to the column either by gravity loading method or by truss method. 60

61. For gravity loading method a platform is constructed over the column and load is applied to the platform by means of sandbags or any other dead loads. The hydraulic jack is placed in between column and loading platform for the application of gradual loading. This type of loading is called reaction loading. At least two dial gauges should be placed at diagonal corners of the plate to record the settlement. The gauges are placed on a platform so that it does not settle with the plate. Apply seating load of .7 T/m 2  and release before the actual loading starts. 61 Plate Load Test Procedure

62. The initial readings are noted. The load is then applied through hydraulic jack and increased gradually. The increment is generally one-fifth of the expected safe bearing capacity or one-tenth of the ultimate bearing capacity or any other smaller value. The applied load is noted from pressure gauge. The settlement is observed for each increment and from dial gauge. After increasing the load-settlement should be observed after 1, 4, 10, 20, 40 and 60 minutes and then at hourly intervals until the rate of settlement is less than .02 mm per hour. The readings are noted in tabular form. After completing of the collection of data for a particular loading, the next load increment is applied and readings are noted under new load. This increment and data collection is repeated until the maximum load is applied. The maximum load is generally 1.5 times the expected ultimate load or 3 times of the expected allowable bearing pressure. 62 Plate Load Test Procedure

63. Limitation of plate load test Base plate not less than 30cm not more than 75cm Soil should be homogeneous, if not then results are incorrect Suitable for cohesionless soil It will give immediate settlement 63

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