Deep Foundations
23 likes5,621 views
The document discusses various types of pile foundations, installation methods, and the mechanics of load-bearing capacity, including axial and lateral loads. It addresses factors influencing the efficiency of pile groups, including spacing and soil conditions, and outlines testing procedures for pile load capacity. Additionally, it explores different theories related to lateral loading on piles and their structural behavior under various conditions.
Deep Foundations
- 1. 1
- 2. 2
- 5. 5
- 6. 6
- 7. 7
- 8. 8
- 9. 9
- 10. 10
- 11. 11
- 13. 13
- 14. 14
- 15. 15
- 16. 16
- 17. 17
- 18. 18
- 19. Pile Driving Piles may be of timber, steel or concrete. When the piles are of concrete, they are to be precast at the Industries They may be driven either vertically or at an angle to the vertical. Piles are driven using a pile hammer. Piles driven by using driving technique have higher bearing capacity compared to bored piles. Driving is usually done to soils which have medium Relative Density. 19
- 20. 20
- 21. 21
- 22. 22
- 23. 23
- 24. Pile Driving Axial Capacity The fundamental equation for axial capacity of pile is Q U = Q b + Q s Where Q U = Ultimate Load carrying capacity of pile Q b =End bearing resistance= q b A b Q s = Skin friction resistance= f s A s Where q b = Ultimate unit bearing capacity at base A b = Bearing area of the pile base f s = Unit skin friction A s = Surface area of the portion of pile embedded in soil. 24
- 25. Installation by Boring These piles are distinguished from drilled piers as small diameter piles. They are constructed by making holes in the ground to the required depth and then filling the hole with concrete. Straight bored piles or piles with one or more bulbs at intervals may be cast at the site. The latter type are called under-reamed piles. 25
- 26. 26
- 27. Load test on piles27 Forces Vertical Load Lateral Load Pull out Load Time Initial Routine
- 28. Test pile It is used in load test and doesn't carry the load of superstructure. Working pile Pile which is driven along with other piles to carry load from the superstructure. 28
- 29. Procedure The test shall be carried out by applying a series of vertical loads on a r.c.c cap over the pile. Load shall be applied by means of a remote controlled hydraulic jack. The test load shall be applied in increments of about 20 percent of assumed safe load. Settlement shall be recorded with at least three dial gauges. Each stage of loading shall be maintained till the rate of movement of pile top is not more than 0.1 mm per hours. 29
- 30. The allowable load on a single pile shall be lesser of the following: • Two-thirds the final load at which the total settlement attains a value of 12mm. • Five percent of the final load at which total settlement equals 10percent of the pile diameter in case of uniform diameter piles and 7.5 per cent of bulb diameter in case of undrreamed piles. 30
- 31. The allowable load on group of piles shall be lesser of the following: • Final load at which settlement attains a value of 25mm. • Two-thirds the final load at which the total settlement attains a value of 40mm. the procedure for routine test should be same as for an initial test with maximum loading and settlement requirements as mentioned earlier. 31
- 32. 32
- 33. 33
- 34. Group Action Of Piles Pile is not used as singularly, beneath a column or a wall , it is used as a group of piles because, eccentric loading pile my fail due to bending stresses. In an actual practice, structural loads are supported by several piles acting as a group. For a column, minimum of three piles are installed in a triangular pattern. For walls, piles are installed in staggered arrangement on both sides of it’s center line. The loads are usually transferred to the pile group through reinforced concrete slab, structurally tied to pile tops such that, the piles act as a unit. Such slabs are called as PILE CAPS. The load carrying capacity of a pile group is not necessarily equal to the sum of load capacity of the individual piles. 34
- 35. 35
- 36. 36
- 37. Estimation of load carrying capacity of the pile group is very complicated problem. Because when piles are spaced in a sufficient distance, a group capacity may approach equal to the sum of individual capacities of each pile . On the other hands, if the piles are closely spaced, stresses transmitted by the piles to the soil my overlap, this may reduce the load carrying capacity of the pile group. The efficiency (Ƞ) of a group of piles is defined as the ratio of the ultimate load of the group to the sum of individual ultimate loads. Ƞg = 𝑸 𝒈𝒖 × 𝟏𝟎𝟎 𝒏 𝑸 𝒖 N=Number of piles in a group Qg(u) =Ultimate load of group of piles. Qu =ultimate load of individual pile . 37
- 38. Ƞg = 𝑸 𝒈𝒖 𝒏 𝑸 𝒖 Thus the group efficiency is equal to the average load per pile to the ultimate load in individual pile. The group efficiency depends on the spacing between the piles . The ideal spacing gives us 100% efficiency. Generally center to center spacing kept between the piles is 2.5B to 3.5B Where B is the diameter of the pile. 38
- 39. For the piles driven in loose and medium dense cohesion less soils, group efficiency Is high. The soil around and between the piles is compacted due to vibration caused during the driving operation. For better result, it is essential to start driving the piles at centre and then work outward. (A) End bearing piles : For driven piles bearing on dense ,compact sand with a spacing equal or greater than 3B , the group capacity is generally taken equal to the sum of individual capacities. Qg = N Qu . (B) Friction piles : The group action of piles in sand is obtained from following expression. Ƞg = Qu 𝑁𝑄 𝑢 ×100 39
- 40. 40
- 41. C) Bored Piles : For the bored piles in sand at conventional spacing of 3B, the group capacity is taken as 2/3 to 3/4 times equal to sum of the individual capacities for both, end bearing and the friction piles. Q q( u) =( 2/3 to3/4 )(NQ u) In bored piles, there is limited densification of the sand surrounding the pile group . Consequently, the efficiency is low. 41
- 42. 42
- 43. MEYERHOF METHOD ; Meyerhof suggested the following empirical relation for the elastic settlement of a pile group in sand. Sand: S g = 9.4 q √BI 𝑵 S g=settlement of group q=load B g=width of group I=influence factor N= standard penetration number 43
- 44. 44
- 45. CLAYEY SOILS Due to consolidation of pile group , sometimes load assumed to spread outward from the edge of the block at an angle 300 to the vertical. settlement of each layer is given by Hi =∆s(i)= 𝑪 𝒄 1 + e0 (i) log ϭ0 + ∆ ϭi ϭ0 Hi =Thickness of i layer eo (i)=initial void ratio of i layer ϭ0 = initial Effective stress ∆ϭi =change in effective stress 45
- 46. Settlement of Pile groups 1.settlement in sand cannot be estimated. 2.Equivalevt raft approach is widely used for calculating the settlement of a pile group in clay. Assumptions to identify the equivalent raft: 1.comman practice is to assume the equivalent raft at a depth of two-third the pile length over an area enclosed by the pile. 2.For bored piles the equivalent raft is assumed at the base of the pile. Pile group in sand: For estimating the settlement of a pile group in sand, the common practice is to extrapolate this form the settlement of an individual test pile measured in a load test. 𝑆 𝑔 𝑆 𝑖 =( 4𝐵+2.7 𝐵+3.6 )2 Where,𝑆 𝑔-settlement of a pile group 𝑆𝑖-settlement of an individual pile 46
- 47. 47
- 48. 48
- 49. Negative skin friction Magnitude of negative skin friction for cohesive soils 𝑭 𝒏 = P𝑳 𝒄 𝒄 𝒂 Where, P = perimeter of pile 𝐿 𝑐 = length of pile 𝑐 𝑎 = unit adhesion = ∝ 𝑐 𝑢 ∝ = adhesion factor 𝑐 𝑢 = un drained cohesion of the compressible layer. for cohesion less soils 𝑭 𝒏 = 𝟏 𝟐 P𝑳 𝒄 𝟐 𝜸𝑲𝒕𝒂𝒏𝜹 Where, K = lateral earth pressure coefficient 𝛿 = angle of friction between pile and soil. 49
- 50. When a pile group passes through a soft, unconsolidated stratum, the magnitude of negative skin friction on the group 𝑭 𝒏𝒈 = n𝑭 𝒏 𝑭 𝒏 = 𝒄 𝒖 𝑳 𝒄 𝑷 𝒈 + 𝜸𝑳 𝒄 𝑨 𝒈 Where, n = number of piles in the group 𝑷 𝒈 = perimeter of the group 𝜸 = unit weight of the soil within the pile group upto a depth 𝐿 𝑐 𝑨 𝒈 = area of pile group within the perimeter 𝑃𝑔 50
- 51. 51
- 52. Laterally Loaded Piles INTRODUCTION Pile foundation are subjected to laterally loaded and moments which under goes wind loads and seismic forces in building. Where as in water earth pressure in retaining wall is water front structure. Piles are assume to resist load and moment in axial compression or tensile only. 52
- 53. 53
- 54. 54
- 55. Plastic theory This theory has been developed for short rigid piles where it has been assumed that the limiting or maximum soil resistance is acting against along the pile length develops. 55
- 56. 56
- 57. Laterally Loaded Piles Elastic theory This theory assume the soil to be replaced by a series of closely spaced independent elastic springs. The equation for a beam on elastic foundation , utilizing the concept of sub grade modulus , hence been solved for different conditions and non dimensional solutions have been given for laterally loaded vertical piles to determine the variation of deflection Bending Moment , soil reaction , shear etc. along entire pile length. The methods based on elastic theory based on the concept of Reese and Matlock Approach for a laterally loaded vertical pile with the soil modulus assumed to increase with depth. 57
- 58. 58
- 59. REESE AND MATLOCK APPROACH Non – dimensional solutions for a laterally loaded piles in a soil deposit in which the sub grade modulus increases linearly with depth. The solutions have been developed for long piles when L/T>5 L is length of the pile and T is the relative stiffness factor T= 𝑬𝑰 𝜼 𝒉 𝟏 𝟓 E and I are the modulus of elasticity and moment of inertia of pile material 𝜼 𝒉 = unit modulus of sub grade reaction or constant of soil modulus 59
- 60. 60
- 61. 61