basics of Pile Foundation expained in simple terms

basics of pile fdn expained in simple terms

1. In situation where the soil at shallow depth is poor, then in order to transmit the load safely, depth of the foundation is increased, till a suitable soil stratum is met. This is called Pile Foundation.

2. Important Characteristics of Pile Foundation Piles are small diameter shaft Piles are generally used in groups Piles transmit the load through End bearing or Frictional Resistance

3. 1. End Bearin g Piles End Bearing Piles are those which transmit majority of load to strong stratum by End bearing Q = Total Load c arrying c apacity of pile Qpg = Bearing Load carrying capacit y o f pile Q = Frictional Load Carrying Capacit y o f pile Q

4. 2. Fr iction Pile Friction Piles are those which transmit majority of load through friction Friction piles are also known as FLOATING PILES

5. Classification of Piles on the Basis of Loadin 1. Compression Pile

6. Classification of Piles on the Basis of Loadin g 2 . T ension Pile Tension Pile Cap Compression Overturning

7. C lassification of Piles on the Basis of Loading 3 . Laterall y Loaded Pile

8. Classification of Piles on the Basis of Loadin g 4. Anchored Pile Resistance by Friction + Bulk Head

9. Classification of Piles on the E ffect of installation on soil D isplacement Piles Driven Piles are displacement piles These displace the soil in lateral and upward direction They can be large displacement piles and small displacement piles

10. Classification of Piles on the Ef f ect of installation on soil 1. D isplacement Piles a) Large displacement piles are in LOOSE SAND and generally displace s soil upto a distance of 3.5 times diameter of Pile b) In case of CLAY , piles are small displacement piles. These Piles remould the soil upto a distance of 2 times of diameter of Pile SAND " z CLA Y , • 3. 5 D 2 p

11. Classification of Piles on the Effect of Installation on Soil 2.Non Displacement Piles These are simply installed in the soil by making Cavity or bore hole in the soil These piles can be Cast in situ Pre Cast Bored Pile

12. Si ngle Pile Load Capacity Q pu = End Bearing Capacity o r Point Bearing Capacity Q f = Frictional Resistance Capacity Q u= Ultimate Load Carrying Capacity Safe Ultimate Load Carr y in g Capacity Qs= Qu FOS

13. N ote 2. 1. For full Mobilisation of Frictional Resistance, the relative movement required is 0.1- 1% of Pile Diameter For full mobilisation of point Bearing Capacity, the relative movement required is 10- 2 0% Pile Diameter

14. Single Pile Capac ity in Sandy Soil 1. D riven Pile in Sand Q Qpu = ( q p ) A base q p = CNc + qNq + ½ B ϒ N ϒ For sand, c=0, Also ½ B ϒ N ϒ is neglible as compared to qNq qp = qNq Where qp = Point bearing resistance q= at the base of pile B= dia of pile Abase= Area of base Effective stress at the level of base surcharge

15. Single Pile Capacity in Sandy Soil 1. D riven Pile in Sand Unit Point resistance increases in direct proportion to the embedded length of the pile, but Several field observations indicate that unit point resistance values increase only upto a limited depth known as Critical Depth of Pile Critical Depth depends on the angle of shearing resistance and diameter of pile 20D 15 D DENSE SA ND LOOSE SAND

16. Single Pile Capacity in Sandy Soil 1. D riven Pile in Sand Frictional Resistance The unit skin friction acting at any depth is equal to the soil pressure acting normal to the pile surface Steel Concrete Timber 20 0.75 φ 0.67 φ 0.5 1 1.5 We know that & σ h = K σ v Qf = frictional stress x frictional area Qf = q f x Af where δ = angle of friction between pile and soil K= Lateral Earth Pressure Constant 1 2 4

17. Single Pile Capacity in Sandy Soil • 2. P re Cast Bored Pile in Sand In pre cast bored pile condition, Qf = frictional stress x frictional area frictional resistance capacity is calculated by taking K =0.5 and δ= φ Point Bearing capacity of bored pile is taken as Half of the Point Bearing Capacity of Driven Pile. ( Qpu )bored pile=   Qf = qf x Af where Qf =K σ avg tan δ K= 0.5 and δ = φ

18. Single Pile Capacity P iles in Clay Piles in Cohesive soil are underreamed piles of large diameter to carry most of the load by virtue of skin friction resistance developed on the pile sha ft Bearing capacity is calculated by total stress approach, using undrained shear strength

19. Single Pile Capacity in Clay P iles in Cla y Q= Qpu + Qf Qpu = ( q p ) A base+ ( qf ) Af q p = CNc + qNq + ½ B ϒ N ϒ For clay, ϕ=0, So N ϒ =0 and cNc >>>> qNq q p = cNc (where Nc= 9 as per Skempton ) q p = 9C

20. Group Pile Ultimate load capacity of Pile group Disturbance in the soil due to installation of Pile overlap the stress and varies Pile Group capacity as compared to n × Q single pile capacit y

21. Group Pile Stress Overlap in Sand In case of Sand, stren gt h of Soil increases due to disturbance occurring during installation, hence Q group > n × Q single In case of clay In case of clay, the strength of soil decrease On remoulding during installation of Pile Group SAND CLAY

22. Ef fi cienc y of Pile Grou p ɳ= Qgroup n × Q single pile For practical purpose we never take efficiency greater than 1 But e fficiency can be greater than 1 for sa n dy soil and less than 1 for clay soils

23. Calculation of This can be treated as a rectangular or square Pile

24. Pile Capacity using Dynamic Pile Formula 1. Engineering News Formula Where W= weight of hammer H= Height of free fall FOS=6 S= Settlement per blow {C= 2.5cm for drop hammer .25 for single acting steam hammer} Q=  

25. Pile Capacity using Dynamic Pile Formula 1. Modified Hilly Formula Qgroup = Where W= weight of hammer c=Total Elastic settlement per blow H= Height of free fall Efficiency per blow S= Settlement per blow  

26. Under Reamed Pile Under Reamed Piles are used in case of expansive soil Bulb is called as UnderReam and diameter of bulb is 2 to 3 times diameter of shaft Minimum distance between the two bulbs is 1.5 × dia of bulb

27. Efficiency Using Empirical Formula ɳ= 1- [ ] ɵ ɵ= ( ) m,n = number of rows and columns  

28. PILE LOAD TEST Pile Load Test is the only direct method for determining allowable load on the pile and considered to be most reliable method because it is in situ test. Pile load test is also used through settlement computation.

29. PILE LOAD TEST Pile load test are divided into two categories: Initial Test Routine Test Initial Test are carried out on test piles to calculate allowable load and to check the settlement at working load before commencement of work on site Routine Tests are done on working piles for the assessment of settlement under working condition According to IS code, for more than 200 piles, there should be minimum of 2 initial tests. Routine test is done on 0.5% to 2% of total number of piles.

30. Pile Load Test Test Pile A pile which is used only in a load test and does not carry the load of superstructure The minimum test load on such piles should be twice the safe load (safe load calculated using static formulae) OR The load st which total settlement attains a value of 10% of pile and 40 mm in case of pile group.

31. WORKING PILE A pile which is drive or cast in situ along with other piles to carry load from the superstructure The maximum test load on such piles should be 1.5 times the safe load ( safe load calculated using static formulae) OR Upto the load at which total settlement Attains a value of 12 mm in case of single pile and 40mm whichever is earlier.

32. Types of Pile Load Test 1. Compression (Vertical) Load Test Vertical load test is carried out to establish load settlement relationship under compression and to determine allowable load on pile 2. Lateral Load Test Lateral load test is carried out to determine safe lateral load on pile 3 . Pull out test (Tension Test) Pull out test is carried out to determine safe tension on pile 4. Cyclic Load Test Cyclic load test is carried out when it is required to determine skin friction and End Bearing separately for the pile It is an initial test.

33. Allowable Load on SINGLE PILE will be lesser of.. Two thirds the final load at which settlement attains a value of 12mm or at specified settlement in exceptional cases Fifty percent of final load at which total settlement equals 10% of pile diameter in case of uniform diameter piles and 7.5% in case of underreamed piles.

34. Allowable Load on SINGLE PILE will be lesser of.. Final load at which total settlement attains a value of 25mm or at specified settlement in exceptional cases Two third of final load at which total settlement attains a value of 40mm

35. Settlement of Pile Foundation Settlement of Pile Group is generally greater than settlement of individual pile for same loading (same load per pile), this is because zone of influence of group of piles is generally more.

36. Settlement of Pile Foundation 1.When Pile is in Uniform Clay Deposit Settlement of pile group cannot be estimated from the data of a load test on a single pile because of the time effect, remolding of soil due to pile driving It is calculated by equivalent raft method The group pile is assumed to act as single large raft Load is assumed to be transferred to a depth 2/3 of pile Length, and then it is assumed to spread out 2 vertical: 1 horizontal to workout the stress increase at mid depth of clay stratum The calculation of settlement is done in similar manner as done in Consolidation Settlement

37. Settlement of Pile in Clay When Pile is in Uniform Clay Deposit H Depth o f clay

38. Settlement of Pile in Clay