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Pile foundation I Uses of piles W 1. To carry vertical load If all the (majority amount) loads are transferred to the pile tips Soft soil...
Pile foundation I Uses of piles W 1. To carry vertical load If all the (majority amount) loads are transferred to the pile tips Soft soil End bearing pile Friction If all the (majority amount) loads are transferred to the soil along the length of pile Hard layer Friction pile End bearing Compaction pile: Short piles used for compacting loose sand. Tu 2. To resist uplift load Tension pile or Uplift: Below some structures such as transmission tower, offshore platform which are subjected to tension. Tension Pile Murthy (2001) 3. To carry inclined and horizontal load (foundation for retaining wall, bridge, abutments and wharves) Laterally loaded piles: Horizontal load acts perpendicular to the pile axis. Murthy (2001) W H Batter piles: Driven at an angle Carry large horizontal load Batter Pile Types of pile Cross-section Shape Mode of Method of Method of Based on Material used Circular Cylindrical load transfer installation forming Steel pile displacement of soil Square Tapered End bearing Driven Pre-cast Displacement piles Timber pile Hexagonal Under-reamed Friction Bored Pre-stressed Non displacement Concrete pile I-section Combined Jetted Cast in situ Composite pile piles H- section Pipe https://in.pinterest.com/pin/680 Steel Pile Based on material used : 043612452541560/ http://www.86steelpipe.com/ Concrete Pile cs/gr-50-steel-pipe-piles.html https://www.slideshare.net/shivamsgandhi/pile-foundation Timber Pile Timber pile: suitable for light loads varies from 100 to 250 kN per pile. Suitable for soft cohesive soil. Concrete Pile: all load condition. Most frequently used piles. Strong, durable. Steel pile: Used to carry heavy load Based on crosssection: a) circular, b) square, c) rectangular, d) hexagonal, e) H- section, f) pipe Rock or very dense sand – H pile and open ended pipe pile (least driving effort) Under the vertical load, the type of pile cross section does not play a important role. However, under horizontal load, square and H section pile perform well as compared to circular pile Pile foundation II Based on Shape: Cylindrical Pile Tapered Pile Underreamed Pile Cohesive soil under laid by a granular soil – Cylindrical pile Loose to medium dense granular soil – Tapered pile (for efficient transfer of load along the length of pile. efficient distribution of pile materials) Expansive soil – Under-reamed pile Under-reamed Pile: 150-200 mm shaft diameter 3 to 4 m long Underreamed portion is 2 to 3 times the shaft dia. Used for expansive soil Punmia (1973) a) Boring by auger b) Under-reaming by under -reamer c) Placing reinforcement cage in position d) Concreting of pile e) Concreting of pile caps Mode of load transfer: End- bearing pile Act as column Transmit the load through a weak soil to a hard stratum The ultimate load carried by pile= load carried by the bottom end Friction pile Do not reach hard stratum Transfer the load through skin friction between embedded soil and pile The ultimate load carried by pile= load transferred by skin friction Combined end- bearing and friction pile The ultimate load carried by pile= load transferred by skin friction + load carried by the bottom end of pile Method of installation Driven Pile: Bored Pile: Driven Pile: loose granular soil (compact the soil, thus increase its shear resistance) Bored pile: best suited to clay soil Jetted pile: used if granular soil are in a very compact state Method of forming Precast concrete piles: Formed in a central casting yard to the specified length, cured and shipped to the construction sites. or If space is available, casting yard may be provided at the site Length upto 20m and precast hollow pipe piles can go up to 60m Shorter piles can carry load up to 600kN, and capacity of longer pile can be as large as 2000KN (in some cases) Prestressed concrete piles: Formed by tensioning high-strength steel (fult =1700 to 1860 MPa ) prestress cables and casting the concrete pile about the cable The prestress cables are cut, when the concrete hardens Cast in situ pile Formed by making a hole in the ground and filling it with concrete If the hole is formed by drilling, then it is called bored cast in situ. If it is formed by driving a metallic shell or a casing into the ground, then it is called driven cast in situ. If during concreting the casing is left in position, then it is termed as cased pile. If the casing is gradually withdrawn, then it is termed as uncased pile. Precast and Prestressed pile: Use in marine structure. Prestressed piles have large vertical load and bending moment capacity and are used in such installation Cast in-situ Pile: Soil of poor drainage quality Suited in places where vibrations are avoided to save the adjoining structures Based on displacement of soil: Displacement Piles : All driven piles are displacement piles as the soil is displaced laterally when the piles is installed. Non-Displacement Piles : Bored piles are non- displacement piles Advantages of precast concrete pile: Piles are cast in controlled environment The required number of piles can be cast in advance Loose granular soil is compacted The reinforcements remain in proper position. Disadvantages of precast concrete pile: Addition reinforcements are required due handling and transportation Special equipments are required for handling and driving Piles can be damaged during handling and transportation If the soil is saturated, then pore water pressure is developed which reduces the shear strength of the soil. Length adjustment is difficult Advantages of cast-in-situ concrete pile: The length of the shell or pile can be increased or decreased No additional reinforcement is required Additional pile can be installed quickly Little chance of damage due to handling and transportation Disadvantages of cast-in-situ concrete pile: Proper quality control Loose granular soil is not compacted significantly A lot of storage space is required for materials Bored cast-in-situ piles: Large diameter pile can be made. Installation can be made without appreciable noise or vibration. Boring may be loosen the granular soil. In uncased pile, concreting is difficult due to the presence of drilling mud. Bored piles are commonly cheaper. Length of the pile can be changed or varied depending the ground condition. Driven cast-in-situ piles: Diameter of the pile can not be made too large. More noise and vibration. Granular soil is compacted. Drilling mud is not required. It is costlier (especially the cased one). Length adjustment is difficult. (Ranjan and Rao, 1991) Typical length and capacities of various piles: Pile Type Pile length Approximate design load (kN) Usual range Maximum Usual range Maximum Timber 10-18 30 150-200 300 Driven precast concrete 10-15 30 300-600 900 Driven prestressed 20-30 60 500-600 900 concrete Cast insitu concrete 15-25 40 300-750 900 (Drilled shell) Concrete cast insitu 15-25 45(large dia.) 600-3000 9000 (large dia.) bulb piles Steel Pile 20-40 Unlimited 300-1000 2500-10000 (small dia.) (large dia.) Composite Pile 20-40 60 300-900 2000 The information can be used only as a guide line during the initial planning and analysis stages Pile foundation III Pile load capacity in compression : a) Static pile load formulae b) Pile load tests c) Pile driving formulae d) Correlation with penetration test data Static pile load formulae The ultimate load capacity of the pile ( Qu) Qu = Qpu + Q f Q pu = Ultimate point load resistance of the pile Qf = Ultimate skin friction Qpu >> Qf point bearing pile or end bearing pile Qf >>Q pu friction pile The ultimate point load can be expressed in the form: Qpu = qpu Ab Ab = sectional area of the pile at its base The ultimate skin friction can be written in the form : Q f = fs As fs = unit skin friction resistance As= surface area of the pile in contact with soil The ultimate load capacity (Q u) can be written in the form Qu = qpu Ab + f s As The general equation for unit point bearing resistance ( qpu) for c-ϕ soil : q pu = cN c + N q + 0.5BN where B = width or diameter of pile σ’ = effective overburden pressure at the tip of the pile, equal to γL N c , Nq, Nγ = bearing capacity factor c = unit cohesion L = length of embedment of pile γ = effective unit weight of soil In a deep foundation , σ'Nq >> 0.5γB Nγ. Hence, the third term is usually neglected q pu = cN c + N q For a granular soil, c=c’=0 q pu = N q For a clay soil, c = c u and ϕu = 0 qpu = cub N c c ub = undrained shear strength at the base of the pile Piles in granular soils: Driven Piles: Tomlinson's / Berezantsev’s Method q pu = ' Nq + 40∘ For a driven piles in sand c = 2 φc – in situ value of angle of shearing resistance If φ > 40˚, Pile driving shall have the effect of reducing the angle of shearing resistance of sand due to dilatancy effect The maximum base or tip or point bearing resistance Berezantsev’s Bearing Capacity factor is limited to 11000 kN/ m2 Murthy (2001) Mayerhof (1976) Solution qpu = ' N q Limiting value for point end bearing qpul = 50Nq tan kN / m 2 for dense sand qpul = 25Nq tan kN / m 2 for loose sand Mayerhof (1976) bearing capacity factors Murthy (2001) Skin friction: f s = h tan( ) f s = K ' tan( ) δ = angle of friction between the pile and the soil K= the lateral earth pressure L σh σh σh = the soil pressure acting normal to the pile surface (horizontal) σ‘ = the effective vertical overburden pressure Ultimate Skin friction resistance ( Qf ) : Q f = fs (av) As Q f = K av ' tan ( ) As σ’av = average effective overburden pressure over the embedded length of the pile Broms (1966) recommends the value of K and δ shown in Table for piles driven into sand Ranjan and Rao, 1991 Murthy (2001) Critical depth: Depend on φ’ value and diameter of pile (D). Critical depth may vary from about 15D in loose to medium sand to 20D in dense sand. Limiting value for skin Resistance in homogenous sand The allowable load Q a : Qu Qa = F Qu = ultimate load F = factor of safety = 2.5 Note: The bored piles in sand have a point bearing or top resistance (qpu ) is 1 / 2 to 1 / 3 of the value of the driven piles. In case of bored pile in sand, the lateral earth pressure coefficient can be calculated as: K = 1-sin . The value of K varies from 0.3 to 0.75 (average value of 0.5). The value is equal to for bored piles excavated in dry soil and a reduced value is considered if slurry has been used during excavation. IS:2911(Part1): 2010 Piles in granular soil 1 n Qu = Ap D N + PD N q + K i PDi tan i Asi 2 i=1 where A p =c/s area of pile tip D= diameter of pile Nq and N γ= bearing capacity factors depending on angle of internal friction PD= effe ctive overburden pressure at pile tip i= any layer between 1 to n layers in which pile is installed and it contributes to positive skin friction Ki= coefficient of earth pressure applicable in i th layer of soil.It depends on the nature of soil strata, type of pile, spacing of pile and its method of construction. For driven piles in loose to dense sand ( = 30 to 40), Ki value in the range of 1 to 2 may be used. For bored piles in loose to dense sand ( = 30 to 40), Ki value in the range of 1 to 1.5 may be used. PDi= effective overburden pressure for i th layer δi= angle of wall friction between soil and pile in i th layer (may be taken as ) Asi= surface area of pile shaft at i th layer Note: As per IS Code [IS:2911(Part1/Sec 1):2010], for piles longer than 15 to 20 times the pile diameter, maximum effective overburden stress at pile tip should correspond to the pile length equal to 15 (if 30) to 20 (if 40) times of the diameter. IS 6403:1981 φ(in Nγ degree) 0 0 Nγ factor can be taken for general shear failure 5 0.45 according to IS 6403. N factor will depend on the nature of soil, type of 10 1.22 piqle, the L/D ratio and its method of construction. 15 2.65 The values applicable for driven piles are given in 20 5.39 this figure. 25 10.88 30 22.40 35 48.03 40 109.41 45 271.76 50 762.89 IS:2911(Part1 / Sec 1): 2010 IS:2911(Part I / Sec2): 2010 Driven precast and cast in situ concrete pile Bored precast and cast in situ concrete pile Pile foundation IV Example: (a) A 15m long, 300 mm diameter pile was driven in a uniform sand (’= 40). The water table is at great depth. Average unit weight of soil is 19 kN/ m3. Calculate the safe load capacity of the pile with F.O.S =2.5. (b) Calculate the safe load capacity of the pile if water table is located at 2m below the ground level. Piles in granular soils: Driven Piles: Tomlinson's / Berezantsev’s Method q pu = ' Nq + 40∘ For a driven piles in sand c = 2 φc – in situ value of angle of shearing resistance If φ > 40˚, Pile driving shall have the effect of reducing the angle of shearing resistance of sand due to dilatancy effect The maximum base or tip or point bearing resistance Berezantsev’s Bearing Capacity factor is limited to 11000 kN/ m2 Murthy (2001) Mayerhof (1976) Solution qpu = ' N q Limiting value for point end bearing qpul = 50Nq tan kN / m 2 for dense sand qpul = 25Nq tan kN / m 2 for loose sand Mayerhof (1976) bearing capacity factors Murthy (2001) Broms (1966) recommends the value of K and δ shown in Table for piles driven into sand Ranjan and Rao, 1991 Murthy (2001) IS:2911(Part1): 2010 Piles in granular soil 1 n Qu = Ap D N + PD N q + K i PDi tan i Asi 2 i=1 where A p =c/s area of pile tip D= diameter of pile Nq and N γ= bearing capacity factors depending on angle of internal friction PD= effe ctive overburden pressure at pile tip i= any layer between 1 to n layers in which pile is installed and it contributes to positive skin friction Ki= coefficient of earth pressure applicable in i th layer of soil.It depends on the nature of soil strata, type of pile, spacing of pile and its method of construction. For driven piles in loose to dense sand ( = 30 to 40), Ki value in the range of 1 to 2 may be used. For bored piles in loose to dense sand ( = 30 to 40), Ki value in the range of 1 to 1.5 may be used. IS 6403:1981 φ(in Nγ degree) 0 0 Nγ factor can be taken for general shear failure 5 0.45 according to IS 6403. N factor will depend on the nature of soil, type of 10 1.22 piqle, the L/D ratio and its method of construction. 15 2.65 The values applicable for driven piles are given in 20 5.39 this figure. 25 10.88 30 22.40 35 48.03 40 109.41 45 271.76 50 762.89 IS:2911(Part1 / Sec 1): 2010 IS:2911(Part I / Sec2): 2010 Driven precast and cast in situ concrete pile Bored precast and cast in situ concrete pile Pile foundation V Example: (a) A 15m long, 300 mm diameter pile was driven in a uniform sand (’= 40). The water table is at great depth. Average unit weight of soil is 19 kN/ m3. Calculate the safe load capacity of the pile with F.O.S =2.5. (b) Calculate the safe load capacity of the pile if water table is located at 2m below the ground level. Piles in granular soils: Driven Piles: Tomlinson's / Berezantsev’s Method q pu = ' Nq + 40∘ For a driven piles in sand c = 2 φc – in situ value of angle of shearing resistance If φ > 40˚, Pile driving shall have the effect of reducing the angle of she aring resistance of sand due to dilatancy effect The maximum base or tip or point bearing resistance Berezantsev’s Bearing Capacity factor is limited to 11000 kN/m 2 Murthy (2001) Mayerhof (1976) Solution qpu = ' N q Limiting value for point end bearing qpul = 50Nq tan kN / m 2 for dense sand qpul = 25Nq tan kN / m 2 for loose sand Mayerhof (1976) bearing capacity factors Murthy (2001) Broms (1966) recommends the value of K and δ shown in Table for piles driven into sand Ranjan and Rao, 1991 Murthy (2001) IS:2911(Part1): 2010 Piles in granular soil 1 n Qu = Ap D N + PD N q + K i PDi tan i Asi 2 i=1 where Ap=c/s area of pile tip D= diameter of pile Nq and Nγ= bearing capacity factors depending on angle of internal friction PD= effe ctive overburden pressure at pile tip i= any layer between 1 to n layers in which pile is installed and it contributes to positive skin friction Ki= coefficient of earth pressure applicable in i th layer of soil.It depends on the nature of soil strata, type of pile, spacing of pile and its method of construction. For driven piles in loose to dense sand ( = 30 to 40), Ki value in the range of 1 to 2 may be used. For bored piles in loose to dense sand ( = 30 to 40), Ki value in the range of 1 to 1.5 may be used. IS 6403:1981 φ(in Nγ degree) 0 0 Nγ factor can be taken for general shear failure 5 0.45 according to IS 6403. 10 1.22 Nqfactor will depend on the nature of soil, type of pile, the L/D ratio and its method of construction. 15 2.65 The values applicable for driven piles are given in 20 5.39 this figure. 25 10.88 30 22.40 35 48.03 40 109.41 45 271.76 50 762.89 IS:2911(Part1 / Sec 1): 2010 IS:2911(Part I / Sec2): 2010 Driven precast and cast in situ concrete pile Bored precast and cast in situ concrete pile Pile foundation VI With and without considering critical length concept: Layered soil Piles in granular soils: Driven Piles: Tomlinson's / Berezantsev’s Method q pu = ' Nq + 40∘ For a driven piles in sand c = 2 φc – in situ value of angle of shearing resistance If φ > 40˚, Pile driving shall have the effect of reducing the angle of she aring resistance of sand due to dilatancy effect The maximum base or tip or point bearing resistance Berezantsev’s Bearing Capacity factor is limited to 11000 kN/m 2 Murthy (2001) Mayerhof (1976) Solution qpu = ' N q Limiting value for point end bearing qpul = 50Nq tan kN / m 2 for dense sand qpul = 25Nq tan kN / m 2 for loose sand Mayerhof (1976) bearing capacity factors Murthy (2001) Broms (1966) recommends the value of K and δ shown in Table for piles driven into sand Ranjan and Rao, 1991 Murthy (2001) IS:2911(Part1): 2010 Piles in granular soil 1 n Qu = Ap D N + PD N q + K i PDi tan i Asi 2 i=1 where Ap=c/s area of pile tip D= diameter of pile Nq and Nγ= bearing capacity factors depending on angle of internal friction PD= effe ctive overburden pressure at pile tip i= any layer between 1 to n layers in which pile is installed and it contributes to positive skin friction Ki= coefficient of earth pressure applicable in i th layer of soil.It depends on the nature of soil strata, type of pile, spacing of pile and its method of construction. For driven piles in loose to dense sand ( = 30 to 40), Ki value in the range of 1 to 2 may be used. For bored piles in loose to dense sand ( = 30 to 40), Ki value in the range of 1 to 1.5 may be used. IS 6403:1981 φ(in Nγ degree) 0 0 Nγ factor can be taken for general shear failure 5 0.45 according to IS 6403. 10 1.22 Nqfactor will depend on the nature of soil, type of pile, the L/D ratio and its method of construction. 15 2.65 The values applicable for driven piles are given in 20 5.39 this figure. 25 10.88 30 22.40 35 48.03 40 109.41 45 271.76 50 762.89 IS:2911(Part1 / Sec 1): 2010 IS:2911(Part I / Sec2): 2010 Driven precast and cast in situ concrete pile Bored precast and cast in situ concrete pile Piles in clay : The ultimate load capacity of pile (Q u): Qu = qpu Ab + f s As In clays, qpu = c u N c and fs = c a = αcu Qu = cub Nc Ab + cu As c ub = undrained cohesion at the base of pile Nc = bearing capacity factor for a deep foundation. For circular and square piles Nc = 9 (proposed by Skempton). Pile must go at least 5D inside the bearing stratum. α = adhesion factor c u = undrained cohesion in the embedded length of pile Values of reduction factor α Murthy (2001) cu (kPa) consistency 0 – 12.5 very soft 12.5-25 soft 25-50 medium 50-100 stiff 100-200 very stiff >200 hard Ranjan and Rao, 1991 Pile foundation VII IS:2911(Part1): 2010 Piles in granular soil 1 n Qu = Ap D N + PD N q + K i PDi tan i Asi 2 i=1 where Ap=c/s area of pile tip D= diameter of pile Nq and Nγ= bearing capacity factors depending on angle of internal friction PD= effe ctive overburden pressure at pile tip i= any layer between 1 to n layers in which pile is installed and it contributes to positive skin friction Ki= coefficient of earth pressure applicable in i th layer of soil.It depends on the nature of soil strata, type of pile, spacing of pile and its method of construction. For driven piles in loose to dense sand ( = 30 to 40), Ki value in the range of 1 to 2 may be used. For bored piles in loose to dense sand ( = 30 to 40), Ki value in the range of 1 to 1.5 may be used. IS:2911(Part1 / Sec 1): 2010 IS:2911(Part I / Sec2): 2010 Driven precast and cast in situ concrete pile Bored precast and cast in situ concrete pile Piles in clay : The ultimate load capacity of pile (Q u): Qu = qpu Ab + f s As In clays, qpu = c u N c and fs = c a = αcu Qu = cub Nc Ab + cu As c ub = undrained cohesion at the base of pile Nc = bearing capacity factor for a deep foundation. For circular and square piles Nc = 9 (proposed by Skempton). Pile must go at least 5D inside the bearing stratum. α = adhesion factor c u = undrained cohesion in the embedded length of pile Values of reduction factor α Murthy (2001) cu (kPa) consistency 0 – 12.5 very soft 12.5-25 soft 25-50 medium 50-100 stiff 100-200 very stiff >200 hard Ranjan and Rao, 1991 Qu The allowable load Q a : Qa = F Qu = ultimate load F = factor of safety = 2.5 Example: A 15 m long pile with diameter 400mm was driven in a homogeneous clay with unconfined compressive strength of 100 kPa. Calculate the ultimate load Carrying capacity of the pile. Example: Layered soil (only Clay) Values of reduction factor α Murthy (2001) cu (kPa) consistency 0 – 12.5 very soft 12.5-25 soft 25-50 medium 50-100 stiff 100-200 very stiff >200 hard Ranjan and Rao, 1991 Piles in cohesive soil [IS:2911(Part1): 2010] n Qu = A p N c c p + i c i Asi i =1 where Ap= c/s area of pile tip Nc = bearing capacity factor may be taken as 9 cp = average cohesion at pile tip αi= adhesion factor for i th layer c i= average cohesion at i th layer Asi= surface area of pile shaft at i th layer Pile foundation VIII Example: Layered soil (Sand-Clay) Load carrying capacity of under-reamed pile in Clay D Qu = cub Nc Ab + c'u As = (9cub ) D12 + c'u As 4 Nc = 9 D1 α = adhesion factor Ab = area of the enlarge base D1 = diameter of the bulb Note: When the bulb is slightly above the tip, Ab is equal to the area of the diameter of the bulb and the projected stem below the bulb is ignored. If bulb is quite high : D For single bulb 2 Qu = (9cub ) D + 9c' ub (D12 − D 2 )+ c' u As 4 4 D1 c ub = unit cohesion at the tip c’ ub = unit cohesion at the bulb level c'u = average cohesion on A s A’s = surfac e area = The length of the shaft equal to 2D above the bulb is usually neglected (As the pile settles, there is possibility of formation of a small gap between the top of bulb) Two or more bulbs D 2 Qu = (9cub ) D + 9c' ub ( 1 D 2 − D 2 )+ c'u As + c"u Asb 4 4 c ub = unit cohesion at the tip D1 c’ ub = unit cohesion at the bulb level As = surface area of the shaft above the top bulb (ignoring 2B length) Asb = surfac e area of the cylinder circumscribing the bulbs between top and bottom bulbs D1 c'u = average cohesion on A s c” u = average cohesion on Asb Pile Load test It is the only direct method for determining the allowable load on piles. It is an in-situ test and the most reliable one also. It is very useful for cohesion less soil. However, for cohesive soil, data from pile load test should be used with caution because of pile driving disturbanc e, pore water pressure development, and inadequate time allowed for the consolidation settlement. Types of load test Vertical load test Lateral load test Pull out test (compression) (Tension) It is carried out to esta blish load- settlement relationship under compression and These two tests are carried out when piles are determine the allowable load on required to resist the lateral loads or uplift loads. pile. Initial test It is to be carried out on test piles to estimate the allowable load, or to predict the settlement at working load. It does not carry any load coming from superstructure. Where there is no specific information about subsoil strata and no past experience, for a project involving more than 200 piles, there should be minimum two initial tests. The minimum load on test piles should be twice the safe load or the load at which total settlement attains a value of 10% of pile diameter for single pile and 40 mm in group. Routine test It is carried out as a check on working pile to assess the displacement corresponding to working load. The minimum no. of routines tests should be half percentage of the piles used. It may vary up to 2 percent or more depending upon the nature of soil strata and importance of structure. A working pile is driven or cast in situ along with other piles to carry the load from superstructure. The load on such piles should be up to 1.5 times the safe load or the load at which the total settlement attains 12mm for single pile and 40 mm for group pile , whichever is earlier. Pile load test Types of Load test Continuous loading Cyclic loading Load is raised to a particular level Continuous increment of load is applied and then dropped to zero, again on the pile head increased to a higher level and reduced to zero. Procedure: As per IS: 2911 part IV (1979) The test shall be carried out by applying the load on a RCC cap over the pile. The load is applied in increment of 20 % of Step 1 the safe load. Settlements are recorded with at least three dial gauges. Step 2 Each stage of loading shall be maintaining till the rate of movement of pile top is not Step 3 more than 0.1 mm /hr. Pile foundation IX The allowable load on a single pile shall be lesser of the following: 2/3rd of final load at which the total settlement attains a value of 12mm. If nothing is specified, then the permissible settlement =12mm. If any other permissible value is specified, then load shall correspond to actual permissible total settlement. 50% of final load at which the total settlement equals to 10% of the pile diameter in case of uniform diameter piles and 7.5% of bulb diameter in case of under reamed piles. The allowable load on a group of piles shall be lesser of the following: Final load at which the total settlement attains a value of 25mm. The permissible settlement is 25mm. 2/3rd of the final load at which the total settlement attains a value of 40mm. Example: The following data was obtained in a vertical pile load test on 300 mm diameter pile. Determine the allowable or safe load as per IS 2911 part IV (1979). Load (kN) Settlement (mm) 0 100 200 300 400 500 600 700 50 2.5 0 100 5.0 10 200 10.0 20 300 17 30 400 28 500 45 40 600 70 50 60 70 80 Vertical cyclic plate load test: It is carried out when it is required to separate the pile load into skin friction and point bearing on single piles of uniform diameter. It is limited to initial tests only. Pile foundation IX The allowable load on a single pile shall be lesser of the following: 2/3rd of final load at which the total settlement attains a value of 12mm. If nothing is specified, then the permissible settlement =12mm. If any other permissible value is specified, then load shall correspond to actual permissible total settlement. 50% of final load at which the total settlement equals to 10% of the pile diameter in case of uniform diameter piles and 7.5% of bulb diameter in case of under reamed piles. The allowable load on a group of piles shall be lesser of the following: Final load at which the total settlement attains a value of 25mm. The permissible settlement is 25mm. 2/3rd of the final load at which the total settlement attains a value of 40mm. Example: The following data was obtained in a vertical pile load test on 300 mm diameter pile. Determine the allowable or safe load as per IS 2911 part IV (1979). Load (kN) Settlement (mm) 0 100 200 300 400 500 600 700 50 2.5 0 100 5.0 10 200 10.0 20 300 17 30 400 28 500 45 40 600 70 50 60 70 80 Vertical cyclic plate load test: It is carried out when it is required to separate the pile load into skin friction and point bearing on single piles of uniform diameter. It is limited to initial tests only. Pile foundation X Dynamic Pile formula Engineering News Record Formula (ENR) Energy input= Work done Qu S = WH From above formula, the allowable pile load is expressed as WH Qa = F (S + C) W= weight of the hammer falling through a height, H S’= Theoretical set= S+C S= real set per blow C= empirical factor allowing reduction in theoretical set due to energy losses F= factor of safety (usually taken as 6) http://hammer.m88play.com/drop-hammer-pile-driver/ WH a) Drop hammer Qa = 6(S + 2.5) WH b) Single acting steam hammers Qa = 6(S + 0.25) (W + ap)H c) Double acting steam hammers Qa = 6(S + 0.25) where W (weight of hammer) and Q a are expressed in kg. H is the height of free fall of hammer in cm. a is the effective area of piston in cm 2 and p is the mean effective steam pressure in kg/cm 2. S is the final set in cm/blow, usually taken as average penetration for the last 5 blows of a drop hammer or 20 blows of a ste am hammer. Example: A 250 diameter pile was driven with a drop hammer of weight 2200 kg and having a free fall of 1.5m. The total penetration of the pile recorded in the last 5 blows was 30mm. Determine the safe pile load using ENR. Modified Hiley Formula Actual Energy delivered= Energy used + Energy losses Whh Qu = S +C2 Whh = S + (C 1 + C 2 + C 3 ) 1 2 where Qu= ultimate driving resistance in tonnes. Safe load is estimated by dividing the ultimate resistanc e by a factor of safety 2.5. W= weight of hammer in tonnes. h= effective fall of hammer, in cm η= efficiency of blow that represents the ratio of energy after impact to striking energy of ram. ηh=hammer efficiency S= final set or penetration per blow in cm. C= total elastic compression= C 1+ C 2 +C 3 When W> Pe and pile is driven into penetrable ground, W + Pe2 = W +P When W< Pe and pile is driven into penetrable ground, 2 W + Pe 2 W − Pe = − W +P W +P where P= weight of pile + anvil+ helmet +follower (if any) in tonnes e= coefficient of restitution of material under impact and ranges from 0 to 1. C1 C2 C3 It is temporary elastic compression of It is temporary elastic It is temporary compression of dolly and packing. compression of pile. soil. Qu = 1.77 Qu L A = 0.675 Qu A = 3.55 where the driving is with 2.5cm thick A where L is length of pile in meter. where A is area of pile in cm2. cushion only on head of pile A is area of pile in cm2. Qu = 9.05 A where the driving is with short dolly upto 60cm long, helmet and 7.5cm thick cushion Murthy (2001) Pile foundation XI Correlation with penetration test data Driven piles in sand 1. Using Cone Penetration resistance The unit point resistance of driven pile q pu = static cone resistanc e q c The skin friction resistanc e for driven piles can also be determined with help of cone penetration resistanc e using Meyerhof(1956) correlation: For Displacement piles, q (av) fs = c kN / m 2 (limited to 100 kN/m2) 2 For H piles, q (av) kN / m 2 (limited to 50 kN/m2) fs = c 4 where q c ( av )= average field value of cone penetration resistance in kg/cm 2 over pile length. Using of static cone penetration data [IS:2911(Part1 /Sec 1):2010] For non homogeneous soil, The ultimate point bearing capacity can be taken as q c0 + q c1 + q c2 q pu = 2 2 qc0 is the average cone resistanc e qc1 is the minimum cone resistanc e qc2 is the average of minimum cone resistanc e Using of static cone penetration [IS:2911(Part1 /Sec 1):2010] Side or skin friction (fs) in kN/m2 qc 2q fs c for clay 25 25 qc q fs c for silty clay and silty sand 100 25 qc 2q fs c for sand 100 100 qc q fs c for coarse sand and gravel 100 150 Example: Determine the allowable load carrying capacity of a 11 m long and 450 mm diameter driven pile constructed in the sand with cone resistance (SCPT) profile as shown in the figure. Pile foundation XII 2. Using N value: The unit penetration resistance of driven pile in sand including H pile can be determined as: q pu = 40N (L D) kN / m 2 where N= standard penetration resistance observed in field without overburden correction L= length of the pile D= diameter of pile For driven piles, qpu is limited to 400 N kN/m2. The skin friction resistance for driven pile in sand can be determined as: For displacement piles: (limited to 100 kN/m2) f s = 2N av kN / m 2 (Driven Piles) For H piles: f s = N av kN / m 2 (limited to 50 kN/m2) where N av = average field value of N along pile length Using of standard penetration data [IS:2911(Part1 /Sec 1):2010] For saturated cohesionless soil, the ultimate load bearing capacity of pile in kN is given by Lb NAs Qu = 40N Ap + For driven piles, qpu is limited to 400 N kN/m2. D 0.5 where N= average N value at tip Lb=length of penetration in bearing strata , in m d= diameter of pile in m Ap= c/s area of pile tip in m 2 N= average N value along pile shaft As= surface area of shaft in m 2 for non plastic silt or very fine sand, Lb NAs Qu = 30N Ap + D 0.6 Bored and cast in situ piles in sand 1 q pu = q pu of driven pile 3 1 fs = f s of driven pile 2 Driven and cast in situ piles in sand For cased pile: q pu and fs can be taken same as that of driven pile. fs = fs of driven pile (if proper compaction of concrete is done) For uncased pile: fs = fs of bored cast in situ (if proper compaction of concrete is not done) Group action of piles: Pile cap Soil Pile group https://www.deltares.nl/en/software/module/d-pile-group-cap- https://theconstructor.org/geotechnical/foundations/pile/page layered-soil-interaction-3/ /2/ Ultimate bearing capacity of pile group≠ sum of all individual piles present in the group. Group efficiency, Qug g = nQu where Qug= ultimate load bearing capacity of pile group Qu= ultimate load bearing capacity of single pile n= no. of piles ✓ ηg< 1 for smaller spacing between piles ✓ ηg >1 for driven piles in loose to medium soil ✓ ηg=1 for larger sp acing of piles Pile group efficiency can be calculated using Converse- Labarre formula: where m= no. of rows of piles m(n − 1) + n(m − 1) n= no. of piles in a row g = 1 − −1 D mn 90 θ= tan S D= Diameter of pile Minimum pile spacing S= Centre to centre spacing Length of pile Friction piles in Friction piles in Point bearing sand clay pile < 12m 3D 4D 3D 12 to 24 m 4D 5D 4D > 24m 5D 6D 5D As per IS: 2911-I-1979 Bearing pile- 2 D Friction pile- 3D Loose sand or fill deposit -2D Pile group in clay Pile may fail in one of the following way By block failure (when spacing is less than 2-3 times diameter of a pile) By individual pile failure ( when piles are spaced wider) The ultimate load capacity of the pile group by block failure is given by: Qug = cub Nc Ab + PbLcu Undrained strength Undrained strength of clay along length of block of clay at base of Bearing capacity pile group Embedded length of pile factor= 9 Perimeter of block c/s area of block The ultimate load capacity of the pile group by individual pile failure is given by: Qug = nQu Example: Determine the spacing of a group of 16 piles with diameter of 300mm such that the efficiency of the pile group is 1. The piles were constructed in uniform clay soil with unconfined compressive strength of 50 kPa. Settlement of a pile group Pile group in clay 1. For the displacement piles or friction piles in homogeneous clay 1− 2 Si = qn B I f E where qn= Net pressure on pile μ= Poisson’s ratio E= young’s Modulus If= Influenc e factor Consolidation settlement p + p Sc = Cc H log10 0 1+ e0 p0 or Sc = mv Hp Where p 0 = initial effective overburden pressure before applying foundation load ∆p= vertical stress at the centre of the layer due to application of load C c = Compression index e0= initial void ratio mv= coefficient of volume compressibility H= thickness of each layer 2. Piles driven into a firm or strong stratum through an overlying clay stratum. 3. For bored piles or end bearing piles bearing on firm stratum Equivalent raft acts at the base of the pile. Pile group in sand Skempton (1953): For same average load Q/pile acting in driven piles, the settlement ratio of group of pile to single pile can be obtained as: S g 4B + 2.7 2 = Si B + 3.6 where B= width of the pile group in ‘meter’ Sg= settlement of pile group Si= settlement of single pile Meyerhof (1959): It is for square pile groups driven in sand Sg S (5 − S 3) = 2 Si 1 1 + r where S= ratio of pile spacing to pile diameter r= no. of rows in the pile group Example: The following data was obtained in a vertical pile load test on 300 mm diameter pile. Determine the allowable or safe load as per IS 2911 part IV (1979). Load (kN) Settlement (mm) 0 100 200 300 400 500 600 700 50 2.5 0 100 5.0 10 200 10.0 20 300 17 30 400 28 500 45 40 600 70 50 60 70 80 Pile Foundation XIII Example: Design a pile group consisting of RCC piles for a column of size 650mm × 650 mm carrying a load of 1500 kN (Total). The exploration data reveal that the sub-soil consists of deposit of clay extending to a greater depth. The other data of the deposit are: Compression index = 0.10, Initial void ratio = 0.9, Saturated unit weight = 20 kN/m3, Unconfined compressive strength= 70kN/m2. Proportion the pile group for the permissible settlement of 40 mm. Design the pile group by considering both bearing and settlement criteria. The water table is considered at the ground level. Use a factor of safety 2.5 against bearing and assume adhesion factor of 0.7. Values of reduction factor α Murthy (2001) cu (kPa) consistency 0 – 12.5 very soft 12.5-25 soft 25-50 medium 50-100 stiff 100-200 very stiff >200 hard Ranjan and Rao, 1991 Fox’s Correction Curves Pile Foundation XIV Negative skin friction: Negative skin friction in single piles The magnitude of negative skin friction, F n for a single pile may be estimated as below: Cohesive soils: Fn = PLcca Where, P= perimeter of pile L c = Length of pile in compressible stratum c a = unit adhesion=αc u α= adhesion factor cu= undrained cohesion of compressible layer Cohesionless soils: 1 Fn = PLc 2 K tan 2 where K=lateral earth pressure coefficient δ= angle of friction between pile and soil (1/2 φ to 2/3φ) Negative skin friction in pile groups The magnitude of negative skin friction, Fng for a pile group passes through soft and unconsolidated soil may be estimated as below: Fng = nFn Higher of value from these two Equation is used in design Fng = cu Lc Pg + Lc Ag where n= number of piles in the group Pg= perimeter of group γ= unit weight of soil within pile group up to a depth of Lc Ag= area of pile group within perimeter P g Ultimate load capacity of a sin gle or a group of piles F.O.S = Working load + negative skin friction Using of static cone penetration data [IS:2911(Part1 /Sec 1):2010] For non homogeneous soil, The ultimate point bearing capacity can be taken as q c0 + q c1 + q c2 q pu = 2 2 qc0 is the average cone resistanc e qc1 is the minimum cone resistanc e qc2 is the average of minimum cone resistanc e
