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Pile foundations

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**Capacity of Single Pile**

Using Theory (c,φ) Using SPT value Using SCPT Value Using Dynamic Formula Pile Load Test Static Formula In-situ Penetration Tests

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**Dynamic Formula - PRINCIPLE**

Energy Workdone Wh Qus W - weight of the driving hammer h height of fall of hammer Wh - energy of hammer blow Q - ultimate resistance to penetration S - pile penetration under one hammer blow Qus - resisting energy of the pile

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**Energy used by pile to move down**

DYNAMIC FORMULA Hileys Formula Engineering News Energy used by pile to move down Energy losses Energy Input

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Hiley’s Formula The energy loss E1 due to the elastic compressions of the pile cap, pile material and the soil surrounding the pile The energy loss E2 due to the interaction of the pile hammer system

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**Hileys Formula - Energy losses**

The energy loss E1 due to the elastic compressions of the pile cap, pile material and the soil surrounding the pile c1 = elastic compression of the pile cap c2 = elastic compression of the pile c3 = elastic compression of the soil.

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**Hileys Formula - Energy Losses**

The energy loss E2 due to the interaction of the pile hammer system Wp = weight of pile Cr = coefficient of restitution

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Hileys Formula where, ηh – Efficiency of the hammer

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**Hileys Formula Elastic compression c1 of cap and pile head**

Pile Material Range of Driving Stress kg/cm2 Range of c1 Precast concrete pile with packing inside cap 30-150 Timber pile without cap Steel H-pile Elastic compression c2 of pile Elastic compression c3 of soil Type of soil c3 Hard Soil Resilient Soil 0.2

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**Hileys Formula Efficiency of pile hammer Hammer Type ηh Drop hammer**

1.00 Single acting Double acting 0.85 Diesel Coefficient of restitution Cr Material Cr Wood pile 0.25 Compact wood cushion of steel pile 0.32 Cast iron hammer on concrete pile without cap 0.40 Cast iron hammer on steel pile without cushion 0.55

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**Engineering News Formula**

W - weight of hammer in kg H - height of fall of hammer in cm s - final penetration in cm per blow (set) C - empirical constant The set is taken as the average penetration per blow for the last 5 blows of a drop hammer or 20 blows of a steam hammer C = 2.5 cm for a drop hammer C = 0.25 cm for single acting hammer

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Problem A 40 x 40 cm reinforced concrete pile 20 m long is driven through loose sand and then into dense gravel to a final set of 3 mm/blow, using a 30 kN single-acting hammer with a stroke of 1.5 m. Determine the ultimate driving resistance of the pile if it is fitted with a helmet, plastic dolly and 50 mm packing on the top of the pile. The weight of the helmet and dolly is 4 kN. The other details are: weight of pile = 74 kN; weight of hammer = 30 kN; pile hammer efficiency ηh = 0.80 and coefficient of restitution Cr = Use the Hiley formula. The sum of the elastic compression C is C = c1 +c2 +c3 = 19.6 mm.

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PILE LOAD TEST

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Pile Load Test Load tests may be carried out on a working pile or a test pile Pile load tests on a single pile or group of piles For the determination of Vertical load bearing capacity Uplift load capacity Lateral load capacity Load test may be of two types Continuous load test. Cyclic load test.

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**Vertical Pile Load Test Assembly**

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**Load – Settlement Curves**

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**Determination of Qu from Load-Settlement Curve**

Qu, can be determined as the abscissa of the point where the curved part of the load-settlement curve changes to a falling straight line Qu is the abscissa of the point of intersection of the initial and final tangents of the load-settlement curve Qa is 50 percent of the ultimate load at which the total settlement amounts to one-tenth of the diameter of the pile for uniform diameter piles. Qa is sometimes taken as equal to two-thirds of the load which causes a total settlement of 12 mm Qa is sometimes taken as equal to two-thirds of the load which causes a net (plastic) settlement of 6 mm

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**Recap - Capacity of Single Pile**

Using Theory (c,φ) Using SPT value Using SCPT Value Using Dynamic Formula Pile Load Test Static Formula In-situ Penetration Tests

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PILE GROUPS

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Some Examples Multistoried Building Resting on Piles

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Some Examples Piles Used to Resist Uplift Forces

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Some Examples Piles used to Resist lateral Loads

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**Pressure isobars of single pile**

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**Pressure Isobars of Group of piles with piles placed farther apart**

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**Pressure Isobars of Group of piles closely spaced**

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**Typical Arrangement of Piles in Groups**

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**Minimum Spacing between Piles**

Stipulated in building codes For straight uniform diameter piles - 2 to 6 d For friction piles – 3d For end bearing piles passing through relatively compressible strata, the spacing of piles shall not be less than 2.5d For end bearing piles passing through compressible strata and resting in stiff clay - 3.5d For compaction piles - 2d.

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Pile Group Efficiency

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**Efficiency of pile groups in sand (Vesic, 1967)**

1. Point efficiency—average of all tests 2. 4 pile group—total efficiency 3. 9 pile group—total efficiency 4. 9 pile group—total efficiency with cap 5. 4 pile group—total efficiency with cap 6. 4 pile group—skin efficiency 7. 9 pile group—skin efficiency

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**CAPACITY OF PILE GROUP Feld’s Rule Converse-Labarre Formula**

Block failure criteria

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FELD'S RULE Reduces the capacity of each pile by 1/16 for each adjacent pile

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**CONVERSE-LABARRE FORMULA**

m = number of columns of piles in a group, n = number of rows, θ = tan-1( d/s) in degrees, d = diameter of pile, s = spacing of piles center to center.

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**PILE GROUP Driven piles Bored piles Pile group in sandy soil**

Pile group in clayey soil

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**Block Failure c = cohesive strength of clay beneath the pile group,**

L = length of pile, Pg = perimeter of pile group, A g= sectional area of group, Nc = bearing capacity factor which may be assumed as 9 for deep foundations.

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Recap Capacity of single pile Capacity of pile group

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