Session 19 – 20 PILE FOUNDATIONS Course : S0484/Foundation Engineering Year : 2007 Version : 1/0 Session 19 – 20 PILE FOUNDATIONS

PILE FOUNDATIONS Topic: Settlement of Piles Laterally Loaded Piles Pull Out Resistance of Piles Pile Driving Formula Negative Skin Friction

S = S1 + S2 + S3 Where: S = total pile settlement SETTLEMENT OF PILES S = S1 + S2 + S3 Where: S = total pile settlement S1 = elastic settlement of pile S2 = settlement of pile caused by the load at the pile tip S3 = settlement of pile caused by the load transmitted along the pile shaft

SETTLEMENT OF PILES Where: Qwp = load carried at the pile point under working load condition Qws = load carried by frictional (skin) resistance under working load condition Ap = area of pile cross section Ep = modulus of elasticity of the pile material L = length of pile  = the magnitude which depend on the nature of unit friction (skin) resistance distribution along the pile shaft.

SETTLEMENT OF PILES Where: qwp = point load per unit area at the pile point = Qwp/Ap D = width or diameter of pile Es = modulus of elasticity of soil at or below the pile point s = poisson’s ratio of soil Iwp = influence factor = r

SETTLEMENT OF PILES Where: Qws = friction resistance of pile L = embedment length of pile p = perimeter of the pile Iws = influence factor

EXAMPLE The allowable working load on a prestressed concrete pile 21 m long that has been driven into sand is 502 kN. The pile data are as follow: - Diameter (D) = 356 mm The area of cross section (Ap) = 1045 cm2 Perimeter (p) = 1.168 m Skin resistance carries 350 kN of the allowable load, and point bearing carries the rest. Use Ep = 21 x 106 kN/m2, Es = 25,000 kN/m2, s = 0.35 and  = 0.62) Determine the settlement of the pile.

EXAMPLE S = S1 + S2 + S3 = 3.35 + 15.5 + 0.84 = 19.69 mm

LATERALLY LOADED PILE

LATERALLY LOADED PILE ELASTIC SOLUTION – EMBEDDED IN GRANULAR SOIL

LATERALLY LOADED PILE

LATERALLY LOADED PILE For L/T  5

LATERALLY LOADED PILE

LATERALLY LOADED PILE

LATERALLY LOADED PILE ELASTIC SOLUTION – EMBEDDED IN COHESIVE SOIL

LATERALLY LOADED PILE

LATERALLY LOADED PILE ULTIMATE LOAD ANALYSIS – MEYERHOF – PILES IN SAND ULTIMATE LOAD RESISTANCE (Qu(g))

LATERALLY LOADED PILE MAXIMUM MOMENT, Mmax DUE TO THE LATERAL LOAD Qu(g) For long (flexible) piles in sand MAXIMUM MOMENT, Mmax DUE TO THE LATERAL LOAD Qg

LATERALLY LOADED PILE ULTIMATE LOAD ANALYSIS – MEYERHOF – PILES IN CLAY ULTIMATE LOAD RESISTANCE (Qu(g))

LATERALLY LOADED PILE MAXIMUM MOMENT, Mmax DUE TO THE LATERAL LOAD Qu(g) For long (flexible) piles MAXIMUM MOMENT, Mmax DUE TO THE LATERAL LOAD Qg

PULL OUT RESISTANCE OF PILES

PULL OUT RESISTANCE OF PILES

PULL OUT RESISTANCE OF PILES EXAMPLE: A concrete pile 50 long is embedded in a saturated clay with cu = 850 lb/ft2. The pile is 12 in. x 12 in. in cross section. Use FS = 4 and determine the allowable pullout capacity of the pile Solution Given cu = 850 lb/ft2  40.73 kN/m2 ’ = 0.9 – 0.00625cu = 0.9 – (0.00625)(40.73) = 0.645

PULL OUT RESISTANCE OF PILES

PULL OUT RESISTANCE OF PILES For dry soils, the equation simplifies to Determine the value of Ku and  from figure 9.36b and 9.36c. Where Tun(all) = allowable uplift capacity and FS is Factor of Safety (a value of 2 – 3 is recommended)

PULL OUT RESISTANCE OF PILES EXAMPLE: a precast concrete pile with a cross section 350 mm x 350 mm is embedded in sand. The length of pile is 15 m. Assume that sand = 15.8 kN/m3, sand = 35o, and the relative density of sand = 70%. Estimate the allowable pullout capacity of the pile (FS = 4) Solution From figure 9.36, for  = 35o and relative density = 70%

PILE DRIVING FORMULA

NEGATIVE SKIN FRICTION Can occur under condition such as: If a fill of clay soil is placed over a granular soil layer into which a pile is driven, the fill will gradually consolidate. This consolidation process will exert a downward drag force on the pile during a period of consolidation If a fill of granular soil is placed over a layer of soft clay. It will induce the process of consolidation in the clay layer and thus exert a downward drag on the pile

NEGATIVE SKIN FRICTION CLAY FILL OVER GRANULAR SOIL

NEGATIVE SKIN FRICTION GRANULAR SOIL FILL OVER CLAY THE UNIT NEGATIVE SKIN FRICTION AT ANY DEPTH FROM z = 0 TO z = L1

NEGATIVE SKIN FRICTION